Examination of an adaptive DFE and MLSE/near-MLSE for fading multipath radio channels

The authors examine the performance over an HF channel of two adaptive equalisers; the decision feedback equaliser (DFE) and the maximum likelihood sequence estimator (MLSE), or an approximation to it (near-MLSE). It is shown that the superiority of the more complex MLSE/near-MLSE over the DFE is not great, and that this advantage is only due to error propagation in the DFE. In the absence of any special arrangement by which the receiver can re-align itself (e.g. periodic training), 16-QAM appears to be the highest feasible signal constellation size. With the periodic insertion of known symbols into the data stream, it is shown that performance can be maintained near the level for which decision errors do not affect the tracking algorithm. With the facility of a request-for-training link to the transmitter, it is shown that performance can be significantly improved for all constellation sizes, with only a small loss in the useful data rate.<>