MIMO Supports for IEEE 802.16m Synchronization Channel IEEE 802.16 Presentation Submission Template (Rev. 9) Document Number: IEEE C802.16m-08/1164r1 Date Submitted: 2008-09-05 Source: Seunghee Han, Sungho Moon, Jin Sam Kwak Voice: +82-31-450-1935 e-mail : {dondai; msungho; samji}@lge.com LG Electronics LG R&D Complex, 533 Hogye-1dong, Dongan-gu, Anyang, 431-749, Korea Venue: Re: PHY: Text; 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: <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 >.

MIMO Supports for IEEE 802.16m Synchronization Channel

Contents MIMO Support Options Comparisons of MIMO Options (CDD and FSTD) Simulation environments Cell ID detection error (AWGN, TU6 120km/h) Beamforming effect in CDD Convergence time Frequency reuse Summary Text Proposals for 16m SDD Annexes

MIMO Supports Description in the SDD Considerations MIMO support is achieved by transmitting SCH subcarriers from known antennas. Antennas are: Cyclic delay diversity (with antenna specific delay values)  CDD Interleaved either within a symbol (multiple antennas can transmit within a single symbol but on distinct subcarriers)  FSTD Across frames (only one antenna transmits in each symbol)  TSTD Or some combination – Actual approach is FFS. Considerations TSTD has problems in the convergence time with low mobility MSs and in power transition time from antenna on/off. TSTD requires multiple time instances to get diversity gain while CDD/FSTD can achieve the diversity in single time instance. CDD and FSTD are promising candidates for 16m SCH in multi-antenna transmissions.

Comparisons of MIMO Options (CDD and FSTD) Commonalities Multi-antenna transmission schemes for 16m SCH Distinguishable channel responses for each antenna  Possible for CE Support of various channel conditions High mobility, Large delay spread, etc CDD Spatial diversity with MS-transparent TxD on the number of antennas can be achievable. Larger spreading gain with longer preamble sequence FSTD Complete separation between antennas Limited frequency diversity can be achievable. Worse correlation property with shorter sequence length

Cell Search Procedure and Simulation Environments Major Parameters (Detailed in Annex A) Search duration : 5 ms (coarse timing synchronization only for one SCH) Multi-antenna transmission: CDD (1/4*Tu shifts for 2Tx, 1/8*Tu shifts for 4Tx), FSTD (Localized type) System bandwidth: 5MHz Carrier frequency offset: random within ±3ppm @ 2.5GHz carrier frequency Number of cell IDs: 256 Sequence length: 212/(frequency reuse) for CDD, 212/(frequency reuse)/(# of Tx antennas) for FSTD Randomly generated BPSK signals (not optimized about PAPR and x-correlation) Cell ID detection: ML hypotheses by non-coherent detection # of cells: 1, 2, 3 <cellA(desired cell): 0dB, cellB: -6dB, cellC: -6dB> Correct Detection Probability Probability to choose a BS which received power is within 3 dB of the BS with the highest received power

Cell ID Detector Full Correlator in Frequency Domain The correlation profiles are calculated coherently for entire sequence length. Since there always exists timing error (particularly large error after coarse timing sync step) and frequency selectivity in practical situation, it doesn’t work. Partial Correlator in Frequency Domain The block-wise correlation profiles are calculated in order to reduce timing error and frequency selectivity. For intra-blocks, coherent summation For inter-blocks, non-coherent summation Differential Correlator in Frequency Domain To reduce timing error and frequency selectivity. In taking differential operations for CDD, a proper interval selection would be needed. (refer to Annex A) Energy Detector using FFT (IFFT) Operation The energies of channel impulse response are aggregated using FFT operation Unfeasible due to large complexity. For example, the ML complexity could be 256*512*log2(512) [=1179648] multiplications even in ignoring energy aggregation operations.  Not considered in the simulation

Cell ID Detector (cont’d) Observations Full correlator is not feasible. The performance for partial correlator depends on the size of blocks. It needs to optimize the parameters. Different correlator works well. The differential correlator is deployed for evaluations.

Cell ID Detection Error (AWGN) 3ppm FO, Practical timing/freq sync, # of cells=1 Both CDD and FSTD work well.

Cell ID Detection Error (AWGN) (cont’d) 3ppm FO, Practical timing/freq sync, # of cells=3 CDD is better than FSTD due to interference randomization. For CDD, Good interference randomization between cells For FSTD, ~0.6dB SNR loss to CDD for 2Tx @1% error rate Severe degradation by ~2.9dB SNR to CDD for 4Tx @ 1% error rate

Beamforming effect in CDD No Coverage Hole Not significant impact of frequency selectivity due to the beam-forming in different subcarriers Constructive subcarriers can be used to maintain the cell-ID detection performance Impact of beamforming steer-like CDD in a certain DoA varies in different subcarriers The overall energies can be maintained. (Annex C) 3ppm FO, Practical timing/freq sync, # of cells=1 No coverage hole has been found for both CDD and FSTD.

Cell ID Detection Error (TU6, 120km/h) 3ppm FO, Practical timing/freq sync 4Tx is better than 2Tx due to spatial diversity gain. CDD is better than FSTD due to frequency diversity gain @ 1% error rate. by ~0.5dB for 2Tx by ~0.6dB for 4Tx

Convergence Time Definition Observation Time interval for cell ID detection error to be less than 1% Observation # of cells The inter-cell interference increases a convergence time. # of transmission antennas Multi-antenna transmission is necessary to reduce the convergence time. Periodicity of 16m SCH If the periodicity of a complete SCH instance is 20 ms, the convergence time will become four times. In case of FSTD Similar trend to the above statements

Frequency Reuse in SCH Pros and Cons Single-Cell Environment Decease in interference from other cells Reduced sequence length (Poor x-correlation property) Single-Cell Environment Reuse 1 is always better than Reuse 3 regardless of CDD and FSTD due to reduced sequence lengths.

Frequency Reuse in SCH (cont’d) Multi-Cell Environment High SNR region (> -10 dB) Reuse 1 is better than reuse 3 due to the sequence lengths. No gain in finding a sector ID due to absence of frequency reuse information Low SNR region (< -10 dB) Reuse 3 is better than reuse 1 due to the reduced interference. Multiple SCH combining is needed for the reliable performance. Multiple SCH Combining # of combined symbols = 5 Reuse 1 is better than reuse 3 even in the low SNR region. With the symbol combining, the x-correlation property is more dominant than interference. FSTD has the same trend regarding frequency reuse.

Summary The CDD transmission of SCH has better performance in terms of the cell ID detection error than FSTD. The proposed CDD transmission shows reasonable convergence times even in multi-cell scenarios. With the symbol combining, a SCH transmission with reuse 1 is better than that with reuse 3 due to the larger sequence length.

Text Proposal for IEEE802.16m SDD ============= Start of text proposal for C80216m-08/003r4================ [Replace the whole section 11.7.2.1.2.5 with the following texts] 11.7.2.1.2.5 MIMO support and channel estimation Figure xx shows the MIMO support in IEEE 802.16m synchronization channel (SCH). Each antenna transmits a cyclic delayed symbol with antenna-specific delay values for IEEE 802.16m MSs to perform full-band channel estimation for each antenna in the whole SCH bandwidth. The number of BS antennas supported for MIMO channel measurements is FFS, depending on the requirements of other 16m SDD content, such as DL MIMO and interference mitigation. Figure xx. Multi-antenna transmission for IEEE 802.16m synchronization channel ================== End of text proposal =============================

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 Carrier frequency offset: random within 0ppm and ±3ppm Frame configuration: All DL signals, 5ms periodicity for Sync channel Number of antennas: 2Tx-1Rx, 4Tx-1Rx Multi-antenna transmission: CDD (1/4*Tu shifts for 2Tx, 1/8*Tu shifts for 4Tx), FSTD (Localized type) Sync channel repetition: 2 (every even subcarrier is nulled) Power boosting to other data channel per antenna on Sync channel: 12.04dB (=16) for CDD, 12.04dB+10*log10(# of Tx antennas)dB (=16*(# of Tx antennas)) FSTD  same total transmit power in SCH

Annex A : Simulation Environments (cont’d) Simulation Descriptions Number of cell IDs: 256 Other data channel modeling: Randomly generated QPSK signals Sequence length for Sync channel: 212 for CDD, 212/(# of Tx antennas) for FSTD  All antennas transmit the same sequences. Sequence type for Sync channel: Randomly generated BPSK signals (not optimized about PAPR and x-correlation) # of cells: 1, 2, 3 <cellA(desired cell): 0dB, cellB: -6dB, cellC: -6dB> All signals from each cell are arrived with same absolute times at MS side. Channel model: AWGN, TU6 (120km/h) Search duration for coarse timing synchronization: 5ms (one sync channel used) Algorithms for coarse timing synchronization: auto-correlation based and further moving averaging to make smooth correlation profile for frequency offset estimation: differential based (auto-correlation based) Cell ID detection: ML hypotheses by non-coherent detection

Annex B : Differential Detector Received signal at k-th subcarrier (assuming non-repeated signals) In AWGN, λ: wavelength, d: antenna spacing The value d is set to d= λ/2 for evaluation. Differential detector Differential operation at Rx Correlation for differential signal

Annex C : Beamforming Effect in CDD The overall energies can be maintained.