Table 1- Classification of OCC, its expected performance, and intended usages
Given that various modulation schemes for OCC systems have been investigated till the date, the proper modulation scheme should depend on both Tx and Rx. Table 1 suggests to classify OCC schemes into four categories under the following considerations:
(i) The method by which the camera Rx samples the modulated light signal. The type of camera acquiring the data is critical to the communication, so the communication system is limited by the camera specifications rather than Tx.
(ii) The characteristics of the modulated light as perceived by the human eye. The light sources and their modulation must be selected appropriately because the primary purpose of a light source is illumination, whereas communication is the secondary concern.
A. High Frame Rate Processing
Figure 1- Infrastructure-to-vehicular reference architecture using traffic lights and high-speed camera
Given that illumination is the primary purpose of a light source, most of the OCC applications serving indoor lighting systems and vehicular environments require flicker mitigation. Ensuring that the light intensity changes are imperceptible to the human eye, the optical modulation rate is set higher than 100 Hz. High frame-rate cameras (such frame rate higher than 1000 fps) can record the intensity changes of the modulated light over image frames.
High-speed camera-based transmission protocols are related to multiple-input-multiple-output (MIMO) technique, which achieves data rates of tens of kbps from LED arrays. Various studies have demonstrated the reliable performance of MIMO space-time coding with high-speed cameras. Although the performance reliability of high-speed cameras based on OCC has been discussed, two unmitigated problems remain including the low detection reliability of LED arrays in mobile situations and the computational complexity of the high-speed cameras. These disadvantages seriously undermine the attractiveness of the existing schemes.
B. Region-of-Interest Signalling
Figure 2 – Example architecture for RoI signaling system employing S2-PSK
Because the OCC technology based on high-frame-rate cameras oversamples the light intensity modulated waveform, the detection of the light sources and the demodulation of data impose a heavy computational burden, especially under mobility. If the region-of-interest (RoI) representing the area of the light sources is detectable and traceable, we can achieve both the desired high frame rate and high-performance data demodulation. Thus, the methodology of RoI-based communication comprises a significant set of the operating modes among the three OCC operating-mode sets introduced in Table 1. Most of the designed PHY-IV modes are based on RoI signaling techniques that support RoI cameras.
The simultaneous transmission of two data streams is newly introduced in the TG7m PHY-VI modes. This development is of particular importance for vehicular applications. The first stream (a low-rate stream for RoI identification) can be modulated via a RoI signaling technique. The second stream (a high-rate data stream) can be modulated via a high-rate MIMO modulation technique and transferred over the selected RoI. In other words, the RoI signaling stream sets up a communication link for the transfer of the high-rate data stream over the selected link.
Both streams can be demodulated using a single camera with two modes (the RoI detection and high-rate data reception modes) or a pair of cameras in which the first camera detects the RoI of multiple light sources and selects the desired link while the second camera demodulates the data at high speed. The RoI signal can be extracted by cameras of either shutter type (global or rolling) with low frame rates.
C. Rolling Shutter-based OCC
Figure 3 – Example of rolling shutter camera receiving data from LED
Despite having low frame rates, rolling-shutter cameras are beneficial for non-flicker systems, as the rolling shutter mechanism sequentially exposes the pixel lines to the incoming light at a high sampling rate. The Rx sampling rate of these systems, which reflects the pixel row sampling rate of the rolling shutter, is typically much greater than the selected optical clock rate (e.g., several kHz), and satisfies the Nyquist rate.
The OCC techniques based on rolling-shutter sampling are given in the PHY-V modes of Table 1. These PHY-V solutions can be divided into two groups. The first group comprises time-domain modulation techniques, such as the C-OOK and PWM/PPM codes. The second group contains frequency-modulation techniques, including the RS-FSK and CM-FSK modes.
Notably, all OCC systems based on rolling-shutter sampling face a tradeoff between the data rate and communication distance. Owing to this tradeoff, the amount of data available per image is proportional to the resolution of the light source on the image. The technical clarification on the data rate-distance tradeoff in rolling shutter OCC will be explained soon in the next post.
D. Screen Modulation
Figure 4 – Example of a camera Rx receiving data from a screen Tx
The modulation of a screen Tx does not require a particular flicker frequency because the human eye is less troubled by the flickering of screen light than by that of LED light. Consequently, the optical clock rate of screen-based OCC systems can be less than 100 Hz. The range of blinking frequencies at which the number of data blocks is clocked out per second is commonly below 20Hz to support typical cameras decoding at low frame rates (e.g., 30 fps).
The data rate of these screen-camera systems can achieve up to Mbps by taking advantage of high spatial resolution at both Tx and Rx. Note that the more data rate is performed by increasing the number of active pixels, the shorter communication distance is supported. Therefore, TG7m screen modes (PHY-VI modes) in Table 1 have shown the interests in the new design of two-dimensional code for minimizing the transmission overhead rather than maximizing the data rate. We will share the open source of both Tx and Rx in Screen-camera communication system that guarantees BER 10-5 performance measured at distance of 3m and perspective views of <45-degree.