3. INTRODUCTION
Wireless communication technology developing fast.
We require high data rate.
Visible light optical communication more attractive
compared to conventional RF band.
Free space optical communication suffers challenges.
New modulation schemes are introduced.
3
4. Property VLC RF
Bandwidth Unlimited,
400nm-700nm
Regulatory, BW
Limited
EMI or RFI No High
Harmless for human
body
NO YES
Power Consumption Relative low Medium
Coverage Distance
Security
Short
More secure
Medium
Not secure
6. Gamma rays ,X-rays ,Ultraviolet light dangerous for the human
body.
Infrared, due to eye safety regulation, can only be used with low
power.
HENCE WE USE THE VISIBLE - LIGHT SPECTRUM.
7. 7
VLC is used for
Vehicle to vehicle communication
Networking in indoor environments
VISIBLE LIGHT COMMUNICATION(VLC)
13. 13
Required in communication system.
Extra parity bits are added to the message.
Receiver can retrieve original data that is corrupted.
Error control coding
14. Low Density Parity Check Codes
LDPC codes were developed by Robert Gallager.
Class of linear block code.
LDPC codes are decoded iteratively.
Error correcting code.
Perform close to Shannon theory.
Characterised by a sparse parity check matrix which
contains only few ‘1’s in comparison to ‘0’s.
LDPC codes are regular LDPC codes if H has uniform
number of 1’s in columns and rows.14
15. LDPC ENCODING
Given some sparse parity check matrix H.
If unsystematic we can use row and column
operations to set H′ = [PT |In−k].
Find the generator matrix G.
Since G = [Ik |P]
Codeword vector ,c = xG.
15
16. MODULATION TECHNIQUES
Amplitude shift keying.
Conventional technique used in optical
communication.
Generation of optical pulse by ON/OFF of LED.
Data rate less.
OOK MODULATION
16
17. Overlapping pulse position modulation
Type of pulse position modulation with overlapping.
Message of duration Tsym divided into symbols
Each pulse covers N time slots.
In normal case we can have Q alphabets.
On overlapping we can transmit J symbols.
J=N(Q-1)+1 symbols.
Bandwidth and throughput efficient.
High data rate.
17
19. Geometric approach to LDPC codes
Tanner Graph has two classes of nodes:
Variable nodes (v): correspond to bit/symbol nodes to
columns of H.
For (n−k)×n matrix H, we have n v nodes.
Check nodes (c): correspond to parity check equations
to the rows of H
We have (n−k )c nodes
19
21. LDPC DECODING
Bit-Flipping Algorithm
Bits are continually flipped .
Step 1: For each received variable nodes vi , calculate the
check node cj . Terminate the algorithm if all cj =0
Step 2: For each check node cj , j = 1,2, . . . consecutively,
we flip the bits of the connected variables nodes {vk} in
order if cj = 1 and converts back to received bits if cj = 0
Step 3: Go back to Step 1
21
22. Assume that an error free received codeword is
V=[10010101].
Since VHT=[0000] this is a valid codeword.
For the matrix
22
23. Received code word
V=10010101 with no
error.VHT=[0000]
If received codeword
V=11010101 is with 1
error.VHT=[1100] .
Flip vk associated
with c0=[v1v3v4v7]
23
24. Flip vk associated with
c1=[v0v1v2v5] since c1=1
SinceSince c2=0 convert the
associated nodes of c2 back
to original mode.
vk=[v2,v5,v6,v7] =[0101]
24
25. Since c3=0 convert the
associated nodes of c3
back to original mode.
vk=[v0,v3,v4,v6] =[1100]
Going back to step 1 we have c0,c1,c2,c3=0 and
algorithm terminates.
We got code word c=10010101 which is the original
error free code word.
25
26. Ratio of number of wrong bits to number of total bits.
Probability of error is the theoretically predicted
expected BER.
The more the signal is affected, the more bits are
incorrect.
When photon count increases BER decreases.
It is a major indicator of the health of the system.
BIT ERROR RATE
26
27. BIT ERROR RATE STUDY
OOK MODULATION
OPPM MODULATION
WITH LDPC CODING27
28. Can provide network access at
Home
Office
Shopping Center
Mining Areas
Vehicle traffic communication
APPLICATIONS
28
29. CONCLUSION
Cost reduced.
No interference on RF signals.
High data rate with less error is achieved.
Safe for health .
Large amount of energy can be saved.
Attractive alternative for mobile computers and
communication.
If this technology put into practical use ,
every bulb act like hotspots to transmit wireless data.
29
30. REFERENCES
30
[1] E. Cho, J. H. Choi, C. Park, M. Kang, M. Shin, Z. Ghassem looy, and C.G.
Lee, “NRZ-OOK signaling with LED dimming for visible light communication
link,” in Proc. NOC, pp. 32–35,IEEE-2011.
[2] J. Grubor, S. Lee, K. Langer, T. Koonen, and J. Walewski, “Wireless High-
Speed Data Transmission with Phosphorescent White-Light LEDs,” in Eur.
Conf. Optical Communication, Berlin, Germany, 2007.
[3] J. Vucic, C. Kottke , S. Nerreter , A. Buttner, K. D. Langer, and
J.W.Walewski, “White light wireless transmission at 200+ Mb/s net data rate by
use of discrete-multitone modulation,” IEEE Photon. Technol.Lett. 21, 1511–
1513 ,2009.
[4] S. Iwasaki, C. Premachandra, T. Endo, T. Fujii, M. Tnimoto, Y.Kimura,
“Visible Light Road-to-Vehicle Communication Using High-Speed Camera,”
Proc. IEEE Intelligent Vehicle Symposium, pp. 13-18,2008
[5] H. M. H. Shalaby, “Capacity and cutoff rate for optical overlapping pulse-
position modulation channels,” IEEE Trans. Commun. 43, 1284–1288 ,1995.