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[ETRI’s Simulation Results for OFDMA Parameters]

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1 [ETRI’s Simulation Results for OFDMA Parameters]
June 2006 doc.: IEEE xxx August 2006 [ETRI’s Simulation Results for OFDMA Parameters] IEEE P Wireless RANs Date: Authors: Notice: This document has been prepared to assist IEEE It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) 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 Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair Carl R. Stevenson as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE Working Group. If you have questions, contact the IEEE Patent Committee Administrator at Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

2 June 2006 doc.: IEEE xxx August 2006 Abstract In this presentation, we show the simulation results for OFDMA parameters in various WRAN environments. First, we present the performances focusing on the timing synchronization and carrier frequency offset estimation using preamble only. Second, we present the performances on channel estimation for various preamble and pilot pattern. Lastly, we present the performances on tracking of frequency offset. Based on our simulation results, we propose the modified preamble and pilot pattern. Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

3 Contents System parameters & OFDMA parameters (for 6MHz)
June 2006 doc.: IEEE xxx August 2006 Contents System parameters & OFDMA parameters (for 6MHz) Frame structure with preamble & pilot pattern Algorithms & operation procedure Top block diagram of PHY simulator System model for synchronization Initial synchronization: Timing & CFO(IFO+FFO) estimation Channel estimation FFO tracking Simulation conditions Simulation results Conclusions Modified frame structure with preamble & pilot pattern Further works Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

4 System Parameters & OFDMA Parameters
June 2006 doc.: IEEE xxx August 2006 System Parameters & OFDMA Parameters Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

5 System Parameters/Single Channel (6MHz)
June 2006 doc.: IEEE xxx August 2006 System Parameters/Single Channel (6MHz) Mode 1K 2K 4K 6K FFT Size 1024 2048 4096 6144 Bandwidth (k = 1, 2, …, 6) k MHz Sampling Factor 8/7 No. of Used Subcarriers (including pilot, but not DC) 140 * k 280 * k 560 * k 840 * k Sampling Frequency 48/7 MHz Subcarrier Spacing 6.696 kHz(***) 3.348 kHz 1.674 kHz 1.116 kHz Occupied Bandwidth 6.696 kHz*140*k 3.348 kHz*280*k 1.674 kHz*560*k 1.116 kHz*840*k Bandwidth Efficiency(*) 93~94 % FFT Time us us us 896 us Cyclic Prefix Time(**) 37.33 us 74.66 us 224 us OFDMA Symbol Time us us us 1120 us (*) Bandwidth Efficiency = Subcarrier Spacing * (Number of Used Subcarriers + 1)/BW (**) It is assumed that cyclic prefix mode is 1/4. (***) Italics indicate an approximated value. Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

6 OFDMA Parameters (2K FFT Mode)
August 2006 OFDMA Parameters (2K FFT Mode) Parameter 1 TV bands 6 7 8 Inter-carrier spacing, DF (Hz) (*) 3348 3906 4464 FFT period, TFFT (ms) (*) 298.66 256.00 224.00 Total no. of sub-carriers, NFFT 2048 No. of guard sub-carriers, NG (L, DC, R) 368 (184,1,183) No. of used sub-carriers, NT = ND + NP 1680 No. of data sub-carriers, ND 1440 No. of pilot sub-carriers, NP 240 No. of sub-carriers per BIN 14 (12 datas + 2 pilots) No. of BIN per subchannel 4 No. of sub-carriers per subchannel 56 (48 datas + 8 pilots) Occupied bandwidth (MHz) (*) 5.628 6.566 7.504 Bandwidth Efficiency (%) (**) 93.8 (*) Italics indicate an approximated value. (**) Bandwidth Efficiency = Subcarrier Spacing * (Number of Used Subcarriers + 1)/BW Sunghyun Hwang, ETRI

7 Frame Structure with Preamble & Pilot Pattern
June 2006 doc.: IEEE xxx August 2006 Frame Structure with Preamble & Pilot Pattern Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

8 Two Approaches of Frame Structure
June 2006 doc.: IEEE xxx August 2006 Two Approaches of Frame Structure In our simulations, we consider two types of frame structure with preamble and pilot pattern. 1st Frame Structure General frame structure With a preamble of one OFDMA symbol and the appropriate pilots The pilots are inserted in the header and data field. 2nd Frame Structure Novel frame structure With a preamble of two OFDMA symbol and no pilots The header and data are purely transmitted without pilots. Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

9 TDD Frame Structure I August 2006 June 2006
doc.: IEEE xxx August 2006 TDD Frame Structure I Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

10 Preamble Pattern Two repetitions within one OFDMA symbol
June 2006 doc.: IEEE xxx August 2006 Preamble Pattern Two repetitions within one OFDMA symbol GI=1/4 (fixed) Preamble shall be modulated using BPSK modulation Used in channel estimation and synchronization NSYM GI NFFT/2 NFFT/2 Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

11 Preamble Pattern Preamble sequence PN sequence generator
August 2006 Preamble Pattern Preamble sequence PN sequence generator Initial states of PN sequence generator: Note that PN sequence has the order of 11 so that the period of preamble sequence is 2047. Total 840 chips are used for each preamble Sunghyun Hwang, ETRI

12 Pilot Pattern of Frame Structure I
June 2006 doc.: IEEE xxx August 2006 Pilot Pattern of Frame Structure I OFDMA Symbol time Pilot Structure for Extendable Channel Estimation Available Pilot Pattern for Channel Estimation BIN Repetition Unit frequency Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

13 TDD Frame Structure II August 2006 June 2006
doc.: IEEE xxx August 2006 TDD Frame Structure II Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

14 Preamble Structure for Frame Structure II
August 2006 Preamble Structure for Frame Structure II OFDMA Symbol Modified Preamble Structure for Extendable Channel Estimation Available Pilot Pattern for Channel Estimation time Pure Data frequency Sunghyun Hwang, ETRI

15 Decision Procedure of Preamble and Pilot Pattern
August 2006 Decision Procedure of Preamble and Pilot Pattern Is it possible to achieve the sufficient performances in initial synchronization using proposed preamble only? Yes No Is it possible to achieve the sufficient performances in tracking without pilots? Reinforce the preamble! If we assume the phase noise model with PSD(0)=-100 dBc/Hz, the answer is ‘Yes’. However, if the phase noise effect becomes more severe, the answer may be ‘No’. Yes No Is it possible to achieve the sufficient performances in channel estimation using proposed preamble only? Is it possible to achieve the sufficient performances in channel estimation using proposed preamble and pilots? Yes No Yes No Use one preamble symbol & Remove all pilots Reinforce the preamble & Remove all pilots Maintain (or slightly modify) the proposed preamble and pilots Reinforce the preamble and(or) the pilots! Frame Structrue II Frame Structure I Sunghyun Hwang, ETRI

16 Algorithms & Operation Procedure
June 2006 doc.: IEEE xxx August 2006 Algorithms & Operation Procedure Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

17 Top Block Diagram of WRAN PHY Simulator
June 2006 doc.: IEEE xxx August 2006 Top Block Diagram of WRAN PHY Simulator Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

18 Signal Model for Synchronization
August 2006 Signal Model for Synchronization Transmitted signal Received signal with carrier frequency offset (CFO) The time-sampled version of the received signal Demodulated symbol at the k-th subcarrier in the i-th OFDM symbol Sunghyun Hwang, ETRI

19 Timing Synchronization
August 2006 Timing Synchronization With the Schmidl’s method Autocorrelation method using the following The metric Timing Sunghyun Hwang, ETRI

20 Timing Synchronization
August 2006 Timing Synchronization Enhanced timing metric to resolve the timing ambiguity in the plateau to protect the timing outside the guard interval Sunghyun Hwang, ETRI

21 CFO(FFO+IFO) Estimation
August 2006 CFO(FFO+IFO) Estimation When the timing is obtained, the received samples corresponding to the preamble are given by FFO estimates using Preamble in time-domain Sunghyun Hwang, ETRI

22 CFO(FFO+IFO) Estimation
August 2006 CFO(FFO+IFO) Estimation Two components in the CFO Only the FFO can be estimated in the time domain Integral frequency offset (IFO) estimation After compensating , the FFT output is given by FFT Sunghyun Hwang, ETRI

23 CFO(FFO+IFO) Estimation
August 2006 CFO(FFO+IFO) Estimation IFO estimation We can obtain the IFO using the correlation of the PN sequence Total CFO estimation range: Sunghyun Hwang, ETRI

24 CFO(FFO+IFO) Estimation Range
August 2006 CFO(FFO+IFO) Estimation Range Requirements on the CFO estimation BS : 2 ppm CPE : 8 ppm Worst case scenario BS - CPE : 10 ppm at the frequency of 862 MHz CFO estimation up to kHz ~ 8.62 kHz Estimation with the proposed preamble Estimation range in the time domain: kHz ~ kHz We should estimate IFO in (-2 ,2) (1 PN offset) Even though the proposed method can estimate the IFO up to 682, we set the estimation range as (-8, 8) at the receiver. Sunghyun Hwang, ETRI

25 FFO Tracking Algorithms
August 2006 FFO Tracking Algorithms FFO estimation using GI in the time-domain FFO estimates using GI in the time-domain Sunghyun Hwang, ETRI

26 FFO Tracking Algorithms
August 2006 FFO Tracking Algorithms FFO estimation using Pilot in the frequency-domain FFO estimates using Pilot in the frequency-domain where, RG is the ratio of GI size to FFT size Sunghyun Hwang, ETRI

27 Channel Estimation Received Signal Vector At Pilot Positions
August 2006 Channel Estimation Received Signal Vector Y: received signal vector in the frequency domain X: diagonal matrix containing data symbols W: AWGN At Pilot Positions XP: diagonal matrix containing pilot symbols HP: channel response at pilot positions Sunghyun Hwang, ETRI

28 LMMSE Channel Estimation
August 2006 LMMSE Channel Estimation Linear minimum mean-squared error (LMMSE) estimation Minimizes the mean-squared error between the channel response and . High computational complexity but good performance. To reduce the complexity in LMMSE estimation, low-rank approximations or partitioned LMMSE may be used. Sunghyun Hwang, ETRI

29 LMMSE Channel Estimation
August 2006 LMMSE Channel Estimation Wiener - Hopf Equation Assume that the estimator is constrained to be a linear function of The problem is to find the matrix K that minimizes the mean-squared error between and the linear estimator The necessary and sufficient condition for the mean-squared error to be minimized is for the estimation error to be orthogonal to each input sample, which is expressed by the Wiener-Hopf equation. where is the noisey pilot estimates, is the variance of the channel noise Wp , and RPP is the autocovariance matrix of the noiseless pilots. Sunghyun Hwang, ETRI

30 LMMSE Channel Estimation
August 2006 LMMSE Channel Estimation LMMSE(Linear Minimum Mean-Squared Error) Estimates where, It also requires to know the covariance matrix of the channel and the average SNR. Sunghyun Hwang, ETRI

31 Synchronization & Ch. Estimation Procedure
June 2006 doc.: IEEE xxx August 2006 Synchronization & Ch. Estimation Procedure Preamble Timing Synchronization FFO estimation IFO estimation FFO tracking Channel Estimation Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

32 Simulation Conditions
June 2006 doc.: IEEE xxx August 2006 Simulation Conditions Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

33 Simulation Conditions
June 2006 doc.: IEEE xxx August 2006 Simulation Conditions Channel coding 802.16e convolutional coding Code rate: 1/2, 2/3, 3/4, 5/6 Modulation Data: QPSK, 16QAM, 64QAM Preamble, Pilot: BPSK No boosting FFT size and GI ratio FFT size: 2K(mandatory) GI ratio: 1/4 Subchannelization No. of used subcarriers for 2K: 1680 (30 subchannels x 56 subcarriers) Subchannel type: Diversity & AMC Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

34 Simulation Conditions
August 2006 Simulation Conditions Channel Modeling AWGN Multipath profiles & Doppler: A, B, C, D Approximation to nearest sampling point Jakes Rayleigh modeling method Multipath Profile A B C D RMS Delay Spread (us) 2.772 1.956 5.692 16.527(*) (*) For WRAN profile D, we assume that the 6-th path has the excess delay of 60 us and relative power of -10 dB Sunghyun Hwang, ETRI

35 Simulation Conditions
August 2006 Simulation Conditions Channel Modeling Multipath profiles & Doppler Channel response of WRAN multipath profile The span of 10 subcarriers Sunghyun Hwang, ETRI

36 Simulation Conditions
August 2006 Simulation Conditions Channel Modeling Carrier frequency offset (CFO) : 8.62 kHz Phase noise Phase noise model in IEEE TGn comparison criteria(doc. IEEE /814r31) where, PSD(0)=-100 dBc/Hz, pole frequency fp=250 kHz, and zero frequency fz= kHz (a) Ideal (b) PSD(0) = -100 dBc/Hz (c) PSD(0) = -65 dBc/Hz Sunghyun Hwang, ETRI

37 June 2006 doc.: IEEE xxx August 2006 Simulation Results 1. Missing probability of preamble starting point 2. MSE of CFO estimate (in acquisition) 3. MSE of FFO estimate (in tracking) 4. Uncoded BER of LMMSE estimation 5. Performance comparison for calculation method of covariance matrix 6. Performance comparison for frequency offset effect 7. Uncoded BER performance using preamble alone 8. Uncoded/Coded BER performance under ideal channel estimation Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

38 Missing Probability of Preamble Starting Point
August 2006 Missing Probability of Preamble Starting Point Window size W=64 Missing probability is defined by the probability that the timing is obtained outside of the guard interval of the preamble. In the IEEE Std section , the detection is expected to be 90% accurate even in fairly good conditions Sunghyun Hwang, ETRI

39 MSE of FFO Estimate (Using Preamble)
August 2006 MSE of FFO Estimate (Using Preamble) Sunghyun Hwang, ETRI

40 MSE of CFO Estimate (Using Preamble)
August 2006 MSE of CFO Estimate (Using Preamble) In the IEEE Std section , CPE shall be synchronized to the BS with a tolerance of maximum 2% of sub-carrier spacing Sunghyun Hwang, ETRI

41 Assumptions for FFO Tracking Simulation
August 2006 Assumptions for FFO Tracking Simulation Residual frequency offset: 2% of subcarrier spacing 2 algorithms for FFO tracking Using guard interval Using pilot Consider the phase noise model in IEEE TGn comparison criteria 2K FFT size & GI ratio of 1/4 Sunghyun Hwang, ETRI

42 MSE of FFO Estimate (Using GI or Pilot)
August 2006 MSE of FFO Estimate (Using GI or Pilot) 2% of sub-carrier spacing Sunghyun Hwang, ETRI

43 Assumptions for Channel Estimation Simulation
August 2006 Assumptions for Channel Estimation Simulation 3 Types of Partitioned LMMSE LMMSE, 1 : Channel estimation using the pilot of each OFDM symbol LMMSE, 3 : Channel estimation using the pilot of 3 OFDM symbols LMMSE, 7 : Channel estimation using the pilot of 7 OFDM symbols Apply the partitioned LMMSE to reduce the complexity in LMMSE estimation (Subcarrier size = 56). Calculation method of covariance matrix Method 1: Pre-calculation assuming exponential model Method 2: Real-time calculation using actual measured model No channel coding employed Initial frequency offset: 2KHz Assume that the CFO estimation using preamble is successful (< 2% of subcarrier spacing) Fine frequency offset tracking loop is ON Sunghyun Hwang, ETRI

44 Uncoded BER Performance (Profile A, QPSK)
August 2006 Uncoded BER Performance (Profile A, QPSK) Sunghyun Hwang, ETRI

45 Uncoded BER Performance (Profile A, 64QAM)
August 2006 Uncoded BER Performance (Profile A, 64QAM) Sunghyun Hwang, ETRI

46 Uncoded BER Performance (Profile C, QPSK)
August 2006 Uncoded BER Performance (Profile C, QPSK) Sunghyun Hwang, ETRI

47 Uncoded BER Performance (Profile C, 64QAM)
August 2006 Uncoded BER Performance (Profile C, 64QAM) Sunghyun Hwang, ETRI

48 Uncoded BER Performance (Profile D, QPSK)
August 2006 Uncoded BER Performance (Profile D, QPSK) Sunghyun Hwang, ETRI

49 Uncoded BER Performance (Profile D, 64QAM)
August 2006 Uncoded BER Performance (Profile D, 64QAM) Sunghyun Hwang, ETRI

50 August 2006 Performance Comparison for Calculation Method of Covariance Matrix (Profile C, QPSK) Sunghyun Hwang, ETRI

51 August 2006 Performance Comparison for Calculation Method of Covariance Matrix (Profile D, QPSK) Sunghyun Hwang, ETRI

52 Performance Comparison for Frequency Offset Effect (Profile A, QPSK)
August 2006 Performance Comparison for Frequency Offset Effect (Profile A, QPSK) Sunghyun Hwang, ETRI

53 Performance Comparison for Frequency Offset Effect (Profile A, 64QAM)
August 2006 Performance Comparison for Frequency Offset Effect (Profile A, 64QAM) Sunghyun Hwang, ETRI

54 Uncoded BER Performance Using Preamble Only (Profile A, QPSK)
June 2006 doc.: IEEE xxx August 2006 Uncoded BER Performance Using Preamble Only (Profile A, QPSK) Sunghyun Hwang, ETRI Sunghyun Hwang, ETRI

55 Uncoded BER Performance Using Preamble Only (Profile C, QPSK)
August 2006 Uncoded BER Performance Using Preamble Only (Profile C, QPSK) Sunghyun Hwang, ETRI

56 Uncoded BER Performance Using Preamble Only (Profile D, QPSK)
August 2006 Uncoded BER Performance Using Preamble Only (Profile D, QPSK) Sunghyun Hwang, ETRI

57 Uncoded/Coded Bit Error Rate in AWGN
August 2006 Uncoded/Coded Bit Error Rate in AWGN Sunghyun Hwang, ETRI

58 Uncoded Error Rate in Fading Channel
August 2006 Uncoded Error Rate in Fading Channel Sunghyun Hwang, ETRI

59 Conclusions From the simulations of initial synchronization
August 2006 Conclusions From the simulations of initial synchronization In the WRAN profile A, B, and C, we obtain 90% preamble detection probability at -4 dB SNR. Even in the WRAN profile D, we obtain 90% preamble detection probability at -2 dB SNR In the WRAN profile A, B, and C, we can synchronize the CPE to the BS within 2 % of sub-carrier spacing at -2 dB SNR Even in the WRAN profile D, we can synchronize the CPE to the BS within 2 % of sub-carrier spacing at 4 dB SNR Sunghyun Hwang, ETRI

60 Conclusions From the simulations of FFO tracking
August 2006 Conclusions From the simulations of FFO tracking The performance using guard interval is better than that using pilot. Here, we assume the GI ratio of 1/4, i.e. 512 subcarriers for 2K FFT size. The performance difference is because the correlation size is different, the number of GI subcarriers is 512 and the number of pilot subcarriers is 240. Another reason is because the pilot in adjacent OFDM symbol has experienced the different channel distortion from the pilot in the previous OFDM symbol. Sunghyun Hwang, ETRI

61 Conclusions From the simulations of LMMSE channel estimation
August 2006 Conclusions From the simulations of LMMSE channel estimation If 7 OFDM symbols are used in LMMSE estimation, there are the performance degradation of 0.2~0.5 dB and 2.0~2.5 dB compared to ideal channel estimation for profile A and profile D, respectively. If 1 or 3 OFDM symbols are used in LMMSE estimation, there are huge performance loss. Therefore, to prevent huge performance loss, it is necessary for pilot symbol to visit every subcarriers. It would be established using preamble or(and) pilot. Regarding the calculation method of covariance matrix, the real-time calculation using actual measured model has a much better performance than that of pre-calculation assuming exponential model. Even though the complexity is increasing, because the WRAN system is fixed, we can decrease the complexity by stopping the training of channel information after several frames. If we use the frequency offset tracking, the performance loss due to residual frequency offset is ignorable. The performance using two preamble alone (without pilots) is similar to the performance using 7 OFDMA symbols (with pilots). Sunghyun Hwang, ETRI

62 Further Works Regarding the phase noise effect
August 2006 Further Works Regarding the phase noise effect We are now performing the simulations for coded BER considering various channel distortions, i.e. AWGN, multipath, doppler, frequency offset, timing offset, phase noise, etc. Based on the coded BER performance with modified preamble and pilot pattern, we will find out how much the performance loss is due to phase noise. We think that the effect of phase noise with PSD(0)=-100 dBc/Hz is ignorable. But in general the phase noise with PSD(0)=-65 to -60 dBc/Hz is assumed, in this case, we may not neglect the performance loss due to phase noise anymore. If the phase noise effect can be ignorable as TGn model, we can select the second novel frame structure. However, if the phase noise effect is significant, we should select the first general frame structure. Regarding the preamble and pilot pattern for Up Link Actually this simulation is focused on the down link operation, however as regards the up link, additionally we should consider the ranging function, the usage of symmetric channel property, the used algorithms in BS. Sunghyun Hwang, ETRI


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