1. Orthogonal Frequency Division Multiplexing for Indoor Optical Wireless Communications using Visible Light LEDs
S. K. Hashemi1, Z. Ghassemlooy1, L. Chao2, and D. Benhaddou3
1Optical Communications Research Group, NCRLab,
Northumbria University, Newcastle upon Tyne, UK
Web site:
2Department of Electronic and Information Engineering,
Hong Kong Polytechnic University
3Engineering Technology Department, College of Technology,
University of Houston, USA
CSNDSP 2008

2. Contents Introduction Optical Wireless Communication (OWC) Links
Combating Inter-symbol Interference
OFDM
Channel Estimation
Simulation results
Conclusions
CSNDSP 2008

3. Introduction Invented more than 40 years ago.
Has been adopted for several technologies:
Asymmetric Digital Subscriber Line (ADSL) services.
IEEE a/g, IEEE a.
Digital Audio Broadcast (DAB).
Digital Terrestrial Television Broadcast: DVD in
Europe, ISDB in Japan
4G, IEEE n, IEEE , and IEEE
CSNDSP 2008

4. LOS Non-LOS OWC Links LOS Rx Tx Rx Tx No multipath Propagation
Only noise is limiting factor
Possibility of blocking
Tracking necessary to maintain LOS link
Non-LOS
Multipath Propagation
Intersymbol interference (ISI)
Difficult to achieve high data date due to ISI Tx Rx
LOS Rx Tx
CSNDSP 2008

5. Techniques to Overcome ISI
Guard slots - in digital modulation (PPM, PIM)
Spread spectrum - at the expense of reduced bandwidth efficiency
Angle diversity - based on multibeam-narrow FOV transceiver
Adaptive decision equalizer
USE OF multi-carriers transmission
In this paper we investigate OFDM system where multipath induced ISI and the effects of background noise are reduced by taking the advantage of narrowband frequency interference, which affects only one of the frequency sub-bands when compare to the single-carrier modulation.
CSNDSP 2008

6. OFDM
OFDM mainly used in RF based wireless communication schemes offering
high data rates capability
high bandwidth efficiency
and capable of dealing with the multipath induced ISI [9]
Has been proposed in optical wireless communication systems [10]
Uses a large number of closely-spaced orthogonal sub-carriers, each modulated at a low symbol rate
CSNDSP 2008

7. (V-OFDM) of Cisco, Iospan,etc.
OFDM - Types
Wideband-OFDM
(W-OFDM) of Wi-LAN
Flash OFDM
from Flarion
Vector OFDM
(V-OFDM) of Cisco, Iospan,etc.
GHz band
Mbps in 40MHz
-- large tone-width
(for mobility, overlay)
-- Freq. Hopping for CCI reduction, reuse
to 5.0MHz BW
-- mobility support
-- MIMO Technology
-- non-LoS coverage, mainly for fixed access
-- upto 20 Mbps in MMDS
Wi-LAN leads the OFDM Forum -- many proposals submitted to
IEEE Wireless MAN
Cisco leads the Broadband Wireless Internet Forum (BWIF)
CSNDSP 2008

8. OFDM Signal 50% bandwidth saving Conventional multicarrier techniques
Frequency
Orthogonal multicarrier techniques
Ch.2
Ch.4
Ch.6
Ch.8
Ch.10
Ch.1
Ch.3
Ch.5
Ch.7
Ch.9
50% bandwidth saving
Frequency
CSNDSP 2008

9. OFDM - Spectrum
The FFT (and its inverse, the IFFT) are used to create a multitude of orthogonal subcarriers using just a single TX.
Subcarrier orthogonality must be preserved in the presence of
timing jitter
frequency offset
fading. T0
Magnitude
Frequency
CSNDSP 2008

10. OFDM System Architecture
Pilot signals uniformly inserted into all symbols (sub-carriers) for channel estimation
for diffuse optical links only.
CSNDSP 2008

11. OFDM
IFFT converts X(k) of length N into a complex time-domain OFDM signal.
In order for the IFFT/FFT to create an ISI-free channel, the channel must appear to provide a circular convolution.
To mitigate the effects of multipath induced ISI, a guard interval of G-sample (or cyclic prefix (CP)) is inserted between symbols.
The length of CP depends on the channel delay spread and is normally considered to be grater than or equal to the channel length (impulse response time) and less than symbol duration
CSNDSP 2008

12. Circular Convolution & DFT/IDFT
Circular convolution allows DFT!
Detection of X (knowing H):
(note: ISI free! Just a scaling by H)
CSNDSP 2008

13. OFDM - Cyclic Prefix (CP)
OFDM signal with CP is x[n]L, and so y[n] = x[n] * h[n].
First v samples of ycp interference from preceding OFDM symbol => discarded.
The last v samples disperse into the subsequent OFDM symbol => discarded.
This leaves exactly L samples at output y, to recover the L data symbols embedded in x.
CSNDSP 2008

14. OFDM
The received OFDM signal propagated through the channel h(n) is given by:
where w[n] is the additive white Gaussian noise, is the photodetector responsivity, and denotes the circular convolution.
Channel models:
Recursive algorithm to take into account higher order reflections [11],
MIMO method for speedy calculation of the impulse response [12].
Mont Carlo ray-tracing algorithm and its modified version uses both the Lambert’s diffuse and the Phong’s models to approximate surfaces with a strong specular component [13-14].
The Sphere model in [15]
Fast iterative model capable of calculating high-order reflections [17].
CSNDSP 2008

15. OFDM - Channel Estimation
With no knowledge of the channel, channel estimation is required for equalization and decoding provided
In one dimensional channel estimation schemes pilot insertion is done in
Block-type - all sub-carriers is used as pilot in a specific period. It uses the least square (LS) or the minimum mean-square error in a slow fading channel [18])
Comb-type - part of the sub-carriers are always reserved as pilot for each symbol, and it uses LS with interpolation and the maximum likelihood in a rapidly changing channel [19]) - Adopted
Block type
Time
Carriers
Come type
Time
Carriers
CSNDSP 2008

16. OFDM – Interpolation Schemes
Pilot symbols uniformly inserted into symbols according to
Information on the exact locations of the pilot and data symbols as well as the values of the pilot symbols are already available at the receiver.
The estimated channel at pilot subcarriers for the LS estimation (no need of prior knowledge of noise variance) is given as:
Interpolation schemes
linear interpolation,
second-order interpolation,
spline cubic interpolation
time domain interpolation which are used in the one dimensional model to extract channel information on the data sub-carriers Hd [18]. In [21, 23] detailed information on the number of pilots used and how to located them as optimal pilot design techniques are given.
CSNDSP 2008

17. OFDM - Simulation Transmitter Receiver time 010101001 0110
OFDM modulation (IFFT)
Channel coding / interleaving
Guard interval
I/Q
Symbol mapping (modulation)
0110
FFT-part
time
1 OFDM symbol
Channel
impulse response:
N symbols
OFDM demod.
(FFT)
Decoding / deinter-leaving
Guard interval removal
Time sync.
I/Q
symbol de-mapping (detection)
Channel est.
Receiver
CSNDSP 2008

18. Simulation Parameters
Parameters Values
No. of Les
Transmitted optical power Ps mW
Centre luminance intensity of LED mcd
Field of view (FOV) o Photodetector surface area Ar cm2
Lambert mode
Concentrator refractive index nr
Photodetector responsivity (A/W)
FFT size
Number of data subcarriers
Number of pilots , 32, 64
Cyclic prefix (CP) ratio /4
The radiation pattern is assumed to be Lambertian
CSNDSP 2008

19. Horizontal Illuminance Distribution
CSNDSP 2008

20. Results - Error Performance for LOS optical wireless OFDM link employing BPSK, QPSK and 16-QAM
CSNDSP 2008

21. Results – Error performance for diffuse OW OFDM link with QPSK & LS channel estimation for a number of pilot symbols
CSNDSP 2008

22. Conclusions
Visible light LED’s was proposed for both lightening and communication system
OFDM technique employed to combat ISI due to the multipath reflections
Due to the variation of the optical channel impulse response in OW links, pilot signals added and the channel is estimated.
We simulated the illumination to meet the ISO for lighting and found the number of LEDs for the simulation parameters.
Performance of the system for both LOS and Diffuse link for BPSK,QPSK and M-QAM was simulated.
CSNDSP 2008

23. Thank you!
CSNDSP 2008

24. Inter-Symbol-Interference (ISI) due to Multi-Path Reflections
Transmitted signal:
Received Signals:
Line-of-sight:
Reflected:
The symbols add up on the channel
 Distortion!
Delays
CSNDSP 2008

25. OFDM Symbols
Group L data symbols into a block known as an OFDM symbol.
An OFDM symbol lasts for a duration of T seconds, where T = LTs.
Guard period > delay spread
OFDM transmissions allow ISI within an OFDM symbol, but by including a sufficiently large guard band, it is possible to guarantee that there is no interference between subsequent OFDM symbols.
The next task is to attempt to remove the ISI within each OFDM symbol
CSNDSP 2008

26. Received LOS Power Maximum_power = 1.2270 [dBm]
Minimum_power = [dBm]
CSNDSP 2008

27. OFDM Systems System FFT Size Number Carriers Channel Spacing kHz
Bandwidth
MHz
Sample
Rate
Symbol
Duration
sec
Data
Mbits/s
HyperLAN/2 64 52 4
312.5
16.25 20
3.2
0.8
6-54
802.11a
16.56
DVB-T
2048
1024
1712
842
4.464
7.643
9.174
224
DAB
8192
1536
1.00
1.536
2.048
24/48/96
msec
3.072
ADSL
256 (down)
64 (up)
36-127
7-28
4.3125
1.104
231.9
CSNDSP 2008