We propose a multilevel brightness modulation (MBM) scheme for optical camera communications, where a LED array and a k-means clustering algorithm are used to encode and decode the data, respectively. The experiment results show that the MBM scheme with a focal length of 18 mm offers longer transmission distance compared with the conventional 4-level undersampled pulse amplitude modulation with subcarrier modulation (UPAMSM) scheme.
Free-space optical communications (FSO) suffers from irradiance fluctuation caused by atmospheric turbulence, which results in optical power loss and consequently decreased signal-to-noise ratio (SNR). The error performances of the FSO based on On-Off Keying (OOK), Differential Phase Shift Keying (DPSK), and Binary Phase Shift Keying (BPSK) schemes in a turbulent atmosphere are presented. The received irradiance after propagating the atmosphere is modeled using the gamma-gamma distribution to evaluate the system error performance in turbulence regimes from weak to strong. The results show that, to obtain a BER of 10-6 at weak turbulence regime, ~15 dB and ~18 dB SNRs are required for BPSK and DPSK, respectively. However, for OOK with a fixed threshold of 0.5 under the same channel condition, OOK reaches an error floor greater than 10-3. The values of SNR required to achieve the same BER increase as the turbulence strength increase to moderate and strong regimes.
We compare the bit error rate (BER) performances of discrete Fourier transform (DFT), discrete cosine transform (DCT) and discrete wavelet modulation (DWT) based orthogonal frequency division multiplexing (OFDM) schemes for visible light communications (VLC). The experiment results show that DWT-OFDM-VLC system with low peak to average power ratio (PAPR) offers better BER performance compared to DFT-OFDM-VLC and DCT-OFDMVLC systems.