IEEE C802.16n-11/0153
Project
IEEE 802.16 Broadband Wireless Access Working Group <http://ieee802.org/16>
Title
Changes on Synchronization Channel for Talk-around Direct Communications
Date
Submitted
2011-09-14
Source(s)
Jihoon Choi, Young-Ho Jung
E-mail:
jihoon@kau.ac.kr, yhjung@kau.ac.kr
Korea Aerospace University
Sungcheol Chang, Seokki Kim, Eunkyung
Kim, Miyoung Yun, Won-Ik Kim,
Sungkyung Kim, Hyun Lee, Chulsik
Yoon, Kwangjae Lim
scchang@etri.re.kr
ETRI
Re:
Call for Comments on the 802.16n AWD
Abstract
This provides AWD text proposals for changes on synchronization channel of talk-around direct
communications
Purpose
To be discussed and adopted by 802.16 TGn
Notice
Release
Patent
Policy
This document does not represent the agreed views of the IEEE 802.16 Working Group or any of its subgroups. It
represents only the views of the participants listed in the “Source(s)” field above. It is offered as a basis for
discussion. It is not binding on the contributor(s), who reserve(s) the right to add, amend or withdraw material
contained herein.
The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution,
and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name
any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole
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contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.16.
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<http://standards.ieee.org/guides/bylaws/sect6-7.html#6> and
<http://standards.ieee.org/guides/opman/sect6.html#6.3>.
Further information is located at <http://standards.ieee.org/board/pat/pat-material.html> and
<http://standards.ieee.org/board/pat>.
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Changes on Synchronization Channel for Talk-around Direct Communications
Jihoon Choi, Young-Ho Jung
Korea Aerospace University
Sungcheol Chang, Seokki Kim, Eunkyung Kim, Miyoung Yun, Won-Ik Kim, Sungkyung Kim, Hyun
Lee, Chulsik Yoon, Kwangjae Lim
ETRI
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1. Introduction
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In this contribution, we propose new sequences for SYNC-CH preamble and define details of SYNC-CH IE.
The SYNC-CH preamble is used to transfer the time and frequency information of a synchronized HR-MS to
neighboring HR-MSs which do not have valid synchronization information. The proposed preamble sequences
provide improved time and frequency estimation accuracy by modifying the conventional sequences shown in
[3]. Through numerical simulations, the performance of the proposed preamble sequences is compared with that
of the conventional sequences. The SYNC-CH IE includes the information related to the TDC link structure and
the TDC frame structure. This contribution presents the SYNC-CH IE with specific fields and defines the pilot
pattern and channel coding method for signal generation in the physical layer.
To support direct communication in IEEE 802.16n, some dedicated resources can be assigned as described in
[1]. By dedicatedly reserving a part of communication resources for infra-structure communications, the
reserved resources are used to provide multiple pairs of direct communication in OFDMA (orthogonal
frequency division multiple access) manner, and the other communication resources are used for infra structure
communication. The resources assigned for direct communication are composed of synchronization channel,
supplementary channel, and dedicated channel [2]. The SYNC-CH (synchronization channel) structure for TDC
(talk-around direct communications) was proposed in [3]. The SYNC-CH is composed of SYNC-CH preamble
and SYNC-CH IE.
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2. Proposed SYNC-CH structure
Subframe
CP
Conventional
SYNC-CH
A
A
A
A
SYNC-CH preamble
Proposed
SYNC-CH
(Seq. 0)
B
B
B
B
SYNC-CH message
B
B
B
SYNC-CH preamble
Proposed
SYNC-CH
(Seq. 1)
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C
-C
C
-C
SYNC-CH message
C
-C
C
SYNC-CH preamble
SYNC-CH message
Figure 1. Proposed SYNC-CH structure in the time domain
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Figure 1 describes the proposed SYNC-CH structure for TDC in the time domain. One SYNC-CH occupies one
802.16m subframe composed of six OFDM (orthogonal frequency division multiplexing) symbols. The first
three OFDM symbols are used for SYNC-CH preamble transmission and the last three OFDM symbols include
the SYNC-CH IE. When the resources for TDC are assigned in the FDM (frequency division multiplexing)
manner, four PRUs including 72 contiguous subcarriers are used to transmit the SYNC-CH.
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2.1 SYNC-CH preamble
The SYNC-CH preamble is used for preamble detection, time offset estimation, frequency offset estimation, and
channel estimation. For joint estimation of time and frequency, the SYNC-CH preamble has repeated patterns.
Suppose that NFFT is the FFT size. While the conventional preamble in [3] has a basic pattern with NFFT samples
in the time domain, the proposed preamble has a basic pattern with NFFT/2 samples in the time domain. The first
OFDM symbol has the CP (cyclic prefix) defined by the time domain preamble pattern. In the proposed
preamble, two kinds of preamble patterns are used in the time domain. In the proposed preamble sequence 0, the
basic preamble pattern is repeated by (6+) times, where  is given by
  2 NCP / N FFT ,
(1)
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where NCP is the CP length. In the proposed preamble sequence 1, the basic preamble pattern and its sign
reversed version are repeated by (3+) times. For both cases, the proposed preamble occupies three OFDM
symbols.
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In the frequency domain, the preamble sequences are defined by the pseudonoise binary codes. To generate the
pseudonoise binary codes, we use the PRBS whose generator polynomial is 1+X1+X4+X7+X15. This PRBS is the
same as that for UL (uplink) ranging code generation of 802.16e and 802.16m, described by Figure 257 of [4].
The PRBS generator is initialized by the seed b14 … b0 = 1,1,0,1,0,1,0,0,0,0,0,0,0,0,0, where b0 is the LSB of
the PRBS seed.
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The binary preamble sequences are defined by the subsequences of the pseudonoise sequence Ck generated by
the PRBS output . The length of each preamble sequence is 72 bits and the number of preamble sequences is
two. Suppose that the first bit of the PRBS output is C0. Then, the preamble sequences are defined as follows.
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0,
k  0, 2, , 70

Sk0  
1  2  Ck , k  1,3, , 71
(2)
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1  2  Ck , k  0, 2, , 70
Sk1  
0,
k  1,3, , 71

(3)
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where S kj is the k-th bit of the j-th preamble sequence. S kj is mapped to the k-th subcarrier among 72
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subcarriers assigned for the TDC link. From the properties of FFT (fast Fourier transform), S k0 is denoted as
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two identical patterns in the time domain, while S k1 is composed of a basic pattern and its sign reversed pattern
in the time domain. The transmit HR-MS selects one of the preamble sequences to generate the SYNC-CH
preamble. The receive HR-MS tries to detect both preamble sequences considering the preamble patterns.
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The proposed preamble can detect wider range of frequency offset in the time domain than the conventional
preamble, because its basic pattern is shorter than that of the conventional preamble. Also, the proposed
preamble is more robust to ICI (inter-carrier interference) by frequency offset, because it only uses 36
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subcarriers at a time.
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2.2 SYNC-CH IE
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SYNC-CH IE is composed of the following fields.
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Table 1. Synchronization channel IE
Field name
Transmitter HR-MS ID
Reference time
Hop count
Reference signal strength
Frame structure information
CRC
Field size
TBD
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2
TBD
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P1
P2
18 contiguous sub-carriers
P1
P2
P1
P2
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3 OFDM
symbols
Figure 2. Pilot structure for OFDM symbols transmitting SYNC-CH IE
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Figure 2 shows the pilot structure for resources utilized for SYNC-CH IE transmission. The resource elements
for SYNC-CH IE is composed of four basic resource blocks, where one basic resource block is a (3 OFDM
symbols)(18 contiguous subcarriers) rectangular region. To support SFBC, pilots for two antenna ports are
assigned.
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SYNC-CH
IE
CRC
addition
Channel
encoding
MIMO
encoder/
precoder
QPSK
modulation
Map to
Sync-CH2
Figure 3. Physical processing block diagram for the SYNC-CH IE
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Figure 3 shows the physical processing block diagram for the SYNC-CH IE. The SYNC-CH IE is appended
with a 16-bit CRC, per the CRC16-CCITT specification in Rec. ITU-T X25. The number of bits including the
16-bit CRC is 64 bits. The resulting sequence of bits is encoded by the TBCC described in 16.3.10.2 with
parameter M=2Kbufsize and Kbursize =3L, where L is the number of information bits. Then the effective code rate is
1/6. The encoded bit sequence is modulated using QPSK. The modulated symbols are mapped to two
transmission streams using SFBC as described in 16.3.6.1.1. The two streams using SFBCare processed and
mapped to the transmit antenna as described in 16.3.6.1.2. Antenna specific symbols at the output of the MIMO
precoder are mapped to the resource elements in the last three OFDM symbols described in 17.3.2.6.3.1.1.
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3. Simulation results
We perform numerical simulations in TDC environments to compare the performance of the proposed
preamble with that of the conventional one in terms of time offset estimation and frequency offset estimation.
For fading channel generation, we developed a M2M (mobile-to-mobile) channel model by modifying the
802.16m EMD MIMO SCM (spatial channel model) described in [5] considering the M2M channel models
proposed in [6] and [7]. The M2M channel model used in the simulation exploits the time domain power profile
of the 802.16m EMD, and considers the mobility of the transmitter. Also, the spatial parameters for the
transmitter have similar statistical characteristics with those for the receiver. Among the channel generation
scenarios of the 802.16m EMD, we used the Bad Urban Macro NLOS channel.
ACI
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¼
Subcarriers for DM
¼
PS
PI
Subcarriers for infra-structure
Frequency
Figure 4. FDM based subcarrier assignment for infra-structure link and TDC link when the per-subcarrier
power of infra-structure link is PI and the per-subcarrier power of DM link is PS.
When the resources for TDC link are allocated in the FDM manner, the subcarriers for TDC link are located
just after those for infra-structure link as shown in Figure 4. Since there is no guard band, the frequency offset
causes the ACI (adjacent channel interference) between the TDC link and the infra-structure link. To generate
the signals for infra-structure link in the simulation, we define the SIR (signal-to-interference ratio) as below.
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PS
(4)
PI
where PS is the per-subcarrier average power of TDC link and PI is the per-subcarrier average power of infrastructure link. In a similar manner, the SNR (signal-to-noise ratio) is defined by
P
SNR  S
(5)
NS
where NS is the per-subcarrier noise power. To consider the worst case performance, we assumed the
synchronization scenario 2-D defined in [3]. Simulation parameters for signal generation and OFDMA
operations are summarized as follows.
SIR 
Table 2. Simulation parameters
Parameter
Carrier frequency
Bandwidth
FFT size
CP size
Sampling rate
Number of transmit antennas
Number of receive antennas
Velocity of transmitter
Velocity of receiver
Moving direction of transmitter
Moving direction of receiver
Timing offset
Frequency offset of transmitter
Frequency offset of
interference
Frequency offset of receiver
Preamble sequence of
conventional SYNC-CH
Preamble sequence of
proposed SYNC-CH
SIR
Value
2.3 GHz
10 MHz
1024
128
11.2 MHz
1
1
30 km/h
30 km/h
/6
-/4
256 samples
9 ppm
2 ppm
-10 ppm
S k0
S k0
-10 dB
Suppose that rk,n denotes the received frequency domain preamble of k-th subcarrier and n-th symbol, where
0k71 and 1n3. To detect the preamble sequence, rk,n is despread as
(6)
yk ,n  rk ,n Skm
The cyclic shift in the time domain is expressed as the phase rotation in the frequency domain. Using the cyclic
property of the SYNC-CH preamble, the timing offset can be estimated as
N
 34

ˆ   FFT arg  ( y2 k 3,1 y2*k 1,1  y2 k 3,2 y2*k 11,2  y2 k 3,3 y2*k 1,3 ) 
(7)
4
 k 0

where arg[x] means the phase of x. Then, the phase rotation by the timing offset can be corrected as
(8)
yˆk ,n  e j 2 kˆ/ NFFT yk ,n .
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In OFDM systems, the normalized frequency offset is defined by
  f  Ts N FFT   i   f
where f is the carrier frequency offset, Ts is the sampling duration, i is the nearest integer to , and f is the
fractional part of . In the TDC link, the normalized frequency offset can be greater than 0.5, thus its integer part
and fractional part are separately estimated. Since the integer part i is denoted as the shift of the preamble
sequence in the frequency domain, it is estimated by using the sequence detection technique considering the
shifted version of preamble sequences. The fractional part f is estimated as
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 35

ˆ
(10)
f 
arg  ( y2 k 1,2 y2*k 1,1  y2 k 1,3 y2*k 1,2 )  .
2
 k 0

To improve the estimation accuracy of time and frequency, three SYNC-CH preambles were accumulated in the
simulation.
Figures 5 and 6 compare the proposed preamble with the conventional one in terms of time and frequency
offset estimation accuracy. The time and frequency estimator using the proposed preamble presents smaller
MSE (mean suqare error) values than the estimator using the conventional preamble. Since the proposed
preamble uses only 36 subcarriers, the ICI by frequency offset is reduced. Thus, the time estimator using the
proposed preamble exhibits huge SNR gain. Also, the frequency offset estimation accuracy is slightly improved
by the benefit of the reduced time offset.
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Conventional
Proposed
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MSE
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-10
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(9)
-5
0
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SNR (dB)
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Figure 5. Comparison of time offset estimation performance
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Conventional
Proposed
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-5
MSE
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-10
-5
0
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SNR (dB)
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Figure 6. Comparison of frequency offset estimation performance
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4. References
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5. Proposed Text for the 802.16n Amendment Working Document (AWD)
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The text in BLACK color: the existing text in the 802.16n Amendment Draft Standard
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The text in RED color: the removal of existing 802.16n Amendment Draft Standard Text
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The text in BLUE color: the new text added to the 802.16n Amendment Draft Standard Text
[1] IEEE C802.16n-11/0051r2, “Dedicated resources allocation for direct communications in IEEE 802.16n,”
March 2011.
[2] IEEE C802.16n-11/0130r1, “Frame structure for talk-around direct communications,” July 2011.
[3] IEEE C802.16n-11/0131r1, “Synchronization channel structure for talk-around direct communications,”
July 2011.
[4] IEEE Std. 802.16-2009, “IEEE Standard for Local and metropolitan area networks; Part 16: Air Interface
for Broadband Wireless Access Systems,” May 2009.
[5] IEEE 802.16m-08/004r5, “IEEE 802.16m evaluation methodology document (EMD),” Jan. 2009.
[6] C. S. Patel, G. L. Stuber, and T. G. Pratt, “Simulation of Rayleigh-faded mobile-to-mobile communication
channels,” IEEE Trans. Commun., Nov. 2005.
[7] M. Patzold, B. O. Hogstad, and N. Youssef, “Modeling, analysis, and simulation of MIMO mobile-to mobile fading channels,” IEEE Trans. Wireless Commun., Feb. 2008.
Note:
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[-------------------------------------------------Start of Text Proposal---------------------------------------------------]
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[Remedy1: Adapt the following change in Section 17.3.2.6 in the 802.16n AWD]
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17.3.2.6 Talk-around Direct Communication
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[note: This contribution provides text proposals for the following sections:
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17.3.2.6.2.1 Frame structure
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17.3.2.6.2.2 Synchronization channel
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17.3.2.6.2.3 Dedicated channel
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17.3.2.6.2.4 Supplementary channel]
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17.3.2.6.2.23 Control structure Synchronization channel
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17.3.2.6.3.1 Synchronization channel
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The Synchronization channel is used for frequency and time synchronization among HR-MSs involved in direct
communications. The location of the synchronization is located at fixed position within dedicated resource
reserved by HR-BS.
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When an HR-MS transmits any channels for direct communication between HR-MSs, the transmitting HR-MS
shall pre-compensate the frequency offset according to the frequency difference between the HR-MS and HRBS. An HR-MS within the coverage of the HR-BS estimates frequency offset with the frequency of the serving
HR-BS. Some HR-MSs can transmit some reference signals to spread the reference frequency of the HR-BS.
The HR-MSs outside of HR-BS coverage can estimate frequency reference by using the propagated reference
signals. If no propagated reference signal can be received, the HR-MS outside of coverage pre-compensate
frequency offset according to the previously estimated offset value which was used when that HR-MS is inside
of HR-BS coverage.
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In addition to the frequency synchronization, the synchronization channel is used for acquiring time
synchronization. Synchronization channel shall be used to estimate the transmission timing of the direct
communication channels to prevent timing offset between the desired signals and interference signals at the
receiver.
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17.3.2.6.3.1.1 Synchronization channel structure
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17.3.2.6.2.2.1 Synchronization channel structure
Subframe
OFDM symbol
period
Sequence j
CP
Sequence j
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Sequence j
Sequence j
Sync-CH preamble
(3 OFDM symbols)
Sync-CH message
(3 OFDM symbols)
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Subframe
OFDM symbol
period
CP
Sequence 0
A
A
A
A
A
A
A
SYNC-CH preamble
(3 OFDM symbols)
Sequence 1
1
2
B
-B
B
-B
SYNC-CH message
(3 OFDM symbols)
B
-B
B
SYNC-CH preamble
(3 OFDM symbols)
SYNC-CH message
(3 OFDM symbols)
Figure 919. Synchronization channel for talk-around direct communication
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Figure 919 describes the synchronization channel structure for direct communication in the time domain. One
synchronization channel occupies one subframe composed of six OFDM symbols. The first three OFDM
symbols are used for SYNC-CH preamble transmission and the last three OFDM symbols include the SYNCCH IE. In the frequency domain, 72 contiguous subcarriers are assigned to transmit the synchronization channel
for direct communication. The SYNC-CH preamble is used for preamble detection, timing offset estimation,
frequency offset estimation, and channel estimation. A preamble sequence with 72 binary codes is mapped to
the 72 subcarriers and the same preamble sequence is repeated during each symbol. The time domain preamble
sequence is obtained by taking IFFT of the sequence mapped to 72 subcarriers. In the frequency domain, a
preamble sequence with 36 binary codes is mapped to 36 subcarriers and remaining 36 subcarriers are not used.
The time domain preamble sequence is obtained by taking IFFT of the frequency domain preamble sequence. In
the time domain, sequence 0 is denoted as repetition of a basic pattern with NFFT/2 samples, where NFFT is the
FFT size, and sequence 1 is composed of a basic pattern with NFFT/2 samples and the sign reversed version of
the basic pattern. The first SYNC-CH symbol is defined by the CP and the time domain preamble sequence.
Second and third SYNC-CH symbols are defined by the repetition of the time domain preamble sequence
without the CP. To limit the preamble length to three OFDM symbols, the time domain preamble sequence is
repeated by (2+) times, where  is given by
  2 NCP / N FFT
where NCP is the CP length. and NFFT is the FFT size.
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17.3.2.6.3.1.2 Preamble sequences for synchronization channel
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17.3.2.6.2.2.2 Preamble sequences for synchronization channel
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The preamble sequences are defined by the pseudonoise binary codes produced by the PRBS used for ranging
code generation. The generator polynomial of the PRBS is 1+X1+X4+X7+X15. The PRBS generator is initialized
by the seed b14 … b0 = 1,1,0,1,0,1,0,0,0,0,0,0,0,0,0, where b0 is the LSB of the PRBS seed. The preamble
sequences are subsequences of the pseudonoise binary sequence Ck generated by the PRBS. The length of each
preamble sequence is 72 bits and the number of preamble sequences is 4. The number of preamble sequences is
two. Each sequence is composed of 36 binary codes and 36 zeros. Suppose that the first bit of the PRBS output
is C0. Then, the preamble sequences are defined as follows.
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S k0  1  2  Ck , 0  k  71
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Sk1  1  2  Ck , 72  k  143
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0,
144  k  146

Sk2  
1  2  Ck , 147  k  215
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0,
216  k  218

Sk3  
1  2  Ck , 219  k  287
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0,
k  0, 2, , 70

Sk0  
1  2  Ck , k  1,3, , 71
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1  2  Ck , k  0, 2, , 70
Sk1  
0,
k  1,3, , 71

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where S kj is the k-th bit of the j-th preamble sequence. When the HR-MS transmitting the synchronization
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channel is within the coverage of the serving HR-BS, S k0 or S k1 shall be used. When the HR-MS transmitting
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the synchronization channel is outside of the HR-BS coverage, S k2 or S k3 shall be used. The transmit HR-MS
selects one of the preamble sequences to generate the SYNC-CH preamble. The receive HR-MS shall be able to
detect all the preamble sequences considering the preamble patterns.
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17.3.2.6.3.1.3 Synchronization channel message
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17.3.2.6.2.2.3 Synchronization channel IE
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Synchronization channel IE is transmitted after channel encoding. The pilot pattern and channel coding method
for the resources for synchronization channel message is FFS. The synchronization channel message IE is
composed of the fields in Table 1247.
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Table 1247. Synchronization channel message IE
Field name
Transmitter HR-MS ID
Reference time
Hop count
Reference signal strength
Frame structure information
CRC
Field size
TBD
2
4 2
TBD
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[note: the feature of the synchronization follows:
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Distributed transmission (The MS decide to transmit the synchronization packet it self)
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Frame Timing and frequency reference by BS is propagated to the MSs out of the service coverage
(using synchronization hop counter as an example)
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17.3.2.6.2.2.3.1 Pilot structure for OFDM symbols transmitting SYNC-CH IE
P1
P2
18 contiguous sub-carriers
P1
P2
P1
P2
3 OFDM
symbols
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Figure xxx. Pilot structure for OFDM symbols transmitting SYNC-CH IE
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Figure xxx shows the pilot structure for resources utilized for SYNC-CH IE transmission. To support SFBC,
pilots for two antenna ports are assigned.
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17.3.2.6.2.2.3.2 Resource mapping of SYNC-CH IE
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Sync-CH
IE
CRC
addition
Channel
encoding
QPSK
modulation
MIMO
encoder/
precoder
Map to
Sync-CH2
Figure yyy. Physical processing block diagram for the SYNC-CH IE
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Figure yyy shows the physical processing block diagram for the SYNC-CH IE. The Sync-CH IE shall be
appended with a 16-bit CRC, per the CRC16-CCITT specification in Rec. ITU-T X25. The number of bits
including the 16-bit CRC is 64 bits. The resulting sequence of bits shall be encoded by the TBCC dscribed in
16.3.10.2 with parameter M=2Kbufsize and Kbursize =3L, where L is the number of information bits. Then the
effective code rate is 1/6. The encoded bit sequence shall be modulated using QPSK. The modulated symbols
shall be mapped to two transmission streams using SFBC as described in 16.3.6.1.1. The two streams using
SFBC shall be processed and mapped to the transmit antenna as described in 16.3.6.1.2. Antenna specific
symbols at the output of the MIMO precoder shall be mapped to the resource elements in the last three OFDM
symbols described in 17.3.2.6.2.1.
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IEEE C802.16n-11/0153
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