The effect of spatial axial ratio variation on QPSK modulation encoded using orthogonal circularly polarized signals

In this work, we show a new method which can directly encode an I (In-phase) and Q (Quadrature-phase) baseband data stream onto a circularly polarized (CP) microwave signal. Here I and Q components are fed directly into a two port CP antenna in order to yield right hand circular polarized (RHCP) or left hand circular polarized (LHCP) waveforms each encoded with zero or 180° phase shift in order to effect QPSK modulation. The CP signal from such an antenna will have deterministic unequal amplitude and phase shifts at different far field spatial locations which if not compensated will degrade the bit-error-rate (BER) performance of the receiver. Simulation and experimental results show the BER performance of the receiver when these two orthogonal CP components have spatially induced phase differences or amplitude imbalances between them. The technique described allows a factor two reduction over the normal QPSK requirement by virtue of the polarization diversity inherent in the scheme.