Abstract:
A radio receiver comprising a compensator arranged to compensate for intersymbol interference in a signal received at the receiver and a configurator arranged to configure the compensator, wherein the compensator comprises a programmable filter and the configurator is capable of configuring the filter in a first mode to operate as an ISI equaliser or in a second mode to implement a RAKE finger set.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the field of digital communications conducted by means of radio frequency (RF) carrier signals. 
       BACKGROUND OF THE INVENTION 
       [0002]    In normal practice, digital signals are converted into streams of modulation symbols, for example using a modulation scheme such as QPSK, and then modulated onto RF carrier signals. Receivers that are configured to handle signals that have been modulated in this way attempt to isolate a wanted received carrier signal and then demodulate the stream of symbols from the RF carrier signal. However, it is likely that the carrier signal will reach the receiver via a number of different paths, with the result that a number of versions of the carrier signal arrive at the receiver, all at different delays. This is the well known phenomenon of multipath propagation, which gives rise to intersymbol interference (ISI) in the demodulated signal. That is to say, the delay between two multipath components can be such that at some given instant, the receiver experiences different symbols from the two paths. It is well known to use an equaliser or a RAKE receiver to compensate or correct for intersymbol interference. 
       SUMMARY OF THE INVENTION 
       [0003]    According to one aspect, the invention provides a radio receiver comprising a compensator arranged to compensate for intersymbol interference in a signal received at the receiver and a configurator arranged to configure the compensator, wherein the compensator comprises a programmable filter and the configurator is capable of configuring the filter in a first mode to operate as an ISI equaliser or in a second mode to implement a RAKE finger set. The invention also consists in a method of compensating for intersymbol interference in a signal received at a receiver, the method comprising configuring a programmable filter and applying the filter to the signal in the compensation of ISI, wherein the configuring step comprises selecting a configuration for the filter from a set including a first filter configuration in which the filter operates as an ISI equaliser and a second filter configuration in which the filter implements a RAKE finger set. 
         [0004]    Thus, the invention provides a relatively compact architecture that can change between RAKE and equaliser solutions to the ISI problem as conditions dictate. 
         [0005]    The radio receiver may be compliant with the WCDMA standards that are maintained by 3GPP. 
         [0006]    The radio receiver may, for example, form part of a handset of a mobile telephone or part of a base station in a cellular telecommunications network. 
         [0007]    Although the invention involves a selection between RAKE and equaliser solutions to the ISI problem, it is to be understood that the invention also extends to the case where an ISI solution is selected from a larger group of available solutions, of which the RAKE and equaliser solutions are two. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    By way of example only, certain embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
           [0009]      FIG. 1  is a block diagram schematically illustrating a mobile telephone handset from the perspective of its role as a radio receiver; 
           [0010]      FIG. 2  is a block diagram schematically illustrating the structure of a finite impulse response filter; 
           [0011]      FIG. 3  is a block diagram schematically illustrating the structure of a cell of the FIR filter depicted in  FIG. 2 ; 
           [0012]      FIG. 4  is a diagram showing a channel impulse response and a chain of cells in an FIR filter that is being configured for use as part of a RAKE receiver; and 
           [0013]      FIG. 5  shows the chain of FIR filter cells as configured in  FIG. 4  feeding into the adder unit of the filter. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    Various of the diagrams in this document illustrate circuits and systems and it will be understood by persons skilled in the field of digital communications that the elements appearing in these figures serve to illustrate functions that are performed in the various circuits and systems and do not necessarily correspond to actual components. 
         [0015]    In general terms, a data signal, comprising a series of bits, that is to be transmitted over the air interface in a WCDMA network is first subjected to forward error correction (FEC) coding. The resulting signal, again comprising a series of bits, is then encoded as a series of constellation symbols belonging to a modulation scheme (and there may be multiple bits of the FEC-encoded signal represented by each modulation symbol), with the symbols then being divided into shorter duration chips by spreading and scrambling processes. The details of this sequence of processes will be familiar to engineers skilled in the field of digital communications and this sequence of processes must be retraced in a receiver in order to recover the transmitted data signal. 
         [0016]      FIG. 1  illustrates schematically a WCDMA handset  10  from the perspective of its role as a wireless signal receiver and shows only those elements necessary for describing the invention. It will be understood by engineers skilled in the field of digital communications that, in practice, the handset  10  will contain other elements besides those shown in  FIG. 1 . 
         [0017]    The handset  10  has an antenna  12  for receiving wireless communications. The antenna  12  picks up radio signals in the vicinity of the handset  10  and supplies them to an RF front end module  14  for processing. The RF front end module  14  uses filtering to isolate an RF signal in a wanted channel of the WCDMA network to which the handset  10  belongs. The RF front end module  14  is also tasked with amplifying the isolated RF signal and demodulating it, for example by direct downconversion, to produce a baseband signal, representing the chip rate signal that was modulated onto an RF carrier in the transmitter. The RF front end module  14  then digitises this baseband signal with a sampling rate that is eight times higher than the chip rate that resulted when the data signal was scrambled and spread during preparation for its transmission. This % 8 oversampled chip rate signal is then fed into a radio data buffer  16 . The % 8 oversampled baseband signal from the radio data buffer  16  is delivered to a finger determination unit  18  and to a downsampling unit  20 . 
         [0018]    The finger determination unit  18  identifies in a known manner a predetermined number of the strongest multipath components within the signal supplied from the radio data buffer  16 . The finger determination unit  18  calculates the RAKE finger positions to a ⅛ chip resolution from the % 8 oversampled baseband signal. The finger determination unit  18  then provides an MRC weights calculation unit  22  with the RAKE finger positions for a purpose that will be described later. In the main signal path, the downsampling unit  20  reduces the degree of oversampling of the signal provided by the radio data buffer  16  from % 8 to % 2. The % 2 oversampled signal provided by the downsampling unit  20  is then supplied to both a channel estimation unit  24  and to a finite impulse response (FIR) filter  26 . 
         [0019]    The channel estimation unit  24  calculates a % 2 oversampled channel impulse response from the % 2 oversampled signal provided by the downsampling unit  20 . Schemes for calculating a channel impulse response from the baseband signal will be well known to engineers skilled in the field of digital communications. The channel impulse response estimate is delivered to a switch  28 . The switch  28  introduces two parallel processing paths that converge in a further switch  30 . These parallel paths provide alternative mechanisms for calculating a set of complex-valued filter coefficients to configure the FIR  26 . 
         [0020]    Switch  28  has A and B outputs and switch  30  has A and B inputs. The switches  28  and  30  operate as a pair and together can assume one of two states. In one state, the switch  28  connects its input to its A output and switch  30  connects its output to its A input. When the switches  28  and  30  are in this state, the handset  10  shall be said to be in RAKE receiver mode. The other state that can be adopted by the switches  28  and  30  is when switch  28  connects its input to its B output and switch  30  connects its output to its B input. When the switches are in this state, the handset  10  shall be said to be in equaliser mode. 
         [0021]    The operation of the handset  10  in equaliser mode shall now be described. 
       Equaliser Mode 
       [0022]    In equaliser mode, the channel impulse response estimate produced by channel estimation unit  24  is supplied via switch  28  to an MMSE weights calculation unit  32 . MMSE weights calculation unit performs the calculations that are necessary to produce the set of filter coefficients that will configure the FIR filter  26  to operate as a minimum mean-square error (MMSE) equaliser. The calculations that are needed to deduce this set of filter coefficients from the channel impulse response estimate provided by channel estimation unit  24 , which include a relatively computationally intensive matrix inversion step, will be known to engineers skilled in the field of digital communications and so will not be described in detail here. 
         [0023]    With the FIR  26  thus programmed, the output of the FIR filter is an equalised version of the % 2 oversampled baseband signal. The equalised % 2 oversampled baseband signal produced by the FIR filter  26  is then supplied to symbol rate conversion unit  34  where the signal undergoes various operations such as despreading, descrambling, fast Hadamard transformation (FHT) and symbol-length accumulation to produce a complex-valued digital signal comprising a stream of symbols. The stream of symbols produced by symbol conversion unit  34  is supplied to a bit rate processor (BRP)  36  where any forward error correction (FEC) coding is decoded to recover a data signal which is then put to its intended use, such as conversion to an analogue audio signal that is played through a loud speaker or rendition as a web page that is shown on an LCD display. 
       RAKE Mode 
       [0024]    In RAKE mode, the % 2 oversampled channel impulse response estimate is provided to the MRC weights calculation unit  22 . It will be recalled that the MRC weights calculation unit  22  also receives as an input the set of finger positions deduced by finger determination unit  18 . The MRC weights calculation unit  22  maps the finger positions onto the channel impulse response estimate. The finger positions are specified to a ⅛ chip resolution but the MRC weights calculation unit  22  nevertheless identifies the samples within the ½ chip resolution channel impulse response estimate that best correspond to the finger positions. Thus, for each finger position, the MRC weights calculation unit  22  identifies a corresponding channel impulse response estimate value. Next, the MRC weights calculation unit  22  deduces a RAKE finger coefficient for each finger position by calculating the complex conjugate of the channel impulse response estimate value that has been mapped to the finger. Thus, a RAKE finger coefficient is deduced for each member of the set of RAKE finger positions. The set of RAKE finger positions, each with its corresponding RAKE finger coefficient, is then deployed in the FIR filter  26  to cause the FIR filter to operate in conjunction with symbol rate conversion unit  34  as a RAKE receiver. Before describing this configuration of the FIR filter  26  in more detail, a brief discussion of the structure of the FIR filter will first be provided. 
         [0025]      FIG. 2  shows the structure of FIR filter  26 . The % 2 oversampled baseband signal s is supplied to a chain of N cells, e.g.  38 . The samples of signal s shift one place to the right along the chain of cells with every clock cycle. Also, each cell in the chain sends an output to an adder  40 . The sum value produced by adder  40  represents the sample of the digital output signal that the FIR filter  26  presents to unit  34  in the current clock cycle. 
         [0026]      FIG. 3  illustrates a typical cell of the chain shown in  FIG. 2 . The sample of signal s that is received from the preceding cell in the chain (or which is presented at the filter&#39;s input in the case of cell  38 ) is supplied both to a one clock cycle delay element  42  and to a multiplier  44 . The output of the delay element provides the input to the next element in the chain. In the multiplier  44 , the input to the cell is multiplied with a so-called “tap coefficient” to produce the output that is passed to the adder  40 . All the cells have this configuration, except the cell numbered N- 1  which does not require the delay element. Each cell in the FIR filter  26  has its own tap coefficient, the tap coefficient of the n th  cell being denoted a n . It is well known that the characteristics of an FIR filter, e.g. its pass band, can be determined by setting these tap coefficients appropriately. 
         [0027]    Returning now to the discussion of RAKE mode operation, the MRC weights calculation unit  22  sets the tap coefficients along the chain to zero except at the positions where RAKE fingers are specified in the aforementioned RAKE finger allocation. At each position along the chain where a RAKE finger falls, the cell is given as its tap coefficient the RAKE finger coefficient deduced for the respective finger. This configuration of the tap coefficients will now be explained further with the help of an example involving  FIG. 4 . 
         [0028]    The bottom part of  FIG. 4  shows a channel impulse response  46  plotting power (vertically) versus time (horizontally). The channel impulse response plot contains three prominent peaks  48 ,  50  and  42 . The time delay between peaks  48  and  50  is τ 0-1  and the time delay between peaks  48  and  52  is τ 0-2 . Consider now the case where the handset  10  is operating in RAKE mode and the MRC weights calculation unit  22  is required to configure the tap coefficients of the FIR filter  26  for RAKE mode operation given the channel impulse response  46  and that RAKE fingers have been allocated to peaks  48 ,  50  and  52  only (fingers  0  to  2 , respectively). 
         [0029]    The strip  54  at the top of  FIG. 4  represents a part of the chain of cells in the FIR filter  26 . It is to be carefully noted, however, that in this figure the chain of cells is shown with signal s flowing through the chain from right to left and not left to right as in  FIGS. 3 and 5 . Each rectangle in the strip  54  represents a cell in the chain. The value shown in each cell represents the tap coefficient of that cell. The MRC weights calculation unit  22  deduces finger coefficients C 0 , C 1  and C 2  for fingers  0 ,  1  and  2  respectively. These finger coefficients are loaded into the chain of cells such that the time offset between the cells containing C 0  and C 1  is τ 0-1  and such that the time offset between the cells containing C 0  and C 2  is τ 0-2 . Besides these cells, all of the FIR filter&#39;s tap coefficients are set to zero. In this way, the FIR filter  26  functions like three RAKE fingers. That this configuration of the FIR filter  26  results in RAKE mode operation will be clearer when  FIG. 5  is considered. 
         [0030]    In  FIG. 5  the chain of cells  54  is shown together with the adder  40  that makes up the FIR filter  26 . In  FIG. 5 , the signal s flows from left to right through the chain  54  of filter cells. As in  FIG. 4 , the values shown in these cells denote the tap coefficients of the cells. Only the paths from the cells containing coefficients C 2 , C 1  and C 0  are shown as feeding into the adder  40  since the paths from the other cells are effectively switched off by their zero-valued tap coefficients. The paths  56 ,  58  and  60  are, in effect, RAKE fingers: each of these paths conveys the % 2 oversampled baseband signal at a time offset relative to the other two paths and each path contains a multiplier, in its respective cell of chain  54 , that applies a respective RAKE finger coefficient to derotate the version of the % 2 oversampled baseband signal s that is travelling along the respective path. The only difference between the representation shown in  FIG. 5  and a traditional RAKE receiver layout is that the symbol rate conversion process is not replicated in each of the paths  56 ,  58  and  60  but is instead performed singly, at a point downstream from the adder  40 , in the symbol rate conversion unit  34 . 
         [0031]    The path  60  represents the earliest RAKE finger, which corresponds to peak  48  in  FIG. 4  and for which RAKE finger coefficient C 0  has been deduced by the MRC weights calculation unit  22 . Path  58  represents a RAKE finger allocated to the next significant multipath component to arrive at the antenna  12 , which is indicated by peak  50  in  FIG. 4 . The RAKE finger of path  58  is delayed by an interval τ 0-1  relative to path  60 . Path  56  represents a RAKE finger allocated to the third, and latest arriving, significant multipath component, which is represented by peak  52  in  FIG. 4  and for which RAKE finger coefficient C 2  was calculated. The version of signal S that travels along the RAKE finger represented by path  56  is delayed by an interval τ 0-2  relative to the leading RAKE finger represented by path  60 . 
         [0032]    The output of the adder  40  of the FIR filter  26  is supplied to the symbol rate conversion unit  34  where the descrambling despreading and accumulation processes that are required to complete the RAKE processing are performed. The stream of symbols produced by symbol conversion unit  34  is supplied to a bit rate processor (BRP)  36  where any forward error correction (FEC) coding is decoded to recover a data signal which is then put to its intended use, such as conversion to an analogue audio signal that is played through a loud speaker or rendition as a web page that is shown on an LCD display. 
         [0033]    In the embodiment described above, the finger determination unit  18  calculates the finger positions for use by the MRC weights calculation unit  22  from the % 8 oversampled baseband signal from the radio data buffer  16 . In one alternative embodiment, the finger determination unit  18  calculates the finger positions by applying a peak detection algorithm to the % 2 oversampled channel impulse response estimate provided by the channel estimation unit  24 .