# Flickering-Free Distance-Independent Modulation Scheme for OCC
## 2 Related Work
Nevertheless, each packet required two delimiters, header, verification bits, and the decoding process involved high-complexity algorithms. Modulation schemes based on undersampling transform the data signal into a passband signal used for switching the LED with a frequency high enough to avoid flickering. Depending on the applied shift keying, they are classified in: frequency, phase, and amplitude shifting. For example, the Undersampled Frequency Shift On-Off Keying (UFSOOK) modulation [27] used one specific frequency, greater than 100 Hz and a multiple of the camera's frame rate for each state (one or zero), whilst [PERSON] et al. [28] defined the two sub-carrier frequencies as a discrete sampling of the camera's frame rate taking the shutter speed as a limitation. [PERSON] et al. in [29] proposed an RGB modulation that combined frequency shifting with OOK signals by comparing two consecutive frames. For each frame, only one band was analyzed per color channel, and two consecutive frames in the same state (on or off) represented a \"1\" bit. The experimental results showed a 100 m link with \(3.8\cdot 10^{-3}\) Bit Error Rate (BER). However, the system's performance was very sensitive to the threshold definition and calibration. In [30], the authors applied a similar scheme over two transmitters, the second emitter's signal was phase-shifted by 90\({}^{\circ}\)The receiver selected the best emitter for each communication process. A BER of \(1.7\cdot 10^{-3}\) was achieved for a 160 m. Nevertheless, the thresholding and calibration issues were not taken into account. In [31], the phase was shifted instead of the frequency, for representing complementary bits separated 180\({}^{\circ}\). This technique reached distances up to 12 m, but it required a synchronization delimiter and the transmission of a \"1\" bit to define the system's demodulation logic. A modification of this modulation was presented in [32], the signal shifted the phase 180\({}^{\circ}\) in the second frame for sending a bit \"1\", avoiding the previous restriction on the demodulation logic. The results showed a feasible 0.5 m link. The light amplitude was changed in [33; 34], avoiding the flickering problem while reaching long distances (60 m). Nevertheless, this method demanded the transmission of all intensity light levels in a predefined order. This technique's data rate was improved by using a Quadrature Amplitude Modulation (QAM) [35], but for distances lower than 1.5 m. Finally, a combination of Pulse Width Modulation (PWM) and amplitude shifting was introduced in [36]. The PWM signals modulated the LED's brightness levels by changing its duty cycle instead of its frequency, while the bit stream was mapped onto an m-ary amplitude modulation constellation. This scheme is highly complex compared to other implementations and reached a modest distance (1 m). Intelligent Transportation System (ITS) applications have been the target of a significant amount of contributions in the field. [PERSON] et al. presented a space-time coding [37] based on a modified Alamouti code [38] which avoid the use of negative signals. For distances where each LED is represented by at least 1 pixel, the demodulation was simple, but longer distances require extra information and complex processing. During the validation, a 48 m link with a BER less than \(1\cdot 10^{-5}\) was achieved. For improving this method, [PERSON] et al. [39] introduced a layered space-time coding: the matrix was divided into submatrices where the diagonal matrices represented the first layer bitstream. A 1 kbps link was established for 210 m with \(3\cdot 10^{-1}\) BER using a 3-layer bitstream. Similarly, [PERSON] et al. [40] presented a layered space-time coding for spatial modulation with different interpolation techniques. The system sent data successfully over a 240 m link. However, both methods required specialized software and hardware due to the process' complexity.