Techniques for Adaptive Equalization of Digital Communication Systems

A previous paper 1 has dealt with the problem of automatic equalization for data transmission systems. In that paper, it was assumed that a finite-length transversal filter was to be used to correct the pulse response of a baseband (VSB) system at the sampling instants. A simple control system was shown which could be used to adjust the tap gains of the transversal filter to optimum positions using a series of test pulses transmitted prior to actual data transmission. After this training period the control system is disconnected, the tap gains remain fixed, 255 256 THE BELL SYSTEM T E C H N I C A L JOURNAL, FEBRUARY 1966 and normal data transmission ensues. This automatic equalization system has been used in conjunction with multilevel vestigial sideband modulation to achieve a rate of 9600 bits-per-second 011 private line voice facilities. 2,3,4 Two limitations of this automatic equalization system are immediately apparent -- it requires that test pulses be transmitted and it must be reset in another training period whenever the channel characteristics change. Other disadvantages of the present mode of operation include the long training period required to establish accurate final settings and the possibility of a nonlinear channel causing the transmission characteristics for data transmission to be slightly different from those for isolated pulse transmission. For these reasons, it has been found advantageous to develop an equalizer capable of deriving its control signals directly from the transmitted data signal itself.