A Multi-Access RoFSO Communication System Incorporating a Microwave Photonic Channel Aggregator

A multi-access radio-over-free-space optical (RoFSO) link incorporating a microwave photonic channel aggregator (MPCA) to increase the optical spectral efficiency is proposed and experimentally demonstrated. In the proposed system, multiple microwave signals received by multiple antennas are aggregated into a single optical signal to be transmitted through a RoFSO link. The MPCA is implemented by controlling the relative phase and amplitude of the microwave signals through electro-optic modulation and polarization combination, in which multiple microwave signals of lower-order modulation formats are aggregated to a single optical signal having a higher-order modulation format, with an increased optical spectral efficiency. The approach is evaluated experimentally. A RoFSO link consisting of an MPCA, a free space link of 1 m, a fiber link of 5 km, and a coherent optical receiver is established. Two, two, and three BPSK microwave signals are, respectively, aggregated into QPSK, 4APSK, and 8QAM optical signals, all at 1 Gbaud, which are transmitted over the RoFSO link. The signals are recovered at the receiver and the error vector magnitudes (EVMs) are measured to be 6.11%, 6.15%, and 6.40%, respectively. To further increase the spectral efficiency, eight BPSK microwave signals are aggregated into two orthogonally polarized 16QAM optical signals, each at 1 Gbaud. Again, after transmission over the RoFSO link, the signals are recovered and the EVMs for the x and y polarization components are measured to be 6.59% and 6.79%, respectively.