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Hierarchy on IEEE 802.16m Synchronization Channel

IEEE 802.16 Presentation Submission Template (Rev. 9) Document Number:

IEEE C802.16m-08/1163Date Submitted:

2008-09-05Source:

Sungho Moon, Seunghee Han, Jin Sam Kwak, Young-Hyoun Kwon Voice: +82-31-450-1935 e-mail : {msungho; dondai; samji; wishwill}@lge.com

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 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.

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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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Hierarchy on IEEE 802.16m Synchronization Channel

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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

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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

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General Terms and Relationships Symbol Timing Detection Algorithms and Signal Forms

Hierarchy and Signal Forms

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

Signal FormsDetection algorithm

Repeated Signal Non-Repeated Signal

Auto-correlation based algorithm Well-Matched N/A

Cross-correlation based algorithm

Not recommended due to ambiguous peaks &

complexityWell-Matched

Signal FormsHierarchy

Repeated Signal Non-Repeated Signal

Hierarchical (two symbols) Matched Well-Matched

Non-Hierarchical (one symbol) Well-Matched Not recommended due to

complexity

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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 cell-edge) For Example,

Super-Frame, 20 ms

Tx. Ant 0

Tx. Ant Ntx-1

a -aCP

...

a' -a'CP

Circular Shift

1 OFDM Symbol

A -ACP

IFFT / FFT

Time Frequency

-1 1 3 5

... ...

-3-5

5 MHz

f

... ...

5 MHz

f

From Other Sectors

5 ms

-1 1 3 5-3-5

Sector/Cell-Common Allocation

...

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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.

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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

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Problem of Hierarchical Structure 2 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 Performance

Double energy should be required for the same performance as non-hierarchical 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.

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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

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Simulation Environments & Procedure

Non-hierarchy A single OFDM symbol for

SCH Total energy for SCH: E

Hierarchical A Two OFDM symbols for

SCH Multiplexing of P-SCH and

S-SCH: TDM Total energy for SCH: 2E.

Hierarchical B Two OFDM symbols for

SCH Multiplexing of P-SCH and

S-SCH: TDM Total energy for SCH: E

Same as Non-hierarchy

Start

Coarse Symbol Timing Acquisition by P-SCH(Auto-correlation based, moving averaging)

Coarse Symbol Timing Acquisition by SCH(Auto-correlation based, moving averaging)

Frequency Offset Estimation & Compensation by P-SCH

Frequency Offset Estimation & Compensation by SCH

Fine Symbol Timing by P-SCH

Channel Estimation by P-SCH and Phase Compensation to S-SCH

Cell ID detection (coherent) Cell ID detection (non-coherent<differential>)

End

For Hierarchical Structure For Non-Hierarchical Structure

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Hierarchy vs. Non-hierarchy (Ideal Assumption) Ideal Assumptions

Ideal timing and frequency sync.

Cell ID Detection Non-coherent detection for

non-hierarchy Coherent detection for

hierarchy Comparisons

With the same energy E used for detections (Hierarchy A and Non-hierarchy)

~3 dB gain for coherent detection

With the same total energy E (Hierarchy B and Non-hierarchy),

Non-hierarchy has a comparable performance to hierarchy.

-18 -16 -14 -12 -10 -8 -6 -4 -2 010

-3

10-2

10-1

100

SNR[dB]

Pro

b. o

f det

ectio

n er

ror

TU6, 120km/h, 1Tx-1Rx, FO=0ppm, Ideal time sync, freq sync off, # of cells=1

Non-hierarchyHierarchyAHierarchyB

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Residual Timing Error Sensitivities Assumptions

Ideal timing = no residual timing

Real timing = Ideal timing ± CP/2

Observations Non-hierarchy (Differential

detection) can compensate for the effect of residual timing error.

For Hierarchy (Coherent detection),

The effect of residual timingerror is significant.

Distorted channel estimation to S-SCH due to OFDM symbols next to P-SCH

-18 -16 -14 -12 -10 -8 -6 -4 -2 010

-3

10-2

10-1

100

SNR[dB]

Pro

b. o

f det

ectio

n er

ror

TU6, 120km/h, FO=0ppm, 1Tx-1Rx, Ideal time sync, Freq sync off, # of cells=1

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)

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Residual Freq Offset Sensitivities Assumptions

FO=0ppm, 0.5ppm, 1.0ppm

FO estimator OFF Observations

No degradation from residual frequency offsets up to 1.0ppm

-18 -16 -14 -12 -10 -8 -6 -4 -2 010

-3

10-2

10-1

100

SNR[dB]

Pro

b. o

f det

ectio

n er

ror

Non-hierarchy, TU6, 120km/h, 1Tx-1Rx, Ideal time sync, Freq sync off, # of cells=1

FO=0.0ppmFO=0.5ppmFO=1.0ppm

-18 -16 -14 -12 -10 -8 -6 -4 -2 010

-3

10-2

10-1

100

SNR[dB]

Pro

b. o

f det

ectio

n er

ror

HierarchyA, TU6, 120km/h, 1Tx-1Rx, Ideal time sync, Freq sync off, # of cells=1

FO=0.0ppmFO=0.5ppmFO=1.0ppm

-18 -16 -14 -12 -10 -8 -6 -4 -2 010

-3

10-2

10-1

100

SNR[dB]

Pro

b. o

f det

ectio

n er

ror

HierarchyB, TU6, 120km/h, 1Tx-1Rx, Ideal time sync, Freq sync off, # of cells=1

FO=0.0ppmFO=0.5ppmFO=1.0ppm

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Symbol Timing Performance Assumptions

Practical Timing and FO=0ppm (FO estimator OFF)

Success: detected timing is within ±(CP/2)

Non-hierarchy: performance for coarse timing

Hierarchy: performance for coarse + fine timing

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.

-18 -16 -14 -12 -10 -8 -6 -4 -2 010

-3

10-2

10-1

100

SNR[dB]

Pro

b. o

f det

ectio

n er

ror

TU6, 120km/h, 1Tx-1Rx, FO=0ppm, Practical time sync, Freq sync off, # of cells=1

Non-hierarchy(SCH energy:E)HierarchyA(SCH energy:2E)HierarchyB(SCH energy:E)

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Hierarchy vs. Non-hierarchy (Practical Assumption) Cell ID detection

Non-coherent detection for non-hierarchy

Coherent detection for hierarchy

Comparisons No gain from coherent

detection due to previously described reasons

The overall cell search performance (sync + cell ID detection) is mainly limited by sync.

With the same total energy E, Non-hierarchy is better than Hierarchy B by ~3dB.

-18 -16 -14 -12 -10 -8 -6 -4 -2 010

-3

10-2

10-1

100

SNR[dB]

Pro

b. o

f det

ectio

n er

ror

TU6, 120km/h, 1Tx-1Rx, 3ppm, Practical time/freq sync,# of cells=1

Non-hierarchy(energy:E)HierarchyA(energy:2E)HierarchyB(energy:E)

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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

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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 ======================

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References[1] IEEE80216m-08/478r3, Design on the Synchronization

Channel for IEEE802.16m.

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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)