An Investigation of Dynamic Subcarrier Allocation in MIMO–OFDMA Systems

In this paper, orthogonal frequency-division multiple-access (OFDMA) systems with dynamic deterministic (as opposed to pseudorandom) allocation of subcarriers to users to exploit multiuser diversity are investigated. Previously published work on dynamic multiuser subcarrrier allocation for OFDMA systems with single-input-single-output (SISO) channels are surveyed. A near-optimal low-complexity algorithm for SISO systems, which is structurally similar to the algorithm by Rhee and Cioffi, is extended to the case of multiple-input-multiple-output (MIMO) systems in this paper. The optimality and adaptability of this algorithm are analyzed by formulating an assignment problem and comparing with one optimal and two extended suboptimal strategies proposed based on previous work. Consideration of a MIMO channel creates further issues for the subcarrier-allocation process. In particular, methods whereby an appropriate subcarrier allocation may be exploited to minimize the effects of correlation in MIMO channels are of considerable interest. Several novel variants of the algorithm (referred to as ldquoschemesrdquo) are proposed and evaluated for MIMO systems employing both space-time block coding (STBC) and spatial multiplexing (SM) in both uncorrelated and correlated fading channels. Simulation results identify the most suitable schemes for both STBC and SM and, in particular, show that substantial improvements in performance (in terms of bit-error rate) in correlated channels can be achieved by means of suitable subcarrier allocation. In uncorrelated channels, the best scheme can offer approximately 7-dB gain over the conventional MIMO channel; in highly correlated channels, even more substantial improvements (>11-dB gain for STBC, >20-dB gain for SM) in performance can also be achieved, demonstrating the ability of a well-designed subcarrier-allocation scheme to mitigate the debilitating effects of correlation on MIMO systems.