[Paper Review] Performance Characterization of Relay-Assisted Wireless Optical CDMA Networks in Turbulent Underwater Channel

Mohammad Vahid Jamali, Farhad Akhoundi+1 more|arXiv (Cornell University)|Aug 17, 2015

Optical Wireless Communication Technologies29 references4 citations

TL;DR

This paper proposes a relay-assisted optical CDMA (OCDMA) network for underwater wireless optical communication (UWOC) to combat absorption, scattering, and log-normal fading in turbulent channels. By using multi-hop transmission with chip detect-and-forward relays and orthogonal codes, the system achieves up to 32 dB BER improvement at 10⁻⁶ for a 90 m link, with optimal relay placement significantly enhancing performance.

ABSTRACT

In this paper, we characterize the performance of relay-assisted underwater wireless optical code division multiple access (OCDMA) networks over turbulent channels. In addition to scattering and absorption effects of underwater channels, we also consider optical turbulence as a log-normal fading coefficient in our analysis. To simultaneously and asynchronously share medium among many users, we assign a unique optical orthogonal code (OOC) to each user in order to actualize OCDMA-based underwater network. The most significant challenge in underwater optical communication is in the ability to extend the short range of its coverage. In order to expand the viable communication range, we consider multi-hop transmission to the destination. Moreover, we evaluate the performance of a relay-assisted point-to-point UWOC system as a special case of the proposed relay-assisted OCDMA network. Our numerical results indicate significant performance improvement by employing intermediate relays, e.g., one can achieve $32$ {dB} improvement in the bit error rate (BER) of $10^{-6}$ using only a dual-hop transmission in a $90$ {m} point-to-point clear ocean link.

Motivation & Objective

• Address the limited range of underwater wireless optical communication (UWOC) due to absorption, scattering, and turbulence.
• Improve system performance in long-distance UWOC by employing multi-hop relay transmission to reduce cumulative path impairments.
• Characterize BER performance of relay-assisted OCDMA-UWOC networks under log-normal fading and turbulent channel conditions.
• Evaluate the impact of relay position and transmitter beam divergence on system performance.
• Demonstrate that chip detect-and-forward relaying with orthogonal codes enables efficient multiple access and reduced interference.

Proposed method

• Model the underwater channel using log-normal fading to represent optical turbulence, combined with absorption and scattering effects.
• Apply optical orthogonal codes (OOC) for multiple access, enabling asynchronous user multiplexing with reduced multiple access interference (MAI).
• Use chip detect-and-forward relaying: relays detect and retransmit chips using Gaussian approximation based on photon counting.
• Formulate end-to-end bit error rate (BER) using statistical channel models, including channel loss coefficient and scintillation index.
• Simulate various network topologies with different hop lengths and beam divergence angles to evaluate performance trade-offs.
• Derive BER expressions for both uplink and downlink transmissions, accounting for MAI and fading effects under varying user counts and code weights.

Experimental results

Research questions

• RQ1 How does multi-hop relay transmission improve BER performance in long-range underwater optical CDMA networks under turbulent conditions?
• RQ2 What is the impact of relay node placement on BER performance in a multi-hop UWOC link?
• RQ3 How do varying transmitter beam divergence angles affect system performance in relay-assisted UWOC systems?
• RQ4 Under what conditions does multiple access interference (MAI) degrade system performance, and when is it mitigated?
• RQ5 To what extent does chip detect-and-forward relaying enhance the viability of UWOC for long-distance applications?

Key findings

• A dual-hop transmission in a 90 m clear ocean link achieves a 32 dB improvement in BER at 10⁻⁶ compared to a direct link, demonstrating significant performance gain.
• Optimal relay placement significantly enhances performance: Mode IV (25 m, 40 m, 25 m hops) outperforms Mode VI (20 m, 50 m, 20 m) by approximately 7.5 dB at BER = 10⁻⁶.
• Increasing the beam divergence angle of first and last hop transmitters degrades system performance, though it offers practical deployment advantages.
• When the number of users is less than or equal to the code weight (M ≤ W), MAI does not affect performance, and uplink and downlink BERs are identical.
• For M > W, uplink BER saturates due to persistent MAI, while downlink BER remains stable due to synchronous transmission and MAI suppression.
• The scintillation index and log-amplitude variance increase with link range and beam divergence, confirming the need for optimized relay placement in long-haul UWOC systems.