Adaptive Reliable Receivers for Optical Wireless communication (ARROW)

在过去的几十年里，移动蜂窝网络中小区尺寸的逐渐缩小和频率重用技术的应用一直是应对容量需求指数增长的主要方法。然而，5G小区的室外部署将需要对回程网络进行大规模扩容。最受欢迎的回程解决方案是基于高度可靠的高速光纤链路；然而，由于安装成本过高，它们的使用仅限于当前回程网络的一小部分。自由空间光通信(FSO)是一种有吸引力的替代解决方案，它提供高容量但经济高效的无线回程连接，而不会干扰射频(RF)通信系统。然而，尽管经过了几十年的技术进步，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)，在低能见度的天气条件下可能会完全失效。因此，重要的问题是，我们是否能够建立高速大气光通信链路，在提供高于射频同行的数据速率的同时，能够在任何天气条件下可靠地运行。Arrow旨在通过将经典和量子光学接收器相结合来解决上述问题，以便在保持高速通信的同时，允许FSO接收器在广泛的灵敏度级别内进行自适应操作。然而，高灵敏度的量子探测器，如单光子雪崩二极管(SPAD)，实际上并不适合地面FSO链路，因为它们可以很容易地在这种链路上经历的高辐照度水平下饱和，而它们的带宽受到死区时间等影响的限制。Arrow公司的混合接收器采用了雪崩光电二极管以及集成到单个芯片中的大量SPAD阵列。阵列的大尺寸有效地缓解了基于SPAD的检测器的饱和问题，同时允许频谱高效的调制，从而显著提高其可实现的数据速率。Arrow接收器将结合经典和量子探测器的功能，使用硬、软光学开关和高效的数字信号处理，以支持基于缓慢变化的天气条件的自适应操作。为了设计高效的交换和信号处理，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级别)以及它们与混合接收器的特性(例如SPAD死区时间、探测器视场和光学分束比)的相互作用来描述混合信道。基于该模型，将提出一些光前端设计和先进的调制和联合编码方案，以提高接收机的数据速率和可靠性。最后，对混合接收机的自适应功能进行了实验验证。Arrow FSO接收器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.