## 5 Discussion for Actual Implementations  
Currently there are two following types of white LEDs on the market: 1-chip LEDs (blue LED chip with yellow phosphor) and 3-chip LEDs (red, green, and blue chips) [25]. The former one is dominant due to its simple mass production. Nevertheless, a major challenge in utilizing these 1-chip white LEDs to achieve high data rate communication is its limited modulation bandwidth, usually in the range of several MHz without equalization. A pre-equalization of a white LED to further increase the modulation bandwidth is needed for high data rate communication systems [26]. A typical indoor visible light communication receiver usually consists of a concentrator, an optical filter, a photodetector, a pre-amplifier, a post-equalizer, and an electrical filter. For NLOS channel such as a diffuse channel, using non-imaging concentrator and corresponding optical filter could effectively enlarge the active receiving area and broaden the FOV to increase the received optical power. However, for the directed LOS channel, the FOV should be designed to be small to reduce the received ambient light noise power. The ambient light noise is usually diffused inside the whole room as background light [21]. Typically, there are two types of photodetectors adopted in an indoor LOS visible light communication system, _i.e._, the photodiode and the image sensor [21]. The photodiode has been widely used when the received optical power is relatively high. The advantages of the photodiode are low price and possible high reception bandwidth. On the other hand, the image sensor is able to provide receiver spatial diversity to enhance detection performance and additional source location information for location-aware services [21]. For application scenarios where multiple LED arrays in a room send different signals to multiple users, using a large FOV photodiode detector may lead to large interference that degrades received SNR. In this case an image sensor would better used as a photodetector [21]. The Media Access Control (MAC) specifications include addressing and channel access control mechanisms to allow for several terminals or readers. The LEDs send their IDs frequently as a type of a beacon signal at different wavelengths. A beacon signal is used to indicate the start of frame and define the number of slots to be allocated for contention, uplink and downlink. There is also non-beacon mode used only for the peer-to-peer (P2P) application. These beacon signals are usually sent at the lowest clock rate C\({}_{1}\) (200 kHz), which is mandatory at both the transmitter and receiver for all devices [27]. For every clock rate, there is an associated set of data rates at the physical layer. This data rate is dependent on the modulation, line coding and Forward Error Correction (FEC) used at the physical layer for a given clock rate. In [27], there is a data rate table rate { C\({}_{i,j}\) } associated with each clock rate C\({}_{i}\) where \(j\) is the data rate index associated with clock rate C\({}_{i}\). The data rate table is a common standard adopted by all VLC devices [27]. \begin{table}
\begin{tabular}{c c c} \hline
**Coordinates** & **Absolute Mean Error (m)** & **RMSE (m)** \\ \hline Rectangular \(x\) & 0.23 & 0.70 \\ Rectangular \(y\) & 0.29 & 0.75 \\ Hexagonal y & 0.16 & 0.60 \\ Hexagonal y & 0.18 & 0.67 \\ \hline \end{tabular}
\end{table} 
Table 2: Absolute mean error and RMSE. The reader is assumed to have a received signal strength indicator (RSSI) unit coupled directly into the microcontroller where the decision is made over which input has the strongest signal. Therefore, the reader can indicate which LEA gives the strongest signal for each position of the reader. A measure of the received signal strength indication (RSSI) is coupled directly into the microcontroller (\(\upmu\)C) where the decision is made over which input has the strongest signal. This information is translated to a binary address associated with the data inputs of the analogue multiplexer [24].