Skip to Main Content
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.