University:
Boston University
Course:
CAS PY 105 | Physics 1
Academic year:

2024
Views:

316

Pages:

19
Author:

insoumissionyj60

An LED-to-LED Visible Light Communication System with Software-Based Synchronization Motivation – Networking Toys • Requirements: • Low bit rate communication • Short distances • Cost-effective • LED-to-LED communication: • • • • • Fulfills requirements Communication is visible No radio waves Reuses already existing components Low-complexity 2 Outline • Motivation • Technology • LED as a Transmitter • LED as a Receiver • Synchronization • Implementation and Platform • Evaluation • Conclusions 3 Transmitter • On-off keying • LED on  symbol ONE • LED off  symbol ZERO • Requirements: • No flickering • Constant brightness • Example: Encode 1 bit directly with a symbol • 1-byte message: 1000 0001  ON OFF OFF OFF OFF OFF OFF ON • Long OFF-sequence changes the perceived brightness • Even longer OFF-sequence introduces flickering 4 Manchester Coding • 0  1 0 ONE ZERO • 1  0 1  ZERO ONE • Uniform distribution of ONE and ZERO symbols • Redundancy 5 LEDs as Light Receivers • LEDs can be used as light receivers [Dietz et al, 2003] • LED used as a receiver: • Reduces complexity • Cost-effective • Limited light sensitivity But • Short range communication • Low data rates still possible 6 Receiver • LED is used in reverse bias  capacitor • LED (cathode) is periodically charged • Measure residual voltage after a certain time period 7 Threshold • Receiver needs a threshold to decode the residual voltage level to a ZERO or ONE • Requirements: • Largest distance from both ZERO and ONE symbol • Adaptive to ambient light • Mean of recent measurements: • Contains ZEROs (ambient light) and ONEs (LED generated light) • ZEROs and ONEs are uniformly distributed (Manchester) • Provides a good threshold if constantly updated 8 Synchronization (1) • Goal: Match the start of a transmitted symbol with the start of a measurement period • Synchronization pattern: Pairs of ONE and ZERO symbols • Shift until measured light for ONE symbols is equal 9 Synchronization (2) synchronize Receiver ONE detected start of measurement offset Transmitter 10 Always «on» • Problem: • LED appears as «off» in reception mode • Not possible to measure the photo-current while the LED is on • Solution: • LED multiplexing  alternating measuring and light emitting slots • No change in transmitter, thanks to Manchester coding (redundancy) • LED perceived as «on»  1 kHz signal 11 Challenges (1) • Receiver only measures light slots: • Ambient light never measured • Computed threshold is equal to the symbol ONE • Decoder cannot detect all ONEs anymore • Halving the computed threshold provides a good approximation (ambient light value 0) 12 Challenges (2) Broken synchronization: • Two consecutive measurements are different for the same amount of received light • Different preconditions for measuring circuit (ADC) • Measurement deviation depends on LED Correction: • Deviation is a linear function • Correction computed from preceding measurement 13 Outline • Motivation • Technology • LED as a Transmitter • LED as a Receiver • Synchronization • Implementation and Platform • Evaluation • Conclusions 14 Hardware Platform • Various Arduino boards for prototyping • ATmega 328P microcontroller • • • • 8 bit 32K program storage 2K RAM 8 ADC pins (multiplexed) • In-house PCB solution • Based on Arduino layout • Smaller form factor for toy integration 15 Implementation • Flexible software implementation (C++, AVR-g++) of physical and MAC layer • Hardware: • ADC • Timers • Software: • Synchronization • Encoding / Decoding • MAC • MAC • Acknowledgments/ Retransmissions • CRC 16 Evaluation • Robustness for typical toy scenarios • Data throughput for: • Different (short) distances • Different data rates • Testbed setup: 17 Evaluation Results 18 Conclusions • Short-range communication system for toys • VLC based on only LEDs • Low-cost • Low-complexity • Software driven physical/MAC layer • Short distance communication • 100-200 b/s data rates 19

An LED-to-LED Visible Light Communication System with Software-Based Synchronization

Related Documents

Recommended Documents

Description

Related Documents

Free up your schedule!

Take 5 seconds to unlock