LDPC-based spaced-time coded OFDMA systems and opportunistic beamforming

We consider a space-time coded OFDMA (orthogonal frequency division multiple access) system, with two or more antennas at the transmitter and only one at each receiver. A technique, called opportunistic beamforming (OBF) is used to improve the throughput of the system. In this schema, the antennas at the transmitter can be induced to modulate the signals with pseudo-random beamforming coefficients. The main purpose of that is to increase the diversity gain channels at the users. The antennas used for this system are called “dumb” antennas. Then, to reduce errors during transmission, we propose a low-density parity-check (LDPC) codes for this system. The LDPC codes can exploit the spatial-diversity and the selective-fading diversity channels in wireless communications. Including, these kind of codes offer low-complexity at the receiver and scalability to the system. Each receiver employs a maximum a posteriori expectation-maximization (MAP-EM) demodulator and a soft LDPC decoder.