Outage in adaptive wireless communication systems

Adaptive signaling schemes are derived that attempt to minimize the outage probability for wireless systems with strict delay constraints, under the assumption of perfect causal channel state information (CSI) at the transmitter and receiver. Using the channel characterization and delay constraint parameters imposed by the application, a recursive set of equations is derived for the outage probability in the case of constant-power adaptive modulation. This set of equations is used to derive an optimal constant-power adaptive uncoded quadrature amplitude modulation (QAM) scheme, and its performance is compared against that of traditional adaptive uncoded techniques designed to maximize average rate subject to a bit error probability constraint. A suboptimal scheme is also presented. Numerical results indicate that the proposed uncoded adaptive schemes significantly outperform adaptive schemes designed to maximize average rate. We also overview results on adaptive coded modulation obtained by Kamath (see Thesis, University of Massachusetts at Amherst, September 2001), which suggest that long-term power adaptation is necessary to achieve gains in outage probability versus non-adaptive coded schemes.