Techniques for Adaptive Equalization of Digital Communication Systems

A previous paper described a simple adjustment algorithm which could be employed to set the tap gains of a transversal filter for the equalization of data transmission systems. An automatic equalizer was shown which used this algorithm during a training period of test pulse transmission prior to actual data transmission. The present paper extends the utility of this automatic equalization system by permitting it to change settings during the data transmission period in response to changes in transmission channel characteristics. Three schemes for accomplishing this adaptive equalization without the use of test signals are described and evaluated analytically. The first such scheme uses periodic estimates of channel response based on the received data signal to adjust or update the transversal filter settings. The second system is entirely digital and employs a sequential testing procedure to make adjustments aperiodically as they are required by changing conditions. The third system uses information obtained from a forward-acting error correction system for the purposes of adaptive equalization. Of the three systems described, the second is not only theoretically superior, but is practically the simplest. Experimental results for this second system are described.