Synchronization for OFDMA System

1. Synchronization for OFDMA System Student: 劉耀鈞 Advisor: Prof. D. W. Lin Time: 2006/3/16

2. Outline • Reference • Introduction to IEEE 802.16-2004 WirelessMan_OFDMA PHY • Synchronization • Downlink synchronization • Uplink synchronization

3. Reference • IEEE Std 802.16-2004, IEEE Standard for Local and Metropolitan Area Networks — Part 16: Air Interface for Fixed Broadband Wireless Access Systems. New York: IEEE, October 2004 • Lin, Meng Ting, “ Fixed and Mobile Wireless Communication Based on 802.16aTDD OFDMA: Transmission Filtering and Synchronization,” June 2003

4. Introduction to IEEE 802.16-2004 WirelessMan_OFDMA PHY WirelessMAN-OFDMA PHY : • Based on OFDM modulation • Designed for NLOS operation in the frequency bands below 11 GHz

5. OFDMA symbol description and parameters • Time domain description : • Tb : useful symbol time • CP:A copy of the last Tgof the useful symbol period, used to collect multipath, while maintaining the orthogonality of the tones

6. OFDMA symbol description and parameters (cont.) • Frequency domain description: An OFDMA symbol is made up of subcarriers: Data subcarriers : for data transmission Pilot subcarriers : for various estimation purposes Null subcarriers : no transmission at all ,for guard bands and DC carrier

7. OFDMA symbol description and parameters (cont.)

8. OFDMA symbol description and parameters (cont.) • Primitive parameters: — BW: This is the nominal channel bandwidth. — Nused: Number of used subcarriers (which includes the DC subcarrier). — n: Sampling factor. This parameter, in conjunction with BW and Nused determines the subcarrier spacing, and the useful symbol time. This value is set to 8/7 as follows: for channel bandwidths that are a multiple of 1.75MHz then n=8/7 else for channel bandwidths that are a multiple of any of 1.25,1.5, 2 or 2.75 MHz then n=28/25 else for channel bandwidths not otherwise specified then n=8/7. — G: This is the ratio of CP time to “useful” time.

9. OFDMA symbol description and parameters (cont.) • Derived parameters: —NFFT : Smallest power of two greater than Nused — Sampling Frequency: Fs = floor(n⋅BW⁄ 8000)× 8000 — Subcarrier spacing: Δf =Fs/ NFFT — Useful symbol time: Tb= 1 ⁄ Δf — CP Time: Tg = G ⋅ Tb — OFDMA Symbol Time: Ts = Tb+ Tg — Sampling time: Tb⁄ NFFT

10. Frame Structure

11. Frame Structure • DL_MAP/UL_MAP : define the access to the DL/UL information (ex: burst profiles, allocation in the subchannel and time axes of the bursts) • TTG (Tx/Rx transition gap) , RTG (Rx/Tx transition gap)

12. Synchronization • Accurate demodulation and detection of an OFDM signal requires carrier orthogonality. • Variations of the carrier oscillator, sample clock or the symbol time affect the orthogonality of the system. • The sample clocks of the users and the base station are assumed to be identical. • Before an OFDM receiver can demodulate the carriers, it has to perform timing synchronization and frequency synchronization.

13. Time offset and frequency offset • If the time offset is smaller than the length of the guard interval minus the length of the channel impulse response, then the orthogonality among carriers is maintained. No ISI and ICI • For large time offset, ISI and ICI occur. • In practical OFDM systems, frequency offset due to oscillator mismatch usually exists between the transmitter and the receiver.

14. Time offset and frequency offset (cont.) • In a mobile environment, the Doppler spread creates similar effects as for the case when all the transmitters have different frequency offsets. • Three effects of a frequency offset: - The amplitude of the FFT output are reduced. - ICI is introduced from other carriers which are now no longer orthogonal. - Introduce a common phase rotation of the subcarriers.  resolved by a channel estimator

15. DL Synchronization • Two synchronization conditions: initial synchronization and normal synchronization.

16. DL Synchronization (cont.) • Initial synchronization • Perform at transmission start • Perform when normal sync failed • Normal synchronization • After initial sync, so we can predict next frame start time • Three kinds of useable information for synchronization: guard interval, pilot carriers (including preamble), and the guard bands.

17. Initial Synchronization • Initial sync should contain 4 stages. • Stage1: symbol time synchronization • Stage2: fractional frequency synchronization • Stage3: integer frequency synchronization • Stage4: frame synchronization

18. Stage I: Symbol Time Synchronization • Use the maximum likelihood criterion to estimate the time and frequency offset. • Under the assumption that received samples are jointly Gaussian, symbol time offset is given by

19. Stage I: Symbol Time Synchronization (cont.) where

20. Stage I: Symbol Time Synchronization (cont.)

21. Stage II: Fractional Frequency Synchronization (cont.) • The ML estimator of the fractional frequency offset is given by • The frequency offset results in an sinusoidal wave in the time domain, and thus the received samples are multiplied by .

22. Stage II: Fractional Frequency Synchronization (cont.) • In AWGN channel, the received sample in the guard time is and the sample in the last part of the useful time is where s(k) is the transmitted signal, N is the FFT size, and n(k) is the noise.

23. Stage III: Integer Frequency Synchronization • The first step is to determine the received OFDM symbol is transmitted from the BS (DL) or other SSs (UL). • DL guard carriers: -1024~-852 and 1024~852 UL guard carriers: -1024~-849 and 1024~849 • A threshold can be set and if any of the carriers {-849, -850, -851, 849, 850, 851} is larger than the threshold, the SS will regard the symbol as the DL symbol.

24. Stage III: Integer Frequency Synchronization (cont.)

25. Stage III: Integer Frequency Synchronization (cont.) • All the guard carriers are checked to see if any of them exceeds the threshold. • If the carrier is detected to be larger than the threshold in the checking procedure, the check is stopped and the frequency is corrected. • An additional check is added to see whether both of the ±851st pilot carriers are larger than the threshold.

26. Stage IV: Frame Synchronization • The FFT outputs are correlated with these 7 possible cases of the reference data. • A frame is determined to start if there are three successive DL symbols with the maximum correlation corresponding to the preamble.

27. Normal Synchronization • After finishing the initial synchronization, the subscriber is able to extract the transmission parameters from the DL MAPs and UL MAPs. • With these parameters, the SS can roughly predict the next symbol and frame start time, so as the normal timing synchronization can be simplified.

28. UL Synchronization • Assume a successful initial synchronization and ranging  low frequency and time offset • No frequency synchronization is done in UL normal transmission • No complex frame synchronization at initialization • BS has to detect the exact UL symbol arrival time ISI free

29. UL Synchronization (cont.) • Using IFFT of the UL preamble in the time domain • Two stages • First stage: use the timing part of the joint ML estimator to detect roughly the symbol start time. • Second stage: use the received signal and the known UL preamble information to find the timing of the first arrival signal.

30. UL Synchronization (cont.) • Using guard interval is not accurate enough  find the exact instant of the first arriving signal

31. UL Synchronization (cont.) • For the UL preamble, the transmitted value of each carrier is given in the BS. • Thus the signal transmitted by each SS in the UL preamble is deterministic and the BS can produce the same signals as all SSs by taking IFFT. • The received samples are correlated with the reference data string which is the IFFT output according to the subchannels used by each SS.

32. UL Synchronization (cont.)

33. UL Synchronization (cont.) • As the user arrival time may vary as much as 50% of the guard interval, we stop the correlation up to 50% of the guard interval earlier than the corresponding detected useful time.

34. UL Synchronization (cont.) • We can find the peak location of each correlator which uses a distinct preamble, then we can know the peak locations of different SSs. • Finally, we compare all these peaks and get the start location of the first coming signal.