# Color-Space-Based Visual-MIMO for V2X Communication +
## 2 Experimental Section  
### Visual-MIMO with Color-Space-Based Modulation  
The first color-space-based modulation scheme, termed color shift keying (CSK), was proposed by the IEEE 802.15.7 task group [4; 5]. However, CSK is not suitable for communications when target color varies with time because it does not involve a systematic manner of coping with changing target color conditions. Here, target color means the wanted color of the LED lighting. In order to overcome this limitation, another color-space-based modulation scheme, termed as generalized color modulation (GCM), was proposed for color-independent visible light communication (VLC) systems [6; 7]. It should be noted that the most distinctive feature and advantage of GCM over the other modulation schemes is color independency. GCM is able to generate any color within a gamut by combining some of the wavelengths or colors. Therefore, through GCM, we may achieve a VLC scheme that can maintain the original color and brightness while performing seamless communication. GCM constructs a constellation diagram in a light color space to represent data symbols. Each constellation point in a color space represents a corresponding color. The target color, _i.e.,_ the color perceivable to human eyes after modulation, is the average of all appropriate constellation points. A simple example of a constellation generation for a target color is illustrated in Figure 2[7]. Here, constellation points in the color space can be arranged using a similar arrangement to that used in RF circular quadrature amplitude modulation (QAM). Supposing an equiprobable symbol transmission, which is reasonable, due to the compensation and interleaving algorithms, the target color can be obtained as the averaged RGB value along a number of symbols as shown in Equation (1). \[(x_{t},\,y_{t})\,=\,\left(\frac{\sum_{i=1}^{N}x_{i}}{N},\,\frac{\sum_{i=1}^{N }y_{i}}{N}\right) \tag{1}\]  
where \((x_{t},\,y_{t})\) denotes the position of a target color and \((x_{i},\,y_{i})\) denotes the position of the \(i_{th}\) symbol. Note that the value of \((x_{t},\,y_{t})\) is more close to the true target color when N is increasing in the probability sense. Figure 1: Illustration of the optical V2V communication system. Figure 3 shows the block diagram of the proposed color-independent visual-MIMO based on the color space [2]. By incorporating GCM into visual-MIMO, we may obtain a better symbol error rate (SER) performance, higher data rate over a larger transmission range, and most importantly, color independency when compared with conventional light-emitting diode (LED) communication. With this scheme, we can obtain a target color at a time without difficulty by the parallel transmission of different symbols (colors) through the LED array. In this case, we know that N in Equation (1) is the number of total LEDs of the LED array. It is also important to note that we may prevent the burst error occurring during the transition of target color change by transmitting symbols in parallel through LED array of color-independent visual-MIMO transmitter in Figure 3. In [8], we have shown that many errors could occur during the transition of an arbitrary target color to another when the transmitter does not send the target color and the receiver is a photo detector (PD) instead of a camera. Furthermore, the proposed system rapidly and easily adapts to the variations in the target color using image processing. Figure 3: Color-space-based, color-independent visual-MIMO transceiving procedure using image processing. Figure 2: Generation example of a constellation diagram for a target color in CIELUV color space (example uses seven LEDs and two bit data symbols). Generally, the capacity of a color-independent visual-MIMO system depends on the number of bits per symbol, the array size (M \(\times\) M), and the frame rate of the camera.