Orthogonal designs optimize achievable dispersion for coherent MISO channels

This work addresses the question of finite block-length fundamental limits of coherently demodulated multi-antenna channels, subject to frequency non-selective isotropic fading. Specifically we present achievability bound for the channel dispersion - a quantity known to determine the delay required to achieve capacity. It is shown that a commonly used isotropic Gaussian input, which is only one of many possible capacity achieving distributions, is suboptimal. Optimal inputs minimizing channel dispersion turn out to include a family of modulation techniques known as orthogonal designs (in particular, Alamouti's scheme). For 8 transmit antennas numerical evaluation shows that up to 40% of additional penalty in delay is incurred by using isotropic codewords (compared to dispersion-optimal architecture exploiting transmit diversity).