Performance of a MIMO system with overlay pilots

A multiple-input multiple-output (MIMO) indoor radio system is studied to identify the origin of a typical performance degradation. When the data and overlay pilot-sequences for the channel estimation are transmitted at the same time, an error floor at high signal-to-noise ratio is normally observed. The floor is caused by channel estimation errors due to the interfering data signal. The cross-talk between the data paths can be calculated, approximately, and it is shown that the shape of the bit error curves can be steered both with the amplitude ratio η between pilot and data signals and with the length L of the sequences. Near-optimum performance can be reached in this way. With L = 16383, for instance, an amplitude ratio of η ≥ 0.15 (0.3) is sufficient for BPSK (16-QAM) modulation in a MIMO system with 8 transmit and 12 receive antennas.