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 crosstalk 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 eta 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 eta >or= 0.15 (0.3) is sufficient for BPSK (16-QAM) modulation in a MIMO system with 8 transmit and 12 receive antennas.