Case Study : Ultra-High-Speed Wireless Transmission via Free-Space Optics

Ultra-High-Speed Wireless Transmission via Free-Space Optics

Abstract

Wireless transmission with high capacity is a critical requirement for next- generation communication systems. We review some of the most recent achievements in the convergence of coherent fibre communications and free- space optics systems, enabling data rates ranging from 400 Gbps to 1 Tbps in this case study.

Introduction

The paper examines the advancement of free-space optics (FSO) systems that have high capacity and can work seamlessly with fiber-based coherent optical transceivers, utilizing advanced modulation techniques. The need for wireless transmission solutions capable of accommodating data-rates exceeding 400 Gbps provided by the latest generation of coherent optical pluggables is crucial. FSO systems are becoming more popular in the research community as they offer a way to avoid complex issues associated with high-frequency RF solutions. The scalability of FSO systems for wavelength-division multiplexing (WDM) applications is impressive, with a recent demonstration of 35-WDM channel 14 Tbps transmission over 220 meters. The paper emphasizes the significance of addressing and mitigating atmospheric turbulence and pointing errors through the utilization of adaptive probabilistic constellation shaping (PCS) modulation.

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Impact of Pointing Errors

The experimental arrangement depicted in Figure 1 was used to investigate the effect of pointing errors on an FSO connection that employs seamless air-to- fiber optical beam reception. The optical beam between the transmitting and receiving fiber collimators was aligned using a set of high-precision stepper motors (Thorlabs ZST200), which were controlled by a processing unit linked to a laptop. When actual communication occurred over the FSO connection, the Tx and Rx components were swapped with a complete coherent optical transmitter and receiver. Figure 2 displays an example of the received optical power characteristics, where the stepper motors were employed to sweep the incoming beam's position across the full 2D axis of the Rx fiber collimator. The results reveal that the FSO link's alignment is incredibly sensitive to pointing errors, as a minor 1 mm shift from the optimal focus point leads to roughly 10 dB power loss on the fiber-collimated optical signal.

Impact of Atmospheric Turbulence

The experimental arrangement employed to evaluate the influence of atmospheric turbulence and weather conditions on the power budget of FSO links. The transmitting (Tx) and receiving (Rx) collimators are spaced out by

around 55 meters, with a concave mirror positioned at the midpoint of the link. Using this outdoor FSO link, we conducted a sequence of prolonged optical power measurements under varying weather conditions, which resulted in the findings presented in Figure 4. In addition to the evident effect of rainfall on the increased variance of the received optical power, we also draw attention to the strong time correlation that defines the FSO system, particularly in the presence of slowly changing weather conditions. The signal's autocovariance displays correlation times of several minutes.

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400G+ Outdoor FSO Demonstration

Using PCS-based adaptive modulation, we achieved 400-600 Gbps transmission rates in an outdoor FSO link. Fig. 5 shows bit-rate adaptability through symbol probability distribution, while Fig. 6 demonstrates that the adaptive PCS technique optimizes bit-rate in real-time, maximizing system capacity and reliability. Fixed solutions are too pessimistic or optimistic, but PCS adapts to changing link conditions.

Terabit Intra-Datacenter FSO Demonstration

We used wideband instrumentation to demonstrate Terabit-capable FSO transmission for intra-datacenter applications, shown in Fig. 1. Pointing errors are a major hurdle, as shown in Fig. 7a, but adaptive PCS technique enables real-time data-rate adaptation with stable NGMI performance and a small operating margin to ensure 100% reliability, as shown in Fig. 7b. This FSO link supports up to 1 Tbps in optimal conditions and remains above 900 Gbps in worst-case scenarios.

Conclusion

FSO offers a high-capacity wireless solution for next-gen networks, including 5G/6G, datacenter interconnects, last-mile access, and backup links. While challenges remain, such as atmospheric turbulence and pointing errors, recent works demonstrate that FSO transmission supporting over 400 Gbps per channel could become a practical solution soon.