Upload
others
View
1
Download
0
Embed Size (px)
Citation preview
ITU Kaleidoscope 2015Trust in the Information Society
A DCO-OFDM System Employing Beneficial Clipping Method
Jiang LiuWaseda University
1
Outline
• Why optical communication
• The challenge of OFDM modulation in optical wireless system
• The proposed beneficial clipping method
• Results and conclusion
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
Current Status of Radio Wireless Communication
– Limited bandwidth
– Interference
– Security
Limited bandwidth
Share
Base station
Relay stationMicrowave oven
Pace maker
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
Comparison between RF and OWC
4
Property of Medium RF OWC
Bandwidth regulated Yes No
Passes through walls Yes No
Multipath distortion Yes Yes
Path loss High High
Dominant noise Other users Background light
RF : Radio Frequency
OWC : Optical Wireless Communication
Adapted from :J. M. Kahn and J. R. Barry, "Wireless infrared communications,“ Proceedings of the IEEE, vol. 85, No. 2, pp. 265-298, 1997.
IEEE 802.15.7 Visible Light Communication
The IEEE 802.15.7 Visible Light Communication Task Group has completed a PHY and MAC standard for Visible Light Communications (VLC).
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
RF Sub-carrierIntensity modulated signal
Pth Pm
Popt(Average power)
Optical intensity
Photocurrent [A]
Optical intensity
Input current [A]RF Input
LED
Photo-detector
6
Intensity modulation and direct detection
(IM/DD)
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
Output Character of LED
7
Ou
tpu
t (w
)
Input (A)Ath
0
-0.02
0.02
0Ad
In optical wiirless systems, the system modulation depth is limited to a narrower range for high transmission speed.
Pmin
Pmax
PMAX
max min
MAX
P Pm
P
Modulation Depth m:
LED nonlinear characteristic
D/A, A/D limited linear range
Nonlinear distortion
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
Experiment results
8
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Input Current /A
Op
tica
l P
ow
ers /W
measurement data
theoretical model
second-order
third-order
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
Peak Clip of LED
9Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
FM signal over optical wireless channel
experiment results
10
Transmitted FM signals
Received FM signals
OFDM signal
0 100 200 300 400 500 600 700 800-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
Time domain signal (no DC)
Am
plitu
de
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
orthogonal frequency division multiplexing
High PAPR (Peak to Average Power)
OW OFDM signal
Light intensity must be unipolar and non-negative. Directed circuit (DC)
should be added.
0 100 200 300 400 500 600 700 800-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
Time domain signal (no DC)
Am
plitu
de
0 100 200 300 400 500 600 700 8000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time domain signal (add DC)
Am
plitu
de
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
orthogonal frequency division multiplexing
Why introduce beneficial clipping to DCO-
OFDM systems?
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
13
In the VLC systems, the system modulation depth is limited toa narrower range for high transmission speed.
However, lower modulation depths will lead to higher system BERs because of the reduction of the valid signal power.
Beneficial clipping method is proposed to enhance the system performance.
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
An example of OFDM signal
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
OFDM signal with DC
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
The beneficical clipping method
16
System analysis:
From the Bussgang theorem, the clipped signal
xc(t) is composed of two parts: the linear
attenuation Kx(t) and the clipping noise nc(t):
( ) ( ) ( )c cx t Kx t n t
The expand process:
A: Maximum signal amplitudeCR: Clipping ratioC: reduced amplitude after clipping
( ) ( )( ) c c
cc
Ax t x tx t
C CR
/CR C A
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
The beneficical clipping method
System analysis:
2
2 2
val
nval nvlc
SNR
The system SNR is determined based on the signal
power (σval)2, the clipping noise power (σnval)
2, and the
optical optical wireless channel noise (σnvlc)2
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
The beneficial clipping method
18
19
Diagram of the IM/DD DCO-OFDM system
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
Results and conclusion
20
BER performances for different clipping ratios and Modulation depth.
SNR (no beneficial clipping) is set as 25dB.
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
Results and conclusion
21
BER performances for different modulation depths.
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
Results and conclusion
22
For a certain modulation depth, the beneficial clipping method can
reduce the system BER and enhance system performance.
In addition, the method also can be used to obtain low modulation
depths for specific BER requirements.
Since more international standards are needed to support the VLC-OFDM
system, in the future we plan to further pursue contributions to the
standardization of VLC system.
23
Thank you!
Any question?
Barcelona, Spain, 9-11 December 2015ITU Kaleidoscope 2015 - Trust in the Information Society
The beneficical clipping method
System analysis:
The transmitted signal in the LEDs is xtr(t),
( ) ( )tr ccx t x t m DC
( ) ( ) ( )
( ) ( )
( ) ( )
elec cc c
c
val val
mx t x t m x t
CR
Km mx t n t
CR CR
x t n t
2
2 2
val
nval nvlc
SNR
The system SNR is determined based on
the valid signal power (σval)2 from xval(t),
the valid clipping noise power (σnval)2
from nval(t), and the optical optical wireless
channel noise (σnvlc)2