Optical wireless network built on white-light LEDs reaches 800Mb/s

Not long ago, the image of a room filled with laptop computers, each streaming a different high-definition (HD)-quality film wirelessly from the Internet, was only a vision. Yet modern wireless local area networks (WLANs) have made this dream a reality. Figure 1 shows an alternate implementation of the same scenario: data transmission via illumination LEDs, a concept known as visible light communication (VLC). The European Commission project OMEGA¹ presented a first-step demonstration of this innovative technology. A 100Mb/s VLC system consisting of 16 ceiling LEDs successfully transmitted four HD videos to four different laptops simultaneously. The receivers were movable across the lit area (∼10m²).

It should be emphasized that VLC, also called optical wi-fi or li-fi (for ‘light-fidelity’), is not intended to replace radio- or physical-link technologies such as WLAN, PowerLAN, and the Universal Mobile Telecommunications System. Rather, it is an option for data links where radio transmission networks are not desired or not possible. Moreover, although a hand or other opaque object placed between the light signal and the receiver impairs the transfer, this presumed disadvantage can equally well be viewed as a security feature.

Figure 1. Artist's conception of a future optical wi-fi implementation (still frame from a video).²

Another advantage of the VLC concept is that low-cost LEDs designed for illumination applications can be modified for data transfer at minimal expense. Many typical commercially available white LEDs consist of a blue LED covered by a white phosphor layer. The slow response of the phosphorescent component limits the LED's modulation bandwidth to a few megahertz. However, we have shown that filtering the phosphorescent portion of the optical spectrum before detection can increase the bandwidth by an order of magnitude, yielding a potential throughput in the hundreds of megabits per second in an indoor environment.³

The main challenge for data transmission with a VLC system remains, however, the LED chip bandwidth itself, which varies between 10 and 20MHz. To circumvent this limitation, different modulation schemes (depending on the throughput requirement) can be applied. Figure 2 is a block schematic showing the setup used in our experiments in on-off keying (OOK) and in discrete multitone (DMT), while Figure 3 shows the achieved data rate as a function of modulation technique and receiver type. In our OOK-based experiments, a pattern generator was used to produce the OOK signal (the dashed lines in Figure 2). After detection at the receiver, an error counter determined the bit error rate (BER). High-speed OOK modulation using a phosphorescent white LED and a simple p-i-n photodiode gave a data rate of 40Mb/s.⁴Optimizing the electronic circuitry increased the data rate to 125Mb/s,⁵ and deploying an avalanche photodiode (APD) in place of the p-i-n nearly doubled the rate to 230Mb/s.⁶

Figure 2. Setup for all experiments involving encoding via either on-off keying (OOK) or discrete multitone (DMT). AMP: Amplifier. AWG: Arbitrary wave generator. dc: LED power supply and bias tee. LPF: Low-pass filter. OSC: Oscilloscope. PC: Personal computer. PD: Photodiode. PRBS: Pseudorandom binary sequence. RGB: Red/green/blue. Rx: Receiver. Tx: Transmitter.

Figure 3. Overview of experimental data rates achieved. The DMT technique, using orthogonal frequency-division multiplexing (OFDM), provides a 10-fold increase over the LED chip bandwidth. APD: Avalanche photodiode. PIN: p-i-n photodiode.

Given the rather modest system bandwidth, transmission at higher data rates requires a spectrally efficient type of modulation. Several of our experiments involved quadrature amplitude modulation on DMT. The DMT technique is familiar from its use in digital subscriber line systems. In wireless applications, it is known as orthogonal frequency-division multiplexing. Besides providing spectral efficiency, DMT allows for bit and/or power loading of individual subcarriers. Implementing basic DMT on a VLC link consisting of a phosphorescent white LED and a low-cost p-i-n photodiode gave a data rate of 230Mb/s.⁷ Replacing the receiver with an APD and adding bit and power loading and symmetrical clipping in DMT raised the operating speed to 513Mb/s, an unprecedented data rate for a single-LED link.⁸Digital signal processing was performed using an arbitrary wave generator and a storage oscilloscope. All experiments were performed at an illuminance level of 400–1000lum at the receiver, which has implications for link range.

In contrast with the single channel provided by phosphorescent white-light LEDs, RGB LEDs (combining discrete red, green, and blue chips in a single housing) enable the use of wavelength division multiplexing (WDM), increasing the overall data rate while offering the potential of service or user separation. The DMT signals in our experiments were processed offline. Each consisted of N=32 subcarriers, to which bit and power loading were applied adaptively to maximize channel quality. Optical filters were added to the receiver front end to separate the WDM channels. Figure 4 shows (a) the response of the analog channels for the three WDM links and (b) the corresponding optimal bit loading masks. The maximum data rates at which the BER did not exceed a 2·10⁻³ forward error correction threshold were ∼ 294 (red), ∼ 223 (green), and ∼ 286 (blue) Mb/s, for a total of ∼ 803Mb/s.⁹

Figure 4. (a) Responses of the red, green, and blue analog channels for the three WDM links and (b) the corresponding optimal bit loading masks. λ: Wavelength.

In conclusion, inexpensive white-light LEDs, despite their relatively low bandwidth, show potential suitability for high-speed transmission in optical wireless data links. Data rates of the magnitude we have demonstrated, surpassing 800Mb/s with WDM, are considered sufficient for the implementation of real-life optical wi-fi. In fact, adaptive modulation techniques could theoretically produce rates of well over 1Gb/s, and this continues to be the focus of our research.