Code Design for Non-Coherent MIMO-OFDM Systems

Recently, the use of coherent space-frequency coding in Orthogonal Frequency Division Multiplexing (OFDM)-based frequency-selective multiple-input multipleoutput (MIMO) antenna fading channels has been proposed [1, 2]. Acquiring knowledge of the fading coefficients in a MIMO channel is already very challenging in the flat-fading case. In the frequency-selective case, estimating the MIMO channel becomes significantly more difficult due to the presence of multiple paths, which results in an increased number of channel coefficients. In this paper, we address code design for non-coherent frequency-selective MIMO-OFDM fading links, where neither the transmitter nor the receiver knows the channel. We derive the design criteria and quantify the maximum achievable diversity gain. We demonstrate that unlike in the coherent case, non-coherent space-frequency codes designed to achieve full spatial diversity in the flat-fading case can fail completely to exploit not only frequency diversity but also spatial diversity when used in frequency-selective fading environments. Such codes are termed “catastrophic”. Finally, we provide explicit constructions of full-diversity (space and frequency) achieving codes and assess their performance through simulation results.