Evaluation of spectrally efficient indoor optical wireless transmission techniques

Optical wireless communications (OWC) has the potential to become a remedy for the shortage of the radio frequency (RF) spectrum. Especially in indoor environments, OWC could enable wireless home networking systems which offload data traffic from existing RF systems. In OWC, data is transmitted by modulating the intensity of light sources, typically incoh erent light emitting diodes (LEDs). Thus, OWC systems employ intensity modulation (IM) an d direct detection (DD) of the optical carrier. Since off-the-shelf LEDs h ave a limited modulation capability, the transmission bandwidth of practical OWC systems is restricted. C onsequently, the available bandwidth has to be used efficiently. In this thesis, spectrally efficient optical wireless transmission techniques a re ev luated. Firstly, multiple transmitter-receiver techniques are investigated. These multiple-inputmultiple-output (MIMO) techniques provide high spectral efficiency, and therefore h igh data rates. Specifically, the MIMO techniques repetition coding (RC), spatial multiplexing (SMP) and sp atial modulation (SM) are analysed for indoor OWC. The performance of these techn iques is evaluated analytically and by means of computer simulations. It is shown that inducing pow er imbalance between the multiple optical transmitters can substantially improve the perfor mance of optical MIMO techniques as the power imbalance improves the differentiability o f the multiple channels. In addition, it is found that link blockage and the utilisation of trans mitters having different optical wavelengths enhance channel differentiability as well. These methods enable the utilisation of optical MIMO techniques under conditions which typically disallo w the application of MIMO schemes due to little differences between the multiple links. Secondly, a novel optical wireless transmitter concept is developed. This concept uses discrete power level stepping to generate intensity modulated optical signals, su ch a orthogonal frequency division multiplexing (OFDM) waveforms. The transmitter consists of several onoff-switchable LED groups which are individually controlled to emit scaled o ptical intensities. As a result, the digital-to-analogue conversion of the signals to be sent is do ne in the optical domain. This method enables the implementation of low-complex and power-efficie nt optical transmitter front-ends – the major shortcoming of conventional optical OFDM tr ansmitters. Thirdly, a novel approach for wireless data transmission within an aircraf t abin is presented. The data is transferred by 2-dimensional visual code sequences. These sequences are displaye d on the in-flight entertainment (IFE) screen and are captured by the built-in camera of a user device which acts as receiver. Transmission experiments within an aircraf t cabin mock-up demonstrate the functionality of the implemented system under realistic conditions, such as ambient illumination and geometric configuration. Altogether, this thesis has analysed the potential of spectrally efficient optic al w reless transmission techniques. It is shown that OWC systems can greatly benefit from th ese echniques. Declaration of originality I hereby declare that the research recorded in this thesis and the thesis its lf wa composed and originated entirely by myself at EADS Innovation Works Germany and in th e Institute for Digital Communications at The University of Edinburgh, with the following exce ptions: The schematics and the circuit board layout of the optical wireless transmitter presented in Chapter 4 have been done by Dr Christoph Heller. The visual code application presented in Chapter 5 uses code segments of the open source libqrencode libraryand of the image processing implementation ZXing which have been adapted by Dr Falk Schubert. Thilo Christian Martin Fath