WO2008094334A1

WO2008094334A1 - A cellular communication system and method of operation therefor

Info

Publication number: WO2008094334A1
Application number: PCT/US2007/083611
Authority: WO
Inventors: David C. Padfield; Gerard T. Foster; Luis Lopes; Chris M. Murphy; Oliver P. Tyce
Original assignee: Motorola, Inc.
Priority date: 2007-01-30
Filing date: 2007-11-05
Publication date: 2008-08-07
Also published as: GB2446192B; GB2446192A; GB0701729D0

Abstract

A cellular communication system comprises a measurement receive processor (101) which receives a measurement report for a measured scrambling code of a downlink pilot signal of a first base station (211) from a remote station (217) served by a second base station (201). The measurement report comprises a first timing indication for the measured pilot scrambling code. A timing processor (103) determines a timing difference indication between the measured scrambling code and a reference scrambling code in response to first and second timing indication for the reference scrambling code. The reference scrambling code may be a pilot signal scrambling code of the second base station (201). An identity processor (105) determines an identity indication for a serving network element for the first base station in response to the timing difference indication.

Description

A CELLULAR COMMUNICATION SYSTEM AND METHOD OF OPERATION

THEREFOR

Field of the Invention

The invention relates to a cellular communication system and in particular, but not exclusively, to cell identification in a Universal Mobile Telecommunication System (UMTS) .

Background of the Invention

In cellular communication systems, handover and cell reselection procedures are typically based on mobile stations measuring and reporting pilot signals transmitted from base stations. For example, in UMTS, the mobile stations receive neighbour lists indicating specific pilot signals and associated pilot signal scrambling codes which should be monitored and reported. Typically, each pilot signal scrambling code of the neighbour list is linked to a specific neighbour cell such that the detection of the pilot signal scrambling code allows the system to uniquely determine which pilot signal has been detected. Accordingly, a Radio Network Controller (RNC) currently supporting a mobile station can directly and uniquely identify the neighbour cell which is being reported. This information can be used for different purposes including the initiation of handovers or gathering of operational statistics used for e.g. network optimisation and planning. However, in some systems and scenarios it is not possible to uniquely identify the cell identity associated with a reported pilot signal scrambling code.

For example, in UMTS the reported cells are identified only by their scrambling code and central frequency. Since there is a finite number of scrambling codes (512), multiple cells must inevitably reuse the same scrambling code over the network as a whole. Resolving scrambling codes to cell identities is therefore non-trivial and is usually achieved using the configured neighbour list. Thus, although the identity can normally be resolved accurately there tends to be situations where the propagation conditions and scrambling code reuse plans are such that a highly reliable identification of the measured cell is not possible based only on the scrambling code. The inability to resolve unequivocally the scrambling code to cell identity in all possible scenarios is a significant disadvantage, for example when performing neighbour list optimization and scrambling code plan improvement. Furthermore, in some scenarios it may result in mobile stations attempting to handover to the wrong cell.

As another example, it has been proposed to transmit neighbour lists including a scrambling code which is shared between a large number of potential neighbour cells thereby making it impossible to differentiate between these cells based on the scrambling code.

Specifically, the capacity of cellular communication systems can be increased by deploying hierarchical cells wherein a macro-cell layer is underlayed by a layer of typically smaller cells (micro-, pico- and/or femto cells) having coverage areas within the coverage area of the macro-cell. The underlay cells have much smaller coverage thereby allowing a much closer reuse of resources.

The current trend is towards introducing a large number of pico-cells to 3G systems. For example, it is envisaged that residential access points may be deployed having a target coverage area of only a single residential dwelling or house. A widespread introduction of such systems would result in a very large number of small underlay cells within a single macro-cell.

However, underlaying a macro-layer of a 3G network with a pico-cell (or micro-cell) layer creates several issues. For example, the introduction of a large number of underlay cells creates a number of issues related to the identification of individual underlay cells when e.g. handing over to an underlay cell. In particular, 3G communication systems are developed based on each cell having a relatively low number of neighbours and extending the current approach to scenarios wherein the mobile station may need to consider large numbers of potential neighbour cells is not practical.

One problem of extending current approaches to scenarios where there are many underlaying pico-cells is how to uniquely and efficiently identify a pico-cell (or micro- cell) . Specifically, it is not practically feasible to assign individual pilot signal scrambling codes to each underlay cell and to identify all potential handover underlay cells as neighbours of the macro-cell as this would require very large neighbour lists. These large neighbour lists would e.g. result in the neighbour list exceeding the maximum allowable number of neighbours in the list, slow mobile station measurement performance as a large number of measurements would need to be made, increased resource usage etc. It would furthermore require significant operations and management resource in order to configure each macro-cell with the large number of neighbours and would complicate network management, planning and optimisation. It has therefore been suggested to share scrambling codes but this results in a target ambiguity and prevents the mobile station from uniquely identifying a potential handover target from the measured scrambling code. For example, if a group of base stations supporting different underlay cells underlaying a given macro-cell use an identical shared pilot signal scrambling code, a mobile station detecting the presence of this shared scrambling code will be aware that a potential handover target has been detected but will not be able to uniquely identify which of the group of underlay cells has been detected.

Hence, an improved cellular communication system would be advantageous and in particular a system allowing increased flexibility, improved suitability for large numbers of potential handover targets/neighbour cells, improved suitability for overlay/underlay handovers, reduced neighbour lists, increased practicality, reduced measurement requirements, facilitated and/or improved handover target detection/identification, improved cell identification for measure scrambling codes and/or improved performance would be advantageous . Summary of the Invention

Accordingly, the Invention seeks to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

According to a first aspect of the invention there is provided a cellular communication system comprising: measurement means for receiving a measurement report for a measured scrambling code of a downlink pilot signal of a first base station serving a first cell from a remote station served by a second base station serving a second cell, the measurement report comprising a first timing indication for the measured pilot scrambling code; timing means for determining a timing difference indication between the measured scrambling code and a reference scrambling code in response to the first timing indication and a second timing indication for the reference scrambling code; and identity means for determining an identity indication for a serving network element for the first base station in response to the timing difference indication.

The invention may allow improved and/or facilitated operation and/or performance in a cellular communication system and may in particular allow improved identification of a cell/serving network element associated with a measured scrambling code of a downlink pilot signal. This may e.g. improve network optimisation etc and may thus improve the performance of the communication system as a whole. For example, the approach may in many embodiments allow identification of a handover target from a group of cells sharing a pilot signal scrambling code thereby improving e.g. handover performance. In particular, the invention may allow improved identification of an underlay target handover cell for a remote station currently served by a macro-cell. The invention may allow highly robust underlay cell identification while using a reduced amount of resources. In particular, the invention may require fewer scrambling codes while still allowing a given number of underlay cells to be identified. The invention may in some embodiments facilitate handover. The invention may in some embodiments facilitate or enable support of large numbers of underlay cells.

The invention may e.g. allow improved handover in a cellular communication system. In particular, the invention may facilitate or improve handovers in systems wherein a remote station may have a large number of possible handover targets. In particular, the invention may allow a reduced number of measurements being required by a remote station to determine a suitable handover target, may allow reduced neighbour lists and/or may reduce the required number of scrambling codes.

The remote station may for example be a User Equipment or a mobile communication unit, e.g. of a 3^rd generation cellular communication system.

The serving network element may be a Radio Network Controller (RNC) which supports the first base station. The first and/or second timing indication may be an indirect indication. For example, the remote station may directly report a measured timing difference between the measured scrambling code and the reference scrambling code thereby providing a combined indication of both the first and second timing indication. The timing difference indication may in some embodiments be determined directly such as a measured timing difference potentially with the inclusion of a timing offset e.g. indicative of a known synchronisation offset between a timing reference for the measured scrambling code and for the reference scrambling code .

According to an optional feature of the invention, the cellular communication system further comprises: a plurality of base stations including the first base station, each base station comprising means for transmitting a pilot signal with a pilot signal scrambling code; means for assigning a relative timing offset for the pilot scrambling codes of the plurality of base stations relative to a timing reference, each pilot scrambling code being assigned a different timing offset; and wherein each of the base stations is arranged to transmit a pilot signal scrambling code with the relative timing offset assigned to the base station and the identity means is further arranged to determine the identity indication by comparing the timing difference to the relative timing offsets of the plurality of base stations .

The feature may allow improved and/or facilitated operation and/or performance. In many embodiments, the feature may allow relative timing of pilot signal transmissions to be controlled such that the timing can effectively and accurately be used to (possibly uniquely) identify the transmitting base station/cell out of the plurality of base station which potentially may use the same scrambling code. Each of the plurality of base stations may be synchronised to the timing reference but the timing reference may not be synchronised with the timing of the reference scrambling code.

The reference scrambling code may be a pilot signal scrambling code of a pilot signal of the second base station. In some embodiments, the identity of a cell for which a downlink pilot signal scrambling code is measured may be determined in response to a timing offset between the measured scrambling code and the timing of the currently serving base station as well as a timing offset between the timing reference and the timing of the currently serving base station.

According to a second aspect of the invention, there is provided a method of operation for a cellular communication system, the method comprising: receiving a measurement report for a measured scrambling code of a downlink pilot signal of a first base station serving a first cell from a remote station served by a second base station serving a second cell, the measurement report comprising a first timing indication for the measured pilot scrambling code; determining a timing difference indication between the measured scrambling code and a reference scrambling code in response to the first timing indication and a second timing indication for the reference scrambling code; and determining an identity indication for a serving network element for the first base station in response to the timing difference indication .

These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment (s) described hereinafter.

Brief Description of the Drawings

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

FIG. 1 illustrates an example of a cell identification apparatus in accordance with some embodiments of the invention;

FIG. 2 illustrates an example of a cellular communication system in accordance with some embodiments of the invention;

FIG. 3 illustrates an example of pilot signal timing in accordance with some embodiments of the invention; and

FIG. 4 illustrates an example of a method of operation for a cellular communication system in accordance with some embodiments of the invention.

Detailed Description of Some Embodiments of the Invention

The following description focuses on embodiments of the invention applicable to a UMTS cellular communication system but it will be appreciated that the invention is not limited to this application but may be applied to many other cellular communication system.

In a conventional UMTS system, the remote station (in UMTS generally referred to as a User Equipment, UE) measures the receive level of downlink pilot signals of neighbour cells included in the neighbour list transmitted to the remote station. Each cell of the neighbour list is indicated by the pilot signal scrambling code which is used by the base station serving the remote station.

The remote station reports the measurements to the fixed network and the RNC serving the remote station typically evaluates the measurements in order to determine whether to initiate a handover. In addition, the measurements may be used for network planning and optimisation.

Normally, a scrambling code reported by a remote station can be correctly resolved to relate to a specific cell identifier using knowledge of which cells are included in the configured neighbour list. However, there are exceptions to this general rule including the following:

Poor code/neighbour list planning can result in a remote station making measurements of a non- neighbour cell which is reusing a code appearing on the current neighbour list. The actual cell referred to by the remote station and the network differs and any handover attempt is likely to be unsuccessful. - remote stations can be instructed to report the detected set of scrambling codes irrespectively of whether these are in the neighbour list - i.e. scrambling codes not appearing in the neighbour list may be reported. In this case, the RNC cannot uniquely identify the source of the detected signal but can only make an estimation of which cell out of the set using a specific scrambling code is the one reported by the remote station.

The neighbour lists are not always readily accessible. For example, when using call trace or a protocol analyzer for troubleshooting or optimization, only the measurement report may be available and not the neighbour list.

However, the inventors have realized that when scrambling codes are reported for handover purposes, synchronisation information is typically included that gives the time difference between the nominal timing of the remote station and the frame timing of the specific cell being reported. The synchronisation information from pairs of cells reported at the same time can be combined together to represent a single measurement representing the observed time difference of arrival (OTDOA) of the signal from the two cells at the remote station. In other cases the OTDOA may be directly reported. Typically, the OTDOA is reported as timing difference between a reported cell and a reference cell and the OTDOA between two cells can be determined from the reported OTDOAs of the two cells relative to the reference cell. The value of the OTDOA is based upon the actual time difference that exists between the two cells, and the propagation delay difference introduced by the remote station's position between the two cells. However, whilst the magnitude of the OTDOA may e.g. be between 0 and

9830400 chips, the propagation delay difference included within individual measurements should vary over a relatively small range (e.g. -50 chips for a cell pair that are separated by ~2 km) .

Because UMTS is normally an unsynchronised system, the OTDOA between each cell pair, is potentially non- stationary and can drift over time - but again the rate of this drift is relatively small compared to the range of the OTDOA. This means that there is a very low probability of there being any two pairs of cells (with zero or one cell in common) in which cells A and C are using the same scrambling code and cells B and D are using the same scrambling code, and the OTDOA between A and B and that between C and D is approximately equal.

This means that when scrambling codes are reused, the OTDOA can be used as a differentiator to determine which of the cells using the scrambling code is actually being measured.

FIG. 1 illustrates an example of a cell identification apparatus which is capable of identifying a cell identity for a measured scrambling code in response to measured OTDOAs. The cell identification apparatus may specifically be part of an RNC that serves a remote station . The cell identification apparatus comprises a measurement receive processor 101 which receives measurement reports from the remote station. The measurement reports comprise signal level measurement information for the scrambling codes of the neighbour list and for each scrambling code it comprises OTDOA information for the reported scrambling codes.

Specifically, at least one measurement report comprises measurement data for a measured scrambling code for a downlink pilot signal of a first base station which serves a first cell being a neighbour cell of the serving cell. The measurement furthermore comprises a first timing indication for the measured scrambling code. The first timing indication for the measured scrambling code may specifically be the OTDOA given as a System Frame Number offset indicating the SFN timing offset between the first cell and the serving cell.

The measurement receive processor 101 is coupled to a timing processor 103 which processes the received measurement data. Specifically, the timing processor 103 determines a timing difference indication between the measured scrambling code and a reference scrambling code. The reference scrambling code is in the specific example the pilot scrambling code of the serving cell but may in other embodiments be a scrambling code of another cell, such as e.g. another neighbour cell or of a synchronisation reference cell. In the specific example, the timing difference may simply be determined as the reported OTDOA i.e. as the SFN-SFN offset between the measured scrambling code (of the measured cell) and the reference scrambling code (of the serving cell) . The timing processor 103 is coupled to an identity processor 105 which is arranged to determine an identity indication for a serving network element for the first base station in response to the timing difference indication. Specifically, the identity processor 105 can compare the determined time difference to a set of known timing offsets between the reference scrambling code and different cells and can select the cell identity which has a timing offset closest to the measured timing difference .

The timing offsets can be determined in different ways in different embodiments. For example, the timing offsets may be manually measured and entered by a network operator at regular intervals, may be determined based on previous measurements or may be known from controlling the transmission times of the cell downlink pilot signal such that the pilot signal scrambling codes are transmitted with known timing offsets that are different for different cells reusing the same code. Various embodiments will be described in the following using different approaches for generating scrambling code time offset information that can be used by the identity processor 105.

In a first example, timing offset knowledge is built up from measurement reports for which the cell identity can be determined with a reasonable reliability and this information is used to determine a cell identity for a measurement which cannot be resolved with a reasonable reliability. In the example, the timing processor 103 receives measurement reports from a relatively large number of remote stations served by the serving cell (which transmits the reference scrambling code) . The measurement reports comprise relative time offset indications between a plurality of cells. Specifically, for all detected scrambling codes, the remote station reports a relative timing offset between the detected scrambling code and the reference scrambling code.

The timing processor 103 then evaluates an identification reliability measure for a cell associated for each relative time offset indication received from the remote station. If this identification reliability measure indicates that the cell identification can be reliably resolved in accordance with any suitable criterion, it proceeds to use the measured relative time offset as an indication of the actual time offset between the identified cell and the serving cell.

Specifically, whenever a scrambling code can be resolved, the OTDOA, the pair of scrambling codes involved, and the resolved cell identifier are recorded in a lookup table. Thus, in an initial phase, cell pair timing offsets are determined for a number of different cell pairs based on the reported timing offsets. The cell pair timing offsets are stored for each cell pair together with the identification of the two cells. For example, the cell pair timing offset for the cell pair of the serving cell and a first cell can be determined by averaging timing information from a large number of neighbour cell measurement reports reporting the scrambling code of the first cell. The cell pairs and timing offsets are then stored in a look-up table which thus comprises general information of timing offsets between different cell pairs .

It will be appreciated that any method of identifying the cell of the reported scrambling code can be used. As a specific example for a UMTS implementation, the sequence of RRC Connection Setup, RRC Measurement Report, RRC Active Set Update and NBAP Radio Link setup/addition/deletion messages within each individual call can be used to match scrambling codes referred to in the RRC messages with cell identifiers referred to in other messages.

The timing processor 103 can directly determine the cell pair timing offset between the serving cell and any cell for which the timing difference relative to the serving cell is reported. However, the timing processor 103 may furthermore determine a cell pair timing offset between two cells in response to two relative timing indications for each of the two cells relative to a third cell. For example, if a measurement report indicates that neighbour cell A has a timing offset of X and neighbour cell B has a timing offset of Y with respect to the serving cell, then the timing processor 103 can determine the cell pair timing offset between cell A and B as X-Y.

The determination of the cell pair timing offset can include a consideration of the propagation delays from the base stations transmitting the pilot signals. For example, a location of the remote station 217 can be determined and used to compensate the measured timing offsets reported by this remote station 117. However, in many embodiments, the propagation delay may be ignored as it is relatively low compared to the intentionally introduced timing offset.

If the identification reliability measure indicates that the cell identification cannot be reliably resolved in accordance with any suitable criterion, the identity processor 105 proceeds to determine the identity of the reported cell using the reported timing difference and the cell timing offsets determined from measurement reports that can be resolved.

Specifically, the identity processor 105 compares the time difference of the measured scrambling code to the stored cell pair timing offset for all pairs that include the cell of the reference scrambling code. Specifically, when the reference scrambling code is that of the serving cell, the reported SFN-SFN timing offset of an unknown scrambling code is compared to all the SFN-SFN timing offsets stored for the serving cell. The cell identity is then selected as the second cell of the cell pair which is closest to the measured timing offset. It will be appreciated that any suitable criterion may be used to determine the cell pair which is closest to the measured timing offset.

Thus, whenever a reported scrambling code remains unresolved, then - for each pairing of this scrambling code with another reported scrambling code - the lookup table is searched using the OTDOA and scrambling code pair to find evidence of the resolution that is implied. The identity processor 105 may not only search the specific cell pairs stored in the look up table but may also compare the measured time difference to the combined cell pair timing offset which results by combining the individual timing offsets in a sequence of cell pairs. For example, if cell pair A, B has a timing offset of X and cell pair B, C has a timing offset of Y, the identity processor 105 may generate the combined timing offset of X+Y for the cell sequence A, B - B, C. If the measured timing difference is close to this value, the cell identity is selected as the cell identity which is only included in one pair of the sequence of cell pairs (i.e. it either begins or ends the sequence) . Furthermore, as only two cells will be included in only one of the pairs of the cell sequence and as one of these is the cell of the reference scrambling code, the cell identity can be uniquely selected.

Thus, the described algorithm uses timing information to overcome the ambiguity that would otherwise exist when multiple resolutions of a particular reused scrambling code are possible. The algorithm allows the basic assumption that a reported scrambling code always belongs to a configured neighbour cell utilizing the scrambling code to be disregarded. The algorithm can therefore resolve scrambling codes even in cases with poorly planned or maintained neighbour lists or unknown neighbour lists. The algorithm can specifically utilize information collected from all remote stations (in all cells) to resolve the scrambling code within a single measurement report. It will be appreciated that any suitable criterion can be used for determining whether the identification reliability measure indicates whether the cell identification cannot be reliably resolved or not. Specifically, the identification reliability measure can be a continuous value which is compared to a threshold. As another example, the identification reliability measure can be a binary value which is e.g. set to 1 if a reliability criterion is met and zero if not. Thus, if this value is above 0.5 the identity is assumed to be resolvable and otherwise it is not.

As a specific example, the identification reliability measure may be determined from the reported signal level. For example, if the received signal level is above a given threshold, it may be assumed that the measured scrambling code is highly likely to be that of a neighbour cell included in the neighbour list whereas if the receive signal level is below the threshold this assumption is considered unreliable. Thus, if the reported receive signal level is below a threshold, the identity is resolved using the measured timing offsets.

In other examples, the timing of the transmitted pilot signal scrambling codes for a group of synchronised cells may be intentionally offset such that the timing offset between the transmitted pilot signal scrambling codes is well known. This information may then be used to resolve the cell identity. In the following, a specific such example will be described where the group of cells use identical pilot signal scrambling codes but with different known relative timing offsets. FIG. 2 illustrates an example of a UMTS communication system in accordance with some embodiments of the invention. In the system, a macro-layer is formed by macro-cells supported by base stations. Furthermore, an underlay layer of pico-cells are supported by a large number of small base stations which henceforth will be referred to as access points. Specifically, each access point may have an intended coverage of a single house or dwelling, and for a typical macro-cell coverage area of 10 to 30 km there may be hundreds or even thousands of pico-cells each supported by an individual access point.

In the system, the macro base stations each have a cell separation code in the form of a scrambling code that is unique within a given region which e.g. may be a reuse area for the cell scrambling codes. Specifically the macro base stations have an assigned pilot scrambling code which is unique within the reuse area such that a set of defined neighbours for each cell always have unique cell scrambling codes. Furthermore, each macro- cell base station has a unique hierarchical network address given by a unique base station ID for a given serving RNC, which itself has a unique RNC ID for a given MSC. Furthermore, each MSC has a unique identity in the network.

Accordingly, the neighbour lists transmitted by the base stations comprise indications of macro-cells which all have different cell scrambling codes. Furthermore, for each macro neighbour cell, a unique network address of the base station supporting the macro-cell can be determined from the detection of a specific neighbour cell pilot signal. Accordingly, a handover to a target macro-cell may be initiated with an explicit and unique identification of the handover target base station.

In contrast, the access points (which in the specific example are base stations supporting pico-cells) use a pilot signal scrambling code which is shared between a plurality of access points within the reuse area and specifically a given neighbour list may comprise indications of shared pilot signal scrambling codes for a plurality of underlay cells that are all considered as neighbours/potential handover targets for the current cell. By sharing a pilot signal scrambling code between a plurality of access points, a significantly reduced number of scrambling codes are required by the system. Furthermore, by keeping the number of scrambling codes low, the number of scrambling codes that must be evaluated by the remote station for handover determination can be reduced substantially thereby reducing the measurement time, power consumption and/or complexity of the remote station.

However, the use of a shared pilot signal scrambling code means that the remote station (or supporting network nodes) cannot uniquely identify the access point which has been detected by the remote station simply from the detected scrambling code. Rather, a remote station detecting a scrambling code does not uniquely identify a given target access point for a handover but at best identifies only a group of access points which all use the same shared pilot signal scrambling code.

In some embodiments, all access points within a coverage area supported by a single macro-RNC may use the same scrambling code. However, it will be appreciated that in other embodiments, a plurality of shared scrambling codes may be available for the access points. Therefore, the access points may be divided into a number of groups with the access points of each group sharing a scrambling code but with different scrambling codes being used for different groups. In such embodiments, the scrambling codes may be allocated to the access points such that a reuse pattern is established with the interference between pico-cells having the same shared scrambling code being reduced or minimised.

In the specific example of FIG. 2, one macro-base station 201 which supports a macro-cell with a typical coverage area of 10-30 kilometres is illustrated. The macro base station 201 is coupled to a macro RNC 203 which is furthermore coupled to other macro base stations (not shown) . The macro RNC 203 is furthermore coupled to a core network 205 which interfaces to other radio access networks and RNCs. In the example, the macro RNC 203 is coupled to a first MSC 207 which is further coupled to a second MSC 209 serving a different set of RNCs than the first MSC 207.

The system furthermore comprises a large number of pico- cell base stations/access points 211, 213 (for clarity only three access points are illustrated in FIG. 2) . Each of the access points 211, 213 supports a pico-cell having a coverage area of typically 10 to 50 meters. The access points 211, 213 implement the required functionality of a UMTS base station in order to support UMTS communications within the pico-cell. However, in contrast to conventional UMTS base stations, the access points 209 use a common shared pilot signal scrambling code. In the system, the access points 211, 213 are residential access points intended to be located in individual subscribers' homes for supporting the subscriber (s) when at home.

The system of FIG. 2 furthermore comprises an access point controller 215 which supports the access points 211, 213. In the specific example, the access point controller 215 specifically assists in routing data between the access points 211, 213 and the core network 205 as well as assisting in providing handover target ambiguity resolution.

The system of FIG. 2 furthermore comprises a remote station 217 which initially is served by the macro base station 201. The remote station 217 monitors the pilot signal scrambling code included in its neighbour list which includes the shared code of the access points 211, 213.

Whilst in access connected mode on the macro cell, the remote station 217 is specifically configured to report a measurement of the shared pilot signal scrambling code to the macro RNC 203. Thus, the macro-RNC 203 may be considered to comprise the measurement receive processor 101 of the apparatus of FIG. 1.

In response to receiving the measurement report, the macro RNC 203 requests the remote station 217 to measure and report the SFN-SFN observed timing difference between the macro cell reference scrambling code and the measured scrambling code, i.e. the shared pilot signal scrambling code . Furthermore, if the measurement report indicates that the cell of the measured shared scrambling code is a suitable handover candidate, the macro-RNC 203 initiates a handover by generating a handover request message.

In the example, the shared scrambling code may be associated with the address (RNC ID) of the access point controller 215. In this case, the macro RNC 203 may determine the preference for a handover (based on the reported pilot signal measurements from the remote station 217) and may accordingly transmit the handover request message to the access point controller 215. Thus, in the example, although the identity of the detected access point 211 is not known, the identity of the access point controller 215 which controls the access points 211, 213 is known and the handover message is transmitted thereto .

The handover request message comprises the measured timing difference and this is used by the access point controller 215 to determine which of the access points have been detected.

In the specific example of a UMTS system, a UMTS RRC Transparent Container of a RANAP Relocation Required message is passed from the macro RNC 203 to the access point controller 215 (arriving for example as a Relocation Request message) . The Transparent Container comprises new Information Elements which in the example comprises the following data:

- the source cell identity; - the SFN-SFN Observed Timing Difference measured by the remote station 217; and

- optionally an indication of the remote station propagation delay.

In the system of FIG. 2, the access point controller 215 is coupled to a scrambling code controller 219 which is arranged to assign different time offsets to the pilot signal scrambling codes transmitted by the access points. Specifically, the scrambling code controller 219 assigns different relative timing offsets to pilot scrambling codes of the access points 211, 213 relative to a timing reference. The timing reference may for example correspond to the timing of one of the pilot signal scrambling codes of the access points.

Each of the access points 211, 213 is arranged to transmit its pilot signal scrambling code with the relative timing offset that has been assigned to it. The relative timing offset assignments may e.g. be communicated to the access points 211, 213 via suitable network messages. The synchronisation of the access points 211, 213 may e.g. be achieved by using a global time reference such as the Global Positioning System (GPS) or Network Time Protocol (NTP) functionality using the backhaul network.

Thus, in the example, the pilot signals of all of the access points 211, 213 are synchronised but have a different timing offset relative to the reference.

However, the macro-base station 201 is not synchronised with the timings of the access points 211, 213. However, as will be described later, several methods can be used to determine the reference timing offset between the two timing domains.

FIG. 3 illustrates an example of the timing of transmitted pilot signal scrambling codes for eight pico- cells (and access points) and one macro cell. In the example, the access point 211, 213 SFN timings for the shared pilot signal scrambling code are configured so that they are evenly spaced across a 256 frame period (2.56s) .

The scrambling code controller 218 and the access points 211, 213 are furthermore coupled to an identity resolve processor 221 which comprises the timing processor 103 and the identity processor 105 of FIG. 1. The identity resolve processor 221 is arranged to determine the identity of the access points 211, 213 by determining the timing difference between the reference scrambling code (the pilot signal scrambling code of the macro station) and the measured scrambling code (the shared pilot signal scrambling code from one of the access points 211, 213) and comparing this timing difference to the relative timing offsets of the access points 211, 213.

In order to make this comparison, the reference timing offset between the macro base station 201 and the timing reference of the access points 211, 213 must be determined.

Specifically, the scrambling code controller 218 provides information of the assigned relative timing offsets to the identity resolve processor 221 which allows the detected cell to be identified if the timing difference is compensated for the reference timing offset.

Thus, in the example, when the handover request message is received, the access points 211, 213 first use the identification of the source macro cell to identity the group of access points 211, 213 which are potential handover targets (e.g. it may be assumed that all access points 211, 213 support underlay cells of the same macro cell such that the group of access points 211, 213 are uniquely linked to the macro cell) .

The access point controller 215 furthermore feeds the received timing indications for the measured scrambling code and the reference scrambling code to the identity resolve processor 221 which proceeds to determine the measured timing difference between the two scrambling codes. In the described example, the handover request message directly comprises an indication of the timing difference in the form of the SFN-SFN timing offset between the measured shared pilot signal scrambling code and the reference scrambling code of the base station.

The determined reference timing offset is used to offset the measured timing difference to compensate for the timing offset between the macro cell timing domain and the access point timing domain. The result is then compared to the known timings of the individual access points 211, 213 given by the relative timing offsets and the closest access point 211, 213 is selected as the unique target access point 211. For example, in FIG. 3, cell 6 is found to be the detected cell. The determined target access point identity is then fed back to the access point controller 215 which proceeds to identify a suitable serving network element for the target access point 211. Specifically, an RNC which supports the handover procedures for the selected target access point 211 is identified and the handover request message is forwarded to this.

In the specific example, each of the access points 211, 213 comprise RNC functionality and the relocation message may therefore be transmitted directly to the target access point 211. The relocation message is in the specific example a UMTS Relocation Request message.

When the target RNC (i.e. the RNC functionality of the access point 211 or an RNC supporting the selected access point) receives the Relocation Request, it proceeds to respond as if the message had been received directly from the macro RNC 203. Specifically, the target RNC can transmit a relocation acknowledge message (e.g. a

Relocation Request Acknowledge) to the macro-RNC thereby initiating a handover procedure generally following a conventional approach. The handover acknowledge message can specifically comprise an address indication for the target RNC thereby allowing the macro RNC 203 to directly interact with the appropriate RNC functionality.

As a specific example, when receiving the Relocation Request message, the target RNC allocates resources in the target pico-cell for the incoming hard handover and returns the matching configuration in e.g. a Physical Channel Reconfiguration message in an RRC transparent container to the macro RNC 203. The macro RNC 203 passes the reconfiguration to the remote station 217 which then attempts to access the target access point 211 using the specified configuration.

The single selected access point 211 then receives an access from the remote station 217 and specifically the access point 211 detects uplink synchronisation at layer 1 and then receives the RRC reconfiguration confirm message from the remote station 217. A relocation detect and relocation complete is then signalled to the core network .

Furthermore, a second rapid relocation (without the remote station 217 being involved) can be executed to relocate the Iu signalling connection from the access point controller 215 to the target RNC.

One way of determining the reference timing offset between the timing reference of the access point timing domain and the timing of the reference scrambling code (e.g. of the timing domain of the macro base station 201) uses an additional pilot signal transmitted by at least one of the access points 211, 213. This second pilot signal has a unique pilot scrambling code for the group of access points 211, 213. Furthermore, the scrambling code is transmitted with a fixed timing relationship to the timing reference. For example, one of the access points 211 can transmit a second pilot signal which is only transmitted from this base station and which has a fixed offset (e.g. of zero) from the timing reference. Accordingly, the access points 211, 213 have different relative timing offsets with respect to this second pilot signal .

In this example, the unique pilot signal scrambling code is included in the neighbour list for the remote station 217 in addition to the shared pilot signal scrambling code. Thus, measurement reports will be received comprising timing indications for both the unique pilot signal scrambling code, the shared pilot signal scrambling code and the reference scrambling code. The reference timing offset between the two timing domains can then directly be found from the timing difference between the unique pilot signal scrambling code and the reference scrambling code. Specifically, the reported SFN-SFN timing offset between the unique pilot signal scrambling code and the pilot signal scrambling code of the macro cell provides the reference timing offset between the two timing domains.

As another example, relocations (such as handovers or cell selection/re-selections) of remote stations typically include exchanging timing information. This information may be used to adjust the reference timing offset by timing information exchanged when a remote station hands over from the macro-cell to one of the access points 211, 213. This approach may be particularly suitable for continuously updating the reference timing offset as handovers based on the previously described address resolution approach can subsequently be used to correct the reference timing offset thereby providing an efficient tracking approach. As another example, one or more of the access points 211, 213 may further comprise a downlink receiver arranged to receive the reference scrambling code. Specifically, the downlink receiver may receive the pilot signal scrambling 5 code of the macro base station 201 and generate a timing indication for this reference scrambling code, e.g. relative to the timing of its own transmitted pilot signal scrambling code.

10 The timing indication is included in a timing messaged which is transmitted to the access point controller 215 and the identity resolve processor 221. The identity resolve processor 221 then determines the reference timing offset from the direct measurement of the timing

15 of the reference scrambling code made by an access point of the synchronised access points 211, 213.

As another example, remote stations connected to one of the access points 211, 213 may be used to report the 20 observed time difference between the pilot signal scrambling code of the serving access point and the timing of the reference scrambling code.

Specifically, when served by one of the access points 25 211, 213, the macro base station 201 can be included in the neighbour list resulting in a reference timing value indicative of the timing of the reference scrambling code relative to the timing of the pilot signal scrambling code of the serving base station being determined. 30 Specifically, the SFN-SFN observed time difference can be determined. The reference timing value is then transmitted to the access point controller 215 and the identity resolve processor 221 which uses it to determine the reference timing offset. Specifically, the reference timing value is offset by the relative timing offset assigned to the serving access point to result in a direct measure of the timing offset between the macro cell timing domain and the access point timing domain.

In some embodiments, the reference scrambling code is not that of a serving base station but may e.g. be another reference scrambling code which can provide a timing reference that can be used to determine the relative timing offset assigned to the specific measured scrambling code.

For example, the reference scrambling code may be a scrambling code of a pilot signal which is transmitted with a fixed relationship to the timing reference of the access point timing domain. Specifically, the reference scrambling code may be a scrambling code of a pilot signal transmitted by one, more or all of the access points 211, 213 and which has a fixed offset to the timing reference.

Also, the reference scrambling code may be a scrambling code of a second pilot signal transmitted by the same access point as the pilot signal which is offset by the assigned relative timing offset. Specifically, the access points 211, 213 may each transmit two pilot signals using two shared pilot signal scrambling codes. However, whereas one pilot signal is transmitted with the same timing offset for all access points 211, 213, the other pilot signal is transmitted with the assigned relative timing offset. Accordingly, the time difference between the two transmitted pilot signals is different for each of the access points 211, 213. Thus, by including both pilot signal scrambling codes in the neighbour list for the remote station 217 the relative timing difference (e.g. the SFN-SFN observed timing offset) is received for both pilot signals thereby allowing the time offset between the two pilot signals to be determined.

This information can then be compared to the known timing offsets for the different access points 211, 213 thereby allowing the identity of the detected access point 211 to be identified.

As the access points 211, 213 may be timing synchronised, the timing offset between the transmitted pilot signals can be set with a high degree of precision e.g. down to chip level (and without a macro or external time base) thereby allowing a low complexity and an accurate system.

It will be appreciated that the system may comprise suitable tracking means for continuously tracking timing drifts in the system. For example, measurement reports may be used to adjust the stored cell pair timing offsets and/or the timing offset used by the access points 211, 213 for the pilot signal transmissions.

FIG. 4 illustrates an example of a method of operation for a cellular communication system in accordance with some embodiments of the invention. The method initiates in step 401 wherein a measurement report for a measured scrambling code of a downlink pilot signal of a first base station serving a first cell is received from a remote station served by a second base station serving a second cell. The measurement report comprises a first timing indication for the measured pilot scrambling code.

Step 401 is followed by step 403 wherein a timing difference between the measured scrambling code and a reference scrambling code is determined in response to the first timing indication and a second timing indication for the reference scrambling code.

Step 403 is followed by step 405 wherein an identity indication for a serving network element is determined for the first base station in response to the timing difference .

It will be appreciated that the above description for clarity has described embodiments of the invention with reference to different functional units and processors. However, it will be apparent that any suitable distribution of functionality between different functional units or processors may be used without detracting from the invention. For example, functionality illustrated to be performed by separate processors or controllers may be performed by the same processor or controllers. Hence, references to specific functional units are only to be seen as references to suitable means for providing the described functionality rather than indicative of a strict logical or physical structure or organization. The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention may optionally be implemented at least partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors .

Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term comprising does not exclude the presence of other elements or steps.

Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also the inclusion of a feature in one category of claims does not imply a limitation to this category but rather indicates that the feature is equally applicable to other claim categories as appropriate.

Furthermore, the order of features in the claims does not imply any specific order in which the features must be worked and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order.

Claims

1. A cellular communication system comprising: measurement means for receiving a measurement report for a measured scrambling code of a downlink pilot signal of a first base station serving a first cell from a remote station served by a second base station serving a second cell, the measurement report comprising a first timing indication for the measured pilot scrambling code; timing means for determining a timing difference indication between the measured scrambling code and a reference scrambling code in response to the first timing indication and a second timing indication for the reference scrambling code; and identity means for determining an identity indication for a serving network element for the first base station in response to the timing difference indication .

2. The cellular communication system of claim 1 further comprising: a plurality of base stations including the first base station, each base station comprising means for transmitting a pilot signal with a pilot signal scrambling code; means for assigning a relative timing offset for the pilot scrambling codes of the plurality of base stations relative to a timing reference, each pilot scrambling code being assigned a different timing offset; and wherein each of the base stations is arranged to transmit a pilot signal scrambling code with the relative timing offset assigned to the base station and the identity means is further arranged to determine the identity indication by comparing the timing difference to the relative timing offsets of the plurality of base stations .

3. The cellular communication system of claim 2 wherein the plurality of base stations are arranged to use a common pilot scrambling code transmitted with the relative timing offsets.

4. The cellular communication system of claim 2 wherein the reference scrambling code is a pilot signal scrambling code of a pilot signal of the second base station .

5. The cellular communication system of claim 4 further comprising timing reference means for determining a reference timing offset between the timing reference and a timing of the reference scrambling code and wherein the identity means is further arranged to determine the identity indication in response to the reference timing offset .

6. The cellular communication system of claim 5 wherein at least one of the plurality of base stations is arranged to transmit a second pilot signal having a unique pilot signal scrambling code that is unique for the plurality of base stations and which has a fixed relationship to the timing reference; and the timing reference means is arranged to determine the reference timing offset in response to measurement reports comprising timing indications of the reference scrambling code and the unique pilot signal scrambling code.

7. The cellular communication system of claim 5 wherein the timing reference means is arranged to determine the reference timing offset in response to timing data of previous relocations of remote stations from the second base station to a base station of the plurality of base stations.

8. The cellular communication system of claim 5 wherein at least one of the plurality of base stations comprises a downlink receiver arranged to receive the reference scrambling code and to transmit a timing message to the identity means, the timing message comprising a reference timing indication for the reference scrambling code; and the timing reference means is arranged to determine the reference timing offset in response to the reference timing indication.

9. The cellular communication system of claim 5 further comprising a supported remote station being served by a serving base station of the plurality of base stations, the supported remote station comprising: means for receiving the reference scrambling code; means for determining a reference timing value indicative of a timing of the reference scrambling code relative to a timing of a pilot signal scrambling code of the serving base station; means for transmitting a timing message to the timing reference means, the timing message comprising the reference timing value; and wherein the timing reference means is arranged to determine the reference timing offset in response to the reference timing value.

10. The cellular communication system of claim 1 wherein the reference scrambling code is a scrambling code of a pilot signal of the first base station and the measurement report further comprises a timing indication for the reference scrambling code.

11. The cellular communication system of claim 1 wherein the measurement means is arranged to initiate a handover request message in response to receiving the measurement report and to transmit the handover request message to an address resolution network element serving the first base station, the address resolution network element comprising the timing means and the identity means and the handover request message comprising the timing indication .

12. The cellular communication system of claim 11 wherein the address resolution network element is arranged to forward the handover request message to the serving network element and the serving network element is arranged to transmit a handover acknowledge message to a radio network controller serving the second base station, the handover acknowledge message comprising an address indication for the serving network element.

13. The cellular communication system of claim 1 wherein the reference scrambling code is that of the second base station and the first timing indication is an indication of a remote station measured timing difference between the measured scrambling code and the reference scrambling code .

14. The cellular communication system of claim 13 further comprising: means for receiving measurement reports comprising relative time offset indications between a plurality of cells; cell pair means for determining cell pair timing offsets of cell pairs in response to the relative time offset indications, each cell pair comprising an identification of two cells and an associated cell pair timing offset indicating a timing offset between the two cells; and wherein the identifying means is arranged to select a first cell pair from a set of cell pairs comprising the second cell in response to comparison between the time difference and the cell pair timing offsets of the set of cell pairs, and to determine the identity of the first cell as an identity of a cell of the first cell pair.

15. The cellular communication system of claim 14 wherein the cell pair means is arranged to determine an identification reliability measure for a cell associated with a relative time offset indication and to ignore the relative time offset indication when determining cell pair timing offsets if the identification reliability measure does not meet a reliability criterion.

16. The cellular communication system of claim 14 wherein the cell pair means is arranged to determine an associated cell identification reliability measure for the first timing indication and wherein the identity means is arranged to determine the identity indication if the associated cell identification reliability measure does not meet a reliability criterion.

17. The cellular communication system of claim 14 wherein the cell pair means is arranged to determine a cell pair timing offset between two cells in response to two relative timing indications for each of the two cells relative to a third cell.

18. The cellular communication system of claim 14 wherein the identifying means is arranged to compare the time difference to a combined cell pair timing offset for a sequence of cell pairs and to determine the identity of the first cell as an identity of a cell which is included in only one pair of the sequence of cell pairs.

19. A method of operation for a cellular communication system, the method comprising: receiving a measurement report for a measured scrambling code of a downlink pilot signal of a first base station serving a first cell from a remote station served by a second base station serving a second cell, the measurement report comprising a first timing indication for the measured pilot scrambling code; determining a timing difference indication between the measured scrambling code and a reference scrambling code in response to the first timing indication and a second timing indication for the reference scrambling code; and determining an identity indication for a serving network element for the first base station in response to the timing difference indication.