The downlink beamforming weights which can suppress interfering signals toward out-of-cell mobile stations for a TDD-based OFDMA system are introduced. First, the downlink beamforming weights are optimally designed based on iteration. Then, the downlink beamforming weights are designed in a sub-optimal way. It is shown that the sub-optimally designed downlink beamforming weights have the same structure as that of the uplink beamforming weights which are derived based on MMSE. The performance of these schemes is compared based on the average receive SINR. The performance of a heuristic scheme which exploits uplink beamforming weights for downlink beamforming weights is also investigated.

In this letter, the diversity-multiplexing tradeoff (DMT) function for a special half-duplex dynamic decode and forward (DDF) relay protocol using two source-antennas, two destination-antennas, and more than two relay-antennas is derived. It is shown that the performance of the DDF relay protocol can be substantially improved by increasing the relay-antenna number, but only for low multiplexing gains.

The analytical derivation of the diversity-multiplexing tradeoff (DMT) for a half-duplex dynamic decode and forward (DDF) MIMO relay protocol has been regarded as an open problem. Recently, however, a minimization problem setting has been found, the solution of which corresponds to the DMT function for a half-duplex DDF MIMO relay protocol. In this paper, the DMT functions for three special half-duplex DDF MIMO relay protocols using two antennas at two of three nodes, source, relay, and destination nodes, and a single antenna at the other node are derived first. Then, the DMT function for a special half-duplex DDF MIMO relay protocol using two antennas at every node is derived. These DDF MIMO relay protocols are compared with one another and with some NAF MIMO relay protocols by simulation.