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

TL;DR

This paper introduces an adaptive modulation scheme for a two-user fading MAC that uses quantized CSI feedback to optimize constellation rotation, improving throughput and error performance without requiring perfect CSI.

Contribution

The paper proposes a novel adaptive modulation scheme utilizing quantized CSI feedback for two-user fading MACs, enhancing performance with minimal feedback overhead.

Findings

Improved throughput over non-adaptive schemes.

Enhanced error performance with the proposed scheme.

Efficient feedback overhead of logarithmic bits.

Abstract

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=2^\lambda $, $ \lambda $ 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 $\lceil \log_2(\frac{M^2}{8}-\frac{M}{4}+2)\rceil + 1 $ bits, for the M-PSK case.