Free-Space Optical Communications with Channel Shortening Filter and Viterbi Equalizer

Abstract

This paper is about signal processing techniques for airborne free-space optical (FSO) wireless communications through clouds. FSO channel is known to be accompanied by multi-scattering, which causes severe intersymbol interference. Unlike traditional FSO systems relying on optical signal processing devices, this research paper investigates the feasibility of digital signal processing schemes applied to FSO. By having a channel shortening equalizer (CSE or TEQ for time equalization) at the transmitter and a Viterbi equalizer at the receiver, reliable communication at low-to-mid level optical thickness values is shown to be feasible. We also propose how existing TEQ algorithms can be modified to fit into TEQ-in-transmitter configurations. Performance is evaluated in BER, and compared with conventional equalizations.

Hybrid Channel Codes for Efficient FSO/RF Communication Systems

“…Although this memory can be made small by choosing a small field of view in the receiver, it cannot be totally removed in practice [37]. Several methods have been proposed in the literature to address the Inter-Symbol Interference (ISI) caused by the channel memory [38], [39]. A well-studied method is to use a multi-carrier scheme such as Orthogonal FrequencyDivision Multiplexing (OFDM) for signal transmission [38], [40].…”

“…This way, the aggregate data rate can be divided among many sub-carriers, and since per channel OFDM symbol rate is much lower, the intrachannel nonlinearities can be completely avoided [38]. Along with OFDM, one can also use equalization to mitigate the ISI, benefiting from the fact that employing OFDM makes it easier to implement equalizers for the resulted narrow-band subchannels [39]. In Section IV, we will show how OFDM can be exploited along with Hybrid Channel Codes to make a robust hybrid FSO/RF system in different channel conditions.…”

Outage Probability and BER Estimation for FSO Links with Truncated Normal Time Jitter and Longitudinal Gaussian Pulse Propagation in Dispersive Media

“…Hence, in this context, the link's length is an important parameter. The bit rate is also important, because the corresponding detection timeslot sets a tolerance limit, beyond which adjacent pulses are overlapped, inducing inter-symbol interference (ISI) [23][24][25].…”