ISS Wireless digital communications for connected objects …alexandre-boyer.fr/alex/enseignement/ISS_Wireless_digital... · Wireless digital communications for connected objects

Wireless digital communications for connected

objects October 2016

Alexandre Boyer

[email protected] - www.alexandre-boyer.fr

PTP Innovative Smart Systems

Objectives of the course

• Operating of radiofrequency digital emitter-

receivers (baseband processing and radio layers)

• Performance evaluation:– Required bandwidth

2

– Required bandwidth

– Capacity of maximal data rate

– Receiver sensitivity

– Link performance

– Radio range evaluation

– Radio/EMC regulations

Case study: Zigbee and radio interface IEEE 802.15.4 – OQPSK 2400 MHz

Zigbee technology

3

• Low Range-WPAN : low data rate (20 – 250 kbps), low power consumption (≈

20 mA for 0.1 % operating time, few µA in stand-my mode), low range (0 dBm,

10-75 m).

• Designes for Internet of Things requirements.

Course contents

• Typical structure of radiofrequency emitter-receivers

• Digital communications – Basics

• Noise and interferences

4

• Noise and interferences

• Data rate limits

• Link performance

• Radio propagation planning – Evaluation of radiorange

• Radio regulations

Some pre-requisites

ISO/OSI Model

5

• Communication systems may be modelled according to ISO/OSI model

• Composed of 7 layers

• Each layer is responsible of one part of the communication standard and

provides services to higher layers

ISO/OSI model – Zigbee communicating node

Some pre-requisites

6

This course:

IEEE 802.15.4

RFID (13.56MHz)

ISM (434MHz +

868 MHz)

GSMGPS

WiFi

VHF30-300MHz

UHF

300-3000MHz

SHF3-30GHz

EHF30-300GHz

HF3-30MHz

MF0.3-3MHz

WiFi

Gigabit

Liaison sous

marine4G

Bluetooth

Radiofrequency spectrum in France

Some pre-requisites

LORA

7

Fréquence (Hz)

100K 1M 10M 100M 1G 10G 100G

Radio AM

Radio OC

CBTV VHF

Radio FM

DVB-T

GSMGPS

DCS

UMTS

Liaison

satelliteWimax

ZigBee

PKE (125 kHz)

� ISM bands (Industrial, Scientific, Medical): 6,765 - 6,795 MHz, 13,553 - 13,567 MHz, 433,05 - 434,79 MHz, 2,4 -

2,5 GHz, 5,725 - 5,875 GHz (Europe).

� GSM900 : 880-915 MHz (UL) and 925-960 MHz (DL). GSM1800 : 1710-1785 (UL) and 1805-1880 MHz (DL)

� UMTS - FDD : 1920-1980 MHz (UL) and 2110-2170 MHz (DL)

� 4G - LTE : 832-862 MHz (UL) and 791-821 MHz (DL). 2500-2570 MHz (UL) and 2620-2690 MHz (DL)

� radio FM : 87,5 – 108 MHz.

� (DVB-T) : band IV 470-606 MHz and band V 606-862 MHz.

� GNSS (Global Navigation Satellite System) : 1559 - 1610 MHz (Band L1, E1 et E2)

Typical structure of radiofrequency

digital emitter-receiver

Innovative Smart System

8

digital emitter-receiver

Antenna

Matching network –

Narrow band filter

RF devices (duplexer,

RF switch)

Transmission

line

RF Front-endBaseband

coding

RF Transceiver

Baseband analog

signals

Ba

seb

an

d

dig

ita

l sig

na

ls

Co

ntr

ol,

sta

tus,

up

Po

we

r su

pp

ly

Configuration

Typical structure of radiofrequency digital emitter-receiver

Structure of RF digital emitter-receiver

Radio channel

9

Baseband

ProcessorPower

management

Ba

seb

an

d

dig

ita

l sig

na

ls

Co

ntr

ol,

sta

tus,

wa

ke-u

p

Power supply

Po

we

r su

pp

ly

Application

processor

Power supply

Da

ta

Example of RF Zigbee module

� ISM 2.4 GHz

� Binary data rate = 250 kbps

� Transmit power = 0 dBm

� Range: 30 – 100 m (indoor / outdoor)

� Receiver sensitivity = -92 dBm

� Current: 10 µA - 50 mA (sleep / transmit)

Typical structure of radiofrequency digital emitter-receiver

10

� Current: 10 µA - 50 mA (sleep / transmit)

Digital communications –


11

Digital communications –

Overview of processing

Digital communications –Overview of processing

Digital source

Source coding Source decoding

Message recipient

Transmitted data preparation Source data recovery

Digital signal processing for transmission/reception

12

Encryption

Canal coding

Line coding

Amplification, filteringTransmission

ChannelFiltering.. Low noise

amplification

Démodulation, Baseband

transposition

Channel decoding

De-encryption

Transmission

NOISEReception = Signal

reconstruction + Logic

state detection

Reception

Modulation, Frequency

transposition

Line decoding

Channel coding� Objectives : reduce the distorsions induced by transmission

channel and noise (internal and interferences) by a

modification of transmitted frames.

� Examples :

� Error detection in received message (CRC field) +

Automatic Repeat reQuest (ARQ)

� Error detection and correction by receiver (Forward Error


13

� Error detection and correction by receiver (Forward Error

Coding).

� Interleaving and scrambling

Interference Technology, Feb 2013Xiaoli Sun, NASA Goddard

Electrical shaping – Line coding

� What is the link between a message symbol and the actual transmitted/received

electrical signals ?

� The link method must provide several advantages:

� Modify the spectrum according to the transmission channel characteristics

� Synchronize receptor clock on incoming binary data steam


14

� Synchronize receptor clock on incoming binary data steam

� Add redundancy for error detection


timeT = 100 ns

A = 1 V

0

Tr = 1 ns

T

Ac

n

T

tn

T

tn

Tn

Tn

T

Ac

r

r

n

τ

π

π

τπ

τπτ

=

>

=+

0

0,sinsin

2

τ = 50 ns

Square signal (periodic)


15

timeT = 100 ns

F = 10 MHz

time

A = 1 V

0

Tr = 1 ns

Tb = 50 ns

Fb = 20 MHz

0 1 0 0 1

Binary signal (random)

Most of the energy within

the range [0; Fb]

FFT

Code Non-Return to Zero (NRZ)



16

� Most of the energy within the range [0;Fb]

� DC component

� No freq. Content at Fb

Biphase binary code (Manchester)

FFT



17

�No DC component

� Energy at Fb � easier synchronization of receptor

�Wider bandwidth than NRZ

Bipolar Return to Zero binary code (RZ)

FFT



18

� No DC component

� No energy at Fb

� Error detection

Frames - Packets� Encapsulation of Payload bits in frames or packets

� Example : structure of Zigbee – IEEE 802.15.4 frame (PHY + MAC)

PHY

Preamble field

(only ‘0’)SFD

PHY header

(frame length) PSDU (PHY payload)

Synchro Header


19

Payload ?

Overhead ?

4 octets 1 octet 1 octet 127 octets max

133 octets max

MAC

Dest

PAN

IDMSDU (MAC payload)

2 oct.

MAC Header

Frame

control

Seq.

Numb.

Dest

Adr

Sour.

PAN

ID

Sour

Adr

ADDR fields

Security

headerFrame

Check Seq.

1 oct. 0/2/8 0/2/80/2 0/2 0/5/6/

10/142 oct.

� Structure of a Zigbee packet – IEEE 802.51.4 in access mode CSMA-CA (Carrier Sense

Multiple Access with Collision Avoidance).

� Weakly loaded network assumption:

CSMA-CA TX Data FrameACK

turnaroundTX ACK Interframe spacing

Frames - Packets


20

Throughput ?

≈ 1.56 ms 4.26 ms 0.64 ms

≈ 7 ms

0.19 ms 0.35 ms

Baseband filtering – pulse shaping

� Necessary to limit bandwidth of transmitted signals

� Baseband filtering based on digital filters (low-pass filters)

� Compromise between bandwidth and time domain waveform


FilteringFiltering

(out-band noise EMITTER

21

Baseband binary signal Baseband

processing

Filtering(pulse shaping)

Modulation

amplification

(out-band noise suppression)

DemodulationBaseband processing

amplification

Transmission channel

EMITTER

RECEPTOR

Baseband binary signal

Filtering(pulse shaping)

Filtering (out-band noise

suppression)

( )2

21

cos

sin

−

=

S

S

S

T

rt

T

rt

T

tctf

π� Example: IEEE 802.15.4 – 2.4 GHz

� Based on a raised cosine filter (roll-off coefficient r =

0.2, M = 2 Mchips/s)

Emission Limit

IEEE 802.15.4 °


Baseband filtering – pulse shaping

22

IEEE 802.15.4

( )rM

B +=°

12

Modulation

� Transmission done on their original frequency band are called baseband

transmission.

� Baseband transmissions are not always the optimal solution, because of

poor characteristics of the transmission channel (noise, attenuation, …).

� Channel for baseband cannot support multi-user transmission !


23

� Channel for baseband cannot support multi-user transmission !

� Modulation leads to a frequency transposition of the signal from the

original bandwidth to anouther, without affecting the carried information.

Modulation – Frequency transposition

Frequency transposition

ModulationB 2B


24

Fréquency

Baseband signalModulated

signal

0 +Fsignal Fcarrier-Fsignal

Demodulation

Fcarrier +FsignalFcarrier -Fsignal

Multiplier

Modulation

� Modulation or frequency transposition is based on a non-linear operation

� Ideal modulation = multiplication.

Modulation – Frequency transposition


25

UM

UP

UE

FrequencyFPFM FP-FM FP+FM

FrequencyFP

Modulation

F1 F2 FP-F2 FP+F2

FP-F1 FP+F1

( ) ( ) ( )( ) ( ) ( )[ ]

( ) ( )( ) ( )( )[ ]ttA

tU

ttttA

tU

ttAtUtUtU

MPPME

MPPME

PMPME

ωωωω

ωωωω

ωω

−++=

−++=

==

coscos2

coscos2

coscos)()(Upper side bandlower side band

Demodulation – Frequency transposition

UE UR

FrequencyFPFM FP-FM FP+FM

Multiplier

Demodulation

2FP-FM 2FP+FM

Filtering

Demodulation


26

UP

FrequencyFPF1 F2 FP-F2 FP+F2

FP-F1 FP+F1

Filtering

Demodulation

( )( ) ( )( ) ( )( )[ ] ( )

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

( ) ( )( ) ( )( )[ ] ( )( ) ( )( )[ ]

( ) ( ) ( )( ) ( )( )tAt

At

AtU

ttA

ttA

tU

ttA

ttA

tU

tttA

tU

tUtUtU

MPPMMD

MPPMPPMPPMD

PMPPPMD

PMPPMD

PED

ωωωωω

ωωωωωωωωωω

ωωωωωω

ωωωωω

−+++=

+−+−+−+++=

−++=

×−++=

=

2cos4

2cos4

cos2

cos2cos4

cos2cos4

coscos2

coscos2

coscoscos2

)()(

� Two types of modulation are distinguished:

� Analog modulation: the baseband signal is continuous

� Digital modulation: the baseband signal is a synchronous digital signal

The differents types of modulations


27

� The baseband signal modifies one or several characteristics of the carrier signal:

� Amplitude

� Frequency

� Phase

� Duty cycle (pulse modulation)

( ) ( ) 1ou0B,tsinBtS p =×= ω

Amplitude Shift Key (ASK or OOK):

Frequency Shift Key (FSK):

10 1 0 1 1

porteuseporteuse

modulantmodulant

État binaire

Amplitude A1A0 A1 A0 A1 A1

Simple digital modulation modulations


28

( ) ( )( ) 1,sin0 ±=×+×= BtBAtS mp ωωFrequency Shift Key (FSK):

Phase Shift Key (PSK):

( ) ( ) 1ou0B,BtsinAtS p0 =×+×= πω

ASK

FSK

PSK

Signal modulé

Fréquence

Signal modulé

F1F0 F1 F0 F1 F1

Phase

Signal modulé

φ 1φ0 φ 1 φ 0 φ 1 φ 1

� What are the criteria to choose a modulation ?

� Nature (analog / digital)

� Power efficiency

Criteria to select a modulation scheme ?


29

� Binary data rate

� Spectral occupancy

� Noise tolerance (minimize error probability for a digital communication)

� Complexity / cost

� Example IEEE802.15.4 - BPSK : M = 1 Mchips/s, Fp = 2.4 GHz, roll-off factor r =

0.2, emission power = 0 dBm

Phase change

Spectral efficiency of IEEE 802.15.4


30

1 symbol (1 µs)

B ≈ 2*M/2*(1+r) = 1.2 MHz

( )B

DHz/s/bits b=η

Spectral efficiency Net binary data

rate

Spectral efficiency of IEEE 802.15.4


31

Modulated signal

bandwidth

� If we suppose 1 bit = 1 chip (spectral spreading neglected) :

HzsbitsBPSK //1=η

How improving the spectral efficiency?

� Limitation of channel bandwidth � limitation of binary data rate.

� Idea to improve the data rate without increasing the bandwidth: transmitting symbols

coded by several bits.

� Digital modulations based on M complex symbols formed by N bits, where

10 1 0 1 1Symbole

Modulation d’amplitude à une porteuse

NM 2=

M-aire digital modulation


32Septembre 2009

( ) NTMTT bbS ×=×= 2log

Symbol duration

Improvement of spectral

efficiency:

1 1 0 1 1

porteuse

Signal modulé

Symbole

1001 11 00 11 10

Porteuse 1

Signal modulé 1

Porteuse 2

Symbole

Signal modulé 2

Modulation d’amplitude à deux porteuses

( ) ( )MB

DHzsbits b

2log// ×=η

Q carrier (Quadrature)

Modulated

� A modulated signal modulé with an amplitude A and a phase φ.

� This signal can be expressed in term of 2 orthogonal basis vectors: cos and sin

functions.

( ) ( )( ) ( ) ( )+=

+= c

tfAtfAts

tfAts

2sin2cos

2cos

ππϕπ

Constellation diagram

I/Q modulator


33

I carrier

(in-phase)

Modulated

signalAmplitude A

Phase φ

AI

AQ

( ) ( ) ( )( )

=+=

+=

+=

I

QQI

QI

cQcI

A

AetAAA

QAIAts

tfAtfAts

arctan

2sin2cos

22 ϕ

ππ

Idea : if a bit modulates one carrier (I or Q), the signal is modulated in

phase and amplitude, and carries 2 bits simultaneously.

QBaseband processing

Binary signal +

Modulated signal

Q channel

I/Q modulator


34

Local oscillator

0°

90°

Carrier

I

processingsignal +

( )tfCπ2cos

signal (amplitude and /or phase)

I channel

� Quadrature Phase Shift Key modulation (QPSK or 4-PSK or 4-QAM)

� 2 bits are transmitted for each symbol, symbol duration = 2×TB

� 4 possible symbols, characterized by different phase states:

• ’11’ � π/4

• ’01’ � 3π/4

Digital M-aire modulation - QPSK


35

• ’01’ � 3π/4

• ’00’ � 5π/4

• ’10’ � 7π/4

I

Q’11’’01’

’00’ ’10’

Constellation diagram

Spectral efficiency IEEE 802.15.4� Example IEEE802.15.4 - QPSK : M = 1 Mchips/s, Fp = 2.4 GHz, roll-off factor r

= 0.2, emission power = 0 dBm


36

?=η

Spectral efficiency more

efficient than BPSK

B ≈ 2*M/2*(1+r) = 1.2 MHz

16-QAM – SNR = 10 dB

Digital M-aire modulation

64-QAM – SNR = 10 dB

Detection thresholdD

ete

ctio

n t

hre

sho

ld


37

EVM

?=η ?=η

EVM

Detection thresholdD

ete

ctio

n t

hre

sho

ld

EVM = Error Vector Modulation

Spectrum spreading - Direct Sequence Spread Spectrum (DSSS)

� Multiplication of the baseband signal by a unique Pseudo-Random Noise PN with a rate

W larger than baseband signal (data rate D) and numerous transitions.

� PN codes belong to a family of orthogonal codes

� Basis element of speading sequence = Chip � W = Chip Rate.

� Effect on signal: Spectrum spreading.

Baseband


38

� Spreading factor:D

WSF =

Spread

signal

W

Baseband

signal

frequency

D

EVM

� The received signal is multiplied by a spreading code to extract the wanted signal and

remove the contribution of other transmitters.

� Use of a correlation receiver, perfectly synchronized with the spread signal.

∫SF

dnnu0

][Signal étalé

Signal Désétalé∫

SFdnnu

0][

Signal étalé

Signal Désétalé



39

codecode Amplifié par

SF

Quasi nul

( )

==D

WSFPG log10log10� Processing gain :

Spread

signal

W

De-spread

signal

Frequency

R

PG

� Advantages:

� Transmission under the noise floor

� Robust to interferences and multipath propagation

� IEEE 802.15.4 OQPSK – 2.4 GHz :

1 Msymb.



40

Baseband

binary data

PHY

Mapping bit

to symbol

Mapping

symbol to chip

OQPSK

modulator

Mdulated and

spread signal

250 kbps 2 Mchip/s62.5 kBds1 Msymb.

phase/s

Spreading factor = ?

Noise and interferences


41

Noise and interferences

Noise & Interferences

� Noise is a random signal, usually thermal origin, that defines the detection

threshold of the receiver

� Random process � the behavior is unpredictible

� The noise is also defined in term of spectral density …

Densitéspectrale de puissance

Noise

42

Densitéspectrale de puissance (W/Hz ou dBW/Hz)

Seuil de bruit

Signal détectable

Signal non détectable

Fréquence

n0

df∫=f

dfnN 00Puissance du bruit :

( ) ( ) ( )

−−==2

22

2

1exp

2

1,

σπσσ µx

µNxf

� …or probability density� Usual noise model: normal or gaussian process

� Widely adopted in telecommunications to model the impact of noise on digital

receivers and estimate their performances


Noise

43

mx

220 XXmNpower σ+=

Temps

Moyenne

Amplitude du bruit (x)

2σ

Densité de probabilité p(x)

Amplitude du bruit (x)

σ = écart-type

( ) ( )

222 σπσ

( ) ( )0

1log10log10

X

P

PxdBX

×=

==( ) ( )

20

0

1

10

log20log20

X

VV

V

VxdBX

×=

==

� Expressing a physical data (voltage, power, electric field) in dB, the ratio between

this data and reference value is computed, and then expressed in a logarithmic

scale.

Décibels - dBm


44

1001 10PP ×=20

01 10VV ×=

( ) 20 log1

VV dBV

V = ×

� Example :

( )

×=

W

PdBWP

1log10

1

0.1

0.01

0.001

10

100

1000

Volts

0

-20

-40

-60

20

40

60

dBV

1

0.1

0.01

0.001

10

100

1000

Watts

0

-10

-20

-30

10

20

30

dBW

� In telecommunication applications, powers are usually expressed in dBmW or dBm

( ) ( )

( ) ( ) ( )( ) ( ) 3030log1010

log10

1log10

3+=+=

×=

×=

− dBWPWPW

WPdBmP

mW

mWPdBmP

mW dBm

Décibels - dBm


45

1

0.1

0.01

0.001

10

100

1000

mW

0

-10

-20

-30

10

20

30

dBm

5 W = dBW

0.5 mW = dBm

-10 dBm = W

80 dBm Typical emission power of a FM radiodiff. Station (50 km range)

55 dBm Typical emission power of a geostationary satellite (band Ku)

43 dBm Typical emission power of a 4G base station in rural environment

24 - 33 dBm Max emission power of a 3G UMTS mobile (class 1 – 3)

Décibels - dBm


46

15 - 20 dBm Emission power of a WiFi access point (IEEE 802.11b/g)

0 dBm Emission power of Zigbee, Bluetooth class 3 (10 m range)

-70 dBm Typical receiving power to ensure correct reception of WiFi packet

-80 dBm Typical receiving power to ensure connection to WiFi network

-100 dBm Sensitivity threshold of WiFi receiver

-127 dBm Typical receiving power of GPS signal

-132 dBm Typical sensitivity threshold of a LoRa ® receiver (1 kBits/s)

-174 dBm Thermal noise floor (B = 1 Hz, 27°c)

• Johnson noise: noise affecting resistance, linked to thermal

movement. Gaussian noise.

• Shot noise: random fluctuation of current.

• 1/f or Flicker noise: due to variation of resistance, linked to presence

4 TRbruitV k B=

2I qIB=

� Numerous noise sources exist:

Intrinsic noise of a digital receiver


47

• 1/f or Flicker noise: due to variation of resistance, linked to presence

of impurities in electronic devices

• Thermal noise kTB, general formulation:

2bruitI qIB=

( ) ( )kTBdBWN log10×=

Power density at ambient temperature ?

Bruit et perturbations

Filtre linéaire

• Discrete memoryless

channel

• Gaussian random

process with a null mean

and σ² variance

2

2

1( ) exp

22

xp x

σπσ

= −

Noise modeling - Canal Additive White Gaussian Noise (AWGN)


48

Signal numérique émis Signal numérique

reçu

Canal de transmission

Filtre linéaire

++

and σ² variance

• White noise

• Model a radio link in

direct visibility with only

intrinsuc thermal noise,

no external

interferences

� Active circuits (amplifiers, mixers, oscillators…) are made of numerous devices able

to produce noise (transistors, diods…).

� Their ability to produce noise is characterized by Noise Figure (NF).

Active circuitNin Nout ( ) ( ) ( )dBmNdBmNdBNFN

NF out −=⇒=

Intrinsic noise of a digital receiver – Noise factor


49

Active circuit

NF

Nin Nout ( ) ( ) ( )dBmNdBmNdBNFN

NNF inout

in

out −=⇒=

1st element 2nd element Nth element

G1

NF1

G2

NF2

GN

NFN

NoutNin

� Cascading several active devices ?

12121

3

1

21 ...

1...

11

−

−++

−+

−+==

N

N

in

out

GGG

NF

GG

NF

G

NFNF

N

NNF

� Example : noise factor for different WLAN and DVB-T receivers

Intrinsic noise of a digital receiver – Noise factor


50

� If we consider only thermal noise:

� Example of a Zigbee receiver IEEE 802.15.4 (ATMEL AT86RF230) : NF = 6 dB, RX

return loss = 10 dB

Intrinsic noise – Noise floor of a IEEE 802.15.4 receiver


51

return loss = 10 dB

� Required sensitivity by the specification IEEE 802.15.4 : less than -85 dBm

� Sensitivity guaranteed by the manufacturer: -101 dBm for PER < 1 %

� To characterize the effect of noise on a signal, we use the

Signal to Noise Ratio (SNR)

� An harmonic signal is detectable if SNR > 0 dB.

Power level

(dBm)

signalSDetected

signal

Undetected

signal

( )

=N

SdBSNR log.10

Signal to noise ratio


Power level

(dBm)

52

noise

signalf

NS

SNR < 0 dB

noise

signal

f

S

N

SNR > 0 dB

Detected

signalsignal

� The noies has a negative impact on analog signal.

� The requirements in term of SNR for analog communications are very stringent.

� Example voice/sound: 45 – 50 dB required. 30 dB : the noise becomes disturbing.

� Noise and electrical disturbance superimpose to the signal .

� For digital transmission, the higher the number of symbol, the harder the

differenciation of symbols.

2 symbols 4 symbols

Amplitude resolution


53

No intersymbol interferences Risk of intersymbol interferences

� If we suppose gaussian white noise, to cancel the risk of reception

error, the maximum number of symbols is given by:

� Maximum decision quantity per moment (in bits) :max 1

SN

N= +

max 2

1( ) log 1

2m

SD bits D

N ≤ = +

Narrowband noise

External noise - interferences


Natural source Man-made noise

Intentional emission

RF jammer

54

Broadband noise

Non intentional emission

Electrostatic discharge

Victim devices



55Septembre 2015

� An external source produces parasitic noise in the transmission channel

� The jamming can be intentional (military, criminal purposes) …

� … or unintentional due to other transmitter using the same channel (co-channel

interference), or adjacent frequency band(adjacent channel interference).

� Concept of Signal to Noise plus Interference Ratio (SNIR) :

S



56Septembre 2015

Signal

Interférences

InterférencesInterférences

f1f1

f1f1

f1 f2 fk

Fréquence

Bande allouée àun opérateur

Sous bande

Co-channel interference in a cellular nework

IN

S

+

� Example: interferences between IEEE 802.11.b (WiFi) and IEEE 802.15.4

Power spectral density

5 mW/MHzWiFi

Channel 1

WiFi

Channel 6WiFi

Channel 11

2 MHz



57

2400

F (MHz)

2412 2425 2437 2450 2462 2475

0.5 mW/MHz

22 MHz

2 MHz

Interferences from 802.15.4 on IEEE 802.11 ?Interferences from 802.11 on IEEE 802.15.4 ?

Increase of noise level for a IEEE 802.15.4 receiver ?

Data rate limitation


58


• Rarely in line of sight

• Multiples reflections due to obstacles, time spread

• Diffusion, diffraction on building edges

• Atmospheric absorption

Radiocommunications are affected by numerous disturbances that make the signal

propagation extremely complex and predictible:


Radiocommunication propagation disturbances

59

Transmission directe

diffusion

réflexion

diffraction

Absorption moléculaire

Forte pluie

1 10 100Fréquence (GHz)

0.1

1.0

10

100

Atténuation (dB/Km)

1000

Pluie moyenne

02 H20

Absorption moléculaire

Forte pluie


0.1

1.0

10

100



0.1

1.0

10

100


1000

Pluie moyenne

02 H20

� In urban environment, radio signal are affected by multipath propagation. The

resulting receiving signal is the sum of direct, reflected, diffracted signals. The

consequence is fast fading.

� Each signal has different characteristic (time of arrival, incidence angle, amplitude,

phase, polarization, frequency).

� The different contributions arrive at different times.

� The sum of all of these contributions (mainly the phase differences) leads to frequency


Radiocommunication propagation disturbances

60

� The sum of all of these contributions (mainly the phase differences) leads to frequency

selective fading (2 up to 30 dB).

transmission Diffusion / diffraction

réflexion

temps

Signal reçu

seuilTrajets multiples

fréquence

Fonction de transfert

seuil

fade

Impulsion

Plusieurs impulsions

Multiple

contributions of the

signal

Frequency selective

fading

Time spread Time spread and « echoes »

Indoor environment Outdoor environment

Impulse response of Hertzian channel


61

(H. Hashemi, « The Indoor Radio Propagation

channel », Proceedings IEEE, vol. 81, no 3, July 1993)

(J. B. Andersen, T. S. Rappaport, S. Yoshida, «

Propagation Measurements and Models for Wireless

Communications Channels», IEEE Communications

Magazine, January 1995)

� Distorsion due to the overlap of two successive symbol, that may lead to a reception

error

� Time spreading, delay, multipath propagation produce ISI.

Transmitted signal transmission

Intersymbol Interferences (ISI)


62

The ISI must be cancelled

Time Time

Received signal

� Required conditions to ensure errorless digital signal transmission:

� Limitation of the binary data rate

� The eye diagram aims at controlling the ISI presence visually.

� Superposition of traces of the received channel over the symbol duration

� The performances of the transmission channel are deduced from the vertical and

horizontal apertures of the eye.

Errorless sampling

Intersymbol Interferences (ISI) - Eye diagram


63

Errorless sampling

T

Intersymbol Interferences (ISI) - Eye diagram


64

T

T

� The information transmission rate depends on the number of symbols per unit of

time. It is related to the transition time of signal in a channel.

� Characterized by the symbol rate expressed in Bauds :

A symbol is supposed constant during T . ( )

TBdM

1=•


Maximum data rate

65

A symbol is supposed constant during TM.

� Digital systems: a symbol among n is coded by a number of bits D :

� The binary data rate is given by:

( )2( ) logD bits n=

What is the maximum data rate that can be transmitted through a

channel without serious errors?

( )MT

BdM =

( ) ( )MT

nDMsbitsD 2log

/ =×=••

5.02

1 =×⇔= SS TBB

TNyquist condition verified if:

� Nyquist condition in time domain: ISI is cancelled if the effect of previous transmitted

symbols cancels at sampling times.

Nyquist frequency criterion


66

2B

BTS 2

1≥ BM 2≤°

Case of ideal low-pass channel: ISI completely cancelled if :

Canal de transmission idéal de largeur de bande B

Bruit additif blanc et gaussien

sortie

S/N� Ideal channel

� The capacity of a channel is the maximum binary data rate that can be transmitted on a

channel without binary error due to ISI.

Capacity of a transmission channel


67

max 2

1( ) log 1

2m

SD bits D

N ≤ = +

! The capacity defines a purely theoretical limit to the channel throughput. To limit

the BER, the following condition must be verified:

BTT

MMrm

×===≤••

211

min

max

( )

+×=×==••

N

SBMDDsbitsC mm 1log/ 2maxmaxmax

CD m ≤•

Filtering effect on ISI

( )r

BM

+=

°

1

2� Example IEEE802.15.4 - QPSK : M = 2 Mchips/s, Fp = 2.4 GHz,

raised cosine filter (r = 0.2), B = 1.2 MHz

� Eye diagram:


68

ISI ? Sampling time?

Link performance


69

Link performance

� Digital signals are sensitive to noise … but less than analog signals.

� The quality of a digital signal is not related to the distorsion of the signal waveform,

but to the ability of the receiver to decode the binary signal correctly.

� Main constraint: Binary Error Rate (BER).

( ) numberbitErroneous

Link performance

Effect of noise on digital communications

70

( )numberbitreceivedTotal

numberbitErroneousBER =%

� We also define Block Error Rate (BLER) or Frame Eror Rate (FER).

� Quality indicator for PHY layer of IEEE 802.15.4 : Packet Error Rate (PER)

( )numberpacketreceivedtotal

numberpacketerroneousPER =%

� The SNR is not the best indicator to measure the degradation of a digital signal.

� If the energy carried by a bit is less than the energy carried by the noise, a binary

error becomes a likely event.

� Case with gaussian white noise:

Link performance

Signal to noise ratio per bit Eb/No

71

� Case with gaussian white noise:

D

SEb =

B

NN =0

Energy per bit Spectral density of noise

D

B

N

S

N

E

B

D

N

E

N

S

o

b

o

b ×=⇔×=

� Extension to the case with interfering signals :

D

SEb =

B

INI

+=0

Energie par bit Densité spectrale de bruit

Link performance

Signal to noise ratio per bit Eb/No

72

Db

BI =0

D

B

IN

S

I

E

B

D

I

E

IN

S

o

b

o

b ×+

=⇔×=+

Amplitude Vin du signal binaire reçu

� A binary signal is transmitted through a symetrical AWGN channel, transmitted to a

binary receptor with a decision threshold λ0. The presence of binary states ‘0’ and ‘1’

is equiprobable.

Link performance

Link between BER and Eb/No – binary signal (AWGN channel)

73

temps

signal binaire reçu

a0

a1

A

Récepteur (seuil de

décision λ0)λ0

Etat binaire transmis a :

0 01

Etat de sortie d :

d = ‘0’ si Vin < λ0

d= ‘1’ si Vin > λ0

Densité de probabilité

2σ 2σ

f(x/a0) f(x/a1)

Link performance


74

Vina0 a1λ0

( ) ( )

−−=

2

20

0 2exp

2

1/

σπσax

axf

( ) ( )

−−=

2

21

1 2exp

2

1/

σπσax

axf



2σ 2σ

( ) ( ) ( ) ( )

( ) ( )∫∫∞−

∞+

+=

===+====0

0

10

10

/2

1/

2

1

/0.0/1.1λ

λ

dxaxfdxaxfP

aadPdPaadPdPP

err

err

Link performance


75

Vina0 a1λ0 a0 a1

λ0

+2σ 2σ


2σPerr

λ0-A0/2

201 aa

A−=

Vin

Vinλ0λ0-A/2

−−= ∫

+ 0

02

2

2exp

2

11

2

1A

err dxx

Pσσπ

( )

−−= ∫

+σ

π

2

0

2

0

exp2

12

1A

err duuP

=σ22

1 AerfcPerr

Link performance


76

==

02

1

N

EerfcBERP b

err

NRZ baseband signal without

any coding

Link performance


77

Binary signal with data rate = 250 Kbits/s. The bandwidth of the baseband signal is

1.2 MHz Compute the minimum signal to noise ratio to ensure that BER < 0.1 %.

� Increasing the number of bits per symbol improves

the spectral efficiency, but it degrades the robustness

to noise and interference.

� AWGN channel conditions:

×=

0N

EerfcBER bβα

Link performance

Effect of the modulation on BER – AWGN channel

78

Link performance

No multipath propagation

No coding

Effect of the modulation on BER – AWGN channel

79

Binary signal with data rate = 250 Kbits/s and a OQPSK modulation. The bandwidth

of the baseband signal is 1.2 MHz Compute the minimum signal to noise ratio to

ensure that BER < 0.1 %.

Link performance

PER estimation for IEEE 802.15.4 –AWGN channel

� Modulation QPSK, D = 250 kbits/s, B = 2 MHz, frame with 1064 bits, AWGN channel, no

coding, no multipath propagation

80

IEEE 802.15.4 requirement: PER < 1 %

Puissance en entrée de Emetteur

Medium de Puissance en

sortie du Récepteur

Bruit

≤Erreur

� From the transmitted power, channel model, noise threshold of the receiver, the

different elements of the channel can be designed to guarantee an errorless

transmission.

Link performance

Link budget

81

entrée de l’émetteur Pe

EmetteurMedium de propagation

sortie du récepteur Pr

Récepteur

Gain Ge

Perte Le

Gain Gr

Perte Lr

Perte de propagation Lp

≤Erreur

binaire ?

rrpeeer LGLGLPP −+−+−=Power budget:

Condition à respecter : thresholdySensitivitPr >

Maximum path loss?

Sensitivity of a digital receiver

( ) insmlossesSNRfloorNoisedBWysensitivit argmin +++=Power

Additional

Sensitivity

threshold

Link performance

Link budget

82

Noise floor

SNRmin

Additional

marginssignal

( ) ( ) ( ) insmBDNENFkTBdBWysensitivit ob arg/log10/log10 ++++=

threshold

Link between 2 Zigbee nodes (IEEE802.15.4 OQPSK 2.4 GHz):

� Emitter and receiver have antenna with gain = 0 dB

� Emitted power = 0 dBm

� Tx/Rx Returnlosses= 3 dB

Link performance

Link budget – IEEE 802.15.4 example

83

� Tx/Rx Returnlosses= 3 dB

� Only thermal noise (T°c = 25°c).

� Receiver noise figure = 8 dB

� Quality requirement: PER < 1 %

Compute the maximum path loss.

Link performance

Link budget – IEEE 802.15.4 example

Emitter

Electrical power (dBm)

Gain emitter antenna (dB)

Losses emitter (dB)

EIRP (dBm)

Bandwidth(MHz)

84

Receptor

Bandwidth(MHz)

Throughput (kBps)

Thermal noise floor @ 300 K (dBm)

Noise figure (dB)

SNR @ PER < 1 % (dB)

Sensitivity receiver (dBm)

Losses receiver (dB)

Gain receiver antenna (dB)

Minimal input power (dBm)

Path loss (dB)

Radio propagation models


85


Estimation of radio range

Free space

Frequency f2

4

×=

c

fd

GGPP ree

r

π

� Ge et Gr : emitter and receiver


Radio range estimation

Free space propagation–Friis formula

86

Pe Pr

� Ge et Gr : emitter and receiver

antenna gains

� c = 3x108 m/s

� Path loss in free space : 2

4

××== fdcGP

GPL

rr

eeP

π

( ) ( )( ) ( )( )MHzfkmddBLP log20log204.32 ⋅+⋅+=



Free space propagation – Friis formula

87

Theoretical radio range of Zigbee ?

Propagation in terrestrial environment (outdoor)



88

Simulation of 3G cell network radio coverage over Rangueil area (2100 MHz )

Attenuation and reflection by walls

Guided propagation along corridors

Propagation in terrestrial environment (indoor)



89

� Rarely in line of sight� Numerous walls and obstacles (furniture, people) � very fast attenuation with the

distance� Dependence on building materials� Non-stationary channel

Diffraction by apertures

Simulation of indoor propagation at 434 MHz

Champ électrique (dBµV/m)

100≈10λ

0

10

-10-20

Fading de Rayleigh ou rapide

Random variations - Slow fading



90

Distance (km)1 10 100

100

80

60

40

20

Modèle terrain plat

0

Masquage des immeubles – fading lent

100 - 1000λ

( )

−=

−

2

2

2 2

10exp

2

1)(

LN

x

LN

LN xpσπσ

β

Shadowing = slow or log-normal fading (σ = 5 à 7 dB in urban environment) :

� If non line of sight propagation, Rayleigh fading model (σ = 2 - 3)

� If line of sight propagation, Rice fading model (ν =0.7 - 1, K = 2 - 10 dB)

0,2

exp)(2

2

2>

−= x

xxxpR σσ

( )

2

202

22

20,

2exp)(

ν

σν

σν

σ

=

>

+−=

K

xx

Jxx

xpR

Random variations - Fast fading



91

22σ=K

Rayleigh – σ²=0.5 Rice – σ²=0.5 et K = 3 dB95 % Quantile ?

Purpose of a propagation model:

� Estimate the radio range of an emitter

� Determine the quality of a received signal according to the distance and the channel

� Compute the interference level when numerous emitters co-exist

� Configure the equipments to ensure a sufficient radio coverage, capacity and quality of service

The model links the path loss L between an emitter and a receiver according to the

Propagation models – General considerations



92

The model links the path loss L between an emitter and a receiver according to the distance, the frequency, propagation channel characteristics.

( )tenvironmenhhdfLPP REER ,,,,−=

� Radio channel modeling is a complex task because of the complexity of propagation

mechanisms.

� Numerous models according to the considered environment and required accuracy

Methods

Propagation models – General considerations



93

Exact but slows Fast but inaccurate

macrocell microcell picocell

Methods

Environmentrural (>10km) urban (~1km) urban dense (<1km) indoor (<100m)

empiricalmixedTheoreticalDiscrete

• frequency• distance• polarization• Antenna height Statistical model

Terrain model

Average attenuation,

Input parameters:Empirical propagation models



94

• Antenna height• Ground

conductivity• Climatic conditions...

Statistical model attenuation, fading

Calibration measurements

(should be validated experimentally)

Example of generic simple empirical model: ( )

+=

00 log.10

d

dnLdBL

� Lo (dB) : average path loss at a reference distance d0

� d0 (m) : reference distance

� d (m) : distance

� n : propagation loss exponent (n=2 for free space, n>2 in terrestrial environment)

� Linear attenuation model

( ) ( ) rrLdBL PP β+= 0

�Lp0 : free space path loss

�Β: empirical lineatrattenuation coefficient (dB/m)

�r distance emitter - receiver (m)

Environnement β (dB/m) @ 1.8 GHz Dense – 1 étage 0.62

Dense – N étages 2.8 Ouvert 0.22

Empirical propagation models – Indoor environments



95

� One slope model

( ) ( ) 00

00 ,log10 rrr

rNrLdBL PP >

+=

�Line of Sight condition up to r0

�Lp0(r0): free space path loss (dB)

�N: empirical attenuation coefficient

�r distance emitter - receiver (m)

Environnement L0(r=1m) (dB)

N

Dense – 1 étage 33.3 4 Dense - 2 étages 21.9 5.2 Dense – N étages 44.9 5.4

Ouvert 42.7 1.9 Couloir 39.2 1.4

� Motley-Keenan model

( ) floorfloorwallWallP LNLNLdBL ++= 0

2D building drawingEmpirical propagation models – Indoor environments



96

� Lpo : free space path loss

� Nwall : number of crossed walls

� Lwall : loss per wall (dB), depending on the wall

materials (10 – 20 dB)

� Nfloor : number of crossed floors

� Lfloor : loss per floor (dB) depending on the floor

materials (10 – 30 dB)

� Furniture ignored

� Reflections and diffractions due to

walls and apertures are also

ignored

Matériau Atténuation moyenne (dB)

Placoplatre 3 Vitre (sanspropriétéathermique) 2

Typical values between 1 and 2 GHz (loss increases with frequency).

Attenuation of building materials



97

Vitre (sanspropriétéathermique) 2Vitre renforcée 8Bois 3Mur en brique d’épaisseur inférieure à 14 cm 4Mur composé de béton d’épaisseur inférieure à10 cm

9

Mur composé de béton d’épaisseur supérieure à25 cm

15

Mur de béton épais (> 25 cm) + grande vitre 11Dalle 23Mur métallique 30

� One floor, dense environment (people,

furniture)

� Load-bearing wall attenuation = 9 dB

� Non load-bearing wall attenuation = 3 dB

Evaluation Zigbee radio range in

indoor environment



2D building drawing

98

Zigbee radio range ?

Radio/EMC regulations


99


� The European directive 99/5/EC (1999) Radio & Telecommunications Terminal

Equipment is applied to all telecom and radio equipments emitting on the range 9 KHz

– 3000 GHz and replaces the CE directive (and also the Low voltage directive about

safety and health of users).

� It demands that all telecom and radio equipments in the European market:

� Comply with safety requirements of theLow Voltage directive (2006/95/CE), EM

limit requirements and EMC requirements of the CE directive (2004/108/EC).


European directive R&TTE

100

limit requirements and EMC requirements of the CE directive (2004/108/EC).

� Radiuo equipments use only the allocated frequency bands to prevent radio

interference

� Mandatory marking:

Mandatory for all equipments

concerned by R&TTE directiveCompulsory warning for

class 2 equipmentsNotified Body

number

Conformity mark Validity area Logo

Conformité EuropéenneEuropean Economic Area

Federal Communications Commission

USA

Voluntary Council for Control of Interference

Japan

China Compulsory

�Non harmonized regulations and

standards in every countries.

�Except some Mutual Recognition

Outside European Union ?


101

China Compulsory Certificate

China

Australian Communications Authority (ACA)

Australia / New Zealand

GOST (State Committee for Quality Control and Standardization)

Former USSR countries

Korea Communications Commission

South Korea

Bureau of Standards, Metrology and Inspection

Taiwan

�Except some Mutual Recognition

Agreements (MRA) to facilitate free

trade.

Example : Radio/EMC regulations for a Zigbee device


102

Fixed device (Far

field)

� Maximum levels defined by ICNIRP, harmonized (nearly) worldwide

Human being exposure to electromagnetic fields

Mobile device (Near

field)


103

ρσ

ρ

2

)/( rmsE

dV

dW

dt

d

dm

dW

dt

dkgWDAS ===28 V/m

61 V/m

61 V/m137 V/m

DAS < 2 W/kg

�Radiation safety zones around a Zigbee access point ?

24 d

PIREP

π=Power density (far-field

condition) (W/m²):

Human being exposure to electromagnetic fields


104

d

PIREE

.60=Electric field (far-field

condition) (V/m):

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