1. 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 JungE-mail: jihoon@kau.ac.kr,jihoon@kau.ac.kr Korea Aerospace University yhjung@kau.ac.kryhjung@kau.ac.kr Sungcheol Chang, Seokki Kim, Eunkyung Kim, Miyoung Yun, Won-Ik Kim, E-mail: scchang@etri.re.krscchang@etri.re.kr Sungkyung Kim, Hyun Lee, Chulsik Yoon, Kwangjae Lim ETRI 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 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. Copyright Policy: The contributor is familiar with the IEEE-SA Copyright Policy. Patent Policy: The contributor is familiar with the IEEE-SA Patent Policy and Procedures: and..html#6sect6.html#6.3 Further information is located at and.http://standards.ieee.org/board/pat/pat-material.htmlhttp://standards.ieee.org/board/pat

2. 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

3. 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

4. Synchronization Channel Structure 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

5. 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 N FFT /2 samples, while the conventional preamble has a basic pattern with N FFT 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

6. Sync-CH Preamble Sequence PRBS (pseudo-random binary sequence) generation –Generator polynomial = 1+X 1 +X 4 +X 7 +X 15 –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

7. Sync-CH Preamble Sequence (cont.) Proposed Sync-CH preamble sequences –2 preamble sequences are defined as follows. where C k is the k-th bit of the PRBS output and 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

8. Sync-CH IE 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. 8 Field nameField size (bits) Transmitter HR-MS IDTBD Reference time2 Hop count2 Reference signal strengthTBD Frame structure information4 CRC16

9. Sync-CH IE (cont.) Physical processing block –The number of information bits including 16-bit CRC is 64 bits. –Channel encoded by TBCC with parameter M=2K bufsize and K bursize =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

10. Performance Evaluation Using the Proposed Sync-CH Preamble 10

11. Timing Offset Estimation Despreading by the m-th preamble sequence where r k,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 where N FFT is the FFT size and arg[x] is the phase of x. Correction of the phase rotation by timing offset 11

12. Frequency Offset Estimation Normalized frequency offset where  f is the carrier frequency offset, T s 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 12

13. Subcarrier Assignment for Simulations –SIR (signal to interference ratio): –SNR (signal to noise ratio): 13

14. 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 14 ParameterValue Carrier frequency2.3 GHz Bandwidth10 MHz FFT size1024 CP size128 Sampling rate11.2 MHz Number of transmit antennas1 Number of receive antennas1 Velocity of transmitter30 km/h Velocity of receiver30 km/h Moving direction of transmitter  /6 Moving direction of receiver -  /4 Timing offset256 samples Frequency offset of transmitter9 ppm Frequency offset of interference2 ppm Frequency offset of receiver-10 ppm Preamble sequence of conventional SYNC-CH Preamble sequence of proposed SYNC-CH SIR-10 dB

15. Timing Offset Estimation Timing accuracy requirement: MSE < 100 Simulation results (SIR=-10dB) 15

16. Frequency Offset Estimation Frequency accuracy requirement: MSE < 4.4  10 -5 Simulation results (SIR=-10dB) 16

17. 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