FREE-SPACE COMMUNICATIONS: Short-range white-light LED datacom reaches 2.1 Gbit/s

Researchers at Fudan University have developed a visible-light communication system based on quasi-balanced detection (QBD) in which opposite signals are applied to odd and even consecutive symbols, allowing a single detector to be used while eliminating second-order signal distortions.

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Visible-light communication (VLC) systems that enable one- or two-way communication between white-light LEDs and computers or mobile devices are drawing the interest of researchers, as the technique could replace wireless Wi-Fi communications and offer increased data rates. To increase bandwidth, VLC developers often use orthogonal frequency-division multiplexing (OFDM), which divides the transmitter’s modulation spectrum into frequency bands. However, OFDM has the problem of second-order nonlinearity distortion that appears when direct detection is used.

Researchers at Fudan University (Shanghai, China) have developed a VLC system based on quasi-balanced detection (QBD) in which opposite signals are applied to odd and even consecutive symbols, allowing a single detector to be used while eliminating second-order signal distortions.1 In addition, the researchers use a red-green-blue (RGB) white LED rather than a blue-emitter/yellow-phosphor LED, which allows them to independently modulate the three colors to create a simple form of wavelength-division multiplexing (WDM).

The QBD-OFDM operation itself happens mostly in the electrical portion of the system. Applying QBD in the OFDM coding allows the measurements of receiver photocurrent as a function of time to be analyzed in a way that enables the removal of second-order intermodulation distortion and DC offset. An additional benefit is a 3 dB improvement in the receiver sensitivity.

For the WDM portion of the setup, a commercial RGB LED with wavelengths of 620, 520, and 470 nm producing a luminous flux of about 6 lm was used. Avalanche photodiodes with a gain of 1 and a sensitivity of 0.42 A/W and covered with R, G, and B filters served as receivers. An arbitrary-waveform generator was used to create the test signals.

The LED output was made low enough so that the LED was not operating in or near saturation. A lens with a 100 mm focal length captured the LED light and delivered it to each detector; the data was recorded by a commercial high-speed digital oscilloscope with a 500 megasample/s recording rate.

Because a real-world VLC link would typically have a distance between transmitter and receiver of 3 m or less, the researchers varied the transmission distance between 0.5 and 2.5 m in 0.5 m steps. The data rate of 2.1 Gbit/s was the result of a WDM combination of red at 750 Mbit/s, green at 650 Mbit/s, and blue at 700 Mbit/s.

Bit-error-rate (BER) measurements were taken for two modulation techniques: QBD-OFDM, and direct-detection optical OFDM (DDO-OFDM). At a 0.5 m distance, the BERs for the so-called sub1 and sub2 signals for the red, green, and blue LEDs were improved by 25.6, 31, 30.3, 25.8, 21.8, and 19.3 dB, respectively.

When used in orthogonal frequency-division multiplexing of RGB white-light LEDs, quasi-balanced
detection (QBD) enables the transmission of data at 2.1 Gbit/s at bit-error rates (BERs) low enough to allow complete bit-error correction.

Using just the green LED for simplicity, the researchers then compared four modulation techniques: QBD-OFDM, DDO-OFDM, asymmetrically clipped optical OFDM (ACO-OFDM), and QBDACO-OFDM. Only the QBD-OFDM and QBD-ACO-DFM were able to provide
BERs of less than 3.8 × 10-3, which, in their setup, is the maximum level at which bit-error correction functions to provide error-free transmission. They provided this performance over the entire experimental range from 0.5 to 2.5 m (see figure). The researchers say the QBD approach is a good one for low-cost VLC networks.

1. Y. Wang et al., Opt. Exp., 21, 23, 27558 (November 18, 2013).

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