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With no CSI at the users, transmission over the two-user Gaussian MultipleAccess Channel with fading and finite constellation at the input, is notefficient because error rates will be high when the channel conditions arepoor. However, perfect CSI at the users is an unrealistic assumption in thewireless scenario, as it would involve massive feedback overheads. In thispaper we propose a scheme which uses only quantized knowledge of CSI at thetransmitters with the overhead being nominal. The users rotate theirconstellation without varying their transmit power to adapt to the existingchannel conditions, in order to meet certain pre-determined minimum Euclideandistance requirement in the equivalent constellation at the destination. Theoptimal modulation scheme has been described for the case when both the usersuse symmetric M-PSK constellations at the input, where $ M=2^\lambda $, $\lambda $ being a positive integer. The strategy has been illustrated byconsidering examples where both users use QPSK or 8-PSK signal sets at theinput. It is shown that the proposed scheme has better throughput and errorperformance compared to the conventional non-adaptive scheme, at the cost of afeedback overhead of just $\lceil \log_2(\frac{M^2}{8}-\frac{M}{4}+2)\rceil + 1$ bits, for the M-PSK case.

An Adaptive Modulation Scheme for Two-user Fading MAC with Quantized Fade State Feedback