1. 1 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 Voice: +82-31-450-1935 e-mail : {msungho; dondai; samji; wishwill}@lge.comsamji; 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. Release: 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 discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.16. Patent Policy: The contributor is familiar with the IEEE-SA Patent Policy and Procedures: and.http://standards.ieee.org/guides/bylaws/sect6-7.html#6http://standards.ieee.org/guides/opman/sect6.html#6.3 Further information is located at and.http://standards.ieee.org/board/pat/pat-material.htmlhttp://standards.ieee.org/board/pat

2. 2 Hierarchy on IEEE 802.16m Synchronization Channel

3. 3 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

4. 4 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

5. 5 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 Forms Detection algorithm Repeated SignalNon-Repeated Signal Auto-correlation based algorithmWell-MatchedN/A Cross-correlation based algorithm Not recommended due to ambiguous peaks & complexity Well-Matched Signal Forms Hierarchy Repeated SignalNon-Repeated Signal Hierarchical (two symbols) MatchedWell-Matched Non-Hierarchical (one symbol) Well-Matched Not recommended due to complexity

6. 6 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,

7. 7 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.

8. 8 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 basedAuto-correlation basedRatio # of complex multiplications51,200,000100,512509 times # of complex additions51,150,000100,511509 times

9. 9 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.

10. 10 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

11. 11 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

12. 12 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.

13. 13 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 timing error is significant.  Distorted channel estimation to S- SCH due to OFDM symbols next to P-SCH

14. 14 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

15. 15 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.

16. 16 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.

17. 17 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

18. 18 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 ======================

19. 19 References [1] IEEE80216m-08/478r3, Design on the Synchronization Channel for IEEE802.16m.

20. 20 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)