With no CSI at the users, transmission over the two-user Gaussian Multiple
Access Channel with fading and finite constellation at the input, is not
efficient because error rates will be high when the channel conditions are
poor. However, perfect CSI at the users is an unrealistic assumption in the
wireless scenario, as it would involve massive feedback overheads. In this
paper we propose a scheme which uses only quantized knowledge of CSI at the
transmitters with the overhead being nominal. The users rotate their
constellation without varying their transmit power to adapt to the existing
channel conditions, in order to meet certain pre-determined minimum Euclidean
distance requirement in the equivalent constellation at the destination. The
optimal modulation scheme has been described for the case when both the users
use symmetric M-PSK constellations at the input, where M=2Ξ», Ξ» being a positive integer. The strategy has been illustrated by
considering examples where both users use QPSK or 8-PSK signal sets at the
input. It is shown that the proposed scheme has better throughput and error
performance compared to the conventional non-adaptive scheme, at the cost of a
feedback overhead of just βlog2β(8M2ββ4Mβ+2)β+1 bits, for the M-PSK case.Comment: 12 pages; 11 figure