A New Photon-Counting MU-MISO Ultraviolet Communication System With MMSE Precoding in Turbulence Channels

Signal-dependent noise and atmospheric turbulence have hindered the implementation of photon-counting systems for ultraviolet (UV) free-space optical (FSO) communications, despite the great potential of the systems in low-power signal applications. This paper designs a new downlink, multi-user, photon-counting UV FSO communication system that minimizes the mean square error (MSE) of the received signals subject to non-negative transmit signals and the total transmit power of the transmitter, under the practical signal-dependent Poisson shot noises of UV photoelectric conversion. We first derive a new closed-form expression for the MSE. Then, we show the convexity of the MSE expression and minimize the MSE using convex optimization techniques. The resultant MMSE precoder can effectively combat severe atmospheric turbulence and mitigate multi-user interference. In a strong Gamma-Gamma turbulence fading channel (
$\alpha =2.1, \beta =2.4$
), it can achieve the average bit error rate (BER) of
$10^{-5}$
at 25 photons/bit in the presence of four users, substantially better than the existing techniques.