Non-orthogonal multiple access in a radio over fiber link based on optical-domain power allocation

Non-orthogonal multiple access (NOMA) in a radio over fiber (RoF) link based on optical-domain power allocation is proposed and demonstrated. The NOMA is implemented at the RoF transmitter where two spectrum-overlapped microwave vector signals with an identical power level are modulated on an optical carrier to generate two orthogonally polarized optical signals. By passing the optical signals through a polarization controller (PC) and a polarizer, the power levels of the two optical signals are controlled to achieve optical power allocation (OPA). The optical signals are then transmitted over a fiber to the receiver. Since the power levels of the two microwave vector signals applied to the modulator are identical and the power allocation is implemented in the optical domain, the nonlinearity due to the higher-power input microwave vector signal is reduced, leading to an increase in the dynamic range. At the receiver, the two optical signals are detected at a photodetector (PD). To demultiplex the two microwave vector signals, a digital signal processing (DSP) algorithm is developed. The proposed approach is evaluated experimentally. The results show that the transmission performance in terms of error vector magnitude (EVM) is improved thanks to the increased dynamic range.