Visible light commuinications using optical OFDM

Thesis

This thesis presents an investigation into designs and developments of indoor visible light communication (VLC) systems using orthogonal frequency division multiplexing (OFDM) schemes. The novel contribution of this thesis is a development of a visible light communication system that incorporates OFDM and imaging-diversity multiple-input multiple-output (MIMO) techniques, which allows robust transmissions on multiple channels at high data rates.

The characteristics of VLC systems are presented, one of which is VLC communications channel exhibits high Signal-to-Noise Ratio (SNR). The major constraint however, is the low modulation bandwidth of typical high power white LED sources.

The performance of OFDM as a modulation scheme is investigated. OFDM offers the possibility for bit and power loading to increase bandwidth efficiency, as well as a straightforward equalisation in time and frequency domains to compensate the low pass frequency response of the LED. This allows transmission rates of up to ~310Mbps at a BER of 2x10-3 on a single-channel link using 16- QAM DCO-OFDM. Further increase in data rates for this transmission is constrained by the available signal power due to a dynamic range limitation at the receiver. An increase in signal power will results in the OFDM waveform to be clipped, which incurs a clipping noise.

MIMO systems offer a linear capacity gain to a number of transmission channels in an ideal configuration. These systems mitigate the dynamic range limitation, as power is divided between multiple channels. The performance of a MIMO transmission system is investigated under the same dynamic range constraint. A 4-channel MIMO transmitter is built using four sets of transmitter components of the single-channel link. An imaging system is used to separate the received optical power onto multiple detectors, and MIMO processing relaxes the requirements for precise mechanical alignments. The experiment has shown that the imaging MIMO system further improves data rates and transmissions of up to ~1.1Gigabit/s at a BER of 2x10-3 are achieved. To the best of the author’s knowledge, this is the highest reported for an indoor VLC MIMO-OFDM transmission. Also presented is an analysis of MIMO system scalability to provide room-wide coverage.

Finally, this thesis presents an extended analysis of several other optical OFDM approaches, and concludes with recommendations of future work to increase the data rates of indoor VLC systems.

Authors: Azhar, A

• Publication date: 2013
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: Oxford University, UK
• Language: English
• Keywords: MIMO; OFDM; VLC
• Subjects: Communications engineering (optical,microwave and radio)