Abstract:
A method and apparatus adapted to adjust threshold values for initiating HO measurements based on terminal or user equipment (UE) performance. The threshold values for initiating the HO measurements and for executing the HO may be determined by the network. Prior to determining whether to proceed with these procedures, the thresholds are adjusted, based on the actual UE performance. This adjustment is achieved by applying an offset in dB that captures the effective SIR improvement of the advanced receiver compared to the standard receiver used to set the thresholds. The offset depends on, among other things, the receiver type used, the number of receiver antennas, the multi-path channel profile, the number of active radio links, and the services/connection modes that are currently active in the UE.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS: 
       [0001]    This application is related to U.S. patent application Ser. No. 11/289,001 filed Nov. 29, 2005. 
     
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX 
       [0003]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0004]    The present invention relates to cellular telecommunication networks. More particularly, and not by way of limitation, the present invention is directed to a system and method for inter-network handover (HO) optimization for terminals or user equipment (UE) using advanced receivers. 
         [0005]    As used herein, the following abbreviations shall have the following meanings: 
         [0006]    BLER block error rate 
         [0007]    BS base station 
         [0008]    CE chip equalizer 
         [0009]    CM compressed mode 
         [0010]    CPICH common pilot channel 
         [0011]    DCCH dedicated control channel 
         [0012]    GRAKE generalized RAKE 
         [0013]    GSM Global System for Mobile communications 
         [0014]    HSDPA high-speed downlink packet access 
         [0015]    HO hand-over 
         [0016]    IRAT inter-RAT 
         [0017]    L 3  Layer  3  RRC 
         [0018]    LMMSE linear minimum mean square 
         [0019]    PDP power delay profile 
         [0020]    RAB radio access bearer 
         [0021]    RAT radio access technology 
         [0022]    RF radio frequency 
         [0023]    RRC radio resource control 
         [0024]    RRM radio resource management 
         [0025]    RSCP received signal code power 
         [0026]    RSSI received signal strength indicator 
         [0027]    SHO soft HO 
         [0028]    SIR signal-to-interference ratio 
         [0029]    SNR signal-to-noise ratio 
         [0030]    UE user equipment 
         [0031]    WCDMA wideband code division multiple access 
         [0032]    A typical cellular mobile communication network (network) consists of many cells, each of which allows a terminal or UE to maintain a connection with the network over a limited geographical area. As the UE moves around in the region covered by the network, at each time moment, it is connected via a radio link to the cell that can provide the connection with “least cost”, where the cost is often related to the signal power needed to transmit over the radio link. Often, but not necessarily, the serving cell is the cell physically closest to the UE. Over time, the network will perform a HO procedure to transfer the UE&#39;s connection from one cell to another. In some systems, the UE can maintain a connection simultaneously over several cells, using several radio links in parallel. This is referred to as SHO. 
         [0033]    In many cases, several different networks, possibly managed by different operators, are active in a given geographical region. Some networks may not provide a substantially complete (e.g., 100%) coverage over the whole region and a UE connected to a specific network may lose its connection in some locations. To avoid an adverse user experience (e.g., a dropped call or data connection), the original network may arrange for an alternative way to continue the connection by seamlessly switching over to another network that can provide coverage at the location. This may be accomplished using (1) another network belonging to the same operator, or (2) a network belonging to another operator, in which case the UE “roams” over to the other operator. 
         [0034]    An example of case (1) is an operator that operates both WCDMA and GSM networks, and switches a WCDMA user over to GSM when the WCDMA coverage becomes weak. This is referred to as an IRAT HO, since the access technology is changed in the HO process. An impending loss of connection in one RAT is not the only reason why an operator may choose to force an IRAT HO. It may also be done if re-distribution of users between the different RATs improves the total capacity of the networks. 
         [0035]    An example of case (2) is the operator that only owns a WCDMA network, and, in the case of deficient coverage, directs the UEs to roam to another operator&#39;s network with whom they have an agreement. This is a cost-efficient way for a new operator to launch their network without requiring 100% coverage from the first day of operation. However, the original operator loses the revenue from the calls after they have roamed to another network. 
         [0036]    A HO to another network is triggered if the useful signal strength, measured by the UE, drops below a certain threshold. This threshold may be continuously signaled by the network to the UE. The network dynamically adjusts the threshold value based on the expected BLER or throughput performance as a function of SIR at the UE, using the knowledge of the current network loading and assuming a standard receiver in the UE. In WCDMA, the standard receiver is a single-antenna RAKE receiver. 
         [0037]    The WCDMA RRM procedures are generally known by practitioners in the art, as is the IRAT HO sequence. 
         [0038]    The UE receiver performance in terms of the experienced BLER, throughput, or the average transmit power expended by the network may be significantly improved by applying more advanced receiver structures. For example, a WCDMA UE, using dual receiver antenna chains and/or applying interference suppression has been shown to provide considerable improvements in the experienced BLER, throughput, or the average transmit power expended by the network. This means that the performance is improved while the UE is in the standard coverage area of the WCDMA network. But in addition to that, advanced UE receivers would allow the coverage area itself to be extended, because the performance limiting (or in more extreme cases, connection-terminating) signal level thresholds are lower and they would be encountered further away from the center of the cell. 
         [0039]    A primary disadvantage of the existing HO management solutions is that the decision to perform the HO to another network is based on comparing the measured signal strength with a fixed threshold that does not account for the actual improved UE performance. As a result, the UEs with advanced receivers are reverted away from the network unnecessarily, which wastes network resources, limits the revenue of the operator, and possibly degrades the user experience. In addition, each HO procedure implies an additional cost to the operator in terms of signaling resources used and potential UE performance degradation due to performing measurements in parallel (e.g. the compressed mode in WCDMA). 
         [0040]    The foregoing disadvantage has been encountered by operators. If, instead of instructing any user below a fixed threshold to roam away of their network, the connections for the UEs with advanced receivers could be maintained all the way to their actual performance limits, the number of roaming occurrences would be reduced significantly. This would mean an expansion of the effective coverage area for users with advanced terminals and improved income for the operator, especially since the users, once roamed away, usually do not automatically return to the original network. 
         [0041]    Thus there is desired a HO decision system that, instead of using a fixed HO threshold, accounts for the actual UE receiver performance. In a related application of Applicant (U.S. Ser. No. 11/289,001, filed Nov. 29, 2005, entitled “Cell Selection in HSDPA Communication Systems), the concept of adjusting HO measurements, like Ec/Io in WCDMA for improving HSDPA cell selection is described. As is known in the art, power sharing is the basis for multi-user access in CDMA systems. In such systems, coverage tests measure the signal (RSCP) to interference (RSSI) ratio, Ec/Io, expressed in dB. Ec/Io is the ratio of the signal energy within one chip duration of the pilot signal to the power spectrum of the interference plus noise. That disclosure describes adjusting Ec/Io to better reflect the HSDPA performance, by taking the PDP as well as an estimate of the cell load into account. However, the concept is basically only suitable for HSDPA inter-frequency handover, and therefore is not adapted to facilitate IRAT/inter-network HO using advanced receivers as disclosed herein. 
         [0042]    It would be advantageous to have a system and method adapted to facilitate IRAT/inter-network HO using advanced receivers that overcomes the disadvantages of the prior art. The present invention provides such a system and method. 
       BRIEF SUMMARY OF THE INVENTION 
       [0043]    The present invention is a system and method adapted to adjust threshold values for initiating HO measurements based on actual UE performance. The threshold values for initiating the HO measurements and for executing the HO are determined by the network. Prior to determining whether to proceed with these procedures, the thresholds are adjusted, based on the actual UE performance. This adjustment is achieved by applying an offset in dB that captures the effective SIR improvement of the advanced receiver compared to the standard receiver used to set the thresholds. The offset depends on, among other things, the receiver type used, the number of receiver antennas, the multi-path channel profile, the number of active radio links, and the services/connection modes that are currently active in the UE. 
     
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0044]    In the following section, the invention will be described with reference to exemplary embodiments illustrated in the figures, in which: 
           [0045]      FIG. 1  is an graph comparing the HO process with example threshold values using the CPICH RSCP as a HO criterion; 
           [0046]      FIG. 2  is a block diagram for a UE according to a first embodiment of the present invention; 
           [0047]      FIG. 3  is a flow chart of the method of the first embodiment of the present invention; 
           [0048]      FIG. 4  illustrates the sequence of operations in the second embodiment of the present invention; 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0049]    In order to more fully understand the operation of the present invention, the operation of an advanced receiver in WCDMA is described. From the beginning of the initial standardization of the WCDMA technology in 3GPP, the standard receiver structure has been a single-antenna RAKE receiver that is able to efficiently collect and utilize signal energy available from a fading multi-path propagation channel. This is achieved by assigning each multi-path component a despreader whose reference copy of the spreading code is delayed equally to the path delay of the corresponding multipath component. The outputs of the despreaders (fingers) are then coherently combined to produce a symbol estimate. The RAKE receiver requires knowledge of the multipath delays and the values of the channel impulse response for all paths. A good measure of performance of the RAKE receiver is the SNR after combining, which is maximized if the signal energy from all paths is utilized. An implicit assumption in RAKE combining is that the impairment (noise and interference) components of the signals seen by each finger are uncorrelated, in which case the RAKE receiver is an optimal linear receiver. 
         [0050]    However, the single-antenna RAKE receiver performance is limited by a number of factors. One performance-limiting aspect is the fading characteristics of the channel, especially on channels with few multipath components. Secondly, there may be considerable correlation between the impairment signals, either due to the effect of the transmitter and receiver pulse shaping or due to the combined effect of many multipath components in the propagation channel. 
         [0051]    In order to improve RAKE receiver performance, a variety of advanced receiver types have been developed. One such advanced receiver type is a dual-antenna receiver, where the signal is received via two separate antennas, RF and front end processing branches. If the two branches are sufficiently separated (electrically and spatially), the dual-antenna RAKE receiver (RAKE 2 ) will then exhibit improved performance, due to the array gain (more signal energy is received) and the diversity gain (the probability of deep fades is reduced). As a result, the BLER performance of the receiver is improved. 
         [0052]    Another advanced receiver type incorporates equalization (e.g., LMMSE CE, GRAKE). Such a receiver takes into account the impairment correlation and, to a considerable degree, undoes the loss of orthogonality caused by the channel. This effectively whitens the unwanted signal component, reducing its power, and improves the SNR after combining. 
         [0053]    The RAKE 2  and GRAKE receivers may also be combined, and the resulting structure (GRAKE 2 ) yields a receiver that combines the diversity and interference suppression in an optimal way, further improving performance. In typical scenarios, GRAKE 2  is the optimal linear receiver for WCDMA. The receiver performance may be improved further by applying non-linear operations, e.g. interference cancellation by subtracting the known interfering signal components before detecting the data symbols of interest. 
         [0054]    Regardless of the advanced receiver type, its performance advantage compared to the RAKE receiver may be characterized by the resulting average combined SIR improvement (ASIR). This improvement is different for different receiver configurations and scenarios, depending on, for example, the receiver type, the number of antennas used, multipath channel profile, whether the UE is in SHO, the RAB/service currently invoked. However, by classifying the scenarios into a manageable set of categories C={c 1 , . . . , c N }, the SIR gain for each category ΔSIR(c 1 ) may be determined and tabulated in advance, e.g. by measurements or simulations. 
         [0055]    To describe the environment in which the foregoing advanced receiver types operate, the HO process from WCDMA to GSM is considered. The WCDMA CPICH RSCP and SNR for each cell in the active set is constantly monitored by the UE. The measurement is updated every 70 ms and may be reported to the network. The network maintains thresholds for when the different HO-related activities should be initiated. The reported values are then compared to the relevant thresholds and, if the measurements indicate that the received signal levels at the UE are becoming too weak, a decision is taken to initiate IRAT measurements. The decision may be taken by the UE or the network, in which case this may be communicated to the UE via a RRC-level “Measurement Control” message. In this example, “event 2d” is triggered and the UE would commence to measure the GSM RSSI (RXLEV) for the closest GSM cell. 
         [0056]    In order to perform measurements on another frequency, the UE enters CM, whereby all the information from the BS is transmitted during a shorter time interval and some percentage (e.g. 50%) of the time may be used by the UE to re-tune to another frequency and carry out the measurements. These results are then also reported to the network in a “Measurement report” message. Typically, once the UE has been instructed to perform the inter-frequency measurements, it will remain in CM until (a) the signal quality improves sufficiently so that no loss of connection is imminent (“event 2f”), or (b) the alternative network is deemed to have better signal quality than the present one (“event 3a”) and a HO is performed. 
         [0057]    The HO process with example threshold values is depicted in graph  100  of  FIG. 1 , using the CPICH RSCP as a HO criterion. As seen therein, in a first embodiment of the present invention, the network broadcasts and the UE has knowledge of the thresholds used for defining the HO-related events. The latest UE measurement at time t is denoted as γ(t) and the threshold for event e is denoted as τ(e) (All SIR values are given in dB.) Then, the UE checks whether the condition(s) for any of the events of interest, eεE, are fulfilled: 
         [0000]        F ( e ,τ( e ),γ( t ))=1 
         [0000]    where F( ) is a set of criteria for triggering the events according to the specifications. An ex-ample of one such instruction could be the event  2   d:    
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         [0058]    If event e is established, the UE sends a message signaling that event to the network. The network may then respond with further instructions to the UE in order to, for example, initiate additional measurements, perform a HO, and the like. 
         [0059]    As mentioned before, the threshold values τ(e) are typically based on the RAKE receiver performance. In the first embodiment of the present invention, the UE will account for the improved receiver performance by introducing a correction term in the decision process identifying whether any events in E are triggered: 
         [0000]        F ( e ,τ( e )− D ,γ( t ))=1 
         [0060]    In one embodiment, D may be determined by defining the current receiver and channel category c(t) and performing a table lookup to find the corresponding average SIR gain, 
         [0000]        D =ΔSIR( c ( t )). 
         [0061]    In another embodiment, two instantaneous SIR values may be computed: the current effective receiver SIR, SIR(t) (which is usually done as part of normal receiver operation) and an additional RAKE receiver SIR SIR RAKE (t), being a low complexity operation, once the channel estimates and combining weights are available. The correction term is then determined as 
         [0000]        D =SIR( t )−SIR RAKE ( t ) 
         [0062]    A block diagram of a UE according to a first embodiment of the present invention is illustrated in  FIG. 2 . As seen therein, the latest UE measurement γ(t) is received at block L 3  decision processor  201 , which also receives τ(e) from the network. The UE measurement γ(t) is supplied from a base band processor  206 , or more precisely from a path searcher  205  which is part thereof. In other implementations, γ(t) could be supplied from some other baseband processing stage. L 3  decision processor  201  checks whether the condition(s) for any of the events of interest, eεE , are fulfilled as noted above. If event e is established, the UE sends a message via modulator  202  and front end transmitter  203  signaling that event to the network. The network may then respond with further instructions to the UE in order to, for example, initiate additional measurements, perform a HO, and the like. 
         [0063]    The control unit  204  will account for the improved receiver performance by introducing a correction term in the decision process of L 3  decision processor  201  identifying whether any events in E are triggered. In one embodiment, D, as communicated to L 3  decision processor may be determined by defining the current receiver and channel category c(t) and performing a table lookup to find the corresponding average SIR gain, as noted above in equation 3. 
         [0064]    The UE of  FIG. 2 , in particular the L 3  decision processor, operates according to the sequence of operations seen in the flow chart  300  of  FIG. 3 . The flow chart of  FIG. 3  checks for all events of interest in E. The process starts in step  301 . In step  302 , the UE receives thresholds used for defining the HO-related events from the network. The threshold for event e is denoted as τ(e). In step  303 , the measurement at time t, denoted as γ(t), is taken. At step  304 , D is determined as described above. Then, in step  305 , the first event of interest is set. In step  306 , the UE checks whether the condition(s) for any of the events of interest, eεE, are fulfilled according to 
         [0000]        F ( e ,τ( e ),γ( t ))=1. 
         [0000]    In step  307 , if event e is established, the UE sends a message signaling that event to the network. The network may then respond in step  308  with further instructions to the UE in order to, for example, initiate additional measurements, perform a HO, and the like. If further events are to be checked, the next event is set at step  309 , which is then checked at step  306 . The process ends at step  310 . 
         [0065]    Note that any subsequent HO-related actions are dictated by the BS in separate commands sent to the UE and are not indicated in the flow chart. 
         [0066]    In a second embodiment of the present invention, the threshold values τ(e) are not broadcast and the UE has no knowledge of the current threshold values. In this embodiment, the UE will send the UE measurement results γ(t) to the network, which then evaluates the relevant conditions for events in E. In other words, the decision process expressed in equation (1) takes place in the network, instead of the UE. 
         [0067]    In the second embodiment of the present invention, the UE accounts for the improved receiver performance by modifying the RSCP value appropriately before reporting it. That is, instead of reporting the actual value of γ(t), it reports 
         [0000]      γ eff ( t )=γ( t )+ D    
         [0068]    The correction term D is determined as in equation (3) or equation (4). 
         [0069]    The block diagram for a UE implementing the second embodiment of the present invention is similar to that seen in  FIG. 2 . The main difference is that instead of performing the L 3  decision process in the UE and sending an event flag to the network, the offset D is applied to the measurement γ(t) and the adjusted measurement is sent it to the network. 
         [0070]      FIG. 4  illustrates the sequence of operations in the second embodiment of the present invention. In some cases, several RABs or services are in progress in the UE simultaneously, which may experience different effective SIR improvement impact due to the advanced receiver. In such a situation, the smallest SIR improvement should be selected as the correction term D. This guarantees that needs of the most sensitive service are accounted for, and avoids undue signal level reduction for any of the services. The flow chart  400  starts at step  401 . In step  402 , the measurement at time t, denoted as γ(t), is taken. At step  403 , D is determined as described above. Then, at step  404 , γeff (τ)=γ(t)+D is reported to the network. At step  405 , the process ends. 
         [0071]    As one option, the RSCP may be used as γ(t), the HO criterion in the disclosed herein. The procedure may alternatively be carried out by using the CPICH Ec/Io measure as the criterion. The adjustment of the alternative threshold(s) would follow the same principles. 
         [0072]    The following advantages are obtained by UEs having advanced receivers in a network applying embodiments of the invention: usable coverage area of each cell in the network is increased for UE-s with advanced receivers; unnecessary instances of HO to other networks are avoided; and UE performance degradation and/or system capacity reduction due to parallel measurements in the UE is minimized. 
         [0073]    The network will obtain the following benefits: the loss of revenue for the original operator due to roaming to another operator&#39;s network is reduced; the available network capacity (i.e. operator&#39;s hardware investment) is more fully utilized; and less network resources are used for HO signaling. 
         [0074]    As will be recognized by those skilled in the art, the innovative concepts described in the present application can be modified and varied over a wide range of applications. For example, the present invention has been described in terms of a HO from WCDMA to GSM. However, the invention is not limited to that context. For example, the described threshold adjustment method may be applied also to HOs between other technologies, between two networks using the same technology, and even to inter-frequency handovers within the same network. The present invention can be implemented wherever it is advantageous to avoid unnecessarily initiating a measurement and HO process that may drain network signaling resources and/or capacity. Accordingly, the scope of patented subject matter should not be limited to any of the specific exemplary teachings discussed above, but is instead defined by the following claims.