Resource Allocation for Downlink Cellular OFDMA Systems: Part II - Asymptotic Analysis and Applications

In a companion paper (see Resource Allocation for Downlink Cellular OFDMA Systems: Part I— Optimal Allocation), we characterized the optimal resource allocation in terms of power control and sub-carrier assignment, for a downlink OFDMA system impaired by multicell interference. In our model, the network is assumed to be one dimensional (linear) for the sake of analysis. We also assume that a certain part of the available bandwidth is likely to be reused by different base stations and that the other part of the bandwidth is shared in an orthogonal way between the different base stations. The optimal resource allocation characterized in Part I is obtained by minimizing the total power spent by the network under the constraint that all users’ rate requirements in nats/s/Hz are satisfied. It is worth noting that when optimal resource allocation is used, any user receives data either in the reused bandwidth or in the protected bandwidth, but not in both (except for at most one pivot user in each cell). In the present paper, we study the asymptotic behavior of the optimal resource allocation as the number of users in each cell tends to infinity. Our analysis allows to characterize the performance of the optimal resource allocation algorithm and to determine the asymptotically optimal pivot-position. As a byproduct of our analysis, we propose a decentralized suboptimal resource allocation algorithm with low complexity and prove that the proposed algorithm is asymptotically optimal when the number of users tends to infinity. Finally, we characterize the optimal value of the frequency reuse factor. Simulations sustain our claims and show that substantial performance improvements are obtained when the optimal value of the frequency reuse factor is used.