Contents I. Introduction
With the increasing development of ultra broadband wireless application the radio frequency (RF) spectrum became congested and scare resources. Thus optical wireless communication (OWC) has drawn considerable attention to the researchers. OWC for indoor application was first proposed by Gfeller and Bapst [1]. In number of applications where higher data throughputs is more of requirement than the mobility, transmission link based on optical wireless would be one of the best options as outlined in [2]–[5]. The performance of OW systems depends on the propagation and type of system used. The basic system types fall into diffuse or line of sight (LOS) systems. In LOS systems, high data rates in the order of Gbit/s can be achieved [6], but the system is vulnerable to blockage/shadowing because of its directionality. In a diffuse OW system, several paths from source to receiver exist, which makes the system robust to blockage/shadwoing. However, the path losses are high and multipaths create inter-symbol interference (ISI) which limits the achievable data rate [7], [8]. There are several advantages of OWC over traditional RF systems, these are: an abundant free spectrum, extremely high communication speed is possible by all network, does not interfere with the over congested RF spectrum. But limitations are: a beam is short ranged, may be harmful for eye. The first limitation can be overcome by wavelength reuse technique, where as eye safety can be ensured by maximum transmit power. It can be classified as line of sight (LOS) and diffuse system. Many researchers have considered diffuse systems for indoor applications it offers robust link and overcomes the problem of shadowing [7], does not require transmitter-receiver alignment and uses the wall or ceiling for multi path reflection [8]. The multipath reflections increases delay spread or inter-symbol interference. Ambient light such as florescent, incandescent light and Compact Florescent Lamp (CFLs) produces channel noise which reduces signal-to-noise plus interference ratio (SNIR). In order to improve the system performance several spot diffusion configuration using multi beam transmitter have been proposed [9]. Multi beam transmitter is place in center of the room and pointed upward. A multi-spot pattern have been generated, illuminated multiple small areas in the ceiling and then reflected multiple spot have been received by receivers. In [10], to improve the bandwidth, reduce the effect of inter-symbol interference, and increase the signal to noise ratio (SNR) when the transmitter operates at a higher data rate under the impact of multipath dispersion, background noise, and mobility in conjunction with an imaging receiver. It proposed different line streaming multi-beam spot diffusion (LSMS) model to gain about 32.3 dB SNR at worst communication path. But the multi path dispersion reduces the performances due to transmitter power and can be improve using power adaptive system by [9]. User mobility is very important aspect of wireless communication especially with today's hand hold devices. As the user device can mobilize with the room then power adaptation will be a great solution to get higher SNR. In this aspect [12] propose a genetic algorithm for multi spot diffuse system in indoor wireless communication. But it is noted from different research that if the diffuse system has a predefined spot for a room and use an adaptive power allocation for beam using calculation of delay spread then it can improve the performance of the OWC. Neural network and Adaptive Linear Equalizers can be a solution in this case for adaptive power distribution. [12] Presents a comparative study of two equalizers, the adaptive linear and the neural equalizer for indoor optical wireless (OW) links using OOK modulation technique to reduce ISI effect.
