Title: Multimode Terminal - Synchronisation
1November 2005 Synchronisation for Multimode
Terminals Prof. Steve McLaughlin University of
Bristol Dr Chris Williams University of Bristol
2Overview
- 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
3Motivation
- 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
4The Channel
5The 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
6Cluster 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
7Spatial 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
8OFDM Synchronisation
9Timing 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).
10Frequency 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)
11Pre-FFT Synchronisation
- Use structural features
- Guard band (frequency)
- Guard interval/cyclic prefix (CP)
- Imposed structure (repeated symbol), e.g. WLAN
12Pre-FFT Synchronisation Methods
- Beek
- ML in AWGN
- Correlation between repeated cyclic prefix
- Time and frequency estimate
- Simplify energy correction term
13Basic 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
14Derivative 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
15Timing Estimate Performance
16Simulation 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
17Performance Eb/N0
- SFN power0dB, SFN delay31 samples
18Performance - SFN delay
- SFN relative power 0dB, Eb/N020dB
Maximum multipath delay exceeds guard interval
19System Level Performance
- Run performance simulations
- Equaliser can have a large impact on the results
20Benefits
- 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.
21Improving Performance
22Reducing 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
23Simulation 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
24Application 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
25Rules SFN delay
- Suppression of variance increase when multipath
delay exceeds CP length - Little loss in performance with short filter
26Frequency Estimation and Mobility
27Mobility Limitations
- In environments with multipath clusters spatially
separated, it may be possible to increase
mobility by synchronisation to each cluster - Time frequency
28Channel 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
29The 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)
30Multiple 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)
31The 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
32Performance 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)
33Conclusions
- 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
34Further 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
36The 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)