# Color-Space-Based Visual-MIMO for V2X Communication +
## 3 Results and Discussion  
In the previous GCM based communication between LED and PD, we used a simple average and moving average of the received symbols in order to obtain the target color information at the receiver [7]. So, many errors could occur during the transition and we had to take an error correction coding including interleaving [7]. Figure 8 shows the transition example of a target color of traffic sign from green to red. The error seems like a burst error in RF communication because it appears as a contiguous sequence of error of target color. However, by using visual-MIMO, we can solve this problem successfully since the target color changes with an average of total colors of LED array at every frame. Figure 8: The transition example of a target color of a traffic sign from green to red under GCM based communication between LED and PD. Figure 7: An example of target color transition in a color-space-based constellation diagram. Figure 9 shows the superior SER performance of the visual-MIMO scheme compared with LED-PD communication. Figure 10 shows the numerical result of the GCM-based visual-MIMO channel capacity _versus_ distance [2]. This result is based on the analytical capacity plots of [1]. In our numerical example, we used the LEA with a size of 16 \(\times\) 16, 4 symbols, and a frame rate of 30 fps. If the distance between the source and destination is short enough to guarantee multiplexing transmission (first stage), the capacity is roughly 10 kbps. Then, when the distance has increased and exceeded the limit of the required bit error rate (BER) performance (second stage), there needs to be a transition from the multiplexing mode to the diversity mode. Although we may meet the BER performance requirement, the loss of capacity is unavoidable. However, by relaying the information message to the destination node (third stage), the source node can communicate with the multiplexing mode, providing a capacity comparable to that of the first stage. Moreover, by performing simultaneous transmissions through the relay and on the direct link (fourth stage), we can also obtain an additional SNR gain because of the path-diversity effect. Figure 10: Numerical results of GCM based visual-MIMO (Channel capacity _vs_. Distance). Figure 9: SER performance comparison between visual-MIMO and LED-PD communication under color variation. Most importantly, by sending the reference color (symbol) as part of the networking information, we may achieve improvements in the SER performance because of noise cancellation. In general, effects of channel noise on the transmitted symbols of the LEA in the same frame (or time) are assumed to be almost same. An example of experimental results is shown in Figure 11. We can see that four symbols are represented in the color space of CIE1931. Note that transmitted symbols and received symbols are denoted by blue dot and red dot, respectively. Without using a reference color, as shown in Figure 11, the probability of symbol error becomes large due to the channel noise. However, if we use a reference color, we may correct the symbol error by compensating the noise. As a reference color, in this experiment, the target color corresponding to the average color of four symbols is used in order not to impair the wanted original color of the lighting. The effect of performance improvement by sending a reference symbol is also shown in Figure 10 by the expected distance gain extending the multiplexing mode. From Figure 10, we see that the system can achieve large multiplexing gains at short distances and at almost all viewing angles, which implies that it is robust to any misalignment between the transmitter and receiver. It is clear that at large distances, because of the noise effect, the LEDs may not be easily resolved even if the transmitter and receiver are properly aligned; hence, at such distances where multiplexing will fail, we can use diversity to satisfy the required BER performance.