A Continuously Adaptive Equalizer for General-Purpose Communication Channels

Automatic equalization techniques have done much to alleviate the deleterious effects of linear distortion in communication channels.1 In particular, equalization schemes such as that suggested in Ref. 2 have permitted significantly increased transmission speeds and better error performance for linear, synchronous data transmission systems. The same improvements can be obtained for arbitrary information transmission systems (not necessarily synchronous or linear) through the 1865 1882 THE BELL SYSTEM TECHNICAL JOURNAL, JULY-AUGUST 190!) use of the generalized equalization techniques described in Ref. 3. The equalization problem becomes more complicated when the linear distortion in the communication channel becomes time varying. In the time-varying case an adaptive equalizer is needed; such an equalizer for the synchronous, linear information transmission case has been discussed in the literature. 4 This paper describes a continuously adaptive equalizer which can be used with any information transmission system: synchronous or asynchronous, using linear or nonlinear modulation. The continuously adaptive equalizer, described here, has the advantage over that described in Ref. 3 that the communication channel is under continuous surveillance by the equalizer controller. If the frequency characteristic of the transmission path should change, the equalizer would immediately begin to compensate for that change. Such a correction would be made regardless of the format being used for information transmission in the channel and independent of whether the channel were being used for information transmission.