Intra-Frame Transmission Adaptation for Fast Fading MIMO-OFDMA Systems

We propose multiple antenna transmission methods for fast fading channel conditions. Proposed methods allocate alternative multiple antenna transmission modes depending on the SNR, modulation order and Doppler frequency to increase reliability, i.e., decrease bit-error-rate (BER). A major difference of our approach from previous works is to allow different transmission modes during a single frame by considering the channel variations within the frame for high mobility scenarios. First method effectively makes use of available channel knowledge by allocating beamforming (BF) as long as channel knowledge is not outdated and switches to space frequency block coding (SFBC) afterwards. Approximate BER expressions for BF and SFBC that are functions of SNR, modulation order, Doppler frequency and initial channel knowledge are derived to be used as a decision metric for mode allocation throughout the frame. Second method adapts rate and transmission mode across symbols in a frame by considering that average channel power is monotonically decreasing as a function of time within the frame when multiuser diversity is exploited. First method performs as good as the better of BF and SFBC over all SNR values. Second method provides additional performance gain over the first method due to more efficient use of better channel conditions.