Orthogonal Differential Modulation for MIMO Multiple Access Channels with Two Users

This paper proposes a differential encoding scheme and several differential decoding schemes for a two-user MIMO system based on orthogonal space-time block codes (OSTBCs) when neither the transmitter nor the receiver knows the channel. We derive low complexity differential decoders for users with two transmit antennas based on three different decoding methods. We deploy dynamic programming in our methods to maintain a low decoding complexity. Simulations demonstrate that our differential schemes using the proposed low complexity decoders provide full transmit diversity. We also present additional differential decoding schemes that achieve full diversity, perform significantly better than the existing schemes, and work for any square OSTBC, but need higher decoding complexity. The proposed schemes work for any number of receive antennas. Moreover, we analyze the diversity of the proposed schemes. To the best of our knowledge, our low complexity schemes are the first low complexity differential schemes for multi-user MIMO systems.