Experimental Investigation of the Generalised Multi-Band Carrier-less Amplitude and Phase Modulation in Visible Light Communications

Werfli, Khald Abdelrhiem (2018) Experimental Investigation of the Generalised Multi-Band Carrier-less Amplitude and Phase Modulation in Visible Light Communications. Doctoral thesis, Northumbria University.

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Abstract

The existing lighting infrastructure for homes and offices features a number of lighting sources that are used in order to provide the required level of illumination. This can be utilised, from the communications point of view, to increase the data throughput in visible light communication (VLC) systems. In general, VLC systems, which utilise white light-emitting diodes (LEDs), only offer a bandwidth limited to the lower MHz region. Therefore, realising VLC-based high data rate communication systems is a challenging task. In the last decade, numerous solutions were proposed to overcome the LED’s limited bandwidth. One of straight forward solutions is to adopt an advanced modulation technique such as carrier-less amplitude and phase (CAP) or orthogonal frequency division multiplexing.
In this thesis, a solution based on multiplexing in both the frequency and space domains is proposed. This thesis investigates and analyses the performance of a new modulation scheme, known as multi-band carrier-less amplitude and phase (m-CAP) modulation, within the context of VLC. The principle of m-CAP is outlined and to further enhance its bandwidth performance the decision feedback equaliser (DFE) is proposed.
An experimental investigation of the m-CAP VLC system shows that a transmission data rate of 31.5 Mb/s over a link span L of 1 m is possible in an indoor environment. Coupling DFE with high order m-CAP systems (i.e., m = 5, 10) is found to be impractical, due to the ability of m-CAP in mitigating the inter-symbol interference imposed by the limited LED’s bandwidth. In addition, the impact of some of the utilised parameters, such as the roll-off factor and the pulse shaping finite impulse response (FIR) filter length, on the performance of m-CAP-VLC links is investigated. The proposed new m-CAP system offers 80%, 75%, ~67% and 50% in system complexity, which is achieved by reducing the number of FIR filters in comparison with the conventional 10, 8, 6, and 4-CAP, respectively. In addition, by adopting a nonuniform distribution of m over the total signal bandwidth Btot (i.e., variably distributing the bandwidth of the allocated sub-carriers so that x sub-carriers are located within the LED’s modulation bandwidth Bmod region, and the remaining y = m-x sub-carriers are spread equally on the across of the roll-off region of the LED’s frequency response), an improvement in the data transmission of ~36% for the 61/5-CAP scheme is attainable in comparison to the conventional 6-CAP system. Results show that, both imaging multiple-input multiple-output (MIMO) and m-CAP have a remarkable capabilities to improve transmission speeds in VLC systems, and hence, here, the two schemes are combined to further improve the net transmission data rate.
In addition, an experimental test-bed of a 4 × 4 imaging MIMO based m-CAP VLC system (i.e., space multiplexing) is developed in order to assess and validate the proposed system performance. Furthermore, an investigation of the link performance as a function of the number of sub-carriers m, L, and signal bandwidth Bsig, is carried out. For all values tested, a maximum data rate of ~249 Mb/s is achieved for m = 20, Bsig = 20 MHz, and L = 1 m, at a bit error rate of 3.2×10-3, using LEDs with ~4 MHz bandwidth.

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