Research On Spatial Modulation And Coding Technology For Visible Light Communications

Visible light communications (VLC) technique continues to attract extensive academic attention as a viable complimentary solution to the overcrowded spectrum of radio frequency (RF) transmission technology. As the fact that lighting equipment is installed nearly everywhere today, it is efficient and realizable to utilize these installations not only for illumination but also for wireless data transmission. In comparison with the traditional RF technology and millimeter wave systems, VLC systems offer a number of unique advantages including a large unlicensed bandwidth, immunity to electromagnetic interference, and an excellent security. However, the bandwidth of the off-the-shelf incoherent-light producing LEDs is limited for visible light, which imposes a restriction on the available bandwidth of practical VLC systems. Therefore, the enhancement of spectral efficiency is in the spotlight of ongoing research activities for VLC technology. In order to provide sufficient illumination, light installations are typically equipped with multiple LEDs. This property can readily be exploited to create optical MIMO communication systems.Spatial Modulation (SM) is a combined MIMO and digital modulation technique, in which the conven-tional signal constellation diagram is extended to an additional dimension, namely the spatial dimension. For a typical SM system, there exist multiple geometrically-separated transmitters but only one of them is active at any given time, where the index of the active transmitter is used to transmit the additional bits. This tech-nique provides an enhanced spectral efficiency by utilizing multiple transmitters while completely avoiding inter-channel interference (ICI), which makes SM a suitable modulation technique for VLC. In this thesis, the spatial modulation technique in VLC systems have been widely studied.Firstly, the thesis gives an overview of the VLC system, including the system structure, transmitting and receiving devices, as well as the visible light channel and modulation methods in VLC. Moreover, three popular MIMO transmission schemes often used in VLC systems, repetition coding (RC), spatial multiplexing (SMP) and spatial modulation are also discussed and compared. Bit error rate (BER) of these three optical MIMO transmission schemes are simulated and compared in indoor VLC environments.Then, the thesis studies the system characteristics of optical spatial modulation (OSM), including both the merits and limitations. Like many MIMO techniques, the performance of OSM heavily relies on the channel characteristics. As for scenarios with high link correlation and low differentiability of the multiple channels, the performance of OSM suffers a strong degradation, which highlights a fundamental drawback of OSM as a technique that is not suitable for the mobile setup. In order to relieve this limitation, we present an improved scheme for typical OSM technique, named generalized optical spatial modulation (GOSM). Simulation results illustrate that the proposed GOSM scheme substantially improves the BER performance of OSM, especially in highly correlated channels. Additionally, we get more insights about the interplay of spatial and signal constellation diagrams in OSM system.Subsequently, the thesis focuses on the power allocation (PA) problem in optical spatial modulation systems. The fundamental principles of PA OSM system and several popular PA algorithms are introduced and discussed. On this basis, a new power allocation algorithm based on the minimum Euclidean distance is proposed. We solve the power allocation problem by sub-problem approximation method and convex optimization. And the performance of the proposed PA scheme and conventional PA schemes are compared through theoretical and numerical analysis. Besides, the numerical results verify that the proposed power allocation algorithm can achieve a performance improvement compared with the popular algorithms.Finally, the channel coding technique for OSM systems is considered in the thesis. We introduce several channel codes that have been studied and utilized in SM systems, such as convolutional code and low density parity check (LDPC) code. Then, the bit-interleaved coded modulation (BICM) scheme of OSM is discussed and the corresponding extrinsic information transfer (EXIT) analysis method is also analyzed. Moreover, the channel coding technique for GOSM systems is also considered.