Study, analysis and application of Optical OFDM, Single Carrier (SC) and MIMO in Intensity Modulation Direct Detection (IM/DD)
Mmbaga, Paul Fahamuel
With the rapid growth of wireless data demands and saturation of radio frequency (RF) capacity, visible light communication (VLC) has become a promising candidate to complement conventional RF communication, especially for indoor short range applications. However the performance of the system depends on the propagation and type of system used. An optical Orthogonal Frequency Division Multiplexing (O-OFDM) together with multiple input multiple output (MIMO) in different scenario and modulation techniques are studied in the thesis. A novel optical wireless communication (OWC) multi-cell system with narrow field of view (FOV) is studied. In this system the intensity modulated beam from four light sources are used for communication. The system allows beams to be concentrated in specific areas of the room to serve multiple mobile devices with low interference and hence increase system capacity. The performance of asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM), direct current biased optical OFDM (DCO-OFDM) and single carrier (SC) modulation are then compared in this system considering single user and multiusers scenarios. The performance of the multi-cell is compared with single cell with wide FOV. It is shown that the capacity for multi-cell system increases with the number of users to 4 times the single user capacity. Also the findings show that multi-cell system with narrow beams can outperform a single wide beam system in terms of coverage area and hence average throughput of about 2.7 times the single wide beam system capacity. One of the impairments in line of sight (LOS) OWC systems is coverage which degrades the performance. A mobile receiver with angular diversity detectors in MIMO channels is studied. The objective is to improve the rank of the channel matrix and hence system throughput. Repetition coding (RC), spatial multiplexing (SMP) and spatial modulation (SM) concepts are used to evaluate throughput across multiple locations in a small room scenario. A novel adaptive spatial modulation (ASM) which is capable of combating channel rank deficiency is devised. Since the receiver is mobile, the channel gains are low in some locations of the room due to the lack of LOS paths between transmitters and receivers. To combat the situation adaptive modulation and per antenna rate control (PARC) is employed to maximise spectral efficiency. The throughputs for fixed transmitters and receivers are compared with the oriented/inclined detectors for different cases. Angular diversity detectors offer a better throughput improvement than the state of the art vertical detectors, for example in ASM angular diversity receiver gives throughput of about 1.6 times that of vertical detectors. Also in SMP the angular detectors offer throughput about 1.4 times that of vertical detectors. SMP gives the best performance compared to RC, SM and ASM, for example SMP gives throughput about 2.5 times that of RC in both vertical detectors and angular diversity receivers. Again SMP gives throughput about 6 times that of SM in both vertical detectors and angular diversity receivers. Also SMP provides throughput about 2 times that of ASM in both vertical detectors and angular diversity receivers. ASM exhibit improvement in throughput about average factor of 3.5 times SM performance in both vertical detectors and angular diversity detectors. As the performance of the system may be jeopardized by obstructions, specular and diffuse reflection models for indoor OWC systems using a mobile receiver with angular diversity detectors in MIMO channels are considered. The target is to improve the MIMO throughput compared to vertically oriented detectors by exploiting reflections from different reflecting surfaces in the room. The throughput across multiple locations in the small room by using RC, SMP and SM approaches is again evaluated. The results for LOS only channels against LOS with specular or diffuse reflection conditions, for both vertical and angular oriented receivers are then compared. The results show that exploiting specular and diffuse reflections provide significant improvements in link performance. For example the reflection coefficient (α) of 0.9 and the antenna separation of 0.6 m, RC diffuse model shows throughput improvement of about 1.8 times that of LOS for both vertical detectors and angular diversity receivers. SM diffuse model shows throughput improvement of about 3 times that of LOS for both vertical detectors and angular diversity receivers. ASM diffuse model shows throughput improvement of about 2 times that of LOS for both vertical detectors and angular diversity receivers. SMP diffuse model shows throughput improvement of about 1.5 times that of LOS for both vertical detectors and angular diversity receiver.