Free space optics for ground-to-train communications

Mohan, Nithin (2023) Free space optics for ground-to-train communications. Doctoral thesis, Northumbria University.

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Abstract

Development of a high-speed railway network such as High Speed 2 in the UK aims to connect the country and is expected to ferry around 300,000 passengers per day. The ubiquitous use of hand-held devices such as smart-phones and laptops would lead to increased demand in the network bandwidth on board high-speed trains (HSTs). Around 66% of young people consider the availability of high-quality internet services on trains to be very important, and close to a third of them would prefer to travel by train if onboard internet services are improved. In today’s hyper-digital era, onboard internet services via WiFi have become less of a handy convenience and more of a necessity for rail passengers at the global level. It is estimated that an 8-carriage train carrying close to 800 passengers in the UK would require 1.2 Gb/s of bandwidth, taking into consideration a monthly mobile data consumption of 20 GB per person. In the UK, CrossCountry and London North-Eastern Railways companies amongst others provide on-board Wi-Fi services, albeit, discouraging consumers from streaming and large file transfers. Additionally, the Wi-Fi user’s access is restricted on CrossCountry trains if their data usage exceeds 70 MB and 100 MB in the standard and first-class cabins, respectively. Furthermore, as more and more passengers access WiFi on trains, operators are struggling to provide quality services that are up to the task, thus discouraging people to use trains. To improve the connectivity for rail passengers, network providers need to make fundamental improvements to the infrastructure that serves the areas along each route.

It is widely understood that RF technologies, with their limited spectrum availability, would fail to handle a high volume of information. Therefore, adopting the complementary optical wireless communication, mostly free space optical systems, has been considered for high-speed trains due to its wide spectrum, higher data rates. However, in outdoor environments, one of the key issues in the intensity modulated/direct detection free space optical communication system is the degradation of the links' performance due to the atmospheric conditions such as fog, snow, smoke, and turbulence. Another key challenge in implementation of FSO for ground-to-train communications is providing a large coverage area, which reduces the deployment costs of establishing a base station and provides uninterrupted internet service with negligible handover delay. Handover management is another key challenge faced by ground to train communications. conventional mobility protocols do not guarantee the quality of service during handovers, but rather focus on maintaining IP session connectivity.

To address the challenges mentioned above, this work proposes, firstly, a coverage model for ground-to-train communication we developed the concept of sectorised base station for ground to train communication spanning a coverage length of 210m and provide an architecture for relayed FSO communication between base stations to minimise handover latency. Secondly, we analysed the performance of polar codes under the effect of weak turbulence to determine the optimum code-rate to achieve a bit error rate (BER) of 10-9. Considering the successive cancellation decoder for scintillation indices of 0.12, 02 and 0.33 the optimum code rate was found to be 0.6, 0.33 and 0.2 respectively, for a confidence limit of 95% using Monte-Carlo simulations. Lastly, a dedicated software-defined-network (SDN)-based mobility architecture has been developed for handover management in ground-to-train (G2T) communication. An FSO experimental testbed was designed using off-the-shelf components which includes Raspberry-Pis to test the performance of the protocol. The results showed that the proposed scheme offered seamless handover with an average data transfer and throughput of 256 Mbps, respectively dropping down to 196 Mbps during the handover time.

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