A comparison of adaptive equalization methods for multiuser detection in OFDM-SS systems

Data transmission over HF channels is subject to phenomena of dispersion, absorption and multiple paths. Performance may be improved by using orthogonal frequency division multiplexing spread spectrum (OFDM-SS) systems. In OFDM-SS systems, transmitting data over slowly varying multi-path channels, both intersymbol interference (ISI) and multiple-access interference (MAI) arise and should therefore be mitigated. Two adaptive nonlinear equalization architectures based on the minimum mean-square error (MMSE) criterion are proposed and examined over such channels. The first receiver architecture is for single-user detection (SUD) consisting of a multiple-layer feed-forward filter (ML-FFF) and a feedback filter (FBF). The ML-FFF acts as a multipath diversity combiner as well as canceller of precursor ISI and main-cursor MAI while the FBF acts as a canceller of post-cursor ISI. The second receiver architecture is for multiuser detection (MUD) where in addition to the first structure, it employs a centralized FBF (CFBF) for cancelling the MAI of previous symbols. Results indicate that an enhancement in capacity is obtained by the multi-user structure over the single-user structure. Additionally, it is demonstrated that both structures perform multipath energy combining (RAKE-like) when the delay spread is in excess of a symbol interval.