A major shortcoming of light-emitting diodes (LEDs) is their highly non-linear optical-power-versus-current characteristic. This non-linearity largely restricts the dynamic range and the transmission power of common optical wireless transmitters. This restriction degrades the performance of optical wireless communication (OWC) systems. In this paper, a novel transmitter concept for OWC is proposed which employs discrete power level stepping. The transmitter consists of several on-off-switchable emitter groups. These groups are individually controlled and emit fixed specific optical intensities in parallel. As optical intensities constructively add up, the total emitted intensity is generated by the sum of the emitted intensities of all activated emitter groups. Therefore, the proposed transmitter solution can generate several discrete optical intensity levels which can be used for optical wireless signal transmission. The transmitter design allows the utilisation of the full dynamic range of LEDs or laser diodes by avoiding non-linearity issues. Moreover, costs and complexity of the optical front-end are significantly reduced as neither a (DAC) nor high-speed current controllers are required. This simple design also provides improved power efficiency. Transmission experiments prove the functionality of the implemented optical transmitter. It is shown that the practical performance of the transmitter closely matches the expected performance determined by computer simulations. Moreover, the implemented optical transmitter is compared to an electrical transmission which provides ideal linearity characteristics, and therefore corresponds to an ideal conventional optical transceiver.
© 2013 IEEE
Thilo Fath, Christoph Heller, and Harald Haas, "Optical Wireless Transmitter Employing Discrete Power Level Stepping," J. Lightwave Technol. 31, 1734-1743 (2013)