[Paper Review] Performance Analysis of FSO System with Spatial Diversity and Relays for M-QAM over Log-Normal Channel

Pranav Kumar Jha, Nitin Kachare+2 more|arXiv (Cornell University)|Sep 16, 2017

Optical Wireless Communication Technologies15 references3 citations

TL;DR

This paper proposes a multi-hop MISO FSO system with decode-and-forward relays and spatial diversity using M-QAM modulation over log-normal fading channels. It analytically derives exact BER expressions and demonstrates that M-QAM with multi-hop MISO relaying achieves up to 47.5 dB SNR gain over SISO-OOK under light fog, significantly outperforming other configurations at equal spectral efficiency.

ABSTRACT

The performance analysis of free space optical communication (FSO) systems using relays and spatial diversity at the transmitter end is presented in this paper. The impact of atmospheric turbulence and attenuation caused by different weather conditions and geometric losses has also been taken into account. The effect of turbulence is modeled over a log-normal probability density function. We present the exact closed form expressions of bit-error rate (BER) for M-ary quadrature amplitude modulation (M-QAM). The FSO system link performance is compared for on-off keying, M-ary pulse amplitude modulation and M-QAM modulation techniques. For relay based free space optical communication systems, M-QAM is proved to be superior than other systems considering the same spectral efficiency for each system. A significant performance enhancement in terms of BER analysis and SNR gains is shown for multi-hop MISO FSO system.

Motivation & Objective

• To analyze the performance of FSO systems under atmospheric turbulence, weather-induced attenuation, and geometric losses using a log-normal fading model.
• To evaluate the impact of spatial diversity (MISO) and relay-based multi-hop transmission on BER performance for M-QAM modulation.
• To compare M-QAM with OOK and M-PAM in terms of spectral efficiency and BER under identical system conditions.
• To quantify the SNR gains of multi-hop MISO FSO systems over SISO and MISO systems with OOK/RCs in clear and foggy weather.
• To validate the superiority of M-QAM in multi-hop MISO FSO systems under realistic atmospheric impairments.

Proposed method

• Models atmospheric turbulence using a log-normal probability density function for irradiance fluctuations.
• Incorporates path loss due to geometric spreading, absorption, and scattering using a normalized path loss coefficient β.
• Derives exact closed-form BER expressions for M-QAM over log-normal channels using statistical channel modeling.
• Employs decode-and-forward (DF) relaying in multi-hop FSO links to improve reliability and extend communication range.
• Applies maximum-likelihood (ML) decoding at the receiver for M-QAM signal detection.
• Compares system performance across configurations: SISO-OOK, MISO-OOK/RCs, multi-hop SISO/MISO with M-PAM and M-QAM, maintaining equal spectral efficiency.

Experimental results

Research questions

• RQ1 How does M-QAM performance compare to OOK and M-PAM in FSO systems with spatial diversity and relays under log-normal fading?
• RQ2 What is the impact of atmospheric conditions such as clear weather and light fog on BER performance in multi-hop FSO systems?
• RQ3 To what extent do multi-hop relaying and MISO diversity reduce BER and improve SNR gain compared to single-hop systems?
• RQ4 How does increasing the number of transmitters affect BER performance in MISO and multi-hop FSO systems?
• RQ5 What is the SNR gain of multi-hop MISO systems using M-QAM over SISO and MISO systems with OOK and RCs under identical spectral efficiency?

Key findings

• The multi-hop MISO FSO system with M-QAM achieves a 22 dB SNR gain at the target BER compared to MISO with OOK and RCs under clear weather with moderate turbulence.
• In light fog conditions, the multi-hop SISO system achieves a 37 dB SNR gain over SISO with OOK, while the multi-hop MISO system achieves a 33 dB SNR gain over MISO with OOK and RCs.
• For a two-relay multi-hop system with 8-QAM, the MISO configuration achieves a 13 dB SNR gain over MISO with OOK and RCs using single transmitter, and surpasses the multi-hop SISO system by 1.2 dB.
• With three transmitters and 8-PAM modulation, the multi-hop MISO system achieves a 19.5 dB SNR gain over MISO with OOK and RCs, and outperforms the multi-hop SISO system by 2.8 dB.
• Under light fog, the two-relay multi-hop system achieves a 10 dB performance improvement over single-hop systems, with SNR gains of 47.5 dB and 42.8 dB for multi-hop SISO and MISO, respectively, over SISO-OOK and MISO-OOK/RCs.
• M-QAM-based multi-hop MISO FSO systems consistently outperform SISO and MISO systems with OOK and RCs, demonstrating superior spectral efficiency and robustness under turbulence and weather-induced fading.