Communication Systems

In this section, I will show different modeling in OWC. First, I will provide an overview the classification of the channel modeling in OWC. Also, this section will discuss the range of transmitter geometries that have been proposed in order to identify the most suitable geometry for use in indoor OW. After that I will show a receiver Architectures in OWC Systems.
We need to Communication channel dictates the performance of any communication system. Design of wireless networks requires an accurate characterisation of the radio channel[1]. Wireless channels due to their unreliable behavior, differ a lot from the wired channels. The received signal strength exhibits random fluctuations in wireless environment due to its time-varying nature. Wireless channel is an inherently hared medium leading to multi-user interference. The random and shared nature makes communication over wireless channels a difficult task. The OWC channel models that can be classified as deterministic and stochastic models.
Deterministic Channel Models: Deterministic channel models are usually based on the detailed description of specific propagation environment, channel scenario, and the position and orientation of the Tx and Rx. The h(t)of the OWC system is obtained using intensive simulations that incorporate details of propagation environments like rooms, offices, buildings, roads, etc. Therefore, these models are site specific, physically meaningful and potentially accurate. Below is a brief review of various approaches of deterministic channel models for OWCs.[2]
Stochastic Channel Models: In stochastic models, the impulse responses of OWC channels are characterized by the law of wave propagation applied to specific Tx, Rx, and scattered geometries, which are predefined in a stochastic fash-ion according to certain probability distributions. Compared to the deterministic, the stochastic approach offers increased flexibility, reduced computational complexity, and considered as non site-specific but with lower accuracy.
Optical Transmitters
The optical transmitter is the most important component of the laser communication system as it performs the following two major functions. First, it is used as a launchpad for the optical signal and in doing so must preserve the optical quality of the signal. Second, it acts as a means to focus the incoming signal onto the imaging optics which in turn relay the signal to the optical detector.
Eye Safety
Eye safety consideration puts limit on the amount of optical power that should be emitted by the transmitter, thus limiting the coverage of an optical wireless system. Both indoor and outdoor optical wireless systems can pose a hazard if LDs are operated at high output power. The eye safety standards are set by International Electro-technical Commission (IEC), where LDs are classified based on their total emitted power into Class 1, 2, 3A and 3B [13]. They dictate that all transmitters must be Class 1 eye safe under all conditions and launch power must not exceed 0.5 mW for the systems employing laser sources. Some LDs operating inside the class 3B category can be made Class 1 safe by passing their beam through a transmissive diffuser, such as a thin plate of translucent plastic. Efficiencies of about 70 percent can be achieved by using such type of diffusers. Computer-generated holograms (CGHs) [17] offer a means to generate arbitrary radiation patterns with efficiencies approaching 100 percent. These holograms break up the wave front of the optical beam in a designated pattern. This diffuses the image of the laser-spot on the retina of the eye.

electrical multiplexing techniques
Time Division Multiple Access (TDMA)
TDMA allows different users to have different time slots in the same frequency channel based on the requested data rates and QoS to transmit their own data. TDMA is used in OWC systems, the digital 2G cellular systems, satellite systems and combat-net radio systems because of high power efficiency. However, the techniques limit the transmission capacity of each user (Kahn &Barry 1997).
optical multiplexing techniques
Wavelength-Division Multiple Access (WDMA)
In WDMA, users can transmit data through a shared channel at the same time using different wavelengths. An optical tunable reception filter can be used to identify the data sent by one user from other wavelengths. A significant disadvantage of such a complex structure is that the hardware is costly. Some indoor OWC systems with WDMA techniques have been proposed (Khan et al. 2012, Wang et al. 2012,Cossu et al. 2012).
Space-Division Multiple Access (SDMA)
SDMA can use parallel spatial pipes next to higher capacity pipes through spatial multiplex-ing to offer enhanced performance. Considering that the optical signal can achieve high angularresolution for its short wavelength, SDM A can make use of alternative dispute resolution(ADR) to discriminate signals from different users and constrain co-channel interference (C-CI) in the same OPA (Carruther & Kahn 2000,Wu et al. 2012). Consequently, the complexity of the structure is accordingly increased.
With upcoming networks, such as the Internet of Things (IOT), the amount of Internet-connected devices is increasing exponentially[1]. However, the radio frequency (RF) spectrum which is currently used in many wireless applications is becoming insufficient due to its inherent limitations, such as spectrum scarcity, and the demand for higher data rates. Optical wireless communications (OWC) is a technology that can potentially support and meet the growing demand for high data rates and has been the subject of interest of researchers, being proposed asa part of sixth generation communication(6G)systems.