  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
cancelation 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.
