Dynamic subcarrier allocation with transmit diversity for OFDMA-based wireless multicast transmissions

We consider the subcarrier allocation problem for multicast transmissions in the downlink for an OFDM-based wireless network. In conventional multicast transmissions, the data rate of a multicast group is dictated by the achievable data rate of the worst case user whose channel condition is the poorest. We develop a new technique called subcarrier pair allocation (SPA) to allocate OFDM subcarriers to multicast groups by exploiting the frequency diversity present in an OFDM system. In the proposed technique, subcarriers are grouped into pairs of two; two identical copies of data are transmitted on each pair of subcarriers. For each user, the achievable data rate is the maximum of the achievable rates between the two subcarriers. The data rate of a multicast group is therefore the minimum of these maximum rates. We derive an expression for the transmission capacity of a multicast group in terms of the group size and the number of groups. Numerical results show that the capacity can be significantly increased by the SPA scheme, especially when the SNR is low and there are a large number of users in a multicast group. In addition, while the transmission capacity saturates as the number of users increases under conventional multicast transmissions, the capacity does not saturate for at least up to a moderate number of users, e.g., 30 users under the proposed scheme.