Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J11/00—Orthogonal multiplex systems, e.g. using WALSH codes
  • H04J11/0069—Cell search, i.e. determining cell identity [cell-ID]
  • H04J11/0093—Neighbour cell search
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W36/00—Hand-off or reselection arrangements
  • H04W36/24—Reselection being triggered by specific parameters
  • H04W36/30—Reselection being triggered by specific parameters by measured or perceived connection quality data
  • H04W36/302—Reselection being triggered by specific parameters by measured or perceived connection quality data due to low signal strength
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W36/00—Hand-off or reselection arrangements
  • H04W36/34—Reselection control
  • H04W36/36—Reselection control by user or terminal equipment

Definitions

• the present invention relates to methods and arrangements in a mobile communication system, in particular to neighbour cell identification activities.
• WCDMA Wideband Code Division Multiple Access
• E-UTRAN Evolved Universal Terrestrial Radio Access Networks
• the UE In WCDMA in active mode the UE is required to detect (or identify) a new intra- frequency cell in at least 800 ms. In addition it should be able to measure and report the downlink quality (i.e. CPICH measurements) of at least 8 cells (one serving and seven neighbor cells), while fulfilling the minimum performance requirements as specified in TS 25.133 [1].
• CPICH measurements the downlink quality
• Idle mode mobility cell reselection Connected mode mobility: handover
• the cell reselection is mainly UE autonomous function without the intervention of its serving cell. But to some extent the UE behavior in this mobility scenario could still be controlled by some broadcasted system parameters and performance specification.
• the handover on the other hand is fully controlled by the network through explicit UE specific commands and by performance specification.
• the mobility decisions are mainly based on the same kind of downlink measurements as will be discussed in more details in relation to the description of the problem.
• Both WCDMA and E-UTRAN are frequency reuse-1 systems. This means the geographically closest neighbor cells operate on the same carrier frequency. An operator may also deploy multiple frequency layers within the same coverage area. Therefore, idle mode and connected mode mobility in both WCDMA and E-UTRAN could be broadly classified into three main categories:
• Intra-frequency mobility Intra-frequency mobility (idle and connected modes) where the UE moves between the cells belonging to the same carrier frequency. This is the most important mobility scenario since it involves less cost in terms of delay due. In addition an operator would have at least one carrier at its disposal that it would like it to be efficiently utilized.
• Inter-frequency mobility is the scenario where the UE moves between cells belonging to different carrier frequencies but of the same access technology. This could be considered as the second most important scenario.
• Inter-RAT mobility is the scenario where the UE moves between cells that belong to different access technologies such as between WCDMA and GSM or vice versa.
• the requirements related to the above measurement tasks are specified.
• WCDMA the specification covers the following is specified to ensure that UE meets at least these requirements.
• Some of these important requirements in active mode include [1]: Identification delay of unknown cells for the given received level of the corresponding synchronization and CPICH signals. The max delay is up to 800 ms for intra-frequency cells in continuous reception mode (i.e. without DRX).
• Minimum number of identified cells (8 including one serving and 7 neighbor cells) for which UE is supposed to report the CPICH measurements with the specified measurement absolute and relative accuracies
• the UE In order to report the downlink measurements of the requirement number of identified cells to the network, the UE needs to regularly look (i.e. identify) for new cells that have become better than the current or old cells.
• cell detection, cell search and cell identification have the same meaning, which is the UE ability to fully synchronize (i.e. finding cell timing, cell identity etc) to previously unknown cell.
• the cell to be identified may or may not belong to the neighbor cell list provided by the network to the UE.
• the UE needs to identify and measure the neighbor cells on regular basis.
• the scheduling of such measurement process is not standardized but is rather UE implementation specific.
• the UE would typically collect measurement samples for different type of measurements at some periodic intervals.
• idle mode the UE does measurement mainly at the paging occasions (i.e. at the wake instances at the end of DRX cycle). Therefore, measurement sampling rate in idle mode is considerably low compared to the connected mode scenario. Due to this reason measurement performance in idle mode becomes much coarser than that would be achievable in connected mode.
• DRX Discontinuous Reception
• WCDMA release 7 a new feature called discontinuous reception (DRX) in connected mode (more specifically in CELL_DCH state) has been specified [5]. This allows UE to save its battery while stay connected since it wakes up only at periodic instances according to the DRX cycle. In release 7 the maximum DRX cycle is 40 ms.
• the DRX feature also implies that UE will mainly collect the measurement samples at the wake up instances or at least less frequently compared to the continuous reception case. Accordingly the measurement requirements have been relaxed in DRX mode in release 7 [1].
• the worst case intra-frequency cell identification in DRX (release 7) may take up to 6 seconds, i.e. if we assume 40 ms DRX and 5% UE activity level [1]. Therefore, DRX in active mode may have some adverse effect on the mobility performance in general and cell identification performance in particular.
• the delay of 6 seconds is still within an acceptable limit for packet data services, which are more delay tolerant. It has been shown that this will affect only up to 10% of the users. This is because in most cases the active set update, where one or more old cells are replaced by the newly identified ones, takes longer than 6 seconds. However, for real time services, even better performance is desirable.
• the active mode DRX may allow network to use up to 2.56 seconds of
• UE in DRX performs measurement and identify cells provided that the minimum received level on synchronization channel (SCH) and common pilot channel (CPICH) on that cell is significantly higher (e.g. 3 dB higher than in case of non DRX). Since cells are relatively stronger therefore on the average the cell identification will be relatively faster.
• SCH synchronization channel
• CPICH common pilot channel
• Absolute thresholds based cell identification delay adjustment The network provides UE with absolute thresholds in terms of CPICH Ec/lo or CPICH RSCP levels and some measurement activity factor. The latter parameter is used to scale the measurement activity (e.g. cell identification time, measurement period) depending upon the CPICH Ec/lo and/or CPICH RSCP reception level of the strongest monitored cell in the active set.
• the UE has to identify a new intra-frequency cell according to the measurement activity factor. Generally this would lead to longer cell identification delay depending upon the signaled activity factor. On the other hand when this condition is no longer met, i.e. if the serving cell quality is weak (below Ec/No or/and RSCP thresholds) then the UE will identify a new cell with the same delay as specified for continuous reception case, i.e. 800 ms worst case delay [1].
• the WCDMA specification [5] allows network in active mode to promptly direct the UE to switch between DRX and non DRX modes through low level signaling.
• one possible solution to speed up the measurement process is to revert to non DRX mode (or very short DRX cycle) when UE enters in cell border region.
• the network can determine whether UE lies in cell border region or not by comparing the UE reported downlink channel quality (e.g. CPICH level) with a certain threshold. Typically between 25-35% users operate in the cell border region. Thus with approach on the average between 65-75% of the time the UE could barely stay in DRX. Since UE cannot fully utilize DRX 1 this approach is not desirable from UE battery saving perspective.
• the present invention aims to provide a method that takes into account the relative performance difference between the serving and the target/neighbour cells.
• the UE shall identify a new cell with relatively lower intensity (or measurement activity) provided that the downlink received signal level (e.g. in terms of CPICH Ec/No or/and SCH Ec/No) from an appropriate selection of neighbour cells (cells belonging to active or candidate set) is not weaker than the serving cell by a certain threshold.
• the UE shall identify a new cell with higher intensity provided the downlink received signal level (e.g. in terms of CPICH Ec/No or/and SCH Ec/No) from said selection of neighbour cells is weaker than the serving cell by a certain threshold.
• the UE shall identify a new cell with relatively lower intensity (or measurement activity) provided that the downlink received signal level (e.g. in terms of CPICH Ec/No or/and SCH Ec/No) from at least N strongest neighbor (cells belonging to active or candidate set) cells is not weaker than the serving cell by a certain threshold.
• the UE shall identify a new cell with higher intensity provided the downlink received signal level (e.g. in terms of CPICH Ec/No or/and SCH Ec/No) from at least N strongest neighbor (or target or cells belonging to active or candidate set) cells is weaker than the serving cell by a certain threshold.
• the UE shall identify a new cell with relatively lower intensity (or measurement activity) provided the downlink received signal level (e.g. in terms of CPICH Ec/No or/and SCH Ec/No) from the weakest neighbor cell in the active or candidate set is not weaker than the serving cell by a certain threshold.
• the UE shall identify a new cell with higher intensity provided the downlink received signal level (e.g. in terms of CPICH Ec/No or/and SCH Ec/No) from the weakest cell belonging to active or candidate set is weaker than the serving cell by a certain threshold.
• the serving cell may not be the strongest. Therefore in yet another set of embodiments the cell identification or detection performance would depend upon the relative difference in the received levels (e.g. in terms of CPICH Ec/No and/or SCH Ec/No) between the strongest and the N strongest target or neighbor cells.
• Figure 1 illustrates a cellular communication network in accordance with the present invention.
• Figure 2 is a flow chart, illustrating a first method in accordance with an aspect of the present invention.
• FIG. 1 illustrates schematically a cellular communications network operating in accordance with the invention.
• the network coverage area is divided into cells 10, 12, 14, 16, 18, 20, 22, etc.
• a user equipment (UE) 24 is located in the cell 12, and is in communication with a base station (eNode B) 26 in that cell, which is therefore the serving cell for that UE.
• eNode B base station
• the UE must make measurements on the signals received from other cells, in order to identify potential neighbour cells.
• the invention will be described herein primarily with reference to a Long term evolution (E-UTRAN) network, but it will be appreciated that it can be applied to many other networks.
• E-UTRAN Long term evolution
• Figure 2 is a flow chart, illustrating a first method in accordance with the present invention.
• step 50 the UE detects the thresholds to be used in the method.
• step 52 the UE makes a comparison between quality measurements that it has made on a reference cell, and quality measurements that it has made on a certain number of other already identified cells. If it is determined in step 54 that a comparison criterion is met, the process passes to step 56, and the cell detection activity or intensity level is increased, while, if it is determined in step 54 that a comparison criterion is not met, the process passes to step 58, and the cell detection activity or intensity level is decreased.
• the cell detection activity or intensity of new cell is determined by the UE after comparing the quality level of the already identified cells with that of the serving cell.
• the network signals at least one relative threshold that can be used by the UE to compare the cells in terms of downlink received levels such as Common pilot channel (CPICH) Ec/No and Synchronization channel (SCH) Ec/No.
• CPICH Common pilot channel
• SCH Synchronization channel
• the threshold could also be a fixed value standardized in the specification.
• the network can also signal a parameter, 'N', i.e. the number of strongest cells whose received levels has to be compared with that of the reference cell.
• the reference cell is the serving cell in the preferred embodiment.
• the comparison is to be done with the best cell.
• this number (N) can also be a standardized value.
• the new cell detection activity is dependent upon whether the difference of the measured quantity between the serving and target cell (s) is larger or smaller than the relative threshold.
• the low or coarse cell detection activity or intensity means that UE mainly detects a new cell during wake up times of the Discontinuous reception (DRX) or over considerably longer period compared to that required in continuous reception scenario.
• High cell detection intensity means on the other hand that UE either identifies the cell continuously or assuming very short DRX cycle. This leads to similar cell detection performance (i.e. cell detection delay) which is achievable in continuous reception scenario.
• Criteria to define relative thresholds cell comparison criterion
• the relative threshold would employ which measurement quantity should be used by the UE to compare cells. In other words the relative threshold would determine the comparison criterion.
• the threshold could be expressed in one or more of the following parameters:
• Neighbour cell measurement level such as Received signal code power (RSCP) and/or Ec/No in WCDMA, or Reference symbol received power (RSRP) and/or
• RSCP Received signal code power
• RSRP Reference symbol received power
• Reference symbol received quality (RSRQ) in E-UTRAN Reference symbol received quality (RSRQ) in E-UTRAN.
• SCH received level for example SCH Signal to Interference and Noise Ratio (SINR),
• one preferred relative threshold to decide the intensity of cell detection is to be expressed in terms of relative CPICH Ec/No and/or SCH Ec/No levels. This is because the cell identification requirements are specified for the given SCH and CPICH received levels.
• the above relative thresholds could be single level or multi-level (M).
• M multi-level
• the network signals only one threshold per criterion. This would lead to two levels of cell detection activity level: normal or high and low.
• the network could signal several relative thresholds per criterion, which would require UE to achieve several cell detection activity levels (i.e. M+1 for M threshold).
• M+1 for M threshold
• Network can also specify how often (i.e. periodicity of comparison) the UE has to compare the cells based on the relative thresholds. This could either be a signalled parameter or can be specified for instance in terms of number of frames or number of DRX cycles or some fixed durations.
• the above thresholds and other related parameters can be signalled by state of the art method using radio resource control (RRC) or any higher layer signalling.
• RRC radio resource control
• the thresholds could be signalled via broadcast channel to all the UE or to specific UE via UE specific control channel.
• different threshold values for the same criterion could be used in idle and active mode.
• the RNC will control the signalling of these thresholds.
• the thresholds are signalled by the base station (or eNode B in E-UTRAN terminology).
• the UE new cell detection activity of a particular target cell will be based on the relative performance difference between target cell(s) and a reference cell. Broadly there are two possibilities:
• the UE always compares the target cells with the serving cell using one or more of the criteria outlined in the section relating to the criteria to define relative thresholds. Based on this comparison the UE will decide whether to detect a new cell with a high or a low intensity.
• both pilot signal quality (RSRQ in E-UTRAN or CPICH Ec/No in WCDMA) and SCH received level (e.g. SCH SINR in E-UTRAN or SCH Ec/No in WCDMA) are used to determine the new cell detection activity or intensity level.
• SCH SINR in E-UTRAN or SCH Ec/No in WCDMA
• ⁇ and ⁇ for RSRQ (or CPICH Ec/No) and SCH SINR (or SCH Ec/No) respectively.
• UE will identify a new cell less frequently (resulting in coarse or longer cell identification delay) if one or both of these measured quantity difference between N best or strongest cells (i.e. in terms of SCH Ec/No and /or CPICH Ec/No in WCDMA) and serving cell S falls below the corresponding threshold.
• N best or strongest cells i.e. in terms of SCH Ec/No and /or CPICH Ec/No in WCDMA
• OPERATOR is set to AND. In an alternative arrangement for both E-UTRAN and WCDMA, OPERATOR is set to OR.
• only one of the measurement quantities (Q or P) could be used in the above algorithm by the UE to decide the measurement activity level on the target cell.
• Q N and P N stand for the respective signal levels received from the weakest of the N best neighbour cells in the active or candidate set.
• the value of N can be signalled to the UE. Varying the value of N can change the overall detection activity.
• the value of N can be stored in the UE.
• Q N and P N stand for the respective signal levels received from the weakest cell in the active or candidate set.
• the UE In the embodiment where the reference cell is the best cell the UE always compare the target or neighbour cells with the best cell, which may not always be the serving cell. As before, the UE uses one or more of the criteria outlined in the section relating to criteria to define relative thresholds. Based on this comparison the UE will decide whether to detect a new cell with a high or a low intensity. This method may however not give sufficient information regarding the target cells, e.g. the strongest cell may have no resources left.
• both pilot signal quality (RSRQ in E-UTRAN or CPICH Ec/No in WCDMA) and SCH received level (e.g. SCH SINR in E-UTRAN or SCH Ec/No in WCDMA) are used to determine the new cell detection activity or intensity level.
• SCH SINR in E-UTRAN or SCH Ec/No in WCDMA
• ⁇ and ⁇ for RSRQ (or CPICH Ec/No) and SCH SINR (or SCH Ec/No) respectively.
• UE will identify a new cell less frequently (resulting in coarse or longer cell identification delay) if one or both of these measured quantity difference between N best or strongest cells (i.e. in terms of SCH Ec/No and /or CPICH Ec/No in WCDMA) and the best cell B falls below the corresponding threshold.
• Q ⁇ and P K stand for the respective signal levels received from the weakest of the N best neighbour cells in the active or candidate set.
• Q K and P K stand for the respective signal levels received from the weakest cell in the active or candidate set.
• FIG. 3 is a flow chart, illustrating a further method in accordance with the present invention.
• step 70 a target cell is identified.
• step 72 a quality of that target cell is compared with a quality of a reference cell. Based on the result of that comparison, in step 74, the measurement activity on that target cell can be adapted.
• the measurement activity or intensity level of an already identified target cell can be adapted based on relative threshold between the serving/best cell and that target cell.
• the relative threshold and other parameters used for the cell detection may not be the same.
• the idea of adaptive measurement activity based on relative threshold and the concept of adaptive cell detection activity based on relative threshold according to the present invention can very well work independently as well as together.
• the UE In DRX mode the UE will search a new cell faster only when it is necessary that is when the active set or candidate set contains relatively weaker cells.
• the network is able to keep track of all the required number of target cells without degrading the measurement performance.
• the UE is still able to save its battery as much as possible while making full use of DRX.
• 3GPP TS 25.133 "Requirements for support of radio resource management (FDD)”.
• 3GPP TS 36.801 "Evolved Universal Terrestrial Radio Access (E-UTRA); Measurement Requirements”.
• 3GPP TS 25.215 “Physical layer measurements (FDD)”.
• E UTRA Evolved Universal Terrestrial Radio Access

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• 2008
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