Abstract:
In a method and a device for determining an active set for a mobile station cell individual offsets (CIO) is determined for a number of cells. The offsets are signaling to the mobile station, which adds corrections to the reference signal measurements based on the cell individual offsets (CIO), thereby forming a corrected reference signal measurement and the active set based on the corrected reference signal measurement.

Description:
TECHNICAL FIELD 
     The present invention relates to a method and a device for determining the active set in a cellular radio system. 
     BACKGROUND 
     One key feature in the Wideband Code Division Multiple Access (WCDMA) standard is the ability to simultaneously establish links to multiple cells. This is feature is commonly referred to as soft handover when the cells are located at different sites, softer handover when the cells are located at the same site or macro diversity etc. The set of cells that a mobile station which may be termed User Equipment (UE) is connected to is commonly denoted the active set. 
     The WCDMA active set is formed based on measurement reports from the User Equipments (UEs). These measurements are performed on a reference signal for each cell. The reference signal is a signal used for measurements and which may also be used for synchronization. In the case of WCDMA, the Common Pilot Channel (CPICH) is used. For all relevant cells, the channel quality/channel condition, such as the received quality or strength is determined. IN the case of WCDMA the measured Ec/No (chip power relative total received signal power) is compared for all relevant cells. Also, the path gain or CPICH level (CPICH Received Signal Code Power, RSCP) may additionally be determined. When determining the active set. 
     Assuming that a UE is connected to a first cell, cell A, and that the UE is gradually approaching a second cell, cell B, the quality of the measured reference signal, such as the CPICH Ec/No, of cell B will at some stage be within a specified window relative the best serving cell, cell A. This triggers an event and the UE sends a measurement report to the serving Radio Network Controller (RNC) that cell B is within range. In response to such a measurement report Cell B is added to the active set of this particular UE via an Active Set Update message transmitted from the RNC to the UE. Also, the cell B has been informed by the RNC that a link to the UE has to be established. 
     In  FIG. 1  such a scenario is schematically depicted. Thus, in  FIG. 1 , the two links to the first and second cells, i.e. cell A and Cell B have equal path gain g 0 . Further it should be noted that the active set is determined based on the signal quality of the measured reference downlink signal. The following events may typically used to determine the active set: 
     A. A new cell has entered reporting range (used when there are room for yet another cell in active set) 
     B. A cell in active set has leaved reporting range 
     C. A new cell is better than a specified active set cell (used when active set is full) 
     D. A different cell is better than previously best cell in active set. 
     If the uplink and downlink path gains of the links to/from the two cells are the same, then the active set, despite only evaluated for the downlink, constitute a good set of cells also for the uplink However, if the uplinks and downlinks are different, and furthermore the downlinks from the two cells are different, the active set might be relevant for the downlink, but much less relevant for the uplink. 
     One such example is when the uplinks and downlinks are unbalanced, for example when low noise amplifiers are used in the uplink to compensate for feeder losses between the Radio Base Station RBS and the antenna. The RBS amplifier feeds the antenna via a feeder cable, which has a certain feeder cable loss. This can be significant, for example in some cases it may be as high as 10-12 dB. In the uplink, the signal is sent from the antenna down to the RBS via a feeder cable causing a similar loss. This could give a very weak signal at the RBS. To compensate for this loss the received signal is amplified by a low noise amplifier directly connected to the antenna. This amplifier is sometimes also referred to as Tower-mounted amplifier (TMA). An exemplary RBS and antenna configuration is illustrated in  FIG. 2 . In  FIG. 2 , the antenna  201  is connected to a TMA  203 . The TMA receives signals from the Antenna  201  on a TMA Antenna port  207 . The TMA  203  is further connected to a Radio Base Station (RBS)  205 . The RBS  205  receives signals from the TMA via RBS port  207 . 
     With a significant feeder loss in cell A, and a negligible loss in cell B, the point where the equal CPICH Ec/No from the two cells is shifted, and the perceived combined downlink feeder loss and path gain is lower at this point. 
     In  FIG. 3  an illustrative example is shown. In the example shown in  FIG. 3 , the path gain is (g 0 +3 dB)−6 dB from cell A including both path gain and feeder loss, and g 0 −3 dB from cell B. However, for the uplink, the gain from this downlink mid point is g 0 +3 dB to cell A, but only (g 0 −3 dB) to cell B. At this downlink mid point, there is probably very little use of cell B. This means that when moving from cell A to cell B, cell A will at some stage be removed from the active set since it becomes worse downlink-wise. However, at this stage, the uplink to cell A can still be relevant, and worth keeping. 
     The active set will in some cases be less relevant for the uplink There are however situations when it is more important to have a relevant active set uplink-wise. One such example is High Speed Packet Access (HSPA), where the High Speed—Downlink Shared Channel (HS-DSCH) is transmitted only from the best cell in the active set, i.e. the serving cell. The serving cell is typically determined to be the cell of the active set providing the best signal quality. On the other hand, the Enhanced Dedicated Channel (E-DCH) is received by all base stations, Node Bs, in the active set, the uplink data rate in a practical case might be limited by the non-serving cells in the active set, i.e. the cells of the active cell other than the serving cell. 
     According to the above, the HS-DSCH is hence only transmitted from the serving cell to the UE and it is therefore important that the cell providing the best down link conditions is the serving cell. Similarly, to obtain good conditions for the E-DCH it is important that the cell providing the best conditions in the up-link is among the cells forming the active set. 
     Hence, there exist a need for a method and a system that is able to provide an active set that is better suited for evolving radio communication systems and which is able to provide a relevant active set for the up-link of a cellular radio communication connection. 
     SUMMARY 
     It is an object of the present invention to overcome or at least reduce some of the problems associated with existing on methods for determining an active set. 
     It is another object of the present invention to provide a method and a device that is capable of providing a relevant active set for the up-link of a cellular radio system. 
     These objects and others are obtained by the method and system as set out in the appended claims. Thus, by forming an active set by determining the cell individual offsets (CIO) and signaling the cell individual offsets to the User Equipments (UE) an active set providing good characteristics for the uplink can be achieved. 
     The signaled cell individual offsets are used by the UE to add corrections to the measured reference signal measurement events, such as CPICH measurements before the event triggering. If the CIOs are set relative the unbalances, the active set will meet the uplink demands on the active set cells. Furthermore, the CIOs can be signaled dedicated, i.e. only to the HSPA-enabled UEs. By using CIOs for all measurements except change of best cell, i.e. event D as described above, it is possible to compile the active set with uplink-relevant cells and the best serving cell in the downlink. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described in more detail by way of non-limiting examples and with reference to the accompanying drawings, in which: 
         FIG. 1  is a view illustrating a UE with links set up to two different cells in accordance with a first exemplary system configuration. 
         FIG. 2  is a view of an RBS and antenna configuration 
         FIG. 3  is a view illustrating a UE with links set up to two different cells in accordance with a second exemplary system configuration 
         FIG. 4  is a flow chart illustrating determination of the active set for a UE. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with the present invention a specific set of CIOs is formed. The CIO can be set according to any suitable algorithm using suitable input data. In addition the CIO may be set manually or as a manual override to a CIO set by an automated device. The specific set of CIOs can preferably be signaled dedicated to a subset of the UEs. By transmitting such a specific set of CIOs to some dedicated UEs an active set relevant for the uplink can be obtained. The remaining UEs, i.e. UE not in the receiving the set of specific CIO subset remain at a normal CIO setting. In one exemplifying embodiment, the specific set of CIO&#39;s is signaled dedicated to HSPA capable UEs whereas UEs not being HSPA capable maintains a normal CIO setting. 
     In another exemplifying embodiment, the specific set of CIOs is signaled dedicated to UEs with certain capabilities, for example EUL UE category  6 , or capable of uplink 16 QAM. 
     Other capabilities that may be used to determine which UEs to signal the specific set of CIOs may be UEs capable of some type of modulation, UEs enabled to handle a specific radio configuration, UEs enabled to handle a certain bit rate or having some type of subscription. 
     In accordance yet another exemplifying embodiment, the specific set of CIO&#39;s is signaled to all UEs. 
     The construction of the specific set of CIO&#39;s is further preferably based on information related to unbalances in the uplink as compared to the downlink. As an example the feeder losses and TMA information can be collected from all cells. 
     In  FIG. 4  a flowchart illustrating steps performed in a cellular radio system network, such as the system depicted in  FIG. 3 , when determining the active set for a particular UE connected to the cellular radio system. 
     First in a step  401 , information related to information relevant to compensate the CPICH measurement with is collected. In particular information related to feeder losses and TMA information of all cells of a cellular radio system or a part thereof is collected. Next, in a step  403  cells associated with feeder losses or TMAs are identified. If a cell is associated with a feeder loss or a TMA, a CIO value for that cell is set to a value reflecting the feeder losses and/or TMA for that cell in a step  405 . In accordance with another example, the set of CIO&#39;s can be set manually, or as an optional manual override to any automatic determination. If on the other hand, a cell is determined not to be associated with any feeder losses or TMA in step  403 , the CIO value for that cell is set to a preset value in a step  407 . The preset value set in step  407  may for example be set to zero. Thereupon, in a step  409 , the CPICH is adjusted by an offset determined by the CIO value assigned in steps  405  and  407 , respectively. If manual settings are applied, or a manual override is applied to the automatically determined values, they are assigned in a step  410 . Finally in a step  411  the active set is determined based on a corrected reference signal measurement. 
     In accordance with one embodiment of the present invention the CIO values determined by the system is only transmitted to a subset of the UEs in the system. For example the CIO value may be transmitted to UEs having HSPA capabilities or some other capabilities. Moreover, the CIO value for a cell may in accordance with one embodiment only be transmitted to a UE if the CIO is above or below some predetermined threshold value. In particular no CIO value for a particular may be transmitted to a UE in case the CIO value is zero. 
     In accordance with one exemplary embodiment the CIO setting when the TMA is available is set to:
 
CIO=min(feeder loss in dB, TMA max gain in dB)
 
and zero for cells without TMA.
 
     In another exemplary embodiment the CIO setting is according to another suitable algorithm based on data available in the system, uplink as well as downlink, configuration data as well as measured live data. In yet another exemplary embodiment, the CIO setting is manual, or as a manual override to any setting determined previously in the process. 
     By using CIO as described above for all triggering events except change of best cell, it is possible to keep the best serving cell downlink-wise in the active set, which is important from an HSDPA performance point of view. To achieve this, the RNC will use the change of best cell reporting to update HSDPA serving cell. 
     When using the specific set of CIOs to HSPA connections, it can also be beneficial to increase the handover margin. This will enable good uplink inter-cell interference control by the use of relative grants to relative many prospective interferers. 
     Similarly, the adaptation of the active set to the uplink could be achieved by CPICH adjustments. The levels at the RBS port  209  can then be adapted by the same amount as CIO defined above. This will give the same CPICH power for all cells at the port  207 . Then, the only option is to use this for all users and connections. This, however, would be at the cost of a capacity loss. In addition, there is a risk that the HSDPA serving cell is different from the best downlink cell. 
     With an active set adapted to the uplink, the uplink RRM will work better, since the relative grants from the EUL scheduler will most likely reach disturbing users to a greater extent than if the active set would be adapted to the downlink. Furthermore, in the case of HSPA, the uplink macro diversity gain could be of higher importance since the downlink transmission is only from best serving cell. Therefore, it is more relevant to adapt the active set to the uplink.