Abstract:
A fixed point finite impulse response (FIR) filter comprising: 1) an input stage for receiving an input signal as a sequence of input samples comprising: i) delay elements connected in series for receiving and shifting N sequential input samples; ii) multipliers, each multiplier receiving a selected one of the N sequential input samples from the delay elements and multiplying the selected input sample by a corresponding coefficient to produce an intermediate product; and iii) a summer for receiving and adding N intermediate products from the multipliers to produce an output sum signal comprising a sequence of output sum samples; and 2) an output stage for truncating k least significant bits (LSBs) from each of the output sum samples, wherein k is a variable number.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention is generally directed to communication devices, and more specifically, to a finite impulse response (FIR) filter that uses adaptive truncation and clipping in a wireless communication device. 
       BACKGROUND OF THE INVENTION 
       [0002]    In conventional wireless code division multiple access (WCDMA) systems, the power of the adjacent channel could be as much as 40.7 dB higher than the in-band signal power received by the base station or mobile station. This significant difference requires that conventional baseband matched filters have a large dynamic range. Conventional fixed point finite impulse response (FIR) filters typically implement truncation and clipping scheme after the correlation block. The truncation and clipping scheme truncates a fixed number of the least significant bits from the correlator outputs and clips the signal peaks at some fixed saturation level. However, to cope with the large dynamic range of the filter input, conventional fixed point FIR filters typically use more output bits than are required in order to avoid system performance degradation. This problem is unique to fixed point FIR filters, since floating point FIR filters do not require clipping and truncation circuits. 
         [0003]    Therefore, there is a need in the art for an improved finite impulse response (FIR) filter that is capable of processing input signals having a potentially large dynamic range without requiring the use of a large number of extra filter output bits to retain system performance. In particular, there is a need in the art for an improved FIR filter having reduced complexity that is able to process input signals having large dynamic ranges wherein size of the FIR filter outputs are optimized for the in-band signal power. 
       SUMMARY OF THE INVENTION 
       [0004]    To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide an improved fixed point finite impulse response (FIR) filter. According to an advantageous embodiment of the present invention, the fixed point FIR filter comprises: 1) an input stage capable of receiving an input signal as a sequence of input samples, the input stage comprising: i) a plurality of delay elements connected in series capable of receiving and shifting N sequential input samples; ii) a plurality of multipliers, each of the multipliers capable of receiving a selected one of the N sequential input samples from the plurality of delay elements and multiplying the selected input sample by a corresponding coefficient to thereby produce an intermediate product; and iii) a summer capable of receiving and adding N intermediate products from the plurality of multipliers to thereby produce an output sum signal comprising a sequence of output sum samples; and 2) an output stage capable of truncating k least significant bits (LSBs) from each of the output sum samples, wherein k is a variable number, to thereby produce a sequence of filtered output samples. 
         [0005]    According to one embodiment of the present invention, the output stage comprises: 1) a variable gain amplifier capable of multiplying each of the output sum samples by a variable gain factor to produce a sequence of shifted output samples, wherein a most significant bit of the each output sum sample is shifted a variable amount to a desired bit position; and 2) a feedforward gain controller capable of determining a power of the each output sum sample and, in response to the determination, adjusting the variable gain factor. 
         [0006]    According to another embodiment of the present invention, the output stage comprises: 1) a variable gain amplifier capable of multiplying each of the output sum samples by a variable gain factor to produce a sequence of shifted output samples, wherein a most significant bit of the each output sum sample is shifted a variable amount to a desired bit position; and 2) a feedback gain controller capable of determining a power of each filtered output sample and, in response to the determination, adjusting the variable gain factor so that an optimum number of the filtered output samples are saturated. 
         [0007]    According to still another embodiment of the present invention, the output stage comprises: 1) a variable truncation unit capable of truncating a variable number, k, of least significant bits from each of the output sum samples to thereby produce a sequence of truncated samples; and 2) a feedforward gain controller capable of controlling the variable truncation unit, wherein the feedforward gain controller determines a power of the each output sum sample and, in response to the determination, adjusts the variable number, k. 
         [0008]    According to yet another embodiment of the present invention, the output stage comprises: 1) a variable truncation unit capable of truncating a variable number, k, of least significant bits from each of the output sum samples to thereby produce a sequence of truncated samples; and 2) a feedback gain controller capable of controlling the variable truncation unit, wherein the feedback gain controller determines a power of the each filtered output sample and, in response to the determination, adjusts the variable number, k. 
         [0009]    According to a further embodiment of the present invention, the output stage further comprises a truncation unit capable of is receiving the sequence of shifted output samples from the variable gain amplifier and truncating a fixed number of least significant bits from each shifted output sample to thereby produce a sequence of truncated samples. 
         [0010]    Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand is that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
           [0012]      FIG. 1  illustrates a wireless communication device according to an exemplary embodiment of the present invention; 
           [0013]      FIG. 2  illustrates a finite impulse response (FIR) filter according to an exemplary embodiment of the prior art; 
           [0014]      FIG. 3  illustrates a finite impulse response (FIR) filter according to a first exemplary embodiment of the present invention; 
           [0015]      FIG. 4  illustrates a finite impulse response (FIR) filter according to a second exemplary embodiment of the present is invention; 
           [0016]      FIG. 5  illustrates a finite impulse response (FIR) filter according to a third exemplary embodiment of the present invention; 
           [0017]      FIG. 6  illustrates a finite impulse response (FIR) filter according to a fourth exemplary embodiment of the present invention; 
           [0018]      FIG. 7A  illustrates a feed-forward calculation block for adaptively determining the truncation value according to an exemplary embodiment of the present invention; and 
           [0019]      FIG. 7B  illustrates a feedback calculation block for adaptively determining the truncation value according to an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]      FIGS. 1 through 7 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged wireless communication system. 
         [0021]      FIG. 1  illustrates wireless communication device  100  according to an exemplary embodiment of the present invention. Wireless communication device  100  is intended to be a generic representation of any type of receiver that may contain a fixed point finite impulse response (FIR) filter using adaptive truncation according to the principles of the present invention. Thus, in one embodiment of the present invention, wireless communication device  100  shown in  FIG. 1  may be a portion of a cellular telephone or a portion of a base station of a wireless network. In an alternate embodiment of the present invention, wireless communication device  100  may be part of a wireless network card in a personal computer (PC) operating in, for example, an IEEE 802.11 compatible wireless local area network (LAN). Those skilled in the art will recognize that the particular details set forth below with respect to wireless communication device  100  are by way of example only and should not be construed so as to limit the scope of the present invention. 
         [0022]    The receive path of wireless communication device  100  comprises antenna  105 , variable gain amplifier (VGA)  110 , radio frequency (RF) filter  115 , quadrature phase shift keying (QPSK) demodulator  120 , analog-to-digital-converters  125 A and  125 B, and automatic gain control (AGC) block  130 . The receive path of wireless communication device  100  also comprises fixed point finite impulse response (FIR) filters  135 A and  135 B and demodulator block  140 . 
         [0023]    Variable gain amplifier (VGA)  110  amplifies the incoming RF signal receive from antenna  105  by an amount determined by gain control signal receive from AGC control block  130 . RF filter  115  then filters the output of VGA  110 . According to an exemplary embodiment of the present invention, RF filter  115  may be any one of several infinite impulse response (IIR) filters that have the primary function of isolating the frequencies of interest (i.e., band selection, channel selection, low-pass filtering) and perform anti-aliasing for ADC-sampling. 
         [0024]    QPSK demodulator  120  then demodulates the filtered RF signal to produce an intermediate frequency (IF) signal or a baseband signal. According to an exemplary embodiment of the present invention, the incoming RF signal is a quadrature phase-shift keying (QPSK) signal and the outputs of QPSK demodulator  120  are an in-phase (I) output signal and a quadrature (Q) output signal. The in-phase (I) output signal from QPSK demodulator  120  is converted from an analog signal to a digital signal by analog-to-digital converter (ADC)  125 A. The quadrature (Q) output signal from QPSK demodulator  120  is converted from an analog signal to a digital signal by analog-to-digital converter (ADC)  125 B. 
         [0025]    The digitized I and Q output signals from ADC  125 A and ADC  125 B are fed back to AGC block  130 . AGC block  130  functions in such a manner that the total power of the in-band signals and the out-of-band adjacent channel signals are maintained at a constant level at the outputs of ADC  125 A and ADC  125 B. The power of the in-band signals at the outputs of ADC  125 A and  125 B is kept constant by AGC block  130  even if there are strong adjacent channel signals. 
         [0026]    The outputs of ADC  125 A and ADC  125 B are filtered by fixed point FIR filter  135 A and fixed point FIR filter  135 B, respectively. FIR filter  135 A and FIR filter  135 B are matched filters, so that only the in-band signals remain at the outputs of FIR filter  135 A and FIR filter  135 B. Since the strengths of the in-band signals at the outputs of ADC  125 A and  125 B vary according to the strength of the adjacent channel signals, the power of signals at the outputs of FIR filter  135 A and FIR filter  135 B also vary. 
         [0027]    Advantageously, since RF filter  115  is typically a 3 rd  or 4 th  order Butterworth filter or Chebycheshev filter that provides only about 18-24 dB attenuation at the center of the adjacent channel, fixed point FIR filter  135 A and fixed point FIR filter  135 B also act as an adjacent channel selectivity filters that provide about 40 dB attenuation at the center of the adjacent channel. 
         [0028]    Next, demodulator  140  demodulates the in-phase baseband signal to thereby recover the symbols of the in-phase baseband signal. Similarly, demodulator  140  demodulates the quadrature baseband signal to thereby recover the symbols of the quadrature baseband signal. The recovered symbols comprise the Data Out signal at the output of demodulator  140 . 
         [0029]      FIG. 2  illustrates fixed point finite impulse response (FIR) filter  200  according to an exemplary embodiment of the prior art. Prior art FIR filter  200  may be used in place of FIR filter  135 A and FIR filter  135 E in  FIG. 1 . FIR filter  200  comprises a chain of N−1 sequential delay (D) elements, including exemplary delay (D) elements  201 ,  202 ,  203 ,  204  and  205 . FIR filter  200  also comprises N multipliers, including exemplary multipliers  211 ,  212 ,  213 ,  214 ,  215  and  216 . FIR filter  200  also comprises summer  220 , least significant bit (LSB) truncation block  230  and saturation block  240 . 
         [0030]    The Data In signal received from ADC  125 A or ADC  125 B comprises a sequence of r-bit digital samples. These r-bit digital samples shift sequentially through the N−1 delay elements, including exemplary delay elements  201 - 205 . The N multipliers, including exemplary multipliers  211 - 216  multiply N sequential samples of the Data In signal by the N filter coefficients c( 0 ), c( 1 ), . . . c(N−1), and c(N). 
         [0031]    The intermediate signal at the output of summer  220  comprises a sequence of m-bit digital samples, where m is greater than r. For example, in an exemplary embodiment of the present invention, r may be 6 bits and m may be from 17 bits to 20 bits. In order to reduce the complexity of FIR filter  200  and subsequent stages of wireless communication device  100 , LSB truncation block  230  truncates (i.e., cuts off) the k least significant bits from the m-bit intermediate signal received from summer  220 . Saturation block  240  compares the (m−k)-bit truncated output from LSB truncation block  230  to a maximum threshold and a minimum threshold and outputs a p-bit output at the Data Out signal. If the (m−k)-bit truncated output from LSB truncation block  230  exceeds the maximum threshold, saturation block  230  outputs a maximum saturation value. It the (m−k)-bit truncated output from LSB truncation block  230  is less than the minimum threshold, saturation block  230  outputs a minimum saturation value. 
         [0032]    For example, let m=17, k=8 and p=6. LSB truncation block  230  drops the nine (9) least significant bits from the 17-bit intermediate signal from summer  220  and outputs a (17-8)=9-bit value to saturation block  240 . The range of the 6-bit output (p=6) from saturation block  240  is from +31 to −32. Saturation block  240  compares each 9-bit value from LSB truncation block  230  to +31 and −32. If the 9-bit value from LSB truncation block  230  is greater than +31, saturation block  240  outputs a maximum saturation value equal to +31 (i.e., 011111 in 2s-complement). If the 9-bit value from LSB truncation block  230  is less than −32, saturation block  240  outputs a minimum saturation value equal to −31 (i.e., 111111 in 2s-complement). If the 9-bit value from LSB truncation block  230  is between +31 and −32 inclusive, saturation block  240  outputs a 6-bit value equal to the 9-bit output of LSB truncation block  230 . 
         [0033]    However, the wide dynamic range of the m-bit output from summer  220  causes problems in the performance of wireless communication device  100 . The above-described operation of FIR filter  200  provides a quantization window having a width of p bits at bit position k. Given a signal power of m bits at the output of summer  220  and a Data Out signal of p bits, the higher the quantization window, the more rounding noise, and the lower the quantization window, the more overflow noise. 
         [0034]    There is an optimal window position for the given power of the input signal of m bits at the output of summer  220 . However, while this optimal window position may change, conventional fixed point FIR filter designs use quantization windows that have a fixed width and a fixed bit position. This leads to performance degradation. The present invention overcomes this problem by providing an apparatus that is capable of performing adaptive truncation, wherein the value of k may be modified in order to truncate a variable number of bits from the output of summer  220 . 
         [0035]      FIG. 3  illustrates fixed point finite impulse response (FIR) filter  135  according to a first exemplary embodiment of the present invention. FIR filter  135  represents one or both of fixed point FIR filters  135 A and  135 B in  FIG. 1 . FIR filter  135  comprises a chain of N−1 sequential delay (D) elements (e.g., shift registers), including exemplary delay (D) elements  201 ,  202 ,  203 ,  204  and  205 . FIR filter  135  also comprises N multipliers, including exemplary multipliers  211 ,  212 ,  213 ,  214 ,  215  and  216 . FIR filter  135  also comprises summer  220 , variable gain amplifier  310 , feed-forward control block  320 , least significant bit (LSB) truncation block  230  and saturation block  240 . The input stages of FIR filter  135 , up to and including summer  220 , operate identically to the input stages of FIR filter  200  and need not be discussed in detail again. 
         [0036]    However, the m-bit output of summer  220  is multiplied by a variable amount of gain by variable gain amplifier  310  before being applied to LSB truncation block  230 . The amount of gain is controlled by feed-forward control block  320 . Feed-forward control block  320  measures the signal strength of the m-bit output of summer  220  and adjusts the gain (G) of amplifier  310  in order to keep the power of the samples entering LSB truncation block  230  in a desired target range. 
         [0037]    For example, if a p=6 bit output (including sign bit) is desired for the Data Out signal and k=7 bits of truncation, then feed-forward control block  320  adjusts the gain of amplifier  310  (up or down), so that the most significant bits of the peaks of the samples entering LSB truncation block  230  are approximately bit positions  12  or  13 , or perhaps bit position  14  (disregarding occasional very large peaks). After the k=7 least significant bits are dropped, the peak values entering LSB truncation block  230  will have their most significant bits in bit positions  5  or  6 , or perhaps bit position  7  (not counting the sign bit). In this manner, most samples at the output of saturation block  240  make full use of the range between +31 and −32 in value without a large number of saturation values being generated. 
         [0038]      FIG. 4  illustrates finite impulse response (FIR) filter  135  according to a second exemplary embodiment of the present invention. As in  FIG. 3 , FIR filter  135  comprises a chain of N−1 sequential delay (D) elements, including exemplary delay (D) elements  201 ,  202 ,  203 ,  204  and  205 . FIR filter  135  also comprises N multipliers, including exemplary multipliers  211 ,  212 ,  213 ,  214 ,  215  and  216 . FIR filter  135  also comprises summer  220 , variable gain amplifier  410 , feedback control block  420 , least significant bit (LSB) truncation block  230  and saturation block  240 . The input stages of FIR filter  135 , up to and including summer  220 , operate identically to the input stages of FIR filter  200  and need not be discussed in detail again. 
         [0039]    As in the case of FIR filter  125  in  FIG. 3 , the m-bit output of summer  220  is multiplied by a variable amount of gain by variable gain amplifier  410  before being applied to LSB truncation block  230 . However, the amount of gain is controlled by feedback control block  420  (rather than a feedforward controller). Feedback control block  420 , discussed below in greater detail, measures the signal strength of the p-bit output saturation block  240  to determine the number of output samples that are saturated and adjusts the gain (G) of amplifier  410  in order to keep the power of the samples entering LSB truncation block  230  in a desired target range. The desired target range reduces the number of output samples in Data Out that are saturated to an optimum level. 
         [0040]      FIG. 5  illustrates finite impulse response (FIR) filter  135  according to a third exemplary embodiment of the present invention. As in  FIGS. 3 and 4 , FIR filter  135  comprises a chain of N−1 sequential delay (D) elements, including exemplary delay (D) elements  201 ,  202 ,  203 ,  204  and  205 . FIR filter  135  also comprises N multipliers, including exemplary multipliers  211 ,  212 ,  213 ,  214 ,  215  and  216 . FIR filter  135  further comprises summer  220 , feed-forward calculation block  520 , least significant bit (LSB) truncation block  530  and saturation block  240 . The input stages of FIR filter  135 , up to and including is summer  220 , operate identically to the input stages of FIR filter  200  and need not be discussed in detail again. 
         [0041]    The m-bit output of summer  220  is applied directly to the input of LSB truncation block  530  (i.e., without gain amplification). However, unlike the above-described LSB truncation block  230 , LSB truncation block  530  truncates a variable number, k, of least significant bits from the m-bit output of summer  220 . The value of k is determined by feed-forward calculation block  520 . Feed-forward calculation block  520 , discussed below in greater detail, measures the signal strength of the m-bit output of summer  220  and adjusts the value of k in LSB truncation block  530  so that the power of the samples exiting LSB truncation block  530  are in a desired target range. 
         [0042]      FIG. 6  illustrates finite impulse response (FIR) filter  135  according to a fourth exemplary embodiment of the present invention. As in  FIG. 3-5 , FIR filter  135  comprises a chain of N−1 sequential delay (D) elements, including exemplary delay (D) elements  201 ,  202 ,  203 ,  204  and  205 . FIR filter  135  also comprises N multipliers, including exemplary multipliers  211 ,  212 ,  213 ,  214 ,  215  and  216 . FIR filter  135  further comprises summer  220 , feedback calculation block  620 , least significant bit (LSB) truncation block  630 , and saturation block  240 . The input stages of FIR filter  135 , up to and including summer  220 , operate identically to the input stages of FIR filter  200  and need not be discussed in detail again. 
         [0043]    The m-bit output of summer  220  is applied directly to the input of LSB truncation block  630  (i.e., without gain amplification). LSB truncation block  630  truncates a variable number, k, of least significant bits from the m-bit output of summer  220 . The value of k is determined by feedback calculation block  620 . Feedback calculation block  620 , discussed below in greater detail, measures the signal strength of the p-bit output saturation block  240  to determine the number of output samples that are saturated and adjusts the value of k in order to keep the power of the samples exiting LSB truncation block  630  in a desired target range. The desired target range reduces the number of output samples in Data Out that are saturated to an optimum level. 
         [0044]      FIG. 7A  illustrates feed-forward calculation block  520  for adaptively determining the truncation value, k, according to an exemplary embodiment of the present invention. Feed-forward calculation block  520  receives the m-bit outputs of summer  220  and periodically generates values of k. It is noted that feed-forward control block  320  functions in a manner that corresponds to the following description of feed-forward calculation block  520 , except that feed-forward control block  320  generates values of gain, G, that are used by amplifier  310 . 
         [0045]    Feed-forward calculation block  520  comprises power estimation block  702 , sum and dump block  704 , filter  706 , and log 2  [X/Threshold1] block  708 . Power estimation block  702  receives the m-bit samples from summer  220  and calculates the power of the samples. Power estimation block  702  may take the absolute value or square value of the signal as the power estimate. Sum and dump block  704  receives the power estimate values from power estimation block  702 , adds consecutive groups of W power estimate values together, divides each sum by W, and outputs the results. In essence, sum and dump block  704  calculates the average value of each group of W consecutive power estimate values received from power estimation block  702 . Thus, the data rate at the output of sum and dump block  704  is 1/W the data rate at the output of power estimation block  702 . 
         [0046]    Filter  706  then filters the average values from the output of sum and dump block  704  to reduce noise and jitter. The smoothed and filtered output of filter  706  is then applied to log 2  [X/Threshold1] block  708 . In an exemplary embodiment of the present invention, the value X represents the bit weight of the most significant bit in the output of filter  706 . For example, if the output of filter  706  is 0000010000001000 binary (1032 decimal), the 11 th  bit is the most significant bit and X equals 1024. The pre-determined Threshold1 value is set so that the correct number of bits, k, are truncated from the output of filter  1024  for a target power output level. 
         [0047]    For example, if Threshold1=64 and X=1024, then [X/Threshold1] equals 16 and the output of log 2  [X/Threshold] block  708  is k=4. If the output of summer  220  is 0001010000001000 binary (5128 decimal) and four bits are truncated from the output of summer  220 , then the input to saturation block  240  is the value 000101000000. 
         [0048]      FIG. 7B  illustrates feedback calculation block  620  for adaptively determining the truncation value according to an exemplary embodiment of the present invention. Feedback calculation block  620  receives the p-bit outputs from saturation block  240  and periodically generates values of k. It is noted that feedback control block  420  functions in a manner that corresponds to the following description of feedback calculation block  620 , except that feedback control block  420  generates values of gain, G, that are used by amplifier  410 . 
         [0049]    Feedback calculation block  620  comprises power estimation block  752 , sum and dump block  754 , adder  756 , filter  758 , decision block  760 , and integration block  762 . Power estimation block  752  receives the p-bit samples from saturation block  240  and calculates the power of the samples. Sum and dump block  754  receives the power estimate values from power estimation block  752 , adds consecutive groups of W power estimate values together, divides each sum by W, and outputs the results. In essence, sum and dump block  754  calculates the average value of each group of W consecutive power estimate values received from power estimation block  752 . Thus, the data rate at the output of sum and dump block  754  is 1/W the data rate at the output of power estimation block  752 . 
         [0050]    Next, adder  756  subtracts a pre-determined Threshold2 value from the power average values at the output of sum and dump block  754 . The Threshold2 value in  FIG. 7B  is different than the Threshold) value in block  708  in  FIG. 7A . The output of adder  756  is an error value that may be equal to 0, may be greater or equal to 1, or may be less than or equal to −1. The error value from adder  756  is filtered and smoothed by filter  758 . The output of decision block  760  has only three values: +1, 0, or −1. If the filtered error value is less than +(Threshold3) and greater than −(Threshold3), then the output of decision block  760  is 0. If the filtered error value is equal to +(Threshold3) or greater, then the output of decision block  760  is +1. If the filtered error value is equal to −(Threshold3) or less, then the output of decision block  760  is −1. The Threshold3 value in decision block  760  is different than the Threshold) value in is block  708  and the Threshold2 block in adder  756 . The Threshold3 value is used to further remove jitter in output k. Thus, the output of decision block  760  is a sequence of +1, 0 and −1 values that are integrated by integration block  762 . The output of integration block  762  is the value k. 
         [0051]    If the p-bit output power from saturation block  240  are too high (i.e., frequent saturations), then the outputs of sum and dump block  754  are consistently higher than the Threshold value on the input of adder  756 . As a result, the error values from adder  756  are consistently greater than or equal to +1 and the outputs of decision block  760  are mostly +1 values. This causes the output of integration block  762  to rise and the value of k increase. This results in a greater number of least significant bits being truncated from the output of summer  220  and the average power of the p-bit outputs of saturation block  240  decreases. Conversely, if the p-bit output values from saturation block  240  are too low, a smaller number of least significant bits are truncated from the output of summer  220  and the average power of the p-bit outputs of saturation block  240  increases. 
         [0052]    In the embodiments illustrated above in  FIGS. 3-6 , the input stage of FIR filter  135  (i.e., delay elements  201 - 205 , multipliers  211 - 216  and summer  220 ) is a direct form realization of an FIR filter. However, those skilled in the art will recognize that FIR filter  135  may be embodied as any type of FIR filter, including, for example, a transpose filter realization. Generally speaking, the input stage of any FIR filter receives input samples having a relatively small number of significant bits and generates outputs samples having a relatively large number of bits. Advantageously, adaptive truncation circuitry according to the principles of the present invention may be easily implemented with any type of FIR filter input stage. 
         [0053]    Although the present invention has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.