Arbitrary signal transmission using an ESPAR antenna

Electronically steerable parasitic array radiator (ESPAR) antennas have been proposed to obtain multiple antenna functionality with only a single radio frequency (RF) chain. However, some signals cannot be transmitted using the ESPAR antennas (EA), because for those signals, the real part of the input impedance (RII) of the EA becomes negative. A negative input impedance implies that the EA cannot be realized and even if it is, it will not transmit that signal. In this work, we obtain approximate signals, which are close to the ideal signals in the mean square error (MSE) sense, for transmission that satisfy the requirement on input impedance. Specifically, we formulate and solve an optimization problem to minimize the MSE between the ideal and the approximate signals taking into account the constraint on the input impedance. We obtain closed-form expression for the approximate currents vector, thus, simplifying the processing at the transmitter. Our simulation results show that the EA system, transmitting the approximate signal, performs nearly the same as the standard multiple RFs antenna (SMA) system.