Use of linear programming for dynamic subcarrier and bit allocation in multiuser OFDM

An adaptive subcarrier allocation and an adaptive modulation for multiuser orthogonal frequency-division multiplexing (OFDM) are considered. The optimal subcarrier and bit allocation problems, which are previously formulated as nonlinear optimizations, are reformulated into and solved by integer programming (IP). A suboptimal approach that performs subcarrier allocation and bit loading separately is proposed. It is shown that the subcarrier allocation in this approach can be optimized by the linear-programming (LP) relaxation of IP, while the bit loading can be performed in a manner similar to a single-user OFDM. In addition, a heuristic method for solving the LP problem is presented. The LP-based suboptimal and heuristic algorithms are considerably simpler to implement than the optimal IP, plus their performances are close to those of the optimal approach.