On coexistence of unicast and multicast traffic in relay-enabled wireless networks

Relay-enabled wireless networks (eg. WIMAX 802.16j) coupled with orthogonal frequency division multiple access (OFDMA) as the air interface technology represent an emerging trend in future wireless infrastructure deployments. While the flexibility of OFDMA allows accommodation of heterogeneous (unicast and multicast) traffic, its combination with relays opens up a multitude of diversity and spatial reuse gains. However, leveraging these benefits to efficiently handle heterogeneous traffic calls for more sophisticated solutions, among which, user scheduling forms a key component. We ask the following specific question in this work: Can unicast and multicast traffic efficiently coexist in an OFDMA-based wireless relay network? We address the question by first identifying the key challenges that make coexistence difficult in the target environment. We show that scheduling of unicast and multicast traffic is tightly coupled even if they are isolated through orthogonal spectral allocations. We also show that the nature of gains that optimize the individual traffic are complementary and hence reveal a fundamental tradeoff in jointly optimizing the system for both unicast and multicast traffic in tandem. Using the insights gained, we propose an integrated scheduling strategy that strikes a good balance in delivering efficient performance to both the unicast and multicast flows. The proposed solution yields gains of over 60% over individual traffic specific strategies.