# Experimental Evaluation of Display Field Communication Based on Machine Learning and Modern Design
## 5 Experimental Evaluation
### Performance According to Transmit Image Size
Consequently, it can be observed that the lower-size image performs better than the large-size image. Although Figure 8 represents the BER of the current communication scheme, the BER at lower PSNR seems equal and cannot be visualized clearly. To spot the error in lower values of BER, we have plotted the error vector magnitude (EVM) in Figure 9. The EVM plots are drawn using the received data bits just before turbo decoding. Moreover, we express the root mean square (RMS) of the EVM in percent by multiplying the ratio by 100. We can observe that the EVM is increasing with increasing PSNR indicating the increasing distance between the measured and ideal points in the received constellation. In other words, it indicates greater BER. Similar to BER plots in Figure 8, we can also observe that the blue channel shows the worse EVM whereas the red and green channels show a similar EVM. In addition, we can observe that as the image size is increasing, the EVM is increasing, showing that the large-size images will produce more errors compared to the small-size images. The reason why the error rate differs depending on the image size depends on the upsampling and downsampling effects. Note that the original image is resized (zoomed or upsampled) to be displayed on a screen. Then, the image is captured from the iPhone, and the display area is extracted from the captured image. Then, the extracted image is resized (or downsampled) to the original size to decode the data. Now, as all the images are upsampled to the same size, if the size of the original image is small, it means that we are transmitting a higher number of multiple pixels per data bit comparatively (due to a  
Figure 8: BER performance according to PSNR and transmit image size. (**a**) \(128\times 128\) pixel size. (**b**) \(256\times 256\) pixel size. (**c**) \(512\times 512\) pixel size. higher upsampling ratio). In other words, one data bit is assigned to more pixels, when the image is zoomed on the screen. That is, zooming automatically adds redundancy to the transmitted data. At the camera receiver, this makes it easy to demodulate the data bits. Therefore, if we reduce the original image size, better error performance is observed. In other words, as the original image size gets smaller, more pixels are utilized to express one data bit making it easier for the camera receiver to decode the data bits. However, it will reduce the achievable data rate. This is obvious from Table 4 that small images can transmit a lesser number of bits comparatively. The authors also want to point out that upsampling effect will fundamentally depend upon the upsampling ratio, i.e., the ratio of display size to the original image size. For example, if we have a bigger display than used in this experiment, it is possible that the \(512\times 512\) pixel image may show better performance. In summary, reducing the original image size is a way to improve the BER in the DFC link. However, it will reduce the data rate. In that case, one can use a huge display keeping the higher image size to take the benefits of both better BER as well as a higher achievable data rate. To get an idea about the upsampling effect, let us take a walkthrough example. As shown in Figure 10, if a \(128\times 128\) pixel image is magnified horizontally and vertically twice to \(256\times 256\) pixels, one pixel of a \(128\times 128\) image becomes equivalent to four pixels. Due to this upsampling effect, the \(128\times 128\) pixel image performs better than the \(256\times 256\) pixel image. Similarly, if we enlarge a \(128\times 128\) pixel image to \(512\times 512\) pixels, one pixel becomes equivalent to 16 pixels. However, note that the image quality looks more distorted on the transmitter display compared to the \(256\times 256\) pixel image. In other words, the smaller the image size, the better the performance, but the worse the image quality. Figure 9: EVM performance according to PSNR and transmit image size. (**a**) \(128\times 128\) pixel size. (**b**) \(256\times 256\) pixel size.