Irina Adriana Stefan

• This thesis focuses on optical wireless communications (OWC), where optical beams are used to transmit information in free space, with wavelengths ranging from infrared (IR) to ultraviolet (UV) including the visible light spectrum. We are particularly interested in visible light communication (VLC) that uses standard o-the-shelf visible light emitting diode (LED) luminaires to transmit broadband data streams. New technical solutions are developed to cope with the increasing demand for wireless data. The well utilised radio frequency (RF) spectrum which is primarily used to provide wireless services is confronted with the problem of a severe spectrum shortage, also referred to as 'spectrum crunch'. A potential solution to this problem is OWC that targets an entirely different part of the electromagnetic spectrum, the infrared and visible light spectrum. For example, indoor OWC systems based on LEDs as light sources and orthogonal frequency division multiplexing (OFDM) as a modulation technique have attracted substantial research interest. In VLC, the white LEDs providing lighting can also be modulated to achieve high-speed wireless data transmission. The combination of lighting design and light communication allows the VLC system to coexist and benefit from the lighting setup present in many indoor environments (e.g. offices, homes, public places). The high signal-to-noise ratio (SNR) stemming from an illuminated room can make VLC very attractive to provide the indoor wireless link instead of using an outdoor radio base station to send the radio frequency signal through multiple walls. Especially with the increasing usage of data over mobile phones and other wireless devices, this technology can be a powerful alternative to complement existing RF wireless systems. In this thesis, key challenges of optical wireless systems have been investigated. We focus on the transmission opportunities given by the current illumination scenario where a single coordinated optical source or multiple optical access points (APs) are installed in the room. First we evaluate the performance of such a VLC system in terms of average area spectral efficiency (ASE). The ASE is derived for optical wireless in a similar way to RF cellular networks and verified through Monte Carlo simulations. More than three orders of magnitude improvement in ASE is achieved by employing a VLC system indoors as compared to state-of-the-art wireless radio systems. While wireless femtocell base stations are a suitable solution for increasing indoor coverage in areas with high user density, VLC has the potential to deliver much higher data rates. This study compares the ASE of both technologies in a real world scenario and demonstrates the great potential of VLC to act as a powerful complementary technology to femtocells indoors. Moreover by allowing different light sources in a room to coexist and exploit the LED light radiation characteristics as well as the particular characteristics of the optical detectors, the improvement in ASE for VLC can be enhanced further. The lighting equipment in a VLC system should also full the function of wireless data transmission. Therefore, a close investigation of the best placement of the lighting system in a room for illumination and communication performance is important for the end-user experience. In this thesis we investigate the transmitter LED arrays design and placement in a room in order to achieve the required lighting distribution and reliable data transmission at the same time. The design of the LED lighting system and placement in the room for VLC is formulated as a constrained optimization problem and applied to a room of 2.5m x 5m x 3m dimension where lighting is provided by two square LED arrays. Based on numerical simulations it is demonstrated that the system performance in terms of average ASE is degraded when fulfilling the lighting conditions compared with the situation where the VLC system targets only the maximization of the communication performance evaluated also in terms of average ASE. Furthermore, a search is performed for the optimal feld of view (FOV) of the detector that maximizes the communication performance and also guarantees the minimum illumination in the room. Another challenge related to the implementation of high-speed VLC is light dimming. This allows an LED system to save energy by controlling the brightness of LEDs. A dimming approach based on amplitude reduction is proposed for VLC systems employing asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) as a modulation scheme. In addition, the impact of the induced distortions due to the nonlinear behavior of the LED as a light source is included in the analysis. The performance of an ACO-OFDM system is investigated in a practical room scenario where various lighting requirements (e.g. brightness levels) are viable. The DC optical power/bias point level must be changed adaptively to maximize the useful dimming range and to maintain a good SNR. By properly selecting the DC optical power and the useful optical signal power, an optimum SNR can be achieved. It is also shown that signal power dependence of induced nonlinear effects clearly limits the maximum achievable data rates. Finally, an air-interface for a cellular optical wireless communication network is developed that is capable of serving multiple users in a point-to-multipoint as well as multipoint-to-point fashion. Physical layer techniques based on OFDM are developed to enable data rates in exceed of 100 Mbps. OFDM is chosen as a modulation technique due to the ability to support high data rates by using high order multi-level quadrature amplitude modulation (M-QAM) without the need of complex channel equalizers. Moreover, OFDM presents an inherent robustness against multipath eects, due to the use of a cyclic prefix (CP) attached to the transmitted OFDM symbols. As a result intersymbol interference (ISI) due to multipath dispersion effects is completely avoided. Multipath dispersion is a significant problem in indoor applications especially at high data rates. Particularly in a diffuse link this problem leads to serious ISI and degrades performance severely. This thesis describes also the digital signal processing algorithms of a complete orthogonal frequency division multiple access (OFDMA) system including forward error correction (FEC), subcarrier allocation, inverse fast Fourier transform (IFFT) operation, synchronization, channel estimation and equalization. Bit error ratio (BER) measurements of the optical wireless link incorporating the Xilinx field programmable gate array (FPGA) board, the analog-to-digital converter (ADC)/digital-to-analog converter (DAC) platform developed by Funkwerk Avionics and the optical tranceiver modules produced by Heinrich Hertz Institute are presented to evaluate the performance of the developed system. Moreover, basic proof-of-concept demonstrations involving 100 Mbps transmission from an access point to two spatially separated users are successfully conducted. For this purpose two separate video streams were transmitted to the two users.