Dynamic subchannel and bit allocation for 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 be decoded at the receiver, the network throughput can be increased by performing subcarrier/bit allocation. In this paper, in order to increase network throughput, we develop the optimum subcarrier/bit allocation method that maximizes the sum of data rate of all the users employing integer programming (IP) which is NP-hard problem. To reduce the complexity, suboptimum two-step algorithm is proposed: firstly, subcarriers are allocated to users under the assumption that the same power is distributed to each subcarrier; in the second step, the number of bits loaded to each subcarrier is determined using the modified Levin-Campello algorithm. Numerical results show that the performance difference between the optimum and suboptimum algorithms is within about 5%, and that total throughput of the proposed algorithm is larger than that of the lowest channel gain (LCG) method where modulation is determined to serve the user with the lowest channel gain.