Abstract:
In a method for improving signal extraction in a code division multiple access (CDMA) telecommunications system, a first iteration of interference cancellation is performed on the basis of bit rates for every signal which are the same as those in a previous frame of the same signal. Filtered and down-converted signals are demodulated in Rake receivers to provide output signals corresponding to decision variables and channel estimates. The decision variables are limited and remodulated and respread prior to the signals being reconstructed using the channel estimates. The reconstructed signals are summed, and each signal is subtracted from the sum to provide an ‘interference’ signal which is then used to obtain the individual signals. Each signal is then demodulated a second time in another Rake receiver to provide a tentative DPDCH signal, a TFI signal, a TPC signal and a SNI signal. The TFI signal is processed to provide a signal indicative of the bit rate which is used to both decode the DPDCH signal providing a data output and to provide an estimate of the bit rate for a subsequent frame of the same signal.

Description:
The present invention relates to improvements in or relating to signal extraction, and is more particularly concerned with extracting a desired signal from a plurality of signals interfering with the desired signal. 
     BACKGROUND OF THE INVENTION 
     The universal mobile telecommunication system (UMTS) terrestrial radio access (UTRA) uses code division multiple access (CDMA) as its multiple access technique. On the uplink (mobile terminal to base station direction), non-orthogonal codes are used in combination with power control. However, because the codes are not orthogonal, the capacity of the uplink is limited by multiple access interference. The UTRA specification provides for the optional use of short codes to allow the use of various receiver techniques in the base station which rely on the fact that the multiple access interference is not noise but is, in fact, other signals. The receiver techniques which operate in this way are generically known as interference cancellation and joint detection. 
     One implementation of interference cancellation operates by first demodulating the data on all of the signals directed to the base station to form estimates of the data. Knowledge of these estimates of the data along with channel estimates allows the generation of delayed approximate replicas of the received signal from each of the mobile terminals. For each wanted signal, the replicas for the other signals are summed together and subtracted from a delayed version of the received composite signal. Thus, at this stage, the interference has been approximately cancelled for that signal. When demodulation (including despreading) is performed, the bit error rate (BER) should be reduced. The whole process can be repeated several times, each time using the improved estimates of the received data to construct the approximate replicas. 
     One implementation of joint detection operates by treating the sum of the signals as a composite signal having travelled over a path with components relating to the individual signal components. This path is then linearly or non-linearly equalised in order to permit demodulation of all of the data over all of the signals. 
     In both interference cancellation and joint detection techniques, it is necessary to have knowledge of the bit rates (and, therefore, the spreading factors) for each of the received signals. In UTRA frequency division duplex (FDD), the signal format consists of frames of 10 ms duration. There are two channels for each signal, namely, the dedicated physical control channel (DPCCH) and the dedicated physical data channel (DPDCH). 
     The DPCCH is a low power constant bit rate channel. It consists of 16 timeslots each comprising pilot symbols, forward error correction (FEC) encoded transport format indicator (TFI) data and transmit power control (TPC) data. The DPDCH consists of time interleaved, FEC encoded data. It has a bit rate which may vary from one frame to the next, the bit rate of which is carried by the TFI data in the DPCCH of the same frame. On the uplink, in a single spreading code transmission, the DPDCH is first spread to become the inphase (I) channel and the DPCCH is spread to become the quadrature (Q) channel. Overall scrambling is then applied to the combined signal. 
     The TFI data is spread out across the frame and cannot be reliably decoded until the whole of the current frame has been received. Thus the bit rate information, for each of the signals, is unavailable until the whole of the current frame has been received. This causes two problems:— 
     First, the reason for applying interference cancellation or joint detection is to increase the system capacity by allowing the reception of signals at a lower signal to noise plus interference ratio than would be possible without using it. This means that before the applicant of interference cancellation, it may be impossible to demodulate the TFI bits, leading to a deadlock situation. This is true even though the DPCCH and DPDCH are transmitted on nominally orthogonal (I and Q) channels since multipath will seriously degrade this orthogonality and because the different signals will be received at the base station with arbitrary mutual carrier phase. 
     Secondly, power control information is generated by making signal to noise plus interference measurements on the DPCCH within the time period of the frame. Thus, if interference cancellation or joint detection cannot be applied until the end of the frame, these measurements will need to be based on the signal to noise plus interference (SNI) without the benefit of interference cancellation or joint detection. If the power control measurement threshold is based on an adequate SNI ratio at this stages then the resultant SNI ratio after the operation of interference cancellation or joint detection will be higher than necessary. On the other hand, attempting to base the power control measurements on there being an adequate SNI ratio after the operation of interference cancellation or joint is problematic because: a) the SNI ratio at the measurement stage will be very low—probably too low to measure, and b) it is not possible to predict, a priori, how effective the interference cancellation or joint detection will be in any given slot. 
     U.S. Pat. No. 5,151,919 (Ericsson) provides a subtractive CDMA demodulation system which optimally decodes a coded system embedded in many other overlapping signals making up a received composite signal. A radio receiver correlated a unique code corresponding o the desired signal to be decoded with the composite signal. WO96/24206 (Nokia) provides a CDMA system in which several users communicate simultaneously on the same frequency band, and in which each user has its own spreading code. For reception of signals, sigal correlators use synchronisation with waveforms of different types to aid decoding. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an improved method of extracting a signal which overcomes the problems mentioned above. 
     In accordance with one aspect of the present invention, there is provided, in a communication system employing coded signals, a method of extracting a desired coded signal from a composite signal comprising the desired signal and one or more interfering coded signals, the method comprising the steps of:—
     a) receiving a composite signal;   b) processing, for each received signal code, individual signals in a first signal processor;   c) determining transport format indicator (TFI) signals using buffer and decoder circuits to provide a bit rate of a frame for at least one interfering signal;   d) dividing the TFI signal path into first and second signal paths;   e) wherein, in a first signal path TFI signals are passed via a latch to provide the first signal processor with a TFI signal, whereby to assign the bit rate determined for said last frame for the next frame; and   wherein, in the second path TFI signals are passed to a further signal processor to adjust the bit rate of an output signal.   

     In accordance with a further aspect of the present invention there is provided, in a communication system employing coded signals, apparatus operable to extract a desired coded signal from a composite signal comprising the desired signal and one or more interfering coded signals, the apparatus comprising:—
     a) receiver means arranged to receive a signal;   b) a first signal processor for processing individual signals, for each received signal code;   c) buffer and decoder circuits for determining transport format indicator (TFI) signals, to provide a bit rate of a frame for at least one interfering signal;   d) a path divider for dividing the TFI signal path into first and second signal paths;
 
wherein, in the first signal path, TFI signals are passed via a latch, to provide the first signal processor with a TFI signal, whereby to assign the bit rate determined for said last frame for the next frame; and
 
wherein, in the second path, TFI signals are passed to a further signal processor to adjust the bit rate of an output signal.
   

     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a block diagram of one embodiment of a part of a base station of a telecommunication system in accordance with the present invention; and 
         FIG. 2  illustrates a block diagram of another embodiment of a part of a base station in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the present invention, a first iteration of interference cancellation or joint detection is performed on incoming signals on the basis that the bit rates for every signal are the same as they were for the same signal in the previous frame. Although this assumption may not be true for all of the signals, it should be true for the vast majority of signals whenever a large number of signals are present. If the frame rate is correct, for example, for 90% of signals, then nominally 90% of the interference would be cancelable. The unsuccessful attempt to cancel the remaining 10% of interference would add a further 10%, leaving the interference at 20% in the idea case. This provides, a 7 dB reduction in interference—a very useful start. 
     It will be appreciated that if the number of active signals is small then the operation of interference cancellation or joint detection will not be needed anyway. 
     As discussed above, in order to demodulate a CDMA signal effectively, the bit rate of that signal needs to be determined. However, due to the interference produced by the presence of other co-channel signals, it is necessary to cancel that interference from the signal before the bit rate can be determined. As discussed above, there are two main methods of eliminating the effects of that interference, namely, interference cancellation and joint detection. The present invention is described below with respect to both these techniques with reference to  FIGS. 1 and 2  respectively. 
     Referring initially to  FIG. 1 , a part of a base station is shown which comprises a receiving (R x ) antenna  100 , a transmitting (T x ) antenna  300 , and processing circuitry for down-converting received signals and up-converting signals for transmission. Antenna  100  receives a plurality of radio signals from a plurality of mobile terminals (not shown) in a telecommunications cell which includes the base station. The received signals are passed from the antenna  100  to a mixer  102  where they are down-converted using the output from a local oscillator  104 . As is the case with CDMA, each radio signal has a unique code so that it can be distinguished from other radio signals received at the same time. The down-converted signals are passed to filter  106  and filtered output  108  is then passed to a bank  110  of Rake receivers. In the illustrated embodiment, four Rake receivers  112 ,  114 ,  116 ,  118  are shown, but it will readily be appreciated that any number of Rake receivers can be utilised according to the receiving capacity of the base station. Each Rake receiver  112 ,  114 ,  116 ,  118  operates on a different code so that each received signal can be individually processed. In this example, Rake receiver  112  operates on code 1, Rake receiver  114  operates on code 2, Rake receiver  116  operates on code 3, and Rake receiver  1118  operates on code 4. 
     Each Rake receiver  112 ,  114 ,  116 ,  118  receives all the plurality of filtered signals, but only demodulates and despreads the signal having the code associated with that receiver and outputs a decision variable signal in accordance with that demodulation and despreading. The decision variable signal from each Rake receiver  112 ,  114 ,  116 ,  118  is then fed to a respective one of decision devices  122 ,  124 ,  126 ,  128  which may be limiting devices. The limited outputs are then fed to respective remodulation/respreading units  132 ,  134 ,  136 ,  138  where the signals are remodulated and respread prior to being fed to respective channel reconstruct filters  142 ,  144 ,  146 ,  148 . 
     Each Rake receiver  112 ,  114 ,  116 ,  118  also outputs a channel estimation signal CE—only the channel estimation signal CE 1  from Rake receiver  112  being shown. Each channel estimation signal CE is input to a respective one of the channel reconstruct filters  142 ,  144 ,  146 ,  148  to enable reconstruction of each demodulated and despread signal. It is to be noted that, although only channel estimation signal CE 1  is shown for clarity, it will be appreciated that Rake receivers  114 ,  116 ,  118  produce respective channel estimation signal CE 2 , CE 3 , CE 4  (not shown) which are fed to corresponding reconstruct filters  144 ,  146 ,  148 . 
     After the signals have been reconstructed, they are summed in summer  150  to form signal  155  which is an estimation of a delayed version of the filtered signal  108  input to the bank  110  of Rake receivers as described above. Signal  155  is then fed to a bank of subtractors  162 ,  164 ,  166 ,  168 . It will be appreciated that a subtractor is provided for each signal which is to be extracted, and more subtractors will be required if more than four signals are to be extracted as described in the illustrated embodiment. Also fed to subtractors  162 ,  164 ,  166 ,  168 , are respective signals  172 ,  174 ,  176 ,  178  which are tapped off the signals entering summer  150 , each signal corresponding to the individual reconstructed signals. Subtractors  162 ,  164 ,  166 ,  168  subtract signals  172 ,  174 ,  176 ,  178  from signal  155  to provide output signals  182 ,  184 ,  186 ,  188  representing the ‘interference’ produced by the presence of the other signals. For example, output signal  182  corresponds to the incoming signal  108  (same as signal  155  as discussed above) minus the signal  172 , that is, the signal having code 1. Similarly, output signal  184  corresponds to signal  108  minus signal  174  (code 2), output signal  186  corresponds to signal  108  minus signal  176  (code 3), and output signal  188  corresponds to signal  108  minus signal  178  (code 4). 
     For clarity, the subsequent processing of signal  182  is described, but it will readily be appreciated that signals  184 ,  186 ,  188  are processed in a similar way. 
     Signal  182  is then passed to a further subtractor  190  where signal  182  is subtracted from a delayed version of signal  108 . As shown, signal  108  is fed to a delay circuit  196  to provide delayed signal  198 . The delay introduced by the delay circuit  196  is equivalent to the time for signal  108  to be processed by the bank of Rake receivers  110 , decision devices  122 ,  124 ,  126 ,  128 , remodulation/respreading units  132 ,  134 ,  136 ,  138 , channel reconstruct filters  142 ,  144 ,  146 ,  148 , summer  150 , and subtractors  162 ,  164 ,  166 ,  168 . Subtractor  190  produces an output signal  192  which represents the signal having code 1. Similarly, output signals  184 ,  186 ,  188  are also passed to subtractors (not shown for clarity) where they are subtracted from delayed signal  198  to provide output signals representing signals having codes 2, 3, and 4. 
     Each output signal corresponding to each of the codes 2, 3, and 4 are also passed to respective further Rake receivers (not shown) and processed in identical fashion to provide the output signals described with reference to code 1 below. 
     Output signal  192  is then passed to a further Rake receiver  202  where it is demodulated and despread to produce output signals  212 ,  222 ,  232 ,  242 . Output signal  212  corresponds to a tentative DPDCH signal which is passed to a DPDCH buffer  252 . The tentative DPDCH signal comprises soft decision variables obtained on the basis of despreading according to the lowest currently available spreading factor. The output from buffer  252  is passed to circuit  262  where the bit rate is adjusted and the DPDCH decoded to provide an output data signal  280 . However, circuit  262  cannot adjust the bit rate and provide the output data signal  280  without knowing the bit rate. Output signal  222  comprises a TFI signal which is passed to a TFI buffer  254 . Output from buffer  254  is passed to circuit  264  where the TFI signal is decoded and the bit rate is determined. Output signal  274  from circuit  264  is passed to circuit  262  to adjust the bit rate and to enable the data signal  280  to be output. Output signal  274  is also passed to a latch  290  which is connected to Rake receiver  112  for inputting the bit rate determined from the last frame. This bit rate is then used as an estimate for the next frame. 
     Signal  232  comprises a TPC signal which is used to control the power which the mobile terminal needs to be transmitted to it by the base station. 
     Signal  242  comprises a SNI signal which is used to provide a measure of the signal to noise plus interference ratio being experienced for the particular signal being received on code 1. Signal  242  is compared with a threshold value in a comparator  272  to generate a series of downlink TPC bits indicating to the relevant terminal whether its power should be reduced or increased. The output from the comparator  272  is modulated in modulator  284 , mixed with other data in multiplexer  286 , spread in spreader  288 , up-converted in mixer  292  fed by local oscillator  294 , and amplified by amplifier  296  before being transmitted by antenna  300 . 
     The other data input to multiplexer  286  will include similarly processed SNI signals from the other further Rake receivers (not shown) corresponding to codes 2, 3 and 4. 
     Referring now to  FIG. 2 , an arrangement utilising joint detection for removal of unwanted signals is shown. Components which have been previously described with reference to  FIG. 1  are referenced the same. 
     In  FIG. 2 , a part of a base station is shown which comprises a receiving (R x ) antenna  100 , a transmitting (T x ) antenna  300 , and processing circuitry for down-converting received signals and up-converting signals for transmission. Antenna  100  receives a plurality of radio signals from a plurality of mobile terminals (not shown) in a telecommunications cell which includes the base station. For ease of explanation, the processing of the received signals is described with reference to four signals each having a unique code, for example, code 1, code 2, code 3, and code 4 as above. The received signals are passed from the antenna  100  to a mixer  102  where they are down-converted using the output from a local oscillator  104 . As is the case with CDMA, each radio signal has a unique code so that it can be distinguished from other radio signals received at the same time. The down-converted signals are passed to filter  106  and filtered output  108  is then passed to a joint detection device  400 . Device  400  processes the filtered output  108  to generate a TFI signal, a TPC signal and an SNI signal for each code—only code 1 and 4 are shown for simplicity, but it will readily be appreciated that codes 2 and 3 are identical. 
     For code 1, device  400  is shown as producing a TFI signal  402 , a TPC signal  404  and a SNI signal  406 . Similarly, for code 4, TFI signal  412 , TPC signal  414  and SNI signal  416  are shown. TFI signals  402 ,  412  are passed to respective circuits  420 ,  430  where they are buffered and decoded. Output signals  422 ,  432  from circuits  420 ,  430  are passed to a second joint detection device  500 . Signals  422 ,  432  are also passed to respective frame latch devices  440 ,  450 , outputs  442 ,  452  therefrom being used to input bit rate information for joint detection device  400  for the next frame of respective signals having code 1 and code 4. 
     TPC signals  404 ,  414  are used to provide information for controlling the power which the mobile terminal needs to be transmitted to it by the base station. SNI signals  406 ,  416  are used to provide an indication to the mobile terminal of the interference being experienced. 
     For clarity, subsequent processing is only shown for signal  406 . Signal  406  is compared with a threshold value in a comparator  272  and if the signal  406  is above the threshold, an output signal is provided which passes through switch  282 , modulated in modulator  284 , mixed with other data in multiplexer  286 , spread in spreader  288 , up-converted in mixer  292  fed by local oscillator  294 , and amplified by amplifier  296  before being transmitted by antenna  300 . 
     The other data input to multiplexer  286  will include similarly processed SNI signals from the other further Rake receivers (not shown) corresponding to codes 2, 3 and 4. 
     Filtered input signals  108  are also fed to a delay circuit  196 . The delay of circuit  196  is chosen to compensate for delays introduced during joint detection in device  400  and buffering and decoding in circuits  420 ,  430 . The delayed signal  198  provides the input to joint detection device  500  as shown. As discussed above, signals  422 ,  432  are used to provide bit rate information which is used to provide DPDCH signals  510 ,  520 ,  530 ,  540  from device  500 . 
     In a further embodiment of the present invention (not illustrated), the first bank  110  of Rake receivers  112 ,  114 ,  116 ,  118  may be replaced with a joint detection device  400  to determine the bit rate so that the signals can be decoded in the further Rake receivers as described above. 
     The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.