1. Introduction

Visible light communication (VLC) based on white light emitting diodes (LEDs) is garnering increasing attention as LEDs are considered to be a major candidate for future illumination [1]. The VLC system offers several advantages such as cost-effective, license-free, electromagnetic interference free and security. There are two types of white-light LED used for lighting: devices which use separate red–green–blue emitters and which use a blue emitter in combination with a phosphor that emits yellow light. Both kinds of white LEDs can be applied for VLC system. The former type enables easy color rendering by adjusting each colour individually, which is very promising for high-speed transmission because of wide bandwidth. The latter type is cost-efficient mainly due to its simple technological design, but the bandwidth is limited to several MHz due to the slow relaxation time of the phosphor. The feasibility of uni-directional VLC systems based on both kinds of white LEDs has been widely investigated [2–5]. The data rate of 3.4Gb/s at a distance below 30cm has been achieved over a RGB white LED by using discrete multi-tone (DMT) modulation and avalanche photodiode (APD) [6]. And the transmission data rate of 200 Mbit/s over a phosphorescent white LED (P-LED) has been reported by using DMT modulation [7]. However, bi-directional VLC transmission is still a main challenge due to the lack of good resolution of the uplink of an indoor VLC system. There have been some reports on bi-directional VLC transmission, such as retro-reflecting link [8] and time-division-duplex [9], but the data rates of uplink are only few Mb/s.

In this paper, we proposed and experimentally demonstrated a novel full-duplex bi-directional VLC system using RGB LED and a commercially available phosphor-based LED in downlink and uplink, respectively. In this demonstration, subcarrier multiplexing (SCM) and wavelength division multiplexing (WDM) are adopted to realize the bi-directional transmission; quadrature amplitude modulation (QAM) and orthogonal frequency division multiplexing (OFDM) modulation are also employed to increase the data rate. Additionally, pre- and post-equalizations are both implemented to compensate the severe channel response of LEDs. For downlink, signals are only modulated on red and green chips, while the blue chip is just lighted by direct current (DC) voltage to maintain white colour illumination. Each LED chip has two SCM channels without channel guardband. A downstream at 575 Mb/s and an upstream at 225 Mb/s after 66-cm free-space transmission are achieved, and the measured bit error rates (BERs) for all channels are under hard-decision FEC limit of 3.8x10⁻³ [10]. We also discuss the interference caused by bi-directional transmission. Moreover, this scheme has good scalability for supporting more terminals and advantage of dynamic traffic reconfiguration by adjusting different bandwidth and modulation orders for uplink and downlink transmissions.

2. Experimental setup

The block diagram of bi-direction VLC System is presented in Fig. 1 . In this scheme, SCM and WDM are employed to realize flexible frequency allocation and bi-directional transmission. We utilize RGB LED (Cree Xlamp MC-E) for downlink and phosphorescent white-light LED (Cree Xlamp XML) for uplink. This type of RGB LED consists of four chips radiating in the wave length regions of 625 nm (red), 530 nm (green) and 455 nm (blue) and white colour, which generates a luminous flux of about 106lm (red: 30.6lm, green: 67.2lm and blue: 8.2lm) at 350mA bias currents with 110° Lambertian emission, and the P-LED generates a luminous flux of about 280lm at 700mA bias currents with 120°Lambertian emission. The red and green lights of RGB LED are used to carry useful information, while the blue light is only supplied with DC to maintain white colour illumination. In this way, the collision between uplink and downlink can be eliminated.

 

Fig. 1 Block diagrams of proposed bi-directional VLC system (AWG: arbitrary waveform generator, EA: electrical amplifier, LPF: low-pass filter, OSC: real-time oscilloscope, DC: direct current, PD: photodiode, DL: downlink, UL: uplink).

Download Full Size | PDF

At the transmitter, the input binary sequences are modulated using QAM format, and then passed to OFDM encoder. Then, the QAM-OFDM signals are up-converted to different subcarriers with center frequency at f1 = 18.75MHz (sub1), f2 = 43.75MHz (sub2) without SCM channel guardband in radio frequency (RF) domain and added up. The bandwidths of all sub-channels are 25MHz. From DC to 5MHz, the transfer curve is not good in this demonstration, and the 25dB bandwidth point is around 50MHz, therefore we choose the signal frequency band from 6.25MHz to 56.25MHz. Moreover, the center frequency and bandwidths of sub-channels can be adjusted to meet the demands of different users. Subsequently, the multiplexed QAM-OFDM signals came from AWG are filtered by a low-pass filter (LPF) and amplified by EA. The electrical QAM-OFDM signals and DC-bias voltage are combined via bias tee, and applied to different LEDs serving as the transmitter. In red colour chip, 64QAM is applied both in sub1 and sub2. However, in green colour chip the modulation format of sub2 is 32QAM, and 64QAM is used in sub1. For uplink, the modulation formats of sub1 and sub2 are 32QAM and 16QAM, respectively. As each sub-channel is independent, it can be used to support multiple users for upstream and downstream transmission.

In this experiment, QAM-OFDM signals which consist of 64 subcarriers are generated by an arbitrary waveform generator (AWG). Up-sampling by a factor 20 is employed, and the sample rate of AWG is 500MS/s. Pre-equalization is used before inverse fast Fourier transform (IFFT) to compensate the distortions of AWG, LED, EA and free-space channel, while training-symbols-based post-equalization is used for other channel impairments such as phase noise. At the receiver, the electrical QAM-OFDM signals are detected by low-cost PDs and recorded by a digital real-time oscilloscope (OSC) with 500MS/s sampling rate. Additionally, in front of each PD, the corresponding optical filter is implemented to filter out the undesired wavelength. Then the received signals are down-converted to baseband and further offline processing which is an inverse procedure of QAM-OFDM encoder.

3. Experimental results and discussions

The experimental setup for the bi-directional VLC system based on LEDs is shown in Fig. 2 . In this experiment, Tektronix AWG 710 (output 1 for uplink) and Tektronix AWG 7122C (output2 for red chip, output 3 for green chip in downlink) are used to generate three different QAM-OFDM signals. And the signals are first amplified by an electrical amplifier (Minicircuits, 25-dB gain) to obtain an appropriate input power of LED. The data is recorded by a commercial high-speed digital oscilloscope (Tektronix TDS 6604) with the maximum bandwidth of 6GHz and the maximum sampling rate of 20GSa/s. A lens (100-mm focus length, 75-mm diameter) was used to collect the light onto the PD. The PD (Hamamatsu S6968, 0.24A/W responsivity at 440 nm) is with 150 mm² active area and about 50MHz bandwidth.The distance between the TX and RX in the different side is 66cm. And the bias currents of P-LED, red chip and green chip are 190mA, 170mA and 160mA.

 

Fig. 2 (a) Experiment setup for the VLC system. (b) downlink tranceiver. (c) uplink tranceiver.

Download Full Size | PDF

First of all, we measured the optical spectra of different LEDs used in this experiment as shown in Fig. 3 . The resolution is 0.35nm. As we can see, the unwanted spectrum component can be filtered out by the corresponding R/G/B optical filter, and the yellow phosphor of the P-LED can also be filtered out by blue filter. The optical spectra of these LED chips with RGB filter are not overlapping, which can be easily concluded that the interference between uplink and downlink will be quite small.

 

Fig. 3 Measured optical spectrum of different LEDs (a) RGB LED. (b) P-LED.

Download Full Size | PDF

The frequency characteristics of the electro-optical-electro channel are measured for all LED chips as shown in Fig. 4 . As we can observe, the frequency responses of blue and red chip of RGB LED are almost the same, and the green chip and the P-LED behave similarly. The bandwidths around 20dB point of the two groups are about 25MHz and 30MHz, respectively. Noting that the higher frequency is fast fading, equalization at frequency domain is needed. According to the channel knowledge, pre-equalization has been designed and applied. The amplitudes of the 64 subcarriers are appropriately pre-equalized. The electrical spectra of the received signals with (w) and without (w/o) equalizations at each wavelength are measured by Spectrum Analyzer HP8562A depicted in Fig. 5 (the spectra with post-equalization are offline processed).We could find that the spectra of each sub-channel are much more flatten after using pre-equalization and post-equalization. The power ratio of sub1 to sub2 is precisely assigned to obtain an optimal performance.

 

Fig. 4 Channel response of individual LED.

Download Full Size | PDF

 

Fig. 5 Electrical spectra of different wavelengths: (a) P-LED (w/o pre-) . (b) red (w/o pre-). (c) green (w/o pre-). (d) P-LED (w pre-). (e) red (w pre-) . (f) green (w pre-). (g) P-LED(w post). (h) red (w post). (i) green (w post).

Download Full Size | PDF

Then, we observe the nonlinearity effect introduced by LED chips. As these two types of LEDs behave similarly on nonlinearity, we take P-LED for discussion. In this demonstration, we just utilize sub-channel1 of P-LED. The input power of P-LED is varied from 8dBm to 20dBm with 2-dB step, and the results are presented in Fig. 6 . As we can see, the optimal input power is 12dBm. A lower input power will reduce the signal-to-noise-ratio (SNR) and cause low modulation depth, while a higher one will cause nonlinearity and clipping. We can also get the same conclusion from the constellation diagrams inserted in Fig. 6. The nonlinearity should be addressed by current source driver instead of voltage source driver or adopt nonlinearity compensation. In this experiment, the input power of P-LED is fixed at 12dBm. The modulation indexes calculated as in Ref [2]. of these three chips are all 1.

 

Fig. 6 Measured BER versus input power of P-LED.

Download Full Size | PDF

Finally, the measured BERs (by comparing the received data and transmitted data) versus different luminous flux of all chips are presented in Figs. 7(a)-(b) . For downlink, the luminous flux of each chip is set at no more than 60% of the maximum luminous flux of each chip with red chip is 30.6lm and green chip is 67.2lm. The illuminances through filters at the receiver side of both links are all below 350lx. It can be seen that all BERs of four channels at downlink and two channels at uplink can be below the pre-FEC limit of 3.8x10⁻³. At the red and green wavelengths, the BERs of sub1 and sub2 are almost the same, while at the blue wavelength the BER of sub1 is around 0.3dB degradation than sub2. The cross talk caused by bi-directional transmission is also analyzed. As the uplink and downlink behave similarly, we choose the uplink for discussion. Figure 7(b) shows the BER performance of uplink with and without downlink. It can be easily seen that there is almost no degradation, which shows that the cross talk is quite low. We also discuss the BER performance versus different distances. As both links behave similarly, we just take uplink for discussion. The results are shown in Fig. 8 . We can find that the BER performance will be degrade as the distance become longer. And the distance can be much longer by employing multiple chips. In this proposed scheme, the same modulation format for the individual sub-channel is adopted for system simplicity. However as the frequency response of the LED is not flatten, one can use adaptive modulation format for different sub-carriers. In this way the maximum data rate would be further improved, and we will do more detailed investigation in this area in the following work.

 

Fig. 7 (a) Measured BERs of downlink. (b) Measured BERs of uplink.

Download Full Size | PDF

 

Fig. 8 Measured BERs of uplink versus distance.

Download Full Size | PDF

4. Conclusion

In this paper, we have reported a bi-directional VLC system based on a commercially available RGB-LED, a P-LED and a low-cost photodiode. A data rate of 225 Mb/s upstream and 575Mb/s downstream transmissions enabled by SCM, WDM and QAM-OFDM has been achieved. Pre- and post-equalization at frequency domain has been adopted to compensate the distortions. A four-user access for downlink and one-user access for uplink can be achieved. The crosstalk of bi-directional transmission is also analyzed. BERs of all channels are under pre-FEC limit of 3.8x10⁻³ after 66-cm free-space transmission. Moreover, the capacity of downlink and uplink can be easily dynamically reconfigured by adjusting the bandwidths and modulation formats of sub-channel. The results show that this scheme is a good candidate for bi-directional transmission in the real VLC system.