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Free-Space Optical Communication and Energy Harvesting in 5G+ Networks

Introduction

Free-space optical communication (FSO) combines the high bandwidth of optical communication with the flexibility of wireless communication. This paper explores how FSO technology, when integrated with unmanned aerial vehicles (UAVs) and energy harvesting techniques, can provide efficient backhaul and fronthaul architectures for 5G and beyond (5G+) networks [1].

System Architecture and Key Devices

The system architecture employs UAVs as relay nodes between base stations and network gateways. Figure 1 illustrates this concept, depicting how FSO links are integrated with UAVs in a 5G+ network environment. This figure illustrates how UAVs establish vertical backhaul/fronthaul connections between base stations and gateways while maintaining line-of-sight communication.

A network architecture diagram showcasing the application of FSO technology in 5G+ scenarios
Figure 1: A network architecture diagram showcasing the application of FSO technology in 5G+ scenarios, including UAV-based FSO link connections to various network devices.
Channel Characteristics and Challenges

FSO communication faces several atmospheric and alignment challenges:

  • Atmospheric Turbulence: The inhomogeneities in air pressure and temperature lead to variations in the refractive index of the air along the transmission path. This phenomenon results in random signal amplitude and phase variations, leading to fading effects.

  • Pointing Errors: Misalignment between the transmitter and receiver leads to significant power losses. Figure 2 illustrates how UAV jitter at different altitudes affects the communication link.

UAV jitter effects at different altitudes,
Figure 2: UAV jitter effects at different altitudes, showing how beam footprint and pointing accuracy influence link stability.
Performance Analysis

System performance is evaluated based on two key metrics: bit error rate (BER) and energy harvesting efficiency. Figure 3 presents a comprehensive BER analysis under various channel conditions.

BER performance curves for OOK and OOK-EH schemes under different channel impairments
Figure 3: BER performance curves for OOK and OOK-EH schemes under different channel impairments, illustrating the impact of various signal-to-noise ratios.
Energy Harvesting Strategies

The energy harvesting module employs an optimized on-off keying (OOK) scheme to enable simultaneous information transmission and energy harvesting. Figure 4 illustrates the receiver architecture integrating communication and energy harvesting modules.

integration of energy harvesting
Figure 4: Receiver block diagram showcasing the integration of energy harvesting and information detection modules.
Practical Implementation Considerations

The practical deployment of the system requires careful consideration of the following parameters:

  1. Beam Divergence: A typical transmitter divergence angle of 1 milliradian is used to balance coverage range and power efficiency.

  2. Receiver Design: An optical circular converging lens with a 10 cm aperture radius and 0.5 A/W responsivity is employed.

  3. Operating Conditions: The system operates at a wavelength of 1550 nm and is influenced by various atmospheric conditions.

Figure 5 illustrates the relationship between link length and system performance under different atmospheric conditions.

relationship between link length and system performance
Figure 5: (a) Optical channel loss and (b) average harvested energy variation with link length, demonstrating performance under different atmospheric conditions.

Results and System Performance

Performance analysis indicates that the proposed system achieves significant energy harvesting while maintaining a reliable communication link. Figure 6 demonstrates the energy harvesting capability under different operating conditions.

Comparison of average harvested energy for OOK and OOK-EH schemes
Figure 6: Comparison of average harvested energy for OOK and OOK-EH schemes under different transmission power levels and operating conditions.
Conclusion

The integration of UAVs and energy harvesting in FSO communication systems offers an effective solution for 5G+ network backhaul/fronthaul connectivity. This system provides additional energy harvesting benefits while ensuring reliable performance, though careful consideration of atmospheric conditions and pointing accuracy is required. Technology is particularly advantageous in urban environments where traditional RF solutions may face bandwidth limitations or interference issues.

Key Advantages of This Technology:

  • High-bandwidth capability with no RF interference

  • Adaptive network topology enabled by mobile UAV nodes

  • Extended UAV operation time through energy harvesting

  • Cost-effectiveness compared to fixed infrastructure

Future research in this field will focus on improving pointing accuracy and enhancing energy harvesting efficiency while maintaining reliable communication performance under diverse atmospheric conditions.

References

[1] Y. Wang and L. Zhang, Eds., "Optical Signal Processing Technologies for Communication, Computing, and Sensing Applications," MDPI Books, 2023. [Online]. Available: https://www.mdpi.com/books/reprint/7078-optical-signal-processing-technologies-for-communication-computing-and-sensing-applications. [Accessed: Dec. 29, 2024]

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