Transcript Slides S802.16n-11/0153

Changes on Synchronization Channel for Talk-around Direct Communications
Document Number:
IEEE S802.16n-11/0153
Date Submitted:
2011-09-20
Source:
Jihoon Choi, Young-Ho Jung
Korea Aerospace University
E-mail: [email protected],
[email protected]
Sungcheol Chang, Seokki Kim, Eunkyung Kim, Miyoung Yun, Won-Ik Kim,
Sungkyung Kim, Hyun Lee, Chulsik Yoon, Kwangjae Lim
ETRI
E-mail: [email protected]
Re:
Call for comments on the 802.16n AWD
Base Contribution:
IEEE C802.16n-11/0153
Purpose:
To be discussed and adopted by 802.16 TGn
Notice:
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
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Introduction
• Resource allocation for TDC (talk-around direct communication)
– Some resources for infra-structure communication are reserved for TDC in the FDM
(frequency division multiplexing) manner.
– The resources assigned for TDC are composed of Sync-CH (synchronization
channel), Ded-CH (dedicated channel), and Sup-CH (supplementary channel).
• Synchronization issues
– Timing offset between TDC and infra-structure link causes ISI (inter-symbol
interference) .
– Frequency offset between TDC and infra-structure link causes ICI (inter-carrier
interference) and ACI (adjacent channel interference).
• Synchronization schemes
– An HR-MS within the HR-BS coverage compensates for its timing and frequency
offsets using the HR-BS reference signals.
– The Sync-CH is used for time and frequency synchronization between direct mode
HR-MSs.
2
Synchronization Requirements
• Timing offset requirements
– The effective CP (cyclic prefix) size is reduced by timing offset.
– In this document, the maximum timing offset is limited to (1/4)(CP size) when
the CP size is 1/8 of the OFDM symbol duration.
(Note: the maximum time delay of 16m EMD channel is about (1/2)(CP size).)
• Frequency accuracy requirements for IEEE 802.16m (or 802.16e)
– BS frequency accuracy: 2 ppm
– MS frequency accuracy within the BS coverage
 0.02 of subcarrier spacing (after frequency offset corrections using RNG-RSP
messages)
– MS frequency accuracy outside the BS coverage
 0.02 of subcarrier spacing (using Sync-CH, Ded-CH preamble, and ranging channel)
3
Synchronization Channel Structure
Subframe
CP
Conventional
SYNC-CH
A
A
A
A
SYNC-CH preamble
Proposed
SYNC-CH
(Seq. 0)
B
B
B
B
SYNC-CH IE
B
B
B
SYNC-CH preamble
Proposed
SYNC-CH
(Seq. 1)
C
-C
C
-C
SYNC-CH preamble
SYNC-CH IE
C
-C
C
SYNC-CH IE
• Synchronization channel
– Includes 6 OFDM symbols.
– The conventional Sync-CH in “IEEE C802.16n-11/0131r1” uses 72 contiguous
subcarriers in the frequency domain.
– The proposed Sync-CH uses 36 subcarriers in the frequency domain (Seq.0
uses only odd subcarriers and Seq.1 uses only even subcarriers).
– In the time domain, first three symbols are used for the Sync-CH preamble and
last three symbols are used for the Sync-CH IE.
4
Sync-CH Preamble
• Sync-CH preamble
– A preamble sequence with 36-bit length is mapped to the 36 subcarriers, and the
same sequence is repeated for 3 OFDM symbols.
– The proposed preamble has a basic pattern with NFFT/2 samples, while the
conventional preamble has a basic pattern with NFFT samples.
– First symbol for the Sync-CH preamble is composed of the CP and the time
domain preamble sequence.
 The time domain preamble for Seq.0 is defined by the repetition of the basic pattern.
 The time domain preamble for Seq.1 is defined by the basic pattern and its sign
reversed version.
– Second and third symbols for the preamble are defined by the repetition of the
time domain preamble sequence without CP.
• Merits of the proposed preamble
– Possible to detect wider range of frequency offset in the time domain.
– More robust to ICI by frequency offset when estimating the time offset in the
frequency domain.
5
Sync-CH Preamble Sequence
• PRBS (pseudo-random binary sequence) generation
–
–
–
–
Generator polynomial = 1+X1+X4+X7+X15
Identical with the PRBS generator for UL ranging codes of 802.16m
Initial seed: b14 … b0 = 1,1,0,1,0,1,0,0,0,0,0,0,0,0,0 (b0 is the LSB)
Note
 The PRBS for the Sync-CH preamble can be simply implemented by changing the initial
seed of the existing PRBS generator for ranging codes.
6
Sync-CH Preamble Sequence (cont.)
• Proposed Sync-CH preamble sequences
– 2 preamble sequences are defined as follows.
0,
k  0, 2, , 70

Sk0  
1  2  Ck , k  1,3, , 71
1  2  Ck , k  0, 2, , 70
Sk1  
0,
k  1,3, , 71

j
where Ck is the k-th bit of the PRBS output and S k is the k-th bit of the j-th
preamble sequence.
– 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 both preamble sequences.
7
Sync-CH IE
• Sync-CH IE
Field size (bits)
TBD
Reference time
2
Hop count
2
Reference signal strength
TBD
Frame structure information
CRC
4
16
• Pilot structure (see the right figure)
– Resource elements for Sync-CH IE are composed of
4 basic resource blocks.
– A resource block is (3 OFDM symbols)(18 subcarriers)
region.
– Pilots for two antenna ports are assigned for SFBC.
P1
P2
18 contiguous sub-carriers
Field name
Transmitter HR-MS ID
P1
P2
P1
P2
3 OFDM
symbols
8
Sync-CH IE (cont.)
• Physical processing block
SYNC-CH
IE
CRC
addition
Channel
encoding
QPSK
modulation
MIMO
encoder/
precoder
Map to
Sync-CH2
– The number of information bits including 16-bit CRC is 64 bits.
– Channel encoded by TBCC with parameter M=2Kbufsize
and Kbursize =3L, where L is the number of information bits.
– Effective code rate = 1/6
– Encoded bits are modulated using QPSK.
– For MIMO transmission, SFBC is used.
9
Performance Evaluation
Using the Proposed Sync-CH Preamble
10
Timing Offset Estimation
• Despreading by the m-th preamble sequence
yk ,n  rk ,n Skm
where rk,n is the frequency domain preamble of k-th subcarrier and n-th symbol.
• Property of FFT/IFFT
– Cyclic shift in the time domain  phase rotation in the frequency domain
• Estimation of timing offset
N FFT
 34

ˆ  
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 ) 
4
 k 0

where NFFT is the FFT size and arg[x] is the phase of x.
• Correction of the phase rotation by timing offset
yˆk ,n  e j 2 kˆ/ NFFT yk ,n
11
Frequency Offset Estimation
• Normalized frequency offset
  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 .
• Estimation of the integer part of frequency offset
– Integer frequency offset i in the time domain
 i - subcarrier shift of the preamble sequence in the frequency domain
– Estimated by detecting the shifted versions of preamble sequences in the
frequency domain.
• Estimation of the fractional part of frequency offset
1
 35

ˆ f 
arg  ( y2 k 1,2 y2*k 1,1  y2 k 1,3 y2*k 1,2 ) 
2
 k 0

12
Subcarrier Assignment for Simulations
ACI
¼
Subcarriers for TDC
¼
PS
PI
Subcarriers for infra-structure
Frequency
– SIR (signal to interference ratio): SIR 
– SNR (signal to noise ratio):
SNR 
P
average subcarrier power of TDC
 S
average subcarrier power of infra-structure
PI
average subcarrier power of TDC PS

noise power per subcarrier
NS
13
S k0
Simulation Environments
•
Fading channel
–
–
•
Bad Urban Macro NLOS of 16m EMD (modified for TDC)
Tx velocity = 30 km/h, Rx velocity 30 km/h
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
14
Timing Offset Estimation
• Timing accuracy requirement: MSE < 100
• Simulation results (SIR=-10dB)
10
4
Conventional
Proposed
3
10
2
MSE
10
1
10
-10
-5
0
5
SNR (dB)
10
15
20
15
Frequency Offset Estimation
• Frequency accuracy requirement: MSE < 4.410-5
• Simulation results (SIR=-10dB)
10
-2
Conventional
Proposed
-3
10
-4
10
-5
MSE
10
-10
-5
0
5
SNR (dB)
10
15
20
16
Conclusion
• Timing offset estimation
– When SIR=-10dB and MSE=100, the proposed preamble has about 4 dB SNR
gain, compared to the conventional preamble.
• Frequency offset estimation
– When SIR=-10dB and MSE=4.410-5 , the proposed preamble has about 0.5 dB
SNR gain, compared to the conventional preamble.
• Note
– The performance can be improved by
 Using more receiver antennas
 Accumulating more preambles
 Employing more elegant estimation algorithms such as closed-loop time and frequency
feedback.
17