We propose several differential decoding schemes for asynchronous multi-user MIMO systems based on orthogonal space-time block codes (OSTBCs), where neither the transmitters nor the receiver has knowledge of the channel. First, we derive novel low complexity differential decoders by performing interference cancellation in time and employing different decoding methods. The decoding complexity of these schemes grows linearly with the number of users. We then present additional differential decoding schemes that perform significantly better than our low complexity decoders and outperform the existing synchronous differential schemes but require higher decoding complexity compared to our low complexity decoders. The proposed schemes work for any square OSTBC, any constant amplitude constellation, any number of users, and any number of receive antennas. Furthermore, we analyze the diversity of the proposed schemes and derive conditions under which our schemes provide full diversity. For the cases of two and four transmit antennas, we provide examples of PSK constellations to achieve full diversity. Simulation results show that our differential schemes provide good performance. To the best of our knowledge, the proposed differential detection schemes are the first differential schemes for asynchronous multi-user systems.

Differential distributed space-time coding (D-DSTC) has been considered to improve both diversity and data-rate in cooperative communications in the absence of channel information. However, conventionally, it is assumed that relays are perfectly synchronized in the symbol level. In practice, this assumption is easily violated due to the distributed nature of the relay networks. This paper proposes a new differential encoding and decoding process for D-DSTC systems with two relays. The proposed method is robust against synchronization errors and does not require any channel information at the destination. Moreover, the maximum possible diversity and symbol-by-symbol decoding are attained. Simulation results are provided to show the performance of the proposed method for various synchronization errors and the fact that our algorithm is not sensitive to synchronization error.

We propose a differential encoding scheme and several differential decoding schemes for asynchronous multiuser MIMO systems based on orthogonal space-time block codes (OSTBCs) where neither the transmitters nor the receiver has knowledge of the channel. We derive novel low complexity differential decoders by performing interference cancelation in time and employing different decoding methods. The decoding complexity of these schemes grows linearly with the number of users. The proposed schemes achieve full diversity and work for any square OSTBC, any number of users, and any number of receive antennas. Simulation results show that our differential schemes provide good performance. To the best of our knowledge, the proposed differential detection schemes are the first differential schemes for asynchronous multi-user systems.

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.