Visible Light Communication: Research Challenges and Solutions

LiFibasedCommunications Networks: ResearchChallenges and Solutions Prof. Abhishek Dixit IIT Delhi Tuesday, February 5, 2019

Contents • Motivations for VLC • Research Challenges and Solutions • Project & Research activities at IITD • Conclusions Visible Light Communication

Motivation for VLC Visible Light Communication

Standardization ITU-T G.vlc IEEE 802.11bb IEEE 802.15.7 IEEE 802.15.7m IEEE 802.15.13 CP-1222 CP-1223 CP-1221 IEC-62943 Li-Fi Networks

Contents • Motivations for VLC • Research Challenges and Solutions • Channel Modelling • Challenges at the Transmitter • Modulation Schemes • Multiplexing Techniques • Indoor Positioning Systems • Media Access Control • Project & Research activities at IITD • Conclusions Visible Light Communication

Multipath Channel Model for VLC Channel impulse response (CIR)[1] • : CIR after k bounces; : total number of LEDs. • S: surface of all reflectors; : area of reflector; : wall reflectivity, m: Lambertian order of emission • For the ithbounce, path loss, angle of incidence, angle of reflection and path length. Fig: Multipath propagation model of diffuse VLC link. [1] K. Lee et al., Indoor Channel Characteristics for Visible Light Communications, 2011. Visible Light Communication

Channel Characterization Channel Parameters[2] Mean excess delay, RMS delay spread, where, Coherence bandwidth, Fig: Channel impulse response for LoS path and multipath. Tx 5 m Rx 5 m [2] F. Miramirkhaniet al., Channel modeling and characterization for visible light communications, 2015. *results based on research work of Rishu Raj (PhD Scholar) and Sonu Jaiswal (M.Tech. Student) Visible Light Communication

Challenges at the Transmitter • Flickering • Dimming support • Optimization of LED semi-angle Flickering Dimming Support Visible Light Communication

Optimization of LED Semi-Angle (b) (a) Fig: Spatial distribution of received optical power for (a) = 70° and (b) = 30° Degree of non-uniformity (DNU) Fig: Effect of changing LED semi-angle on average received power, Pavg and degree of non-uniformity, DNU. [3] K. Saxena, R. Raj and A. Dixit, A novel optimization approach for transmitter semi-angle and multiple transmitter configurations in indoor visible light communication links, ICCCNT 2018. Visible Light Communication

Optimization of LED Semi-Angle (LoS path) Optimization Function, F Fig: Variation in optimal LED semi-angle with different values of exponents αandβ. Fig: F-plot for optimization of semi-angle with four LED panels and α = β = 1. [3] K. Saxena, R. Raj and A. Dixit, A novel optimization approach for transmitter semi-angle and multiple transmitter configurations in indoor visible light communication links, ICCCNT 2018. Visible Light Communication

Dimming Based Modulation Schemes Variable On-Off Keying (VOOK) Variable Pulse Position Modulation (VPPM) • Dimming is achieved by filling the non data portion of symbol with filler bits • Maintains constant data rate • Provides dimming by adjusting pulse width. • Duty cycle is proportional to required dimming level Fig: VPPM symbols for bit ‘0’ and bit ‘1’ with (a) 60 % and (b) 20 % dimming Fig: VOOK symbols for (a) 90 %, (b) 70 %, (c) 30 % and (d) 10 % dimming Visible Light Communication

Dimming Based Modulation Schemes Multiple Pulse Position Modulation (MPPM) • The transmitter sends optical pulses during anyw out of n number of slots (1 ≤ w ≤ n) • Number of symbols that can be transmitted for a given n and w is nCw • For a fixed value of n, dimming can be achieved by varying w Overlapping Pulse Position Modulation (OPPM) • Special case of MPPM where the transmitter sends optical pulses during w number of consecutive slots • Number of symbols that can be transmitted for a given n and w is (n – w + 1) • (a) (b) (c) Fig: Sample waveforms for (a) 4-PPM, (b) MPPM {n = 6, w = 3} and (c) OPPM {n = 6, w = 3} Visible Light Communication

Contents • Motivations for VLC • Research Challenges and Solutions • Channel Modelling • Challenges at the Transmitter • Modulation Schemes • Multiplexing Techniques: OFDM and NOMA • Indoor Positioning Systems • Media Access Control • Project & Research activities at IITD • Conclusions Visible Light Communication

Orthogonal Frequency Division Multiplexing Add DC Bias and/or Clipping P/S and CP addition S/P and Mapping Hermitian Symmetry DAC and LPF Data IFFT LED Transmitter Optical Channel Receiver PD Decoding and P/S CP removal and S/P DC biased optical OFDM (DCO - OFDM ) • DC bias is added, energy inefficient • Better performance when higher spectral efficiency required Asymmetrically clipped optical OFDM (ACO - OFDM) • Negative part of the signal is clipped, energy efficient. • Preferred with low order constellations (low spectral efficiency) Filter and ADC FFT Output Noise Visible Light Communication

Comparison of DCO-OFDM and ACO-OFDM Fig: CCDF plots for comparison of PAPR performance Fig: Comparison of BER performance *results based on research work of Gaurav Pandey (PostDoc Fellow) and MahendraBhadoria (M.Tech. Student) Visible Light Communication

Performance analysis with different orders of M-QAM Fig: BER performance of DCO-OFDM for varying order (M) of M-QAM Fig: BER performance of ACO-OFDM for varying order (M) of M-QAM *results based on research work of Gaurav Pandey (PostDoc Fellow) and MahendraBhadoria (M.Tech. Student) Visible Light Communication

Non-Orthogonal Multiple Access • users are multiplexed in the power domain by assigning distinct power levels to different users depending upon their channel conditions • uses superposition coding at the transmitter and successive interference cancellation (SIC) at the receiver • achieves superior spectral efﬁciencies • all users can use the entire available bandwidth of the system Fig: Block diagram of basic NOMA scheme with K users.[4] [4] L. Yin et al. , Performance Evaluation of Non-Orthogonal Multiple Access in Visible Light Communication, 2017. Visible Light Communication

Power Allocation Schemes in NOMA Gain Ratio Power Allocation (GRPA)[5] Normalized Gain Difference Power Allocation (NGDPA)[5] where : electrical power allocation for nth user : LOS optical channel gain between LED and nthuser [5] C. Chen et al., On the Performance of MIMO-NOMA-Based Visible Light Communication Systems, 2018. Visible Light Communication

Power Allocation Schemes in NOMA 1 m LED1 LED2 User2 User1 r R = 2 m • Table: System parameters Fig: Achievable sum rate and sum rate gain of NGDPA over GRPA. *results based on research work of Rishu Raj (PhD Scholar) and Vipul Yadav (M.Tech. Student) Visible Light Communication

Performance Evaluation of NOMA Fig: BER performance of VLC system with and without NOMA when r/R = 0.3. *results based on research work of Rishu Raj (PhD Scholar) and Vipul Yadav (M.Tech. Student) Visible Light Communication

Contents • Motivations for VLC • Research Challenges and Solutions • Channel Modelling • Challenges at the Transmitter • Modulation Schemes • Multiplexing Techniques • Media Access Control • Project & Research activities at IITD • Conclusions Visible Light Communication

Media Access Control in VLC Problem with VLC MAC • VLC MAC works on CSMA/CA similar to WiFi and • follows distributed approach mainly • is not optimized • has QoS issues • slotted CSMA/CA in standard[6] suggests centralized MAC, but is not preferred/used Solutions • Use centralized based MAC protocols • Better QoS control [6] IEEE Std 802.15.7-2011, Sep. 2011. Li-Fi Networks

Comparison of distributed vs centralized Table : Simulation Parameters Fig: Mean delay vs the normalized load Distributed Centralized

Li-Fi Project (Subproject of 5G TestBed) IIT Delhi Funding: Department of Telecommunication (DoT), Ministry of Communications, Government of India. End Date: March, 2021 Visible Light Communication

Team Structure Li-Fi Networks

Objectives • Goal of the project is to develop a Li-Fi test-bed • Real-time bi-directional communication channel (link length 3.5 m) • Downstream (VLC): at data rate of 500 Mbps • Upstream (Infrared): at data rate of 100 Mbps • Fast handovers to support user mobility of less than 5 km/h. • MAC layer to ensure latency below 100 ms and packet loss rate below 5% for a user density of 1 persons/5m2. Li-Fi Networks

What would be our contribution? Li-Fi Networks

Technical specifications of the project Li-Fi Networks

Test bed setup for Version 0 Light emitting diode and lens assembly Lens, photo-detector and receiver circuit Transmitter circuit Li-Fi Networks

Results Input Waveform (data rate 1 Mbps) Output Waveform Li-Fi Networks

Results (BER Calculation) Li-Fi Networks

Test bed setup for Version 0 Spatial multiplexing 5 cm TRANSMITTER CIRCUIT RECEIVER CIRCUIT Plano-convex lens OSCILLOSCOPE DATA (NRZ) 1.5 m RECEIVER SIDE TRANSMITTER SIDE Li-Fi Networks

Conclusions • VLC is a promising technology – data rates, health safe, cheap • Targets to get up to 500 Mb/s with right modulation and multiplexing solutions • We achieved up to 1 Gb/s with OOK Visible Light Communication

Thank You ! Visible Light Communication

Visible Light Communication: Research Challenges and Solutions