Towards Equalization of Mixed Multi-user OFDM Signals Over a Doubly-Dispersive Channel

This work explores the recovery of mixed signals, which have been transmitted by multiple users over a doubly-dispersive channel using OFDM modulation, and received by a multi-antenna system. We consider multiple users transmitting QPSK signal blocks over a shared OFDM frequency band and consider two cases, with respect to the location of signal data sources. The first case will involve collocated users transmitting blocks of QPSK signals to be received by the multi-antenna system, and the second case will involve non-collocated mobile users transmitting QPSK signal blocks to the multi-antenna system. On each OFDM subcarrier, transmitted symbols from multiple users are mixed thereby providing a low probability of interception (LPI) scenario. A modified OFDM receiver architecture is proposed and used to separate and recover these user signals at the base station. A Least square equalizer is applied to the signal data blocks at the receiver. These received signal blocks have also been corrupted by additive white Gaussian noise (AWGN) and inter-symbol interference (ISI) after transmission through a Rayleigh fading Doppler channel. The channel state information is estimated using pilots, and this gives a baseline performance of the proposed multi-user OFDM receiver.