Transcript IEEE C802.16m-08/1163

Hierarchy on IEEE 802.16m Synchronization Channel
IEEE 802.16 Presentation Submission Template (Rev. 9)
Document Number:
IEEE C802.16m-08/1163
Date Submitted:
2008-09-05
Source:
Sungho Moon, Seunghee Han, Jin Sam Kwak, Young-Hyoun Kwon
e-mail : {msungho; dondai; samji; wishwill}@lge.com
Voice: +82-31-450-1935
LG Electronics
LG R&D Complex, 533 Hogye-1dong, Dongan-gu, Anyang, 431-749, Korea
Venue:
IEEE 802.16m-08/033, Call for Detailed Physical Layer Comments
Purpose:
This contribution proposes SDD text for SCH based on ToC in IEEE 802.16m-08/003r4.
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
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1
Hierarchy on IEEE 802.16m Synchronization
Channel
2
Contents




Synchronization Channel (SCH) Functionalities
General Terms and Relationships
Non-Hierarchical vs. Hierarchical
Simulation Results
 Residual timing/frequency offset sensitivities
 Symbol timing performance
 Hierarchy vs. Non-hierarchy
 Summary
3
SCH Functionality
 Synchronization
 Timing synchronization
 Symbol, frame, and super-frame
 Carrier frequency and frequency offset
 Cell Identification and Additional Information
 Cell or sector ID distinction
 Essential information for BCH decoding should be detected during cell
search.
 Channel Estimation
 Allow estimation of channels for multiple transmit antennas
 Measurement Functions
 RSSI measurement
 Noise power estimation
4
General Terms and Relationships
 Symbol Timing Detection Algorithms and Signal Forms
Signal Forms
Repeated Signal
Non-Repeated Signal
Auto-correlation based algorithm
Well-Matched
N/A
Cross-correlation based algorithm
Not recommended due to
ambiguous peaks & complexity
Well-Matched
Detection algorithm
 Hierarchy and Signal Forms
Signal Forms
Repeated Signal
Non-Repeated Signal
Hierarchical
(two symbols)
Matched
Well-Matched
Non-Hierarchical
(one symbol)
Well-Matched
Not recommended due to
complexity
Hierarchy
 Possible Structures



NH structure: (Non-Hierarchical, Repeated signal form, Auto-correlation based algorithm)
H structure 1 : (Hierarchical, Non-repeated signal form, Cross-correlation based algorithm)
H structure 2 : (Hierarchical, Repeated signal form for P-SCH, Auto-correlation based algorithm)
 Hybrid Type

Hierarchical with one symbol
5
Non-Hierarchical Structure
 Definition
 Only a single type of 16m synchronization symbol which may be in addition to the
legacy 16e preamble
 No needs of 16e preamble as part of the SCH functionality
 Main Features




One OFDM symbol through multiple antennas (CDD, FSTD, or TSTD)
Every other subcarrier : Null  2x repeated signal in time
Auto-correlation based detection algorithm
Sector/cell-common allocation (due to maintain the 2x repeated signal in the celledge)
Super-Frame, 20 ms
 For Example,
5 ms
IFFT / FFT
Time
1 OFDM Symbol
CP
Tx. Ant 0
a
Frequency
5 MHz
-a
CP
...
A
-A
...
f
...
...
Circular
Shift
-5 -3 -1 1 3 5
Sector/Cell-Common
Allocation
From Other Sectors
CP
Tx. Ant Ntx-1
a'
...
-a'
...
-5 -3 -1 1 3 5
5 MHz
6
f
Hierarchical Structures
 Definition
 More than one type of SCH symbols exist within a super-frame.
 This may or may not use the legacy 16e preamble as one level of hierarchy.
 Main Features
 The P-SCH can be used for initial acquisition.
 Hierarchical structure 1 : Cross-correlation based detection algorithm
 Hierarchical structure 2 : Auto-correlation based detection algorithm
 The S-SCH can be used for fine synchronization, cell/sector identification
(ID), and channel measurements.
 The P-SCH can be used as a phase reference for S-SCH detection.
7
Problem of Hierarchical Structure 1
 Overhead
 Additional resource (secondary sync. channel) compared to the non-hierarchical
structure
 # of symbols for SCH (16e + 16m) in a super-frame
 Total 8 symbols in the legacy-disabled mode
 Total 12 symbols in the legacy-support mode without reusing of 16e preamble
 Complexity in Timing Detection
 Cross-correlation based algorithm
 Sharpen peak in the condition of very small frequency offsets
 Require separate step only for updating correlation metric every sample
 Comparisons (# of multiplications and additions) [1]
 Replica-based detection (cross-correlation based)
 Auto-correlation based detection
 50,000 samples during a radio frame
Cross-correlation based
Auto-correlation based
Ratio
# of complex multiplications
51,200,000
100,512
509 times
# of complex additions
51,150,000
100,511
509 times
8
Problem of Hierarchical Structure 2
 Overhead
 Additional resource (secondary sync. channel) compared to the nonhierarchical structure
 # of symbols for SCH (16e + 16m) in a super-frame
 Total 8 symbols in the legacy-disabled mode
 Total 12 symbols in the legacy-support mode without reusing of 16e preamble
 Performance
 Double energy should be required for the same performance as nonhierarchical one.
 With coherent detections of S-SCH using P-SCH, the performance will be
degraded due to
 Frequency offsets in practical environments
 Composite channel from adjacent cells at cell edge
 The overall performance depends on timing sync.
 The only benefit of coherent detection will disappear.
9
Problem of Hybrid Structure
 Complexity
 With non-repeated P-SCH (cross-correlation based algorithm)
 Require separate steps only for updating correlation metrics every sample  large
complexity
 Multiplexing of Data Channel with SCH
 Separate processing for multiplexing will be required (e.g. 2 times of 256FFT
for SCH and a single 512FFT for data in the 5MHz)
 A number of guard subcarriers will be required due to orthogonality
destruction.
 Performance Degradation in Cell ID Detection
 The sequence length will become half in a given amount of information.  it
will result in an increase of detection error rate and false alarm rate.
 Similar performance degradation to the previous Hierarchical 1 and 2
structures
10
Simulation Environments & Procedure
Start
For Non-Hierarchical Structure
For Hierarchical Structure
Coarse Symbol Timing Acquisition by P-SCH
(Auto-correlation based, moving averaging)
Coarse Symbol Timing Acquisition by SCH
(Auto-correlation based, moving averaging)
 Non-hierarchy
 A single OFDM symbol for SCH
 Total energy for SCH: E
 Hierarchical A
Frequency Offset Estimation & Compensation by
P-SCH
Frequency Offset Estimation & Compensation by
SCH
Fine Symbol Timing by P-SCH
 Two OFDM symbols for SCH
 Multiplexing of P-SCH and SSCH: TDM
 Total energy for SCH: 2E.
 Hierarchical B
Channel Estimation by P-SCH and Phase
Compensation to S-SCH
Cell ID detection (coherent)
Cell ID detection (non-coherent<differential>)
End
11
 Two OFDM symbols for SCH
 Multiplexing of P-SCH and SSCH: TDM
 Total energy for SCH: E  Same
as Non-hierarchy
Hierarchy vs. Non-hierarchy (Ideal Assumption)
 Ideal Assumptions
 Ideal timing and frequency sync.
 Cell ID Detection
TU6, 120km/h, 1Tx-1Rx, FO=0ppm, Ideal time sync, freq sync off, # of cells=1
0
10
 Non-coherent detection for nonhierarchy
Non-hierarchy
HierarchyA
HierarchyB
 Comparisons
 With the same energy E used for
detections (Hierarchy A and
Non-hierarchy)
Prob. of detection error
 Coherent detection for hierarchy
-1
10
-2
10
 ~3 dB gain for coherent
detection
 With the same total energy E
(Hierarchy B and Nonhierarchy),
-3
10
 Non-hierarchy has a comparable
performance to hierarchy.
12
-18
-16
-14
-12
-10
-8
SNR[dB]
-6
-4
-2
0
Residual Timing Error Sensitivities
 Assumptions
 Ideal timing = no residual timing
 Real timing = Ideal timing ± CP/2
 Non-hierarchy (Differential
detection) can compensate for the
effect of residual timing error.
 For Hierarchy (Coherent
detection),
 The effect of residual timing
error is significant.
Prob. of detection error
 Observations
TU6,
120km/h, FO=0ppm, 1Tx-1Rx, Ideal time sync, Freq sync off, # of cells=1
0
10
Non-hierarchy(No residual timing error)
Non-hierarchy(timing error of CP/2)
HierarchyA(No residual timing error)
HierarchyA(timing error of CP/2)
HierarchyB(No residual timing error)
HierarchyB(timing error of CP/2)
-1
10
-2
10
 Distorted channel estimation to SSCH due to OFDM symbols next to
P-SCH
-3
10
13
-18
-16
-14
-12
-10
-8
SNR[dB]
-6
-4
-2
0
Residual Freq Offset Sensitivities
 Assumptions
Non-hierarchy,
TU6, 120km/h, 1Tx-1Rx, Ideal time sync, Freq sync off, # of cells=1
0
10
FO=0.0ppm
FO=0.5ppm
FO=1.0ppm
Prob. of detection error
 FO=0ppm, 0.5ppm, 1.0ppm
 FO estimator OFF
 Observations
 No degradation from residual
frequency offsets up to 1.0ppm
-1
10
-2
10
-3
10
-10
-8
-6
-4
-2
0
SNR[dB]
HierarchyB,
TU6, 120km/h, 1Tx-1Rx, Ideal time sync, Freq sync off, # of cells=1
0
10
FO=0.0ppm
FO=0.5ppm
FO=1.0ppm
-1
Prob. of detection error
Prob. of detection error
HierarchyA,
TU6, 120km/h, 1Tx-1Rx, Ideal time sync, Freq sync off, # of cells=1
0
10
FO=0.0ppm
FO=0.5ppm
FO=1.0ppm
10
-2
10
-3
10
-18
-18
-16
-14
-12
-16
-14
-12
-1
10
-2
10
-3
-16
-14
-12
-10
-8
SNR[dB]
-6
-4
-2
0
14
10
-18
-10
-8
SNR[dB]
-6
-4
-2
0
Symbol Timing Performance
 Assumptions
 Practical Timing and FO=0ppm (FO
estimator OFF)
 Success: detected timing is within
±(CP/2)
TU6,
120km/h, 1Tx-1Rx, FO=0ppm, Practical time sync, Freq sync off, # of cells=1
0
10
Non-hierarchy(SCH energy:E)
HierarchyA(SCH energy:2E)
HierarchyB(SCH energy:E)
 Observations
 The performance for cell ID
detection without a residual timing
error is -10.5dB @10^-2.
 The performance for timing sync.
with ±(CP/2) is approximately 0 dB
@10^-2.
 The overall cell search performance
(sync + cell ID detection) much more
depends on the timing performance
than the pure cell ID detection.
Prob. of detection error
 Non-hierarchy: performance for
coarse timing
 Hierarchy: performance for coarse +
fine timing
-1
10
-2
10
-3
10
15
-18
-16
-14
-12
-10
-8
SNR[dB]
-6
-4
-2
0
Hierarchy vs. Non-hierarchy (Practical Assumption)
 Cell ID detection
 Non-coherent detection for nonhierarchy
 Coherent detection for hierarchy
0
TU6, 120km/h, 1Tx-1Rx, 3ppm, Practical time/freq sync,# of cells=1
10
 Comparisons
Non-hierarchy(energy:E)
HierarchyA(energy:2E)
HierarchyB(energy:E)
 No gain from coherent detection
due to previously described reasons
Prob. of detection error
 The overall cell search performance
(sync + cell ID detection) is mainly
limited by sync.
 With the same total energy E, Nonhierarchy is better than Hierarchy
B by ~3dB.
-1
10
-2
10
-3
10
16
-18
-16
-14
-12
-10
-8
SNR[dB]
-6
-4
-2
0
Summary
 Problems of Hierarchical Structure 1 (= Hierarchical, Non-repeated
signal form, Cross-correlation based algorithm)
 Doubled overhead compared to non-hierarchical structures
 Huge increase in complexity for timing synchronization
 Problems of Hierarchical Structure 2 (= Hierarchical, Repeated
signal form for P-SCH, Auto-correlation based algorithm)
 Doubled overhead compared to non-hierarchical structures
 Doubled energy for the same performance as non-hierarchical one
 No benefit of coherent detection in the practical environments
 Problems of Hybrid Structures (= Hierarchical within one symbol)
 Difficulty to be multiplexed with data
 Performance degradation on cell ID detection
17
Text Proposal for IEEE802.16m SDD
============ Start of text proposal for C80216m-08/003r4 ==============
[Adopt the following texts and remove other sentences in section 11.7.2.1.2.1.1, and
delete section 11.7.2.1.2.1.5]
11.7.2.1.2.1.1 Hierarchy
No hierarchy of synchronization channel (SCH) exists. The SCH is
transmitted with a span of one OFDM symbol through multiple transmit
antennas, in order to enable time- and frequency-synchronization and
cell-ID detection.
=================== End of text proposal ======================
18
References
[1] IEEE80216m-08/478r3, Design on the Synchronization Channel for
IEEE802.16m.
19
Annex A : Simulation Environments
 Simulation Parameters















Carrier frequency: 2.5GHz
System bandwidth: 5MHz
Sampling factor: 28/25
Sampling frequency: 5.6MHz
Subcarrier spacing: 10.9375kHz
FFT size: 512
CP length: 1/8*Tu, where Tu is effective OFDM symbol duration
Number of used subcarriers: 424
Number of guard subcarriers: 88
Number of antennas: 1Tx-1Rx
Sync channel repetition: 2 (every even subcarrier is nulled)
Number of cell IDs: 256
Other data channel modeling: Randomly generated QPSK signals
Sequence length for Sync channel: 212
Sequence type for Sync channel: Randomly generated BPSK signals (not
optimized about PAPR and x-correlation)
20