Visible Light Communication (VLC) Low-Complexity Visible Light Networking with LED-to-LED Communication. Application: Toy-to-Toy Communication

Transcription

1 Introduction Visible Light Communication (VLC) Low-Complexity Visible Light Networking with LED-to-LED Communication Domenico Giustiniano, Nils Ole Tippenhauer, Stefan Mangold VLC is an emerging technology, it uses the visible light spectrum light emitters (LEDs) transport information wirelessly VLC is a low-power, low-cost alternative to traditional short-range wireless RF communications Wireless Days IFIP Conference, Dublin, November 212. Image: Bytelight 2 / 23 Motivation LED-to-LED communication Motivation Application: Toy-to-Toy Communication Most of the research on VLC has focused on wideband white LEDs used in ambient illumination. Wideband: 4 nm Narrowband and visible LEDs have received little attention. Narrowband: nm (a) Police car approaching... (b) Car detecting the message and reacting! 1) Light is safe. 2) We can control the communication through the directivity and the visual field of light propagation this makes it interactive (in contrast to infrared or RF). 3 / 23 4 / 23

2 Summary Summary of the Rest of the Talk Principles We investigate the limit of communication using a single LED and microcontroller (Arduino) for applications that require low PHY rate. We introduce an efficient MAC protocol to access the channel CSMA/CA is here inefficient. We study and resolve the problem of light flicker. Previous work does not consider that the light flicker when 1 LEDs operates in ad-hoc network and 2 the LED is also a receiver We implement and evaluate the solution in a network of up to four transceivers communicating on the same visible color. The brightness of LEDs can be adapted rapidly, at speeds orders of magnitude higher than for conventional light emitting devices. By varying the intensity of the LED light source, data messages can be communicated through visible light to a receiver sensitive to light. In our approach, the receiver is the same LED used for transmission. Only one microcontroller and one LED. 5 / 23 6 / 23 Transmission Reception Station transmits the bits using the binary OOK modulation: bit = symbol = ZERO bit = 1 symbol = ONE V th ZERO ONE 7 / 23 8 / 23

3 Light Intensity Reception Flicker transmitting data V ca (T ) > V th symbol = ZERO V ca (T ) < V th symbol = ONE. Ø ÒØ ÁÒØÖ ¹ Ø Ö ÆÓ Ø ÒØ ÁÒØ Ö¹ Ø Ö V th ZERO ONE Single light channel to transmit and receive data flicker effects. Intra- and inter-data flickers 8 / 23 9 / 23 Frequency of Intensity Modulation and Energy symbols Visual emotion/ feedback Flickering region VLC Frequency symbols Transmitter: carry information (bit) from the transmitter to the intended receiver. Receiver: either receive any data from stations in range or listen the channel before transmitting any data. Only one station is allowed to output energy in each Symbol. Energy symbols No data is transmitted. Any station is free to output energy to the optical medium. 1 / / 23

4 Scheme Frequency analysis ÝÑ ÓÐ Ø Ñ T E D D E Time (c) and Energy symbols. ONE ZERO ZERO ONE Time (d) Reception. ZERO ONE ZERO ONE Time (e) Transmitter, bit 1. Output Voltage [V(dB)] Observed average Flicker region Frequency [Hz] Output Voltage [V(dB)] Observed average Flicker region Frequency [Hz] CFF=Critical flicker frequency=166 Hz T < 1 4CFF = 1.5 ms 12 / / 23 LED noise Transmission and Reception with One LED T ZERO [ms] T ZERO [ms] Darkness B5 Y5 G5 R5 B3 Y3 G3 R3 With Ambient Noise B5 Y5 G5 R5 B3 Y3 G3 R3 Flicker Region 1/(4CFF) No Flicker Region Flicker Region 1/(4CFF) No Flicker Region Symbol ONE Charging: Reception: Symbol ZERO Using blue and red LEDs, we can transmit with symbol duration below T = 1.5 ms, without being affected by flicker and LED noise. We use T = 512 µs unless otherwise stated. Long time wasted in collisions due to low PHY rate Key idea: sense the channel transmitting a ZERO symbol 14 / / 23

5 CSMA/CD MAC access protocol packet Station B Station A RBT RBT new RBT Message a 1 Message a 1 new RBT RBT cont'd Message b 1 Message b 1 Detection is possible at the transmitter using Symbols ZERO, where light can be sensed. t 12 bit 8 bit 8 bit 24 bit 128 bit 16 bit Sync SFD Length MHR CRC Software-based synchronization protocol. State machine for MAC protocol. All implemented in Arduino. hex file of about 11 kb over the 3 kb available. 16 / / 23 Two Contending LEDs Three Contending LEDs 18 / / 23

6 Full flickering elimination MAC Throughput 9 Throughput [b/s] CSMA/CA,4 byte CSMA/CD,4 byte CSMA/CA,16 byte CSMA/CD,16 byte Number of Stations 2 / / 23 CSMA/CD MAC Throughput Conclusion Conclusion From renewal theory, the maximum system throughput S max is: S max = E[P] E[T m ] = T =.45/T (1) For T = 512 µs, S max = 878 b/s, very close to our experiments. We obtain a Jain s fairness index of.98 1, that indicates that both our protocol and prototype implementation are fair. Short-range free-space LED-to-LED communication enables a wide variety of entertainment applications. These applications benefits from: the explicit visual feedback to the user when pointing a device, the system simplicity and the low-cost nature of the LED-based systems. We proposed and evaluated an LED-to-LED communication system, including physical layer transmission and networking protocols. Our evaluation addressed technical challenges, such as the elimination of flicker and detection of collisions. 22 / / 23