Adaptive symbol-rate free-space-optical communications

2003

Abstract This work investigates the achievable information rate with the state-of-the-art turbo coding and intensity modulation/direct detection for outdoor long-distance free-space optic (FSO) communications. The channel under weak atmospheric turbulence is modeled as a log-normal intensity fading channel where on-off keying makes it look asymmetric. While no effort is made to spectrally match the code to the asymmetry of the channel, the decoding strategy is optimally adjusted to match to the channel response.

Atmospheric turbulence has made a significant contribution in free-space optical (FSO) communications' areas of research. Assuming slowly varying channel, a feedback can be implemented to overcome the problem of fading. In comparison with all former published works, in this paper, we apply an algorithm to reduce average power consumption by regulating transmitter Erbium Doped Fiber Amplifier (EDFA) gain given channel state information (CSI). As a benchmark, a simple but non practical power control algorithm is introduced and discussed in this paper. To make the algorithm more practical, the quantized counterpart of the algorithm is introduced and its performance is compared to continuous one. It is shown by consuming 4dB more power than the continuous algorithm, we can simply implement a practical quantized power control algorithm. The statistical analysis of the proposed adaptive algorithms is performed, considering a complex model of the channel, including a low power transmitting laser, EDFA statistical model, channel fading, channel attenuations, receiver lens, photodetector model and all sources of optical and electrical noise. It is shown the proposed algorithm brings significant improvements over its non-adaptive counterpart.

IEEE Transactions on Communications, 2020

In this paper, we propose a practical adaptive coding modulation scheme to approach the capacity of free-space optical (FSO) channels with intensity modulation/direct detection based on probabilistic shaping. The encoder efficiently adapts the transmission rate to the signal-to-noise ratio, accounting for the fading induced by the atmospheric turbulence. The transponder can support an arbitrarily large number of transmission modes using a low complexity channel encoder with a small set of supported rates. Hence, it can provide a solution for FSO backhauling in terrestrial and satellite communication systems to achieve higher spectral efficiency. We propose two algorithms to determine the capacity and capacity-achieving distribution of the scheme with unipolar M-ary pulse amplitude modulation (M-PAM) signaling. Then, the signal constellation is probabilistically shaped according to the optimal distribution, and the shaped signal is channel encoded by an efficient binary forward error correction scheme. Extensive numerical results and simulations are provided to evaluate the performance. The proposed scheme yields a rate close to the tightest lower bound on the capacity of FSO channels. For instance, the coded modulator operates within 0.2 dB from the M-PAM capacity, and it outperforms uniform signaling with more than 1.7 dB, at a transmission rate of 3 bits per channel use.

Due to ability to provide very high-speed transmission of data, without the use of any physical channel, free space optics (FSO) is an emerging as a promising communication technology. In FSO, the atmosphere ants as the channel and its turbulent nature causes degradation in bit error rate performance. This paper presents the performance evaluation of FSO communication for through Rayleigh fading environment. The equations for Bit Error Rate and Outage Probability are derived for BPSK, QPSK and 16-QAM modulation techniques. Simulation results has been presented out to calculate the FSO communication system performances derived for BPSK, QPSK and 16-QAM modulation techniques.

IEEE Communications Surveys & Tutorials, 2014

Optical wireless communication (OWC) refers to transmission in unguided propagation media through the use of optical carriers, i.e., visible, infrared (IR), and ultraviolet (UV) bands. In this survey, we focus on outdoor terrestrial OWC links which operate in near IR band. These are widely referred to as free space optical (FSO) communication in the literature. FSO systems are used for high rate communication between two fixed points over distances up to several kilometers. In comparison to radio-frequency (RF) counterparts, FSO links have a very high optical bandwidth available, allowing much higher data rates. They are appealing for a wide range of applications such as metropolitan area network (MAN) extension, local area network (LAN)-to-LAN connectivity, fiber backup , backhaul for wireless cellular networks, disaster recovery, high definition TV and medical image/video transmission, wireless video surveillance/ monitoring, and quantum key distribution among others. Despite the major advantages of FSO technology and variety of its application areas, its widespread use has been hampered by its rather disappointing link reliability particularly in long ranges due to atmospheric turbulence-induced fading and sensitivity to weather conditions. In the last five years or so, there has been a surge of interest in FSO research to address these major technical challenges. Several innovative physical layer concepts, originally introduced in the context of RF systems, such as multiple-input multiple-output communication, cooperative diversity, and adaptive transmission have been recently explored for the design of next generation FSO systems. In this paper, we present an up-to-date survey on FSO communication systems. The first part describes FSO channel models and transmitter/receiver structures. In the second part, we provide details on information theoretical limits of FSO channels and algorithmic-level system design research activities to approach these limits. Specific topics include advances in modulation, channel coding, spatial/cooperative diversity techniques, adaptive transmission, and hybrid RF/FSO systems.

Optical and Quantum Electronics, 2020

In this paper, the analytical channel capacity of the free space optical (FSO) system under weak, moderate and strong turbulence is derived. The FSO channel statistics follows the log-normal distribution under weak turbulence and follows the Gamma-Gamma distribution under moderate and strong turbulence. The three generally used adaptive transmission schemes in FSO communication system are as follows (1) optimal simultaneous power and rate adaptation, (2) optimal rate adaptation with constant transmit power, and (3) truncated channel inversion with a fixed rate. The closed-form approximations for the average ergodic channel capacity for these adaptive transmission schemes under different atmospheric turbulence without any pointing error are derived. Simulation results depict the excellent agreement of the proposed analytical models with the Monte Carlo simulation.

Journal of Lightwave Technology, 2023

In this paper, to achieve higher robustness against atmospheric turbulence for high-capacity free-space optical (FSO) communications, an adaptive multi-modal FSO transceiver has been designed and experimentally demonstrated. We show that based on the dynamically estimated channel state information, modulation formats and power for different transmit modes can be adaptively allocated at the transmitter side. Meanwhile, at the receiver side, we show that the most suitable multi-input/multi-output decoder can be selected and employed to meet the requirement of forward error correction at the minimal expense of power consumption. By employing time-division multiplexed transmitter and receiver emulation and a spatial light modulator for turbulence emulation, an aggregate data rate of 590 Gbit/s/wavelength has been achieved when suffering from strong atmospheric turbulence, verifying the feasibility of the proposed adaptive transceiver over a turbulent FSO link. Moreover, to demonstrate practical applicability, all key devices such as transponder, multiplexer, and demultiplexer are commercially available in this work. Index Terms-Atmospheric turbulence, free-space optics (FSO), mode-division multiplexing (MDM), multiple-input multiple-output (MIMO), optical wireless communication. I. INTRODUCTION he demand for internet traffic, which is currently attributed to the increasing popularity of 5G services, big data, cloud Manuscript received xx xx, 2022.

Iet Communications, 2009

Turbulence fading is one of the main impairments affecting the operation of free-space optical (FSO) communication systems. The authors study the performance of FSO communication systems, also known as wireless optical communication systems, over log-normal and gamma -gamma atmospheric turbulenceinduced fading channels. These fading models describe the atmospheric turbulence because of its very good agreement with experimental measurement data. Closed-form expressions for the average (ergodic) capacity and the outage probability are derived for both statistical models. Another contribution of this work is a study of how the performance metrics are affected by the atmospheric conditions and other parameters such as the length of the link and the receiver's aperture diameter. The derived analytical expressions are verified by various numerical examples and can be used as an alternative to time-consuming Monte-Carlo simulations.

2014

In this paper, we made a comprehensive simulative study on the performance assessment of a Subcarrier intensity-modulated Free Space Optical (FSO) Communication system. The proposed system under investigation consider a communication link between a base station and a mobile unit using light wave transmission through free space in consideration with Atmospheric turbulence effect. The FSO system implements various types of channel coding schemes such as LDPC, Turbo, Cyclic, BCH and Reed-Solomon. From MATLAB based simulated study on synthetic data transmission, it is found a quite noticeable impact on implementing different types of channel coding schemes on performance enhancement of the presently considered FSO system. The system is also capable of showing its robustness in retrieving transmitted data in spite of atmospheric turbulence effect under QAM digital modulation and BCH channel coding scheme.

—In recent years, free space optical (FSO) communication has gained significant importance owing to its unique features: large bandwidth, license free spectrum, high data rate, easy and quick deployability, less power and low mass requirement. FSO communication uses optical carrier in the near infrared (IR) and visible band to establish either terrestrial links within the Earth's atmosphere or inter-satellite/deep space links or ground-to-satellite/satellite-to-ground links. It also find its applications in remote sensing, radio astronomy, military, disaster recovery, last mile access, back-haul for wireless cellular networks and many more. However, despite of great potential of FSO communication, its performance is limited by the adverse effects (viz., absorption, scattering and turbulence) of the atmospheric channel. Out of these three effects, the atmospheric turbulence is a major challenge that may lead to serious degradation in the bit error rate (BER) performance of the system and make the communication link infeasible. This paper presents a comprehensive survey on various challenges faced by FSO communication system for both terrestrial and space links. It will provide details of various performance mitigation techniques in order to have high link availability and reliability of FSO system. The first part of the paper will focus on various types of impairments that poses a serious challenge to the performance of FSO system for both terrestrial and space links. The latter part of the paper will provide the reader with an exhaustive review of various techniques used in FSO system both at physical layer as well as at the upper layers (transport, network or link layer) to combat the adverse effects of the atmosphere. Further, this survey uniquely offers the current literature on FSO coding and modulation schemes using various channel models and detection techniques. It also presents a recently developed technique in FSO system using orbital angular momentum to combat the effect of atmospheric turbulence.