oa Design and Performance Analysis of VLC-based Transceiver

Background

As the number of handheld devices increases, wireless data traffic is expanding exponentially. With the ever increasing demand of higher data rate, it will be very challenging for the system designers to meet the requirement using limited Radio Frequency (RF) communication spectrum. One of the possible remedies to overcome this problem is the use of freely available visible light spectrum [1].

Introduction

This paper proposes an indoor communication system by utilizing Visible Light Communication (VLC). VLC technology utilizes visible light spectrum (750–380 nm) not only for illumination but with an additive advantage of data communication [2]. Visible Light Communication exploits high frequency switching capabilities of Light Emitting Diodes (LEDs) to transmit data. A receiver generally containing a Photo Diode receives signals from optical source and can easily decode the information being transmitted. In practical systems, usage of CMOS imager as a receiver containing an array of Photo Diodes is preferred over a single Photo Diode. Such receiver will enable multi-target detection and multi-channel communication resulting in more robust transceiver architecture [3].

Method

This work demonstrates a real-time transceiver implementation for Visible Light Communication on FPGA. A Pseudo Noise (PN) sequence is generated that will act as input data for the transmitter. Direct Digital Synthesizer (DSS) is implemented for the generation of carrier signal for modulation purpose [4]. Transmitter utilizes On-Off-Keying (OOK) for modulation of incoming data due to its simplicity [5]. The modulated signal is then converted into analog form using Digital-to-Analog (DAC) converter. An analog driver circuit is connected with digital transmitter which is capable of driving an array of Light Emitting Diodes (LEDs) for data transmission. Block level architecture of VLC Transmitter is shown in Fig. 1.

The receiver architecture uses analog circuitry including Photo Diodes for optical detection and Operational Amplifiers for amplification of received signal. Analog-to-Digital (ADC) conversion is performed before transmitting data back to FPGA for demodulation and data reconstruction. Figure 2 demonstrates the architecture of VLC Receiver.

Results and Conclusion

The system is implemented and tested using Xilinx Spartan 3A series FPGA [6]. Basic transceiver implementation utilizes data rate of 1Mbps with a carrier frequency of 5 MHz. However, in VLC, data rate and carrier frequency directly affects the optical characteristics including color and intensity of LEDs. Therefore, different data rates and modulation frequencies are evaluated for optimum data transmission with minimal effects on optical characteristics of LEDs. System complexity in terms of hardware resources and performance analysis including Bit Error Rate (BER) under varying conditions is also compared.

Results demonstrate that it is feasible to establish a low data rate communication link for indoor applications ranging up to 10 m using commercially available LEDs. Integrating a CMOS imager at receiver end will enable a VLC based Multiple-Input-Multiple-Output (MIMO) communication link that can serve multiple channels, maximizing to 1 channel per pixel [3]. Higher data rates are also achievable by utilizing high data rate modulation techniques (OFDM) at the expense of computational complexity and hardware resource utilization [7].

One of the possible implications of this work could be the implementation of VLC based Indoor positioning and navigation system. It can be a potential benefit for large constructions involved in public interactions including but not limited to hospitals, customer support centers, public facilitation offices, shopping malls and libraries. The system will largely utilize existing infrastructure of indoor illumination with added advantage of data communication.

The study also proposes extension of this work for utilization of VLC in outdoor applications. However, more robust algorithms are required for outdoor communication due to the presence of optical noise and interference caused by weather and atmospheric conditions. Robustness of existing algorithm can be increased by integrating Direct Sequence Spread Spectrum (DSSS) together with OOK for modulation. However, further research work is required to evaluate the performance, complexity and robustness of this system under realistic conditions.

References

[1]Cisco Visual Networking Index, “Global Mobile Data Traffic Forecast Update, 2012–2017,” CISCO, White Paper, Feb. 2013.

[2]Terra, D. Inst. de Telecomun., Univ. de Aveiro, Aveiro, Portugal, Kumar, N. Lourenco, N. Alves, L.N.,” Design, development and performance analysis of DSSS-based transceiver for VLC”, EUROCON - International Conference on Computer as a Tool (EUROCON), 2011 IEEE.

[3]“Image Sensor Communication”. VLCC Consortium.

[4]Xilinx DDS Compiler IP Core “http://www.xilinx.com/products/intellectual property/

dds_compiler.html#documentation”

[5]Nuno Lourenço, Domingos Terra, Navin Kumar, Luis Nero Alves, Rui L Aguiar, “Visible Light Communication System for Outdoor Applications”, 8th IEEE, IET International Symposium on Communication Systems, Networks and Digital Signal Processing

[6]Xilinx Spartan-3A Starter Kit “http://www.xilinx.com/products/boards-and-kits/

hw-spar3a-sk-uni-g.html”

[7]Liane Grobe, Anagnostis Paraskevopoulos, Jonas Hilt, Dominic Schulz, Friedrich Lassak, Florian Hartlieb, Christoph Kottke, Volker Jungnickel, and Klaus-Dieter Langer, “High Speed Visible Light Communication Systems”, IEEE Communications Magazine, December 2013.