Adaptive Reliable Receivers for Optical Wireless communication (ARROW)

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


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.

Until now we have 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 (APD) and single-photon avalanche diode (SPAD) array receiver also has the potential to combat the adverse weather conditions. We have done some investigations on employing such hybrid receiver for high speed links. In particular, we modelled the response of a SPAD array to high-speed communication by involving the effects of inter-symbol interference (ISI) which is commonly ignored in the literature for low speed data transmission. We have also proposed the use of photon arrival time information in designing an optimal detection scheme for SPAD-based photon counting receivers.

In addition, to operate the switch between two channels smartly, the statistical relationship between weather conditions and received background light is required. Until now, some initial results of the solar irradiance and sky radiance against the wavelength were achieved based on the weather data provided by Met office. The bit error rate of the hard-switching hybrid receiver was demonstrated which justified the effectiveness of the proposed hybrid receiver on keeping the link availability in a wide range of weather conditions.

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 of SPAD based receiver in the presence of ISI.
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. Developing new technique on mitigating ISI can be employed in the practical SPAD-based communication systems to improve the bit error rate and reduce the number of SPADs required to achieve a target performance. This technique will allow the SPAD receivers to operate at a sub-dead-time regime which can significatly increase the acheivable data rate.
Sectors Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics

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 Computer model/algorithm 
Year Produced 2019 
Provided To Others? No  
Impact It is demonstrated that significant BER performance can be achieved by using the proposed scheme. 
Title Modelling the communication performance of hybrid optical receiver 
Description The bit error rate of a novel hybrid optical receiver is simulated which can maintain the required availability over a wide range of weather conditions. 
Type Of Material Computer model/algorithm 
Year Produced 2019 
Provided To Others? No  
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 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 Computer model/algorithm 
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 atteunations under adverse weather conditions. 
Type Of Material Computer model/algorithm 
Year Produced 2020 
Provided To Others? No  
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. 
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