www.mobilevce.com © 2004 mobile vce november 2005 synchronisation for multimode terminals prof....
TRANSCRIPT
www.mobilevce.com
© 2004 Mobile VCE
November 2005
Synchronisation for Multimode Terminals
Prof. Steve McLaughlinUniversity of Bristol
Dr Chris WilliamsUniversity of Bristol
www.mobilevce.com
© 2004 Mobile VCE
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
www.mobilevce.com
© 2004 Mobile VCE
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
www.mobilevce.com
© 2004 Mobile VCE
The Channel
www.mobilevce.com
© 2004 Mobile VCE
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
www.mobilevce.com
© 2004 Mobile VCE
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
-200 -150 -100 -50 0 50 100 150 2002.6
2.8
3
3.2
3.4
3.6
3.8
4
4.2
4.4
4.6Centre 1, FDB 2
Direction of Arrival/
Tim
e of
Arr
ival
/ s
-133
-128
-123
-118
-113
-108
2.6 2.8 3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.60
0.5
1
1.5
2
2.5
3
3.5
4
4.5x 10
-6 Multipath power delay profile
Delay (us)
Pat
h am
plitu
de
www.mobilevce.com
© 2004 Mobile VCE
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
www.mobilevce.com
© 2004 Mobile VCE
OFDM Synchronisation
www.mobilevce.com
© 2004 Mobile VCE
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 =2k/N (k-carrier index, -time offset, N-FFT size).
Negative timing error tolerable up to Nyquist limit (density of pilots, 1 in 12).
Current symbol Next symbol Previous symbol
Timing window too advanced
Timing window OK
Timing window too delayed
ISI
CP CP CP
www.mobilevce.com
© 2004 Mobile VCE
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)
Transmitter signal
Receiver signal
www.mobilevce.com
© 2004 Mobile VCE
Pre-FFT Synchronisation
Use structural features
Guard band (frequency)
Guard interval/cyclic prefix (CP)
Imposed structure (repeated symbol), e.g. WLAN
CP A A IA IA IA B B B B IB C C
(a) Broadcast burst preamble
CP C C
(b) Downlink burst preamble
CP B B B B IB C C
(c) Uplink burst preamble (short)
CP B B B B B B B B B IB C C
(d) Uplink burst preamble (long), and direct link burst preamble
www.mobilevce.com
© 2004 Mobile VCE
Pre-FFT Synchronisation Methods
Beek ML in AWGN Correlation between repeated cyclic prefix Time and frequency estimate Simplify energy correction term
22
2
122
1*
)()(2
1)(
)()()(
:
)())(2cos()(),(
ns
s
Nm
mku
Nm
mku
g
g
Nkrkrm
Nkrkrm
where
ff
www.mobilevce.com
© 2004 Mobile VCE
Basic Correlation Technique
MLF derived for AWGN channel by Beek
DVB-T : N=2048, G=512
Focus on timing estimate error
0
100
200
300
400
500
600
0 200 400 600 800 1000 1200 1400
Sample
MLF
0
100
200
300
400
500
600
700
0 200 400 600 800 1000 1200 1400
Sample
MLE
0
200
400
600
800
1000
1200
0 200 400 600 800 1000 1200 1400
Sample
MLE
AWGN Multipath
A=0.4
Multipath
A=1.5
Tg
www.mobilevce.com
© 2004 Mobile VCE
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)
-1500
-1000
-500
0
500
1000
1500
0 200 400 600 800 1000 1200 1400
Sample
MLE/
Diff
Derivative
Smoothed
derivative
Linear projection (opt)
www.mobilevce.com
© 2004 Mobile VCE
Timing Estimate Performance
www.mobilevce.com
© 2004 Mobile VCE
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
www.mobilevce.com
© 2004 Mobile VCE
Performance – Eb/N0
SFN power=0dB, SFN delay=31 samples
0
20
40
60
80
100
120
140
0 5 10 15 20 25
Eb/No (dB)
Mea
n t
imin
g e
rro
r (s
amp
les) Beek
Derivative
2nd D - zero
0
20
40
60
80
100
0 5 10 15 20 25
Eb/No (dB)
Tim
ing
err
or
std
. d
ev.
(sam
ple
s)
Beek
Derivative
2nd D - zero
www.mobilevce.com
© 2004 Mobile VCE
Performance - SFN delay
SFN relative power = 0dB, Eb/N0=20dB
-20
0
20
40
60
80
100
120
0 20 40 60 80
SFN delay (samples)
Mea
n t
imin
g e
rro
r (s
amp
les) Beek
Derivative
2nd D - zero
0
20
40
60
80
100
120
140
0 20 40 60 80
SFN delay (samples)
Tim
ing
err
or
std
. d
ev.
(sam
ple
s)
Beek
Derivative
2nd D - zero
Maximum multipath delay exceeds guard interval
www.mobilevce.com
© 2004 Mobile VCE
System Level Performance
Run performance simulations Equaliser can have a large impact on the results
0.00001
0.0001
0.001
0.01
0.1
1
5 10 15 20
Eb/No (dB)
BE
R
Beek
Derivative
No correction0.000001
0.00001
0.0001
0.001
0.01
0.1
1
0 10 20 30 40 50 60 70
Delay (samples)
BE
R
Beek
Derivative
No correction
www.mobilevce.com
© 2004 Mobile VCE
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.
www.mobilevce.com
© 2004 Mobile VCE
Improving Performance
www.mobilevce.com
© 2004 Mobile VCE
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
G U
www.mobilevce.com
© 2004 Mobile VCE
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
Timing EstimatorRules detection &
replacementMedian/FIR filters
Timing Estimate
From correlator A(tE) B(tE) C(tE)
For ‘after’ replacement
For ‘before’ replacement
www.mobilevce.com
© 2004 Mobile VCE
Application of Rules
-40
-30
-20
-10
0
10
20
30
40
50
0 5 10 15 20 25
Eb/No (dB)
Mea
n t
imin
g e
rro
r (s
am
ple
s)
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25
Eb/No (dB)
Tim
ing
err
or
std
. dev
. (sa
mp
les)
-40
-20
0
20
40
60
80
0 5 10 15 20 25
Eb/No (dB)
Mea
n t
imin
g e
rro
r
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25
Eb/No (dB)
Mea
n t
imin
g e
rro
r
-40
-30
-20
-10
0
10
20
30
40
50
0 5 10 15 20 25
Eb/No (dB)
Mea
n t
imin
g e
rro
r
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25
Eb/No (dB)
Mea
n t
imin
g e
rro
r
-40
-30
-20
-10
0
10
20
30
40
50
0 5 10 15 20 25
Eb/No (dB)
Mea
n t
imin
g e
rro
r
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25
Eb/No (dB)
Mea
n t
imin
g e
rro
r-40
-20
0
20
40
60
80
0 5 10 15 20 25
No rules (short)
Hard limit (short)
Before filter (short)
After filter (short)
Beek (short)
No rules (Long)
Hard limit (long)
Before filter (long)
After filter (long)
Beek (long)
No RulesHard limitReplace estimate with previous pre-filter valueReplace estimate with previous post-filter value
All 3 rules are used consistently
www.mobilevce.com
© 2004 Mobile VCE
Rules – SFN delay
Suppression of variance increase when multipath delay exceeds CP length
Little loss in performance with short filter
0
5
10
15
20
25
30
35
40
0 10 20 30 40 50 60
Delay (samples)
Tim
ing
err
or
std
. d
ev.
No rules (Long)
Hard limit (long)
Before filter (long)
After filter (long)
Beek (long)
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60
Delay (samples)
Tim
ing
err
or
std
. d
ev.
No rules (short)
Hard limit (short)
Before filter (short)
After filter (short)
Beek (short)
After filter (long)
www.mobilevce.com
© 2004 Mobile VCE
Frequency Estimation and Mobility
www.mobilevce.com
© 2004 Mobile VCE
Mobility Limitations
In environments with multipath clusters spatially separated, it may be possible to increase mobility by synchronisation to each cluster
Time & frequency
Cellular
Cellular
Tx1
Tx2
Doppler Spectrum (single antenna)
Frequency{ {
Spread forTx1
Spread forTx2
Combined spread
Doppler Spectrum
Frequency{
Combined spread (afterremoving individual
shifts)
www.mobilevce.com
© 2004 Mobile VCE
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
www.mobilevce.com
© 2004 Mobile VCE
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)
www.mobilevce.com
© 2004 Mobile VCE
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)
www.mobilevce.com
© 2004 Mobile VCE
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
www.mobilevce.com
© 2004 Mobile VCE
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)
www.mobilevce.com
© 2004 Mobile VCE
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
www.mobilevce.com
© 2004 Mobile VCE
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
www.mobilevce.com
© 2004 Mobile VCE
Thank you !
For further information please contact:
Dr Chris WilliamsE-mail: [email protected]: +44 117 331 5049
www.mobilevce.com
© 2004 Mobile VCE
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)
Cluster (i) path distribution
Path (j)
Fd,max-Fd,max
j
(i)
Doppler spread (i) = Fd,max*(i/360)
Doppler shift (j) = Fd,max * cos((j))