Experimental studies to be performed as part of the exercise include investigation of the process ofadaptive correction of dispersive channel by means of LMS algorithm, for transmission with 128-QAM (Quadrature Amplitude Modulation) modulation. The transmission with 128-QAM modulationcompatible with the V.17 standard will be considered. According to this standard, in the transmitter,data are compressed and then pass through a scrambler. The task of the scrambler is the mixing of bitsof information to eliminate any repetitions of bit sequence and to whiten the spectrum of thetransmitted signal. This treatment eliminates the spectrum bands, which could contribute to theformation of crosstalk in a telephone links. The so formed bit stream goes to a mapper, in whichgroups of successive bits (from 2 to 7 bits) are assigned to the symbols from the constellation of thefixed modulation.
The symbol coordinates modulate the amplitude and phase of the in-phase andquadrature carriers and the QAM modulation signal is formed.In the receiver (Fig. 1), the original signal is restored by the quadrature demodulation from thefrequency of the carrier and low pass filtering using filters matched to the shape of the transmittedpulses. At the output of the demodulator, a series of complex values x(n) carrying information aboutthe amplitude and phase of the transmitted sequence of symbols is created. The complex signal x(n)passes through the adaptive filter of equalizer, eliminating InterSymbol Interference (ISI), and reachesa detector, wherein the estimates of the received symbols d(n) are determined. Each symbol is replacedby a group of bits representing it (from 2 to 7 bits), according to the defined constellation.
In this way,the bit stream is created. Then, the bit stream passes through a descrambler (the system correspondingto the scrambler in the transmitter). The last stage of processing is decompressing of the image.