On the Performance Gains of Cooperative Transmission Concepts in Intensity Modulated Direct Detection Visible Light Communication Networks

Visible light communication networks are currently constrained in range by the divergent source characteristics of the light emmiting diode (LED) and in achievable rate by the modulation bandwidth of the LED transmitter; however, as device advancements improve the limitation on rate becomes inter-symbol interference (ISI). The key contributor to ISI is the non-uniform time of arrival (TOA) of all light source transmissions. Data is conveyed using intensity modulation/direct detection, as such a time of arrival synchronization is not required of the light carrier, but rather of the baseband modulation sequence (data carrier). The concept of cooperative transmission is shown to improve achievable rate; however, communication range does not benefit as well in such networks. Cooperative transmission is a scheme in which a collection of transmission sources pool together their resources to send a commonly shared message in a manner that allows all data transmissions to arrive synchronously at the intended receiver. Although, LEDs emit incoherent light, it supposed that the a collection of LEDs can acheive time synchronization in its intensity modulated waveforms by altering the delays in which the LEDs change state. An elegant one-bit feedback procedure for RF beam-steering is adapted to acheive time of arrival synchronization of the data carrier over the visible light channel. It is assumed that all LED sources share the same clock; however, the sources and a user terminal do not. Data carrier clock synchronization between sources and a user terminal can be obtained using a Manchester line code or a mutual synchronization architecture to adapt to a master group of transmission sources fairly quickly. It is shown that cooperative transmit beamforming can lower the impact of ISI on in the visible light channel as device capability matures to require such an architecture.