Path selection for spatial multiplexing gain with multiple relay stations in MIMO-OFDMA systems

The LOS components of the base station (BS) can provide a high SNR to mobile stations (MSs), but they cannot achieve a high spatial multiplexing gain in the MIMO-OFDMA system. The spatial multiplexing gain is obtained by degrees of freedom between MIMO antenna arrays. MIMO channels should be uncorrelated to achieve the maximum degrees of freedom, and it depends on rich scattering environments which are needed in making the channel spatially selective. For this environments, we deploy multiple relay stations (RSs) that can provide uncorrelated transmit antenna paths, called "paths" to each MS. In this paper, we select proper paths from BS and RSs to achieve the maximum degrees of freedom with the desired SNR under a LOS MIMO channel. To do this, the proposed scheme selects paths by the order of decreasing SNR among uncorrelated paths until the maximum degrees of freedom. By the simulation, we compare the proposed scheme to the system with and without RSs. We observe that the proposed scheme achieves a higher throughput of 33% and 53% for 50 MSs than the system with and without RSs, respectively.