Patent Publication Number: US-7715858-B2

Title: Secondary station and method of operating the station

Description:
The present invention relates to a secondary station for use in a radio communication system and further relates to a method of operating the secondary station. While the present specification describes a system with particular reference to the emerging Universal Mobile Telecommunication System (UMTS), it is to be understood that the techniques described are equally applicable to use in other mobile radio systems. 
     There are two basic types of communication required between a Base Station (BS) and a Mobile Station (MS) in a radio communication system. The first is user traffic, for example speech or packet data. The second is control information, required to set and monitor various parameters of the transmission channel to enable the BS and MS to exchange the required user traffic. 
     In many radio communication systems accurate power control is important. This is particularly so in systems employing spread spectrum Code Division Multiple Access (CDMA) techniques, because many communication channels share the same bandwidth and so transmission at too high a power in any one channel reduces the signal to noise ratio in all the other channels. Uplink power control, of signals transmitted to a Base Station (BS) from a Mobile Station (MS), is particularly important. It ensures that the BS receives signals from different MSs at approximately the same power level for a given data rate and quality of service, while minimising the transmission power required by each MS. Downlink power control, of signals transmitted by the BS to a MS, is required so that the MS receives signals from the BS with a low error rate while minimising transmission power, to reduce interference with other cells and radio systems. 
     In a UMTS embodiment, power control is normally operated in a closed loop manner. For uplink power control the BS determines the required changes in the power of transmissions from a MS and signals these changes to the MS by means of Transmit Power Control (TPC) commands. To minimise overheads, a TPC command typically instructs the MS to increase or decrease its power, with the change in power being a step of predetermined size. However, in some systems a TPC command may also determine the step size to be used. 
     A MS generally communicates with a single BS. During the course of a call the MS may wish to investigate transferring to another BS, for example when the quality of the communication link deteriorates as the MS moves away from its BS, or when the relative traffic loading of different cells requires adjusting. The process of transferring from one BS to another is known as handover. In a version of this process known as soft handover, the MS engages in communication with a plurality of BSs (known as the “active set” of BSs) to determine to which BS, if any, it should transfer. When the MS is engaged in this process it will receive TPC commands from each of the BSs. An example of a strategy for determining what change in power to make based on the received TPC commands is disclosed in International Patent Application WO 00/36762. 
     A problem with power control during soft handover is that there is a limited amount of time available to receive, decode and process the power control commands. For example, in UMTS there is a period of 416 chips (approximately 108 μs) after the arrival of the first TPC command during which the received commands need to be decoded and processed to determine the magnitude and direction of the required power change. This period is followed by a period of 50 μs during which the transmission power change should be made. 
     This problem is made worse because in UMTS soft handover there may be a time difference of up to 148 chips (38.5 μs) between the arrival times of the first signals from each BS. When the signal from a BS is received via several downlink paths and the information from the paths is combined (for example using a Rake receiver), a further delay is introduced of up to the worst-case delay spread between paths. In a UMTS system this could reduce the available processing time by up to 20 μs. The combined effect of soft handover and delay spread can therefore be to reduce the available processing time by half. This allows very little flexibility for scheduling of the necessary processing tasks in a receiver, particularly for transceiver architectures having significant processing delay through use of vector processors. 
     An object of the present invention is therefore to maximise the time available in a MS for processing of power control commands. 
     According to a first aspect of the present invention there is provided a secondary station for use in a radio communication system comprising a plurality of primary stations, the secondary station having means for engaging in a soft handover process, in which the secondary station communicates simultaneously with at least two primary stations, and receiver means for receiving transmitted signals from the at least two primary stations as a plurality of multipath signals, wherein means are provided for processing an initial subset of the multipath signals to provide an initial estimate of what action should be taken by the secondary station, and for processing multipath signals received after the initial subset to update the estimate. 
     According to a second aspect of the present invention there is provided a method of operating a secondary station comprising engaging in a soft handover process, in which the secondary station communicates simultaneously with at least two primary stations, receiving transmitted signals from the at least two primary stations as a plurality of multipath signals, processing an initial subset of the received multipath signals to provide an initial estimate of what action should be taken by the secondary station, and processing multipath signals received after the initial subset to update the estimate. 
     The present invention is based upon the recognition, not present in the prior art, that by making an initial estimate of the required power change before all multipath signals have been processed, improved flexibility in scheduling information processing in a MS is possible. 
    
    
     
       Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: 
         FIG. 1  is a block schematic diagram of a radio communication system; 
         FIG. 2  is a block schematic diagram of a Rake receiver having five fingers; 
         FIG. 3  is a block schematic diagram of a radio communication system with a MS in the process of soft handover; 
         FIG. 4  is a graph showing the amplitude A of received signals against time t from receipt of the first signal; and 
         FIG. 5  is a flow chart showing a method of processing multipath signals in accordance with the present invention. 
     
    
    
     In the drawings the same reference numerals have been used to indicate corresponding features. 
     Referring to  FIG. 1 , a radio communication system comprises a primary station (BS)  100  and a plurality of secondary stations (MS)  110 . The BS  100  comprises a microcontroller (μC)  102 , transceiver means (Tx/Rx)  104  connected to antenna means  106 , power control means (PC)  107  for altering the transmitted power level, and connection means  108  for connection to the PSTN or other suitable network. Each MS  110  comprises a microcontroller (μC)  112 , transceiver means (Tx/Rx)  114  connected to antenna means  116 , and power control means (PC)  118  for altering the transmitted power level. Communication from BS  100  to MS  110  takes place on a downlink frequency channel  122 , while communication from MS  110  to BS  100  takes place on an uplink frequency channel  124 . 
     The transceiver means  114  in a MS  110  may include a Rake receiver. Such a receiver, well known to those skilled in the art, is designed to detect a CDMA signal transmitted over a dispersive multipath channel. A block schematic diagram of a five-finger Rake receiver is shown in  FIG. 2 . Signals received via the antenna  116  are down-converted to baseband and supplied as a first input to five mixers  202 . A signal generator (SIG)  204  generates a local copy of a signal encoded with the same spreading code as that used by the BS  100 . This signal is supplied as a second input to the first mixer  202   a . The same signal, delayed by delay means (DEL)  206   b , is supplied as a second input to the second mixer  202   b , and similarly to mixers  202   c , 202   d , 202   e  delayed further by delay means  206   c , 206   d , 206   e  respectively. 
     By adjusting the phase of the generated signal according to the output of a channel estimator and the delays applied by the delay means  206 , five versions of the same transmitted signal received by five different paths having different delays can be handled. The received signals have their amplitudes multiplied by a weight factor proportional to their respective received signal strengths by attenuators (ATT)  208 , and are then summed by adding means (SUM)  210 . The combined signal is then integrated by integration means (INT)  212  over successive symbol periods to determine the received symbols, which symbols are supplied to the remainder of the receiver for further processing. If signals are received via more than five different paths, the phase of the signal generator  204  and the delays introduced by the delay means  206  are adjusted to match the five strongest received paths (or those with the best signal to interference ratio). 
     A MS  110  engaged in a soft handover process is illustrated in  FIG. 3 , the MS  110  having three two-way communication channels  326   a , 326   b , 326   c  with three respective BSs  100   a ,  100   b ,  100   c . In a given time slot the MS  110  receives TPC commands from each of BSs  100   a ,  100   b ,  100   c . If the received signals are processed by a Rake receiver having n fingers, it is conventional for each of the n strongest signals to be allocated to a finger. 
     An example of a set of signals received via different paths is shown in  FIG. 4  as a graph of the amplitude A of a plurality of signals  402  against the time of arrival t relative to the time of arrival of the first received signal  402   a . If the signals are processed by a six finger Rake receiver, Rake fingers would be allocated to signals  402   a ,  402   b ,  402   c ,  402   f ,  402   g  and  402   h , while signals  402   d  and  402   e  would be ignored. 
     However, the applicants have determined that such an allocation strategy may not be optimum during soft handover, in view of the limited processing time available. Instead it is proposed, in accordance with the present invention, that Rake fingers are allocated to signals based on time of arrival information, either alone or in combination with signal strength information. This overcomes the situation that a Rake finger is allocated to a signal which has arrived too late to be incorporated in the decision-making process for the next power change, in which case such a finger is effectively wasted. 
     Hence, in the example shown in  FIG. 4 , the MS  110  determines that any signals received after time t 1  will be too late to be used in determining the next power change. Consequently, the allocation of Rake fingers may be modified from that normally used, as described above, with the fingers being allocated instead to signals  402   a ,  402   b ,  402   c ,  402   d ,  402   e  and  402   f . This allocation therefore ignores strong late signals ( 402   g , 402   h ), which arrive after a predefined window of arrival for the first significant path. Such late-arriving signals could optionally be used in determining the power control change to be made in a subsequent slot. 
     The MS  110  may also employ additional techniques, in addition to or instead of the modified allocation of Rake fingers described above, to increase the time available for processing power control commands. One such technique is for the MS  110  to start making its power change on the basis of an initial estimate of the required power change, made before all information from received signals is available. If necessary, the power change could then be corrected based on further received signals. This technique would yield benefits in flexibility for scheduling of processing tasks in the MS  110 , provided that a correction to the implemented power change was only required in a small proportion of cases. 
     In some soft handover situations, for example when a reliable down command is received from the earliest BS  100 , there is no need to wait for further power control commands to arrive. In other cases, a suitable strategy might be always to reduce power when a down command, whether reliable or not, is received from the earliest BS  100 , and similarly for increasing power in response to an up command subject to the power change being corrected in the event that a reliable down command is received later. Such a strategy would meet the requirements of the UMTS specification. 
     On occasions when the direction of the power control step did require correction, such correction would be likely to extend beyond the allocated 50 μs period for making power changes. In itself this is not a problem, provided that the average uplink transmission power for the remainder of the slot after the power change is not affected to such an extent that it falls outside permitted tolerances, and provided that the error vector magnitude does not exceed permitted tolerances. The error vector magnitude is defined in UMTS as the root mean square (rms) error vector between the transmitted waveform and a closely-matched reference waveform. 
     In systems other than UMTS, particularly where information from different BSs  100  during soft handover might affect the required magnitude of the power step, the strategy of starting the power change based on an initial estimate could have greater benefits. For example, the power change could be implemented by means of an initial coarse power change in the RF part of the transceiver  114  with the remaining fine tuning of the transmission power being achieved by adjusting the amplitude of the baseband signals for transmission once the remaining power control commands had been processed. 
     Another technique is to modify the timings of transmissions from a BS  100 . According to the UMTS specification, the MS  110  notifies a BS  100  if the time of arrival of its signals drifts outside a range of typically ±148 chips relative to signals from other BSs  100  (or alternatively relative to a fixed offset from the timing reference for uplink transmissions). The BS  100  can adjust its transmission timing in steps of 256 chips. By reporting a received signal as having arrived outside the acceptable time limit, even if it has not, the MS  110  can arrange the timing of downlink transmissions from a plurality of BSs  100  so as to improve power control command processing. The MS  110  may also decide not to report a weak signal which arrives outside the time limit, for example to avoid any consequent changes to the timing reference. 
     For example, if the signal from the first-received BS  100  was consistently weaker than the signals from other BSs  100  received later, the MS  110  could report one or more of the stronger, later signals as having arrived late, so that the UMTS network arranged for its timing to be advanced by 256 chips, thereby ensuring that the strongest signal was received first. This algorithm would significantly improve application of the initial estimation method described above. 
     As an alternative, or in addition, to the above technique, the MS  110  could report as late any downlink signal received more a predetermined amount late, for example 74 chips (i.e. half of the 148 chip timing tolerance), so as to maximise the time available for power control command processing. 
     A flow chart illustrating a method in accordance with the present invention of processing multipath signals is shown in  FIG. 5 . The method starts, at step  502 , with a MS  110  beginning a soft handover process. The MS  110  continues, at step  504 , to receive an initial subset of the multipath signals from a plurality of BSs  100  until such time as the MS  110  schedules processing of the received signals, at step  506 . The processing provides an initial estimate of the required power change, which estimated power change is implemented by the MS  110  at step  508 . Multipath signals received, at step  510 , after processing of the initial subset are also processed, at step  512 . The results of this processing enable the initial estimate of the required power change to be corrected (if necessary), after which the MS  110  corrects its transmission power as required, at step  514 . 
     Although the above description relates to a Rake receiver, it will be apparent that the present invention is equally applicable to any receiver capable of resolving a plurality of multipath signals. Further, although the above description relates to reception of power control commands via a plurality of multipath signals the present invention is also applicable to other transmissions having tight time constraints for decoding. An example of such a transmission is feedback information for controlling transmit diversity of BSs  100  in a UMTS system. 
     From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of secondary stations and component parts thereof, and which may be used instead of or in addition to features already described herein. It will be appreciated that certain features of the invention which are, for clarity, described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment may also be provided separately or in any suitable subcombination. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of features during the prosecution of the present application or of any further application derived therefrom. 
     In the present specification and claims the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. Further, the word “comprising” does not exclude the presence of other elements or steps than those listed.