High-Speed VLC Links Sample Clauses

High-Speed VLC Links. During the past few years, growing insights were gained into the efficient implementation of VLC data transmission using LEDs initially developed for illumination purposes. In general, there are two main types of white LEDs commonly used for lighting: the phosphorescent type consisting of a blue LED chip plus a yellow phosphor layer, and the multi-color type consisting of three (in some cases four) individual chips, mostly red, green and blue (hence: RGB). While the phosphorescent type allows for cost-efficient installations mainly due to its simple driver design, it provides only a few MHz modulation bandwidth caused by the slow response time of the phosphorescent material. However, we were able to demonstrate that the bandwidth can be enhanced by an order of magnitude up to 20 MHz by suppressing the phosphorescent portion of the optical spectrum with the aid of a “blue filter” at the receiver [5]. In contrast, the white-light RGB-type LEDs enable three individual color channels each providing approximately 15 MHz bandwidth. By using three drivers in parallel, wavelength division multiplexing (WDM) can be realized. However, the advantage of an increased aggregate data rate is achieved at the expense of higher costs. We demonstrated that, although illumination LEDs are not developed for data transmission, they offer significant potential for high-speed communications. Starting with phosphorescent LEDs and simple On-Off Keying (OOK) modulation, which allowed 100-200 Mb/s data rates, the transmission speeds were increased continuously by applying more spectrally efficient modulation formats such as adaptive Orthogonal Frequency Division Multiplexing (OFDM) alias Discrete Multitone Transmission (DMT1), an idea originally proposed in [49]. We continued working on sophisticated design and transmission improvements enabling up to 800 Mb/s using a standard high-brightness RGB-LED combined with a commercial Si-PIN-photodiode in a single- color transmission mode [50]. These results confirmed that ordinary RGB-LEDs designed for illumination can deliver data rates up to the Gb/s range based on DMT modulation with bit and power loading for throughput maximization beyond conventional 3-dB-bandwidth limitations. A real boost of the throughput can be achieved with RGB-LEDs using WDM. The principle is shown in Figure 28. WDM for VLC was studied in detail in [49] using a commercially available high-brightness white-light RGB-LED as optical source and WDM pass-band filters ...
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