Adaptive Modulation in Spectrum-Sharing Systems with Delay Constraints

In this paper, we consider variable-rate variable-power MQAM modulation employed under delay quality-of-service (QoS) constraints over spectrum-sharing channels. In particular, we assume two users sharing the spectrum with one of them having a primary access to the band, and the other, known as secondary user, constrained by interference limitations imposed by the former. We study the performance of the secondary user´s link employing adaptive MQAM modulation scheme when, on top of the above-mentioned interference constraint, the secondary user is also required to satisfy a statistical delay QoS constraint. Considering two modulation schemes, namely, continuous MQAM and discrete MQAM with restricted constellations, we obtain the effective capacity of the secondary user´s link, and derive the optimum power allocation scheme that maximizes the effective capacity in each case. Numerical simulations are conducted to corroborate our theoretical results.