Adaptive Reliable Receivers for Optical Wireless communication (ARROW)

Lead Research Organisation: University of Edinburgh
Department Name: Sch of Engineering

Abstract

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.

Planned Impact

Economic impacts:

The fixed and mobile data traffics in the UK are set to increase, respectively, at rates of 100% every two years and 25% to 42% per year. ARROW will lay an important foundation to support the efficient expansion of UK's digital communication infrastructure by developing high-speed alternative solutions based on the emerging but affordable optical wireless technology. ARROW receivers are attractive solutions for both terrestrial (e.g., backhaul and broadband connectivity) and satellite-to-ground applications where atmospheric effects can be limiting.

ARROW will potentially impact photonics, and telecommunication industries within the manufacturing sector by providing new designs and applications that can be well adopted inline with the large expansion of 5G networks nationally and internationally. Our partnership with ST Microelectronics will help us to accelerate such impacts by translating the research conducted in the project into higher technology readiness levels and ultimately generating economic impact through licensing or spin-out formations.

A recent research reports that increased broadband speeds alone could add £17 billion to UK output by 2024, which strongly suggests that a commercial success of ARROW can deliver potential economical impacts on many small and large industries particularly within the service sector that rely on digital economy. This would include the banking system, streaming service providers, social media service providers, cloud computing providers, online retailers, etc.

Societal Impacts:

With the advent of Internet of things, the dependence of our daily life to the quality and robustness of Internet connections become more apparent. Another important societal issue that we are facing today is the migration of a significant portion of daily social interactions into social media. This amplifies the expectations of the general public for ubiquitous data connectivity with high levels of quality of service. Therefore, ARROW can potentially impact the society and the general public by contributing to the improvement of mobile and broadband networks as our today's day-to-day activities, more than ever, depend on data connectivity. In general, the project promotes the use of light as a natural and economical medium for carrying data supporting high-speed, ubiquitous, and secure data connectivity, which would be of wide societal interest.

Impacts on the professional community:

ARROW's impacts will be further extended by contributing into the development of expert researchers/engineers in the emerging field of optical wireless communication. In particular, the project will be an important step in the professional development of the research associate and PhD students who would be involved in this project. Furthermore, ARROW will disseminate knowledge to the wide community of engineers working in telecommunications, optoelectronics, photonics and quantum technology. This knowledge will be in the form of fundamental theoretical models, novel designs and experimental data.
 
Description Free-space optical communication (FSO) offers wireless connectivity with high data rates and low system complexity, however it is significantly influenced by infrequent adverse weather conditions, e.g., fog. This project aims to design high-speed optical wireless communication links that can reliably operate over all weather conditions while providing high data rates. We first designed a hybrid radio frequency (RF) / FSO link by using a game theoretic spectrum trading process. In the proposed system, the FSO link source can borrow a portion of licensed RF spectrum from one of the surrounding RF nodes to establish a RF link to support optical link when it is non-functional due to the adverse weather conditions. It is demonstrated that the proposed system has the potential to enhance the FSO link availability towards the carrier-class requirement. Alternatively, hybrid classical photodetector (PD) and single-photon avalanche diode (SPAD) array receiver also has the potential to combat the adverse weather conditions. We have proposed a novel highly sensitive dual-mode receiver comprising these two types of receiver. To operate the switch between two modes smartly, the statistical relationship between weather conditions and received background light has been investigated. The bit error rate and achievable data rate of the proposed hybrid receiver was demonstrated confirming its effectiveness in keeping the link availability in a wide range of weather conditions. The proposed receiver can be employed not only in FSO systems but also in other optical wireless communication (OWC) systems, e.g., visible light communication (VLC) and underwater wireless optical communications (UWOC), to enhance the performance.
Furthermore, some novel techniques have been proposed in this project to effectively mitigate the dead time effects of SPAD which can also be used in general SPAD-based OWC systems. We modeled the response of a SPAD array to high-speed communication by incorporating the effects of inter-symbol interference (ISI) which is commonly ignored in the literature for low speed data transmission. We have proposed for the first time the use of photon arrival time information in designing an optimal detection scheme for SPAD receivers. The use of the variable optical attenuator (VOA) is also proposed to optimally adjust the incident photon rate of SPAD array to maintain its performance. Moreover, we propose to use time-gated SPAD receivers in OWC systems and the optimal gate-ON time interval has been investigated. We have also developed a laboratory based experimental setup that has verified the performance gains promised by some of our proposed techniques particularly for optimal detection in the presence of dead-time-induced ISI and the performance analysis of the dual-mode SPAD/PD receiver.
Exploitation Route The current findings of the project on hybrid RF/FSO links can inspire the researchers to design more efficient and cost-effective hybrid RF/FSO links to improve the reliability and availability of the current commercial FSO links. The proposed new techniques on mitigating dead time effects of SPAD can be employed in commercial SPAD-based communication systems to improve the bit error rate and achievable data rate and reduce the number of SPADs required to achieve a target performance. Moreover, a number of innovations were proposed that can be considered for commercialization of robust optical receiver for different applications such terrestrial and satellite FSO communications as well as underwater communications. In particular, a UK patent application has been filed in 2020 to protect one of the above mentioned innovations.
Sectors Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics

 
Title Demonstration of the high speed SPAD-based optical wireless communication systems 
Description In recent decades, optical wireless communication (OWC) attracts more and more attention in both research and industry communities. To improve the receiver sensitivity in the lower-power regime, single-photon avalanche diodes (SPAD) can be employed. However, SPAD suffers from strong nonlinearity effects which limits its achievable data rate. We have experimentally demonstrated that a well-designed OFDM scheme adapted to SPAD nonlinearity can achieve a data rate of 5 Gbps which is significantly higher than the highest data rate achieved in the literature. This record data rate is achieved by the mitigation of SPAD nonlinear distortion through nonlinear equalizer based on Volterra series model, the optimization of peak-to-average power ratio by controlling OFDM clipping level, and finally the employment of adaptive bit and energy loading algorithm. 
Type Of Material Improvements to research infrastructure 
Year Produced 2021 
Provided To Others? Yes  
Impact The developed method of improving the data rate of SPAD-based OWC systems can give insights on the application of SPAD in the future high-speed OWC links. 
URL https://arxiv.org/abs/2111.07123
 
Title Demonstration of the hybrid SPAD/PD optical wireless communication receiver 
Description To achieve a resilient optical wireless communication (OWC) receiver that can operate in highly dynamic environment, we have experimentally demonstrated a hybrid SPAD/PD receiver which takes advantage of both high sensitivity of SPAD detector in low power regimes and high SNR of PD detector in high power regimes. The proposed receiver can adapt to the instantaneous channel condition. By adjusting the operation mode and transmittance of the variable optical attenuator with the change of the channel condition, the receiver can always achieve the best performance when operated in the dynamic environment. 
Type Of Material Improvements to research infrastructure 
Year Produced 2021 
Provided To Others? Yes  
Impact The proposed technology offers an alternative solution for enhancing the resilience of wireless communication in various applications including Air (e.g., reliable and high-speed airborne free-space optical (FSO) communication among drones, high-altitude platforms, aircraft, satellites and ground), Land (e.g., FSO backhaul from the gateway to deployed station, land to air FSO links) and Maritime (e.g., ship to shore FSO links as well as underwater optical communications). 
 
Title Effective techniques to mitigate the Inter-symbol interference (ISI) in SPAD receivers 
Description In the literature, some equalisation techniques used for traditional RF communication systems are employed for mitigating the effects of inter-symbol interference (ISI) in optical wireless communication (OWC) systems with single-photon avalanche diodes (SPADs). However, different from the traditional communication systems, the channel expression of SPAD receivers is inherently non-linear due to its unique characteristics. Therefore, all of the equalisation methods originally designed for systems with linear channel expressions are sub-optimal and cannot result in significant performance improvement in systems with SPAD receiver. This motivates us to design a novel ISI mitigation method for SPAD-based systems. We have designed a new ISI mitigation technique based on the special characteristics of SPADs. 
Type Of Material Improvements to research infrastructure 
Year Produced 2020 
Provided To Others? Yes  
Impact It is demonstrated that significant BER performance can be achieved by using the proposed scheme. 
 
Title Modelling the communication performance of hybrid SPAD/PD optical receiver for FSO systems 
Description A novel hybrid SPAD/PD optical receiver adapted to the change of weather conditions is proposed. The bit error rate and achievable data rate of the proposed receiver are simulated which demonstrate that the proposed receiver can maintain the required availability over a wide range of weather conditions. 
Type Of Material Improvements to research infrastructure 
Year Produced 2020 
Provided To Others? Yes  
Impact The designed hybrid receiver can significantly reduce the probability of non-functional link due to the adverse weather conditions. In addition, it is beneficial from low complexity and low cost. The proposed design of hybrid receiver has great potential of being employed in the commercial free-space optical links. 
 
Title Modelling the communication performance of optical wireless communication system with time-gated SPAD receivers 
Description SPAD-based receivers suffer from significant dead-time induced intersymbol interference (ISI) and strong background light. The time-gated SPAD receivers can be employed to combat these effects and improve the communication performance. 
Type Of Material Improvements to research infrastructure 
Year Produced 2020 
Provided To Others? No  
Impact It is demonstrated that the gate-ON time interval can be optimized to achieve the best performance. Our extensive performance analysis illustrates the superiority of the time-gated SPAD receiver over the traditional free-running receiver in terms of the BER performance and the tolerance of background light. 
 
Title Modelling the response of a SPAD array to high-speed communication 
Description The channel model of an optical wireless communication system with single-photon avalanche diode (SPAD) array receiver is established in MATLAB. In most of the studies in the literature, the effects of dead time and dead-induced inter-symbol interference are ignored by assuming that the symbol duration is much longer than dead time. However, in high-speed data transmission, the symbol duration is on the order of a few to tens of nanosecond which is comparable to the typical dead time of SPADs. In this scenario, the decrease of detected photon counts and increase of inter-symbol interference induced by dead time can significantly degrade the communication performance. A channel model of SPAD receiver involving these effects was established which can be employed to simulate the accurate communication performance in practical system with SPAD receiver. 
Type Of Material Improvements to research infrastructure 
Year Produced 2018 
Provided To Others? No  
Impact It is a more accurate model for modelling the communication performance of the SPAD-based optical wireless communication systems. 
 
Title Modelling the signal and background light atteunation under adverse weather conditions 
Description The performance of FSO links are significantly influenced by the adverse weather conditions which changes not only the received optical signal power but also the received ambient light from the sky or sun. Although there are several models in the literature which describe the signal power attenuation in fog conditions, they are all based on some approximations which might be inaccurate under some weather conditions. In addition, there are lack of the models for the ambient lgiht attenuations in the literature. This motivates us to come up with a proper model to describe both the signal and background light attenuations under adverse weather conditions. 
Type Of Material Improvements to research infrastructure 
Year Produced 2020 
Provided To Others? Yes  
Impact An accurate model of the light attenuations ( both signal and background light) under various weather conditions can help people to simulate the communication performance of the practical FSO links more accurately. 
 
Title SPAD-based optical wireless communication system with signal pre-distortion and noise normalization 
Description We propose a novel SPAD-based OWC system in which the non-linear distortion caused by dead time can be successfully eliminated by the pre-distortion of the signal at the transmitter and for the transformed signal at the receiver, the originally signal dependent noise becomes signal independent so that the conventional signal detection techniques designed for AWGN channels can be employed to decode the signal. 
Type Of Material Improvements to research infrastructure 
Year Produced 2020 
Provided To Others? No  
Impact Our numerical results demonstrate the superiority of the proposed system compared to the state-of-the-art systems in terms of BER performance and achievable data rate. 
URL https://arxiv.org/abs/2101.09333
 
Description Collaborations with STMicroelectronics 
Organisation STMicroelectrics
Country Switzerland 
Sector Private 
PI Contribution We proposed a novel optimal detection scheme for SPAD-based FSO receivers.
Collaborator Contribution In order to implement our propsoed scheme experimentally, a SPAD board from Prof Robert Henderson's group in Institute for Integrated Micro and Nano Systems was employed. This design of the board is in collaboration with STMicroelectronics.
Impact Based on this collaboration, a journal paper titled 'Optimal Photon Counting Receiver for Sub-Dead-Time Signal Transmission' was finished and submitted to Journal of Lightwave Technology. This collaboration is not multi-disciplinary.
Start Year 2019
 
Title ADAPTIVE HYBRID OPTICAL DETECTION 
Description A receiving device (12) for receiving an optical communication signal, wherein the optical communication signal comprises an encoded or modulated signal, the device comprising: one or more photodetectors (12) configured to produce photodetector signals in response to detecting photons; one or more further photodetectors (14) configured to produce further photodetector signals; a controller (16) configured to select an operational mode of the receiving device in dependence on at least a light level, wherein the operational mode is one of at least a first mode in which a demodulation or decoding process is performed on the photodetector signals and a second mode in which the demodulation or decoding process is performed on the further photodetector signals, and a photon count limiter (18) associated with the one or more photodetector for controlled limiting of the photon count of the one or more photodetectors in dependence on at least a light level. 
IP Reference WO2022034055 
Protection Patent application published
Year Protection Granted 2022
Licensed No
Impact In this patent, we invent a hybrid SPAD/PD receiver which takes advantage of both high sensitivity of SPAD detector in low power regimes and high SNR of PD detector in high power regimes. The proposed receiver can adapt to the instantaneous channel condition. By adjusting the operation mode and transmittance of the variable optical attenuator with the change of channel condition, the receiver can always achieve the best performance when operated in the dynamic environment. The proposed receiver
 
Description Disseminating the projects in the Disruptive Wireless Communication Paradigms: The Road Towards 6G, Indian Institute ofInformation Technology Guwahati, India, 2021 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Presented the results of the project before a group of students and academics and professionals from in the Disruptive Wireless Communication Paradigms: The Road Towards 6G, Indian Institute ofInformation Technology Guwahati, India, 2021. The even was held virtually and sparked questions and discussions during and after the event.
Year(s) Of Engagement Activity 2021
URL https://sites.google.com/view/dwcp-6g/home?authuser=0
 
Description Disseminating the projects results before a group of students and academics from Beihang University, China in 2020. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Undergraduate students
Results and Impact More than 50 undergraduate and post graduate students attended along with a number of academics from Beihang university and that sparked questions and discussion afterwards. This event was held virtually.
Year(s) Of Engagement Activity 2020
 
Description GLOBECOM 2021 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact In GLOBECOM 2021, a flagship conference in the area of digital communications, we have presented our work titled 'Reliable Optical Receiver for Highly Dynamic Wireless Channels: An Experimental Demonstration'. The audiences are the researchers and students from the universities and industries all over the world.
Year(s) Of Engagement Activity 2021
 
Description IEEE 5G World forum, Montreal, Canada 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Delivered an invited talk on "Resilient Free-Space Optical Communication for 5G and Beyond" for professionals, acedemic and industrial audience attending the IEEE 5G World Forum held in Montreal, Canada. The invited talk was delivered in the OWC topical session of the IEEE 5G World forum. The talk was attracted interests on how a resilient design can be developed for the next generation of optical wireless communication systems.
Year(s) Of Engagement Activity 2021
URL https://attend.ieee.org/wf-5g/optical-wireless-communication-owc/
 
Description WCNC 2020 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact In 2020 IEEE Wireless Communications and Networking Conference (WCNC) we have presented our work titled 'Spectrum trading in hybrid RF/FSO communications: a stackelberg game approach'. The audiences are the researchers and students from the universities and industries all over the world.
Year(s) Of Engagement Activity 2020