IEEE C802.16n-11/0131r1
Project
IEEE 802.16 Broadband Wireless Access Working Group <http://ieee802.org/16>
Title
Synchronization Channel Structure for Talk-around Direct Communications
Date
Submitted
2011-07-19
Source(s)
Jihoon Choi, Young-Ho Jung
E-mail:
jihoon@kau.ac.kr, yhjung@kau.ac.kr
Korea Aerospace University
Sungcheol Chang, Eunkyung Kim,
Sungkyung Kim, Won-Ik Kim, Miyoung
Yun, 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 synchronization channel structure 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
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contained herein.
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<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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IEEE C802.16n-11/0131r1
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Synchronization Channel Structure for Talk-around Direct Communications
Jihoon Choi, Young-Ho Jung
Korea Aerospace University
Sungcheol Chang, Eunkyung Kim, Sungkyung Kim, Won-Ik Kim, Miyoung Yun, Hyun Lee, Chulsik
Yoon, Kwangjae Lim
ETRI
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1. Introductions
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In this contribution, we define the Sync-CH (synchronization channel) structure for direct communication.
The Sync-CH is composed of Sync-CH preamble and Sync-CH message. The Sync-CH preamble is used to
transfer the time and frequency information of a HR-MS inside a HR-BS coverage to a HR-MS outside the HRBS coverage. Also, the Sync-CH preamble is transmitted to initialize a DM communication link between HRMSs outside the HR-BS coverage. The Sync-CH message includes the information related to the DM link
structure and the DM frame structure. To assess the proposed Sync-CH preamble structure, simulations results
are presented in terms of the preamble detection performance, the timing estimation error, and the frequency
estimation error.
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2. Scenarios for initial synchronization
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]. As discussed in [3], timing offset exceeding the CP (cyclic
prefix) size and carrier frequency offset of a HR-MS cause ICI (inter-carrier interference), ISI (inter-symbol
interference), and ACI (adjacent channel interference) between the DM (direct mode) communication link and
the infra-structure communication link.
If the resources for DM link and infra-structure communication are divided in FDM (frequency division
multiplexing) manner, there is some possibility of mutual interference between DM links and infra-structure
links due to different arrival time and carrier frequency of desired signal and interference signal. In the aspect of
initial synchronization using the Sync-CH preamble, the following four operation scenarios are considered.
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A. Synchronized HR-MS to synchronized HR-MS in the same HR-BS coverage
HR-MS1
R1
1
HR-BS
R1
HR-MS2
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Figure 2-1. Initial synchronization when a synchronized HR-MS transmits the preamble to other HR-MS
in the same HR-BS coverage
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In this case, both HR-MS1 and HR-MS2 are synchronized to HR-BS using the R1 infra-structure
communication link. According to the frequency accuracy requirements defined in [3], the MS
frequency offset is within 0.02 of subcarrier spacing. Therefore, the frequency offset between HR-MS1
and HR-MS2 is negligible. Suppose that HR-MS1 is near HR-BS and HR-MS2 is located in the cell
edge of HR-BS. Then, there exists some timing offset between the R1 link and the DM link due to the
round-trip delay difference. Thus, the timing offset needs to be estimated by using the Sync-CH
preamble.
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B. Synchronized HR-MS to synchronized HR-MS in a different HR-BS coverage
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HR-BS1
R1
HR-MS1
RMM
HR-MS2
R1
HR-BS2
Figure 2-2. Initial synchronization when a synchronized HR-MS transmits the preamble to other HR-MS
in a different HR-BS coverage
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In this scenario, HR-MS1 is synchronized to HR-BS1, while HR-MS2 is synchronized to HR-BS2.
According to the frequency accuracy requirements defined in [3], the BS frequency offset is within 2
ppm of the carrier frequency. In other words, the frequency offset between HR-MS1 and HR-MS2 is up
to 4 ppm. Also, there exists some timing offset between HR-MS1 and HR-MS2, because the round-trip
delay of HR-MS1 is different from that of HR-MS2, and HR-MS1 and HR-MS2 are synchronized to
different HR-BSs. Thus, both the timing offset and the frequency offset need to be estimated by using
the Sync-CH preamble.
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C. Synchronized HR-MS to non-synchronized HR-MS
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HR-BS
R1
HR-MS1
RMM
HR-MS2
Figure 2-3. Initial synchronization when a synchronized HR-MS transmits the preamble to a nonsynchronized HR-MS
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In this case, HR-MS1 is synchronized to HR-BS and HR-MS2 is outside the HR-BS coverage. HRMS1 transmits the Sync-CH preamble to transfer the timing and frequency information of HR-BS to
HR-MS2 to HR-MS2. Since HR-MS2 is not synchronized to any HR-BS, HR-MS2 estimates the timing
and frequency of HR-MS1 through the Sync-CH preamble. When a HR-MS is free running, the carrier
frequency offset is generally up to 10 ppm. Since the frequency offset of HR-BS is up to 2 ppm, the
frequency offset between HR-MS1 and HR-MS2 reaches up to 12 ppm. In addition, the timing offset is
up to the half of the OFDM symbol duration of 802.16n.
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D. Non-synchronized HR-MS to non-synchronized HR-MS
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HR-MS1
RMM
HR-MS2
Figure 2-4. Initial synchronization when a non-synchronized HR-MS transmits the preamble to a nonsynchronized HR-MS
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In this scenario, both HR-MS1 and HR-MS2 are outside the HR-BS coverage. In other words, neither
HR-MS1 nor HR-MS2 is connected to the infra-structure network. The DM communication link can be
used in this case to share information between HR-MS pairs or enable an emergency call. As mentioned
in Section II-C, the frequency offset of a free running HR-MS is up to 10 ppm. Thus, the frequency
offset between HR-MS1 and HR-MS2 reaches up to 20 ppm. In addition, the timing offset is up to the
half of the OFDM symbol duration of 802.16n, because there is no reference timing information. In this
case, HR-MS2 estimates the timing and frequency information of HR-MS1 using the Sync-CH preamble,
and it is synchronized with HR-MS1 by adjusting its time and frequency.
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3. Synchronization channel structure
Subframe
OFDM
symbol
CP
Data channel
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Synchronization channel
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Sync-CH preamble
Sync-CH message
Figure 3-1. Synchronization channel structure including Sync-CH preamble and Sync-CH message
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Figure 3-1 describes the synchronization channel structure for DM communications in the time domain. One
Sync-CH occupies one 802.16m 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 Sync-CH message.
When the resources for DM communication are assigned in the FDM manner, four PRUs including 72
contiguous subcarriers are used to transmit the synchronization channel. The Sync-CH preamble is used for
preamble detection, timing offset estimation, frequency offset estimation, and channel estimation. For joint
estimation of time and frequency, the Sync-CH preamble has repeated patterns. Specifically, a preamble
sequence with length 72 is defined in the frequency domain and the time domain preamble sequence is obtained
by IFFT. The first symbol of the preamble is defined by the CP and the time domain preamble sequence. Second
and third symbols are defined by the repetition of the time domain preamble sequence without the CP. To make
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the preamble occupy three OFDM symbols, the time domain preamble sequence is repeated by (2+) times,
where  is given by
  2 NCP / N FFT ,
(1)
where NCP is the CP length and NFFT is the FFT size.
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3.1 Preamble sequences
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The preamble sequences are defined by the pseudonoise binary codes in the frequency domain. 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 ranging code generation of 802.16e and 802.16m, described by Figure 257 of [3]. 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 4.
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
(2)
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Sk1  1  2  Ck , 72  k  143
(3)
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144  k  146

Sk2  
1  2  Ck , 147  k  215
(4)
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0,
216  k  218

Sk3  
1  2  Ck , 219  k  287
(5)
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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 DM link. S k0 and S k1 are used for the scenarios 2-A, 2-B, and 2-C, while S k2 and
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S k3 are used for the scenario 2-D. Note that the first three bits of S k2 and S k3 are not used to avoid the ACI
between the DM link and the infra-structure link.
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3.2 Synchronization channel message
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Synchronization channel message 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 is
composed of the following fields:
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Field name
Transmitter HR-MS ID
Hop count
Frame structure information
CRC
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Field size
TBD
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Table 3.1 Synchronization channel message
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4. Performance evaluation using the proposed Sync-CH preamble
To assess the performance of the proposed Sync-CH preamble in terms of preamble detection probability,
timing offset estimation, and frequency offset estimation, we perform numerical simulations in DM
communication environments. 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 [4] considering the
M2M channel models proposed in [5] and [6]. 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 Urban Macro NLOS channel.
ACI
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¼
Subcarriers for DM
¼
PS
PI
Subcarriers for infra-structure
Frequency
Figure 4-1. FDM based subcarrier assignment for infra-structure link and DM 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 DM link are allocated in the FDM manner, the subcarriers for DM link are located
just after those for infra-structure link as shown in Figure 4-1. Since there is no guard band, the frequency offset
causes the ACI between the DM 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.
P
SIR  S
(6)
PI
where PS is the per-subcarrier average power of DM 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
(7)
NS
where NS is the per-subcarrier noise power. To consider the worst case performance, we assumed the
synchronization scenario 2-D. Since the infra-structure MSs are synchronized with the BS, the maximum
frequency offset of interference signals was set to 2 ppm. Simulation parameters for signal generation and
OFDMA operations are summarized as follows.
Table 4-1. Simulation parameters
Parameter
Carrier frequency
Bandwidth
FFT size
Value
2.3 GHz
10 MHz
1024
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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
Sync-CH preamble sequence
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11.2 MHz
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30 km/h
30 km/h
/6
-/4
256 samples
9 ppm
2 ppm
-10 ppm
S k2
-10 dB
SIR
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Subframe
Received
interference signal
Sync-CH preamble
Received
Sync-CH
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Sampled preamble
at receiver
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Sync-CH message
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1st symbol
2nd symbol
3rd symbol
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timing offset
Figure 4-2. Sync-CH preamble sampling and OFDM symbol mapping when the receiver has a timing offset.
Figure 4-2 shows the received Sync-CH preamble and the interference signal. Due to the timing offset, the
receiver starts sampling the preamble with some time delay and computes three symbols corresponding to the
Sync-CH preamble in the frequency domain. 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
(8)
yk ,n  rk ,n Skm
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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
 71

ˆ   FFT arg  ( yk ,1 yk*1,1  yk ,2 yk*1,2  yk ,3 yk*1,3 ) 
(9)
2
 k 1

where arg[x] means the phase of x. Then, the phase rotation by the timing offset can be corrected as
(10)
yˆk ,n  e j 2 kˆ/ NFFT yk ,n .
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To determine if the preamble sequence S km was transmitted, the SINR (signal to interference plus noise ratio)
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corresponding to S km is computed as
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PT  PN
PN
where PT and PN are defined by
SINRm 
(11)
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PT   (| yk ,1 |2  | yk ,2 |2  | yk ,3 |2 )
(12)
k 0
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(13)
 (| yk ,1  yk 1,1 |2  | yk ,2  yk 1,2 |2  | yk ,3  yk 1,3 |2 ) .
2 k 1
Given a threshold , we determine that S km was transmitted if SINRm > . In general,  is designed by taking
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into account the false alarm probability. Note that only 69 subcarriers are used when S k2 or S k3 is transmitted.
In OFDM systems, the normalized frequency offset is defined by
  f  Ts N FFT   i   f
(14)
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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 DM 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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 71

(15)
ˆ f 
arg  ( yk ,2 yk*,1  yk ,3 yk*,2 )  .
2
 k 0

To improve the sequence detection probability and the estimation accuracy of time and frequency, multiple
preambles can be used. In the simulation, we compared the performances when the number of preambles is 1, 2,
and 3.
Figures 4-3 and 4-4 show the sequence detection probability. When SNR=-2 dB and three preambles were
used, the detection probability is 0.98 and 0.80 when the false alarm probability is 0.01 and 0.001, respectively.
Figures 4-5 and 4-6 present the maximum timing estimation error and the maximum frequency estimation error,
respectively. The maximum estimation errors were computed as
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PN 
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 max  E[ˆ   ]  3 E[(ˆ   )2 ]
(16)
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 max  E[ˆ   ]  3 E[(ˆ   )2 ] .
(17)
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Considering multiple hops and channel delay spread, we define the maxum allowed timing offset as 32 samples.
When SNR > -3 dB and three preambles were used, the maximum timing estimation error is less than 32
samples. The frequency accuracy requirement for MS is defined by 0.02 of the subcarrier spacing [3]. This
requirement is satisfied when SNR > 2 dB and three preambles were used.
As mentioned in simulation environments, we considered the worst case. Thus, the performances shown in this
section can be improved by using more receiver antennas, accumulating more preambles, and employing more
elegant estimation algorithms.
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Detection Probability
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0
10
-1
10
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2
3
-2
-8
-6
-4
-2
0
2
SNR (dB)
Figure 4-3. Preamble sequence detection probability when the false alarm probability is 0.01.
Detection Probability
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Number of preambles = 3
Number of preambles = 2
Number of preambles = 1
0
-1
Number of preambles = 3
Number of preambles = 2
Number of preambles = 1
-2
-8
-6
-4
-2
0
2
SNR (dB)
Figure 4-4. Preamble sequence detection probability when the false alarm probability is 0.001.
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Max Timing Error
Number of preambles = 1
Number of preambles = 2
Number of preambles = 3
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1
0
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-10
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-8
-6
-4
-2
SNR (dB)
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Figure 4-5. Maximum timing estimation error
Max Normalized Frequency Error
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1
Number of preambles = 1
Number of preambles = 2
Number of preambles = 3
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0
10
-1
10
-2
-10
-8
-6
-4
-2
SNR (dB)
0
2
4
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Figure 4-6. Maximum normalized frequency estimation error
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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-11/0130, “Frame structure for talk-around direct communications,” July 2011.
[3] IEEE Std. 802.16-2009, “IEEE Standard for Local and metropolitan area networks; Part 16: Air Interface
for Broadband Wireless Access Systems,” May 2009.
[4] IEEE 802.16m-08/004r5, “IEEE 802.16m evaluation methodology document (EMD),” Jan. 2009.
[5] C. S. Patel, G. L. Stuber, and T. G. Pratt, “Simulation of Rayleigh-faded mobile-to-mobile communication
channels,” IEEE Trans. Commun., Nov. 2005.
[6] 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 Physical layer
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17.3.2.6.2.1 Frame structure
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17.3.2.6.2.2 Physical structure
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17.3.2.6.2.3 Control structure]
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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
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)
Figure xx1. Synchronization channel for direct communication
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Figure xx1 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 Sync-CH
message. 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
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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. 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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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. 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
18
Sk1  1  2  Ck , 72  k  143
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0,
144  k  146

Sk2  
1  2  Ck , 147  k  215
20
0,
216  k  218

Sk3  
1  2  Ck , 219  k  287
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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.
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17.3.2.6.3.1.3 Synchronization channel message
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Synchronization channel message 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 is
composed of the fields in Table xxx.
Field name
Transmitter HR-MS ID
Hop count
Frame structure information
CRC
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Field size
TBD
4
4
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Table xxx. Synchronization channel message
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IEEE C802.16n-11/0131r1
1
[note: the feature of the synchronization follows:
2
-
Distributed transmission (The MS decide to transmit the synchronization packet it self)
3
4
-
Frame Timing and frequency reference by BS is propagated to the MSs out of the service coverage
(using synchronization hop counter as an example)
5
6
[-------------------------------------------------End of Text Proposal----------------------------------------------------]
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