Efficient algorithm for proportional fairness scheduling in multicast OFDM systems

In multicast orthogonal frequency division multiplexing (OFDM) systems, the difference in link conditions of users complicates adaptive modulation because modulation should be adjusted to serve the user who experiences the worst channel condition. If we assume that the multicast data are separated into layers and any combination of the layers can he decoded at the receiver, the network throughput can be increased by performing subcarrier/bit allocation [C. Suh et al., 2004]. In addition, if we consider the concept of proportional fairness (PF), the fairness factor can be increased while minimizing total throughput degradation. In this paper, we formulate the optimization problem for PF scheduling and show that this problem is NP-hard one requiring large complexity. To reduce the complexity, we propose a simple heuristic algorithm for PF scheduling by separating subcarrier allocation and bit loading. Numerical results show that the performance difference between the optimum and proposed algorithms is within about 5%, and that PF scheduling may be the best solution for multicast scheduling if we consider both total throughput and fairness.