Abstract
Free-space optical communication (FSOC) provides high-capacity wireless connections, without exhausting scarce RF resources, between airborne platforms constituting aerial fronthaul/backhaul network for 5G and beyond. However, accurate pointing, acquisition, and tracking (PAT) of narrow-divergence optical beams between the transmitter and receiver has long been a major technical challenge of FSOC systems. We introduce an adaptive beam control technique to facilitate the PAT and improve the link availability.
1. Introduction
Airborne platforms are auto-controlled or remotely-operated aerial vehicles operating in high altitudes, such as aerostats, balloons, and drones. The utilization of airborne platforms as a new infrastructure for 5G-and-beyond wireless communication networks has been the subject of considerable interest internationally in recent years [1]-[5]. There are two main reasons for this trend. Ultra-dense heterogeneous small cells are perceived as a key enabler for modern wireless communication networks. In remote areas, however, the installation of terrestrial infrastructure with a large number of small-cell base stations would be highly costly and time-consuming. Airborne platforms, serving as network nodes, can offer substantial benefits in this regard. Also, ultra-high network availability is one of the key motivating trends behind the 5G evolution. In case of emergency or disaster, for example, airborne platforms can undoubtedly provide rapid operational availability for fast bridging and filling network gaps on an ad hoc basis [1].
The 5G network is expected to support 1000-fold increase in data traffic capacity and 10-fold increase in throughput in comparison to 4G systems [6]. To fulfill the expectations, high-speed (e.g., tens of Gbps) communication systems should be realized to transmit massive data traffic between airborne fronthaul/backhaul links. Also, long-distance transmission (e.g., 10 to a few hundred kilometers) is needed to set up the aerial network and improve the network coverage for remote areas. Optical fiber is the best option in terms of data rate and transmission distance among current communication technologies. However, it is not possible to deploy optical fibers for airborne communications. The existing RF and microwave fronthaul/backhaul links rely on frequency bands between 6-60 GHz. However, these bands have been becoming congested in many countries [3]. Limited transmission range and interference with existing terrestrial services are also non-negligible technical issues of RF and microwave technology [2].
The free-space optical communication (FSOC) systems, thanks to their carrier frequency in the range of a few hundreds of THz, can deliver data at a rate of hundreds of Gbps over a long distance. The high carrier frequency also allows FSOC systems to avoid interference problems without exhausting scarce RF and microwave bands. Airborne platforms can be designed to operate in the stratosphere, where the atmospheric conditions are conducive to light propagation. At this high altitude, light scattering and attenuation from clouds, fog, and aerosols are much lower than at the ground level. Thus, FSOC systems could potentially be an ideal option for airborne interconnections, as shown in Fig. 1. 5G data services can be provided to the relatively unexplored regions of the world by constructing an aerial mesh network of airborne platforms placed at an altitude of approximately 20 km or above, where FSOC systems can be used for the interconnections between platforms
Figure 1. An example of airborne FSOC system for 5G networks.
2. Challenges
The pointing, acquisition, and tracking (PAT) required for airborne FSOC systems is technically challenging due to narrow divergence angle of optical beam and the movements/vibrations of airborne platforms [7]. It is worth mentioning that any PAT techniques should work within tight constraints of airborne platforms on size, weight, and power (SWaP).
The PAT, in general, comprises two phases, coarse and fine PAT. The major role of the coarse PAT is to achieve a rough alignment between the transmitter and receiver. A typical procedure for coarse PAT is to scan an area where the receiver is expected to be located using a beacon light having a relatively large divergence angle. When the receiver detects the beacon signal, it sends the acknowledgment immediately back to the transmitter side. The transmitter employs a separate optical beam with a narrow divergence angle for fine PAT and improves the alignment before sending data to the receiver.
The fine PAT is used to achieve a higher accuracy of the alignment between the transmitter and the receiver. It operates continuously to maintain the alignment. This is because the alignment could be disrupted, for example, by pointing errors and/or angular-of-arrival (AoA) fluctuations. A leading cause of pointing error is transmitter vibrations. Pointing error gives rise to a random movement of beam footprint on the receiver aperture plane. AoA fluctuations could arise from the vibrations at the receiver. They manifest themselves as spot motion or image dancing at the focal plane of the receiver. Thus, both pointing errors and AoA fluctuations severely degrade the performance of FSOC systems during the operation of fine PAT.
Despite the PAT procedure, it is inevitable that there still exists misalignment between the transmitter and receiver, and consequently the received optical signal suffers from frequency-independent fading. This can make or break the link availability of airborne FSOC systems.
3. Solutions
We have recently studied an adaptive beam control technique to facilitate the PAT [8]-[10]. The major purpose of this technique is to enhance the link availability degraded by pointing errors and AoA fluctuations even in the presence of PAT. We propose to employ variable focus lenses to adjust the beam sizes for a practical realization of the adaptive beam control technique. In this scheme, a variable focus lens at the transmitter produces an optical beam having a large divergence angle for coarse PAT. Thus, a single optical beam can be used both for PAT and data transmission. This beaconless scheme could help to simplify transmitter and receiver realization in FSOC systems. For the fine PAT, optimum beam sizes at both transmitter and receiver are realized by using variable focus lenses for mitigating the adverse effects of pointing errors and AoA fluctuations. The major benefits of this realization include that the electrically-adjustable variable focus lens allows the system to be implemented compactly, without resorting to mechanical movements that would be bulky, heavy, and prone to failure. Thus, this scheme would be suitable for airborne applications having tight SWaP constraints.
Figure 2. Adaptive beam control technique-based PAT for inter-airborne FSOC systems.
We also propose a new design of the receiver using an integrated beam position sensor and data photo-detector (PD). Besides data detection, this device provides feedback information about the beam location at the receiver, which is used for PAT and adaptive beam control. There are several benefits from the proposed design. First, this integrated device enables a compact design for the receiver. Second, the receiver sensitivity can be improved by the amount of insertion loss of a beam splitter, which is used in typical PAT subsystems. Third, it ensures precision in acquiring information for feedback control since both the sensor and data PD are placed on the same plane. In the conventional design where discrete sensor and data PD are used, an actual beam position of the data signal could be different from the readout from the sensor.
Fig. 2 shows a brief structure of the proposed scheme. At the transmitter, a single light source is employed for both PAT and data transmission. The optical signal generated from the source is first fed to a variable focus lens. The optical beam is then sent to a steering mirror to direct it towards the receiver.
At the receiver, another variable focus lens is used to collect the optical signal. A beam steerer is used to control the direction of the optical beam after the lens. The beam steerer can be implemented by using a fast steering mirror, micro-electro-mechanical systems, liquid crystal, or optical phased array. An integrated beam positioning sensor and data PD is located on the focal plane. In this device, the PD is placed in the middle of the beam positioning sensor. A good example of the beam position sensors is a quadrant detector composed of four PDs. Each of these PDs produces a photocurrent proportional to the power of the incident beam. The quadrant detector has two outputs: one provides the power differences between quadrants, and the other provides the total power detected by the beam position sensor. The former is sent to an AoA tracking controller. This controller adjusts the beam steerer at the receiver to minimize the beam displacement on the focal plane. The latter is sent back to the transmitter through the feedback channel. A low-power radio link, for example, can be used as the feedback channel between the transmitter and receiver. A PAT controller at the transmitter controls the beam steerer at the transmitter based on the feedback information to realize coarse and fine PAT.
It is noted that some information, such as airborne locations, transmit power, size of receiver aperture, and receiver sensitivity, should be exchanged between the transmitter and receiver through the feedback channel before the PAT starts. When the coarse PAT is initiated, the transmit beam size is enlarged. During communications, beam sizes at both transmitter and receiver are adjusted optimally for the system performance based on feedback and local information.
4. Conclusions
Free-space optical communication has the advantages of delivering high-speed data over a long wireless channel without exhausting scarce RF resources. On the other hand, the usage of airborne platforms for the implementation of aerial communication networks offers cost-effectiveness and rapid deployment. Thanks to joining those advantages, airborne FSOC systems could be an essential part of flexible, high-capacity fronthaul/backhaul networks for 5G+. However, the PAT is particularly challenging in inter-airborne FSOC systems. In this regard, the adaptive beam control technique could be applied to support a compact and robust PAT.
Other airborne FSOC systems, such as airborne-to/from-satellite and airborne-to/from-ground communications, could be also considered for 5G+ networks. In this case, there are more arising issues including strong atmospheric turbulence and long-range PAT. In addition to the presented techniques, more research from different fields, such as communications, photonics, and mechanical engineering, should be conducted to address the issues in the future.
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