Equalization of a dynamic channel with forward error correction using an adaptive precoder

In high-speed communications systems the task of channel equalization can be quite demanding. One of the most common and most powerful equalization algorithms is the Decision Feedback Equalizer (DFE). In certain cases though, the DFE is not powerful enough to give the required performance margin on its own, and a Forward Error Correction (FEC) scheme must be employed to give an extra boost in performance. The introduction of the FEC in itself creates problems for the DFE which is compensated for by the use of a precoder. In steady state, this does not affect the overall performance of the system, but in a situation where the channel is subject to change, the precoding scheme runs into difficulty. This paper describes a modification to the conventional fixed precoder to allow the precoder to adapt slowly in a dynamic channel environment. Also shown here is a new method of dealing with a dynamic channel by maintaining an active DFE in the receiver, purely for the purpose of adapting the precoder coefficients during runtime. The technique is applied to Digital Subscriber Line (DSL) transmission at 1.544 Mbit/s, using coded 16-PAM modulation.