Adaptive equalizer for digital signals affected by distorsions with time varying characteristics

Claim 2

Method for the adaptive equalization of digital signals, according to which the signals coming from the transmission channel (1) to be equalized are sampled and encoded and the digital samples are then filtered in a transversal filter (EQ) having powers-of-two coefficients, characterized in that a block of said samples is extracted at the output of said transversal filter (EQ) and the above powers-of-two coefficients are adaptively computed in real time by effecting for each coefficient at a frequency inferior or equal to the symbol frequency the difference between the filtered samples and the corresponding decided symbols, then by computing the modulo and sign of the difference above and using the sign to effect an operation of exclusive OR with the sign of the received signal, the exclusive-OR result being used to compute a first parameter β i (j), where the subscript i refers to the ith transversal filter coefficient and index j denotes the jth iteration of the computing process of jth coefficient, by assigning β i (j) a value equal to 1 or 0.5 if the exclusive OR result is equal to logic value 0 or +1 respectively and if the ith transversal filter coefficient at the ith iteration c i (j) is positive, assigning β i (j) a value equal to 0.5 or 1 if the exclusive-OR result is equal to 0 or +1 respectively and if c i (j) is negative, and then using the value of parameter β i (j) thus computed and the modulo of c . (j) to obtain a threshold parameter S.(j) according to the following formula: I : where S 0 and S 1 are two pre-determined constants on which the convergence speed of coefficient computing algorithm depends, then said threshold parameter S i (j) being compared with the modulo of said difference, and if this modulo is greater, allowing a counting index to be incremented or decremented according to whether the exclusive OR result is +1 or 0 respectively and, when the index above exceeds a pre-determined value, to be reset and to enable the modification of the coefficient c l of the transversal filter, whose algebraic value is rendered greater or smaller if the above exclusive OR result is 0 or +1 respectively, and to be then sent to the transversal filter (EQ).

Claim 3

An adaptive equalizer for digital signals, comprising a digital-to-analog converter (AD) for converting the input analog signal into a series of digital samples, a transversal filter (EQ) for filtering the samples above and a decision circuit (CD) for extracting the corresponding symbols, characterized in that it comprises: - a difference device (DD), for effecting the difference between the serial signals supplied by the transversal filter and the signals supplied by the decision circuit; - a sign detecting device (SE), connected to the output of the difference device and equipped with as many outputs as the coefficients of the transversal filter are; - a number of exclusive-OR logic gates (XR) equal to the coefficient number, which receive at an input the signal present at an output of the sign detecting device and at the other input the sign of the received signal extracted from the corresponding sign cell of the transversal filter (EQ); - a number of devices (DS) for determining thresholds equal to the coeffocoent number, each device presenting an input connected to the output of the corresponding exclusive-OR logic gate, an input connected to a computing unit (UC) and two inputs which can be accessed from the outside for loading constants ( S O , S 1 ); - a modulo extracting device (ME), connected to the difference device output and equipped with as many outputs as the transversal filter coefficients are; - a number of comparison circuits (DC), equal to the coefficient number, each circuit presenting an input connected to the output of the corresponding device for computing the thresholds and the other input to the corresponding output of the modulo extracting device; - a number of up-down counters (CL) each for incrementing or decrementing said counting index according to the logic value of the signal supplied by the corresponding exclusive-OR logic gate, when it is enabled by the corresponding comparison circuit, the counting modulo being equal to said pre-determined number (K), which, once reached, causes each counter to be reset and to emit an enabling signal for a corresponding computing unit (UC); - a number of computing units (UC), each apt to compute a coefficient (c i (j)) of the transversal filter (EQ), to determine a parameter on the basis of the signal received from the corresponding exclusive-OR logic gate and to decide, on the basis of the value of said determined parameter and of the end-of-counting signal arriving from the corresponding counter, whether to double, halve the previous coefficient or let it unchanged.

Claim 4

An adaptive equalizer for digital signals, comprising a digital-to-analog converter (AD) for converting the input analog signal into a series of digital samples, a transversal filter (EQ) for filtering the samples above and a decision circuit (CD) for extracting the corresponding symbols, characterized in that it comprises: - a difference device (DD), for effecting the difference between the serial signals supplied by the transversal filter and the signals supplied by the decision circuit; - a sign detecting device (SE), connected to the output of the difference device and equipped with as many outputs as the coefficients of the transversal filter are; - a number of exclusive-OR logic gates (XR) equal to the coefficient number, which receive at an input the signal present at an output of the sign detecting device and at the other input the sign of the received signal extracted from the corresponding sign cell of the transversal filter (EQ); - a number of devices (DS) for determining thresholds equal to the coeffocoent number, each device presenting an input connected to the output of the corresponding exclusive-OR logic gate, an input connected to a computing unit (UC) and two inputs which can be accessed from the outside for loading constants ( S O , S 1 ); - a modulo extracting device (ME), connected to the difference device output and equipped with as many outputs as the transversal filter coefficients are; - a number of comparison circuits (DC), equal to the coefficient number, each circuit presenting an input connected to the output of the corresponding device for computing the thresholds and the other input to the corresponding output of the modulo extracting device; - a number of up-down counters (CL) each for incrementing or decrementing said counting index according to the logic value of the signal supplied by the corresponding exclusive-OR logic gate, when it is enabled by the corresponding comparison circuit, the counting modulo being equal to said pre-determined number (K), which, once reached, causes each counter to be reset and to emit an enabling signal for a corresponding computing unit (UC); - a number of computing units (UC), each apt to compute a coefficient (c i (j)) of the transversal filter (EQ), to determine a parameter on the basis of the signal received from the corresponding exclusive-OR logic gate and to decide, on the basis of the value of said determined parameter and of the end-of-counting signal arriving from the corresponding counter, whether to double, halve the previous coefficient or let it unchanged.