In order to advance VLC technology toward
future commercial applications, more recently
we developed the first bidirectional real-time
high-speed rate-adaptive VLC system. It operates
in half-duplex mode based on time-division
duplex (TDD). The idea is presented schematically
in Fig. 4. Data transmission is based on a
rate-adaptive OFDM modulation and demodulation
scheme. The transceivers are equipped with
tailored VLC transmitter and receiver modules.
As the VLC channel is based on intensity modulation
and direct detection, a real-valued positive
waveform is needed. Here, DC-biased DMT is
applied to obtain a unipolar (positive valued)
time domain signal at the transmitter, while any
potentially remaining negative signal amplitudes
are clipped at the expense of an increased error
rate. Possible bit errors are handled by integrated
forward error correction (FEC). The VLC
transmitter is primarily composed of a newly
developed LED current driver and an off-theshelf
high-power visible-light LED. The VLC
receiver comprises a transimpedance amplifier
(TIA) and a commercially available high-speed
Si-PIN-photodiode. These new modules have
significantly increased the modulation bandwidth
of our optical link up to 180 MHz.