Ultra-Wideband (UWB 2):
Physical Layer Options and Receiver
Structures
Outline of Course
Fundamentals
1. Fundamentals of short/medium range
wireless communication 1
–
–
–
2.
Fundamentals of short/medium range
wireless communication 2
–
–
–
3.
digital transmission systems
equivalent baseband model
digital modulation and ML-detection
fading channels
diversity
MIMO wireless
Fundamentals of short/medium range
wireless communication 3
–
Multicarrier modulation and OFDM
Systems I: OFDM based broadband access
4. WLAN 1: IEEE 802.11g, a
5. WLAN 2: IEEE 802.11n
6. Vehicular Networks
Systems II: Wireless short range access
technolgies and systems
7. UWB 1: Promises and challenges of Ultra
Wideband Systems
8. UWB 2: Physical Layer options
9. Wireless Body Area Network case study:
UWB based human motion tracking
10. The IEEE 802.15x family of Wireless
Personal Area Networks (WPAN):
•
•
•
Bluetooth,
ZigBee,
UWB
Systems III: RF identification (RFID) and
sensor networks
12. RFID 1
13. RFID 2
14. RFID 3
15. Summary and Conclusions
Communication Technology Laboratory
Wireless Communication Group
2
Outline
•
Physical Layer Options
– UWB Impulse Radio
– Direct Sequence UWB
– UWB Multiband
•
Receiver Structures
– RAKE Receiver
– Transmitted Reference Receiver
– Energy Detector
•
Appendix
– UWB Multiband
– IEEE 802.15.3a Multipath Model
3
Ultra-Wideband Impulse Radio
(UWB-IR)
4
UWB-IR:
Modulation and MA Options
- Modulation schemes: PPM, BPSK (BPAM), PAM, OOK, …
- MA schemes: TH-MA, DS-MA, …
5
Peer-to-Peer Scenario:
• In the following, we discuss UWB-IR modulation
schemes in peer-to-peer communication:
One transmitter
One receiver
• Only one transmitter and one receiver
• No interferer
 No need for a MA scheme.
Picture from [Weisenhorn, IZS, 2004]
6
Similarities Among UWB-IR Systems:
• Application of very short duration pulses with 2ns  Tp  0.13ns ,
occupying a very large bandwidth of 500MHz  Bp  7.5GHz.
• In contrast to UWB-MB, the whole band is used in one block.
• Each symbol consists of
Tp
pulses g (t )  Repetition coding
g (t )
Time
• One pulse per frame ( Tf )
• Very low duty cycle
7
Most Popular UWB-IR Modulations:
Binary Pulse Position Modulation
(BPPM)
Symbol ‘0’ Symbol ‘1’
Binary Pulse Amplitude Modulation
(BPAM)
(Binary Phase Shift Keying (BPSK))
Symbol ‘-1’
Time
Time
Symbol ‘1’

s t  
 N f 1
  g t  kT  jT
k  j 0
s
f
 bk  ,
Ts  N f  Tf , Tf  2 and bk  0,1.
• Modulation of pulse position
• Extension to any M-ary PPM
possible with: Tf  M 
s t  

N f 1
  b g t  kT
k  j 0
Ts  N f  Tf
and
k
s
 jT f

bk  1
• Modulation of pulse polarity
Pictures from [Giannakis, CEWIT, 2003]
8
Other Types of PAM:
Pulse Amplitude Modulation (PAM)
On-Off Keying (OOK)
Symbol ‘1’
Symbol ‘0’
Time
Symbol ‘2’
s t  
Symbol ‘1’
 N f 1
  b g t  kT  jT 
k  j 0
k
Ts  N f  Tf and
s
Time
f
s t  
 N f 1
  b g t  kT
k  j 0
bk 1,2
k
s
 jT f

Ts  N f  Tf and bk 0,1
• Modulation of amplitude
• Extension to any M-ary PAM possible
Pictures from [Giannakis, CEWIT, 2003]
9
Example of BPPM:
s t  
 N f 1
  g t  kT  jT
s
k  j 0
s(t)
t
Tf
bk  0
s(t)
bk  1
 bk  
g(t)
• transmitting
• transmitting
f
Ts

t
Tf
Ts
Ts  N f  T f i.e. Ts  4  T f
T f  M   i.e. T f  2  
10
Example of BPAM:
s t  

N f 1
  b g t  kT
k  j 0
s(t)
• transmitting
 jT f

g(t)
t
Ts
s(t)
bk  1
s
Tf
bk  1
• transmitting
k
t
Tf
Ts
Ts  N f  Tf i.e. Ts  4  Tf
11
Uncoordinated Multiple Access Scenario:
 Multiple access (MA) scheme required
to reduce interference!
[Weisenhorn, IZS, 2004]
12
Direct Sequence Spread Spectrum (DSSS):
Conventional Principle
DSSS signal
Data signal
„Chip“ sequence
Time domain
Frequency domain
Data signal
convolution
*
Pseudo-Random sequence
Spread data signal
13
Direct Sequence in UWB-IR (1):
DS data signal
„Chip“ sequence
Data signal
Spectral Lines due to Rep. Coding
1
1
1
1
Time domain
Frequency domain
Data signal
*
Pseudo-Random sequence
Randomized data signal
14
Direct Sequence in UWB-IR (2):
s ( A)  t  
 b  g t  kT  jT  c
k 

s
( B)
N f 1

(A)
k
f
( A)
j
Note: can also be
combined with PPM
N f 1
t    bk  g t  kTs  jTf  c(jB)
 B
k 
c 
c 
j 0
s
j 0
( A)
j
User A specific binary pseudo-random sequence (PN) of length
Nf
(B)
j
User B specific binary pseudo-random sequence (PN) of length
Nf
User A
User B
Tf
...
...
Time
Pictures from [Giannakis, CEWIT, 2003]
15
DS-UWB Compared to DSSS:
• DS in UWB-IR is very similar to DSSS in conventional
systems:
– Data bit is spread over multiple consecutive pulses.
– Pseudo-random code is used to separate users (MA).
– Spectrum is smoothed very efficiently.
but:
– In UWB-IR-DS the code rate equals the pulse rate.
 Spectrum is not significantly spread by the DS.
16
Time-Hopping Multiple Access:
s ( A)  t  
s( B) t  
( A)
( A)
b
g
t

kT

jT

c

 k 
s
f
j Tc 
k 
j 0

N f 1
 b  g t  kT  jT
k 
c 
c 
N f 1

( B)
k
j 0
s
f
Note: can also be
combined with PPM
 c(j B )Tc 
( A)
j
User A specific Tf / Tc  -ary pseudo-random sequence (PN) of length N f
(B)
j
User B specific Tf / Tc -ary pseudo-random sequence (PN) of length
User A
Nf
User B
Tf  5  Tc
...
...
Time
Pictures from [Giannakis, CEWIT, 2003]
17
Time-Hopping Properties:
• Data bit is spread over multiple consecutive pulses.
• Pseudo-random code is used to separate users (MA).
• User separation also possible in non-coherent
receivers such as the energy detector.
• Spectral smoothening not as effective as with DS.
18
UWB Receivers
Outline
 Matched filter
 Receiver structures

Rake

Transmitted reference

Energy detector
20
Communication Technology Laboratory – Wireless Communications Group
20
Introduction
 Pulse based UWB
 Transmitter and receiver for UWB are said to be very simple due to
no need of Mixers, RF Oscillators and PLLs
 For transmitters this assumption holds probably
 But receivers are probably more complex as often assumed since
energy has to be captured from all multipaths
21
Communication Technology Laboratory – Wireless Communications Group
21
Matched Filter
 Transmission of a single pulse s(t) with duration T
 n(t) is a white Gaussian noise process of zero mean and power spectral
density N0/2
 Receiver consists of a linear time-invariant filter g(t) and a sampler
n(t )
t= T
single pulse
s (t )
g (t )
y (t )
y (T )
receiver
The matched filter g(t) = s(-t+T) is a time reversed and delayed version of the
input signal s(t). It maximizes the SNR at the sampling instant T.
22
Communication Technology Laboratory – Wireless Communications Group
22
UWB System with Multipath Channel
TX
RX
s (t )
1
t
TP
r (t ) = h(t )* s (t )+ n(t )
h(t )
1
- 1
- 1
(noiseless)
23
Communication Technology Laboratory – Wireless Communications Group
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Matched Filter for Multipath Channel I

Optimum receiver: correlator or matched filter
g (t )= s (t )* h(t )
n (t )
s (t )
h (t )

r (t )
N
g (t )= s (t )* h (t )= s (t )* å hi ×d(t - t i )=
i= 1
r t 
y
N
å
hi ×s (t - t i )
i= 1
y1  t 
s  t  1 
h1
+

y
yN t 
s t   N 
hN
24
Communication Technology Laboratory – Wireless Communications Group
24
Matched Filter for Multipath Channel II
r t 
y1  t 
s  t  1 
h1
+

y
yN t 
s t   N 
r t 
hN

y1
s  t  1 
h1

s t   N 
+
y
yN
correlator in
each branch
„RAKE“
hN
25
Communication Technology Laboratory – Wireless Communications Group
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ARAKE (All RAKE)

Optimum receiver with unlimited resources

Combines all N resolved multipath components

Number of resolvable components N increases with bandwidth => large number of
RAKE fingers
0.6
N
yA   hk   r  t   s  t   k  dt
0.4
k 1
t
0.2
0
-0.2
-0.4
-0.6
-0.8
0
500
1000
1500
2000
Communication Technology Laboratory – Wireless Communications Group
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26
SRAKE (selective RAKE)

Also referred as selection combining (SC)

Only subset of resolved multipath components is processed

Selects the L strongest paths

Better performance than a single path receiver

Requires the knowledge of the instantaneous values of all multipath components
0.6
0.4
yS   hk   r  t   s  t   k  dt
0.2
kL
0
t
L  indices of L strongest paths
-0.2
-0.4
-0.6
L=6
-0.8
0
500
1000
1500
27
2000
Communication Technology Laboratory – Wireless Communications Group
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Impact on the design of WBAN´s
Number of RAKE fingers using a SRAKE

Antennas placed on the front side of the body in
15cm steps

Collecting 75% of the whole energy
(front side measurements)
2 fingers @ 15cm

20 fingers @ 90cm
1
0.8
P/Ptotal

Short distance multihop increases the energy
that can be captured with a simple RAKE
0.6
15cm
30cm
45cm
60cm
90cm
0.4
0.2
0
5
10
15
Number of strongest paths
20
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Communication Technology Laboratory – Wireless Communications Group
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PRAKE (Partial RAKE)

Sometimes also referred as nonselective combining (NSC)

Collects the energy from the M first multipath components

These multipath components must not be the best, e.g. in NLOS environment

Compared to SRAKE no selection mechanism is required

Needs only to find the first M multipath components => complexity reduction
0.6
0.4
0.2
M
yP   hk   r  t   s  t   k  dt
0
k 1
t
-0.2
-0.4
-0.6
-0.8
0
M=6
29
500
1000
1500
2000
Communication Technology Laboratory – Wireless Communications Group
29
Selective nonselective Combining (SC-NSC)

Only the strongest path is tracked

The K-1 paths following the strongest path are chosen for the remaining path delays

SC-NSC is better suited for NLOS channels (where the direct path with the shortest
delay, i.e. the first path, is attenuated) than PRAKE/NSC since the strongest path can
be tracked
0.6
0.4
0.2
yP 
0
k0  K 1

k  k0
hk   r  t   s  t   k  dt
t
k0  arg max  hk
-0.2
k

-0.4
-0.6
K=6
-0.8
0
500
1000
1500
2000
Communication Technology Laboratory – Wireless Communications Group
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30
Conclusions on Rake Receivers
 ARAKE is an optimum receiver

Realization of a matched filter

High complexity
 Complexity reduction by using only a fraction of all paths

Performance degradation
 Channel estimation necessary

Amplitudes and delays have to be known
 Simpler receiver structures without channel estimation would be desirable
31
Communication Technology Laboratory – Wireless Communications Group
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Transmitted Reference Receiver
32
Communication Technology Laboratory – Wireless Communications Group
32
Principles of Transmitted Reference Systems
 2 pulses (=1 doublet) are transmitted for one symbol

1st pulse is the reference pulse, which is used as template

2nd pulse is the data pulse
 Implicit channel estimation since both pulses pass the same channel
 Channel has to be invariant over 1 doublet only
 BPF required for noise reduction
 Noisy template for correlation
 Information rate usually drops by 50 % since half of the pulses are used as
reference
33
Communication Technology Laboratory – Wireless Communications Group
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TR PAM I
 Information in the amplitude of the data pulse
Reference pulses
a 1
TP
a0
t
T
TP
T
Data pulses
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Communication Technology Laboratory – Wireless Communications Group
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TR PAM II
 TX energy higher since two pulses are needed for 1 bit
 Correlation of reference and data pulse
r (t )

BPF
T Tp
T
T Tp
d
 r t   r t  T  dt
T
T
 Performance depends on the time of integration Tp
 Performance degradation if inter-pulse interference exists
35
Communication Technology Laboratory – Wireless Communications Group
35
Integration Duration I


BER performance depends on the integration duration

If integration duration is too short, not enough energy can be captured

If integration duration is too long, the CIR is decayed so much that the noise term gets dominant
Channel models from IEEE 802.15.3a

LOS (Line of Sight)

NLOS (Non-Line of Sight)
LOS
NLOS
1.2
0.2
0.15
0.9
0.1
0.6
0.05
0.3
0
-0.05
0
-0.1
-0.3
-0.6
0
-0.15
20
40
60
[ns]
80
100
-0.2
0
20
40
Communication Technology Laboratory – Wireless Communications Group
60
[ns]
80
100
36
36
Integration Duration II
 Body area network measurements around the torso
 In general, shorter integration duration for LOS links than for NLOS
37
Communication Technology Laboratory – Wireless Communications Group
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Energy Detector
38
Communication Technology Laboratory – Wireless Communications Group
38
Energy Detector I
 Energy detector (ED) collects energy from multipaths

Integrates the energy of the receive signal
 Non-coherent receiver structure

No antipodal signaling possible, e.g. BPSK

Usually used with pulse position modulation (PPM)
 No explicit channel estimation is necessary
 Begin and end of the integration interval has to be known
39
Communication Technology Laboratory – Wireless Communications Group
39
Energy Detector II
Pulse Position Modulation (2 PPM)
a0
kT
a 1
 k  1T
TI
TI
TP
r (t )
BPF
t
2
TP

kT T p
kT

kT TI Tp
kT TI
kT TI Tp

dk 
r  t  dt 
2
kT TI
kT T p

r 2  t dt
kT
1, if d k  0
aˆk  
 0, otherwise
 Integration of noise on the position where no pulse is located
40
Communication Technology Laboratory – Wireless Communications Group
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Performance Comparison
 Real measured channels around the human body (15cm distance  quasi LOS)
0
10
TR
MF
ED
-1
BER
10
-2
10
-3
10
-4
10
0
2
4
8
6
E /N
b
10
12
14
0
 Performance of TR and ED similar but about 6 dB worse than MF
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Communication Technology Laboratory – Wireless Communications Group
41
Conclusions
 TR and ED are much simpler than an ARAKE

No explicit channel estimation necessary
 Position of the receive signal in time domain has to be known accurately
 Performance is worse than ARAKE

Performance strongly depends on integration duration

Integration duration is for LOS channels usually shorter than for NLOS
channels
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Communication Technology Laboratory – Wireless Communications Group
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Appendices
UWB Multiband (Certified Wireless USB)
IEEE 802.15.3a Multipath Model
43
Ultra-Wideband Multi-Band
(UWB-MB)
44
UWB-Multiband OFDM
•
•
•
•
Spectrum is divided into sub-bands
Serial transmission over the sub-bands
Application of TF codes for piconet separation
Strongly promoted by industry (Wireless USB,
WiMedia, MBOA)
45
ECMA-368 (MB-OFDM Standard): Basic Idea
• Split overall spectrum into 14 bands of 524MHz
bandwidth
• Serial transmission of OFDM symbols over the bands
• OFDM symbol:
– 128 point FFT/IFFT independent of data rate
– Modulation: QPSK or DCM
• Information is coded across several bands (TF codes) to
achieve frequency diversity and piconet separation.
• Zero-padded suffix:
– Robustness against multi-path
– Time to switch band
46
ECMA-368: Bandplan
• Overall band of 7.5GHz is split into14 bands:
– Bandwidth: 524MHz
– Separation: 524MHz
• Bands are grouped into 5 band groups
• Several TF codes for each band group  several piconets
• Band groups are managed by FDMA:
– Better SOP performance
[ECMA-386, 2006]
47
Code Map of Band Group 1:
Fixed Frequency Interleaved Channels (FFI)
Time Frequency Interleaved Channels (TFI)
[ECMA-386, 2006]
48
EMCA-368: Rate Independent Parameters
• 8 OFDM tones are set to zero.
[ECMA-386, 2006]
49
ECMA-368: Rate Dependent Paramters
[ECMA-386, 2006]
50
QPSK versus DCM:
• Quadrature Phase Shift Keying (QPSK)
• Dual Carrier Modulation (DCM):
– Two different 16-QAM mappings
– Two different carriers
– Frequency diversity
Subcarrier 1
4 bits
Subcarrier 50
[ECMA-386, 2006]
51
Should One Go Multi-Band?
•
•
Pros
–
Flexible band selection
•
•
–
–
–
–
Easy to fit to spectral masks
NBI mitigation
Power efficient
Implementation by COTS
Suitable for IC integration
Suited and strongly promoted for
HDR systems (e.g. Wireless USB)
Cons
–
–
–
–
Not low complexity
Not low power
High rate sampling
Small advantage over other
systems, e.g. 802.11n
[Giannakis, CEWIT, 2003]
52
IEEE 802.15.3a Multipath Model
53
IEEE 802.15.3a Multipath Model (1)
The proposed model uses the following definitions:
Tl = the arrival time of the first path of the l-th cluster
k,l = the delay of the k-the path within the l-th cluster relative to the first path arrival time Tl
 = cluster arrival rate
 = ray arrival rate, i.e., the arrival rate of path within each cluster
.
t
54
IEEE 802.15.3a Multipath Model (2)
•
•
•
•
Log-normal (rather than Rayleigh) distribution for the multipath gain magnitude
Independent fading assumed for each cluster as well as each ray within the cluster
Real valued passband model
Target channels: CM 1 (LOS 0-4m), CM 2 (NLOS 0-4m), CM 3 NLOS 4-10m, CM 4
(Extreme NLOS)
Discrete time impulse responses:
L
K
hi (t )  X i  ki ,l  (t  Tl i   ki ,l )
l 0 k 0
Xi
 ki ,l
Shadowing coefficient
Tl i
Delay of the lth cluster
 ki ,l
Delay of the kth multipath component relative to the lth cluster arrival time
Multipath gain coefficients
Tl i
55
IEEE 802.15.3a Multipath Model (3)
•
Shadowing coefficient
•
Multipath gain coefficients
– Multipath amplitude sign
– Ray power
Exponential decay
20log10( X i )  Normal(0, x2 )
 k ,l  pk ,l l  k ,l
(  k , l  n1  n2 ) / 20
 l  k ,l  10
n1  Normal(0, 12 ) Cluster fading
n2  Normal(0, 22 )Ray fading
pk ,l  1 (equiprobable)
20log10(l  k ,l )  Normal( k ,l , 12   22 )
 k ,l 
10ln( 0 )  10Tl /   10 k ,l / 
ln(10)

( 12   22 ) ln(10)
20
2
 / 
E   l  k ,l    0 e Tl /  e k ,l


•
Poisson cluster and ray arrival
p Tl Tl 1    exp   Tl  Tl 1   ,



 0 mean energy of the first
path of the first cluster
l 0

p  k ,l  ( k 1),l   exp    k ,l   ( k 1),l  ,
k 0
56
IEEE 802.15.3a Multipath Model (4)
Target Channel
Characteristics
Mean excess delay (nsec) (  m )
CM 1
CM 2
CM 3
5.05
10.38
14.18
RMS delay (nsec) (  rms )
5.28
8.03
14.28
CM 4
25
NP10dB
NP (85%)
24
36.1
35
61.54
Model Parameters
 (1/nsec)
 (1/nsec)


 1 (dB)
0.0233
2.5
7.1
4.3
3.3941
0.4
0.5
5.5
6.7
3.3941
0.0667
2.1
14.00
7.9
3.3941
0.0667
2.1
24.00
12
3.3941
3.3941
3.3941
3.3941
3.3941
3
3
3
3
Model Characteristics
Mean excess delay (nsec) (  m )
5.0
9.9
15.9
30.1
RMS delay (nsec) (  rms )
5
8
15
25
NP10dB
NP (85%)
Channel energy mean (dB)
Channel energy std (dB)
12.5
20.8
-0.4
2.9
15.3
33.9
-0.5
3.1
24.9
64.7
0.0
3.1
41.2
123.3
0.3
2.7
 2 (dB)
 x (dB)
57