High speed light emitting diode (LED) based visible light communication (VLC) system is restricted by the limited LED bandwidth, low detector sensitivity, linear and nonlinear distortions. Thus, single and two cascaded constant-resistance symmetrical bridged-T amplitude hardware pre-equalizers, carrier-less amplitude and phase (CAP) modulation and a three-stage hybrid post-equalizer are investigated to increase transmission data rate for a high speed LED based VLC system. The schemes utilized by the hybrid post-equalizer are modified cascaded multi-modulus algorithm (M-CMMA), Volterra series based nonlinear compensation algorithm and decision-directed least mean square algorithm (DD-LMS). With these technologies, we successfully implement high speed CAP32, CAP64 and CAP128 VLC experimental system over 1m free space transmission with bit error rate (BER) under the hard decision-forward error correction (HD-FEC) threshold of $3.8\times 10^{-3}$. System performance improvement employing these key technologies is also validated through experimental demonstration. With the superposition of each stage of the hybrid post-equalizer, system performance will be improved. And utilizing two cascaded hardware pre-equalizer will provide more accurate channel compensation than single pre-equalizer.

In this letter, we proposed a constant-resistance symmetrical bridged-T amplitude equalizer for high-speed visible light communication (VLC) system. Using the hardware equalizer, we successfully demonstrated a gigabit per second VLC transmission over 80-cm free space based on a RGB LED. The measured bit error rates (BERs) for the signals in 64-quadratic-amplitude modulation (64QAM) single carrier modulation at 1.05-Gb/s, 64QAM-orthogonal frequency division multiplexing (OFDM) at 1.05 Gb/s, and bit and power loading OFDM modulation at 1.42 Gb/s, are under 7% pre-forward error correction (pre-FEC) limit of 3.8 × 10 ^-3 , clearly validating the feasibility of the proposed equalizer. Compared with the system without using the equalizer, the BER performance of the VLC systems can be improved at least by 1 order of magnitude. Moreover, the equalizer can be easily integrated into amplifier or LED because of its compact size and easy installation for just using passive component. To the best of our knowledge, it is the highest data rate with longest transmission distance using preequalization circuit.

In this paper, we proposed a large-coverage underwater visible light communication (VLC) system utilizing quadrature amplitude modulation-discrete multitone (QAM-DMT) and integrated $2\times 2$ positive-intrinsic-negative (PIN) array reception to achieve equal-gain combining (EGC) receiving. The transmitter consists of a lens group and blue-emitting silicon substrate light-emitting diode peak emission, the wavelength of which is 458 nm. After 1.2 m underwater transmission, a data rate of over 1 Gbit/s is successfully achieved with a spot of 25 cm in diameter and 11 cm alignment tolerances. A data rate of 1.8 Gbit/s is also achieved when using parallel light reception. Both are the highest data rates to our knowledge so far in a large-coverage underwater VLC system, which shows the benefit and feasibility of using an integrated PIN array with QAM-DMT modulation and EGC.

Contradictions among receiver optical power, bandwidth of photodiode, and transmission distance are ubiquitous in visible light communication (VLC) systems. In order to moderate such contradictions and improve the performance of VLC systems, we proposed a space diversity receiver for a light-emitting diode (LED)-based VLC system and experimentally demonstrated the highest data rate for an RGB LED with a $1.4Gbit/s$ data rate employing two detectors located in different space positions at the same time. The maximal ratio combining algorithm was used in the VLC system for the first time. As a result, the bit error ratios of the receivers can be reduced by 48.08 dB (red LED), 12.04 dB (green LED), and 27.48 dB (blue LED) compared with any separate detector. The performance of the VLC system was significantly improved. It turned out that it was highly desirable to use multiantennas in the VLC system.