November 2006
doc.: IEEE 802.22-05/0263r0
IEEE P802.22 Wireless RANs
Contribution for IEEE 802.22 WRAN Systems
Sensing Scheme for DVB-T
Date: 2006-11-10
Author(s):
Name
Linjun Lv
Zhou Wu
Mingwei Jie
Company
Huawei Technologies
Huawei Technologies
Huawei Technologies
Address
Phone
email
Shenzhen, China
86-755-28973119
lvlinjun@huawei.com
Shenzhen, China
86-755-28979499
wuzhou@huawei.com
Shenzhen, China
86-755-28972660
jiemingwei@hauwei.com
Soo-Young Chang
Huawei Technologies
Davis, CA, U.S.
1-916 278 6568
sychang@ecs.csus.edu
Jianwei Zhang
Huawei Technologies
Shanghai, China
86-21-68644808
zhangjianwei@huawei.com
Jun Wang
UESTC
Chengdu, China
86-28-83206693
junwang@uestc.edu.cn
Shaoqian Li
UESTC
Chengdu, China
86-28-83202174
lsq@uestc.edu.cn
Chengdu, China
86-13096307946
honey_puma@sina.com
86-13880765377
ocean_chl@163.com
86-13568886741
truedream82321@sina.com
Qihang Peng
Lei Chen
Meng Zeng
UESTC
UESTC
Chengdu, China
UESTC
Chengdu, China
Abstract
Contents
In this contribution, some schemes to detect the existence of DVB-T signals are proposed. Simulation results are provided
to show the performances of all these schemes. From these results, it is proved that the DVB-T signals can be effectively
detected by utilizing the characteristics of the pilots inserted in the OFDM symbols.
Notice: This document has been prepared to assist IEEE 802.22. It is offered as a basis for discussion and is not binding on the
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accepts that this contribution may be made public by IEEE 802.22.
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Submission
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Soo-Young Chang, Huawei
November 2006
doc.: IEEE 802.22-05/0263r0
List of Figures
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
The generation of PRBS sequence ...................................................................................................6
OFDM Structure ...............................................................................................................................7
Sliding Correlation Based on CP ......................................................................................................7
Sliding Correlation Based on time domain of Pilot in CP ................................................................8
Simulation Result (sensing time is 17.92ms, false alarm rate is 0.1) ...............................................9
Simulation Result (sensing time is 17.92ms, false alarm rate is 0.01) ...........................................10
Submission
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Soo-Young Chang, Huawei
November 2006
doc.: IEEE 802.22-05/0263r0
List of Tables
Table 1 The duration of CP for 8MHz channel .................................................................................................4
Table 2 Location of continual pilot carriers ......................................................................................................5
Submission
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Soo-Young Chang, Huawei
November 2006
doc.: IEEE 802.22-05/0263r0
1. Reference
[1]
[2]
[3]
[4]
[5]
ETSI EN 300 744, “Digital Video Broadcasting (DVB): Framing structure, channel coding and modulation for
digital terrestrial television”.
J.–J. Van de Beek, M. Sandell, and P. O. Borjesson, “ML estimation of time and frequency offset in OFDM
systems”, IEEE trans. Signal Processing, vol. 45, pp. 1800-1805, July 1997.
D. Landstrom, S. K. Wilson, J. – J. Van de Beek, Per Odling, and P. O. Borjesson, “Symbol time offset estimation
in coherent OFDM systems”, IEEE trans. On communications, vol. 50, No. 4, April, 2002.
M. D. Duncan, and A. S. Robert, “Acquisition of spread spectrum signals by an adaptive array”, IEEE trans. On
acoustics, speech, and signal processing, vol. 37, No. 8, Aug. 1989.
D. Landstrom, S. K. Wilson, J. – J. Van de Beek, Per Odling, and P. O. Borjesson, “Synchronization for a DVB-T
receiver in presence of co-channel interference”, PIMRC, IEEE, vol. 5, pp. 2307-2311, Sept. 2002.
2. Introduction to DVB-T Signal
Digital Video Broadcasting-Terrestrial (DVB-T), the European Standard on digital TV radio, has already been adopted
by more than 30 countries, so it is reasonable to develop DVB-T sensing algorithms for IEEE 802.22 WRAN system.
There are some distinct characteristics of DVB-T signal, such as OFDM, Cyclic Prefix (CP), Pilot symbols etc, several
sensing algorithms based on these characteristics are proposed in this document. They are cyclic prefix based sliding
correlation detection,, time domain pilot signals ,based sliding correlation, time domain pilot signals in cyclic prefix
based sliding correlation and multi-antennas detection. In this document, all of these algorithms are described
corresponding simulation results are presented.
DVB-T 0 adopts Coded Orthogonal Frequency Division Modulation (COFDM) scheme. The number of OFDM
subcarriers is as large as 2K or 8K. Each OFDM symbol contains 6817 data subcarriers in the 8K mode and 1705 data
subcarriers in the 2K mode respectively. Also, the OFDM symbol is composed of two parts: the data subcarriers and the
pilot subcarriers. According to the locations of the pilot subcarriers, the pilot is divided to two parts: continual pilot and
scattered pilot. The location of continual pilot subcarriers within each OFDM symbol is fixed, whereas the location of
scattered pilot carriers within each OFDM symbol is changed regularly. Meanwhile, in order to eliminate inter-symbol
interference caused by multipath propagation, a cyclic prefix is inserted at the beginning of each OFDM symbol.
2.1 The Cyclic Prefix
To protect DVB-T signal against inter-symbol interference (ISI), a cyclic prefix which copy the last part of the symbol is
added at the beginning of each OFDM symbol. The duration of CP for the 2K and 8K mode is listed in Error!
7 / 64  s for 8MHz
Reference source not found. respectively, where T is the elementary period, which equal to
channel.
Table 1 The duration of CP for 8MHz channel
Mode
8K mode
CP
1/4
Duration of
symbol part
8192  T
2048  T
896 s
224 s
Duration of
cyclic prefix
Submission
2K mode
1/8
1/16
2048  T 1024  T
112 s
224 s
512  T
56 s
page 4
1/32
256  T
28 s
1/4
512  T
56 s
1/8
256  T
28 s
1/16
128  T
14 s
1/32
64  T
7 s
Soo-Young Chang, Huawei
November 2006
Symbol
duration
doc.: IEEE 802.22-05/0263r0
10240  T
1120 s
9216  T
1008 s
8704  T
952 s
8448  T
924 s
2560  T
280 s
2304  T
252 s
2112  T
2176  T
231 s
238 s
2.2 Location of scattered pilot carriers
There are 68 OFDM symbols in one frame, for the symbol of index
for which index
l (ranging from 0 to 67), the scattered pilot carriers
k belongs to the subset
k  K min  3  (l mod 4)  12 p p integer,p  0, k [ K min; K max ]


(1)
p is an integer that takes all possible values greater than
where k is the frequency index of the OFMD subcarriers and
[ K min; K max ]
k
or equal to zero, provided that the resulting value for
does not exceed the valid range
.
2.3 Location of continual pilot carriers
In addition to the scattered pilots described above, 177 continual pilots in the 8K mode and 45 in the 2K mode are
inserted according to Error! Reference source not found.. Where “continual” means that they occur on every symbol.
Table 2 Location of continual pilot carriers
2.4 The generation and modulation of pilot
The continual and scattered pilot are generated by a Pseudo Random Binary Sequence (PRBS),
wk
, corresponding to
their respective carrier index k . The PRBS sequence is generated according to Error! Reference source not found.The
PRBS is initialized so that the first output bit from the PRBS coincides with the first active carrier. A new value is
generated by the PRBS on every used subcarrier (whether or not it is a pilot).
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November 2006
doc.: IEEE 802.22-05/0263r0
Figure 1
The generation of PRBS sequence
The polynomial for the PRBS generator shall be:
X 11  X 2  1
(2)
The pilot cells are always transmitted at the “boosted” power level. Thus the corresponding modulation is given by:
Re cm,l ,k   3 / 4  2(1/ 2  wk )
Im cm ,l ,k   0
where
m is the frame index, k is the frequency index of the subcarriers and l is the time index of the OFDM symbols.
3. Detection of DVB-T Signals
3.1 Single Antenna Scenario
The problem of determining whether there is a DVB-T signal or not on the specific spectrum band can be categorized
into the following binary hypothesis test with the null hypothesis H0 corresponding to DVB-T signal absent and the
alternative hypothesis H1 corresponding to DVB-T signal existing, that is
Lh 1

H1 : r  k    hl s  k  l   n  k 
l 0

H : r  k   n  k 
 0
where
r k 
(3)
represents the kth sample of received signal,
s k 
and
hl  l  0,
, Lh  1
are transmitted signal and the lth
nk 
path fading coefficient of the channel respectively, and the additive white Gaussian noise (AWGN)
is modeled as
independent complex Gaussian random variables.
Based on this detection model and the characteristics of DVB-T signal described above, we proposed the following
effective schemes to detect the existence of DVB-T signals.
3.1.1 CP Based Sliding Correlation
Let’s assume that there are N carriers in a OFDM symbol and the length of CP is L . Therefore, in time domain, the
Submission
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Soo-Young Chang, Huawei
November 2006
doc.: IEEE 802.22-05/0263r0
length of a OFDM symbol is N  L . As shown in Error! Reference source not found., the first L samples are the
same to the last L samples in the same OFDM symbol. Consequently, there exits correlation between these two parts for
this construction.
CP
0
N  L 1
N
L
Figure 2
OFDM Structure
From the description and analysis above, the following detection statistics is derived for DVB-T signal detection:
 B _ CP 
  L 1
 r  k  r  k  N 

k
(4)
where  ranges from o to N  L  1 . The corresponding implementation diagram for this scheme is illustrated in Error!
Reference source not found.。
r k 
Conjunction
Moving Sum
Absolute
Max
Comparision
Dealy-N
Figure 3
Sliding Correlation Based on CP
3.1.2 Detection Based on IFFT of Pilot
As described in the previous section, there are a number of pilots (including both continuous ones and scattered ones).
This significant characteristic can be utilized for its existence detection.
N
There are totally N carriers in an OFDM symbol in the frequency domain. Let p represent the number of the pilot
N  Np
subcarriers. The transmitted DVB-T Signal can be divided into two parts: the first part is made up of
data
carriers and it can be expressed as
st  k  
1
N

n0, , N 1 \ 
xn e j 2 kn / N
(5)
in time domain , the second part is made up of
is given as
mk  
1
N
 p e
n
Np
pilot subcarrier and the corresponding representation in time domain
j 2 kn / N
n
(6)
Therefore the binary hypothesis test in (3) can be transformed into the following equations:
Lh 1

H1 : r  k    hl  st  k  l   m  k  l    n  k 
l 0

H : r  k   n  k 
 0
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November 2006
doc.: IEEE 802.22-05/0263r0
3.1.2.1 Time Domain Pilot Based Sliding Correlation
 P1 
  N  L 1

r  k  m  k   
k
(8)

 is the time delay between the transmitter and the receiver, r  k  is the conjunction of receiving signal r  k  . In this
algorithm, it is similar to the situation in Section 3.1.1, the value of  is unknown, so the sliding correlation between
r k 
and
mk 
is necessary. The implementation diagram is very similar to Error! Reference source not found.
3.1.2.2 Time domain Pilot in Cyclic Prefixes Based Sliding Correlation
 P2 
  L 1
  r  k   r  k  N   m  k   

k
(9)
It is obvious that the length of summation in (8) is N  L  1 . In order to reduce the length of this operation, thus
reducing the complexity of the whole detection process, and considering the CP inherited in DVB-T signals, we propose
 r  k   r  k  N 
another DVB-T signal detection algorithm, as illustrated in (9), where

is the conjunction of
r k   r k  N  
, is the time delay between the transmitter and the receiver, the implementation diagram of this
scheme is depicted in Error! Reference source not found..
r k 
Moving Sum
Conjucntion
Absolute
Comparision
m k  
Delay N
Figure 4
Sliding Correlation Based on time domain of Pilot in CP
3.1.3 Simulation results
For our simulations, we choose 2K mode of 8MHz bandwidth DVB-T signal, A cyclic prefix with 56 s length is added
at the beginning of each OFDM symbol, and the resulted total OFDM symbol duration is 280 s . The channel
parameters is set according to the annex B of the DVB-T standard document “ETSI EN 300 744” 0 , which is a 20 paths
Rayleigh fading channel. The sensing time for all the simulations is 17.92ms , that means the duration of 64 OFDM
symbols. The simulation results are shown in Error! Reference source not found. and Error! Reference source not
found. in terms of ROC curves.
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November 2006
Figure 5
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Simulation Result (sensing time is 17.92ms, false alarm rate is 0.1)
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November 2006
doc.: IEEE 802.22-05/0263r0
Figure 6
Simulation Result (sensing time is 17.92ms, false alarm rate is 0.01)
3.2 Multi-Antenna Scenario
As multiple antennas utilized at Base Station (BS) is one possible solution, thus making multi-antenna detection scenario
available.
The detection model in multi-antenna scenario is

H1 : x  n  k   hs  k   w  n  k 


H 0 : x  n  k   w  n  k 
x
where M 1 vector
(10)

s  h
w 
is the received signal vector from M receiver antennas and
, and
are transmitted
n

0,1,
,
N

1
N
r
signal, channel fading coefficients vector and noise vector respectively. Here
( r is the length of
k  0,1, , Nd  1 N d  N  L
received signal) and
(
).
The detection statistics is
c  n 
where
m

m 
2
1
2
H
rˆxm
 n  Rˆ xx1  n  rˆxm  n 
(11)
H
denotes conjunction transpose operation and
N  L 1
 mk 
k 0
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Soo-Young Chang, Huawei
November 2006
rˆxm  n  
N  L 1
ˆ  n 
R
xx
doc.: IEEE 802.22-05/0263r0
 x n  k  m k 

k 0
(13)
N  L 1
 x n  k  x n  k 
H
k 0
(14)
3.2.1 Simulation Results
The simulation setup is same as before except that the sensing time is set to be 1.12ms , that means 4 times OFDM
symbol duration and the number of receiver antennas is two.
Figure 1 Simulation Result for 2 antenna detection (sensing time is 1.12ms)
4. Conclusion
In this document, several algorithms are proposed to detect the existence of DVB-T signals. Simulation results are also
provided to shown the performances of all these schemes. From these results, it can be concluded that the DVB-T signals
can be effectively detected by utilizing the characteristics of the pilots inserted in the OFDM symbols.
Submission
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Soo-Young Chang, Huawei