A review of gallium nitride LEDs for multi-gigabit-per-second visible light data communications

The field of visible light communications (VLC) has gained significant interest over the last decade, in both fibre and free-space embodiments. In fibre systems, the availability of low cost plastic optical fibre (POF) that is compatible with visible data communications has been a key enabler. In free-space applications, the availability of hundreds of THz of the unregulated spectrum makes VLC attractive for wireless communications. This paper provides an overview of the recent developments in VLC systems based on gallium nitride (GaN) light-emitting diodes (LEDs), covering aspects from sources to systems. The state-of-the-art technology enabling bandwidth of GaN LEDs in the range of >400 MHz is explored. Furthermore, advances in key technologies, including advanced modulation, equalisation, and multiplexing that have enabled free-space VLC data rates beyond 10 Gb/s are also outlined

High Speed Optical Wireless Communication Systems.

Visible light communication (VLC) systems have become promising candidates to complement conventional radio frequency (RF) systems due to the increasingly saturated RF spectrum and the potentially high data rates that can be achieved by VLC systems. Over the last decade, significant research efforts have been directed towards the development of VLC systems due to their numerous advantages over RF communication systems, such as the availability of simple transmitters (light emitting diodes, (LEDs)) and receivers (silicon photo detectors), better security at the physical layer, improved energy efficiency due to the dual functionally (i.e., illumination and communication) and hundreds of THz of license-free bandwidth. However, several challenges face VLC systems to achieve high data rates (multi gigabits per second). These challenges include the low modulation bandwidth of the LEDs, inter symbol interference (ISI) due to multipath propagation, co-channel interference (CCI) and the light unit (i.e., VLC transmitter) should be ‘‘ON’’ all the time to ensure continuous communication. This thesis investigates a number of techniques to design robust highspeed indoor VLC systems that support a single user and multi-users. Light engines composed of RYGB laser diodes (LDs) are used for communication and illumination. The main goal of using LDs is to enable the VLC system to achieve high data rates while using simple modulation techniques (such as on off keying (OOK)), which adds simplicity to VLC systems. Three VLC systems based on the computer generated holograms (CGHs) are proposed in this thesis, which are single beam static CGH-VLC system, static CGH-VLC system and adaptive CGH-VLC system. Whereas in the first and the second systems a single photodetector is used (added simplicity), an imaging receiver is used in the third one to obtain spatial multiplexing. We consider the lighting constraints where illumination should be at an acceptable level and should consider diffuse reflections (up to second order) to find the maximum data rate that can be offered by each system. In the first system, the CGH is used to produce one fixed broad beam from the best light unit and focus it to a specific area on the communication floor. In the second system, the CGH generates 100 beams (all these beams carry same data) from the best transmitter and directs these beams to an area of 2 m × 2 m on the communication floor. In the third system, the CGH is used to generate eight beams from the best transmitter and steer these beams to the receiver’s location. In addition, each one of these eight beams carries a different data stream. This thesis also presents an indoor VLC system in conjunction with an imaging receiver with parallel data transmission (spatial multiplexing) to reduce the effects of inter-symbol-interference (ISI). To distinguish between light units (transmitters) and to match the light units used (to convey the data) with the pixels of the imaging receiver, we proposed the use of subcarrier multiplexing (SCM) tones. Each light unit transmission is multiplexed with a unique tone. At the receiver, a SCM tone decision system is utilised to measure the power level of each SCM tone and consequently associate each pixel with a light unit. We proposed a high data rate single user VLC system based on wavelength division multiplexing (WDM) in this thesis. An imaging diversity receiver (IMDR) is used as an optical receiver. Based on the location of the IMDR, each colour of the RYGB LDs sends a different data stream at a different rate where the variable data rates are attributed mainly to the different power levels assigned to the colours to yield the desired white colour. Each pixel of the IMDR is covered by a specific colour optical filter. WDM in conjunction with the SCM tones are used to realise a high data rate multi-user VLC system in this thesis. The SCM tones are used to allocate an optimum transmitter to each user and to calculate the co-channel interference (CCI). Two novel optical receivers are used to evaluate the performance of the VLC systems: an array of non-imaging receivers (NI-R) and an array of non-imaging angle diversity receivers (NI-ADR). This thesis proposes a multi-branch transmitter (MBT) as a solution that can improve the VLC system performance over an indoor channel and support multi-user operation. The MBT has many transmitter branches (TBs) where each branch is directed to a specific area. By reducing the semi angle of each TB, the effect of multipath propagations is reduced and the received optical power is improved. The performance of the MBT is evaluated with a single user VLC system using a wide field of view (W-FOV) receiver and then with an angle diversity receiver (ADR). The results show that this system can provide a data rate of 4 Gb/s and 10 Gb/s when using wide FOV receiver and ADR, respectively. In addition, the performance of the MBT is evaluated in a multi-user scenario. We used the MBT with WDM and SCM tones to realise a high data rate multiuser indoor VLC system. Four colour laser diodes (RYGB LDs) are used as sources of illumination and data communication. One colour of these four colours is used to convey the SCM tones at the beginning of the connection to set up the connection. When the connection is set up, the data is transmitted in parallel through the RYGB LDs

MIMO MC-CDMA systems over indoor optical wireless communication channels

Optical wireless communication systems offer a number of advantages over their radio frequency counterparts. The advantages include freedom from fading, freedom from spectrum regulations and abundant bandwidth. The main limitations of optical wireless systems include background noise attributed to natural and artificial light sources and multipath propagation. The former degrades the signal to noise ratio while the latter limits the maximum achievable data rate. This thesis investigates the use of transmit power adaptation in the design of optical wireless spot-diffusing systems to increase the power associated with the main impulse response components, resulting in a compact impulse response and a system that is able to achieve higher data rates. The work also investigates the use of imaging diversity receivers that can reject the background noise components received in directions not associated with the signal. The two techniques help improve the optical wireless system performance. The multibeam transmitter and the multi-detector angle diversity receiver or imaging receiver form a multiple input multiple output (MIMO) system. The work also investigates additional methods that can improve the performance such as transmitter beam angle adaptation, and improved modulation and coding in the form of multi-carrier code division multiple access (MC-CDMA). Furthermore, the work investigates the robustness of a link design that adopts the combination of these methods in a realistic environment with full mobility.EThOS - Electronic Theses Online ServiceGBUnited Kingdo