Reducing Overhead for Low-Power Optical Wireless Communications

We demonstrate on-off-keying optical wireless transmissions according to the IEEE P802.15.13 PM-PHY, in which we replace the 8b10b line-coding by guided and non-guided data scramblers and compensate the remaining high-pass distortions through a frequency domain equalizer. The cyclic prefix length is identified as a critical parameter in balancing performance and transmission overhead, especially at low symbol rates. This is due to the high-pass characteristics of AC-coupled frontends causing baseline variation, effectively creating very long impulse responses. With a cyclic prefix length of 800 ns, 8b10b can be replaced by a scrambler and overhead is reduced by 50 %, while still maintaining an error-free transmission for all symbol rates from 12.5 to 200 MBd.

I. Introduction

Optical Wireless Communications (OWC) are a promising technology for the interconnection of mobile and fixed devices in production facilities: Light enables tight control of interference and offers inherent security, as light signals can simply be spatially confined. However, the multi-carrier waveforms commonly used to increase throughput at high signal-to-noise ratio (SNR) levels, such as orthogonal frequency division multiplexing (OFDM), come with a high peak-to-average power ratio (PAPR) and require power amplifiers with a large dynamic range, which is an issue especially in mobile devices. Furthermore, the most commonly used method of adapting OFDM signals to a non-negative form suitable for optical transmission is to simply add a large bias, which drains energy in the transmitter.