Effect of Optical Loss and Antenna Separation in 2$\,\times\,$2 MIMO Fiber-Radio Systems

We study the impact of fiber-optic loss due to realistic cable deployment and optical connector loss variation and distance between the transmitter antennas in a radio-over-fiber transmission system with 2 × 2 MIMO. We evaluate an upper bound on the system capacity based on measured values of the error vector magnitude and the condition number of the channel matrix. For this, assuming a Rayleigh fading environment, we derive conditional probability density functions of eigenvalues of a 2 × 2 Wishart matrix. We compare the obtained results with another approach, which is based on IEEE 802.11 relations between the allowed relative constellation error and the achievable data rate for an individual stream. Trends predicted by both models agree very well. Our analysis shows that (i) about 1 meter spatial separation of fiber-fed transmitter antennas is optimum and results in the maximized system performance and (ii) the maximum tolerable optical power imbalance in two fiber optic links is about 6 dB.