Dynamically adaptive equalizer system and method

Abstract: An adaptive filter or equalizer for digital data communication includes, a first cascade circuit connected to operate as a digital finite impulse response (FIR) filter, the first cascade circuit having a first plurality of input taps for application of filter weighting signals thereto, a single input for samples of input data, and an output for filtered data. A second cascade circuit, which has the inverse canonical form, has a second plurality of input taps for receiving signal samples of input data, a single input for receiving error signals, and an output for yielding a succession of weighting signals. A sequencing circuit applies data signals to successive cascade stages of the second cascade circuit in a time sequence which is the reverse of that of the weight signals applied to the plurality of input taps of the first cascade circuit, such that the second cascade circuit functions as a weighting signal generator for the first cascade circuit. Storage registers at the inputs of the first and second cascade circuits are updated by weight and data signals, respectively, serve to multiplex application of single inputs to multiple cascade stages. An error signal is derived from the output of said first cascade circuit and applied to the error input of the second cascade circuit.

Description: BACKGROUND OF THE INVENTION: This invention relates to the design and circuit implementation of adaptive equalizers, which are used in modern electrical communication to equalize (i.e., compensate for) linear amplitude and phase distortions which occur naturally within the useful frequency band of practical transmission circuits. If these distortions are compensated, as is well known in the art, a given circuit is capable of correctly received transmission of information at substantially higher rates and over virtually unlimited transmission distances. In particular, the invention relates to dynamically adaptive equalizers, which employ the signals transmitted through a communication circuit or channel for measurement of its distortion. Such circuits alter their amplitude and phase characteristics with time to minimize the measured error at the receiving end of the circuit. Adaptive equalizers are required for use in connection with circuits whose distortion properties are unknown at the time of initiation of transmission, or which for any reason may change during transmission. Representative circuits include those having variable multiple paths, radio transmissions carrying digital voice, data and other signals, and switched telephone lines. In typical adaptive equalizer operation, communication may be established using a repeated signal carrying no information. The equalizer adjusts rapidly to compensate for the transmission circuit distortion. After information transmission is begun, the information signals are then used continuously for iterative dynamic correction of the filter characteristics. More specifically, this invention relates to adaptive equalizers implemented using digital sampling and computation processes to effect equalization of transmission lines, or to remove signal distortion due to variable and multiple paths, in radio transmissions carrying digital voice, data and other signals. RELATED ART STATEMENT Digitally operating adaptive equalizers are well known in the electrical communications art. All equalizers are, in their basis, electrical filters, and adaptive equalizers are based on electrically adaptable filters, typically taking the form of finite impulse response filters, some forms of which are described in Watanabe, U.S. Patent 4,771,395, September 13, 1988. In addition to an dynamically adaptable electrical filter, an adaptive equalizer requires a means to produce the series of control inputs (termed "weights") which define the filtered output in time when responding to a single input pulse of known amplitude. This means is conventionally referred to as a weight generator, and its outputs as weights, though they are actually measures of the filter's response to a pulse input at evenly spaced time intervals. The weight generator, in turn, uses as its input an error signal which is derived by comparing the output of the filter with expected value(s) of the output. Since these circuits are used in digital data transmission, the desired output signals have a very limited number of values. Previous adaptive equalizers use identical circuits for the FIR filter and weight generator (WG) functions. In contrast to this prior art, the present invention uses the inverse canonical structure as a weight generator. As an example of an earlier adaptive equalizer, the Mobile Link 1/2 Receiver Program (MLRP) receiver, a product of the Applicant hereof, uses the Zoran 891 FIR filter chip for both the FIR and WG functions of its adaptive equalizer. Others are also believed to be using Zoran for both FIR and WG as applied to adaptive equalizers. The Zoran chip is second-sourced by Harris as the HSP43891. White, U.S. Patent 4,524,424 describes one configuration wherein a Transversal Filter (of which a finite impulse response filter is a particular example) derives weight signals as a set of parallel outputs from a Tap Weight Computer comprising multipliers, integrators, and time delays. Typical of such prior-art adaptive equalizers, White's invention incorporates a multiplicity of electrical connections between the weight generator and the filter, each transferring a weight signal to one cascade stage of the filter. BRIEF DESCRIPTION OF THE INVENTION An object of the present invention is to provide an adaptive digital filter (equalizer) whose design makes use of similar circuits for the digital elements of the greatest complexity, i.e. the filter and the inverse canonical structure as the weight generator. The practical benefits are reduced design time and reduced production cost for the equalizer. These benefits apply to adaptive filters having both filter and weight generator on a single semiconductor chip, as well as to ones in which filter and weight generator comprise separate chips or collections of chips and other components. Another object of the invention is to minimize the number of additional integrated circuits required ("glue chips" to construct a family of adaptive equalizers from a multiplicity of integrated circuits of the same design. Still another object of the invention is to employ a weight generator circuit having a single output to drive multiple weighting inputs of the adaptive filter. Yet another object of the invention is to employ low-cost integrated digital circuits capable of operation at significantly higher speeds than are required to handle information signals, and through multiplex use of their outputs achieve significant reduction in cost and complexity of an adaptive filter for those information signals. These objects are realized, in one embodiment of our invention, through novel circuits incorporating commercially available integrated digital circuits. In another embodiment, they are realized through large-scale integrated circuits which incorporate identical subcircuits for weight generator and adaptive filter functions. An adaptive filter or equalizer according to this invention includes a first cascade circuit connected to operate as a digital finite impulse response filter, the first cascade circuit having a first plurality of input taps for application of filter weighting signals thereto, a single input for samples of input data, and an output for filtered data. The adaptive equalizer also includes a second cascade circuit, which is the inverse canonical form to the first cascade circuit and having a second plurality of input taps for receiving signal samples of input data, a single input for receiving error signals, and an output for yielding a succession of weighting signals. A sequencing circuit applies data signals to successive cascade stages of the second cascade circuit in a time sequence which is the reverse of that of the weight signals applied to the plurality of input taps of the first cascade circuit, such that the second cascade circuit functions as a weighting signal generator for the first cascade circuit. Preferably, storage registers at the inputs of the first and second cascade circuits are updated by weight and data signals, respectively, and serve to multiplex application of single inputs to multiple cascade stages. Preferably, an error signal is derived from the output of said first cascade circuit and applied to the error input of the second cascade circuit. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, advantages and features of the invention will become more clear when considered with the following specification and accompanying drawings wherein: FIG. 1 depicts a representative form of the configurable circuit used as a common basis for the filter and associated weight generator, called "inverse canonical form", FIG. 2a depicts an adaptive filter configuration of this circuit, FIG. 2b depicts a weight generator configuration of this circuit, FIG. 3 depicts the interconnection of the circuits of Figures 2a and 2b to form an adaptive equalizer, FIG. 4 depicts interconnection of two pairs of the same circuits to form an adaptive equalizer with doubled number of filter elements, FIG. 5 displays results of a computer simulation of the operation of our invention, and FIG. 6 illustrates an implementation of the invention with 16T/2 spaced taps and N/2 diversity channels using 4N equalizer units. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 depicts a single electronic circuit suited for either filter or weight generator application. Although the example shown includes N=four cascaded stages, it should be understood that any suitable multiplicity of stages could be used. The central elements of the circuit include a plurality of digital multipliers 10-1, 10-2, 10-3, 10-4, whose outputs 11-1, 11-2, 11-3, 11-4, are signed products of a first signal input B and a set of signed numeric values derived from a like plurality of storage registers 12-1, 12-2, 12-3, 12-4. The multipliers may be implemented in a variety of ways, for example using either fully parallel-by-digit binary operation (equivalent to look-up in a multiplication table stored in read-only memory), serial bit-by-bit multiplication, or any other logically correct signed integer multiplication logic. The product generated by each multiplier forms one input to a digital adder 13-1, 13-2, 13-3 and 13-4, whose other input is derived from the adder to its left, but delayed one clock cycle by a conventional delay circuits 14-1, 14-2, 14-3 and 14-4, shown as T. The delayed output of the right-most adder 13-4 forms the output of the circuit. A common application of such filters is to detection of phase- and amplitude-modulated signals. In this case, each of the inputs and weight values shown is a time series of pairs of values, one representing the in-phase or real component (I) and the other the quadrature or imaginary component (Q) of the complex baseband representation of a signal. Hence, each register in fact comprises a pair of registers (one for the I and the other for the Q value). Likewise the adder comprises two adders and the multiplier four multipliers plus two adders, to correctly combine the in-phase and quadrature terms. The technique works equally well with real number valued signals. Digital numeric values stored in registers 12-1, 12-2, 12-3, 12-4, are derived from an electronic switch 15, shown schematically in the figures. The electronic switch connects input A to one of the storage registers 12-1...12-4 during each clock cycle, at which time input A updates the value stored in that register. Between clock pulses, the switch 15 advances to left or right one register. After advancing to the left or right end of the series of registers, the switch next moves to the register at the opposite end to continue its cycle. The direction of advance of switch 15 is arranged to be statically configured, as for example by application of a voltage to a controlling terminals or by multiplexer logic circuits (not shown). It is to be understood that each storage register 12-1...12-4, may be designed to store a signed number having up to some selected number of binary digits, and that multipliers 10-1...10-4 and adders 13-1...13-4 are designed to deal with corresponding numbers of binary digits. As an example, the multipliers 10-1...10-4 might be designed to combine pairs of 8-bit inputs into 16-bit products, and adders 13-1...13-4 to hold output values with up to 20 bits. Input C is used when the circuit is connected to another to extend the number of filter taps, as in the example depicted in Figure 4, and when the circuit is used as a weight generator, as shown in Figure 2b. Conventional digital clock pulses, generated in a circuit external to those shown, are used to control and sequence the circuit's operations. Although by the convention used in the diagrams, for clarity of disclosure, no clock input is shown, each part of the circuit receives common clock pulses. Likewise, each part of the circuit is energized by electric power, supplied by an external power supply which is omitted in the diagram convention used for clarity of disclosure. In the FIR filter configuration of Figure 2a, the signal input B comprises the input data signal, a succession of signed integer digital values, which may for example be derived by sampling an analog input signal for each clock pulse. The second input A, in this case, is a sequence of weighting values, which update the value in each register 16-1, 16-2, 16-3 and 16-4 in turn, sequencing to the left in this configuration. This circuit thus carries out correctly the function of a FIR filter, whose output Y n at time sample (i.e., clock pulse) n , is given by where x n = filter input at time sample n y n = filter output at time sample n w i = ith tap weight { i =0,1,2,...,N-1} N = number of tap weights (e.g. 4 in the example depicted) Each of the values x n , y n , and w i comprises inphase and quadrature components, which are combined according to rules of combination of complex numbers. In particular, the in-phase component of a sum is the sum of the in-phase components, and likewise for the quadrature components. The in-phase component of a product is the product of the in-phase components less the product of the quadrature components. The quadrature component of a product is the product of the first in-phase and second quadrature components, plus the product of the second in-phase and first quadrature components. Figure 2b illustrates the same circuit, here configured to operate as a weight generator for use in an adaptive filter. Configuration for this application requires that the second input, used to update the registers 2, be switched in left-to-right sequence, so that each of the N weights is correctly updated by the error signal. In addition, the output of the adder chain is fed back to its input C, in this case. The correct increments (differences from the weight value at the previous sample time) for weight generator outputs at sample time n are products of the detected error at time sample n and the conjugated input data values at samples n , n +1, ..., n + N -1, as given by the expression Δ w i,n = e n x n+i where Δ w i,n = increment of i th tap weight {i=0,1,2,...,N-1} at sample time n * = denotes complex conjugate operation e n = error input at sample time n x n = data input at sample time n The incremental tap weight values are combined with the sum of previous increments by the adders, such that the output tap weight for tap i at time sample n is given by An essential part of the configuration of the weight generator circuit is the connection of its output D internally or externally, as shown in Figure 2b, back to its input C . Thence the weights circulate through the shift register formed by the adders and delays T, and are incrementally updated continually, in that process. Figure 3 depicts the connection of the circuits of Figures 1a and 1b to form an adaptive equalizer incorporating the invention by connecting the weight output ( D ) of the weight generator configured circuit to input A of the filter-configured circuit. The data samples enter input B of the FIR filter 30, are filtered using the latest set of weights, and then output on port D. The weights are loaded into input A of the FIR filter at a rate of one weight per clock. Input C of the FIR ASIC is tied off to zero. The data samples also enter input A of weight generator 31 (they are conjugated internally) and are correlated with the error on input B to produce updated weights which exit the weight generator 31 on port D. Port D of the weight generator is also then tied around to input C internally for the weight accumulation function. The error generation and other auxiliary functions are performed externally. Data input and error are derived from signal detection circuits in a conventional manner. Figure 4 depicts the connection of four of the same circuits to form an adaptive equalizer having double the number of taps as that in Figure 3. In this case, the output D of the FIR circuit is connected to the C input of the corresponding circuit on the right. Cascading in a like manner can occur for any length. The data input is applied to the rightmost WG circuit without added delay. Each WG circuit cascaded leftward receives its data input delayed by 4 additional clocks indicated in the left WG by TD. The delay is shown as done internally for the embodiment shown. It could be external and applied between the data input bus and input A. Figure 5 reproduces outputs of a computer simulation which displays (c) Input data, (b) Equalizer error, (a) Equalizer output and (d) weight signals, characterized in the simulation as Inverse canonical weight bus signals. The significant aspect of the traces generated by the computer simulation is that the output (a), after a necessary period of automatic and dynamic adjustment of the weighting values, becomes essentially identical to the input (c) except for a delay. The error values (b), initially large, reduce after the adjustment period to near zero. The weight-bus data shown, when applied to the weight values stored in the registers 2 of the FIR filter-configured circuit(s), represent a situation in which a single weight attains a high value (the equivalent of unity) while all others remain near zero. Figure 6 shows an array of finite impulse response (FIR) filters constituted by application specific integrated circuit (ASIC) chips, and weight generators WG also in ASICs cascaded in the following dimensions: Number of taps: 8 Number of samples per symbol T/2 Number of diversity channels N/2 This demonstrates expandability of the system in various dimensions. While the description above describes a particular class of applications, the invention is applicable to any adaptive filter, equalizer or other electrical filter application in which the uncorrupted and undistorted signal has a format from which can be derived an error signal whose reduction to zero represents the removal of corruption and distortion.

Claims: An adaptive equalizer comprising: a first cascade circuit (30) connected to operate as a digital finite impulse response filter, said first cascade circuit having a first plurality of input taps for application of filter weighting signals thereto, a single input (B) for samples of input data, and an output (D) for filtered data; a second cascade circuit (31), which is in the inverse canonical form and having a second plurality of input taps for receiving conjugate samples of said input data, a single input (B) for receiving error signals, and an output (D) for yielding a succession of weighting signals; and sequencing circuit means for applying data signals to successive cascade stages of said second cascade circuit in a time sequence which is the reverse of that of the weight signals applied to the plurality of input taps of said first cascade circuit, such that said second cascade circuit functions as a weighting signal generator for said first cascade circuit. The equalizer defined in claim 1 further including storage registers (12-1, 12-2, 12-3, 12-4, 16-1, 16-2, 16-3, 16-4) at said input taps of the first and second cascade circuits, said storage registers being updated by weight and data signals, respectively, and serving to multiplex application of single inputs to multiple cascade stages. The equalizer defined in claims 1 or 2 further including means to derive an error signal from the output (D) of said first cascade circuit (30) and apply said error signal to the error input (B) of said second cascade circuit (31). An equalization process to compensate for amplitude and phase distortions in a digital communication channel, comprising: providing a first cascade circuit (30) connected to operate as a digital finite impulse response filter, said first cascade circuit having a first plurality of input taps for application of filter weighting signals thereto, a single input (B) for samples of input data, and an output (D) for filtered data; providing a second cascade circuit (31), said second circuit having the inverse canonical structure of said first cascade circuit and having a second plurality of input taps for receiving signal samples of input data, a single input (B) for receiving error signals, and an output (D) for yielding a succession of weighting signals; and applying data signals to successive cascade stages of said second cascade circuit in a time sequence which is the reverse of that of the weight signals applied to the plurality of input taps of said first cascade circuit, such that said second cascade circuit functions as a weighting signal generator for said first cascade circuit. The process defined in claim 4 further including providing storage registers (12-1, 12-2, 12-3, 12-4, 16-1, 16-2, 16-3, 16-4) at said input taps of the first and second cascade circuits, updating data in said storage registers by weight and data signals, respectively. The process defined in claims 4 or 5 further including deriving an error signal from the output (D) of said first cascade circuit (30) and applying said error signal to the error input (B) of said second cascade circuit (31).

Inventors: CURRIVAN BRUCE J, OHLSON JOHN E, CURRIVAN, BRUCE J., OHLSON, JOHN E.
Applicants: STANFORD TELECOMM INC, STANFORD TELECOMMUNICATIONS, INC
Application year: 1993
Main classification: H04B7
IPC codes: H04B3, H03H17, H03H15, H03H21