Ultra-Wideband (UWB 2): Physical Layer Options and Receiver Structures.

Ultra-Wideband (UWB 2): Physical Layer Options and Receiver

Structures

2Communication Technology LaboratoryWireless Communication Group

Outline of Course

Fundamentals

1. Fundamentals of short/medium range wireless communication 1

– digital transmission systems– equivalent baseband model– digital modulation and ML-detection


– fading channels– diversity– MIMO wireless


– 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

3

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

4

Ultra-Wideband Impulse Radio (UWB-IR)

5

UWB-IR:

Modulation and MA Options

- Modulation schemes: PPM, BPSK (BPAM), PAM, OOK, …

- MA schemes: TH-MA, DS-MA, …

6

Peer-to-Peer Scenario:

• In the following, we discuss UWB-IR modulation schemes in peer-to-peer communication:

• Only one transmitter and one receiver• No interferer

No need for a MA scheme.

One transmitter One receiver

Picture from [Weisenhorn, IZS, 2004]

7

Similarities Among UWB-IR Systems:

• Application of very short duration pulses with , occupying a very large bandwidth of .

• In contrast to UWB-MB, the whole band is used in one block.

• Each symbol consists of pulses Repetition coding

• One pulse per frame ( )• Very low duty cycle

500MHz 7.5GHzpB

Time

( )g t

2ns 0.13nspT

fT

pT ( )g t

8

Most Popular UWB-IR Modulations:

Binary Pulse Position Modulation (BPPM)

Binary Pulse Amplitude Modulation (BPAM)(Binary Phase Shift Keying (BPSK))

Time

Symbol ‘’

Symbol ‘-1’

Time

Symbol ‘1’

1

0

,fN

s f kk j

s t g t kT jT b

,s f fT N T 2fT and

1

0

fN

k s fk j

s t b g t kT jT

s f fT N T and 1kb

• Modulation of pulse position• Extension to any M-ary PPM possible with:

• Modulation of pulse polarity

Pictures from [Giannakis, CEWIT, 2003]

Symbol ‘’

fT M

0,1 .kb

9

Other Types of PAM:

Pulse Amplitude Modulation (PAM)

Time

Symbol ‘’

Symbol ‘’

1

0

fN

k s fk j

s t b g t kT jT

s f fT N T 1,2kb and

On-Off Keying (OOK)

Time

Symbol ‘’

Symbol ‘’

1

0

fN

k s fk j

s t b g t kT jT

0,1kb s f fT N T and


• Modulation of amplitude• Extension to any M-ary PAM possible

10

Example of BPPM:

1

0

fN

s f kk j

s t g t kT jT b

g(t)

. . 4

. . 2

s f f s f

f f

T N T i e T T

T M i e T

• transmitting

• transmitting

Ts

t

s(t)

1kb

0kb

Ts

Tf

t

s(t)

Tf

11

Example of BPAM:

Tf

Ts

t• transmitting

• transmitting

Ts

t

g(t)

s(t)

s(t)

1kb

. . 4s f f s fT N T i e T T

Tf

1kb

1

0

fN

k s fk j

s t b g t kT jT

12

Uncoordinated Multiple Access Scenario:

[Weisenhorn, IZS, 2004]

Multiple access (MA) scheme requiredto reduce interference!

Multiple access (MA) scheme requiredto reduce interference!

13

Direct Sequence Spread Spectrum (DSSS): Conventional Principle

Data signal

Pseudo-Random sequence

Spread data signal

Time domain

Frequency domain

Data signal „Chip“ sequence

DSSS signal

*

convolution

14

Direct Sequence in UWB-IR (1):

Data signal

Pseudo-Random sequence

Randomized data signal

Time domain

Frequency domain

Data signal „Chip“ sequence

DS data signal

*

1 1 1 1

Spectral Lines due to Rep. Coding

User B specific binary pseudo-random sequence (PN) of length

User A specific binary pseudo-random sequence (PN) of length

15

Direct Sequence in UWB-IR (2):

1

(A)

0

( ) ( )fN

k s fA A

jk j

s t b g t kT jT c

1

( ) ( )

0

fNB

k sB

f jk j

Bs t b g t kT jT c

Time

... ...

User A User B

fT

( )jAc fN

( )jBc fN

( )jAc

( )jBc


Note: can also be combined with PPM

16

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.

17

Time-Hopping Multiple Access:

1

( )

0

( ) ( )f

A AN

Ak s f j c

k j

s t b g t kT jT c T

1

( )

0

( ) ( )f

B BN

Bk s f j c

k j

s t b g t kT jT c T

Time

... ...

User A User B

5f cT T

User A specific -ary pseudo-random sequence (PN) of length ( )jAc fN

( )jBc User B specific -ary pseudo-random sequence (PN) of length fN


Note: can also be combined with PPM

/f cT T

/f cT T

18

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.

UWB Receivers

Communication Technology Laboratory – Wireless Communications Group

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20

Outline

Matched filter

Receiver structures

Rake

Transmitted reference

Energy detector


21

21

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


22

22

Matched Filter

( )s t

( )n t

( )g t

t T=

( )y T( )y t

single pulse

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

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.


23

23

UWB System with Multipath Channel

TXTX RXRX

t

( )s t ( ) ( ) ( ) ( )r t h t s t n t= * +( )h t

PT ( )noiseless

1

1-

1

1-


24

24

Matched Filter for Multipath Channel I Optimum receiver: correlator or matched filter

( )s t ( )h t( )r t

( )n t ( ) ( ) ( )g t s t h t= *

y

( ) ( ) ( ) ( ) ( ) ( )1 1

N N

i i i ii i

g t s t h t s t h t h s td t t= =

= * = * × - = × -å å

1h

Nh

+ y 1s t

Ns t

r t 1y t

Ny t


25

25

Matched Filter for Multipath Channel II

1h

Nh

+ y 1s t

Ns t

r t1y

Ny

1h

Nh

+ y 1s t

Ns t

r t 1y t

Ny t

correlator in each branch

„RAKE“


26

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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

1

N

A k kk t

y h r t s t dt

0 500 1000 1500 2000-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6


27

27

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

L = 6

L

L indices of strongest paths

S k kk t

y h r t s t dt

L

0 500 1000 1500 2000-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6


28

28

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

0 5 10 15 20

0.2

0.4

0.6

0.8

1

Number of strongest paths

P/P

tota

l

15cm30cm45cm60cm90cm

Short distance multihop increases the energy

that can be captured with a simple RAKE

Short distance multihop increases the energy

that can be captured with a simple RAKE


29

29

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

M = 6

1

M

P k kk t

y h r t s t dt

0 500 1000 1500 2000-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6


30

30

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

K = 6

0

0

1

0 arg max

k K

P k kk k t

kk

y h r t s t dt

k h

0 500 1000 1500 2000-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6


31

31

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


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32

Transmitted Reference Receiver


33

33

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


34

34

TR PAM I

PT

T

PT

T

0a 1a

t

Reference pulses

Data pulses

Information in the amplitude of the data pulse


35

35

TR PAM II

TX energy higher since two pulses are needed for 1 bit

Correlation of reference and data pulse

pT T

T

( )r t

pT T

T

d r t r t T dt

T

BPFBPF

Performance depends on the time of integration p

Performance degradation if inter-pulse interference exists


36

36

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)

0 20 40 60 80 100-0.6

-0.3

0

0.3

0.6

0.9

1.2

[ns]0 20 40 60 80 100

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

[ns]

LOS NLOS


37

37

Integration Duration II

Body area network measurements around the torso

In general, shorter integration duration for LOS links than for NLOS


38

38

Energy Detector


39

39

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


40

40

Energy Detector II

1, if 0ˆ

0, otherwisek

k

da

pkT T

kT

( )r t

2 2I p p

I

kT T T kT T

k

kT T kT

d r t dt r t dt

BPFBPF2

I p

I

kT T T

kT T

PT

ITkT

PTIT 1k T

Pulse Position Modulation (2 PPM)

0a 1a

t

Integration of noise on the position where no pulse is located


41

41

Performance Comparison

Real measured channels around the human body (15cm distance quasi LOS)

0 2 4 6 8 10 12 1410

-4

10-3

10-2

10-1

100

Eb/N

0

BE

R

TRMFED

Performance of TR and ED similar but about 6 dB worse than MF


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42

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

43

AppendicesUWB Multiband (Certified Wireless USB)

IEEE 802.15.3a Multipath Model

44

Ultra-Wideband Multi-Band (UWB-MB)

45

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)

46

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

47

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]

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Code Map of Band Group 1:

Fixed Frequency Interleaved Channels (FFI)

Time Frequency Interleaved Channels (TFI)

[ECMA-386, 2006]

49

EMCA-368: Rate Independent Parameters

• 8 OFDM tones are set to zero.

[ECMA-386, 2006]

50

ECMA-368: Rate Dependent Paramters

[ECMA-386, 2006]

51

QPSK versus DCM:

• Quadrature Phase Shift Keying (QPSK)• Dual Carrier Modulation (DCM):

– Two different 16-QAM mappings– Two different carriers– Frequency diversity

Subcarrier 1

Subcarrier 50

[ECMA-386, 2006]

4 bits

52

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]

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IEEE 802.15.3a Multipath Model

54

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

55


• 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

l

K

k

ilk

il

ilkii TtXth

0 0,, )()(

ilk , Multipath gain coefficients

ilT Delay of the lth cluster

Delay of the kth multipath component relative to the lth cluster arrival time

Shadowing coefficient

ilk , i

lT

iX

56


• Shadowing coefficient

• Multipath gain coefficients

– Multipath amplitude sign

– Ray power

Exponential decay

• Poisson cluster and ray arrival

lkllklk p ,,,

),0(Normal)(10log20 2xiX

, 1 (equiprobable)k lp

),(Normal)(10log20 22

21,, lklkl

//0

2

,,lkl eeE T

lkl

20

)10ln()(

)10ln(

/10/10)ln(10 22

21,0

,

lkl

lk

T

1 1

, ( 1), , ( 1),

exp , 0

exp , 0

l l l l

k l k l k l k l

p T T T T l

p k

20/)(

,21,10

nn

lkllk

),0Normal( 211 n

),0Normal( 222 n

Cluster fadingRay fading

0 mean energy of the first path of the first cluster

57

Target Channel Characteristics

CM 1 CM 2 CM 3 CM 4

Mean excess delay (nsec) ( m ) 5.05 10.38 14.18

RMS delay (nsec) ( rms ) 5.28 8.03 14.28 25 NP10dB 35 NP (85%) 24 36.1 61.54 Model Parameters (1/nsec) 0.0233 0.4 0.0667 0.0667 (1/nsec) 2.5 0.5 2.1 2.1 7.1 5.5 14.00 24.00 4.3 6.7 7.9 12

1 (dB) 3.3941 3.3941 3.3941 3.3941

2 (dB) 3.3941 3.3941 3.3941 3.3941

x (dB) 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 12.5 15.3 24.9 41.2 NP (85%) 20.8 33.9 64.7 123.3 Channel energy mean (dB) -0.4 -0.5 0.0 0.3 Channel energy std (dB) 2.9 3.1 3.1 2.7


Ultra-Wideband (UWB 2): Physical Layer Options and Receiver Structures.

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