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Figure 6: Channel Equalisation.  In order to equalise (or inverse identify the telephone channel) an identical pseudo-random binary sequence is set up at both ends of the telephone call. This is equivalent to the implicit connection shown. If indeed the error does adapt to e(k) = 0 then d(k) = y(k) = s(k-D) where D is the delay of the signal through the channel. Therefore after an initial training mode we can switch to data transmission mode and the adaptive filter will perform (zero-forcing) equalisation. Data equalisation for cable and mobile channel equalisation has a number of varied implementations and additional processing elements to produce decision feedback equalisers, blind equalisers using decision directed mode and so on.