Application of randomization techniques to space-time convolutional codes

Abstract

This correspondence introduces a new approach to design space-time convolutional codes (STCCs) with large constellation size in systems with any number of transmit antennas. Our design procedure is based on utilizing quadrature phase-shift keying (QPSK) STCCs as component codes, and, consequently, unlike existing techniques, the search space does not grow exponentially with the constellation size. Our approach is further based on the fact that an n x m multiple-input multiple-output (MIMO) system is equivalent to n distinct 1 x m systems. By employing a common design for each individual 1 x m system, we arrive at an approach whose complexity does not grow with the number of transmit antennas. To describe our approach, we first demonstrate that a system employing an STCC can be implemented with only a single transmit antenna when there are multiple receive antennas. The idea is to transmit more than one symbol from a single transmit antenna during a symbol period by superimposing the encoded symbols on top of each other. This objective is achieved by inducing randomness into the system, that creates additional channel paths, called virtual paths. The design of the distributions of the induced random variables is studied for slow Rayleigh and Rician fading channels by utilizing an upper bound on the pairwise block error probability. Simulation results evaluate the performance of this technique for the case of two transmit antennas and several different number of receive antennas, a spectral efficiency of 4 b/s/Hz for slow Rayleigh and Rician fading channels.