Spatial Multiplexing of OFDM Signals With QPSK Modulation Over ESPAR

An electronically steerable parasitic array radiator (ESPAR) transmitter, employing one active and one parasitic antenna element, which can simultaneously transmit two orthogonal frequency division multiplexing (OFDM) signals with quadrature phase shift keying (QPSK) modulation via spatial multiplexing, is developed. In particular, a minimum mean square error (MMSE) scheme for quantizing the parasitic load impedance is developed to reduce the number of distinct load impedance values that should be realized for spatial multiplexing over ESPAR. To this end, the probability density function of the parasitic load impedance is derived, and the realizable region of the load impedance is analyzed. It is observed that the resistance of the active antenna element should be large enough to transmit OFDM signals. The characteristics of two existing ESPAR antennas with different mutual coupling matrices are examined, and it is observed that the one with the larger active antenna resistance is suitable for OFDM transmission. The bit error rate (BER) and the receive error vector magnitude performances of the proposed ESPAR systems with 6/8-bit quantizers and zero-forcing (ZF) receiver are compared with those of the corresponding ideal multiple-input multiple-output (MIMO) system through computer simulation. The results demonstrate that the performances of the proposed ESPAR with the 8-bit quantizer and the ideal MIMO systems can be almost identical, whereas the proposed ESPAR with the 6-bit quantizer exhibits some performance loss.