UWB flexible assets in radio, access, and network …acts.ing.uniroma1.it/Talks/080327-DiBenedetto-WPNC.pdfUWB flexible assets in radio, access, and network design INFOCOM Department

WPNC 2008

The UWB Groupat the

University of RomeLa Sapienza

UWB UWB flexible assets flexible assets ininradio, access, and networkradio, access, and network

designdesign

INFOCOM DepartmentINFOCOM DepartmentUniversity of Rome La SapienzaUniversity of Rome La Sapienza

ItalyItaly

Maria-Gabriella Di BenedettoMaria-Gabriella Di Benedetto

WPNC 2008

The UWB Groupat the


OutlineOutline

UWB Impulse Radio features and framework ofapplication

Flexible radio

Flexible MAC

Flexible routing

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UWB Impulse Radio featuresUWB Impulse Radio features

Impulse radio UWB signals are obtained by transmittingvery short pulses with typically no Radio Frequenciesmodulation

(In communication systems, “very short” refers to a duration of thepulse that is typically about a few hundred picoseconds)

This technique goes under the name of Impulse Impulse Radio (IR)Radio (IR)

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Time duration of a pulse is smaller than original symbol duration

Contrarily to conventional Spread Spectrum, increased bandwidth isnot provoked by spreading sequences, but rather by the

VeryVery short shortpulsepulse

Ultra-WideUltra-WideBandwidthBandwidth

energy is spread over a large bandwidth

extremely short pulse duration that induces ultra-wide bandwidth

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Impulse Radio systems differ in terms of modulation and coding

User data can modulate pulse amplitude with binary antipodal variations(Pulse Amplitude Modulation PAM)

User data can turn pulses on and off(On Off Keying OOK)

User data can dither pulse position(Pulse Position Modulation PPM)

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On top of modulation, symbols are encoded using Time Hopping THcodes for pulse shaping and user differentiation

0 0.2 0.4 0.6 0.8 1 1.2 1.4

x 10-7

-1

-0.5

0

0.5

1

1.5x 10

-3

Time [s]

Amplitude [V]

0 0.2 0.4 0.6 0.8 1 1.2 1.4

x 10-7

-1

-0.5

0

0.5

1

1.5x 10

-3

Time [s]

Amplitude [V]

0 1 2 3 4 5 6 7 8 9 10

x 109

10-30

10-25

10-20

10-15

Frequency [Hz]

PSD in logarithmic units

0 1 2 3 4 5 6 7 8 9 10

x 109

10-40

10-35

10-30

10-25

10-20

10-15

10-10

Frequency [Hz]

PSD in logarithmic units

PPM IR UWB PPM IR UWB signal without codingsignal without coding

PPM IR UWB PPM IR UWB signal with signal with TH TH codingcoding

Power Power Spectral Spectral DensityDensity

Power Power Spectral Spectral DensityDensity


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0 0.2 0.4 0.6 0.8 1 1.2 1.4

x 10-7

-1

-0.5

0

0.5

1

1.5x 10

-3

Time [s]

Amplitude [V]

�

s(t) = PTXTS pw (t ! jTS ! c j ! a j")j

#

PTX is the average transmitted powerTS is the pulse repetition periodpw(t) is the energy-normalized pulse shapecj<TS is the TH code value for pulse jaj is the data symbol carried by pulse jε is the PPM shift

We focus on a typical signal format: Time Hopping coding TH andbinary Pulse Position Modulation PPM

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UWB Impulse Radio framework of applicationUWB Impulse Radio framework of application

Standard for low-rate WPANs:

multi-month to multi-year battery life

data rates of 20-250 kbps

power management for low powerconsumption

low complexity

.4.4

.4.4aaSame as above, plus:

location enabled: high precisionranging/location (at least 1 meteraccuracy)

ultra low power

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UWB Impulse Radio in the IEEE 802.15.4a standardUWB Impulse Radio in the IEEE 802.15.4a standard

In March 2005, TG4a selected two optional PHYs:

– UWB Impulse Radio (unlicensed UWB spectrum)

– Chirp Spread Spectrum (unlicensed 2.4 GHz spectrum)

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A Flexible RadioA Flexible Radio

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Flexible Flexible radio design: radio design: example example of of applicationapplication

• Different waveforms can be selected for transmission.• Waveforms lead to different spectral shapes for the transmitted signals, so that

the UWB signal can be adapted to different interference scenarios.• We suppose up to 6 possible different pulses

! """=j jjSwSTX acjTtpTPts )()( #

w = w = 1, 1, ……, 6, 6

M.-G. Di Benedetto, G. Giancola and M.D. Di Benedetto, " Introducing Consciousness in UWBnetworks by Hybrid Modelling of Admission Control," Mobile Networks and Applications,vol.11 no.4, pp. 521-534 ISSN: 1383-469X, ACM/Springer Journal on Mobile Networks andApplications, Special Issue on "Ultra Wide Band for Sensor Networks" , 2006.

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Scenario: a self-organizing network of low-power, low cost and lowrate IR-UWB devices (IEEE 802.15.4a like devices)

802.15.4anetwork

Multi User

Interference

External

Interference

External

Interference

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Frequency

W1(f) W2(f) W3(f) W4(f) W5(f) W6(f)

Energy Spectral Density

6 pulses are available for transmission

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• All nodes communicate through oneelected node, denoted as the ConsciousNode of the network (CNode)

• The CNode plays the role of networkcoordinator

CNode

ActiveNode

Active

NodeActive

Node

• Time Hopping (TH) coding is used for identifying users• Power control at the CNode is assumed for all uplink connections

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Performance of a link between one active node and the CNode is expressed by

�

BER = (1/2)erfc SNR /2( )

�

SNR =1

Rb

PRX

!p (w) + "m

2(w)(N #1)PRX

PTX is the average received powerRb is the bit rateηp(w) is the variance of noise (thermal noise + external interference)collected for one single pulse [w = 1,2,…,6]σm

2(w) is a Multi User Interference weight [w = 1,2,…,6]N is the number of active users

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Evaluation Evaluation of of transmission parameterstransmission parameters

• Based on environment sensing, CNode estimates the ηp(w) andσm

2(w)

• Then, it computes the value of the minimum power Pmin that mustbe received from each node in order to guarantee SNR ≥ SNR0(minimum reference value required for synchronization)

�

Pmin (w) =!p (w)

TS

1

SNR0

"#m

2(w)(N "1)TS

$

% &

'

( )

�

SNR =1

Rb

PRX

!p (w) + "m

2(w)(N #1)PRX

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Evaluation Evaluation of of transmission parameterstransmission parameters

• Since Pmin depends on the adopted waveform, the waveformpw*(t) that better adapts to the environment is the one leading tothe smallest Pmin(w) value

• CNode can thus determine:– the waveform pw*(t) to be used by active nodes in the network– the corresponding Pmin(w*)

�

Pmin (w) =!p (w)

TS

1

SNR0

"#m

2(w)(N "1)TS

$

% &

'

( )

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The The three levels three levels of of flexibilityflexibility

IntermediateFlexibility

FullFlexibility

NoFlexibility

The CNode is always capable to select theoptimum pulse shape among the 6 availablewaveforms

The CNode has reduced capabilities, sinceit is capable to select the pulse shape withina sub-set of the 6 available waveforms

The CNode randomly selects a waveform atthe beginning of network operation and doesnot perform any further selection duringnetwork lifetime

We simulated three different types of Cnode

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ResultsResults

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

x 10-3

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8x 10

-9

Time [s]

Energy [J]

No Cognition

Intermediate Cognition

Full Cognition

Interference pattern: 20 external interferers generating narrowband signals located @ 2, 4,6 and 8 GHz switching on and off, with average active and silent time of 100 µs

IntermediateFlexibility

Full Flexibility

No Flexibility

TimeTime

Spen

t Ene

rgy

Spen

t Ene

rgy

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A Flexible MACA Flexible MAC

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IR UWB flexible MAC designIR UWB flexible MAC design• IR Ultra Wide Band based on time hopping and impulse radio is

characterized by:– Low probability of pulse collision– Accurate ranging (theoretical limit far below 1 cm)

Uncoordinated, Wireless, Baseborn medium access for UWBcommunication networks (UWB)2

• Impulse Radio features with respect to MUI and synchronizationform the basis for the definition of an UWB-tailored MAC algorithm

M.-G. Di Benedetto, L. De Nardis, M. Junk,G. Giancola, " (UWB)^2: Uncoordinated, Wireless,Baseborn, medium access control for UWB communication networks," Mobile Networks andApplications, vol. 10 no.5, pp. 663-674. Mobile Networks and Applications special issue onWLAN Optimization at the MAC and Network Levels, 2005.

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(UWB)(UWB)22 MacMac

• For TH-IR, the probability of successful packettransmission for uncoordinated transmission of severalusers and in the presence of MUI is fairly high.

• Based on this result, (UWB)2 was the first MAC protocolto propose a pure Aloha approach

• For synchronization purposes (UWB)2 foresees thepresence of a synchronization trailer in each transmittedpacket The (UWB)2 approach was proposed and adopted with

large majority of votes within the IEEE 802.15.4a group

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(UWB)(UWB)22 MAC: design choices MAC: design choices

Key assumptions Design Choices

TH-CDMA:Common signaling code available

to all devices+

Dedicated data code unique foreach transmitter

No Carrier Sensing: pure Aloha(with TH coding)

Synchronization is achieved on apacket-by-packet basisSimple Synchronization Hardware

Low Data Rate and rare packets(peak rate ≤ 1 Mb/s,

average rate ≤ 20 Kb/s)

Time Hopping Impulse Radio withGHz bandwidth

Need for broadcast packets

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t0+ τ + Δ + τ + Δ

+ τ

Tx Rxt0

t0 + τ

t1=t0+τ +Δ

Time Time

The LE -> LC -> DATA packet exchange allows both Tx and Rx terminals to determine their distance

2 0

2TxRx

t td c c!

" "#= =

3 1

2RxTx

t td c c!

" "#= =

DATA

LE

LC

(UWB)(UWB)22 MAC: Transmission and Ranging procedure MAC: Transmission and Ranging procedure

t2=t0+τ +Δ

t3 + τt0+ τ + Δ + τ + Δ =

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(UWB)(UWB)22: Performance analysis: Performance analysis

• (UWB)2 was tested by simulation in a typical Low Data Rate scenario:– user rates in the range 10-100 kb/s– 1 Mb/s on the wireless channel

• Channel effect and Multi User Interference were taken into account

• Two performance indicators were considered:– Throughput - % of transmitted packets, that are correctly received– Delay - Time gap between first transmission of a packet and its correct

reception, including retransmissions of collided packets– Packet Error Probability

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Throughput and delay as a function of number of terminals

Throughput Delay


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Packet Error Probability as a function of number of terminals

Packet Length: 2000 bitsPulse Rate: 1 Mpulses/sNs = 1TM = 1 ns

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A Flexible networkA Flexible network

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Flexible routingFlexible routing

• Is it feasible to design a routing strategy that adapts its path selectioncriterion to internal and external network conditions?

• What is the impact of such a routing strategy in a power-constrained,interference-prone UWB network, in terms of:

• Network performance• Network lifetime

??M.-G. Di Benedetto and L. De Nardis, " Cognitive routing in UWB networks," invited paper, IEEEInternational Conference on UWB 2006 ICUWB 2006, Boston, Massachusetts, USA, September 24-27, 2006.

UWB UWB allows allows accurate accurate ranging accuracy ranging accuracy and power managementand power management

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1. Define a routing metric that determines the cost of a linkbased on internal and external conditions

2. Select the minimum cost route

�

UWBCost x,y( ) = cSync t( ) ! Sync x,y( ) + cPower t( ) ! Power x,y( ) +

+cMUI t( ) ! MUI x,y( ) + cReliability t( ) ! Reliability x, y( ) +

+cTraffic t( ) !Traffic y( ) + cDelay t( ) !Delay x,y( ) +

+cAutonomy t( ) ! Autonomy y( ) + cCoexistence t( ) !Coexistence y( )

x

y

UWBCost(x,y)

The following metric was defined

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Autonomy term

Coexistence term

�

Autonomy y( ) =1!Residual Energy y( )

Full Energy y( )

�

Coexistence y( ) =Measured External Interference y( )

Maximum Interference y( )

Delay term

�

Delay x,y( ) =1

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Analysis by simulationAnalysis by simulation

• Three different coefficient sets

• Generation of external interferers– fc=3.5 GHz, B=20 MHz, Pt=10 mW– Random activity factor a in the interval (0,1]– Random position– Death/birth of interferers every 100 sec

Coefficient Set 1 Set 2 Set 3

CDelay 1 0.0001 0.0001

CAutonomy 0 1 0

Ccoexistence 0 0 1

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Effect on PowerEffect on Power

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Effect Effect on Multi on Multi User InterferenceUser Interference

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AcknowledgmentAcknowledgment

• This work was partially supported by the FP6Integrated Project PULSERS (PervasiveUltra-wideband Low Spectral Energy RadioSystems) Phase II (contract FP6-027142).

http://www.pulsers.eu/

UWB flexible assets in radio, access, and network …acts.ing.uniroma1.it/Talks/080327-DiBenedetto-WPNC.pdfUWB flexible assets in radio, access, and network design INFOCOM Department

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