Effective Service Capacity Analysis Of Opportunistic Multi-Carrier Ofdma Systems

Exact queuing-theoretic modeling of wireless systems is tough due to the complex service processes that arise from the interaction of the wireless channel with state-of-the-art signal processing algorithms. Nevertheless, with the rise of video applications like streaming and telephony in upcoming fourth generation cellular networks, such analysis is important for devising admission control strategies. In this paper we apply recent insights into approximation approaches for queuing systems to opportunistic OFDMA systems and study different approaches for applying the modulation type to OFDM subcarriers. In particular, we provide analytical closed-form expressions for the delay distribution of three opportunistically scheduling OFDMA systems (adaptive modulation, link adaptation per terminal, link adaptation over all subcarriers). After validating these expressions, we evaluate the different system designs numerically. We find that although adaptive modulation systems have a higher complexity, their queuing performance is only slightly superior to systems with link adaptation per terminal. Furthermore, the systems feature in general an optimal number of terminals that can be supported in the cell, leveraging multi-user diversity with limited available resources. Nevertheless, these optima change from scheme to scheme.