Indoor VLC system with multiple LEDs of different path lengths employing space-time block-coded DMT/CAP modulation [invited]

We experimentally demonstrate a practical visible light communication (VLC) system of 3.2-m transmission using spectrally efficient discrete multitone (DMT) or carrierless amplitude phase (CAP) modulation. The transmitters consist of two light emitting diodes (LEDs) spaced 2 m apart, the arrangement of which corresponds to a common case for full-range illumination in indoor environments, and this work also investigates the influence of the difference in transmission path lengths from the LEDs to a single receiver. In the overlapped illumination area, the delay interference between the LEDs could result in up to about 40% capacity reduction using quaternary CAP or bit-loaded DMT signals, when two LEDs simply transmit the same signal. To mitigate the impact of the delay interference, Alamouti space-time block coding (STBC) is employed for both DMT and CAP signals, and the combination of CAP and STBC is proposed for the first time to the authors' best knowledge. Employing STBC, we successfully demonstrate interference-tolerant VLC over 3.2-m air transmission, and the maximum data rates for DMT and CAP signals are 220 and 150 Mbps, respectively.