Channel Estimation and Equalization of Mixed Signals for a MIMO-OFDM Multiple- Access System

In this work we consider the problem of recovering a set of 16-QAM symbols that have been simultaneously transmitted by multiple users to a base station over a Rayleigh fading AWGN channel, using OFDM modulation. At the base station multiple antennas receive a mixture of the transmitted user symbols. We present a pilot-aided non-orthogonal multiple access (NOMA) OFDM scheme, which provides an estimate of the channel frequency response (CFR) between each user and each receive antenna. Our proposed scheme has been implemented and demonstrated for a MIMO communication system with two to four users, and varying numbers of receive antenna. The SER performance of the MIMO scheme is evaluated based on running Monte Carlo simulations over varying SNRs, specifically for a two-user system transmitting to five receive-antenna. We outline a method to determine the minimum and the least upper bound on the number of pilot blocks, which is required to effectively estimate the channel frequency response (CFR) with this scheme. We also investigate the performance of the equalization scheme for the MIMO-OFDM system with different numbers of OFDM subcarriers.