Low-Density MIMO Codes

Multiple antenna systems are promising approaches to increase the data rate of wireless communication systems. There, modulated symbols are multiplexed on multiple transmission antennas. Typically an outer error correcting code is used additionally to ensure a desired quality of service for a given data rate. An appropriate communications performance can be achieved by iterative decoding, where probabilistic information are exchanged between outer decoder and the MIMO demodulator. However, in this case the MIMO demodulator has to calculate probabilistic soft-output values for each bit which can be very complex. In this paper we present a new class of codes named low-density MIMO codes which reduces the complexity of the MIMO soft-demodulator inherently by code design without decreasing the data rate of the overall system. This is achieved by trading-off parity check constraints of the outer code for the benefit of the MIMO soft-demodulation process. With this technique outstanding communications performance can be achieved which are demonstrated for design examples of a 16-QAM 4times4 antenna system. For a quasi-static fading channel we show that we can approach the capacity bound by 1.25 dB at an outage probability of FER = 10^-2 with a codeword length of just 2000 bits. This is to the best of our knowledge the best communications performance reported so far.