LED-to-LED communication: a bidirectional low-complexity system based on RGB LEDS as receiver

Galal, Miriam (2020) LED-to-LED communication: a bidirectional low-complexity system based on RGB LEDS as receiver. Doctoral thesis, Northumbria University.

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Abstract

This thesis studies, designs and characterises a low-complexity bidirectional LED-to-LED communication system employing only off-the-shelf tricolour light emitting diodes (LEDs) as transmitters and receivers with the aim of providing the required connectivity for contemporary small Internet-of-things (IoT) devices. In today’s interconnected world of Internet of Things devices, it is imperative to have low-complexity and small size communication modules to enable seamless communication between such small devices over short distances and with low data rates without exhausting the already over-crowded radio frequency spectrum. While a regular visible light communication (VLC) system containing an LED and a photodiode is a viable option, it provides only one-directional communication. To enable an uplink, a repetition of the downlink is usually provided, doubling the cost, complexity and space of the original circuits. Hence, an LED-to-LED solution maximises the benefit of VLC and imitates the idea of having one antenna for transmitting and receiving, known in radio frequency (RF) technologies.

In order to design an LED-to-LED system, the LED is characterised as transmitter and receiver both qualitatively and empirically. The results confirmed the feasibility of the LED as a wavelength-selective and directional photodetector with a field of view of 18° and low responsivity of 0.2 mA/W under zero bias. Interestingly, the experiments also reveal that the LED could even detect light when slightly forward biased. Based on the identified characteristics of the LED, a current-mirror-based LED driver, a transimpedance amplifier and an analogue switch are designed and separately evaluated to mitigate the low responsivity of the LED as photodetector. The VLC channel is also characterised and the interference due to the most common contemporary ambient light sources is evaluated. The interference of each light source is mathematically modelled based on the empirical results. The overall LED-to-LED link is practically implemented and optimised to reach data rates up to 200 kbps at distances up to 7 cm with the help of an optimised matched filter at the detection phase. The analogue switch completes the system by providing the option of a bidirectional half-duplex channel while avoiding flickering. At a switching frequency of up to 300 Hz, the LED-to-LED system could deliver error-free data transmission at speeds up to 100 kbps in a back-to-back configuration.

The findings of this thesis indicate that a bidirectional low-complexity VLC system depending only on RGB LED for transmission and photodetection is not only feasible, but also a very viable option for short-range, low-speed communication between small IoT devices with high potential of commercial deployment.

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