Patent GB-1511870-A (GB)
Title: Equalizer
IPC: H03H17/00
Filed: 1975-11-11
Abstract: 1511870 Adaptive equalizers INTERNATIONAL BUSINESS MACHINES CORP 11 Nov 1975 [21 Feb 1975] 46491/75 Heading H4R In an equalizer for a phase shift keyed transmission system over a communication channel which introduces phase and amplitude distortions so that a received signal is equivalent to the sum of the transmitted signal and a distortion signal representative of the phase and amplitude distortion, the phase # 0  of a received signal is detected and from the received phase is subtracted a phase error (# 0 -# 0 ) where # 0  is an approximation of the value of the phase α 0  of the transmitted signal and the phase error (# 0 -# 0 ) is defined as sin (# 0 -# 0 ) = ## 0  cos# 0 -#u 0  sin # 0  where ## 0  and #u 0  are approximate values of the in phase and quadrature components in a cartesion co-ordinate system of the distortion signal. Having determined the phase approximation # 0  an estimate # 0  is made of the value of the transmitted phase α 0 . As described the receiver incorporates a conventional phase demodulator (44, Fig. 5, not shown) which produces a binary signal representative of the phase # 0  of the received signal which is applied to a conversion unit CU, Fig. 6, of the equalizer of the invention. A read only store 60 provides Cos# 0  and Sin# 0  values, corresponding to the values of # 0  on line 56, which are applied to multipliers 66 and 67 fed on their other inputs 95, 93 with ## 0  and #u 0  which are the in phase and in quadrature components of the approximate distortion signal produced by the cross coupled transversal filter 81 from the estimated in phase and in quadrature values of the transmitted signal # 0  and # 0 . The outputs of multipliers 66 and 67 and fed to a subtractor to derive a value Sin (# 0  - α 0 ) = #v 0  Cos # 0 -#u 0  Sin # 0 . It being shown that Sin # 0  Cos α 0  - Cos # 0  Sin α 0  = ##v 0  Cos # 0 -#u 0  Sin # 0 . A second read only memory 70 converts the value Sin (# 0 -# 0 ) to (# 0  - # 0 ) which after combination with the input phase signal # 0  in subtractor 71 provides an output # 0  equivalent to an approximation of the transmitted phase α 0 . A detector unit 74 receives the approximate phase # 0  and finds the closest of the possible transmitted phases as the value # 0  which is output on line 78 and is also fed into a converter 77 to provide corresponding quadrature related components # 0  and # 0  on lines 79 and 80 as input to the cross coupled transversal filter 81 which in normal manner synthesizes the approximate trailing distortion on the received signal. The multiplier coefficients of the transversal filter C l -C i -C k  and D l -D i -D N  are continuously updated using a correlation technique with the apparatus of Fig. 7, which shows the apparatus for the adjustment of the multipliers at one tapping point of the filter, i.e. for C i  and D i . It is shown that the errors associated with the values of the coefficients C, and D i  are where the bar over the rhs indicates the long term average. The apparatus of Fig. 7 utilizes a zero forcing technique in which only the signs of the terms in the expressions are considered and it is shown then that The apparatus of Fig. 7 evaluates these expressions using as inputs the various outputs of the apparatus of Fig. 6 and evaluating dα 0  as the difference between the approximate value and the estimated value of the transmitted phase, i.e. the phase error signal, from the signals at the input to and output from the detector 74. The long term mean of the expressions is derived by the integrators 108 and 109 and reversible counters 110 and 111 sum the signs of d(D i  + C i ) and d(D i  - C i ) to obtain the values D i  - C i  and D i  - D i  which are summed and differenced to obtain D i  and C i  which are entered into the respective registers 114 and 115 for application to the multipliers on the i'th tap of the delay devices 82 and 87. The equalizer operates on digital signals from samples of the input signal fed through an A to D converter.