MIMO coding and linear equalization schemes for asynchronous visible light communication system
Date of Issue2016-02-02
School of Electrical and Electronic Engineering
Photonics Research Centre
In this thesis, we introduce a new form of indoor wireless communication: visible light communication (VLC). Both the traditional RF communication and visible light communication are propagated through free space, hence many coding schemes for RF communication could also be applied to VLC. MIMO coding is suitable for VLC for two reasons: first, there are always multiple illumination sources in a room in practice, these light sources could work as multiple transmitters for MIMO VLC; secondly, the shadowing effect is very common in a VLC system, and MIMO coding could address this problem very effectively. Many MIMO coding schemes such as Orthogonal Space-Time Block Code (OSTBC) and Shift-Orthogonal Space-Time Block Code (SOSTBC) have been reported. These coding strategies are suitable under different conditions. In order to apply these orthogonal MIMO schemes, negative-value symbols need to be transmitted, but the light signal in low-cost VLC system is mostly intensity modulated with energy detection (particularly when the LED is not in the illumination mode but is still desired to be in the communication mode), so we cannot transmit negative symbols in the transmitters directly. To address this problem, we investigate special modulation schemes for intensity modulation to represent the negative symbols, and compare their performances with other transmission schemes under different channels. Furthermore, there are some practical system problems such as timing misalignment and multipath effect caused by un-synchronized transmitters and reflections. So the MIMO coding schemes should be adapted with equalization for these channels. Besides the SOSTBC mentioned above, for intensity modulated VLC systems we could also use Repetition Coding with Single-Carrier Frequency Domain Equalization (RC with SC-FDE) and Repetition Coding with Single-Carrier Over-Sampling Frequency Domain Equalization (RC with SC OS-FDE) to deal with these problems. In this thesis, these schemes are further extended for asynchronous channel with fractional delays. Finally we realize the VLC system on a hardware test bed. To build the MIMO VLC experimental test bed, we use an arbitrary waveform generator as a signal generator, and a digital oscilloscope as a signal receiver. The optical transmitter is a cool white LED, and the optical receiver is a wide bandwidth photodetector. Other necessary accessories include bias tees and constant voltage source, both of which are used to provide a DC bias voltage for the LED. The three MIMO schemes mentioned above are all realized in the experiment. Experimental BER results are collected, with low transmission rate (1Mb/s) and high transmission rate (10Mb/s), and with different asynchronous channel environment (integer delays and fractional delays). The experimental results largely verify the theories.