Optical code-division multiple-access embedded with a polarisation diversity scheme for radio-over-fibre transmissions

This study presents a spectral polarisation-coding optical code-division multiple-access (SPC-OCDMA) system for radio-over-fibre (RoF) transmissions. The proposed system implements encoding and double-balanced detection processes using a polarisation diversity scheme to enlarge the transmitted radio frequency (RF) signal space. By combining the quasi-orthogonality of Walsh-Hadamard codes with arrayed waveguide grating (AWG) routers and a polarisation beam splitter (PBS), the proposed coder/decoder (codec) pair can encode-decode multiple Walsh-Hadamard code words and retain the ability of multiple access interference (MAI) cancellation. This study presents a series of investigations to evaluate the sensitivity of the proposed structure to the degree of matching in the state of polarisations (SOPs) of the transmitted light in the encoder and decoder, respectively. Results show that when the polarisation states are fully matched between the encoder and the decoder, the proposed system achieves a carrier-to-noise ratio (CNR) improvement of up to 3-dB compared with that obtained from a complementary spectral polarisation coding (SPC) system with a single-balanced detector. Numerical results demonstrate that maintaining the system with an orthogonal SOP for each radio base station (RBS) effectively reduces the phase-induced intensity noise (PIIN).