Linear Time-Packing Detectors for Optical Feeder Link in High Throughput Satellite Systems

Abstract

This paper studies the performance of linear detectors when used to recover the information transmitted on a time-packed optical feeder link of a High Throughput Satellite (HTS) system. More precisely, a real-valued time-packed M-ary Pulse Amplitude Modulation (M-PAM) signal is used to modulate the intensity of the laser diode beam with an external Mach-Zehnder Modulator (MZM). At the receiver side, the signal samples are recovered with a photo-detector, and the sequence of payload bits to be encapsulated into the 5G radio frame can be optimally recovered with the aid of a Viterbi equalizer. However, as the modulation order of the M-PAM signal grows, as well as the overlapping factor of time-packing increases and/or the roll-off factor of the pulse-shaping filter decreases, the number of trellis states that are needed to recover the transmitted bit sequence successfully grows notably. To address this issue, this paper studies the performance of two low-complexity linear equalization strategies, namely truncated Minimum Mean Square Error (MMSE) and adaptive MMSE equalizers. As expected, though being sub-optimal, both linear detectors are able to mitigate the impact of the Inter-Symbol Interference (ISI) that time-packing introduces, enabling a better spectral efficiency when compared to baseline M-PAM transmissions without overlapping.