2. The IROW room setup and channel characteristics

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.

LED

Light Hologram

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.

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

However, this type of hologram cannot be controlled.

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

126 Optical Fiber and Wireless Communications

and angles. This makes APA-LSMS system design very challenging.

In order to study the impact of directive ambient light noise sources and multipath dispersion, as well as their effect on the received data flow, consideration was given to an unoccupied rectangular room that had no furnishings, with dimensions of 8 m 4 m 3 m (length width height). Researchers in Ref. [9] have studied and investigated the power reflected in indoor IROW system. The study found that the light reflected on either ceiling or wall is Lambertian in nature (mode n = 1). They also found that the wall reflected power by 80% whereas ceiling by only 30%. In this chapter, we consider that the rays reflected from door and windows are similar to those coming from walls. The reflecting elements from walls and ceiling can be modelled by dividing the surfaces into a small square shape, which can operate as secondary Lambertian transmitter with n = 1.

The accuracy of the impulse response profile is controlled by the size of the reflecting elements. Therefore, element sizes of 5 cm 5 cm for the first order reflections and 20 cm 20 cm for the second order reflections are employed for all arrangements. Previous work studied the received optical power within an indoor environment. They found that most of the received optical power is located within the two first order reflections (1st and 2nd). Third order and higher reflections are highly attenuated [10, 11]. Hence, two bounces are considered in our calculations. All the proposed systems use an upright transmitter with 1 W optical power. Furthermore, the significant signal to noise ratio (SNR) improvement of the hologramproposed systems is used to reduce the transmit power to 80 mW reducing the power density on the adaptive hologram and helping eye safety.

In OW communication links, intensity modulation with direct detection (IM/DD) is considered the most viable approach. The indoor OW IM/DD channel can be fully specified by its impulse response h(t), and it can be modelled as a baseband linear system given by

$$I(t, Az, \text{ell}) = \sum\_{m=1}^{M} \text{Rx}(t) \otimes h\_{\text{m}}(t, Az, \text{ell}) + \sum\_{m=1}^{M} \text{Rn}(t, Az, \text{ell}).\tag{1}$$

where I(t, Az, El) is the current instantaneous due to m reflecting elements, El and Az are the directions of arrival in the elevation and azimuth angles, t is the absolute time, x(t) is the optical power transmitted, ⊗ denotes convolution, M is the total number of receiving elements, R is the photodetector responsivity and n(t, Az, El) is the background noise. The delay spread is a good tool to measure signal spread due to multipath propagation. The delay spread can be written as [12, 13]:

$$DS = \sqrt{\left(\sum\_{\forall i} (t\_i - \mu)^2 P\_{r\_i}^2\right) \Big/ \sum\_{\forall i} P\_{r\_i}^2} \tag{2}$$

where the time delay ti is associated with the received power Pri (Pri reflects the impulse response h(t) behaviour) and μ is the mean delay given by

$$
\mu = \sum\_{\forall i} t\_i P\_{r\_i}^2 \Big/ \sum\_{\forall i} P\_{r\_i}^2. \tag{3}
$$

The delay spared is deterministic for a given stationary transmitter-receiver and reflecting elements' positions. The delay spread can change for a given transmitter-receiver location when the reflecting elements moves or an object is entering and leaving the environment. However, the impact of such a change is not considered in this work and has not been investigated by other researchers.

The SNR of the received signal can be calculated by taking into account the powers associated with logic 0 and logic 1 (PS<sup>0</sup> and PS1), respectively. The SNR is given by [14]:

$$\text{SNR} = \left(\frac{\text{R}(P\_{s1} - P\_{s0})}{\sigma\_0 + \sigma\_1}\right)^2 \tag{4}$$

$$
\sigma\_0 = \sqrt{\sigma\_{pr}^2 + \sigma\_{bn}^2 + \sigma\_{s0}^2} \text{ and } \sigma\_1 = \sqrt{\sigma\_{pr}^2 + \sigma\_{bn}^2 + \sigma\_{s1}^2} \tag{5}
$$

where σ<sup>2</sup> pr represents the receiver noise, which is a function of the design used for the preamplifier; σ<sup>2</sup> bn represents the background shot noise component and σ<sup>2</sup> <sup>s</sup><sup>0</sup> and σ<sup>2</sup> <sup>s</sup><sup>1</sup> represent the shot noise associated with the received signal (PS<sup>0</sup> and PS1), respectively. The signal-dependent noise (σ<sup>2</sup> si) is very small due to the weak received optical signal, see the experimental results reported in Ref. [15]. In this study, we used the PIN-FET transimpedance preamplifier proposed in Ref. [16]. The background shot noise calculations can be found in Ref. [3]. Nine branches angle diversity receiver is used to reduce the impact of multipath dispersion. In this work, we employed the non-imaging angle diversity receiver design proposed in Ref. [8]. We considered maximum ratio combining (MRC) scheme. Calculations of MRC method can be found in our previous work in Refs. [1, 19].

In order to consider the impact of background noise, we use eight light bulbs in the room. These lights are used for illuminations. However, in OW receiver, the signals arrived from each light are considered as undesired signals which can be modelled as background shot noise. In this study, we assumed that Philips PAR 38 Economic' (PAR38) was used as spotlight in which each unit of light radiates 65 W within a narrow beam width. The light from these units can be modelled as a Lambertian radiant intensity with order nl = 33.1 [11]. Additional simulation parameters are given in Table 1.
