Fir digital filter for high-speed communications systems

Abstract: A finite impulse response digital filter for a digital communications system has successive virtual taps, the successive virtual taps having tap weight coefficients c₁ to c₅, respectively, where c₁ = c₅, c₂ = c₄, and c₃ is the coefficient of a virtual center tap. The filter comprises an add-and-­ delay multiplier circuit having three physical taps with tap weight coefficients d₁, d₂ and d₃, where d₁ = c₁ + c₂ = c₄ + c₅, d₂ = c₂ + c₃ = c₄ + c₃, and d₃ = c₁ = c₅. The circuit comprises tap weight multipliers (1, 2, 3), delay elements (4, 6, 8, 10) connected to the tap weight multipliers to define the physical taps, and adders (5, 7, 9, 11). The arrangement is such that a first digital output sequence comprising d₁ × a i-2 + d₂ × a i-1 + d₃ × a i appears at the output of one of the adders and a second digital output sequence comprising d₃ × a i-2 + d₂ × a i-1 + d₁ × a i appears at the output of another adder, where a i represents a digital sample at a given instant, and a i-1 and a i-2 represent digital samples which are delayed for intervals T and 2T, respectively, with respect to a i , where T is the reciprocal of the symbol rate of digital samples of the communications system. The first and second output sequences are alternately sampled at intervals T and coupled to an output terminal of the digital filter.

Description: The present invention relates generally to digital filters, and more specifically to a high-speed digital filter suitable for digital radio transmission systems. Due to the sharp cutoff capability and high cost performance, digital filters are increasingly used in many applications, instead of analog filters. Digital filters are of two types, i.e., IIR (infinite impulse response) filter and FIR (finite impulse response) filter. For digital radio, FIR low-pass filters are employed because of their linear operating characteristic for input-output phase relationship. Since the shift registers that comprise a digital filter are the primary source of aliasing, or fold-over distortion noise, they are clocked at twice the symbol rate for oversampling the incoming signal. The clock rate of the shift registers thus determines the maximum operating speed of a digital filter. If higher transmission speed is desired for a system, the clock rate of the digital filters must be increased. However, the operating speed of a digital filter is limited to the maximum operating speed of the integrated circuit components that comprise the shift registers and other constituent parts of the filter. One approach would be to design a digital filter with two parallel subsystems each being operated respectively on two clock sequences each being out of phase by π radian with respect to the other. However, the hardware cost of the filter is twice as much as the current FIR digital filter. An FIR filter for two data streams including a demultiplexes at the output is disclosed in the document EP-A-0 180 989. It is therefore an object of the present invention to provide a finite impulse response digital filter which is capable of operating at higher speeds to meet the high speed requirement of digital communications systems without increasing hardware cost. This object is solved with the features of the claims. The tap weights have symmetric values with respect to the center tap point value c m+1 , and therefore, a relation c j = c 2n+2-j holds. The present invention will be described in further detail with reference to the accompanying drawings, in which: Fig. I is a block diagram of a prior art digital filter; Fig. 2 is a timing diagram associated with the prior art digital filter; Fig. 3 is a block diagram of a digital filter according to a first embodiment of the present invention; Fig. 4 is a timing diagram useful for describing the operation of the present invention; and Fig. 5 is a block diagram of a second embodiment of this invention. Before going into the details of the present invention, it is appropriate to describe a prior art FIR digital filter having an odd-number of tap weight coefficients. In Fig. 1, input digital samples with a symbol rate f s are supplied to the tapped delay line consisting of shift registers 36 to 39 defining successive tap points to which tap weight multipliers 31 to 35 are respectively connected. Shift registers 36 to 39 are clocked at twice the symbol rate f s to oversample the incoming signal. The successively delayed digital samples are multiplied with tap weight coefficients c₁ to c₅ respectively by the multipliers and summed altogether by an adder 40 to produce an output sequence b i . The filter has a roll-off characteristic which is symmetric with respect to the center tap. Therefore, the relations c₁ = c₅ and c₂ = c₄ hold. Since shift registers 36-39 are driven at twice the speed of digital samples travelling through the tapped delay line, a digital sample a₁ appears at the inputs of multipliers 34 and 35 at a given instant t₁ as illustrated in part (a) of Fig. 2, and similarly a succeeding digital sample a₂ appears at the inputs of multipliers 32 and 33 and the next sample a₃ appears solely at the input of multiplier 31. The summation of these multiplied digital samples results in an output sample b₁ of duration 1/2f s as indicated in part (b) of Fig. 2. At time t₂ which is displaced by interval 1/2f s with respect to t₁, digital sample a₁ appears only at the input of multiplier 35, and digital sample a₂ appears at the inputs of multipliers 33 and 34 and digital sample a₃ appears at the inputs of multipliers 31 and 32, resulting in an output sample b₂. In like manner, at time t₃, digital samples a₂ and a₃ appear simultaneously at the inputs of multipliers 34, 35 and 33 and 32, with digital sample a₄ appearing solely at the input of multiplier 31, and at time t₄, samples a₃ and a₄ appear simultaneously at the inputs of multipliers 34, 33 and 32 and 31, with sample a₂ appearing solely at the input of multiplier 35. Therefore, the output digital samples b i can be represented as follows: b₁ = (c₄ + c₅) a₁ + (c₂ + c₃) a₂ + c₁ × a₃ b₂ = c₅ × a₁ + (c₃ + c₄) a₂ + (c₁ + c₂) a₃ b₃ = (c₄ + c₅) a₂ + (c₂ + c₃) a₃ + c₁ × a₄ b₄ = c₅ × a₂ + (c₃ + c₄) a₃ + (c₁ + c₂) a₄ It is seen that like tap weight coefficients appear cyclically at sampling intervals 1/2f s . If the following is given: d₁ = c₄ + c₅ d₂ = c₂ + c₃ d₃ = c₁ b₁ and b₃ can be represented as: b₁ = d₁ × a₁ + d₂ × a₂ + d₃ × a₃ b₃ = d₁ × a₂ + d₂ × a₃ + d₃ × a₄ Since c₅ = c₁ and c₄ = c₂, d₁, d₂ and d₃ are also given by: d₁ = c₁ + c₂ d₂ = c₃ + c₄ d₃ = c₅ Therefore, b₂ and b₄ can be represented as follows: b₂ = d₃ × a₁ + d₂ × a₂ + d₁ × a₃ b₄ = d₃ × a₂ + d₂ × a₃ + d₁ × a₄ In general terms, d j = c j + c j+1 = c 2n+1-j + c 2n+2-j , and the center tap coefficient d m+1 = c₁ = c 2n+1 , where j is in the range between 1 and 2n+1 for coefficients "c" and between 1 and 2m+1 for coefficients "d", and n ≧ 2 and m ≧ 1. Therefore, the odd-numbered output digital samples can be represented by d j × a i + d j+1 × a i+1 + ····· + d 2m+1 × a 2m+1 and the even-numbered output digital sample is represented by d 2m+1 × a i + d 2m × a i+1 + ····· + d j+1 × a 2m + d j × a 2m+1 . It follows from the foregoing that the output of the digital filter is divided into first and second groups. In the first group, output samples are sequentially multiplied by tap weights d₁, d₂ and d₃ according to their order of occurrence, while in the second group, output samples are sequentially multiplied by the tap weights in sequence which is opposite to the order of occurrence of the first group. Referring now to Fig. 3, there is shown a FIR digital filter according to the present invention. The digital filter of the present invention has at least five virtual tap weights which are represented by at least three physical tap weights d₁, d₂ and d₃ just described, and is so constructed that it generates output samples b₁ and b₃ which appear in a first output sequence A i at alternate intervals with output samples b₂ and b₄ which appear in a second output sequence B i . An input digital sample a i at symbol rate f s is applied to an input terminal 10 to which are connected the input terminals of tap weight multipliers 1, 2 and 3 having tap weight coefficients d₁, d₂ and d₃, respectively. For generating the output sequences A i and B i , first and second delay-and-adder circuits are provided. The first delay-and-adder circuit comprises a shift register 4, an adder 5, a shift register 6 and an adder 7. The second delay-and-adder circuit comprises a shift register 8, an adder 9, a shift register 10 and an adder 11, all of which are connected in order opposite to that of the first delay-and-adder circuit. All shift registers 4, 6, 8 and 10 are clocked by a clock source 20 at intervals T=1/f s . The output terminal of multiplier 1 is connected to the input of shift register 4 and to an input terminal of adder 11. Shift register 4 delays the input digital sample for a duration T equal to the reciprocal of the symbol rate f s and produces a digital output sample d₁ × a i-1 which is supplied to an input of adder 5 to which the output of multiplier 2 is supplied. Therefore, adder 5 produces an output sample d₁ × a i-1 + d₂ × a i which is applied to and delayed for interval T by shift register 6 for introduction of a further delay, producing an output sample represented by d₁ × a i-2 + d₂ × a i-1 . Adder 7 sums the output of shift register 6 with the output of multiplier 3, generating an output A i represented by a sum of samples d₁ × a i-2 + d₂ × a i-1 + d₃ × a i at one input of a switch 12. On the other hand, shift register 8 introduces a delay interval T to the output of multiplier 3 and applies a delayed output sample d₃ × a i-1 to adder 9 which sums it with the output of multiplier 2, producing an output d₃ × a i-1 + d₂ × a₁ at the input of shift register 10. Shift register 10 delays it for an additional interval T, presenting a delayed output sample d₃ × a i-2 + d₂ × a₁₋₁ which is summed by adder 11 with the output of multiplier 1 to produce an output sequence B i represented by d₃ × a i-2 + d₂ × a i-1 + d₁ × a i at the other input of switch 12. As illustrated in Fig. 4, during a first interval 1/f s the output sequence A i comprises a sum of d₁ × a₁, d₂ × a₂ and d₃ × a₃ and the output sequence B i comprises a sum of d₃ × a₁, d₂ × a₂ and d₁ × a₃. During a second interval 1/f s , the output sequence A i comprises a sum of d₁ × a₂, d₂ × a₃ and d₃ × a₄ and the sequence B i comprises a sum of d₃ × a₂, d₂ × a₃ and d₁ × a₄. Switch 12 is a sampling means which is driven by the clock source 20 at intervals T=1/f s to sample the output sequence A i for a first half duration T/2 and sample the output sequence B i for a second half duration. The sampled half-duration signals are multiplexed at the output of switch 12 into a single stream and coupled to the filter output terminal 21. As a result, the sampling rate of the digital filter of the invention is one-half the sampling rate of the digital filter of the type as shown in Fig. 3. Since the symbol rate of the incoming signal is equal to the Nyquist rate, and each of the output sequences is sampled for duration 1/2f s , the incoming signal is double-oversampled. Therefore, the shift registers are no longer the source of aliasing noise and hence it is not necessary to operate them at twice the symbol rate. The present invention provides a digital filter which is simple in configuration and is capable of processing incoming signals having a symbol rate higher than f s . Fig. 5 is an illustration of a modified form of the digital filter of the invention. In this modification, shift registers 51 and 52 are connected in series from an input terminal 50 to define successive taps to which digital multipliers 53, 54 and 55 of a first set are connected, respectively, and digital multipliers 56 and 57 of a second set are connected to those taps to which multlpliers 53 and 55 are connected. Multipliers 53, 54 and 55 have tap weight coefficients d₃, d₂ and d₁, respectively, and multipliers 56 and 57 have coefficients d₁ and d₃, respectively. The outputs of multipliers 53-55 are connected to a first adder 58 to produce the output sequence A i and the outputs of multipliers 56, 54 and 57 are connected to a second adder 59 to produce the second output sequence B i . The output sequences are alternately sampled at intervals T=1/f s for half duration T/2 by a switch 60 in response to clock pulses from source 61 for oversampling the incoming signal at twice the symbol rate f s and multiplexed for application to an output terminal 62. Similar to the first embodiment, shift registers 51 and 52 are clocked at intervals T=1/f s by clock source 61 so that digital samples are delayed for unit interval T by each of the shift registers. Successively arriving incoming digital samples at symbol rate f s are therefore delayed by shift registers 51 and 52, producing samples a i , a i-1 and a i-2 which are multiplied respectively by multipliers 53-55, producing a weighted output sample d₃ × a i from multiplier 53, a second output sample d₂ × a i-1 from multiplier 54, and a third output sample d₁ × a i-2 from multiplier 55. These weighted samples are summed by adder 58 into the output sequence A i = d₁ × a i-2 + d₂ × a i-1 + d₃ × a i . Likewise, digital samples a i and a i-2 are further multiplied by multipliers 56 and 57, respectively, producing a first output sample d₁ × a i from multiplier 56, a second output sample d₂ × a i-1 from multiplier 54, and a third output sample d₃ × a i-2 from multiplier 57. These weighted samples are summed by adder 59, resulting in the output sequence B i = d₃ × a i-2 + d₂ × a i-1 + d₁ × a i .

Claims: A finite impulse response digital filter for a digital communications system, the filter including (2n + 1) successive virtual taps respectively having tap weight coefficients c j , where n is integer and equal to or greater than 2 and j is in the range between 1 and 2n+1, where c j = c 2n+2-j , comprising: add-and-delay multiplier circuit means having (2m + 1) physical taps having tap weight coefficients d j , where m is integer and equal to or greater than unity and d j = c j + c j+1 = c 2n+1-j + c 2n+2-j , and d m+1 = c₁ = c 2n+1 , said circuit means comprising a plurality of tap weight multipliers (1,2,3), and a plurality of delay elements (4,6,8,10) connected to said multipliers to define said physical taps, and a plurality of adders (5,7,9,11), the arrangement of said circuit means being such that a first digital output sequence A i comprising d j × a i + d j+1 × a i+1 + ····· + d 2m+1 × a 2m+1 appears at the output of one of said adders (7) and a second digital output sequence B i comprising d 2m+1 × a i + d 2m × a i+1 + ····· + d j+1 × a 2m + d j × a 2m+1 appears at the output of another of said adders (11), where a i represents a digital sample at a given instant, and a i+1 represents a digital sample displaced by an interval T with respect to said digital sample a i , where T is the reciprocal of the symbol rate of digital samples of said communications system; and means for sampling (12) said first output sequence at intervals T for a first half duration and sampling said second output sequence at intervals T for a second half duration and multiplexing the sampled sequences into a single sequence for coupling to an output terminal (21) of said digital filter. A finite impulse response digital filter as claimed in claim 1, wherein each of said delay elements (4,6,8,10) is a shift register clocked at intervals T. A finite impulse response digital filter as claimed in claim 1 or 2, wherein said add-and-delay multiplier circuit means comprises: a plurality of tap weight multipliers (1,2,3) connected to an input terminal (10) of said digital filter to form successive physical taps for multiplying an input digital sample with said physical tap weight coefficients d₁, d₂ .... d k and d k+1 to produce weighted output digital samples (d₁, d₂ .... d k and d k+1 ) × a i ; a first delay-and-adder circuit including a series of alternately series-connected delaying and summing elements, each of said delaying elements (4,6) delaying each of the weighted output digital samples for interval T to produce successively delayed samples, and each of said summing elements (5,7) summing the outputs of said multipliers and said delaying elements to produce a first group of output digital samples d₁ × a i-k , d₂ × a i-(k-1) , ..... d k × a i-1 and d k+1 × a i as said first output sequence; and a second delay-and-adder circuit including a series of alternately series-connected delaying and summing elements, each of said delaying elements (8,10) delaying each of the weighted output digital samples for interval T to produce successively delayed samples, and each of the last-mentioned summing elements (9,11) summing the outputs of said multipliers and the last-mentioned delaying elements to produce a second group of output digital samples d k+1 × a i-k , d k × a i(k-1) , ..... d₂ × a i-1 and d₁ × a i as said second output sequence. A finite impulse response digital filter as claimed in claim 2 or 3, wherein each of the first, second, third and fourth delay means (4,6,8,10) comprises a shift register which is clocked at intervals T. A finite impulse response digital filter as claimed in any one of claims 1 to 4, wherein said add-and-delay multiplier circuit means comprises: delay elements (51,52) successively connected to an input terminal (50) of said filter to define successive physical taps for delaying each of successively arriving incoming digital samples for interval T; a plurality of tap weight multipliers (53-57) connected respectively to said successive physical taps for multiplying successively delayed digital samples a i-k , a i-(k-1) ,..... a i-1 and a i with said physical tap weight coefficients d₁, d₂ ..... d k and d k+1 , respectively, to produce a first group of weighted output digital samples, and for multiplying digital samples a i-k , a i-(k-1) ,..... a i-1 and a i with said physical tap weight coefficients d k+1 , d k ..... d₂ and d₁, respectively, to produce a second group of weighted output digital samples; and a first adder (58) for summing the weighted output digital samples of firtst group to produce said first output sequence; and a second adder (59) for summing the weighted output digital samples of said second group to produce said second output sequence. A finite impulse response digital filter as claimed in claim 5, wherein each of said delay means (51,52) comprises a shift register which is clocked at intervals T.

Inventors: EISAKU SASAKI, EISAKU, SASAKI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1990
Main classification: H03H17
IPC codes: H03H17, H04L27