Resource allocation for sum-power minimization in multiuser OFDM with user rate constraints

This paper studies the resource allocation optimization problem under user rate constraints in a single cell multiuser orthogonal frequency division multiplexing system. Since the optimization problem does not have a convexity structure, it is numerically very difficult to solve and the optimal solution is computationally very expensive. We propose two efficient suboptimal solutions, i) based on Lagrange dual decomposition (LDD), and ii) based on separating the subcarrier and power allocation. The former solution employs subgradient method to optimize the dual variables. When the number of subcarriers is very large, it is shown that the solution obtained from LDD is optimal. However, because of non-convexity of the original optimization problem, the optimality is not guaranteed. The latter approach disjoints the subcarrier and power allocation. The subcarrier allocation is performed based on proportional data rate requirements, and finally optimal power allocation is performed under given subcarrier allocation. For both of the solutions, the achieved objective values are very close to the optimal solution, and have lower computational complexity.