Optimal power control for discrete-rate link adaptation schemes with capacity-approaching coding

In wireless communications, bandwidth is a scarce resource. By employing link adaptation we achieve bandwidth-efficient wireless transmission schemes. We propose a variable-power transmission scheme for slowly varying flat-fading channels using a fixed number of codes. Assuming that capacity-achieving codes for AWGN channels are available, the proposed power adaptation scheme maximizes the average spectral efficiency (ASE) for any finite number N of available rates. We show that the power adapted transmission scheme, using just four different rates, achieves a spectral efficiency within 0.15 bits/s/Hz of the Shannon capacity for continuous rate and power adaptation. Further, when restricted to N optimally chosen rates, introducing power adaptation has significant ASE and outage probability gains over a constant power scheme.