November 2005 Synchronisation for Multimode Terminals Prof. Steve McLaughlin University of Bristol

1. November 2005 Synchronisation for Multimode Terminals Prof. Steve McLaughlin University of Bristol Dr Chris Williams University of Bristol

2. Overview • Motivation • Channels • Review of OFDM synchronisation • Robust timing synchronisation in multipath and single frequency networks • Performance of timing estimators • Timing variance reduction • Multiple antennas for synchronisation • Increased mobility results • Conclusions and future directions

3. Motivation • Synchronisation for OFDM (multimode) • Enhance mobility • Particularly for current broadcast standards • Robust in multipath environments • … and single frequency networks • Signals from different transmitters arrive in clusters • Efficient data transmission • Reduction of the required guard time for OFDM • Focus on processing that is common to the different standards • … and make it more efficient / less complex

4. The Channel

5. The Channel - in time • Two classes of channel: • Single transmitter • Multiple (on channel) transmitter – single frequency network for broadcast (OFDM) • Model SFN with independent multipath clusters, with relative delay and power as parameters • For SFN effective delay spread a function of transmitter spacing as well as the environment • Potentially, long effective delay spreads a problem • Clustering also appears in the spatial domain

6. Experimental evidence for multipath clustering, even with single transmitter But typically less than 3 or 4 clusters Urban SIMO trials in Bristol Some clustering evident Can this be exploited? Multipath clusters may not be separable in time Cluster Statistics

7. Spatial Characteristics • Evenly select 12 channels from one measurement run • Search for 1,2 or 3 ‘beams’ to find maximum energy collected related to beam width (5º grid) • e.g. 2 beams of 90 degrees loses less than 1dB • Limited loss for coarser search grid • Doppler spread characteristics related to cluster parameters 1 beam 2 beams 3 beams

8. OFDM Synchronisation

9. Timing Synchronisation • Positive timing error introduces ISI and ICI, so much less tolerance. • Pre-FFT coarse timing correction • Timing offset induces phase offset given by f=2pkD/N (k-carrier index, D-time offset, N-FFT size). • Negative timing error tolerable up to Nyquist limit (density of pilots, 1 in 12).

10. Frequency Synchronisation • Introduces ICI • More critical for OFDM • Integer and fractional parts • Pre-FFT correction for fractional part (NDA) • Post-FFT correction for integer part (NDA/DA)

11. Pre-FFT Synchronisation • Use structural features • Guard band (frequency) • Guard interval/cyclic prefix (CP) • Imposed structure (repeated symbol), e.g. WLAN

12. Pre-FFT Synchronisation Methods • Beek • ML in AWGN • Correlation between repeated cyclic prefix • Time and frequency estimate • Simplify energy correction term

13. Basic Correlation Technique • MLF derived for AWGN channel by Beek • DVB-T : N=2048, G=512 • Focus on timing estimate error Tg AWGN Multipath A=0.4 Multipath A=1.5

14. Derivative Based Methods • Other techniques too dependent on the actual channel characteristics • Derivative of MLE is maximum near first peak, and has edge shortly after (or negative going zero crossing of 2nd-Derivative) Derivative Linear projection (opt) Smoothed derivative

15. Timing Estimate Performance

16. Simulation Parameters • DVB-T System, 2k mode • Pilot structure & coding (RS & convolutional) • Short cyclic prefix: • 64 samples (1/32 useful symbol) • Model (LOS) proposed by Bug • Less impact of the equaliser • Two multipath clusters (2 SFN Tx) • Estimate filters: 15pt median, 16 pt averaging FIR • No ‘rules based’ processing • See deliverables and ICR for NLOS short CP, and long CP results

17. Performance – Eb/N0 • SFN power=0dB, SFN delay=31 samples

18. Performance - SFN delay • SFN relative power = 0dB, Eb/N0=20dB Maximum multipath delay exceeds guard interval

19. System Level Performance • Run performance simulations • Equaliser can have a large impact on the results

20. Benefits • For the same CP length, longer multipath delay spreads can be tolerated without the system becoming synchronisation limited. • In broadcast scenarios, this would allow transmitters to be place further apart, reducing infrastructure costs or giving more flexibility in transmitter positioning. • For new air interface designs, a shorter CP may be used from the view of synchronisation, hence improving spectrum efficiency. • Derivative method is applicable to repeated symbol preambles for summing over half the preamble length, and for OFDMA.

21. Improving Performance

22. Reducing Estimate Variance • Some estimates have large error • … particularly for short cyclic prefix • Have used longer median and FIR filters • Possible to use knowledge of the correlation and derivative peaks to bound the estimates • Correlation peak within CP • 1. Start of symbol before peak • 2. Start of symbol after peak position minus CP • 3. Start of symbol after derivative peak

23. Simulation Parameters • Bug UN2 (NLOS) channel, DVB-T 2k mode • Estimate filters (per symbol): • Short – 5pt median, 8pt FIR • Long – 15pt median, 16pt FIR • Approach with estimate outside bounds: • Hard limit • Replace previous pre-filter estimate • Replace previous post-filter estimate

24. Application of Rules All 3 rules are used consistently Hard limit No Rules Replace estimate with previous pre-filter value Replace estimate with previous post-filter value

25. Rules – SFN delay • Suppression of variance increase when multipath delay exceeds CP length • Little loss in performance with short filter

26. Frequency Estimation and Mobility

27. Mobility Limitations • In environments with multipath clusters spatially separated, it may be possible to increase mobility by synchronisation to each cluster • Time & frequency

28. Channel Considerations • On the premise spatial clusters exist: • Each cluster will have Doppler offset • Doppler spread proportional to angular spread • Greater cluster angular separation in this model implies larger offset differences (& v.v.) • No great angular discrimination required (3-4 antennas OK) • Assumptions weak with local scattering – but unlikely to be travelling fast • Clusters may have time separation, but not always • For different transmitters (SFN), each will have independent carrier offset • Spatial discrimination seems the best way forward

29. The Process • Separate signal into clusters • Estimate frequency (& time) offset for each cluster • Correct each for frequency offset • Combine (weighted?) & pass to FFT • Timing correction before or after combination? • Before – N estimates, each signal individually corrected • Potential to reduce delay spread – easier equalisation or reduced CP length, etc. • After – Combine N estimates, from previous discussion need to choose the earliest one (if branch power exceeds a threshold)

30. Multiple Antenna Processing • How to separate clusters? • Could do DoA estimation & then signal separation • For small terminals may make more sense to have directional elements (on 4/6 edges) & process each non-adaptively • More antennas (directional) – more Doppler spread reduction (but beware!) • In a multimode terminal use MIMO capability (same frequency band?), even if not MIMO processing • With MIMO/diversity processing can still do frequency/time correction prior to FFT (1 for each channel)

31. The Model • AGC on each antenna (equal SNR) • Common timing correction (earliest) • Power weighted signal combining • Sectored antennas • Antennas are co-located, so limited additional diversity gain

32. Performance for different Doppler • Opposed signals can be separated, allowing higher Doppler shifts • 2 equal power clusters, angles 20, -160, angular spread 45, Bug UN2 (Eb/N0=16dB)

33. Conclusions • Method for improved timing synchronisation (patent application filed) • New derivative based method outperforms the peak detection method • System performance limited by equaliser • “Rules” processing reduces variance and may allow shorter estimation filters • Proposed to use multiple antennas to improve mobility • Benefits demonstrated • Performance degraded when a cluster is split between branches • Controlling the directionality and beam width would help • Real channels to be investigated

34. Further Reading • D-WE2.1.1/2.3.1 Architectures, Link Enhancement and Synchronisation Techniques for Multimode Baseband Terminals • Part 2 on fundamentals of synchronisation, and review of synchronisation for OFDM • D-WE2.3.4 Synchronisation for multimode terminals • Comparison of pre-FFT synchronisation methods, including first derivative method • ICR-WE2.3.1 Enhancements to Synchronisation for OFDM • Rules-based enhancements, and second derivative comparison • ‘Robust OFDM timing synchronisation in multipath channels’, submitted to IEEE Trans. Veh. Tech • ‘Robust OFDM timing synchronisation’, Elect. Letts., Vol. 41, No. 13, pp. 751-752, 23 June 2005 • ‘Synchronisation in a receiver ‘, patent application GB0419399.1

35. Thank you ! For further information please contact: Dr Chris Williams E-mail: chris.williams@bristol.ac.uk Tel: +44 117 331 5049

36. Channel paths processed separately Each cluster has mean angular offset & spread Each cluster has Bug power delay profile Angular distribution of paths is Laplacian Path angles change randomly (av. Once every 20 sym) Each path has classical Doppler spectrum Doppler spread is proportional to angular spread (all paths) Doppler offset scaled according to angle of arrival (ea. Path) The Model