application of pulse compression technique to generate ... · • independently from the chirp the...

UWB Forum at Graz University of Technology 05 May 2011 - Graz, Austria

Application of pulse compression technique togenerate IEEE 802.15.4a-compliant UWB IR

pulse with increased energy per bit

Tamas Istvan Krebesz

Dept. of Measurement and Inf. Systems

Budapest Univ. of Tech. and Economics

Budapest, HUNGARY

The Faculty of Information Technology,

Pazmany Peter Catholic University,

Budapest, HUNGARY

Low coverage of UWB IR is caused by the limited energy per bit transmitted

Main questions to be answered?

• What pose limitations on energy per bit?

• What techniques can be used to increase the bit energy?

Budapest University of Technology and Economics


CONTENTS:

1. Main features of Ultra-WideBand Impulse Radio and their consequences

2. Pulse compression in a LOS noise free channel

3. Pulse compression in multipath channel

4. Noncoherent receivers for UWB applications

5. Windowing of UWB pulse to suppress unwanted sidelobes

6. Conclusions



1 Main features of Ultra-WideBand Impulse Radio and

their consequences

• duration of UWB pulses set extremely short (nanosec)⇒ ultra-wideband carrier (min. 500 MHz)⇒ one pulse carries low Eb and so low coverage (few meters)1

⇒ the use of real UWB applications may be prevented

How can Eb be increased?

• increasing the pulse power- limitations imposed by FCC

• increasing the pulse duration- limitation imposed by IEEE 802.15.4a

1G. Kolumban, F. C. M. Lau, and C. K. Tse, ”UWB radio: From an idea to implementations,” invited tutorialat 2010 IEEE International Conference on Ultra-Wideband, in Proc. of ICUWB’10 Tutorial Session, Nanjing China,September 20–23, 2010.



1.1 Regulation of the Federal Communications Commission

• The FCC Regulations impose a limit on both the peak and average powersof UWB carrier pulses transmitted

• The low-data rate UWB systems, considered here are peak power limited 2

• The FCC peak power limit says 3: ”There is a limit on the peak level of theemissions contained within a 50-MHz bandwidth centered on the frequencyat which the highest radiated emission occurs · · · That limit is 0 dBm EIRP”

Note: the FCC peak power limit is not directly applied to the modulatedUWB signals, instead, the output of a bandpass filter is specified

2K. Witrisal et. al., “Noncoherent Ultra-Wideband Systems: An Overview of Recent Research Activities,” IEEE

Signal Processing Magazine, vol. 26, no. 4, pp. 48–66, July 20093Federal Communications Commission, Part 15 of the Commission Rs Rules Regarding Ultra-Wideband

Transmission Systems; Subpart F, FCC–USA, Online: <http://sujan.hallikainen.org/FCC/FccRules/2009/15/>



1.2 IEEE 802.15.4a Standard

• IEEE 802.15.4a Standard 4 allows the use of chirp for UWB carrier

• Independently from the chirp the Standard defines the transmitted pulseshape to be constrained by the shape of its cross-correlation function with aroot raised cosine pulse with roll-off factor β = 0.6:

Note: The above two specificationsare considered here while Eb isincreased by the application ofpulse compression technique

Root-raised cosine reference pulse:

−1 −0.5 0 0.5 1

x 10−8

−0.2

0

0.2

0.4

0.6

0.8

1

Time [s]R

efer

ence

pul

se a

mpl

itude

4IEEE Std 802.15.4a-2007, IEEE Computer Society, LAN/MAN Standards Committee, 2007



2 Pulse compression in a LOS noise free channel

• In UWB IR the coverage is limited by the low Eb

How should the Eb be increased while peak power cannot be higher and theresolution coming from short duration should be kept?

• The problem is similar when high-resolution radar with long range has to beimplemented

Solution: an FM modulated pulse with long duration is transmitted toassure a high pulse energy and the duration of received signal iscompressed by a matched filter to achieve the required resolution

It will be shown: • the pulse duration may be increased considerably pro-vided that the FCC bandwidth requirements and FCCpower limits are met

• the correlation requirement of IEEE St. 802.15.4a withthe prescribed pulse after compression is met



2.1 Basic idea of pulse compression

• FM is applied to the carrier frequency of the transmitter to generate theradiated pulse xt(t)

• the received signal xr(t) is fed into a matched filter characterized by itsimpulse response h(t)

• the compressed pulse xcomp(t) appears at the output of matched filter

Rampsignal

FMmodulator

Matchedfilterh(t)

MultipathAWGNchannel

xcomp(t)xr(t)xt(t)

• Linear chirp is applied: f(t) = f0 + µt f0 - start frequency, µ - chirp rate

• relation between RF BW of pulse and chip duration Tc: µ = (2B)/Tc

• impulse response of matched filter: h(t) = cos{2π[

f0(Tc − t) + µ2(Tc − t)2

]

}



2.2 Design equations

• determine UWB chirp pulse duration Tc(=100 ns) considering low probabilityof signal collision but assure a large Eb and so coverage

• determination of pulse duration Tcomp(=2.43 ns) (width of main lobe) ofUWB chirp pulse after pulse compression, i.e. the matched filter output

• assure that the IEEE 802.15.4a standard specification on the cross-correlationis satisfied, i.e. main lobe of xcorr is greater than 0.8

• attainable peak power is determined by FCC peak power limit, 1 mW

The pulse compression rate is: R = 2Tc

Tcomp

From R the chirp rate is: µ =R

Tc

Recall: µ =2B

Tc

⇒ RF BW can be calculated



2.3 Waveforms in UWB chirp IR system - transmitter

• UWB chirp pulse is a constant envelope signal, the duration is 100 ns

• Its spectrum is smooth and free from spikes

• FCC peak power limit is checked at the output of a 50 MHz filter

Spectrum of the UWB chirp pulse

4 4.5 5 5.5 6

x 109

10−4

10−3

10−2

10−1

100

101

Frequency (Hz)

Nor

mal

ized

pow

er

Output of the FCC filter

1 2 3 4 5 6 7 8 9

x 10−8

−0.25

−0.2

−0.15

−0.1

−0.05

0

0.05

0.1

0.15

0.2

0.25

Time [s]

50 M

Hz

FC

C fi

lter

outp

ut



2.3 Waveforms in UWB chirp IR system - receiver

• the duration of the received UWB pulse is compressed by a matched filter

• the compressed UWB chirp has a sinc-like envelope, its correlation with theIEEE reference pulse exeeds 0.8

−5 0 5

x 10−9

−80

−60

−40

−20

0

20

40

60

80

Time [s]

Rel

ativ

e am

plitu

de to

rad

iate

d pu

lse

The compressed UWB chirp pulse

at the output of matched filter

−1 −0.5 0 0.5 1

x 10−8

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Time [s]C

ross

−co

rrel

atio

n m

agni

tude

Cross-correlation of the IEEE reference pulse

with the envelope of compressed UWB chirp pulse



3 Pulse compression in multipath channel

3.1 Noise-free three-ray multipath channel

Path No. Gain Excess delay

#1 0 dB 0 ns

#2 -3 dB 15 ns

#3 -6 dB 50 ns

• the duration of the radiated UWB chirp pulse is 100 ns

⇒ the three received pulses overlap each other:

0 0.5 1 1.5 2

x 10−7

−2.5

−2

−1.5

−1

−0.5

0

0.5

1

1.5

2

2.5

Time [s]

Norm

alize

d amp

litude

to 1st re

ceive

d wav

eform



3.1 Noise-free three-ray multipath channel (cont.-ed)

• let the overlapped signal be processed by a matched filter

• the pulses are well separated in the time domain

• even the parameters of the multipath channel can be identified

0 0.5 1 1.5 2

x 10−7

−2.5

−2

−1.5

−1

−0.5

0

0.5

1

1.5

2

2.5

Time [s]

No

rma

lize

d a

mp

litu

de

to

1st

re

ceiv

ed

wa

vefo

rm

0 0.5 1 1.5 2 2.5 3

x 10−7

−80

−60

−40

−20

0

20

40

60

80

Time [s]

Re

lativ

e a

mp

litu

de

to

1st

re

ceiv

ed

wa

vefo

rm



3.2 Noisy three-ray multipath channel

• UWB communication established in a noisy three-ray multipath channel

• Signal-to-Noise Ratio (SNR) is -3.4 dB

• the other parameters of the multipath channel are the same as in thenoise-free case

0 0.5 1 1.5 2

x 10−7

−6

−4

−2

0

2

4

6

Time [s]

Nor

mal

ized

am

plitu

de to

1st

rec

eive

d w

avef

orm

Received signal in the noisy three-ray channel

2 3 4 5 6 7 8

x 109

10−3

10−2

10−1

100

Frequency (Hz)

Nor

mal

ized

pow

er

Spectrum of the received signal



3.2 Noisy three-ray multipath channel (cont.-ed)

• the noisy signal processed by a matched filter

• the matched filter compresses the UWB pulse in time and due to theprocessing gain of pulse compression it improves the SNR considerably

• the three received UWB pulse components that are hidden by channel noiseand multipath propagation become clearly distinguishable

0 0.5 1 1.5 2

x 10−7

−6

−4

−2

0

2

4

6

Time [s]

Nor

mal

ized

am

plitu

de to

1st

rece

ived

wav

efor

m

Received signal in the noisy three-ray channel

0 0.5 1 1.5 2 2.5 3

x 10−7

−80

−60

−40

−20

0

20

40

60

80

Time [s]

Rel

ativ

e am

plitu

de to

1st

rece

ived

wav

efor

m

Compressed UWB chirp signal



4 Noncoherent receivers for UWB applications• coherent detectors are not feasible in UWB impulse radio 5

• one possibility is the use of an envelope detector

• the envelop detector placed after the matched filter

• each component of the received signal is well separated in the time domaindespite the bad propagation condition

0 0.5 1 1.5 2 2.5 3

x 10−7

0

10

20

30

40

50

60

70

80

90

Time [s]

Relat

ive am

plitud

e to 1

st rece

ived w

avefo

rm

5K. Witrisal et. al., “Noncoherent Ultra-Wideband Systems: An Overview of Recent Research Activities,” IEEESignal Processing Magazine, vol. 26, no. 4, pp. 48–66, July 2009



5 Windowing of UWB pulse to suppress unwanted sidelobes• as a consequence of the application of pulse compression technique unwanted

sidelobes appear in the time function of the compressed signal

• the cross correlation of the sidelobes with the IEEE reference pulse shouldnot exceed 0.3 otherwise the peak sidelobe level has to be controlled bywindowing

• a 3-term Blackmann Harris window is used here for demonstration

0 0.2 0.4 0.6 0.8 1

x 10−7

−1

−0.8

−0.6

−0.4

−0.2

0

0.2

0.4

0.6

0.8

1

Time [s]

Norm

alize

d am

plitu

de

Transmitted UWB chirp signal windowed by 3-term Blackmann Harris window



5.1 Effects of the windowing

• Sidelobes are significantly reduced

⇒ in radar applications it is important to avoid false detection

⇒ in UWB communication it can also be considered as a loss in Eb

• the envelope of the UWB chirp signal is not constant anymore therefore linear amplifier has

to be used

• the main lobe of the UWB chirp signal becomes wider so the resolution is reduced

• the duration of the windowed compressed

signal is 2.5 times longer compared to the

compressed signal without windowing

• the longer compressed pulse duration re-

sults in worse resolution at the receiver

but no side lobes appear−5 0 5

x 10−9

−80

−60

−40

−20

0

20

40

60

80

Time [s]

Relat

ive a

mpli

tude

to ra

diate

d wa

vefo

rm

Windowed UWB chirp signal after compression



6 Conclusions• increasing the coverage is a must in UWB IR radio → the contradiction among the (i)

large energy per bit, (ii) short duration and (iii) limited peak power is resolved by pulse

compression

The pulse compression of UWB IR carriers has four important advantages:

• the duration of radiated UWB chirp pulse can be increased considerably and the enlarged

Eb assures a large radio coverage

• due to the pulse compression, compatibility with IEEE Std. 802.15.4a is preserved

• in multipath channels the UWB pulses overlap each other. After compression the overlapped

UWB chirp pulses become separated

• pulse compression has a processing gain that improves the signal-to-noise ratio.

Disadvantages:

• pulse compression always generates sidebands that can be reduced by windowing

⇒ windowing introduce an amplitude modulation and the windowed UWB chirp pulses will

not be a constant envelope signal anymore


application of pulse compression technique to generate ... · • independently from the chirp the...

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