WO2014156261A1 - Communication system - Google Patents

Communication system Download PDF

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Publication number
WO2014156261A1
WO2014156261A1 PCT/JP2014/051591 JP2014051591W WO2014156261A1 WO 2014156261 A1 WO2014156261 A1 WO 2014156261A1 JP 2014051591 W JP2014051591 W JP 2014051591W WO 2014156261 A1 WO2014156261 A1 WO 2014156261A1
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WO
WIPO (PCT)
Prior art keywords
communication device
cell
mobile communication
base station
candidate
Prior art date
Application number
PCT/JP2014/051591
Other languages
French (fr)
Inventor
Thomas Delsol
Original Assignee
Nec Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nec Corporation filed Critical Nec Corporation
Publication of WO2014156261A1 publication Critical patent/WO2014156261A1/en

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W36/00—Hand-off or reselection arrangements
    • H04W36/0005—Control or signalling for completing the hand-off
    • H04W36/0055—Transmission or use of information for re-establishing the radio link
    • H04W36/0061—Transmission or use of information for re-establishing the radio link of neighbour cell information
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W36/00—Hand-off or reselection arrangements
    • H04W36/0005—Control or signalling for completing the hand-off

Definitions

  • the present invention relates to a communication system and to components thereof for providing communication services to mobile or fixed communication devices.
  • the invention has particular but not exclusive relevance to handover procedures from/to user equipment in Long Term Evolution (LTE) communication systems currently being developed by the 3 rd Generation Partnership Project (3 GPP).
  • LTE Long Term Evolution
  • a base station i.e. evolved NodeB, eNB
  • RAN Radio Access Network
  • CN core network
  • UEs User Equipment
  • LPN Low Power Node
  • Heterogeneous Networks or HetNets.
  • NPL 1 3 GPP standard TS 36.331 vl 1.1.0
  • handover may fail due to:
  • the handover procedure may fail more regularly.
  • the present invention therefore aims to provide an improved communication system and improved components of the communication system which overcome or at least alleviate one or more of the above issues.
  • the invention provides a mobile communication device for use in a communication network comprising a plurality of base stations each operating at least one cell, the communication device comprising means for communicating with a serving cell of the cells, which cell may be operated by a serving base station of the plurality of base stations; means for receiving, from the serving base station, information identifying a plurality of candidate cells for handover of the mobile communication device; means for selecting a target cell, from the plurality of candidate cells, to which handover of the mobile communication device may be attempted; and means for initiating handover of the mobile communication device to the selected target cell.
  • the mobile communication device may further comprise means for performing measurements relating to at least one of the plurality of cells; and means for sending, to the serving base station, a measurement report based on the measurements wherein the information identifying the plurality of candidate cells may be sent by the serving base station responsive to the measurement report.
  • the target cell selection means may be configured to select a target cell based on the measurements.
  • the target cell selection means may be configured to select a target cell based on measurements performed by the measurement performing means after the measurement report has been sent.
  • the measurement performing means may be configured to perform at least one measurement relating to each candidate cell.
  • the measurement performing means may be configured to perform at least one further measurement in response to the reception of information identifying a plurality of candidate cells by the receiving means and the target cell selection means may be configured to select a target cell based on the at least one further measurement.
  • the mobile communication device may further comprise means for receiving, from the serving base station, cell selection criteria for use by the selecting means in selecting a target cell from the plurality of candidate cells wherein the target cell selection means may be configured to select a target cell based on the cell selection criteria.
  • the target cell selection means may be configured to apply the cell selection criteria to at least one measurement associated with a candidate cell of the plurality of candidate cells.
  • the cell selection criteria may comprise an offset value associated with a candidate cell of the plurality of candidate cells and the target cell selection means may be configured to apply the offset value to a measurement associated with the candidate cell associated with the offset value.
  • the cell selection criteria may comprise an offset value associated with a frequency and the target cell selection means may be configured to apply the offset value to a measurement associated with a candidate cell which operates at the frequency.
  • the cell selection criteria may comprise a hysteresis value associated with a candidate cell of the plurality of candidate cells and the target cell selection means may be configured to apply the hysteresis value to a measurement associated with the candidate cell associated with the hysteresis value.
  • the cell selection criteria may comprise a penalty value associated with a candidate cell of the plurality of candidate cells and the target cell selection means may be configured to apply the penalty value to a measurement associated with the candidate cell associated with the penalty value.
  • the target cell selection means may be configured to use the cell selection criteria and the measurements to determine a priority for each of the candidate cells, and the target cell for handover of the mobile communication device may be selected based on the priority (e.g. to be the highest priority target cell).
  • the mobile communication device may further comprise means for determining that a handover failure has occurred, the selecting means may be operable to select, responsive to a determination that a handover failure has occurred, a further cell, from the plurality of candidate cells, to which a connection procedure for the mobile communication device may be attempted and means for initiating the connection procedure to the selected further cell.
  • the connection procedure may comprise an RRC connection re-establishment procedure.
  • the connection procedure may comprise a handover procedure.
  • the selecting means may be operable to select a plurality of cells, including the target cell, from the plurality of candidate cells, to which handover of the mobile communication device may be attempted and the initiating means may be operable to initiate handover of the mobile communication device to each of the selected plurality of cells.
  • the information identifying a plurality of candidate cells may be received by the receiving means in a handover command.
  • the information identifying a plurality of candidate cells may comprise information identifying at least three candidate cells.
  • the information identifying a plurality of candidate cells may comprise a candidate cell list.
  • the information identifying a plurality of candidate cells may identify each candidate cell using a Cell ID associated with the candidate cell.
  • the information identifying a plurality of candidate cells may identify each candidate cell using a frequency associated with the candidate cell.
  • the invention provides a base station for use in a communication network comprising a plurality of neighbouring cells each operated by a neighbouring base station, the base station comprising: means for communicating with a mobile communication device located within a cell operated by the base station; means for selecting, from said plurality of neighbouring cells, a plurality of candidate cells for handover of the mobile communication device; means for generating information identifying said plurality of candidate cells; and means for sending said information identifying said plurality of candidate cells to the mobile communication device.
  • the base station may further comprise means for receiving a measurement report from the mobile communication device, the measurement report based on measurements relating to at least one of the neighbouring cells wherein the candidate target cell selecting means may be configured to select the plurality of candidate target cells based on the measurement report.
  • the base station may further comprise means for receiving information relating to communication conditions prevailing in at least one neighbouring cell from the at least one neighbouring base station wherein the candidate cell selecting means may be configured to select the plurality of candidate cells based on the information relating to communication conditions prevailing in the neighbouring cells received from the at least one neighbouring base station.
  • the information relating to communication conditions prevailing in the at least one neighbouring cell received from the at least one neighbouring base station may comprise cell load information.
  • the base station may comprise means for sending a handover command to each of the candidate cells, informing each candidate cell to prepare for handover of the mobile
  • the base station may further comprise means for receiving a handover request acknowledge message from each of the candidate cells responsive to the respective handover command.
  • the handover request acknowledge message may comprise information relating to the respective candidate cell.
  • the information relating to the respective candidate cell may comprise at least one of: cell frequency; cell ID; cell load; prevailing communication conditions.
  • the base station may further comprise means for generating cell selection criteria for use by the selecting means in selecting a target cell from the plurality of candidate cells and means for sending the cell selection criteria to the mobile communication device.
  • the cell selection criteria may comprise an offset value associated with a candidate cell of the plurality of candidate cells.
  • the cell selection criteria may comprise an offset value associated with a frequency.
  • the cell selection criteria may comprise a hysteresis value associated with a candidate cell of the plurality of candidate cells.
  • the cell selection criteria may comprise a penalty value associated with a candidate cell of the plurality of candidate cells.
  • the criteria may be generated based on the measurement report.
  • the criteria may be generated based on the prevailing conditions in each candidate cell.
  • the information identifying the plurality of candidate cells may comprise a candidate cell list.
  • the base station may be operable to order the candidate cell list in a prioritised order.
  • the prioritised order may be based on the prevailing conditions in each respective candidate cell (e.g. the load in each candidate cell).
  • the sending means may be configured to send the information identifying the plurality of candidate cells in a handover command.
  • the sending means may be configured to send the cell selection criteria in the handover command.
  • the information identifying a plurality of candidate cells may comprise information identifying at least three candidate cells.
  • the information identifying a plurality of candidate cells may identify each candidate cell using a Cell ID associated with the candidate cell.
  • the information identifying a plurality of candidate cells may identify each candidate cell using a frequency associated with the candidate cell.
  • the invention provides a communication system
  • the base station comprises; means for communicating with the mobile communication device; means for selecting, from said plurality of neighbouring cells, a plurality of candidate cells for handover of the mobile communication device; means for generating information identifying said plurality of candidate cells; and means for sending said information identifying said plurality of candidate cells to the mobile communication device; and wherein the mobile communication device comprises: means for communicating with the base station; means for receiving, from said base station, information identifying a plurality of candidate cells for handover of the mobile communication device; means for selecting a target cell, from said plurality of candidate cells, to which handover of the mobile communication device is to be attempted; and means for initiating handover of the mobile communication device to the selected target cell.
  • the invention provides a method performed by a mobile communication device in a communication network comprising a plurality of base stations each operating at least one cell, the method comprising: communicating with a serving cell of said cells, which cell is operated by a serving base station of said plurality of base stations; receiving, from said serving base station, information identifying a plurality of candidate cells for handover of the mobile communication device; selecting a target cell, from said plurality of candidate cells, to which handover of the mobile communication device is to be attempted; and initiating handover of the mobile communication device to the selected target cell.
  • the invention provides a method performed by a base station in a communication network comprising a plurality of neighbouring cells each operated by a neighbouring base station, the method comprising: communicating with a mobile communication device located within a cell operated by the base station; selecting, from said plurality of neighbouring cells, a plurality of candidate cells for handover of the mobile communication device; generating information identifying said plurality of candidate cells; and sending said information identifying said plurality of candidate cells to the mobile
  • the invention provides a method performed in a
  • the method comprising: the base station: communicating with the mobile communication device; selecting, from said plurality of neighbouring cells, a plurality of candidate cells for handover of the mobile communication device; generating information identifying said plurality of candidate cells; and sending said information identifying said plurality of candidate cells to the mobile communication device; and the mobile
  • the invention provides a mobile communication device for use in a communication network comprising a plurality of base stations each operating at least one cell, the communication device comprising: a transceiver operable to: communicate with a serving cell of said cells, which cell is operated by a serving base station of said plurality of base stations; and receive, from said serving base station, information identifying a plurality of candidate cells for handover of the mobile communication device; and a processor operable to: select a target cell, from said plurality of candidate cells, to which handover of the mobile communication device is to be attempted; and initiate handover of the mobile communication device to the selected target cell.
  • a transceiver operable to: communicate with a serving cell of said cells, which cell is operated by a serving base station of said plurality of base stations; and receive, from said serving base station, information identifying a plurality of candidate cells for handover of the mobile communication device
  • a processor operable to: select a target cell, from said plurality of candidate cells, to which handover of the mobile communication device is
  • the invention provides a base station for use in a communication network comprising a plurality of neighbouring cells each operated by a neighbouring base station, the base station comprising: a transceiver operable to: communicate with a mobile communication device located within a cell operated by the base station; and a processor operable to: select, from said plurality of neighbouring cells, a plurality of candidate cells for handover of the mobile communication device; and generate information identifying said plurality of candidate cells; and wherein the transceiver is further operable to: send said information identifying said plurality of candidate cells to the mobile communication device.
  • Another aspect of the present invention provides a computer program product comprising computer implementable instructions for causing a programmable computer device to become configured as a communication device as described above or as a base station as described above.
  • the invention also provides a corresponding system, methods and computer software products that may be provided on a carrier signal or on a recording medium, such as a CD, DVD or the like.
  • Fig. 1 schematically illustrates a mobile telecommunication system including a cell currently serving a mobile communication device along with neighbouring cells;
  • Fig. 2 is a block diagram illustrating the main components of a mobile communication device forming part of the system shown in Fig. 1 ;
  • Fig. 3 is a block diagram illustrating the main components of a base station forming part of the system shown in Fig. 1 ;
  • Fig. 4 is a timing diagram showing signalling messages exchanged between the mobile communication device, a serving base station and further base stations;
  • Fig. 5 is a timing diagram showing signalling messages exchanged between the mobile communication device, the serving base station and further base stations, where handover failure occurs;
  • Fig. 6 is an alternative timing diagram showing signalling messages exchanged between the mobile communication device, the serving base station and further base stations, where handover failure occurs;
  • Fig. 7 is a further timing diagram showing signalling messages exchanged between the mobile communication device, the serving base station and further base stations;
  • Fig. 8 is a flow chart illustrating a method, performed by the serving base station, of selecting candidate target cells
  • Fig. 9 is a flow chart illustrating a method, performed by the mobile communication device, of selecting a target cell from the candidate target cells.
  • Fig. 1 schematically illustrates a mobile (cellular) telecommunication system 1 including a mobile communication device 3 and base stations 5, comprising a plurality of base stations 5-1 to 5-4.
  • cells 1 to 4 are macro cells while cells 3 and 4 are small cells.
  • Each base station 5 is coupled to a core network 7 via an 'S 1 ' interface and the core network 7 is also coupled to other networks (e.g. the Internet) via one or more gateways (not shown).
  • the core network 7 includes, amongst other things, a mobility management entity (MME), a home subscriber server (HSS), a serving gateway (SGW) and a Packet Data Network (PDN) Gateway (PGW), which have also been omitted for sake of simplicity.
  • MME mobility management entity
  • HSS home subscriber server
  • SGW serving gateway
  • PGW Packet Data Network Gateway
  • PGW Packet Data Network Gateway
  • the 'SI ' interface between the base stations 5 and the core network 7 might utilise, for example, a high speed, high bandwidth communication link, such as an optical fibre link and the like.
  • An 'X2' interface (not shown) is also provided between neighbouring base stations 5 to facilitate data exchange between them and to compute eNB neighbour list.
  • FIG. 1 As those skilled in the art will appreciate, whilst one mobile communication device 3 and four base stations 5 are shown in Fig. 1 for illustration purposes, additional user equipment and/or base stations may be present in a deployed system.
  • mobile communication device 3 is located within cell 1 , and base station
  • 5-1 acts as a serving cell for the mobile communication device 3. Connection of the mobile communication device 3 with a serving base station 5-1 allows the mobile communication device 3 to perform communication actions such as registering with the network, updating its registration, and establishing a data session or a voice call.
  • the mobile communication device 3 when the mobile communication device 3 approaches the edge of cell 1 , or when the signal conditions within this cell begin to deteriorate, for example due to path loss, physical environment or power limitation, it becomes necessary for the mobile communication device 3 to handover to another cell, such as cell 2, cell 3, cell 4, or any other suitable cell.
  • another cell such as cell 2, cell 3, cell 4, or any other suitable cell.
  • the serving base station 5-1 receives information from the mobile communication device 3 which it uses to determine when handover becomes necessary for the mobile
  • the information received by the base station 5-1 could be any information measured by the mobile communication device 3 or information deduced from the measurements performed by the mobile communication device 3.
  • the mobile communication device 3 might classify its neighbour cells in order of received power.
  • the mobile communication device 3 is therefore configured (by or via base station 5-1) to perform signal measurements with respect to its neighbouring cells and/or serving cell and to report the results based on these measurements when certain predetermined signal conditions are met.
  • the mobile communication device 3 is configured to perform signal measurements periodically, according to a pre-defined periodicity. Criteria defining the predetermined signal conditions for measurement reporting are provided by the serving base station 5-1. In some instances, the reporting signal conditions will not be met, in which case no measurement report will be sent. Therefore, after having sent the measurement report to the network following event trigger, the mobile communication device 3 will continue to monitor the neighbour cells but may not report these measurements to the serving base station 5-1, depending on the reporting signal conditions set by the network.
  • the mobile communication device might determine signal conditions in any given cell by measuring either one of a Reference Signal Receive Power (RSRP), a Reference Signal Receive Quality (RSRQ), Received Signal Strength Indicator (RSSI) and a Received Signal Code Power (RSCP) of that cell - depending on the access technology used in that cell.
  • RSRP Reference Signal Receive Power
  • RSSI Received Signal Strength Indicator
  • RSCP Received Signal Code Power
  • RSRP Reference Signal Received Power
  • measurement is defined as the linear average of the received power of the resource elements carrying cell-specific reference signals within the considered measurement frequency bandwidth.
  • RSRQ Reference Symbol Received Quality
  • RSRP Received Signal Strength Indicator
  • the E-UTRA carrier RSSI is the linear average of the total received power in OFDM symbols containing the reference symbols.
  • the 3GPP standard TS 36.331 vl 1.1.0 defines measurement reporting triggering related to eight different event types (e.g. Events Al to A6, Bl, and B2) that the base station may configure for user equipment within its cell(s). It will be appreciated that additional events may be defined in future version of the specification.
  • event report describing the event type and associated parameters is triggered. For example, an event may occur if measured signal conditions of a neighboring cell become better than a pre-defined threshold, or measured signal conditions of the serving cell becomes worse than a pre-defined threshold.
  • the generation of the configuration parameters for configuring the signal measurements (for example their periodicity) and the reporting signal conditions may take place in the base station 5-1, or in the core network 7, in which case the configuration parameters are forwarded to the base station 5-1 which in turn forwards the configuration parameters to the mobile communication device 3.
  • the serving base station 5-1 receives the results of any measurement (i.e. an indication that a configured event has occurred) using RRC signalling or the like.
  • the serving base station 5-1 determines, based on the reported measurements, that handover is necessary, the serving base station 5-1 proceeds to select a plurality of candidate target cells using the received measurement results.
  • the plurality of candidate target cells may, for example, be cells which provide more favourable signal conditions to the mobile communication device 3.
  • the serving base station 5-1 After the serving base station 5-1 has selected a plurality of candidate target cells for handover of the mobile communication device 3, the serving base station 5-1 advantageously provides the mobile communication device 3 with a list of the selected plurality of candidate target cells and in this embodiment also provides the mobile communication device 3 with further criteria for use, by the mobile communication device 3, to select a handover target cell from the list of candidate target cells.
  • the mobile communication device 3 is configured to select one of the candidate target cells in the candidate target cell list provided by the base station 5-1 to be the target cell to which the mobile communication device 3 will attempt to hand over (e.g. taking account of any further criteria provided by the base station and any local factors appropriately).
  • allowing the mobile communication device 3 to select a target cell from a candidate target cell list rather than relying on the serving base station 5-1 to select the target cell allows the mobile communication device 3 to recover from a handover failure more quickly, for example by selecting a further cell from the candidate target cell list to attempt connection with, thereby reducing the need for any additional signalling with the serving and/or other base stations (e.g. to establish a new target cell to which to direct handover).
  • the mobile communication device 3 allows the mobile communication device make the selection taking into account currently prevailing local communication conditions as characterised by its most recent measurements of signal conditions, even if these measurements were not reported to the serving base station 5-1 in a measurement report.
  • the mobile communication device 3 can also make use of previously measured or previously reported measurements, which may, for example, allow the mobile communication device to assess its mobility. As a result, the likelihood of failure of handover of the mobile communication device 3 from the serving cell 5-1 to the target cell selected by the mobile communication device 3 is minimised.
  • Fig. 2 is a block diagram illustrating the main components of the mobile communication device 3 shown in Fig. 1.
  • the mobile communication device 3 includes transceiver circuitry 31 which is operable to transmit signals to, and to receive signals from, the base station 5 and/or other mobile communication devices (not illustrated in Fig. 1) via at least one antenna 33.
  • the mobile communication device 3 may of course have all the usual functionalities of a conventional mobile communication device (such as a user interface 35) and this may be provided by any combination of hardware, software and firmware, as appropriate.
  • the operation of the transceiver circuit 31 is controlled by a controller 37 in accordance with software stored in memory 39.
  • the software includes, among other things, an operating system 41, a
  • communications control module 43 a measurement module 45, a reporting module 47, a target cell selection module 48 and a handover module 49.
  • the communications control module 43 is operable to handle (e.g. generate, send and receive) control signals for controlling the connections between the mobile communication device 3 and other user equipment or various network nodes, such as the serving base station 5-1.
  • the measurement module 45 is operable to perform desired signal measurements (e.g. CRS or CSI-RS measurements), to determine associated signal quality values (e.g. RSRP and RSRQ) and to determine when particular configured event conditions (e.g. Al to B2 or R0 to R6) have been met.
  • desired signal measurements e.g. CRS or CSI-RS measurements
  • associated signal quality values e.g. RSRP and RSRQ
  • particular configured event conditions e.g. Al to B2 or R0 to R6
  • the reporting module 47 is operable to generate and send information (e.g. by triggering a measurement report) to the serving base station 5-1, when one of the configured events has occurred (e.g. based on the results of signal measurements performed by the measurement module 45).
  • the information sent includes an indication of each neighbouring cell to which the event relates and/or any further information that may assist the serving base station 5-1 in selecting a plurality of candidate target cells.
  • the target cell module 48 is configured to use the candidate target cell list received by the mobile communication device 3 from the serving base station 5-1 to select a target cell to which the mobile communication device 3 will attempt to handover. Preferably, this selection is made based on criteria received from the serving base station 5-1 and in view of the latest available measurements performed by the mobile communication devices measurement module 45. The target cell selected by the target cell selection module 48 is notified to the handover module 49.
  • the handover module 49 is operable to perform handover of the mobile communication device 3 from serving base station 5-1 to the target cell selected by the target cell selection module.
  • Fig. 3 is a block diagram illustrating the main components of one of the base stations 5 shown in Fig. 1, such as serving base station 5-1.
  • the base station 5 includes transceiver circuitry 51 which is operable to transmit signals to, and to receive signals from, the mobile communication device 3 via at least one antenna 53.
  • the base station 5 is also operable to transmit signals to and to receive signals from nodes in the core network 7 (such as the MME or the SGW) and other base stations, via a network interface 55.
  • the operation of the transceiver circuit 51 is controlled by a controller 57 in accordance with software stored in memory 59.
  • the software includes, among other things, an operating system 61, a communications control module 63, a measurement configuration module 65, a candidate target cell module 67 and a handover command module 69.
  • the communications control module 63 is operable to control communications between the base station 5 and the mobile communication device 3, and the network devices such as the MME, the HSS, the SGW, the PGW, and neighbour base stations.
  • the measurement configuration module 65 is operable to control the mobile
  • the measurement configuration module 65 is also operable to pass the relevant information from the measurement report to the candidate target cell module 67 for the purposes of identifying a plurality of candidate target cells.
  • the candidate target cell module 67 is operable to use information received in a measurement report from the mobile communication device 3 to identify a plurality of candidate target cells for handover of the mobile communication device 3. For example, the target cell list module may determine that target cell 2 and target cell 3 as illustrated in Fig. 1 have measured signal strengths which are above a threshold and therefore cells 2 and 3 qualify as candidate target cells. After identifying a plurality of candidate target cells for the candidate target cell list, the candidate target cell module 67 informs the handover command module 69 of the chosen candidate target cells.
  • the handover command module 69 is operable to receive the chosen candidate target cells from the candidate target cell module 67.
  • the handover command module 69 generates a handover command message for transmission to the mobile communication device 3, in which the handover command message includes a list of the candidate target cells chosen by the candidate target cell module 67. This allows the mobile communication device 3, after receipt of the handover command generated by the handover candidate command module 69, to select a target cell from the candidate target cell list included with the handover command.
  • the handover command module 69 may also additionally include, within the handover command, criteria for use in the selection of a target cell from the candidate target cell list by the mobile communication device 3. Alternatively, these criteria may be provided separately by the base station 5-1.
  • the mobile communication device 3 and the base station 5 are described for ease of understanding as having a number of discrete modules (such as the communications control modules, the reporting module, and the handover command module). Whilst these modules may be provided in this way for certain applications, for example where an existing system has been modified to implement the invention, in other applications, for example in systems designed with the inventive features in mind from the outset, these modules may be built into the overall operating system or code and so these modules may not be discernible as discrete entities. These modules may also be implemented in software, hardware, firmware or a mix of these.
  • Fig. 4 illustrates a handover procedure according to one embodiment.
  • the mobile communication device 3 selects a target cell for handover based on a candidate target cell list received from the serving base station and the latest measurements taken for neighbouring cells.
  • the mobile communication device 3 transmits a measurement report to the serving base station 5-1, providing the base station 5-1 with the results of measurements performed by the mobile communication device 3 with respect to the cells neighbouring the serving cell (e.g. cell 2, cell 3, and cell 4) and with respect to the serving cell (cell 1).
  • the serving cell e.g. cell 2, cell 3, and cell 4
  • Triggering of transmission of the measurement report by the mobile communication device 3 is caused when the mobile communication device 3 determines that a preconfigured event has occurred, for example one of events Al to B2 as described previously.
  • measurement report may, for example, include respective reference signal received power (RSRP) and/or reference symbol received quality (RSRQ) measurements relating to each of the measured cells (e.g. Cells 1 to 4).
  • RSRP reference signal received power
  • RSRQ reference symbol received quality
  • the serving base station 5-1 receives the measurement report transmitted in step 401 and identifies a plurality of candidate target cells for handover of the mobile
  • Cells 2 and 3 operated by base stations 5-2 and 5-3 respectively are selected as candidate target cells by the serving base station 5-1 (along with any other cells (not shown) that meet the criteria for selecting a target cell).
  • the serving base station 5-1 transmits a handover command to each of the neighbouring base stations, operating an identified candidate target cell, using the associated X2 interface (e.g. the neighbouring base station 5-2, the neighbouring base station 5-3, and each of any other identified candidate base stations).
  • the associated X2 interface e.g. the neighbouring base station 5-2, the neighbouring base station 5-3, and each of any other identified candidate base stations.
  • the base station 5-2 transmits a Handover Request Acknowledge message to the serving base station 5-1 over the X2 interface.
  • the Handover Request
  • Acknowledge message includes a transparent container containing a 'mobilityControlInfo' information element (IE) which provides information, relating to base station 5-2, for controlling mobility, such as its cell frequency and physical channel configuration.
  • IE 'mobilityControlInfo' information element
  • the serving base station 5-1 provides the base stations 5-2 and 5-3 with information relating to the mobile communication device 3 from which the mobile communication device's context, e.g. its RRC context, can be derived.
  • the mobile communication device's context e.g. its RRC context
  • the serving base station 5-1 is configured to generate, at step 415, a handover command which, in this embodiment, comprises an RRC connection reconfiguration message including a list of candidate target cells as identified by the serving base station 5-1 in step 402.
  • the candidate target cell list is provided in the form of a dedicated information element for providing the list and associated control information (referred to as 'enh-mobilityControl-InfoList' in this example).
  • the dedicated information element includes a control information element (also referred to here as 'mobilityControlInfo') based on the corresponding information element provided by each of the candidate target cell's base station 5-2 and 5-3 in the Handover Request Acknowledge message, in order to allow the UE to access either of the candidate target cells.
  • the handover command also includes, in this example, information specifying criteria for use, by the mobile communication device 3, in selecting a target cell from the candidate target cell list.
  • the base station includes, in this embodiment, information in the handover command that allows the mobile communication device 3 to select the candidate target cells in a priority order using the base station's own criteria, albeit that the priority order may change in dependence on the latest measurements performed by the mobile communication device 3.
  • the information may specify a respective offset and/or hysteresis parameter, for each candidate cell, that may be applied to the latest results of measurements for that cell in order to reduce and/or increase the relative priority attributed to that cell compared to other candidate cells.
  • the serving base station 5-1 transmits the generated handover command to the mobile communication device 3.
  • the mobile communication device 3 performs periodic measurements of the neighbouring cells 2 to 4 and serving cell 1 (at steps 407 and 417) and subsequently determines that no preconfigured event has occurred. As a result, no new measurement report is generated and sent to the serving base station 5-1, following steps 407 and 417, but the mobile communication device nevertheless maintains the measurements.
  • the mobile communication device 3 selects a target cell from the candidate target cell list by determining the most suitable cell based on the results of the latest
  • measurements in this case the measurements made at step 417, taking any criteria specified in the handover command message into account.
  • the mobile communication device 3 transmits a handover confirm message to the target base station selected in step 421, in this case the base station 5-2.
  • the base station 5-2 transmits a UE Context Release message to the base station 5-1, effectively informing the base station 5-1 that a successful handover has taken place, and of the identity of the candidate target cell to which handover has successfully completed.
  • the base station 5-1 transmits a handover cancel message to the base station of each candidate target cell to which the mobile communication device 3 has not been handed over to, in this example base station 5-3.
  • Figs. 5 to 7 illustrate further exemplary handover procedures.
  • Fig. 5 illustrates a further timing diagram of an exemplary handover procedure, similar to that of Fig. 4, in which handover failure occurs. Steps shown at 501 to 521 are generally equivalent to the equivalently numbered steps 401 to 421 in Fig. 4 (albeit that the steps of Fig. 5 are prefixed with a "5" rather than a "4"). Therefore, in the interests of brevity, steps 501 to 521 will not be described further here. [0114]
  • Handover failure may be established, for example, by expiry of a "T304" timer as defined in 3 GPP TS 36.331.
  • the mobile communication device 3 selects a cell from the candidate target cell list for R C connection re-establishment, excluding the cell to which handover was attempted at step 523, in this case cell 2.
  • Fig. 5 illustrates only two candidate target cells, and therefore in this embodiment the mobile communication device 3 selects the remaining candidate target cell - cell 3. It is noted that there are likely to be more candidate target cells than those illustrated, and therefore in these cases the selection of a target cell by the mobile communication device 3 is made by determining the most suitable remaining candidate target cell based on the latest measurements, such as the measurements made at step 517, or measurements performed subsequent to step 517 (not illustrated).
  • the mobile communication device 3 sends an RRC connection
  • the base station 5-3 sends an associated RRC connection re-establishment complete message to the mobile communication device 3.
  • the base station 5-3 transmits a UE Context Release message to the base station 5-1.
  • the base station 5-1 transmits a handover cancel message to the candidate target cells to which the mobile communication device 3 has not been handed over to, in this example base station 5-2.
  • Fig. 6 is a timing diagram of an exemplary handover procedure, similar to that of Fig. 5, in which handover failure also occurs. However, instead of performing re-establishment, the mobile communication device 3 transmits a handover confirm message to a cell selected from the candidate target cell list.
  • steps 601 to 625 of Fig. 6 are generally equivalent to steps 501 to 525 of Fig. 5 (albeit that the steps of Fig. 6 are prefixed with a "6" rather than a "5") and in the interests of brevity, steps 601 to 625 will not be described further here.
  • mobile communication device 3 selects a cell from the candidate target cell list for handover, excluding the cell to which handover was attempted at step 523, in this case cell 2.
  • the selection is made by determining the most suitable cell based on the latest measurements, in this case the measurements made at step 617. As explained above in relation to Fig. 6 illustrates only two candidate target cells, and therefore in this embodiment the mobile communication device 3 selects the remaining candidate target cell - cell 3.
  • the mobile communication device 3 transmits a handover confirm message to the target base station selected in step 621, in this case the base station 5-2.
  • the base station 5-3 transmits a UE Context Release message to the base station 5-1, informing the base station 5-1 that a successful handover has taken place.
  • the base station 5-1 transmits a handover cancel message to the candidate target cells to which the mobile communication device 3 has not been handed over to, in this example base station 5-2.
  • the embodiment illustrated in Fig. 6 is advantageous because it allows the mobile communication device 3 to access a target cell using the mobilityControlInfo information relating to the target cell present in the enh-mobilityControl-InfoList received in the Handover Command at step 619.
  • a new timer in addition to timers T304 and T310 may be used in order to allow relevant radio access to be kept valid, during the duration of the timer, so that the radio access can be used by the mobile communication device.
  • Fig. 7 is a timing diagram illustrating a similar method to Fig. 4, but where the mobile communication device 3 sends handover confirm messages to multiple candidate target cells. This method advantageously increases the chances that a successful handover will be completed, as the likelihood of handover failure to two or more different candidate target cells is
  • the mobile device is able to monitor several radio links.
  • the serving eNBl takes the final decision as to which cell (of those to which the mobile communication device has sent handover confirm messages) the UE will be handed over (for example based on the priority order that the serving eNB receives the UE context release messages from the target cells).
  • the UE may indicate in the handover confirm message the ranking or last measurements that the UE has performed to facilitate the serving eNBl decision.
  • the UE will continue to monitor each target eNB to receive the next message to know which has become the new serving eNB.
  • Fig. 8 illustrates a method, performed by the serving base station 5-1, of generating a candidate target cell list and cell selection criteria.
  • the serving base station 5-1 receives a measurement report from the mobile communication device 3.
  • the measurement report includes an RSRP and or an RSRQ for each of the neighbouring cells of the serving base station 5-1.
  • the base station 5-1 compares the RSSI of each neighbouring cell to a preconfigured threshold value. If the RSSI is greater than the preconfigured threshold value then the serving base station 5-1 designates the cell as a candidate target cell.
  • the base station 5-1 transmits a Handover Command to each of the candidate target cells. Subsequently, at step 807 the base station 5-1 receives a Handover Request Acknowledge message from each base station operating a candidate target cell, responsive to the respective Handover Command.
  • Each Handover Request Acknowledge message includes a 'mobilityControlInfo' information element (IE) which provides information relating to the respective candidate target cell.
  • the Handover Request Acknowledge message further includes a cell load indication (for example, as part of the 'mobilityControlInfo' information element (IE) or in a separate IE) although it will be appreciated that a load indication may be provided in separate signalling.
  • the base station 5-1 ranks the candidate target cells in priority order according to their cell load indication, with those cells having a lower cell load being allocated a higher ranking (for example, the cell with the lowest load achieves the highest ranking).
  • the base station 5-1 uses the rankings of the candidate target cells to generate cell selection criteria, the criteria including a penalty value allocated to each candidate target cell based on its ranking, where the higher a cell is ranked, the lower a penalty value it is assigned.
  • the base station 5-1 generates a candidate target cell list, in the form of an enh-mobilityControl-InfoList, identifying each of the candidate target cells.
  • the base station 5-1 sends the candidate target cell list and the cell selection criteria to the mobile communication device 3.
  • the allowability of handover to a particular candidate cell effectively is verified if the measured receiver power is greater than a minimum required receiver power as modified by an offset.
  • Hrxlev is a Cell Handover receiver level value (dB)
  • rxlevmeas is the measured cell receiver level value (RSRP) averaged over a period rxlevmin is the minimum required receiver level in the cell (dBm)
  • rxlevminoffset is an offset to be applied effectively to either favour or penalize a measured receiver level in accordance with a network configuration (e.g. for offloading of some cells or to favour a macro cell). This may be provided by the base station to the UE for each candidate cell.
  • the UE sends a Handover failure to the serving cell to indicate that there is no suitable cell.
  • the UE ranks the cell according to the criteria [(RSRP or RSRQ measurement) - the offset] calculating the RSRP and RSRQ values using averaged RSRP or RSRQ results.
  • W 201 [(RSRP or RSRQ measurement) - the offset] calculating the RSRP and RSRQ values using averaged RSRP or RSRQ results.
  • the UE selects the cell ranked most highly as the best cell for handover.
  • Hysteresis can be used to verify the criterion H is valid over a given period.
  • Fig. 9 illustrates a method, performed by the mobile communication device 3, of selecting a target cell from the candidate target cell list.
  • the mobile communication device 3 receives the candidate target cell list and the cell selection criteria from the base station 5-1.
  • the mobile communication device 3 retrieves the most recent measurements made for each of the candidate target cells, comprising an RSRP/RSRQ for each cell.
  • the mobile communication device 3 uses the cell measurements and the cell selection criteria to rank the candidate target cells in priority order, using the cell's RSSI and applying the penalty value associated with the cell given by the cell selection criteria.
  • the mobile communication device 3 selects the highest ranking candidate target cell as the target cell to which handover will be attempted.
  • the embodiments described above allow the selection of the best target cell by the mobile communication device from a candidate target cell list, which is advantageous over, for example, selection of a target cell by the network, because the mobile communication device can select a new candidate target cell from the list if handover fails, reducing recovery time from handover failure.
  • allowing the mobile communication device to select the target cell means the mobile communication device can take into consideration the most recent measurements available, and use the cell selection criteria given by the base station (e.g. penalty values). This increases the likelihood of a successful handover (or reestablishment procedure), and minimises the likelihood of ping-pong of the mobile communication device between cells.
  • These improvements in cell mobility are especially effective in a HetNets comprising multiple small cells, because these present a challenging network environment for handover procedures, where for example signalling conditions can change significantly over small distances and time scales.
  • the above described embodiments provide an improved handover performance, for example reducing handover failure, in any cellular communication network.
  • the base station is referred to as an eNB, the base station could be any other entity having similar functionality, for example a UE relay (or other relay node) acting as a base station, or relying information from a base station.
  • a UE relay or other relay node acting as a base station, or relying information from a base station.
  • the link provided between the base stations and the mobile communication device might use any 3 GPP technologies, such as GSM, UMTS, LTE, and the like.
  • a different (e.g. non-3GPP) communication technology might be used, such as Wi-Fi, CDMA, WiMAX, and the like.
  • the mobile communication device also supports other technologies than LTE (e.g. 3GPP technologies such as UMTS & HSPA or non-3 GPP technologies such as WiFi, WiMAX, CDMA) its transceiver circuit and communications control module might be implemented differently than described above.
  • small cell base stations e.g. femtocell base stations
  • gateways such as femto gateways (FGWs)
  • Iuh Iu home
  • the base stations are connected to a Radio Network Controller (RNC) via an "Iub" interface.
  • RNC Radio Network Controller
  • the measurements made by the mobile communication device might alternatively or additionally comprise measuring the strength of cell reference signals (CRS) or channel state reference signals (CSI-RS) broadcast by the neighbouring cells.
  • CRS cell reference signals
  • CSI-RS channel state reference signals
  • the mobile communication device might measure the value of, for example, RSRP and/or RSSI and/or RSRQ.
  • RSRP RSRP and/or RSSI and/or RSRQ.
  • other parameters might be measured, such as RSCR Measurements are to be expressed in dBm in case of RSRP, or in dB in case of RSRQ.
  • the mobile communication device may be required to perform RSRP and RSRQ measurements of at least 4 inter-frequency identified cells per E-UTRA carrier and to monitor up to at least 3 E-UTRA carriers. Therefore, an E-UTRA mobile communication device may be capable of measuring at least 12 inter-frequency cells.
  • the base station receives the results of any measurement using RRC signalling.
  • RRC signalling any type of signalling might be used, either additionally or alternatively.
  • the mobile equipment could receive a handover command from a plurality of nodes and send back a handover confirm to one or more nodes.
  • the handover command could also be adapted to include information that could be used to rank and select the best suitable cell.
  • the mobile equipment could be served by a UE relay acting as a base station or relaying information from a base station.
  • the base station 5-1 selects candidate target cells based on their respective RSSI
  • the base station may select candidate target cells using any appropriate method.
  • the plurality of candidate target cells may, for example, be cells which provide the most favourable signal conditions to the mobile communication device 3 or which the base station predicts will have favourable signal conditions as a result of UE mobility, for example where the mobile communication device is travelling towards the cell.
  • the serving base station 5-1 may identify candidate target cells by means other than using information received by the mobile telephone 3.
  • the base station 5-1 may use the X2 interface to communicate with neighbouring base stations (e.g. 5-2, 5-3 and 5-4) in order to identify candidate target cells.
  • the neighbouring base stations may transmit an indication of their current cell load via the X2 interface (or via the mobile communication device), allowing the base station 5-1 to select the candidate target cells based on their load, optionally in combination with the RSSI for each neighbouring cell provided by the mobile communication device.
  • the core network 7 rather than the serving base station 5-1 may select the candidate target cells using any of the described methods or any other methods.
  • the serving base station may select the candidate target cells using UE-measured signal conditions in the neighbouring cells and/or the serving cell 1 , where the signal conditions are modified by a pre-defined offset value.
  • RSSI is one example of measurements which the base station 5-1 and mobile communication device 3 can use to identify candidate target cells and a target cell - any other measurement may be used, such as RSRP and/or RSRQ as detailed above.
  • the serving base station 5-1 may inform the mobile telephone 3 of the single identified target cell.
  • the mobile telephone 3 may be configured to handover to the identified target cell, or it may be configured to decide whether to handover to the identified target cell based on the most recent measurements performed by the mobile telephone 3.
  • the serving base station 5-1 sends an enh-MobilityControlInfoList to mobile communication device 3
  • the serving base station could instead provide the mobile communication device 3 with any information identifying a plurality of candidate target cells.
  • the information sent by the serving base station could comprise a subset of the information received from the candidate target cells, such as part of a mobilityControlInfo IE for each cell, and/or the information sent the mobile communication device 3 could be refined in some manner to reduce size of the message.
  • Candidate target cells may be identified in the list using their operating frequencies, cell IDs or any other suitable information.
  • the selection, by the mobile communication device, of a target base station for handover (e.g. step 421), or for RRC connection re-establishment (e.g. step 526), is described as using the latest measurements (e.g. those made at step 417 or 517). However, such selections can be made using previous measurements in combination with or instead of the latest measurements, such as those made at step 407, which may for example allow improved determination of telephone mobility.
  • the mobile communication device could optionally perform additional measurements after receiving the handover command.
  • the mobile communication device could perform additional measurement if a measurement was not performed within 5 ms of receiving the handover command, and the overall time between the mobile communication device receiving a handover command and sending a handover confirm message may be configured not to exceed 15 ms.
  • a 'handover timer' parameter might be configured to define the maximum time that may elapse between taking the measurements and initiating a handover to a target cell selected from the candidate target cell list.
  • an alternative option could be to allow the mobile communication device more time to perform additional measurement when judged necessary by the base station 5-1, in which case a new network timer could be configured to allow this extra processing time in the mobile communication device.
  • the mobile communication device 3 selects the remaining candidate target cell (cell 3) for re-establishment/handover.
  • the mobile communication device 3 selects the remaining candidate target cell (cell 3) for re-establishment/handover.
  • the mobile communication device 3 selects the remaining candidate target cell (cell 3) for re-establishment/handover.
  • the mobile communication device 3 selects the remaining candidate target cell (cell 3) for re-establishment/handover.
  • the mobile communication device compares the latest signal measurements of cell 3 with the target cell selection criteria provided by the base station 5-1, and if the cell 3 does not constitute a suitable target cell, then the mobile communication device either selects another cell without guidance, or returns to the initial serving cell 1.
  • the candidate target cells comprise three or more candidate target cells, in order to allow selection between at least two remaining candidate target cells should the first handover attempt fail, taking into account the most recent measurements and cell selection criteria.
  • Figs. 8 and 9 described exemplary cell selection criteria comprising a penalty value associated with each candidate target cell, other criteria may be used additionally or alternatively. Any criteria should preferably be used to compare all the candidate target cells to ensure the mobile communication device selects a target cell reliably. For example, in a given network environment different mobile communication devices should select the same target cell when having identical criteria set by the network and identical radio environment.
  • the cell selection criteria may comprise a frequency specific offset to be applied each candidate target cell of a given frequency. This would give priority to certain frequencies, determined by the base station 5-1 as being preferable (e.g. having a low possibility of interference).
  • the cell selection criteria may comprise a cell specific offset to be applied to a candidate target cell, and preferably a cell specific offset for each candidate target cell. This option allocates priority to particular cells, for example cells which are determined to have a low load.
  • the cell specific offset may be allocated in accordance with a preconfigured load balancing algorithm, formulated to achieve load balancing across the cells in the network.
  • the cell selection criteria may comprise a hysteresis parameter to be applied to a candidate target cell, or to be applied when comparing one candidate target cell with another cell, for example the current serving cell.
  • a hysteresis parameter can help to prevent the mobile communication device from selecting a cell with an unstable RSRP and/or RSRQ level.
  • the cell selection criteria may comprise a penalty or offset value which could be dependent on the mobile communication device speed. This would ensure that the mobile communication device will avoid selection of a small cell as a target cell when the mobile communication device is travelling too fast.
  • the offsets i.e. frequency specific offset or cell specific offset
  • penalty and hysteresis parameters are preferably expressed in dB, or alternatively (or additionally) in dBm or in any other suitable unit (or unitless).
  • One or several of these parameters or additional parameters may also be defined to penalize small cells in comparison with macro cell.
  • the priority associated with each candidate target cell by the base station could be implicitly indicated from the order of the candidate target cells in the in the Handover Command message, i.e. the first listed candidate target cell (identified e.g. by its frequency or cell ID) has the highest priority for selection.
  • a new algorithm to take into account any of the cell selection criteria described could be configured in the mobile communication device. This algorithm would take into consideration all the relevant information sent by the network (e.g. the candidate target cell and cell selection criteria) along with additional mobile communication device inputs such as the latest
  • the cell selection criteria (or a subset of the cell selection criteria) and/or the candidate target cell list (or a subset of it) could be given by the network (e.g. the serving base station 5-1) in a System Information Block (SIB), in dedicated signalling such as measurement
  • SIB System Information Block
  • the information provided in the Handover Command message could be processed by the serving base station 5-1 to minimize size of this information, to keep the size of the Handover Command as small as possible, allowing it to be received by the UE in a reasonable time.
  • no cell selection criteria are sent to the mobile
  • the mobile communication device is configured to provide an indication of its source cell in the handover confirm message, in order to allow the target cell to retrieve UE context from the source cell. This minimises the negative impact of the target base station operating the target cell not having received a handover command from the serving base station due to backhaul latency.
  • the mobile telephone 3 when the mobile telephone 3 determines that a handover failure has occurred (e.g. due to expiry of the T304 timer), the mobile telephone 3 starts a "T310" timer. If the mobile telephone 3 is able to select a further target cell for handover from the candidate target cell list, the mobile telephone is configured to stop timer T310.
  • the mobile telephone 3 transmits a handover confirm message to a further cell selected from the candidate target cell list, after handover failure.
  • a new timer in addition to timers T304 and T310 may be used in order to allow relevant radio access to be kept valid, during the duration of the timer, so that the radio access can be used by the mobile communication device.
  • the mobile telephone 3 could be pre-programmed with such a new timer, and/or the new timer could be sent to the mobile telephone 3 by the serving base section 5-1.
  • the mobile telephone may be configured to attempt to connect to one or multiple target cells selected from the candidate target cell list, until expiry of the new timer.
  • the serving base station 5-1 and candidate target base stations may also use the new timer, and release resources associated with the mobile telephone 3 upon expiry of the new timer.
  • a target eNB could also be a UE relay. It is foreseen that 3 GPP will define new type of cell which will be "UE relay". One of the functionality of the UE relay would be to provide coverage in some area.
  • the base station 5-1 may provide the mobile communication device 3 with
  • the base station 5-1 may provide the mobile communication device 3 with configuration data defining the required periodicity of the measurement reports to be sent from the mobile communication device to the serving base station (i.e. step 401 is repeated at predefined intervals).
  • a mobile communication device based telecommunication system was described.
  • the signalling techniques described in the present application can be employed in other communication systems.
  • mobile communication devices as examples of user equipment, other communication nodes or mobile communication devices may also be used for example, personal digital assistants, laptop computers, web browsers, e-book readers, personal computers implementing 3 GPP technology, machine type communication (MTC) devices, modem devices included in routers (e.g. a MIFI - LTE WIFI router), etc. may be used without departing from the scope of the invention.
  • MTC machine type communication
  • modem devices included in routers e.g. a MIFI - LTE WIFI router
  • the mobile communication devices and the base stations will each include transceiver circuitry.
  • this circuitry will be formed by dedicated hardware circuits.
  • part of the transceiver circuitry may be implemented as software run by the corresponding controller.
  • the software modules may be provided in compiled or un-compiled form and may be supplied to the base station or the relay station as a signal over a computer network, or on a recording medium. Further, the functionality performed by part or all of this software may be performed using one or more dedicated hardware circuits.
  • MobilityControlInfo OPTIONAL - Cond HO dedicatedlnfoNASList SEQUENCE (SIZE(1..maxDRB)) OF
  • radioResourceConfigDedicated RadioResourceConfigDedicated OPTIONAL Cond HO-toEUTRA
  • enh-MobilityControUnfoList :: SEQUENCE (SIZE (l..maxMobilityControlInfo)) OF MobilityControlInfo
  • a new MobilityControlInfo information element may be defined, using a different coding to allow reduction of the size of the information element (IE) of the
  • the network e.g. the base station
  • the network must be able to detect 3 GPP UE release implementation, or support of the feature must be provided explicitly in a UE capability (E.G. FGI bit (Feature Group Indicators bit) or a UE Radio Capability).
  • E.G. FGI bit Feature Group Indicators bit
  • UE Radio Capability a UE Radio Capability
  • the present invention can be materialized by a program for causing a computer such as a CPU (Central Processing Unit) to execute the processes shown in Figs. 4 to 9.
  • a computer such as a CPU (Central Processing Unit) to execute the processes shown in Figs. 4 to 9.
  • CPU Central Processing Unit
  • Non-transitory computer readable media include any type of tangible storage media.
  • Examples of non-transitory computer readable media include magnetic storage media (such as floppy disks, magnetic tapes, hard disk drives, etc.), optical magnetic storage media (e.g. magneto-optical disks), CD-ROM, CD-R (compact disc recordable), CD-R/W (compact disc rewritable), and semiconductor memories (such as mask ROM, PROM (programmable ROM), EPROM (erasable PROM), flash ROM, RAM (random access memory), etc.).
  • the program may be provided to a computer using any type of transitory computer readable media. Examples of transitory computer readable media include electric signals, optical signals, and electromagnetic waves. Transitory computer readable media can provide the program to a computer via a wired communication line (e.g. electric wires, and optical fibers) or a wireless communication line.

Abstract

A communication system (1) comprising a base station (5) having a plurality of neighbouring cells and a mobile communication device (3) located within a serving cell operated by the base station. The base station (5) is operable to select, from the plurality of neighbouring cells, a plurality of candidate cells for handover of the mobile communication device (3). The base station (5) generates information identifying the selected candidate cells and sends the information to the mobile communication device (3). The mobile communication device (3) comprises a transceiver (31) for communicating with the base station (5) and for receiving the information identifying the plurality of candidate cells. The mobile communication device (3) selects a target cell, from the plurality of candidate cells, to which handover of the mobile communication device (3) is to be attempted and to initiate handover of the mobile communication device (3) to the selected target cell.

Description

DESCRIPTION
Title of Invention
COMMUNICATION SYSTEM
Technical Field
[0001]
The present invention relates to a communication system and to components thereof for providing communication services to mobile or fixed communication devices. The invention has particular but not exclusive relevance to handover procedures from/to user equipment in Long Term Evolution (LTE) communication systems currently being developed by the 3rd Generation Partnership Project (3 GPP).
Background Art
[0002]
In 3GPP LTE networks, a base station (i.e. evolved NodeB, eNB) of a Radio Access Network (RAN) transmits data and signalling between a core network (CN) and User Equipment (UEs) located within the base station's coverage area.
[0003]
Recent developments in communication networks have seen increased deployment of small cells or Low Power Node (LPN), such as pico cells and femto cells, which have a smaller coverage area than existing macro cells. Networks comprising a number of different cell types, for example a network comprising a macro cell and a femto cell, are referred to as
Heterogeneous Networks, or HetNets.
[0004]
Currently, the average geographic density of macro cells is 5-7 macro cells per square kilometre. However, it is predicted that the number of small cells in urban areas will reach 40 small cells per square kilometre. The increasing geographic density of small cells presents challenges in achieving successful and efficient handover procedures in HetNets.
Citation List
Non Patent Literature
[0005]
NPL 1: 3 GPP standard TS 36.331 vl 1.1.0
Summary of Invention
Technical Problem
[0006]
Current 3GPP proposals allow handover of a UE from a serving eNB to a target eNB. However, due to various environmental parameters the handover procedure may fail.
[0007]
For example, handover may fail due to:
- A sudden change of the radio environment.
- High Neighbour Interference (if proper frequency spacing between the cells to avoid neighbour interference was not taken into account).
- High mobility (if the handover process in the cell doesn't accurately take
speed/direction into account).
- No Dominant neighbour cell (if cell sites are designed poorly there might be areas where neighbours are received at the same level and some neighbours randomly look good for handover for a certain amount of time. Such situation is very problematic because handover decisions will be difficult, and unsuccessful handovers will occur frequently).
[0008]
Due to the increasing implementation of small cells, the handover procedure may fail more regularly.
[0009]
The present invention therefore aims to provide an improved communication system and improved components of the communication system which overcome or at least alleviate one or more of the above issues.
Solution to Problem
[0010]
According to one aspect, the invention provides a mobile communication device for use in a communication network comprising a plurality of base stations each operating at least one cell, the communication device comprising means for communicating with a serving cell of the cells, which cell may be operated by a serving base station of the plurality of base stations; means for receiving, from the serving base station, information identifying a plurality of candidate cells for handover of the mobile communication device; means for selecting a target cell, from the plurality of candidate cells, to which handover of the mobile communication device may be attempted; and means for initiating handover of the mobile communication device to the selected target cell.
[0011]
The mobile communication device may further comprise means for performing measurements relating to at least one of the plurality of cells; and means for sending, to the serving base station, a measurement report based on the measurements wherein the information identifying the plurality of candidate cells may be sent by the serving base station responsive to the measurement report.
[0012]
The target cell selection means may be configured to select a target cell based on the measurements.
[0013]
The target cell selection means may be configured to select a target cell based on measurements performed by the measurement performing means after the measurement report has been sent.
[0014]
The measurement performing means may be configured to perform at least one measurement relating to each candidate cell.
[0015]
The measurement performing means may be configured to perform at least one further measurement in response to the reception of information identifying a plurality of candidate cells by the receiving means and the target cell selection means may be configured to select a target cell based on the at least one further measurement.
[0016]
The mobile communication device may further comprise means for receiving, from the serving base station, cell selection criteria for use by the selecting means in selecting a target cell from the plurality of candidate cells wherein the target cell selection means may be configured to select a target cell based on the cell selection criteria.
[0017]
The target cell selection means may be configured to apply the cell selection criteria to at least one measurement associated with a candidate cell of the plurality of candidate cells.
[0018]
The cell selection criteria may comprise an offset value associated with a candidate cell of the plurality of candidate cells and the target cell selection means may be configured to apply the offset value to a measurement associated with the candidate cell associated with the offset value.
[0019]
The cell selection criteria may comprise an offset value associated with a frequency and the target cell selection means may be configured to apply the offset value to a measurement associated with a candidate cell which operates at the frequency. [0020]
The cell selection criteria may comprise a hysteresis value associated with a candidate cell of the plurality of candidate cells and the target cell selection means may be configured to apply the hysteresis value to a measurement associated with the candidate cell associated with the hysteresis value.
[0021]
The cell selection criteria may comprise a penalty value associated with a candidate cell of the plurality of candidate cells and the target cell selection means may be configured to apply the penalty value to a measurement associated with the candidate cell associated with the penalty value.
[0022]
The target cell selection means may be configured to use the cell selection criteria and the measurements to determine a priority for each of the candidate cells, and the target cell for handover of the mobile communication device may be selected based on the priority (e.g. to be the highest priority target cell).
[0023]
The mobile communication device may further comprise means for determining that a handover failure has occurred, the selecting means may be operable to select, responsive to a determination that a handover failure has occurred, a further cell, from the plurality of candidate cells, to which a connection procedure for the mobile communication device may be attempted and means for initiating the connection procedure to the selected further cell.
[0024]
The connection procedure may comprise an RRC connection re-establishment procedure.
[0025]
The connection procedure may comprise a handover procedure.
[0026]
The selecting means may be operable to select a plurality of cells, including the target cell, from the plurality of candidate cells, to which handover of the mobile communication device may be attempted and the initiating means may be operable to initiate handover of the mobile communication device to each of the selected plurality of cells.
[0027]
The information identifying a plurality of candidate cells may be received by the receiving means in a handover command. [0028]
The information identifying a plurality of candidate cells may comprise information identifying at least three candidate cells.
[0029]
The information identifying a plurality of candidate cells may comprise a candidate cell list.
[0030]
The information identifying a plurality of candidate cells may identify each candidate cell using a Cell ID associated with the candidate cell.
[0031]
The information identifying a plurality of candidate cells may identify each candidate cell using a frequency associated with the candidate cell.
[0032]
According to another aspect, the invention provides a base station for use in a communication network comprising a plurality of neighbouring cells each operated by a neighbouring base station, the base station comprising: means for communicating with a mobile communication device located within a cell operated by the base station; means for selecting, from said plurality of neighbouring cells, a plurality of candidate cells for handover of the mobile communication device; means for generating information identifying said plurality of candidate cells; and means for sending said information identifying said plurality of candidate cells to the mobile communication device.
[0033]
The base station may further comprise means for receiving a measurement report from the mobile communication device, the measurement report based on measurements relating to at least one of the neighbouring cells wherein the candidate target cell selecting means may be configured to select the plurality of candidate target cells based on the measurement report.
[0034]
The base station may further comprise means for receiving information relating to communication conditions prevailing in at least one neighbouring cell from the at least one neighbouring base station wherein the candidate cell selecting means may be configured to select the plurality of candidate cells based on the information relating to communication conditions prevailing in the neighbouring cells received from the at least one neighbouring base station.
[0035]
The information relating to communication conditions prevailing in the at least one neighbouring cell received from the at least one neighbouring base station may comprise cell load information.
[0036]
The base station may comprise means for sending a handover command to each of the candidate cells, informing each candidate cell to prepare for handover of the mobile
communication device.
[0037]
The base station may further comprise means for receiving a handover request acknowledge message from each of the candidate cells responsive to the respective handover command.
[0038]
The handover request acknowledge message may comprise information relating to the respective candidate cell.
[0039]
The information relating to the respective candidate cell may comprise at least one of: cell frequency; cell ID; cell load; prevailing communication conditions.
[0040]
The base station may further comprise means for generating cell selection criteria for use by the selecting means in selecting a target cell from the plurality of candidate cells and means for sending the cell selection criteria to the mobile communication device.
[0041]
The cell selection criteria may comprise an offset value associated with a candidate cell of the plurality of candidate cells.
[0042]
The cell selection criteria may comprise an offset value associated with a frequency.
[0043]
The cell selection criteria may comprise a hysteresis value associated with a candidate cell of the plurality of candidate cells.
[0044]
The cell selection criteria may comprise a penalty value associated with a candidate cell of the plurality of candidate cells.
[0045]
The criteria may be generated based on the measurement report.
[0046] The criteria may be generated based on the prevailing conditions in each candidate cell.
[0047]
The information identifying the plurality of candidate cells may comprise a candidate cell list.
[0048]
The base station may be operable to order the candidate cell list in a prioritised order.
[0049]
The prioritised order may be based on the prevailing conditions in each respective candidate cell (e.g. the load in each candidate cell).
[0050]
The sending means may be configured to send the information identifying the plurality of candidate cells in a handover command.
[0051]
The sending means may be configured to send the cell selection criteria in the handover command.
[0052]
The information identifying a plurality of candidate cells may comprise information identifying at least three candidate cells.
[0053]
The information identifying a plurality of candidate cells may identify each candidate cell using a Cell ID associated with the candidate cell.
[0054]
The information identifying a plurality of candidate cells may identify each candidate cell using a frequency associated with the candidate cell.
[0055]
According to another aspect, the invention provides a communication system
comprising a base station having a plurality of neighbouring cells each operated by a
neighbouring base station and a mobile communication device located within a serving cell operated by the base station; wherein the base station comprises; means for communicating with the mobile communication device; means for selecting, from said plurality of neighbouring cells, a plurality of candidate cells for handover of the mobile communication device; means for generating information identifying said plurality of candidate cells; and means for sending said information identifying said plurality of candidate cells to the mobile communication device; and wherein the mobile communication device comprises: means for communicating with the base station; means for receiving, from said base station, information identifying a plurality of candidate cells for handover of the mobile communication device; means for selecting a target cell, from said plurality of candidate cells, to which handover of the mobile communication device is to be attempted; and means for initiating handover of the mobile communication device to the selected target cell.
[0056]
According to another aspect, the invention provides a method performed by a mobile communication device in a communication network comprising a plurality of base stations each operating at least one cell, the method comprising: communicating with a serving cell of said cells, which cell is operated by a serving base station of said plurality of base stations; receiving, from said serving base station, information identifying a plurality of candidate cells for handover of the mobile communication device; selecting a target cell, from said plurality of candidate cells, to which handover of the mobile communication device is to be attempted; and initiating handover of the mobile communication device to the selected target cell.
[0057]
According to another aspect, the invention provides a method performed by a base station in a communication network comprising a plurality of neighbouring cells each operated by a neighbouring base station, the method comprising: communicating with a mobile communication device located within a cell operated by the base station; selecting, from said plurality of neighbouring cells, a plurality of candidate cells for handover of the mobile communication device; generating information identifying said plurality of candidate cells; and sending said information identifying said plurality of candidate cells to the mobile
communication device.
[0058]
According to another aspect, the invention provides a method performed in a
communication system comprising a base station having a plurality of neighbouring cells each operated by a neighbouring base station and a mobile communication device located within a serving cell operated by the base station the method comprising: the base station: communicating with the mobile communication device; selecting, from said plurality of neighbouring cells, a plurality of candidate cells for handover of the mobile communication device; generating information identifying said plurality of candidate cells; and sending said information identifying said plurality of candidate cells to the mobile communication device; and the mobile
communication: receiving, from said base station, information identifying a plurality of candidate cells for handover of the mobile communication device; selecting a target cell, from said plurality of candidate cells, to which handover of the mobile communication device is to be attempted; and initiating handover of the mobile communication device to the selected target cell.
[0059]
According to another aspect, the invention provides a mobile communication device for use in a communication network comprising a plurality of base stations each operating at least one cell, the communication device comprising: a transceiver operable to: communicate with a serving cell of said cells, which cell is operated by a serving base station of said plurality of base stations; and receive, from said serving base station, information identifying a plurality of candidate cells for handover of the mobile communication device; and a processor operable to: select a target cell, from said plurality of candidate cells, to which handover of the mobile communication device is to be attempted; and initiate handover of the mobile communication device to the selected target cell.
[0060]
According to another aspect, the invention provides a base station for use in a communication network comprising a plurality of neighbouring cells each operated by a neighbouring base station, the base station comprising: a transceiver operable to: communicate with a mobile communication device located within a cell operated by the base station; and a processor operable to: select, from said plurality of neighbouring cells, a plurality of candidate cells for handover of the mobile communication device; and generate information identifying said plurality of candidate cells; and wherein the transceiver is further operable to: send said information identifying said plurality of candidate cells to the mobile communication device.
[0061]
Another aspect of the present invention provides a computer program product comprising computer implementable instructions for causing a programmable computer device to become configured as a communication device as described above or as a base station as described above.
[0062]
The invention also provides a corresponding system, methods and computer software products that may be provided on a carrier signal or on a recording medium, such as a CD, DVD or the like.
Advantageous Effects of Invention
[0063]
According to the present invention, it is possible to provide improved techniques for overcoming or at least alleviating an issue that due to various environmental parameters the handover procedure may fail.
Brief Description of Drawings
[0064]
Embodiments of the invention will now be described, by way of example only, with reference to the attached figures in which:
[Fig- 1]
Fig. 1 schematically illustrates a mobile telecommunication system including a cell currently serving a mobile communication device along with neighbouring cells;
[Fig. 2]
Fig. 2 is a block diagram illustrating the main components of a mobile communication device forming part of the system shown in Fig. 1 ;
[Fig- 3]
Fig. 3 is a block diagram illustrating the main components of a base station forming part of the system shown in Fig. 1 ;
[Fig. 4]
Fig. 4 is a timing diagram showing signalling messages exchanged between the mobile communication device, a serving base station and further base stations;
[Fig- 5]
Fig. 5 is a timing diagram showing signalling messages exchanged between the mobile communication device, the serving base station and further base stations, where handover failure occurs;
[Fig. 6]
Fig. 6 is an alternative timing diagram showing signalling messages exchanged between the mobile communication device, the serving base station and further base stations, where handover failure occurs;
[Fig- 7]
Fig. 7 is a further timing diagram showing signalling messages exchanged between the mobile communication device, the serving base station and further base stations;
[Fig. 8]
Fig. 8 is a flow chart illustrating a method, performed by the serving base station, of selecting candidate target cells;
[Fig. 9]
Fig. 9 is a flow chart illustrating a method, performed by the mobile communication device, of selecting a target cell from the candidate target cells.
Description of Embodiments
[0065]
Table 1 below shows the acronyms used throughout the description:
[Table 1]
Figure imgf000012_0001
Table 1 : Set of acronyms
[0066]
(Overview)
Fig. 1 schematically illustrates a mobile (cellular) telecommunication system 1 including a mobile communication device 3 and base stations 5, comprising a plurality of base stations 5-1 to 5-4. As indicated by the relative sizes of cells 1 to 4 (operated by base stations 5-1 to 5-4 respectively), cells 1 and 2 are macro cells while cells 3 and 4 are small cells. Each base station 5 is coupled to a core network 7 via an 'S 1 ' interface and the core network 7 is also coupled to other networks (e.g. the Internet) via one or more gateways (not shown). The core network 7 includes, amongst other things, a mobility management entity (MME), a home subscriber server (HSS), a serving gateway (SGW) and a Packet Data Network (PDN) Gateway (PGW), which have also been omitted for sake of simplicity. The 'SI ' interface between the base stations 5 and the core network 7 might utilise, for example, a high speed, high bandwidth communication link, such as an optical fibre link and the like. An 'X2' interface (not shown) is also provided between neighbouring base stations 5 to facilitate data exchange between them and to compute eNB neighbour list.
[0067] As those skilled in the art will appreciate, whilst one mobile communication device 3 and four base stations 5 are shown in Fig. 1 for illustration purposes, additional user equipment and/or base stations may be present in a deployed system.
[0068]
As illustrated, mobile communication device 3 is located within cell 1 , and base station
5-1 acts as a serving cell for the mobile communication device 3. Connection of the mobile communication device 3 with a serving base station 5-1 allows the mobile communication device 3 to perform communication actions such as registering with the network, updating its registration, and establishing a data session or a voice call.
[0069]
However, when the mobile communication device 3 approaches the edge of cell 1 , or when the signal conditions within this cell begin to deteriorate, for example due to path loss, physical environment or power limitation, it becomes necessary for the mobile communication device 3 to handover to another cell, such as cell 2, cell 3, cell 4, or any other suitable cell.
[0070]
Further reasons for handover becoming necessary may include, for example, the presence of regions where the signals from the serving base station 5-1 cannot be decoded by the mobile communication device 3. Also, in uplink, because a base station will generally
communicate with many mobile communication devices, its resources may become limited or insufficient. Such limited cell coverage or lack of base station resources results in fewer services for users and on-going communication/applications may suffer, for example experiencing increased latency.
[0071]
The serving base station 5-1 receives information from the mobile communication device 3 which it uses to determine when handover becomes necessary for the mobile
communication device 3, and to identify a plurality of suitable candidate target cells. The information received by the base station 5-1 could be any information measured by the mobile communication device 3 or information deduced from the measurements performed by the mobile communication device 3. For example, the mobile communication device 3 might classify its neighbour cells in order of received power.
[0072]
The mobile communication device 3 is therefore configured (by or via base station 5-1) to perform signal measurements with respect to its neighbouring cells and/or serving cell and to report the results based on these measurements when certain predetermined signal conditions are met.
[0073]
In this embodiment, the mobile communication device 3 is configured to perform signal measurements periodically, according to a pre-defined periodicity. Criteria defining the predetermined signal conditions for measurement reporting are provided by the serving base station 5-1. In some instances, the reporting signal conditions will not be met, in which case no measurement report will be sent. Therefore, after having sent the measurement report to the network following event trigger, the mobile communication device 3 will continue to monitor the neighbour cells but may not report these measurements to the serving base station 5-1, depending on the reporting signal conditions set by the network.
[0074]
Depending on the technology used, the mobile communication device might determine signal conditions in any given cell by measuring either one of a Reference Signal Receive Power (RSRP), a Reference Signal Receive Quality (RSRQ), Received Signal Strength Indicator (RSSI) and a Received Signal Code Power (RSCP) of that cell - depending on the access technology used in that cell.
[0075]
RSRP (Reference Signal Received Power) is equivalent to the signal strength
measurement and is defined as the linear average of the received power of the resource elements carrying cell-specific reference signals within the considered measurement frequency bandwidth.
[0076]
RSRQ (Reference Symbol Received Quality) is used to represent the cell quality and is defined as the ratio of RSRP to E-UTRA carrier RSSI (Received Signal Strength Indicator).
[0077]
The E-UTRA carrier RSSI is the linear average of the total received power in OFDM symbols containing the reference symbols.
[0078]
The 3GPP standard TS 36.331 vl 1.1.0 defines measurement reporting triggering related to eight different event types (e.g. Events Al to A6, Bl, and B2) that the base station may configure for user equipment within its cell(s). It will be appreciated that additional events may be defined in future version of the specification. When a particular event occurs (i.e. measured signal conditions meet the associated predetermined criteria) transmission of an event report describing the event type and associated parameters is triggered. For example, an event may occur if measured signal conditions of a neighboring cell become better than a pre-defined threshold, or measured signal conditions of the serving cell becomes worse than a pre-defined threshold.
[0079]
The generation of the configuration parameters for configuring the signal measurements (for example their periodicity) and the reporting signal conditions may take place in the base station 5-1, or in the core network 7, in which case the configuration parameters are forwarded to the base station 5-1 which in turn forwards the configuration parameters to the mobile communication device 3.
[0080]
The serving base station 5-1 receives the results of any measurement (i.e. an indication that a configured event has occurred) using RRC signalling or the like. When the serving base station 5-1 determines, based on the reported measurements, that handover is necessary, the serving base station 5-1 proceeds to select a plurality of candidate target cells using the received measurement results. The plurality of candidate target cells may, for example, be cells which provide more favourable signal conditions to the mobile communication device 3.
[0081]
After the serving base station 5-1 has selected a plurality of candidate target cells for handover of the mobile communication device 3, the serving base station 5-1 advantageously provides the mobile communication device 3 with a list of the selected plurality of candidate target cells and in this embodiment also provides the mobile communication device 3 with further criteria for use, by the mobile communication device 3, to select a handover target cell from the list of candidate target cells. The mobile communication device 3 is configured to select one of the candidate target cells in the candidate target cell list provided by the base station 5-1 to be the target cell to which the mobile communication device 3 will attempt to hand over (e.g. taking account of any further criteria provided by the base station and any local factors appropriately).
[0082]
Advantageously, allowing the mobile communication device 3 to select a target cell from a candidate target cell list rather than relying on the serving base station 5-1 to select the target cell allows the mobile communication device 3 to recover from a handover failure more quickly, for example by selecting a further cell from the candidate target cell list to attempt connection with, thereby reducing the need for any additional signalling with the serving and/or other base stations (e.g. to establish a new target cell to which to direct handover).
[0083] Furthermore, allowing the mobile communication device 3 to select a target cell from a list of candidate target cells allows the mobile communication device make the selection taking into account currently prevailing local communication conditions as characterised by its most recent measurements of signal conditions, even if these measurements were not reported to the serving base station 5-1 in a measurement report. The mobile communication device 3 can also make use of previously measured or previously reported measurements, which may, for example, allow the mobile communication device to assess its mobility. As a result, the likelihood of failure of handover of the mobile communication device 3 from the serving cell 5-1 to the target cell selected by the mobile communication device 3 is minimised.
[0084]
(Mobile communication device)
Fig. 2 is a block diagram illustrating the main components of the mobile communication device 3 shown in Fig. 1. As shown, the mobile communication device 3 includes transceiver circuitry 31 which is operable to transmit signals to, and to receive signals from, the base station 5 and/or other mobile communication devices (not illustrated in Fig. 1) via at least one antenna 33. The mobile communication device 3 may of course have all the usual functionalities of a conventional mobile communication device (such as a user interface 35) and this may be provided by any combination of hardware, software and firmware, as appropriate. The operation of the transceiver circuit 31 is controlled by a controller 37 in accordance with software stored in memory 39. The software includes, among other things, an operating system 41, a
communications control module 43, a measurement module 45, a reporting module 47, a target cell selection module 48 and a handover module 49.
[0085]
The communications control module 43 is operable to handle (e.g. generate, send and receive) control signals for controlling the connections between the mobile communication device 3 and other user equipment or various network nodes, such as the serving base station 5-1.
[0086]
The measurement module 45 is operable to perform desired signal measurements (e.g. CRS or CSI-RS measurements), to determine associated signal quality values (e.g. RSRP and RSRQ) and to determine when particular configured event conditions (e.g. Al to B2 or R0 to R6) have been met. When event conditions are determined to have been met, the measurement module 45 identifies, and indicates to the reporting module 47, each neighbouring cell to which the event relates. [0087]
The reporting module 47 is operable to generate and send information (e.g. by triggering a measurement report) to the serving base station 5-1, when one of the configured events has occurred (e.g. based on the results of signal measurements performed by the measurement module 45). The information sent includes an indication of each neighbouring cell to which the event relates and/or any further information that may assist the serving base station 5-1 in selecting a plurality of candidate target cells.
[0088]
The target cell module 48 is configured to use the candidate target cell list received by the mobile communication device 3 from the serving base station 5-1 to select a target cell to which the mobile communication device 3 will attempt to handover. Preferably, this selection is made based on criteria received from the serving base station 5-1 and in view of the latest available measurements performed by the mobile communication devices measurement module 45. The target cell selected by the target cell selection module 48 is notified to the handover module 49.
[0089]
The handover module 49 is operable to perform handover of the mobile communication device 3 from serving base station 5-1 to the target cell selected by the target cell selection module.
[0090]
(Base station)
Fig. 3 is a block diagram illustrating the main components of one of the base stations 5 shown in Fig. 1, such as serving base station 5-1. As shown, the base station 5 includes transceiver circuitry 51 which is operable to transmit signals to, and to receive signals from, the mobile communication device 3 via at least one antenna 53. The base station 5 is also operable to transmit signals to and to receive signals from nodes in the core network 7 (such as the MME or the SGW) and other base stations, via a network interface 55. The operation of the transceiver circuit 51 is controlled by a controller 57 in accordance with software stored in memory 59. The software includes, among other things, an operating system 61, a communications control module 63, a measurement configuration module 65, a candidate target cell module 67 and a handover command module 69.
[0091]
The communications control module 63 is operable to control communications between the base station 5 and the mobile communication device 3, and the network devices such as the MME, the HSS, the SGW, the PGW, and neighbour base stations.
[0092]
The measurement configuration module 65 is operable to control the mobile
communication device, to cause it to perform desired signal measurements (for example CRS or CSI-RS measurements) and to send associated information (e.g. in a measurement report) when a configured event has occurred. The measurement configuration module 65 is also operable to pass the relevant information from the measurement report to the candidate target cell module 67 for the purposes of identifying a plurality of candidate target cells.
[0093]
The candidate target cell module 67 is operable to use information received in a measurement report from the mobile communication device 3 to identify a plurality of candidate target cells for handover of the mobile communication device 3. For example, the target cell list module may determine that target cell 2 and target cell 3 as illustrated in Fig. 1 have measured signal strengths which are above a threshold and therefore cells 2 and 3 qualify as candidate target cells. After identifying a plurality of candidate target cells for the candidate target cell list, the candidate target cell module 67 informs the handover command module 69 of the chosen candidate target cells.
[0094]
The handover command module 69 is operable to receive the chosen candidate target cells from the candidate target cell module 67. The handover command module 69 generates a handover command message for transmission to the mobile communication device 3, in which the handover command message includes a list of the candidate target cells chosen by the candidate target cell module 67. This allows the mobile communication device 3, after receipt of the handover command generated by the handover candidate command module 69, to select a target cell from the candidate target cell list included with the handover command.
[0095]
The handover command module 69 may also additionally include, within the handover command, criteria for use in the selection of a target cell from the candidate target cell list by the mobile communication device 3. Alternatively, these criteria may be provided separately by the base station 5-1.
[0096]
In the above description, the mobile communication device 3 and the base station 5 are described for ease of understanding as having a number of discrete modules (such as the communications control modules, the reporting module, and the handover command module). Whilst these modules may be provided in this way for certain applications, for example where an existing system has been modified to implement the invention, in other applications, for example in systems designed with the inventive features in mind from the outset, these modules may be built into the overall operating system or code and so these modules may not be discernible as discrete entities. These modules may also be implemented in software, hardware, firmware or a mix of these.
[0097]
(Handover Procedure)
Fig. 4 illustrates a handover procedure according to one embodiment. In summary, in this embodiment the mobile communication device 3 selects a target cell for handover based on a candidate target cell list received from the serving base station and the latest measurements taken for neighbouring cells.
[0098]
At step 401 the mobile communication device 3 transmits a measurement report to the serving base station 5-1, providing the base station 5-1 with the results of measurements performed by the mobile communication device 3 with respect to the cells neighbouring the serving cell (e.g. cell 2, cell 3, and cell 4) and with respect to the serving cell (cell 1).
[0099]
Triggering of transmission of the measurement report by the mobile communication device 3 is caused when the mobile communication device 3 determines that a preconfigured event has occurred, for example one of events Al to B2 as described previously. The
measurement report may, for example, include respective reference signal received power (RSRP) and/or reference symbol received quality (RSRQ) measurements relating to each of the measured cells (e.g. Cells 1 to 4).
[0100]
At step 402 the serving base station 5-1 receives the measurement report transmitted in step 401 and identifies a plurality of candidate target cells for handover of the mobile
communication device 3. In this example, Cells 2 and 3 operated by base stations 5-2 and 5-3 respectively are selected as candidate target cells by the serving base station 5-1 (along with any other cells (not shown) that meet the criteria for selecting a target cell).
[0101]
In steps 403 and 405 the serving base station 5-1 transmits a handover command to each of the neighbouring base stations, operating an identified candidate target cell, using the associated X2 interface (e.g. the neighbouring base station 5-2, the neighbouring base station 5-3, and each of any other identified candidate base stations).
[0102]
At steps 409 and 411 the base station 5-2 transmits a Handover Request Acknowledge message to the serving base station 5-1 over the X2 interface. The Handover Request
Acknowledge message includes a transparent container containing a 'mobilityControlInfo' information element (IE) which provides information, relating to base station 5-2, for controlling mobility, such as its cell frequency and physical channel configuration.
[0103]
Subsequently, at step 413 the serving base station 5-1 provides the base stations 5-2 and 5-3 with information relating to the mobile communication device 3 from which the mobile communication device's context, e.g. its RRC context, can be derived.
[0104]
The serving base station 5-1 is configured to generate, at step 415, a handover command which, in this embodiment, comprises an RRC connection reconfiguration message including a list of candidate target cells as identified by the serving base station 5-1 in step 402. The candidate target cell list is provided in the form of a dedicated information element for providing the list and associated control information (referred to as 'enh-mobilityControl-InfoList' in this example). In this example, the dedicated information element includes a control information element (also referred to here as 'mobilityControlInfo') based on the corresponding information element provided by each of the candidate target cell's base station 5-2 and 5-3 in the Handover Request Acknowledge message, in order to allow the UE to access either of the candidate target cells.
[0105]
The handover command also includes, in this example, information specifying criteria for use, by the mobile communication device 3, in selecting a target cell from the candidate target cell list. The base station includes, in this embodiment, information in the handover command that allows the mobile communication device 3 to select the candidate target cells in a priority order using the base station's own criteria, albeit that the priority order may change in dependence on the latest measurements performed by the mobile communication device 3. For example, the information may specify a respective offset and/or hysteresis parameter, for each candidate cell, that may be applied to the latest results of measurements for that cell in order to reduce and/or increase the relative priority attributed to that cell compared to other candidate cells.
[0106] At step 419 the serving base station 5-1 transmits the generated handover command to the mobile communication device 3.
[0107]
In the meantime, at some point between steps 401 and 419, in this example, the mobile communication device 3 performs periodic measurements of the neighbouring cells 2 to 4 and serving cell 1 (at steps 407 and 417) and subsequently determines that no preconfigured event has occurred. As a result, no new measurement report is generated and sent to the serving base station 5-1, following steps 407 and 417, but the mobile communication device nevertheless maintains the measurements.
[0108]
At step 421 the mobile communication device 3 selects a target cell from the candidate target cell list by determining the most suitable cell based on the results of the latest
measurements, in this case the measurements made at step 417, taking any criteria specified in the handover command message into account.
[0109]
At step 423 the mobile communication device 3 transmits a handover confirm message to the target base station selected in step 421, in this case the base station 5-2.
[0110]
At step 425 the base station 5-2 transmits a UE Context Release message to the base station 5-1, effectively informing the base station 5-1 that a successful handover has taken place, and of the identity of the candidate target cell to which handover has successfully completed.
[0111]
At step 427 the base station 5-1 transmits a handover cancel message to the base station of each candidate target cell to which the mobile communication device 3 has not been handed over to, in this example base station 5-3.
[0112]
(Alternative Handover Procedures)
Figs. 5 to 7 illustrate further exemplary handover procedures.
[0113]
Fig. 5 illustrates a further timing diagram of an exemplary handover procedure, similar to that of Fig. 4, in which handover failure occurs. Steps shown at 501 to 521 are generally equivalent to the equivalently numbered steps 401 to 421 in Fig. 4 (albeit that the steps of Fig. 5 are prefixed with a "5" rather than a "4"). Therefore, in the interests of brevity, steps 501 to 521 will not be described further here. [0114]
However, at step 523 transmission of a handover confirm message from the mobile communication device 3 to the target base station 5-2 fails. At step 525 it is established that handover failure has occurred. Handover failure may be established, for example, by expiry of a "T304" timer as defined in 3 GPP TS 36.331.
[0115]
At step 526 the mobile communication device 3 selects a cell from the candidate target cell list for R C connection re-establishment, excluding the cell to which handover was attempted at step 523, in this case cell 2. For simplicity Fig. 5 illustrates only two candidate target cells, and therefore in this embodiment the mobile communication device 3 selects the remaining candidate target cell - cell 3. It is noted that there are likely to be more candidate target cells than those illustrated, and therefore in these cases the selection of a target cell by the mobile communication device 3 is made by determining the most suitable remaining candidate target cell based on the latest measurements, such as the measurements made at step 517, or measurements performed subsequent to step 517 (not illustrated).
[0116]
At step 527 the mobile communication device 3 sends an RRC connection
re-establishment message to the base station 5-3, and at step 529 the base station 5-3 sends an associated RRC connection re-establishment complete message to the mobile communication device 3.
[0117]
At step 531, the base station 5-3 transmits a UE Context Release message to the base station 5-1.
[0118]
At step 533 the base station 5-1 transmits a handover cancel message to the candidate target cells to which the mobile communication device 3 has not been handed over to, in this example base station 5-2.
[0119]
Fig. 6 is a timing diagram of an exemplary handover procedure, similar to that of Fig. 5, in which handover failure also occurs. However, instead of performing re-establishment, the mobile communication device 3 transmits a handover confirm message to a cell selected from the candidate target cell list.
[0120]
It can be seen that steps 601 to 625 of Fig. 6 are generally equivalent to steps 501 to 525 of Fig. 5 (albeit that the steps of Fig. 6 are prefixed with a "6" rather than a "5") and in the interests of brevity, steps 601 to 625 will not be described further here.
[0121]
Accordingly, at step 626 mobile communication device 3 selects a cell from the candidate target cell list for handover, excluding the cell to which handover was attempted at step 523, in this case cell 2. The selection is made by determining the most suitable cell based on the latest measurements, in this case the measurements made at step 617. As explained above in relation to Fig. 6 illustrates only two candidate target cells, and therefore in this embodiment the mobile communication device 3 selects the remaining candidate target cell - cell 3.
[0122]
At step 627 the mobile communication device 3 transmits a handover confirm message to the target base station selected in step 621, in this case the base station 5-2.
[0123]
At step 631, the base station 5-3 transmits a UE Context Release message to the base station 5-1, informing the base station 5-1 that a successful handover has taken place.
[0124]
At step 633 the base station 5-1 transmits a handover cancel message to the candidate target cells to which the mobile communication device 3 has not been handed over to, in this example base station 5-2.
[0125]
The embodiment illustrated in Fig. 6 is advantageous because it allows the mobile communication device 3 to access a target cell using the mobilityControlInfo information relating to the target cell present in the enh-mobilityControl-InfoList received in the Handover Command at step 619.
[0126]
In this example, a new timer in addition to timers T304 and T310 may be used in order to allow relevant radio access to be kept valid, during the duration of the timer, so that the radio access can be used by the mobile communication device.
[0127]
Fig. 7 is a timing diagram illustrating a similar method to Fig. 4, but where the mobile communication device 3 sends handover confirm messages to multiple candidate target cells. This method advantageously increases the chances that a successful handover will be completed, as the likelihood of handover failure to two or more different candidate target cells is
significantly less that the likelihood of handover failure to a single selected target cell. [0128]
In this embodiment the mobile device is able to monitor several radio links. The serving eNBl takes the final decision as to which cell (of those to which the mobile communication device has sent handover confirm messages) the UE will be handed over (for example based on the priority order that the serving eNB receives the UE context release messages from the target cells). The UE may indicate in the handover confirm message the ranking or last measurements that the UE has performed to facilitate the serving eNBl decision. The UE will continue to monitor each target eNB to receive the next message to know which has become the new serving eNB.
[0129]
(Generating Candidate Target Cell List and Cell Selection Criteria)
Fig. 8 illustrates a method, performed by the serving base station 5-1, of generating a candidate target cell list and cell selection criteria.
[0130]
At step 801 the serving base station 5-1 receives a measurement report from the mobile communication device 3. In this embodiment, the measurement report includes an RSRP and or an RSRQ for each of the neighbouring cells of the serving base station 5-1. At step 803, the base station 5-1 compares the RSSI of each neighbouring cell to a preconfigured threshold value. If the RSSI is greater than the preconfigured threshold value then the serving base station 5-1 designates the cell as a candidate target cell.
[0131]
At step 805, the base station 5-1 transmits a Handover Command to each of the candidate target cells. Subsequently, at step 807 the base station 5-1 receives a Handover Request Acknowledge message from each base station operating a candidate target cell, responsive to the respective Handover Command. Each Handover Request Acknowledge message includes a 'mobilityControlInfo' information element (IE) which provides information relating to the respective candidate target cell. In this embodiment the Handover Request Acknowledge message further includes a cell load indication (for example, as part of the 'mobilityControlInfo' information element (IE) or in a separate IE) although it will be appreciated that a load indication may be provided in separate signalling.
[0132]
At step 809, the base station 5-1 ranks the candidate target cells in priority order according to their cell load indication, with those cells having a lower cell load being allocated a higher ranking (for example, the cell with the lowest load achieves the highest ranking). At step 811, the base station 5-1 uses the rankings of the candidate target cells to generate cell selection criteria, the criteria including a penalty value allocated to each candidate target cell based on its ranking, where the higher a cell is ranked, the lower a penalty value it is assigned.
[0133]
At step 813, the base station 5-1 generates a candidate target cell list, in the form of an enh-mobilityControl-InfoList, identifying each of the candidate target cells. At step 815, the base station 5-1 sends the candidate target cell list and the cell selection criteria to the mobile communication device 3.
[0134]
A possible, and purely illustrative example of, method for ranking candidate cells is summarised briefly below.
[0135]
The allowability of handover to a particular candidate cell effectively is verified if the measured receiver power is greater than a minimum required receiver power as modified by an offset.
[0136]
Specifically, handover criterion (H) for a candidate cell, indicates handover to a cell is permitted when Hrxlev > 0 for which Hrxlev = rxlevmeas - rxlevmin - rxlevoffset.
[0137]
where
Hrxlev is a Cell Handover receiver level value (dB)
rxlevmeas is the measured cell receiver level value (RSRP) averaged over a period rxlevmin is the minimum required receiver level in the cell (dBm)
rxlevminoffset is an offset to be applied effectively to either favour or penalize a measured receiver level in accordance with a network configuration (e.g. for offloading of some cells or to favour a macro cell). This may be provided by the base station to the UE for each candidate cell.
[0138]
In case, no cell meets the handover criterion then the UE sends a Handover failure to the serving cell to indicate that there is no suitable cell.
[0139]
In case, one or several cells meet the criterion H then the UE ranks the cell according to the criteria [(RSRP or RSRQ measurement) - the offset] calculating the RSRP and RSRQ values using averaged RSRP or RSRQ results. W 201
[0140]
The UE then selects the cell ranked most highly as the best cell for handover.
[0141]
Hysteresis can be used to verify the criterion H is valid over a given period.
[0142]
(Selection of a Target Cell from Candidate Target Cell List)
Fig. 9 illustrates a method, performed by the mobile communication device 3, of selecting a target cell from the candidate target cell list.
[0143]
At step 901 the mobile communication device 3 receives the candidate target cell list and the cell selection criteria from the base station 5-1.
[0144]
At step 903, the mobile communication device 3 retrieves the most recent measurements made for each of the candidate target cells, comprising an RSRP/RSRQ for each cell.
[0145]
At step 905, the mobile communication device 3 uses the cell measurements and the cell selection criteria to rank the candidate target cells in priority order, using the cell's RSSI and applying the penalty value associated with the cell given by the cell selection criteria.
[0146]
At step 907, the mobile communication device 3 selects the highest ranking candidate target cell as the target cell to which handover will be attempted.
[0147]
In summary, the embodiments described above allow the selection of the best target cell by the mobile communication device from a candidate target cell list, which is advantageous over, for example, selection of a target cell by the network, because the mobile communication device can select a new candidate target cell from the list if handover fails, reducing recovery time from handover failure. Also, allowing the mobile communication device to select the target cell means the mobile communication device can take into consideration the most recent measurements available, and use the cell selection criteria given by the base station (e.g. penalty values). This increases the likelihood of a successful handover (or reestablishment procedure), and minimises the likelihood of ping-pong of the mobile communication device between cells. These improvements in cell mobility are especially effective in a HetNets comprising multiple small cells, because these present a challenging network environment for handover procedures, where for example signalling conditions can change significantly over small distances and time scales.
[0148]
(Modifications and Alternatives)
Detailed embodiments have been described above. As those skilled in the art will appreciate, a number of modifications and alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein.
[0149]
Although particularly advantageous with HetNets including small cells, the above described embodiments provide an improved handover performance, for example reducing handover failure, in any cellular communication network.
[0150]
While in the above embodiment the base station is referred to as an eNB, the base station could be any other entity having similar functionality, for example a UE relay (or other relay node) acting as a base station, or relying information from a base station.
[0151]
In the above embodiments, communications protocols and interfaces conforming to the 3GPP LTE standards are described. However, it will be appreciated that the use of other communication standards is also possible. For example, the link provided between the base stations and the mobile communication device might use any 3 GPP technologies, such as GSM, UMTS, LTE, and the like. Alternatively, a different (e.g. non-3GPP) communication technology might be used, such as Wi-Fi, CDMA, WiMAX, and the like. If the mobile communication device also supports other technologies than LTE (e.g. 3GPP technologies such as UMTS & HSPA or non-3 GPP technologies such as WiFi, WiMAX, CDMA) its transceiver circuit and communications control module might be implemented differently than described above.
[0152]
In particular, while it is described above that an X2 interface is provided between bases stations 5, in other embodiments small cell base stations (e.g. femtocell base stations) are connected to one or more gateways (such as femto gateways (FGWs)) via an Iu home (Iuh) interface.
[0153]
In some embodiments, for example UTRA implementations, the base stations are connected to a Radio Network Controller (RNC) via an "Iub" interface.
[0154]
The measurements made by the mobile communication device might alternatively or additionally comprise measuring the strength of cell reference signals (CRS) or channel state reference signals (CSI-RS) broadcast by the neighbouring cells. When measuring LTE cells, the mobile communication device might measure the value of, for example, RSRP and/or RSSI and/or RSRQ. In case of cells implementing a different wireless standard (e.g. CDMA), other parameters might be measured, such as RSCR Measurements are to be expressed in dBm in case of RSRP, or in dB in case of RSRQ.
[0155]
The mobile communication device may be required to perform RSRP and RSRQ measurements of at least 4 inter-frequency identified cells per E-UTRA carrier and to monitor up to at least 3 E-UTRA carriers. Therefore, an E-UTRA mobile communication device may be capable of measuring at least 12 inter-frequency cells.
[0156]
In the above embodiments, the base station receives the results of any measurement using RRC signalling. However, it will be appreciated that other type of signalling might be used, either additionally or alternatively.
[0157]
It will be appreciated that, in another embodiment, the mobile equipment could receive a handover command from a plurality of nodes and send back a handover confirm to one or more nodes. The handover command could also be adapted to include information that could be used to rank and select the best suitable cell.
[0158]
Further, the mobile equipment could be served by a UE relay acting as a base station or relaying information from a base station.
[0159]
Candidate Target Cell Selection:
While in Fig. 8 the base station 5-1 selects candidate target cells based on their respective RSSI, the base station may select candidate target cells using any appropriate method.
[0160]
The plurality of candidate target cells may, for example, be cells which provide the most favourable signal conditions to the mobile communication device 3 or which the base station predicts will have favourable signal conditions as a result of UE mobility, for example where the mobile communication device is travelling towards the cell.
[0161]
The serving base station 5-1 may identify candidate target cells by means other than using information received by the mobile telephone 3. The base station 5-1 may use the X2 interface to communicate with neighbouring base stations (e.g. 5-2, 5-3 and 5-4) in order to identify candidate target cells. The neighbouring base stations may transmit an indication of their current cell load via the X2 interface (or via the mobile communication device), allowing the base station 5-1 to select the candidate target cells based on their load, optionally in combination with the RSSI for each neighbouring cell provided by the mobile communication device.
[0162]
Furthermore, the core network 7 rather than the serving base station 5-1 may select the candidate target cells using any of the described methods or any other methods.
[0163]
The serving base station may select the candidate target cells using UE-measured signal conditions in the neighbouring cells and/or the serving cell 1 , where the signal conditions are modified by a pre-defined offset value.
[0164]
Also, RSSI is one example of measurements which the base station 5-1 and mobile communication device 3 can use to identify candidate target cells and a target cell - any other measurement may be used, such as RSRP and/or RSRQ as detailed above.
[0165]
When the serving base station 5-1 determines, based on, for example, measurement reports received from the mobile telephone 3, that only a single neighbouring cell is a suitable candidate target cell, then the base station 5-1 may inform the mobile telephone 3 of the single identified target cell. The mobile telephone 3 may be configured to handover to the identified target cell, or it may be configured to decide whether to handover to the identified target cell based on the most recent measurements performed by the mobile telephone 3.
[0166]
Candidate Target Cell List:
While, in step 419 of Fig. 4 and equivalent steps (e.g. suffixed "19") of other Figs., the serving base station 5-1 sends an enh-MobilityControlInfoList to mobile communication device 3, the serving base station could instead provide the mobile communication device 3 with any information identifying a plurality of candidate target cells. For example, the information sent by the serving base station could comprise a subset of the information received from the candidate target cells, such as part of a mobilityControlInfo IE for each cell, and/or the information sent the mobile communication device 3 could be refined in some manner to reduce size of the message.
[0167] Candidate target cells may be identified in the list using their operating frequencies, cell IDs or any other suitable information.
[0168]
Target Cell Selection:
The selection, by the mobile communication device, of a target base station for handover (e.g. step 421), or for RRC connection re-establishment (e.g. step 526), is described as using the latest measurements (e.g. those made at step 417 or 517). However, such selections can be made using previous measurements in combination with or instead of the latest measurements, such as those made at step 407, which may for example allow improved determination of telephone mobility. The mobile communication device could optionally perform additional measurements after receiving the handover command. For example, depending on potential performance requirements defined by 3 GPP, the mobile communication device could perform additional measurement if a measurement was not performed within 5 ms of receiving the handover command, and the overall time between the mobile communication device receiving a handover command and sending a handover confirm message may be configured not to exceed 15 ms. For this purpose, a 'handover timer' parameter might be configured to define the maximum time that may elapse between taking the measurements and initiating a handover to a target cell selected from the candidate target cell list.
[0169]
However, an alternative option could be to allow the mobile communication device more time to perform additional measurement when judged necessary by the base station 5-1, in which case a new network timer could be configured to allow this extra processing time in the mobile communication device.
[0170]
In Figs. 5 and 6, only two candidate target cells are shown, and therefore in this embodiment the mobile communication device 3 selects the remaining candidate target cell (cell 3) for re-establishment/handover. However, in an alternative embodiment, the mobile
communication device compares the latest signal measurements of cell 3 with the target cell selection criteria provided by the base station 5-1, and if the cell 3 does not constitute a suitable target cell, then the mobile communication device either selects another cell without guidance, or returns to the initial serving cell 1.
[0171]
Preferably, the candidate target cells comprise three or more candidate target cells, in order to allow selection between at least two remaining candidate target cells should the first handover attempt fail, taking into account the most recent measurements and cell selection criteria.
[0172]
Cell Selection Criteria:
Although Figs. 8 and 9 described exemplary cell selection criteria comprising a penalty value associated with each candidate target cell, other criteria may be used additionally or alternatively. Any criteria should preferably be used to compare all the candidate target cells to ensure the mobile communication device selects a target cell reliably. For example, in a given network environment different mobile communication devices should select the same target cell when having identical criteria set by the network and identical radio environment.
[0173]
The cell selection criteria may comprise a frequency specific offset to be applied each candidate target cell of a given frequency. This would give priority to certain frequencies, determined by the base station 5-1 as being preferable (e.g. having a low possibility of interference).
[0174]
The cell selection criteria may comprise a cell specific offset to be applied to a candidate target cell, and preferably a cell specific offset for each candidate target cell. This option allocates priority to particular cells, for example cells which are determined to have a low load. In particular, the cell specific offset may be allocated in accordance with a preconfigured load balancing algorithm, formulated to achieve load balancing across the cells in the network.
[0175]
The cell selection criteria may comprise a hysteresis parameter to be applied to a candidate target cell, or to be applied when comparing one candidate target cell with another cell, for example the current serving cell. Such a hysteresis parameter can help to prevent the mobile communication device from selecting a cell with an unstable RSRP and/or RSRQ level.
[0176]
The cell selection criteria may comprise a penalty or offset value which could be dependent on the mobile communication device speed. This would ensure that the mobile communication device will avoid selection of a small cell as a target cell when the mobile communication device is travelling too fast.
[0177]
The offsets (i.e. frequency specific offset or cell specific offset), penalty and hysteresis parameters are preferably expressed in dB, or alternatively (or additionally) in dBm or in any other suitable unit (or unitless).
[0178]
One or several of these parameters or additional parameters may also be defined to penalize small cells in comparison with macro cell.
[0179]
Furthermore, the priority associated with each candidate target cell by the base station could be implicitly indicated from the order of the candidate target cells in the in the Handover Command message, i.e. the first listed candidate target cell (identified e.g. by its frequency or cell ID) has the highest priority for selection.
[0180]
A new algorithm to take into account any of the cell selection criteria described could be configured in the mobile communication device. This algorithm would take into consideration all the relevant information sent by the network (e.g. the candidate target cell and cell selection criteria) along with additional mobile communication device inputs such as the latest
measurements of the network environment.
[0181]
The cell selection criteria (or a subset of the cell selection criteria) and/or the candidate target cell list (or a subset of it) could be given by the network (e.g. the serving base station 5-1) in a System Information Block (SIB), in dedicated signalling such as measurement
configuration, in a R C connection reconfiguration message or in any similar message.
[0182]
Furthermore, the information provided in the Handover Command message (e.g. cell selection criteria and/or candidate target cell list) could be processed by the serving base station 5-1 to minimize size of this information, to keep the size of the Handover Command as small as possible, allowing it to be received by the UE in a reasonable time.
[0183]
In alternative embodiments, no cell selection criteria are sent to the mobile
communication device 3 by the serving base station 5-1, and only a candidate target cell list is provided.
[0184]
Handover Procedures:
In another possible embodiment (not illustrated in the above figures), the mobile communication device is configured to provide an indication of its source cell in the handover confirm message, in order to allow the target cell to retrieve UE context from the source cell. This minimises the negative impact of the target base station operating the target cell not having received a handover command from the serving base station due to backhaul latency.
[0185]
In some embodiments, when the mobile telephone 3 determines that a handover failure has occurred (e.g. due to expiry of the T304 timer), the mobile telephone 3 starts a "T310" timer. If the mobile telephone 3 is able to select a further target cell for handover from the candidate target cell list, the mobile telephone is configured to stop timer T310.
[0186]
In Fig. 6, the mobile telephone 3 transmits a handover confirm message to a further cell selected from the candidate target cell list, after handover failure. In this case, a new timer in addition to timers T304 and T310 may be used in order to allow relevant radio access to be kept valid, during the duration of the timer, so that the radio access can be used by the mobile communication device. The mobile telephone 3 could be pre-programmed with such a new timer, and/or the new timer could be sent to the mobile telephone 3 by the serving base section 5-1. The mobile telephone may be configured to attempt to connect to one or multiple target cells selected from the candidate target cell list, until expiry of the new timer. The serving base station 5-1 and candidate target base stations may also use the new timer, and release resources associated with the mobile telephone 3 upon expiry of the new timer.
[0187]
Further Modifications and Alternatives:
It has to be noted that a target eNB could also be a UE relay. It is foreseen that 3 GPP will define new type of cell which will be "UE relay". One of the functionality of the UE relay would be to provide coverage in some area.
[0188]
The base station 5-1 may provide the mobile communication device 3 with
configuration data defining the required periodicity of the measurements to be performed by the mobile communication device (i.e. step 407 is repeated at predefined intervals). Also, the base station 5-1 may provide the mobile communication device 3 with configuration data defining the required periodicity of the measurement reports to be sent from the mobile communication device to the serving base station (i.e. step 401 is repeated at predefined intervals).
[0189]
In the above embodiments, a mobile communication device based telecommunication system was described. As those skilled in the art will appreciate, the signalling techniques described in the present application can be employed in other communication systems. Although the above embodiments described mobile communication devices as examples of user equipment, other communication nodes or mobile communication devices may also be used for example, personal digital assistants, laptop computers, web browsers, e-book readers, personal computers implementing 3 GPP technology, machine type communication (MTC) devices, modem devices included in routers (e.g. a MIFI - LTE WIFI router), etc. may be used without departing from the scope of the invention.
[0190]
In the embodiments described above, the mobile communication devices and the base stations will each include transceiver circuitry. Typically this circuitry will be formed by dedicated hardware circuits. However, in some embodiments, part of the transceiver circuitry may be implemented as software run by the corresponding controller.
[0191]
In the above embodiments, a number of software modules were described. As those skilled in the art will appreciate, the software modules may be provided in compiled or un-compiled form and may be supplied to the base station or the relay station as a signal over a computer network, or on a recording medium. Further, the functionality performed by part or all of this software may be performed using one or more dedicated hardware circuits.
[0192]
Exemplary Implementation of Invention:
A possible implementation of the new parameters that need to be added in TS36.331 can be represented as follows (new part in italics):
[0193]
RRCConnectionReconfiguration-r8-IEs ::= SEQUENCE {
measConfig MeasConfig OPTIONAL, ~ Need ON
mobilityControlInfo MobilityControlInfo OPTIONAL, - Cond HO dedicatedlnfoNASList SEQUENCE (SIZE(1..maxDRB)) OF
DedicatedlnfoNAS OPTIONAL, - Cond nonHO
radioResourceConfigDedicated RadioResourceConfigDedicated OPTIONAL,— Cond HO-toEUTRA
securityConfigHO SecurityConfigHO OPTIONAL, - Cond HO
nonCriticalExtension RRCConnectionReconfiguration-v890-IEs OPTIONAL } RRCConnectionReconfiguration-v890-IEs ::= SEQUENCE {
lateNonCriticalExtension OCTET STRING OPTIONAL, ~ Need OP nonCriticalExtension RRCConnectionReconfiguration-v920-IEs OPTIONAL } RRCConnectionReconfiguration-v920-IEs ::= SEQUENCE {
otherConfig-r9 OtherConfig-r9 OPTIONAL, - Need ON
fullConfig-r9 ENUMERATED {true} OPTIONAL, - Cond HO-Reestab nonCriticalExtension RRCConnectionReconfiguration-vl020-IEs OPTIONAL } RRCConnectionReconfiguration-vl020-IEs ::= SEQUENCE {
sCellToReleaseList-rlO SCellToReleaseList-rlO OPTIONAL, - Need ON sCellToAddModList-rlO SCellToAddModList-rlO OPTIONAL, - Need ON nonCriticalExtension SEQUENCE { } OPTIONAL - Need OP }
[0194]
New Part:
RRCConnectionReconfiguration-vlxxx-IEs ::= SEQUENCE {
enh-mobilityControlInfoList enh-MobilityControUnfoList OPTIONAL, - Need ON }
enh-MobilityControUnfoList ::= SEQUENCE (SIZE (l..maxMobilityControlInfo)) OF MobilityControlInfo
[0195]
It is noted that this is only an example and that other implementations are possible.
[0196]
For example, a new MobilityControlInfo information element may be defined, using a different coding to allow reduction of the size of the information element (IE) of the
enh-MobilityControlInfoList.
[0197]
Also the above implementation example was described to take into account coding of the previous 3 GPP release for backward compatibility. However, a new coding of the message could be used and the "enh-MobilityControlInfoList" could be sent without the legacy
"MobilControlInfo" information. In this case, the network (e.g. the base station) must be able to detect 3 GPP UE release implementation, or support of the feature must be provided explicitly in a UE capability (E.G. FGI bit (Feature Group Indicators bit) or a UE Radio Capability).
[0198]
Various other modifications will be apparent to those skilled in the art and will not be described in further detail here.
[0199]
For example, the present invention can be materialized by a program for causing a computer such as a CPU (Central Processing Unit) to execute the processes shown in Figs. 4 to 9.
[0200]
The program can be stored and provided to a computer using any type of non-transitory computer readable media. Non-transitory computer readable media include any type of tangible storage media. Examples of non-transitory computer readable media include magnetic storage media (such as floppy disks, magnetic tapes, hard disk drives, etc.), optical magnetic storage media (e.g. magneto-optical disks), CD-ROM, CD-R (compact disc recordable), CD-R/W (compact disc rewritable), and semiconductor memories (such as mask ROM, PROM (programmable ROM), EPROM (erasable PROM), flash ROM, RAM (random access memory), etc.). The program may be provided to a computer using any type of transitory computer readable media. Examples of transitory computer readable media include electric signals, optical signals, and electromagnetic waves. Transitory computer readable media can provide the program to a computer via a wired communication line (e.g. electric wires, and optical fibers) or a wireless communication line.
[0201]
This application is based upon and claims the benefit of priority from United Kingdom Patent Application No. 1305728.6, filed on March 28, 2013, the disclosure of which is incorporated herein in its entirety by reference.
Reference Signs List
[0202]
1 TELECOMMUNICATION SYSTEM
3 MOBILE TELEPHONE
5 (5-1 to 5-4) BASE STATION
7 CORE NETWORK
31 TRANSCEIVER CIRCUIT
33 ANTENNA
35 USER INTERFACE
37 CONTROLLER
39 MEMORY
41 OPERATING SYSTEM
3 COMMUNICATIONS CONTROL MODULE
5 MEASUREMENT MODULE
7 REPORTING MODULE
8 TARGET CELL SELECTION MODULE 49 HANDOVER MODULE
51 TRANSCEIVER CIRCUIT
53 ANTENNA
55 NETWORK INTERFACE
57 CONTROLLER
59 MEMORY
61 OPERATING SYSTEM
63 COMMUNICATIONS CONTROL MODULE
65 MEASUREMENT CONFIGURATION MODULE
67 CANDIDATE TARGET CELL MODULE
69 HANDOVER COMMAND MODULE

Claims

[Claim 1]
A mobile communication device for use in a communication network comprising a plurality of base stations each operating at least one cell, the communication device comprising: means for communicating with a serving cell of said cells, which cell is operated by a serving base station of said plurality of base stations;
means for receiving, from said serving base station, information identifying a plurality of candidate cells for handover of the mobile communication device;
means for selecting a target cell, from said plurality of candidate cells, to which handover of the mobile communication device is to be attempted; and
means for initiating handover of the mobile communication device to the selected target cell.
[Claim 2]
The mobile communication device according to claim 1, further comprising:
means for performing measurements relating to at least one of said plurality of cells; and
means for sending, to said serving base station, a measurement report based on the measurements;
wherein the information identifying the plurality of candidate cells is sent by the serving base station responsive to said measurement report.
[Claim 3]
The mobile communication device according to claim 2, wherein said target cell selection means is configured to select a target cell based on said measurements.
[Claim 4]
The mobile communication device according to claim 3, wherein said target cell selection means is configured to select a target cell based on measurements performed by the measurement performing means after said measurement report has been sent.
[Claim 5]
The mobile communication device according to any of claims 2 to 4, wherein said measurement performing means is configured to perform at least one measurement relating to each candidate cell.
[Claim 6]
The mobile communication device according to any of claims 2 to 5;
wherein said measurement performing means is configured to perform at least one further measurement in response to the reception of information identifying a plurality of candidate cells by the receiving means; and
wherein said target cell selection means is configured to select a target cell based on said at least one further measurement.
[Claim 7]
The mobile communication device according to any of claims 1 to 6, further comprising:
means for receiving, from said serving base station, cell selection criteria for use by the selecting means in selecting a target cell from said plurality of candidate cells;
wherein said target cell selection means is configured to select a target cell based on said cell selection criteria.
[Claim 8]
The mobile communication device according to claim 7, wherein said target cell selection means is configured to apply said cell selection criteria to at least one measurement associated with a candidate cell of the plurality of candidate cells.
[Claim 9]
The mobile communication device according to claim 8, wherein said cell selection criteria comprises an offset value associated with a candidate cell of the plurality of candidate cells; and
wherein the target cell selection means is configured to apply said offset value to a measurement associated with the candidate cell associated with the offset value.
[Claim 10]
The mobile communication device according to claim 8 or 9, wherein said cell selection criteria comprises an offset value associated with a frequency; and
wherein the target cell selection means is configured to apply said offset value to a measurement associated with a candidate cell which operates at said frequency.
[Claim 11]
The mobile communication device according to any of claims 8 to 10, wherein said cell selection criteria comprises a hysteresis value associated with a candidate cell of the plurality of candidate cells; and
wherein the target cell selection means is configured to apply said hysteresis value to a measurement associated with the candidate cell associated with the hysteresis value.
[Claim 12]
The mobile communication device according to any of claims 8 to 11, wherein said cell selection criteria comprises a penalty value associated with a candidate cell of the plurality of candidate cells; and
wherein the target cell selection means is configured to apply said penalty value to a measurement associated with the candidate cell associated with the penalty value.
[Claim 13]
The mobile communication device according to any of claims 7 to 12, wherein said target cell selection means is configured to use the cell selection criteria and the measurements to determine a priority for each of the candidate cells, and wherein the target cell for handover of the mobile communication device is selected based on said priority (e.g. to be the highest priority target cell).
[Claim 14]
The mobile communication device according to any of claims 1 to 13, further comprising:
means for determining that a handover failure has occurred;
wherein said selecting means is operable to select, responsive to a determination that a handover failure has occurred, a further cell, from said plurality of candidate cells, to which a connection procedure for the mobile communication device is to be attempted; and
means for initiating said connection procedure to the selected further cell.
[Claim 15]
The mobile communication device according to claim 14, wherein said connection procedure comprises an R C connection re-establishment procedure.
[Claim 16]
The mobile communication device according to claim 14, wherein said connection procedure comprises a handover procedure.
[Claim 17]
The mobile communication device according to any of claims 1 to 16:
wherein said selecting means is operable to select a plurality of cells, including said target cell, from said plurality of candidate cells, to which handover of the mobile
communication device is to be attempted; and
wherein said initiating means is operable to initiate handover of the mobile
communication device to each of the selected plurality of cells.
[Claim 18]
The mobile communication device according to any of claims 1 to 17, wherein said information identifying a plurality of candidate cells is received by said receiving means in a handover command.
[Claim 19]
The mobile communication device according to any of claims 1 to 18, wherein said information identifying a plurality of candidate cells comprises information identifying at least three candidate cells.
[Claim 20]
The mobile communication device according to any of claims 1 to 19, wherein said information identifying a plurality of candidate cells comprises a candidate cell list.
[Claim 21]
The mobile communication device according to any of claims 1 to 20, wherein said information identifying a plurality of candidate cells identifies each candidate cell using a Cell ID associated with the candidate cell.
[Claim 22]
The mobile communication device according to any of claims 1 to 21 , wherein said information identifying a plurality of candidate cells identifies each candidate cell using a frequency associated with the candidate cell.
[Claim 23]
A base station for use in a communication network comprising a plurality of neighbouring cells each operated by a neighbouring base station, the base station comprising: means for communicating with a mobile communication device located within a cell operated by the base station;
means for selecting, from said plurality of neighbouring cells, a plurality of candidate cells for handover of the mobile communication device;
means for generating information identifying said plurality of candidate cells; and means for sending said information identifying said plurality of candidate cells to the mobile communication device.
[Claim 24]
The base station according to claim 23, further comprising:
means for receiving a measurement report from the mobile communication device, said measurement report based on measurements relating to at least one of said neighbouring cells; wherein said candidate target cell selecting means is configured to select said plurality of candidate target cells based on said measurement report.
[Claim 25]
The base station according to claim 23 or 24; further comprising means for receiving information relating to communication conditions prevailing in at least one neighbouring cell from said at least one neighbouring base station;
wherein said candidate cell selecting means is configured to select said plurality of candidate cells based on said information relating to communication conditions prevailing in said neighbouring cells received from said at least one neighbouring base station.
[Claim 26]
The base station according to any of claims 23 to 25, wherein said information relating to communication conditions prevailing in said at least one neighbouring cell received from said at least one neighbouring base station comprises cell load information.
[Claim 27]
The base station according to any of claims 23 to 26, further comprising:
means for sending a handover command to each of the candidate cells, informing each candidate cell to prepare for handover of the mobile communication device.
[Claim 28]
The base station according to claim 27, further comprising:
means for receiving a handover request acknowledge message from each of the candidate cells responsive to the respective handover command.
[Claim 29]
The base station according to claim 28, wherein said handover request acknowledge message comprises information relating to the respective candidate cell.
[Claim 30]
The base station according to claim 29, wherein said information relating to the respective candidate cell comprises at least one of: cell frequency; cell ID; cell load; prevailing communication conditions.
[Claim 31]
The base station according to any of claims 23 to 30, further comprising:
means for generating cell selection criteria for use by the selecting means in selecting a target cell from said plurality of candidate cells; and
means for sending said cell selection criteria to the mobile communication device.
[Claim 32]
The base station according to claim 31, wherein said cell selection criteria comprises an offset value associated with a candidate cell of the plurality of candidate cells.
[Claim 33] The base station according to claim 31 or 32, wherein said cell selection criteria comprise an offset value associated with a frequency.
[Claim 34]
The base station according to any of claims 31 to 33, wherein said cell selection criteria comprises a hysteresis value associated with a candidate cell of the plurality of candidate cells.
[Claim 35]
The base station according to any of claims 31 to 34 wherein said cell selection criteria comprises a penalty value associated with a candidate cell of the plurality of candidate cells.
[Claim 36]
The base station according to any of claims 31 to 35, wherein said criteria is generated based on said measurement report.
[Claim 37]
The base station according to any of claims 31 to 36, wherein said criteria is generated based on said prevailing conditions in each candidate cell.
[Claim 38]
The base station according to any of claims 23 to 35, wherein said information identifying said plurality of candidate cells comprises a candidate cell list.
[Claim 39]
The base station according to claim 38, wherein the base station is operable to order the candidate cell list in a prioritised order.
[Claim 40]
The base station according to claim 39, wherein said prioritised order is based on the prevailing conditions in each respective candidate cell (e.g. the load in each candidate cell).
[Claim 41]
The base station according to any of claims 23 to 38, wherein said sending means is configured to send said information identifying said plurality of candidate cells in a handover command.
[Claim 42]
The base station according to claim 41, wherein said sending means is configured to send said cell selection criteria in the handover command.
[Claim 43]
The base station according to any of claims 23 to 42, wherein said information identifying a plurality of candidate cells comprises information identifying at least three candidate cells.
[Claim 44]
The base station according to any of claims 23 to 43, wherein said information identifying a plurality of candidate cells identifies each candidate cell using a Cell ID associated with the candidate cell.
[Claim 45]
The base station according to any of claims 23 to 44, wherein said information identifying a plurality of candidate cells identifies each candidate cell using a frequency associated with the candidate cell.
[Claim 46]
A communication system comprising a base station having a plurality of neighbouring cells each operated by a neighbouring base station and a mobile communication device located within a serving cell operated by the base station;
wherein the base station comprises:
means for communicating with the mobile communication device;
means for selecting, from said plurality of neighbouring cells, a plurality of candidate cells for handover of the mobile communication device;
means for generating information identifying said plurality of candidate cells; and
means for sending said information identifying said plurality of candidate cells to the mobile communication device; and
wherein the mobile communication device comprises:
means for communicating with the base station;
means for receiving, from said base station, information identifying a plurality of candidate cells for handover of the mobile communication device;
means for selecting a target cell, from said plurality of candidate cells, to which handover of the mobile communication device is to be attempted; and
means for initiating handover of the mobile communication device to the selected target cell.
[Claim 47]
A method performed by a mobile communication device in a communication network comprising a plurality of base stations each operating at least one cell, the method comprising: communicating with a serving cell of said cells, which cell is operated by a serving base station of said plurality of base stations;
receiving, from said serving base station, information identifying a plurality of candidate cells for handover of the mobile communication device;
selecting a target cell, from said plurality of candidate cells, to which handover of the mobile communication device is to be attempted; and
initiating handover of the mobile communication device to the selected target cell.
[Claim 48]
A method performed by a base station in a communication network comprising a plurality of neighbouring cells each operated by a neighbouring base station, the method comprising:
communicating with a mobile communication device located within a cell operated by the base station;
selecting, from said plurality of neighbouring cells, a plurality of candidate cells for handover of the mobile communication device;
generating information identifying said plurality of candidate cells; and
sending said information identifying said plurality of candidate cells to the mobile communication device.
[Claim 49]
A method performed in a communication system comprising a base station having a plurality of neighbouring cells each operated by a neighbouring base station and a mobile communication device located within a serving cell operated by the base station the method comprising:
the base station:
communicating with the mobile communication device;
selecting, from said plurality of neighbouring cells, a plurality of candidate cells for handover of the mobile communication device;
generating information identifying said plurality of candidate cells; and sending said information identifying said plurality of candidate cells to the mobile communication device; and
the mobile communication:
receiving, from said base station, information identifying a plurality of candidate cells for handover of the mobile communication device;
selecting a target cell, from said plurality of candidate cells, to which handover of the mobile communication device is to be attempted; and
initiating handover of the mobile communication device to the selected target cell.
[Claim 50]
A mobile communication device for use in a communication network comprising a plurality of base stations each operating at least one cell, the communication device comprising: a transceiver operable to:
communicate with a serving cell of said cells, which cell is operated by a serving base station of said plurality of base stations; and
receive, from said serving base station, information identifying a plurality of candidate cells for handover of the mobile communication device; and
a processor operable to:
select a target cell, from said plurality of candidate cells, to which handover of the mobile communication device is to be attempted; and
initiate handover of the mobile communication device to the selected target cell.
[Claim 51]
A base station for use in a communication network comprising a plurality of neighbouring cells each operated by a neighbouring base station, the base station comprising: a transceiver operable to:
communicate with a mobile communication device located within a cell operated by the base station; and
a processor operable to:
select, from said plurality of neighbouring cells, a plurality of candidate cells for handover of the mobile communication device; and
generate information identifying said plurality of candidate cells; and wherein the transceiver is further operable to:
send said information identifying said plurality of candidate cells to the mobile communication device.
[Claim 52]
A computer implementable instructions product comprising computer implementable instructions for causing a programmable computer device to perform the method of claim 47 or 48.
PCT/JP2014/051591 2013-03-28 2014-01-17 Communication system WO2014156261A1 (en)

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