An investigation of carrier recovery techniques for space-time coded MIMO wireless systems

In this paper we investigate and propose two methods for pilot-tone-aided carrier recovery in space-time coded systems with transmit and receive diversity for high-rate applications. We assume that due to mismatches in local oscillators, there is a small relative frequency and phase offset between the two transmit antennas. The first approach relies on an FFT for initial carrier frequency and phase recovery at the receiver, plus a simple decision-directed circuit for estimating initial phase error and canceling accumulative phase error due to carrier frequency estimation. This is a non-tracking technique that prevents accumulation of phase error and has no burst length limitation. The second technique also uses an FFT for initial estimation, but uses a DPLL for tracking at the receiver, and thus requires lower FFT resolution (as few as 64 points). Simulations results are shown for a (2,2) spatial DEMUX system with two transmit and two receive antennas. The simulation results show that the latter technique is more robust. With smaller FFT and shorter pilot duration, the estimated phase error due to low resolution is resolved by use of a reduced constellation algorithm. All simulations assume a narrow-band channel for simplicity, but these results can also apply to broadband channels, using a different space-time coding algorithm.