Light emitting diode designs and modulation schemes for dual illumination and visible light communication applications

Emerging dual use applications of light emitting diode (LED) for general illumination (lighting) and free space optical (FSO)/visible light communication (VLC) require optimization of the basic device design, device interconnections and/or chip level packaging architectures. Understanding the fundamental technological constraints for simultaneous illumination and communication using high-flux visible-wavelength light emitting diodes (LEDs) is necessary for designing such dual-use systems. In this research, two LED architectures, namely, high current (HC) and high voltage (HV) types have been compared for their 3dB optical bandwidth at operating conditions necessary for general illumination. The contributions of the intrinsic as well as extrinsic device impedances and circuit parasitics on the fundamental switching speeds of these packaged devices have been analysed. The 3 dB optical bandwidth of the phosphor based white HC and HV LEDs have been found to be typically in the range of 3-8 MHz due to the long optical decay time constant of the phosphors. It has been shown that one could achieve significantly higher optical bandwidth using the emission from the blue GaInN/GaN LED alone. In this process, however, a large amount of optical power (~80%) is sacrificed when filtering out the phosphor emission from the white LEDs leading to significant decrease in signal-to-noise ratio (SNR) of the VLC channel in the system. The 3 dB optical bandwidth in the range of 30-35 MHz has been observed using the blue emission from the GaInN HC and HV LEDs without the phosphor contribution. The bandwidths of these LEDs were also estimated from the measured values of series resistance and diode capacitance. The parasitic resistances and capacitances from the LED driver circuit, LED package in conjunction with the series resistance and junction capacitance of the LED result in an order of magnitude reduction in the bandwidth from the expected bandwidth of the LED alone. The design considerations of driver circuits for power conversion and high-speed switching make the HV LED configuration preferable for the dual-usage paradigm. To increase the aggregate bandwidth of visible light communication (VLC) systems in the 100 Mbps-multi-Gbps range, a coded inverse multiplexing architecture has been developed. The new scheme uses multiple channels of same frequency and different duty cycles with On-Off Keying (OOK). This system has been shown to deliver bandwidths as high as 50 MHz. It is capable of bandwidth up to 400 MHz using existing commercial LEDs and drivers with reduced circuit parasitics. Coded inverse multiplexing can also be deployed with color tunable red-green-blue-yellow (RGBY) light fixtures for a multi-channel VLC network. This scheme when implemented using Red-Green-Blue (RGB) LEDs for general lighting had minimal impact on correlated color temperature (~ 35 K) for communication at modulation rates ~ 50 MHz. Since this scheme is a multiple input, single output (MISO) scheme, compact receivers can be used and the scaling up of bandwidth can be done by adding more LED channels to the transmitter.