In mode division multiplexing (MDM) free space optical (FSO) communication system, the atmospheric turbulences such as fog, rain, and haze cause adverse effects on system performance. This paper investigates the mitigation of atmospheric turbulences of FSO using MDM and decision feedback equalizer (DFE) with minimum mean square error (MMSE) algorithm. The implementation of the MMSE algorithm is used to optimize both the feedforward and the feedback filter coefficients of DFE. The proposed system comprises three parallel 2.5Gbit/s channels using Hermite-Gaussian modes. A data rate of 7.5Gbit/s over 40 m, 800 m, 1400 m, and 2km under medium fog, rain, haze, and clear weather, respectively, has been achieved. In addition, it is noticed that the link distance is reduced while increasing the attenuation. The simulation results revealed that a DFE improves the performance MDM FSO system while maintaining high throughput and desired low bit error rate.

Mitigation of Atmospheric Turbulences Using Mode Division Multiplexing based on Decision Feedback Equalizer for Free Space Optics

Abstract Free space optics (FSO) systems use the atmosphere as a propagation medium. However, a common problem is atmospheric turbulence, including fog, rain, and haze that emerges between the transmitter and the receiver from time to time. These adverse weather conditions impose power loss on the optical signal, producing distortion and degrading bit error rate (BER) and throughput. To reduce the effect of atmospheric turbulence, this paper proposes a decision feedback equalizer (DFE) with minimum mean square error (MMSE) in mode division multiplexing (MDM) for the FSO system. The DFE with varying tap counts is investigated. The MMSE algorithm is utilized to optimize both the feedforward and feedback filter coefficients of the DFE. The proposed system consists of four parallel 2.5 Gbps channels that use Hermite-Gaussian (HG) modes. The results show that the DFE equalization scheme successfully transmits 10 Gbps over 40 m, 800 m, 1400 m, and 2 km in medium fog, medium rain, medium haze, and clear weather. Performance is analyzed in terms of BER and eye diagrams and compared with the traditional model. Based on BER and eye diagram results, DFE improves the outdoor FSO system immunity to distortion in medium fog, medium haze, medium rain, and clear weather while maintaining high throughput and desired low BER.

https://www.tandfonline.com/doi/pdf/10.1080/23311916.2022.2034268?needAccess=true

Performance improvement of mode division multiplexing free space optical communication system through various atmospheric conditions with a decision feedback equalizer

Free Space Optical (FSO) Communication is the transmission of data using optical beam through free space. It requires the line of sight condition to be maintained between the transmitter and receiver in order to transmit data. High data rates, license-free spectrum and low deployment cost are some of the advantages of FSO link. But the attenuation due to atmospheric conditions is a major factor responsible for the degradation of performance of FSO link. Depending upon the weather conditions of a particular place, the performance of FSO system varies accordingly. So in this paper, we have analysed the attenuation coefficient for different weather conditions. The effects of rain, snow and fog on the performance of FSO link has been analyzed in terms of attenuation coefficient along with the study of turbulent nature of free space optical communication link employing Gamma-Gamma Turbulence Model.

Performance Evaluation of Free Space Optical (FSO) Communication Link: Effects of Rain, Snow and Fog

In this work, a novel dual-hop relay-assisted hybrid free space optical/radio frequency (FSO/RF) communication system is presented. In this system, RF signal is transmitted from two antennas, and then forwarded by a single antenna relay through FSO channel. This is the first time that performance of using Alamouti coding (AC) or antenna selection (AS) at the transmitter of a hybrid FSO/RF system is investigated. FSO link has gamma-gamma atmospheric turbulence, and in order to get closer to the actual results, the effect of pointing error is also considered. For the first time closed-form expressions are derived for bit error rate and outage probability P out of the proposed system and validated through MATLAB simulations. Results indicate that in this structure, there is slight performance difference between AC and AS schemes. Hence, due to more complexity, power consumption and latency of AC, AS is recommended. Dual-hop, hybrid FSO/RF system significantly improves performance and reliability of the system, and is particularly suitable for long-range applications that direct RF communication between source and destination is not possible. Considering these advantages this structure is particularly suitable for mobile communications which has power and processing limitations.

Performance analysis of hybrid FSO/RF communication systems with Alamouti coding or antenna selection

Availability analysis of terrestrial free space optical (FSO) link using visibility data measured in tropical region

The advancement of the terrestrial free space optical communication system (FSO) has shown a drastic increase. But despite the various advantages provided by FSO communication, there are various issues related to the design of optical links. To overcome the problems like scattering, diffraction, turbulence, etc., different techniques were developed, and one of the most widely employed techniques is aperture averaging. The aperture averaging is the main focus of this paper and using on–off (OOK) modulation technique, the bit error rate (BER) performance curves have been analysed for varying data rates and aperture sizes at the receiver using optisystem software.

Performance Analysis in Free Space Optical Communication System Using Aperture Averaging

Free space optics (FSO) is emerging as a viable complementary technology to address the need for larger bandwidth and high data rate at affordable cost. FSO communication systems face severe link availability and reliability challenges under different weather conditions, and this is a limitation for the wide-scale acceptability of the FSO technology. The main objective of this paper is to analyze the impact of fog, snow, and rain on FSO link, and hence evaluate the performance of the FSO system for various weather conditions. It is analyzed that bit error rate (BER) and link margin of FSO system are very poor for heavy fog, rain, and snow. However, decreasing the data rate for a particular weather condition can improve both these performance parameters.

Saloni Bhatia

Papers

Performance Analysis in Free Space Optical Communication System Using Aperture Averaging

Performance Evaluation of Free Space Optical Link Under Various Weather Conditions