Complexity analysis and heuristic algorithms for radio resource allocation in OFDMA networks

In this paper, we address the problem of allocating users to radio resources (i.e. subcarriers) in the downlink of an OFDMA system. In particular, we consider a multi-format resource allocation problem (MF-RAP) in which the link adaptation adjusts the spectral efficiency for each user-subcarrier pair, i.e. for each radio link, in order to minimize the total transmission power while fulfilling a rate request for each user. We propose an integer linear programming (ILP) formulation of the problem and exhaustively discuss the computational complexity. Specifically, we prove that the problem is NP-hard in the strong sense and demonstrate that it is hard to be approximated in polynomial time within a constant factor. Hence, we present heuristic approaches that achieve reasonably good solutions in the general case. Computational experiences show that, in comparison with a commercial state-of-the-art ILP optimization solver, the proposed algorithms are effective in terms of solution quality and CPU times.