Equalizer of preset type for quadrature amplitude modulated signals

Claim 1

An equalizer for equalizing demodulated signals obtained by demodulating quadrature amplitude modulated signals sent thereto through a transmission line having an impulse response characteristics from a transmitting end for series of communication code pulses generated thereat at a predetermined code pulse interval T, comprising a two-dimensional transversal filter having variable gain units of complex variable gains Ck and responsive to said demodulated signals for producing equalized signals, the letter k being representative of at least a predetermined number of a plurality of consecutive integers including zero, a clock pulse generator for producing clock pulses whose repetition period is substantially equal to said code pulse interval, and means for presetting said variable gains in response to a limited number of test signals supplied to said equalizer, each of said test signals being representative of said impulse response characteristics and given by a complex signal z(t), where t represents time for each of said test signals, said complex signal having the greatest complex amplitude z0 substantially at t 0, the greatest complex amplitudes of said test signals being spaced with respect to time at least said code pulse interval multiplied by the number of said integers, said gain presetting means comprising: means responsive to said clock pulses and a quasi-equalized complex signal s(t) produced by said transversal filter responsive to each of said test signals for producing complex error signals representative of errors e(kT) of said quasiequalized signal at time points kT, means responsive to each of said test signals for holding a greatest amplitude complex signal representative of the greatest amplitude of each test signal, means responsive to each of the error signals produced responsive to each of said test signals and the greatest amplitude signal held for each test signal for producing an adjustment signal representative of a complex amount of adjustment dh determined in accordance with a set of rules given by dh D - when - -Re(z(0).e*(hT))> Im(z(0).e*(hT)) , dh -D - when - Re(z(0).e*(hT))> Im(z(0).e*(hT)) , dh jD - when - Im(z(0).e*(hT))> Re(z(0).e*(hT)) , dh -jD - when - -Im(z(0).e*(hT))> Re(z(0).e*(hT)) , where h is that one of the integers h which gives the error e(hT) represented by said each error signal, D is a predetermined amount, j is the imaginary unit, Re and Im represent the real and the imaginary parts, and e*(htT) is the conjugate complex number of the last-mentioned error, and means responsive to said clock pulses and said adjustment signal for adjusting that one of said variable gains by the amount of adjustment dh represented by the last-mentioned adjustment signal which has the same suffix h as the last-mentioned amount of adjustment.