Improved space-time coding for multiple antenna multicasting

Abstract

Multicasting is the general method of conveying the same information to multiple users. For multicasting applications over multiple-input multiple-output (MIMO) channels, it is of great importance to ensure that all users experience an adequate level of performance while utilizing the transmit diversity available in MIMO channels. One technique, called max-min beamforming, provides a way to increase the received signal-to-noise ratio (SNR) of the worst-case user. However, due to its limited degrees of freedom, this method suffers severe performance degradation when the number of users increases. Alternatively, space-time codes can be employed to transmit data simultaneously to all users. By introducing additional degrees of freedom in the time domain, open-loop orthogonal space-time block coding (OSTBC) schemes achieve better performance than max-min beamforming when the number of users is large. In this paper, a precoded OSTBC scheme is proposed that optimizes the performance of the worst-case user. By pre-multiplying the OSTBC codeword by a precoding matrix, the transmitter can adapt to current channel conditions which results in a higher worst-case receive SNR. Simulation results demonstrate that the proposed scheme performs universally better than open-loop OSTBC schemes and outperforms the max-min beamforming scheme for a large number of users.