Consortium leader PETER PAZMANY CATHOLIC UNIVERSITY€¦ · • It is defined as a technique of...

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 1

Development of Complex Curricula for Molecular Bionics and Infobionics Programs within a consortial* framework**

Consortium leader

PETER PAZMANY CATHOLIC UNIVERSITYConsortium members

SEMMELWEIS UNIVERSITY, DIALOG CAMPUS PUBLISHER

The Project has been realised with the support of the European Union and has been co-financed by the European Social Fund ***

**Molekuláris bionika és Infobionika Szakok tananyagának komplex fejlesztése konzorciumi keretben

***A projekt az Európai Unió támogatásával, az Európai Szociális Alap társfinanszírozásával valósul meg.

PETER PAZMANY

CATHOLIC UNIVERSITY

SEMMELWEIS

UNIVERSITY

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 2

Peter Pazmany Catholic University

Faculty of Information Technology

Ad hoc Sensor Networks

Digital modulation

www.itk.ppke.hu

Érzékelő mobilhálózatok

Digitális moduláció

Dr. Oláh András

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 3

Lecture 3 review• Signal propagation overview

• Path loss models

• Log Normal Shadowing

• Narrowband Fading Model

• Wideband Multipath Channels

Ad hoc Sensor Networks: Digital modulation

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 4


Outline• Advantage of digital modulation• Bandwidth of signals• ISI-free system requirements• IQ modulator• Constellation and “eye” diagrams• Tradeoff between spectral efficiency and power efficiency• Linear and constant envelope modulation scheme• Spread Spectrum Modulation

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 5


Structure of a wireless communications link

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 6


Modulation• It is defined as a technique of mapping the information signal

to a transmission signal (modulated signal) which is bettersuited for the operating medium(i.e. the wireless channel).

• The transmitted radio signal can be described as

• By letting the transmitted information change the amplitude,the frequency, or the phase to carry the information we get thethree basic types of digital modulation techniques:– ASK (Amplitude Shift Keying)– FSK (Frequency Shift Keying)– PSK (Phase Shift Keying)

( ) ( ) ( )( )cos 2s t A t ft tπ= + ΘAmplitude Frequency Phase

Constant amplitude

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 7


Modulation (cont’)

( ) ( ) ( )( )cos 2s t A t ft tπ= + Θ

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 8


Advantage of digital communications• It allows information to be “packetized”

– It can compress information in time and efficiently send as packets through network.

– In contrast, analog modulation requires “circuit-switched” connections that are continuously available.

• Inefficient use of radio channel if there is “dead time” in information flow.

• It allows error correction to be achieved– Less sensitivity to radio channel imperfections.

• It enables compression of information.– More efficient use of channel.

• It supports a wide variety of information content.– Voice, text and email messages, video can all be represented as digital

bit streams.

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 9


Digital modulation• Better performance and more cost effective than analog modulation methods

(AM, FM, etc.)• Performance advantages:

– the digital transceivers are muchcheaper, faster and morepower-efficient thananalog transceivers;

– higher data rates are achieved compared to analog with the same signal bandwidth;– powerful error correction techniques make the signal much less susceptible to noise

and fading, and equalization can be used to mitigate ISI [→see later];– more efficient multiple acces strategies (spread spectrum techniques applied to

digital modulation can remove or combine multipath, resist interference, and detectmultiple users simultaneously);

– better security and privacy for digital systems;– combination of multiple information types (voice, data, & video) in a single

transmission channel;– implementation of modulation/demodulation functions usingDSP software (instead

of hardware circuits).

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 10


Digital modulation (cont’)• Choice of digital modulation scheme• Many types of digital modulation methods → small differences• Performance factors to consider (corresponding metrics)

– low Bit Error Rate (BER) at low S/N (BER performance)– resistance to interference and multipath fading– high data rate– high spectral efficiency (SE [bps/Hz])– easy and cheap implementation of mobile units (Receiver complexity)– transmission power amplifier linearity requirements (linearity)– efficient use of battery power in mobile unit (Power efficiency, PE)

• No existing modulation scheme can simultaneously satisfy all of these requirements.

• Each one is better in some areas with trade-offs of being worse in others.

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 11


Bandwidth of a signal: the conceptMany definitions depending on application. Recall from

DSP course

FCC definition (99%)

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 12


Frequency ranges of a some natural signals

Biological S

ignals

Type of Signal Frequency Range [Hz]

Electroretinogram 0 - 20

Pneumogram 0 - 40

Electrocardiogram (ECG) 0 -100

Electroenchephalogram (EEG) 0 - 100

Electromyogram 10 - 200

Sphygmomanogram 0 - 200

Speech 100 - 4000

Seicmic signalsSeismic exploration signals 10 - 100

Eartquake and nuclear explosion signals 0.01-10

Electromagnetic signals

Radio bradcast 3x104 - 3x106

Common-carrier comm. 3x108 - 3x1010

Infrared 3x1011 - 3x1014

Visible light 3.7x1014 - 7.7x1014

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 13


A simplified communication modellRecall from ICT course

PROBLEM:Bandlimited channel !

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 14


Digital signal transmission over analog channelRecall from ICT course

PROBLEM:1. Nyquist pulseare noncausal and of infinite duration.2. We cannot implement the ideal lowpass filter in practice.3. It decays very slowly (~1/t).

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 15


ISI-free system requirementsRecall from ICT course

0

1 0

0 otherwisel ll

g δ== =

( ) ( )FTg t G f→

( ) ( )FT S1

1n

ng nT G f G f

T T → = + =

∑ 12

fT

≤

Nyquist criterion for ISI-free communication: (Not to mention the ISI caused by the coherence

bandwidth of the wireless channel.)

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 16


ISI-free system requirements (cont’)• Nyquist criterion:

• Observations:– To satisfy the Nyquist criterion, the channel bandwidthB must be at

least 1/(2T)– For the minimum bandwidth the impulse response is Nyquist pulse.– The pulse shapeg(t) fulfills the Nyquist criterion if it is center-

symmetric for 1/2T: (basis pulse shapping)

Recall from ICT course

( ) ( )FT S1

1n

ng nT G f G f

T T → = + =

∑

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 17


ISI-free system requirements (cont’)Nyquist Pulse

( ) ( )Nsin f T

g tf T

ππ

=

B=1/TB ~ 1/T

Raised-Cosine Pulse

( ) ( ) ( )( )RC 2

sin cos

1 2

t T t Tg t

t T t T

π αππ α

=−

α=0

B=(1+α)1/T


2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 18


Nyquist criterion with matched filteringRecall from ICT course

T R

11

n

n nG f G f

T T T + + =

∑

( )( )

R

2

RminB

G fB

G f df−∫

GR(f) = GT(f)* matched filter

( ) ( ) ( ) 2T RG f G f G f=

Uncorrelated ηk :

( ) { } 00k l k

N k lR k E

k lη η−

== = ≠

( ) ( ) ( )20 R 0S f N G f N G fη = =

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 19


Error probability


( )BPSKBER SNR= Φ −

{ } { } ( ) { } ( )( ){ } ( ) ( ){ } ( )

{ } { }0 0 0

ˆ ˆ ˆPr Pr 1 1 1 Pr 1 1 1

Pr sgn 1 1 1 Pr sgn 1 1 1

1 1 1 1Pr 1 0.5 Pr 1 0.5 1

2

BER y y y y p y y y p y

y y p y y y p y

N N N

η η

η η

= ≠ = = = − = − + = − = = =

= + = = − = − + + = − = = =

= ≥ + < − = − Φ + Φ − = Φ −

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 20


• It describes the ability of a modulationtechnique to preserve the quality ofdigital messages at low power levels(low SNR):

required PE = Eb / N0for a certain BER (e.g. 10-3)

where Eb : energy/bit andN0 : noisepower/bit

• Tradeoff between signal power andfidelity:– asEb / N0 ↓, thanBER ↑

• It depends on the particular type ofmodulation employed.

Receiver sensitivity or power efficiency (PE)

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 21


Bandwidth efficiency or spectral efficiency (SE)• Ability of a modulation technique to accommodate data in a

limited BWSE = R / B,

whereR is the data rate,B is the systembandwidth.

• Trade of between R data rate and B bandwidth:– as R ↑, than B ↑

• For a digital signal

ss

1 so as , and R B R T B

T∝ ∝ → ↑ ↑

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 22


M-ary Keying• each pulse or “symbol” havingm finite states represents

n = log2 M bits/symbol– e.g.M = 0 or 1 (2 states)→ n = 1 bit/symbol (binary)– e.g.M = 0, 1, 2, 3, or 4 (4 states)→ n = 2 bits/symbol

• E.g.: when a systemis changed frombinary to 4-ary:– In the case of binary: "0" = - 1V and "1" = 1 V– In the case of 4-ary: "0" = - 1V, "1" = - 0.33V, "2" = 0.33 V, "3" = 1V

• What would be the newdata rate compared to the old data rate ifthe symbol periods were kept constant?

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 23


• Most famous result in communication theory.

– B : bandwidth– C : channel capacity (bps) of real data (not

retransmissions or errors)– To produce error-free transmission, some of the

bit rate will be taken up using retransmissions orextra bits for error control purposes.

– Lower bit error rates from higher power resultsmore real data

– As noise powerPN increases, the bit rate wouldstill be the same, butSEmax decreases.

• SEmax is fundamental limit that cannot beachieved in practice.

Maximum SE: Shannon’s theorem (1948)Recall from ICT course

Claude Elwood Shannon (1916-2001)

S bmax 2 2

N 0

log 1 log 1P EC R

SEB P N B

= = + = +

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 24


Fundamental trade-off between SE and PE• If SE improves thenPE deteriorates (or vice versa)

– One may need to waste more power to get a better data rate.– One may need to use less power (to save on battery life) at the expense of

a lower data rate.

• SE vs. PE is not the only consideration, we use other factors toevaluate, e.g.:– resistance to interference and multipath fading;– easy and cheap implementation in mobile unit;– etc.

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 252011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 25

Ad hoc Sensor Networks: Wireless channel characterization and models

• The canonical form of a band passtransmitted radio signal is

where ej2πft is the carrier factor.• The signals(t) can be written as

• We will define the following quantities

• The complex envelope ofs(t) is nowwritten as

and

Convex envelope theory

( ) ( ) ( )( ) ( ) ( ){ }jj2πfcos Re e e tts t A t t t A tω Θ= + Θ =

( ) ( ) ( )( ) ( ) ( ) ( )( ) ( )cos cos sin sins t A t t t A t t tω ω= Θ − Θ

( ) I Qjs t s s= +ɶ

( ) ( ){ }j2πRe e fts t s t= ɶ

Recall from Chapter 3

( ) ( ) ( )( )( ) ( ) ( )( )

I

Q

cos

sin

s t A t t

s t A t t

= Θ

= Θ

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 26


Constellation diagrams

• Plot I/Q samples on x-y axis• The constellation diagram provides a sense of

how easy it is to distinguish between differentsymbols

• Assign each I/Q symbol to a set of digital bits(eg. Gray code)

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 27


Constellation diagrams

• Noise corrupts sampled I/Q values• The points in the constellation diagram no

longer consist of single dots for each symbol

• What is the best way to match received I/Qsamples with their corresponding symbols?(Detection)

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 28


Constellation diagrams properties• Distance between signals is related to differences in modulation

waveforms– Large distance→ easy to discriminate→ goodBER at lowSNR– Power Efficient related to density

• Occupied BW↓ as number of signal states↑– If we can represent more bits per symbol, then we need less BW for a

given data rate.– Small separation → “dense” → more signal states/symbol → more

information/Hz !!– Bandwidth Efficient

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 29


• Key idea: wrap signal back onto itself inperiodic time intervals and retain alltraces– Similar to the action of an oscilloscope

• Increasing the number of symbolseventually reveals all possible symboltransition trajectories– It shows the ISI present as well as timig

jitter present.• Eye diagram allows visual inspection of

the impact of sample time and decisionboundary choices– Large eye opening implies less

vulnerability to symbol errors

Eye diagrams

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 30


Modulation schemes

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 31


Binary Phase Shift Keying (BPSK)

• Phase transitions force carrier amplitudeto change from “+” to “−”.– Amplitude varies in time.

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 32


Quaternary Phase Shift Keying (QPSK)

• Four different phase states in one symbol period• Two bits of information in each symbol• double theSE of BPSK→ or twice the data rate in

same signal BW• samePE (same BER at specifiedEb/N0)

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 33


Transmit power amplifier

When a modulation signalencounters a nonlinearity the signalbecomes distorted and its occupiedfrequency bandwidth increases(spectrum re-growth). The mostsignificant source of nonlinearitycomes from the transmission PA.

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 34


Offset QPSK

QPSK OQPSK

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 35


Quaternary Phase Shift Keying (QPSK)

• OQPSK ensures there are fewer baseband signaltransitions applied to the RF amplifier, helpseliminate spectrum regrowth after amplification.

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 36


Frequency Shift Keying (FSK)

• Constant Envelope as compared to AM– Linear: Amplitude of the signal varies according to

the message signal.– Constant Envelope: The amplitude of the carrier is

constant, regardless of the variation in the messagesignal. It is the phase that changes.

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 37


M-ary Phase Shift Keying (MPSK)

• The SE↑ with M↑• ThePE ↓ with M↑

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 38


M-ary QAM

• Basic trade-off: Better bandwidth efficiency atthe expense of power efficiency– More bits per symbol time→ better use of

constrained bandwidth– Need much more power to keep constellation

points far enough apart for acceptable bit errorrates.

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 39


M-ary FSK• Frequencies are chosen in a special way so

that they are easily separated at thedemodulator (orthogonality principle).

• M-ary FSK transmitted signals:

– fc = nc / 2T for some integernc– The M transmitted signals are of equal energy

and equal duration

• The SE of an M-ary FSK signal↓ with M↑• The PE↑ with M↑

– Since M signals are orthogonal, there is nocrowding in the signal space

s2( ) cos ( )

0 0,1,...,

i c

Es t n i t

T T

t T i M

π = + ≤ ≤ =

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 40


Modulation scheme comparisons

Given a modulation scheme and a targetedBER then the communication system designercan determine the SE (spectral efficiency) and the PE (Eb/N0 required to maintain theaverageBER target).

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 41


• The transmitter expands (spreads) signal Bs bandwidth many times with a p(t) spreading code and the signal is then collapsed (despread) in receiver side with the same code.

• Other signals created with other codes just appear at the receiver as random noise.• Processing Gain (PG)= Bs /BT

Spread Spectrum Modulation (SSM)

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 42


Spread Spectrum Modulation (SSM) advantage• Resistant to narrowband interference.• It allows multiple users with different codes to share same the

wireless channel– no frequency reuse needed– rejects interference from other users

• It combats multipath fading → if a multipath signal is received with enough delay (more than one chip duration), it also appears like noise.

• As number of simultaneous users ↑ the SE↑

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 43


Spreading codes• Signal spreading is done by multiplying the data signal by a

pseudo-noise (PN) code or sequence– the pseudo-noise signal looks like noise to all observers except those who

know how to recreate the sequence.

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 44


Spreading codes: PN codes• Binary sequence with randomproperties→ noise-like (called

"pseudo-noise" because they are not noise technically)

• ≈ equal #’s of 1’s and 0’s• Very low correlation between time-shifted versions of same

sequence

• Very low cross-correlation between different codes– each user assigned unique code that is approximately orthogonal to all

other codes

– the other users’ signals appear like random noise!

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 45


Type of spread spectrum modulation • Direct Sequence (DS)

– Multiply baseband data by PN code (same as above)– Spread the baseband spectrum over a wide range.– The Rx spread spectrum signal

– where m(t) : the data sequence and p(t) the PN sequence

• Frequency Hopping (FH)– Randomly change fc with time– In effect, this signal stays narrowband but moves around a lot to use a wide band of

frequencies over time.

– Hopset: the set of possible carrier frequencies– Hop duration: the time during between hops– Classified as fast FH or slow FH

• fast FH: more than one frequency hop during each Tx symbol• slow FH : one or more symbol are Tx in the time interval between frequency hops.

( )2( ) ( ) ( )cos 2 si cs

Es t m t p t f t

Tπ θ= +

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 46


Spread spectrum modulation and the multiple access• With Spread Spectrum Modulation, users are able to share a common

band of frequencies yielding a multiple access technique– TDMA: Users share a band of frequencies, but use a different time slot– FDMA: Users share a band of frequencies, but use a different slice of

frequency– SSM enables CDMA (Code Division Multiple Access): Users share a band of

frequencies and a number of time-slots, but each use a different spreading code.

2011.11.28.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 47

Summary• Nyquist criterion for ISI-free communication.• No existing modulation scheme simultaneously satisfies all of

these requirements well.• Given a modulation scheme and a targetedBER then the

communication systemdesigner can determine the SE(spectral efficiency) and the PE (Eb/N0 required to maintain theaverageBER target).

• With Spread SpectrumModulation, users are able to share acommon band of frequencies a multiple access technique(CDMA)

• Next lecture: Detection and channel equalization


Date post:	23-Oct-2020
Category:	Documents
Upload:	others
View:	1 times
Download:	0 times

Consortium leader PETER PAZMANY CATHOLIC UNIVERSITY€¦ · • It is defined as a technique of...

Documents