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LiFi based Communications Networks: Research Challenges and Solutions
Prof. Abhishek Dixit IIT Delhi Wednesday, April 17, 2019
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Visible Light Communication
Contents Motivations for VLC Research Challenges and Solutions Project & Research activities at IITD Conclusions 17-Apr-19 Visible Light Communication
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Visible Light Communication
Contents Motivations for VLC Research Challenges and Solutions Project & Research activities at IITD Conclusions 17-Apr-19 Visible Light Communication
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Visible Light Communication
Motivation for VLC Unlicensed Visible Spectrum High Capacity High Security Provision of Spatial Reuse High Energy Efficiency Easy Implementation High Safety Low Cost 17-Apr-19 Visible Light Communication
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Standardization ITU-T G.vlc IEEE 802.11bb IEEE 802.15.7 IEEE 802.15.7m
CP-1222 CP-1223 CP-1221 IEC-62943 Wednesday, April 17, 2019 Li-Fi Networks
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Visible Light Communication
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 17-Apr-19 Visible Light Communication
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Multipath Channel Model for VLC
Channel impulse response (CIR)[1] β(π‘)= π=1 π πΏπΈπ· π=0 β β (π) (π‘) β (π) (π‘)= π π π πΏ 1 πΏ 2 β― πΏ π+1 ππππ‘ πΌ π+1 π πΉππ πΏ π‘β π· 1 + π· 2 +β―+ π· π+1 π π π΄ πππ πΏ 1 = π΄ ref π+1 cos π π½ 1 cos πΌ 1 2π π· 1 2 , πΏ 2 = π΄ ref cos π½ 2 cos πΌ 2 π π· πΏ π+1 = π΄ PD cos π½ π+1 cos πΌ π+1 π π· π+1 2 π (π) (π): CIR after k bounces; π΅ π³π¬π« : total number of LEDs. S: surface of all reflectors; π¨ πππ : area of reflector; π: wall reflectivity, m: Lambertian order of emission For the ith bounce, π³ π : path loss, πΆ π : angle of incidence, π· π : angle of reflection and π« π : path length. Fig: Multipath propagation model of diffuse VLC link. [1] K. Lee et al., Indoor Channel Characteristics for Visible Light Communications, 2011. 17-Apr-19 Visible Light Communication
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Channel Characterization
Channel Parameters[2] Mean excess delay, π= π=1 π π π,π π‘ π,π + π=1 π π πππ,π π‘ πππ,π π π RMS delay spread, π π
ππ = π 2 β π 2 where, π 2 = π=1 π π π,π π‘ π,π 2 + π=1 π π πππ,π π‘ πππ,π π π Coherence bandwidth, π΅ πΆ = 1 π π
ππ Tx Rx 5 m Fig: Channel impulse response for LoS path and multipath. Channel Parameters LoS Path (k=0) Multipath (k = 3, π = 0.8) π (ns) 13 14.02 π π
ππ (ns) 3.48 Bc (MHz) inf 287.7 [2] F. Miramirkhani et 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) 17-Apr-19 Visible Light Communication
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Visible Light Communication
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 17-Apr-19 Visible Light Communication
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Challenges at the Transmitter
Flickering Dimming support Optimization of LED semi-angle Flickering Dimming Support 17-Apr-19 Visible Light Communication
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Optimization of LED Semi-Angle
(b) Fig: Spatial distribution of received optical power for (a) π 1/2 = 70Β° and (b) π 1/2 = 30Β° Degree of non-uniformity (DNU) π· ππ β π πππ₯ π πππ , π· ππ β₯1 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. 17-Apr-19 Visible Light Communication
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Optimization of LED Semi-Angle (LoS path)
Optimization Function, F πΉβ π ππ£π πΌ π· ππ’ π½ , πΌβ₯0, π½β₯0 Fig: Variation in optimal LED semi-angle with different values of exponents Ξ± and Ξ². Fig: F-plot for optimization of semi-angle with four LED panels and Ξ± = Ξ² = 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. 17-Apr-19 Visible Light Communication
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Visible Light Communication
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 17-Apr-19 Visible Light Communication
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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 β0β and bit β1β with (a) 60 % and (b) 20 % dimming Fig: VOOK symbols for (a) 90 %, (b) 70 %, (c) 30 % and (d) 10 % dimming 17-Apr-19 Visible Light Communication
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Dimming Based Modulation Schemes
Multiple Pulse Position Modulation (MPPM) The transmitter sends optical pulses during any w out of n number of slots (1 β€ w β€ 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 β 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} 17-Apr-19 Visible Light Communication
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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 17-Apr-19 Visible Light Communication
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Orthogonal Frequency Division Multiplexing
Data S/P and Mapping Hermitian Symmetry IFFT P/S and CP addition DAC and LPF Add DC Bias and/or Clipping Optical Channel Filter and ADC CP removal and S/P FFT Decoding and P/S Output Transmitter Receiver Noise LED PD 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) 17-Apr-19 Visible Light Communication
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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 Mahendra Bhadoria (M.Tech. Student) 17-Apr-19 Visible Light Communication
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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 Mahendra Bhadoria (M.Tech. Student) 17-Apr-19 Visible Light Communication
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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 efο¬ciencies 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. 17-Apr-19 Visible Light Communication
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Power Allocation Schemes in NOMA
Gain Ratio Power Allocation (GRPA) [5] π π = π» π+1 π» π+1 π π+1 Normalized Gain Difference Power Allocation (NGDPA) [5] π π = π» 1 β π» π+1 π» π π π+1 where π π : electrical power allocation for nth user π» π : LOS optical channel gain between LED and nth user [5] C. Chen et al., On the Performance of MIMO-NOMA-Based Visible Light Communication Systems, 2018. 17-Apr-19 Visible Light Communication
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Power Allocation Schemes in NOMA
LED1 LED2 User1 User2 R = 2 m r 1 m Table: System parameters Parameter Value Room Dimensions 5 m Γ 5 m Γ 3 m Transmitter-receiver separation 2.15 m Modulation bandwidth 10 MHz Responsivity of photo-detector 0.53 Transmitted optical power per LED 10 W Semi-angle of LED 60Β° Physical area of each detector 1 cm2 FOV of photo-detector 70Β° 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) 17-Apr-19 Visible Light Communication
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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) 17-Apr-19 Visible Light Communication
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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 17-Apr-19 Visible Light Communication
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Visible Light Communication
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 17-Apr-19 Visible Light Communication
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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 , Sep Wednesday, April 17, 2019 Li-Fi Networks
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Comparison of distributed vs centralized
Table : Simulation Parameters S.No. Parameters Non-Converged Converged 1. Size of ACK and POLL - 64 bytes 2. CSMA/CA slot time 20 Β΅s 3. CSMA/CA SIFS 10 Β΅s 4. CSMA/CA DIFS 50 Β΅s 5. Number of devices 8 6. Guard Time 1 Β΅s 7. Line Rate 1 Gb/s 8. Size of REPORT and GATE 9. Queue Size 1 Mb Centralized Distributed Fig: Mean delay vs the normalized load 4/17/2019
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Visible Light Communication
Contents Motivations for VLC Research Challenges and Solutions Project & Research activities at IITD Conclusions 17-Apr-19 Visible Light Communication
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Visible Light Communication
Li-Fi Project (Subproject of 5G TestBed) IIT Delhi Funding: Department of Telecommunication (DoT), Ministry of Communications, Government of India. End Date: March, 2021 17-Apr-19 Visible Light Communication
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Expected team strength
Team Structure S.No. Designation Number of candidates 1. Project Engineer 3 2. Ph.D. student 1 3. Sr. Project Assistant 4. Project Associate 2 5. Project Attendant Current team strength 8 Expected team strength 12 Wednesday, April 17, 2019 Li-Fi Networks
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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. Reference for walking speed (Gait Cycle) Wednesday, April 17, 2019 Li-Fi Networks
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What would be our contribution?
Characteristics PHY β I PHY β II PHY β III IITD Data rate 11.67 β Kbps 1.25 β 96 Mbps 12 β 96 Mbps 500 Mbps Modulation OOK VPPM 4-CSK 8-CSK 16-CSK OOK, OFDM, QAM Multiplexing - Spatial multiplexing, OFDM, NOMA etc. MAC Distributed Centralized Wednesday, April 17, 2019 Li-Fi Networks
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Technical specifications of the project
Characteristics Final Version 3 Version 0 Downlink speed 500 Mbps 1 Mbps Uplink speed 10 Mbps - Receiver Sensitivity (10-3 BER) -20 dBm -13 dBm Transmitted communication power 0.2 W 1 W Link length 3.5 m 3 m MAC layer Centralized controller, mobility, horizontal handover. Delivery date 31st March, 21 31st March, 19 Subsystem finish date 31st December, 20 31st December, 18 Integration time Nov, 20 β Jan, 21 Dec, 18 β Mar, 19 Wednesday, April 17, 2019 Li-Fi Networks
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Test bed setup for Version 0
Light emitting diode and lens assembly Lens, photo-detector and receiver circuit Transmitter circuit Wednesday, April 17, 2019 Li-Fi Networks
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Results Input Waveform (data rate 1 Mbps) Output Waveform
Wednesday, April 17, 2019 Li-Fi Networks
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Results (BER Calculation)
Wednesday, April 17, 2019 Li-Fi Networks
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Test bed setup for Version 0
Spatial multiplexing 5 cm TRANSMITTER CIRCUIT RECEIVER CIRCUIT Plano-convex lens DATA (NRZ) OSCILLOSCOPE 1.5 m TRANSMITTER SIDE RECEIVER SIDE Wednesday, April 17, 2019 Li-Fi Networks
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Visible Light Communication
Contents Motivations for VLC Research Challenges and Solutions Project & Research activities at IITD Conclusions 17-Apr-19 Visible Light Communication
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Visible Light Communication
Conclusions VLC is a promising technology β 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 17-Apr-19 Visible Light Communication
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Visible Light Communication
Thank You ! 17-Apr-19 Visible Light Communication
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