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Orthogonal Frequency Division Multiplexing for Indoor Optical Wireless Communications using Visible Light LEDs. S. K. Hashemi 1 , Z. Ghassemlooy 1 , L. Chao 2 , and D. Benhaddou 3 1 Optical Communications Research Group, NCRLab, Northumbria University, Newcastle upon Tyne, UK - PowerPoint PPT Presentation
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CSNDSP 20081
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: http://soe.unn.ac.uk/ocr
2Department of Electronic and Information Engineering,
Hong Kong Polytechnic University3Engineering Technology Department, College of Technology,
University of Houston, USA
Contents
• Introduction• Optical Wireless Communication (OWC) Links• Combating Inter-symbol Interference • OFDM• Channel Estimation• Simulation results• Conclusions
CSNDSP 20082
CSNDSP 20083
Introduction
• Invented more than 40 years ago.• Has been adopted for several technologies:
– Asymmetric Digital Subscriber Line (ADSL) services.– IEEE 802.11a/g, IEEE 802.16a.– Digital Audio Broadcast (DAB).– Digital Terrestrial Television Broadcast: DVD in
Europe, ISDB in Japan– 4G, IEEE 802.11n, IEEE 802.16, and IEEE 802.20.
CSNDSP 2008
OWC Links
Non-LOS
Multipath Propagation Intersymbol interference (ISI) Difficult to achieve high data date due to
ISI
Non-LOS
Multipath Propagation Intersymbol interference (ISI) Difficult to achieve high data date due to
ISI
RxRxTxTx
LOSLOS
No multipath PropagationOnly noise is limiting factorPossibility of blockingTracking necessary to maintain LOS link
TxTx
RxRx
CSNDSP 20085
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 20086
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 20087
OFDM - Types
Wideband-OFDM(W-OFDM) of Wi-LAN
www.wi-lan.com
Flash OFDMfrom Flarion
www.flarion.com
Vector OFDM(V-OFDM) of Cisco, Iospan,etc.
www.iospan.com
-- 2.4 GHz band-- 30-45Mbps in 40MHz-- large tone-width (for mobility, overlay)
-- Freq. Hopping for CCI reduction, reuse-- 1.25 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 802.16 Wireless MAN•Cisco leads the Broadband Wireless Internet Forum (BWIF)
CSNDSP 20088
OFDM Signal
Ch.1
Ch.2 Ch.3 Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.10
Ch.3 Ch.5 Ch.7 Ch.9Ch.2 Ch.4 Ch.6 Ch.8 Ch.10
Ch.1
Conventional multicarrier techniques
Orthogonal multicarrier techniques
50% bandwidth saving
Frequency
Frequency
CSNDSP 20089
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.
Frequency
Magnitude
T0
CSNDSP 200810
OFDM System Architecture
Pilot signals uniformly inserted into all symbols (sub-carriers) for channel estimationfor diffuse optical links only.
)}1(,),1(),0({ ppppP NXXX
CSNDSP 200811
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
))}(({)( nkXIFFTnx N
CSNDSP 200812
Circular Convolution & DFT/IDFT
• Circular convolution:
• Detection of X (knowing H):
(note: ISI free! Just a scaling by H)
• Circular convolution allows DFT!
CSNDSP 200813
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 200814
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].
][][][.][ nwnhnsnyr
CSNDSP 200815
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
Tim
eCarriers
Tim
e
Carriers
Block type
Come type
CSNDSP 200816
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.
1,,1,0)(
)()(ˆ p
p
pp Nm
mX
mYmH
})),/((),1(),1)/(()0(),0({ pNdNFloordXpXpNdNFloordXdXpX
CSNDSP 200817
-60 -40 -20 0 20 40 60-50
-40
-30
-20
-10
0
10
f [MHz]
pow
er s
pect
rum
mag
nitu
de [
dB] OFDM spectrum for N
FFT = 128, N
w in = 12, N
guard = 24, oversampling = 1
0 20 40 60 80 100 120 140 160 180 200-0.2
-0.1
0
0.1
0.2time domain signal (baseband)
sample nr.
imaginaryreal
OFDM - Simulation
OFDM modulation
(IFFT)
Channel coding /
interleaving
Guard interval
I/Q I/QSymbol mapping
(modulation)
Transmitter
N symbols
OFDM demod. (FFT)
Decoding / deinter-leaving
Guard interval removal
Time sync.
I/Q I/Q
symbol de-mapping
(detection)
Channel est.
ReceiverFFT-part
time
1 OFDM symbol
Channel impulse response:
0101010010110
CSNDSP 200818
Parameters Values
No. of Les 4Transmitted optical power Ps 20.0 mWCentre luminance intensity of LED 730 mcdField of view (FOV) 60o Photodetector surface area Ar 1 cm2
Lambert mode 0.6461Concentrator refractive index nr 1.5Photodetector responsivity 0.53 (A/W)FFT size 256Number of data subcarriers 128Number of pilots 16, 32, 64Cyclic prefix (CP) ratio 1/4The radiation pattern is assumed to be Lambertian
Simulation Parameters
01
23
45
0
1
2
3
4
50
1
2
3
Hei
ght[
m]
Length[m]Width[m]
2.5m
(3.5,3.5)m
(1,1)m
(3.5,1)m
(1,3.5)m
LEDs interval: 1cm
CSNDSP 200819
Horizontal Illuminance Distribution
01
23
45
0
1
2
3
4
5200
400
600
800
1000
1200
1400
x[m]
Distribution of horizantal illuminance [lx]
y[m]
Illum
inan
ce[lx
]
Number of LEDs60 x 60 (4 set)
CSNDSP 200820
Results - Error Performance for LOS optical wireless OFDM link employing BPSK, QPSK and 16-QAM
0 5 10 15 202010
-4
10-3
10-2
10-1
100
Eb/N
o(dB)
SE
ROFDM over AWGN Channel
BPSK SimulationBPSK TheoryQPSK SimulationQPSK Theory16-QAM Simulation16-QAM Theory
CSNDSP 200821
Results – Error performance for diffuse OW OFDM link with QPSK & LS channel estimation for a number of pilot symbols
0 5 10 15 20 2510
-3
10-2
10-1
100
Eb/N
0(dB)
SE
RNpilot=16
Npilot=32Npilot=64
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 200822
CSNDSP 200823
Thank you!
CSNDSP 200824
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 200825
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 200826
Received LOS Power
01
23
45
0
1
2
3
4
5-6
-4
-2
0
2
x[m]
Distribution of Received Power [dBm]
y[m]
Rec
eive
d P
ower
[dB
m]
Maximum_power = 1.2270 [dBm]
Minimum_power = -5.2891 [dBm]
CSNDSP 200827
OFDM Systems
System FFT Size
NumberCarriers
ChannelSpacing
kHz
BandwidthMHz
SampleRateMHz
SymbolDuration
sec
Data
Rate
Mbits/s
HyperLAN/2 64 524
312.5 16.25 20 3.20.8
6-54
802.11a 64 524
312.5 16.56 20 3.2 0.8
6-54
DVB-T 20481024
1712842
4.464 7.643 9.174 224 0.68-14.92
DAB 20488192
1536 1.00 1.536 2.048 24/48/96msec
3.072
ADSL 256 (down)64 (up)
36-1277-28
4.3125 1.104 1.104 231.9 0.64-8.192
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