1. Introduction

In general, holography is the storage of the phase and amplitude information of a wavefront. It is usually used as an approach to create three-dimensional (3D) images of objects through interference between a wavefront diffracted and a coherent reference beam. In optical wireless communications (OWC), the hologram is a transparent or reflective device that is used to

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Figure 1. Holographic diffuser with uniform intensities that cover desired area.

spatially modulate the phase or amplitude of the energy passing through it. Figure 1 illustrates the effect of a diffusing hologram on a set of rays from a light source. Here, the light source (light emitting diode (LED)) is split into a number of beams that cover the desired area.

Holograms can be produced from mathematical description or physical object. In mathematical approach, any wavefront can be generated. If mathematical method is implemented with a computer, the hologram is called a computer generated hologram (CGH). A ground glass diffuser can be given as a simple example of physical object diffuser, where ground glass can be placed at the output of a laser to change it from a point source to a large area source. However, this type of hologram cannot be controlled.

Beam steering has been widely studied in wireless communication systems to maximise the signal to noise (SNR) at the receiver [1, 2]. It is also considered as an attractive option in optical wireless communication (OWC) systems to enhance the system performance [3, 4]. New adaptive technique using beam steering is introduced in visible light communication (VLC) links in Ref. [5]. The goal is to maximise the SNR at the receiver in all possible locations within an indoor environment. Simulations results have shown that high data rate up to 20 Gb/s can be achieved by partially steering some of the beams towards the receiver location. Multiinput-multi-output (MIMO) infrared (IR) links employing beam steering method has been introduced in Ref. [6]. Furthermore, demonstration of IR-linked energy transmission using beam forming along with a spatial light modulator (SLM) is shown in Ref. [7]. An efficient power and angle adaptation technique is proposed in Refs. [1–4] in order to help the IR optical wireless (IROW) transmitter to optimise the diffusing spots distribution. These methods (power and angle adaptations) are able to enhance the received signal strength level, regardless of the receiver's location, the receiver's field of view (FOV) and transmitter's position. A significant performance improvement using beam angle and beam power adaptation in a line strip multi-beam system (APA-LSMS) is shown in Refs. [1, 2]. However, a cost has to be paid due to the complex adaptation requirements. The adaptive APA-LSMS transmitter needs to generate a single spot and scanning with all the possible locations (around 8000 locations) in the room in order to find the receiver and then generate the hologram with optimum powers and angles. This makes APA-LSMS system design very challenging.

In this chapter, we aim to point out the impairments of IROW links and propose new efficient solutions beyond those reported in Refs. [1–4]. We report an adaptive hologram selection method employing simulated annealing (SA) to generate diffusing spots (multi-beam). The proposed system is pre-calculated and stored all the holograms in memory. Each stored hologram is suited for a given transmitter and receiver location. Hence, it eliminates the need to calculate holograms real time at each transmitter-receiver location. We model fast angle and power adaptive holograms (FAPA-Holograms) and fast delay, angle and power adaptive holograms (FDAPA-Holograms) mobile OW systems, in conjugation with angle diversity receivers [8]. The conventional diffuse system (CDS) and line strip multi-beam systems (LSMS) are studied for comparison purposes. The ultimate goal of the proposed systems: FAPA-Holograms and FDAPA-Holograms is to reduce the time required to generate hologram at optimum transmitter and receiver location as well as to enhance the overall system performance such as SNR and channel bandwidth in a typical indoor environment. A significant improvement can be obtained by increasing the scanned stored holograms in our systems to approach the original power and angle adaptive methods proposed previously in Refs. [1, 2]. However, increasing the number of scanned stored holograms leads to an increase in the computation time needed to find the best hologram. To overcome this issue, we introduce a divide and conquer (D&C) algorithm to select the best hologram among a finite vocabulary of holograms, hence speed up the adaptation process associated with these adaptive systems [8]. High data rates of 2.5 and 5 Gb/s are considered for the FAPA-Holograms and FDAPA-Holograms systems.

The remainder of this chapter is organised into the following sections: Section 2 presents The IROW room setup and channel characteristics. Section 3 presents the proposed systems' configurations. Section 4 introduces the simulation results and discussion of the IROW systems. Finally, conclusions are drawn in Section 5.
