Dynamic subchannel and bit allocation in multiuser OFDM with a priority user

We consider multiuser orthogonal frequency division multiplexing (OFDM) with adaptive subcarrier allocation and adaptive modulation, especially when there is one priority user who must be provided with a fixed data rate. We develop the optimum subcarrier/bit allocation method that minimizes total transmission power employing integer programming (IP) which is an NP-hard problem. To reduce the complexity, a suboptimum two-step algorithm is proposed: firstly, subcarriers are allocated to the priority user and then the remaining subcarriers are distributed to other users considering the best channel gain for each subcarrier; in the second step, using the Levin-Campello algorithm, the bits are loaded into the priority user and the other users separately. Numerical results show that total transmission power of the proposed optimum/suboptimum algorithms is significantly smaller than that of fixed modulation. In addition, the difference of total transmission power between the optimum and suboptimum algorithms is within about 0.5 dB when the number of subcarriers is 64 and the required data rate of the priority user is identical to the average required data rate of each user.