Optical camera communication (OCC) serves as a significant complement to existing radio frequency (RF) communication methods in the realm of outdoor vehicle-to-everything (V2X) communication due to its superior anti-interference capability, security, and relatively low deployment cost. However, the inherent limitations of image sensor imaging mechanisms and manufacturing processes make OCC technology exhibit a considerable gap in communication rate and reliability compared to traditional visible light communication (VLC) techniques. Existing research has explored higher-order modulation schemes and resource multiplexing to enhance the communication performance of OCC. Nevertheless, current higher-order data modulation schemes struggle to simultaneously achieve simplicity, flicker-free operation for human eyes, and flexible scalability. Furthermore, the exposure effect of optical cameras is a critical factor affecting the reliability of OCC, with its impact being more pronounced for higher-order modulation schemes. Consequently, this adverse effect must be effectively suppressed to ensure reliable communication. We proposed a higher-order modulation scheme with binary amplitude suitable for OCC and constructed a two-dimensional constellation space in the frequency and phase domains, enabling simple implementation of non-flickering and flexibly scalable higher-order data modulation. Additionally, a demodulation algorithm based on frequency domain equalization is presented to mitigate inter-symbol interference and frequency-selective attenuation caused by the camera exposure effect.

First, an exposure model for a rolling shutter camera was established, which characterized the exposure effect as a low-pass filter and described its distortion of optical pulses as inter-symbol interference and frequency-selective attenuation. A joint phase-frequency shift keying modulation method was presented by leveraging the constant amplitude envelope and evenly distributed characteristics of both frequency shift keying (FSK) and phase shift keying (PSK). This method constructed a two-dimensional constellation space in the frequency and phase domains to achieve flexible higher-order modulation. The waveform of each modulated symbol consists of square waves with different frequencies and phase offsets, ensuring ease of implementation. The data frame structure was designed, comprising Q data symbols, q header symbols, and q tail symbols to facilitate synchronization at the receiver and eliminate inter-symbol interference. Based on this frame structure, a corresponding data demodulation algorithm was provided with the core steps therein, including a frequency domain equalizer and a symbol demodulator. The former calculated the optimal equalization filter coefficients under the minimum mean square error (MMSE) criterion to compensate for the frequency-selective attenuation experienced by the received signal and enhance data demodulation performance. The latter estimated the constellation point corresponding to the transmitted symbol from the pixel value sequence after frequency equalization, followed by constellation de-mapping to obtain the payload bits. Finally, the influence of imaging parameters of the rolling shutter camera, such as exposure time and readout time, on the system transmission performance was briefly analyzed.

The symbol error rate (SER) performance of the proposed joint phase-frequency shift keying modulation system was evaluated using Monte Carlo simulations. The influence of different camera exposure time parameters on the average SER of the system was compared. The results demonstrate that a longer camera exposure leads to more significant inter-symbol interference and frequency-selective attenuation, thereby considerably affecting demodulation reliability. However, the introduction of a cyclic prefix and a frequency domain equalization algorithm effectively mitigates this adverse effect. The average SER performance of the system is compared when employing constellation diagrams with different frequency and phase modulation orders, indicating that the system reliability gradually decreases as the number of constellation points increases. Moreover, this influence is more pronounced for an increase in the frequency modulation order compared to the phase modulation order. Furthermore, the average SER performance of the joint phase-frequency shift keying modulation system is compared when using image sensors with different readout time. Although using an image sensor with a shorter readout time yields better SER performance, it comes at the cost of reduced pixel utilization efficiency. Finally, the SER performance of four higher-order modulation schemes with binary amplitude is compared under the same exposure time, modulation order, and symbol rate. The simulation results show that the proposed joint phase-frequency shift keying modulation scheme outperforms the other higher-order modulation schemes.

We propose a joint phase-frequency shift keying modulation scheme for OCC along with a demodulation algorithm based on frequency domain equalization. This modulation scheme utilizes binary-amplitude square waves as carriers to achieve flicker-free and scalable higher-order modulation. Compared to existing higher-order modulation techniques, it offers lower implementation complexity and better illumination compatibility. At the receiver, by integrating frequency domain equalization into the demodulator structure, inter-symbol interference and frequency-selective attenuation caused by the image sensor exposure effect are effectively suppressed. Through numerical simulations, we compare the SER performance of the proposed scheme under different exposure time parameters and elucidate the negative influence of the exposure effect on data transmission reliability, while also confirming the effectiveness and necessity of employing a frequency domain equalization algorithm in the demodulator. Based on the low-pass filtering characteristic of the exposure effect, we explore and validate the significant influence of the constellation design, particularly the selection of frequency combinations, on the system's SER. By relating the readout time parameter of the rolling shutter image sensor to the equivalent sampling frequency in an oversampling modulation scheme, the trade-off between communication effectiveness and reliability under imaging resolution constraints is verified. Furthermore, under the same modulation order and symbol rate, the proposed modulation exhibits superior reliability compared to traditional modulation schemes such as PSK, FSK, and pulse position modulation (PPM).