ColorBars: Increasing Data Rate Of LED-to-Camera .

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ColorBars: Increasing Data Rate of LED-to-CameraCommunication using Color Shift KeyingPengfei Hu, Parth H. Pathak, Xiaotao Feng, Hao Fu and Prasant MohapatraACM CoNEXT 2015

Outline Visible Light Communication (VLC) background LED-to-camera communication, applications Limitations of existing approaches Data rate and reliability Our approach - Color shift keying Challenges and ColorBars system design Color flicker, inter-frame loss and receiver diversity Performance evaluation RGB LEDs, Android and iPhone

VLC - Background Utilize 430 - 790 THz spectrum for communicationTransmitter - LEDsReceiver - photodiode,image sensor

LED-to-Camera Communication Transmitter identity and information association Users can identify an LED transmitter and receive information specificallytransmitted by that LED Difficult with most types of RF communication and NFC Applications Cameras on smartphones and wearables like smart-glasses In retail stores, user can visualize LED on merchandise and receive moreinformation about it Location-specific broadcast - LED lights in an office can broadcast floor mapand navigation information

Rolling Shutter CMOS image sensors Matrix of photodiodes Commonly used in mobile devices Rolling shutter phenomenon Scan one column of pixels at a time Subsequent scanning enables communication Example: ON-OFF Keying

Limitations of Existing Schemes Current LED-to-camera communication techniques have many limitations Low data rate Low reliability due to inter-frame gap Human perceivable flickering Two existing approachesON-OFF Keying (OOK)-Ambient lightinterferenceON-OFF flickeringFrequency Shift Keying (FSK)-Longer symbolduration 12 bytes/sec. [1,2]

Our Approach Color Shift Keying (CSK) Use different colors as datasymbols Especially suitable for camerareceivers How? Use RGB tri-LED instead of whiteLED (Blue LED with yellowphosphor coating) Pulse Width Modulation formixing R, G and B colors

CSK Constellation Design Originally proposed in IEEE 802.15.7 standard Constellation based on CIE 1931 color space8 CSK16 CSK

Challenges - CSK for LED-to-Camera Communication Color flicker LED’s primary purpose is illumination Guaranteeing white light even when using CSK Avoiding human perceivable color or color change

Challenges - CSK for LED-to-Camera Communication Color flicker LED’s primary purpose is illumination Guaranteeing white light even when using CSK Avoiding human perceivable color or color change Inter-frame data loss Inter-frame gap of camera receivers Unidirectional communication Reliable data delivery

Challenges - CSK for LED-to-Camera Communication Color flicker LED’s primary purpose is illumination Guaranteeing white light even when using CSK Avoiding human perceivable color or color change Inter-frame data loss Inter-frame gap of camera receivers Unidirectional communication Reliable data delivery Receiver diversity Differences in cameras based on color filters, types and arrangements Same transmitted color captured differently by different cameras Reduce demodulation errors

Challenges - CSK for LED-to-Camera Communication Color flicker LED’s primary purpose is illumination Guaranteeing white light even when using CSK Avoiding human perceivable color or color change Inter-frame data loss Inter-frame gap of camera receivers Unidirectional communication Reliable data delivery Receiver diversity Differences in cameras based on color filters, types and arrangements Same transmitted color captured differently by different cameras Reduce demodulation errors

Avoiding Color Flicker Temporal summation Human eye “accumulates” incoming photons for a period of time until itsaturates The time period is called “critical duration” Luminance and color perception The perceived color is the average of the received colors during the criticalduration due to the temporal summation (Bloch’s law of vision)

Avoiding Color Flicker Difficult to ensure equal proportion of R, G and B within critical duration Dedicated illumination symbols Inserted periodically Amount depends on symbol frequency, also subjectiveHigher symbol freq. more demodulation errors10 volunteers1000 Sym./sec.3000 Sym./sec.

Challenges - CSK for LED-to-Camera Communication Color flicker LED’s primary purpose is illumination Guaranteeing white light even when using CSK Avoiding human perceivable color or color change Inter-frame data loss Inter-frame gap of camera receivers Unidirectional communication Reliable data delivery Receiver diversity Differences in cameras based on color filters, types and arrangements Same transmitted color captured differently by different cameras Reduce demodulation errors

Inter-frame Data Loss Camera receiver Data transmitted during inter-frame gap is lost Unidirectional communication from LED to camera Reed-Solomon error correction coding RS(n,k) can correct t bit errors where 2t n - k, n is the size of the codewordand k is number of data bitsSize of a symbol(in bits)illumination ratioNumber of symbolsreceived between twoconsecutive framesNumber of symbols lostbetween two consecutiveframes

Packetization Data, parity and illumination symbols are encapsulated in a packet Packet header includes- Flag indicating data or calibration packet- Size of the packet Size of a packet is chosen to be total size of a frame and inter-frame gap

Challenges - CSK for LED-to-Camera Communication Color flicker LED’s primary purpose is illumination Guaranteeing white light even when using CSK Avoiding human perceivable color or color change Inter-frame data loss Inter-frame gap of camera receivers Unidirectional communication Reliable data delivery Receiver diversity Differences in cameras based on color filters, types and arrangements Same transmitted color captured differently by different cameras Reduce demodulation errors

Receiver DiversityCIELab color spaceCamera diversity Camera exposure time and ISO also affect the captured color Calibration packets Transmitted periodically Includes all symbols of current CSK modulation Receiver uses the symbol colors for matching during demodulation

Demodulation Demodulation uses CIELab color space Color is converted from RGB to {a, b} Brightness is non-uniformly distributed inthe captured frame Dimension L is removedCIELab color space Colors of calibration packets used formatching (Euclidean distance in a,b-plane)

Performance Evaluation - Experiment Setup Transmitter BeagleBone Black (1 GHz processor, 512MB memory) Off-the-shelf RGB tri-LED Maximum frequency of color change approx. 4500 Hz PWM controller for color generation Receiver Nexus 5 (Android) and iPhone 5S (iOS) Nexus 5 - Android app for decoding, demodulation and error correction iPhone 5S - captured video processed offline

Inter-frame Loss Ratio Cameras on Nexus 5 and iPhone5S Frame rate - 30 frames per secondTransmission SymbolRate (Hz)Received Symbol Rate (Hz)Nexus 5iPhone 7.7340003060.672431.01Avg. Inter-frame LossRatio0.230.37

Symbol Error Rate (SER) Fraction of symbols incorrectly demodulated Low SER for 4 and 8 CSK even at high symbol frequency High inter-symbol interference at 16 and 32 CSKNexus 5iPhone 5S

Throughput Number of received symbols without any error correction Excluding the illumination symbols Lower throughput of iPhone 5S due to its larger inter-frame gapNexus 5iPhone 5S

Goodput RS decoding and error correction Lower goodput of iPhone 5S is due to higher overhead of parity bits (larger interframe gap)Nexus 55.2 KbpsiPhone 5S2.5 Kbps

Conclusions and Future Work CSK is an attractive scheme for LED-to-camera communication Higher achievable data rate than OOK and FSK Design of ColorBars Flicker free operation, reliable communication Open challenges and future work Larger LED - camera distance- programmable LED with higher lumens Optimizing CSK constellation based on other factors- Camera type, ambient light

Thank you!Questions/comments?

Avoiding human perceivable color or color change Inter-frame data loss Inter-frame gap of camera receivers Unidirectional communication Reliable data delivery Receiver diversity Differences in cameras based on color filters, types and arrangements Same transmitted

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