Adaptive Reliable Receivers for Optical Wireless communication (ARROW)

移动的蜂窝网络中小区大小的逐渐缩小和应用频率复用技术已经是在过去几十年中科普容量需求的指数增长的主要方法。然而，5G小区的室外部署需要对回程网络进行大规模扩展。最优选的回程解决方案基于高度可靠和高速的光纤链路;然而，由于压倒性的安装成本，它们的使用仅限于当前回程网络的一小部分。自由空间光（FSO）通信是一种有吸引力的替代解决方案，其提供高容量但具有成本效益的无线回程连接，而不干扰射频（RF）通信系统。然而，尽管有几十年的技术进步，FSO链路仍然存在可用性问题，在恶劣天气条件下偶尔会出现长时间中断。这是因为经典的高速FSO接收器（如雪崩光电二极管（APD））可能在低能见度天气条件下完全失效。因此，重要的问题是，我们是否可以建立高速大气光通信链路，使其能够在所有天气条件下可靠地运行，同时提供超过其射频同行的数据速率。ARROW旨在通过结合经典和量子光学接收来解决上述问题，以允许FSO接收器在广泛的灵敏度水平范围内自适应操作，同时保持高速通信。然而，诸如单光子雪崩二极管（SPAD）的高灵敏度量子检测器实际上不适合于地面FSO链路，因为它们可以容易地在这样的链路处经历的典型高辐照度水平下饱和，而它们的带宽受到诸如死区时间的影响的限制。ARROW的混合接收机采用APD沿着，并将大量SPAD集成到单个芯片中。阵列的大尺寸有效地缓解了基于SPAD的检测器的饱和问题，同时允许可以显著提高其可实现的数据速率的频谱高效调制。ARROW接收机将结合联合收割机的经典和量子探测器的功能，使用硬和软光学开关和有效的数字信号处理，以支持基于缓慢变化的天气条件的自适应操作。为了设计有效的切换和信号处理，我们将开发一个准确但易于处理的理论模型，该模型根据不同的大气效应（例如，可见度和背景光水平）以及它们与混合接收器的特性的相互作用（例如，SPAD死时间、检测器视场和分光比）。基于此模型，将提出一些光前端设计和先进的调制和联合编码方案，以提高数据速率和接收机的可靠性。最后，混合接收机的自适应功能将被实验证明和验证。ARROW FSO接收机预计将为各种实际链路几何形状和地理位置提供运营商级可用性。

The gradual shrinkage of cell sizes in mobile cellular networks and applying frequency reuse techniques has been the main approach to cope with the exponential growth of capacity demands over the last few decades. However, the outdoor deployment of 5G cells will require a large-scale expansion of the backhaul network. The most preferred backhaul solution is based on highly reliable and high-speed fibre optic links; however, their use is limited to a fraction of the current backhaul network because of overwhelming installation costs. Free space optical (FSO) communication is an attractive alternative solution that provides high-capacity but cost-effective wireless backhaul connectivity without interfering with radio frequency (RF) communication systems. However, despite decades of technological advances, FSO links still suffer from availability issues in the form of occasional long outages in adverse weather conditions. This is because classical high-speed FSO receivers such as avalanche photodiodes (APDs) may totally fail under low visibility weather conditions. The important question is, therefore, whether we can build high-speed atmospheric optical communication links that can reliably operate over all weather conditions while providing data rates beyond their RF counterparts. ARROW aims to address the question above by combining classical and quantum optical receptions to allow for adaptive operation of FSO receivers within a wide range of sensitivity levels while keeping high-speed communication. However, highly sensitive quantum detectors such as single photon avalanche diodes (SPADs) are not practically suitable for terrestrial FSO links as they can easily saturate at typically high irradiance levels experienced at such links while their bandwidth is limited by effects such as dead time. ARROW's hybrid receiver employs an APD along with a large array of SPADs integrated into a single chip. The large size of array effectively relaxes the saturation issue of the SPAD-based detector while allowing for spectrally efficient modulations that can significantly improve its achievable data rate. ARROW receivers will combine the functionality of the classical and quantum detectors using hard and soft optical switching and efficient digital signal processing to support adaptive operation based on the slow varying weather condition. In order to design efficient switching and signal processing, we will develop an accurate but tractable theoretical model that describes the hybrid channel in terms of different atmospheric effects (e.g., visibility and background light level) and their interaction with the hybrid receiver's characteristics (e.g., SPAD dead time, detectors field of view, and optical splitting ratio). Based on this model, a number of optical frontend designs and advanced modulation and joint coding schemes will be proposed to enhance both data rate and reliability of the receiver. Finally, the adaptive functionalities of the hybrid receiver will be experimentally demonstrated and validated. ARROW FSO receivers are expected to provide carrier grade availability for a wide range of practical link geometries and geographical locations.