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Multimode Terminal - Synchronisation

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Less impact of the equaliser. Two multipath clusters (2 SFN Tx) ... Equaliser can have a large impact on the ... System performance limited by equaliser ' ... – PowerPoint PPT presentation

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Title: Multimode Terminal - Synchronisation


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
Cluster Statistics
  • 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

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
    f2pkD/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 N2048, G512
  • Focus on timing estimate error

Tg
AWGN
Multipath A0.4
Multipath A1.5
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 power0dB, SFN delay31 samples

18
Performance - SFN delay
  • SFN relative power 0dB, Eb/N020dB

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/N016dB)

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_at_bristol.ac.uk Tel 44 117 331
5049
36
The Model
  • 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)
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