ECE537/5 #1Spring 2009© 2000-2009, Richard A. Stanley

ECE537 Advanced and High Performance Networks

5: Wireless Factors and Mobile Networking

Professor Richard A. Stanley, P.E.

ECE537/5 #2

Overview of Tonight’s Class

• Student presentations/discussions on wireless networking

• Review of last time

• Issues in mobile networking implementations

ECE537/5 #3

Last time…

• Wireless networking is growing rapidly in importance

• There are many “special” considerations for wireless networking

• Unlike most wired networking, physical layer effects play a large in proper design of a network and its protocols

ECE537/5 #4

Future Direction

• “Secure information sharing is what is needed within DoD networks.”

» Mr. David M. Wennergren» Deputy Assistant Secretary of Defense for

Information Management and Technology & DoD Deputy Chief Information Officer

» AFCEA NOVA lunch, 9 Oct 2009

• What does this mean for our networking designs and implementations?

ECE537/5 #5

Delay Spread Issues

• As we saw last week, delay spread puts a limit in signaling speeds in wireless systems

• Thus, estimating delay spread is important to system design

• As it turns out, not only is the RMS mean spread important, but so is the variance

• I have put a paper on this topic on the web page

ECE537/5 #6

Delay Spread Estimation

From Schober et al.

ECE537/5 #7

Atmospheric Absorption

From: http://www.mike-willis.com/Tutorial/gases.htm

ECE537/5 #8

Water Refraction

ECE537/5 #9

Noise• In a communication system, S/N is a function

of:Transmitter output powerGain of Transmit and Receive antennasPath lossReceiver noise

• To characterize the receiver alone, Friis introduced Noise Figure which characterized the degradation in S/N by the receiver.

• Noise Figure of a receiver is the ratio of the S/N at its input to the S/N at its output

ECE537/5 #10

Noise

• Thermal noise (Johnson Noise) exists in all resistors and results from the thermal agitation of free electrons therein– The noise is white (flat with frequency)– The power level of the noise is directly proportional to the absolute

temperature of the resistor – The level is precisely en

2=4kTRB (V2), or 4kTR (V2/Hz)Where,

– –k is Boltzman’s constant =1.38x10-23Joules/ºK– –T is the absolute temperature of the resistor in ºK– –R is the value of the resistance in Ohms– –B is the effective noise bandwidth

– The available noise power is en2/4R = kTB

• Thermal noise in the resistance of the signal source is the fundamental limit on achievable signal sensitivity

ECE537/5 #11

Noise Temperature

• Every body having a temperature > 0º K emits electromagnetic radiation

• Amount of energy emitted is directly proportional to temperature– As temperature decreases, amount of radiation is

lowered, and the frequency peak shifts to lower frequencies

• Noise Temperature is an equivalent (not actual) temperature for a body that generates the same amount of noise

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Noise Factor and Temperature

ECE537/5 #13

Noise Temperature Effects

ECE537/5 #14

Benefits of Sleep

ECE537/5 #15

Effect of Neighborhood Size

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So What?

• All these effects drive decisions on modulation schemes and protocols that would not have come into play with a traditional, wired network

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Wired Wireless

• Attenuation is stable• Phase not well-controlled,

but temporally stable

• Propagation times predictable, relatively invariant

• Group fading, if any

• No Doppler shift

• Attenuation unstable• Phase component is time-

varying and only statistically predictable

• Propagation times not predictable except statistically, highly varying

• Frequency-selective fading common

• Doppler shift for moving terminals

ECE537/5 #18

What to Do With a Shared Medium?

• Channel Partitioning, by time, frequency or code– Time Division,Code Division, Frequency Division

• Random partitioning (dynamic), – ALOHA, CSMA, CSMA/CD

– Carrier sensing: easy in some technologies (wire), hard in others (wireless)

– CSMA/CD used in Ethernet

ECE537/5 #19

Modulation Schemes

• Why modulation?

• What techniques and how do they relate to channel characteristics?

ECE537/5 #20

Simple Digital Modulation

ECE537/5 #21

QPSK

Note the Gray coding

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If Four are Good…

ECE537/5 #23

16 QAM

ECE537/5 #24

Performance

ECE537/5 #25

64 QAM

ECE537/5 #26

Distorted QAM

ECE537/5 #27

Dealing With Distortion

• Lengthen the pulses (i.e. reduce the signaling speed)

• Simplify the signal constellation

• Equalize the circuit– Different approaches for wired and wireless– Why?

ECE537/5 #28

OFDM

• Distributes data over a large number of carriers that are spaced apart at precise frequencies– Spacing provides the “orthogonality”

– Prevents the demodulators from seeing frequencies other than their own

• Benefits of OFDM:– high spectral efficiency

– resiliency to RF interference

– lower multi-path distortion than single-frequency carrier systems

ECE537/5 #29

OFDM

• OFDM: a form of MultiCarrier Modulation. • Different symbols are transmitted over different subcarriers• Spectra overlap, but signals are orthogonal.• Example: Rectangular waveform -> Sinc spectrum

ECE537/5 #30Spring 2009© 2000-2009, Richard A. Stanley

OFDM Transmission

• Transmission of QAM symbols on parallel subcarriers

• Overlapping, yet orthogonal subcarriers

cos(ct+ st)

cos(ct)

cos(ct+ ist)

cos(ct+ (N-1)st)

User symbols

Ser

ial-

to-

par

all

el = Ser

ial-

to-

Par

alle

l

I-F

FT

Par

alle

l-to

-S

eria

l

ECE537/5 #31

OFDM Subcarrier Spectra

• OFDM signal strength versus frequency.

• Rectangle <- FFT -> Sinc

• before channel

• after channel

Frequency

ECE537/5 #32

Applications

• Fixed / Wireline:

• ADSL Asymmetric Digital Subscriber Line

• Mobile / Radio:– Digital Audio Broadcasting (DAB)

– Digital Video Broadcasting - Terrestrial (DVB-T)

– Hiperlan II

– Wireless 1394

– WiMAX WiFi

ECE537/5 #33

The Wireless Multipath Channel

ECE537/5 #34Spring 2009© 2000-2009, Richard A. Stanley

ECE537/5 #35

The Mobile Multipath Channel

• Delay spread • Doppler spread

Frequency Time

FT

Frequency

FT

Frequency

Time

ECE537/5 #36Spring 2009© 2000-2009, Richard A. Stanley

Effects of Multipath Delay and Doppler

Frequency

Tim

e

Narrowband

Frequency

Tim

e

OFDMWideband QAM

Frequency

Tim

e

ECE537/5 #37Spring 2009© 2000-2009, Richard A. Stanley

Effects of Multipath (II)

Frequency

Tim

e

+-+--+-+

DS-CDMA

Frequency

Tim

e +

-

-

FrequencyHopping

Frequency

Tim

e + - + -

+ - +-

+ - +-

MC-CDMA

ECE537/5 #38

MC-CDM BER analysis

• Rayleigh fading channel– Exponential delay spread

– Doppler spread with uniform angle of arrival

• Perfect synchronization • Perfect channel estimation, no estimation of ICI• Orthogonal codes• Pseudo MMSE (no cancellation of ICI)

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BER for MC-CDMABER for BPSK versus Eb/N0

• (1) 8 subcarriers

• (2) 64 subcarriers

• (3) infinitely many subcarriers

• (4) 8 subc., short delay spread

• (5) 8 subc., typical delay spread

1 0 -5

1 0 -4

1 0 -3

1 0 -2

1 0 -1

5 1 0 1 5L o cal-m e an E n /N 0

E b /N 0 E b /N o (d B )

(1 )

(2 )

(3 )

(4 )

(5 )

A v g. B E R

A W G N

O F D M

Local-mean Eb/N0

ECE537/5 #40

Capacity

Capacity per dimension versus local-mean EN/N0, no Doppler.

-5 0 5 10 15 20 25 30 35 400

1

2

3

4

5

6

7

Local-mean En/N0 (dB)

Cap

acity

: B

its p

er S

ubca

rrie

r

-* : Rayleigh

* : MC-CDMA

- : LTI

Non-fading, LTI

Rayleigh

MC-CDM

ECE537/5 #41

OFDM and MC-CDMA in a rapidly time-varying channel

Doppler spread is the Fourier-dual of a delay spread

ECE537/5 #42

Doppler Multipath Channel

• Describe the received signal with all its delayed and Doppler-shifted components

• Compact this model into a convenient form, based on time-varying amplitudes.

• Make a (discrete-frequency) vector channel representation

• Exploit this to design better receivers

ECE537/5 #43

Mobile Multipath Channel

• Collection of reflected waves, each with

• random angle of arrival

• random delay

• Angle of arrival is uniform

• Doppler shift is cos(angle)

• U-shaped power density spectrum

Doppler Spectrum

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ICI caused by Doppler

•

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 10

-4

10-3

10-2

10-1

100

Normalized Doppler [fm/fsub]

Pow

er, V

aria

nce

of IC

I

P0

P1 P2 P3

Po

we

r o

r va

ria

nce

of I

CI

Doppler spread / Subcarrier Spacing

Neighboring subcarrier2nd tier subcarrier

3rd tier subcarrier

ECE537/5 #45

BER in a mobile channel

0 5 10 15 20 25 30 35 4010

-7

10-6

10-5

10-4

10-3

10-2

10-1

100

Antenna Speed (m/s)

Lo

cal-M

ea

n B

ER

for

BP

SK

OFDM, 10 dB

MC-CDMA, 20 dB 30 dB

MC-CDMA, 10 dB

OFDM, 20 dB

OFDM, 30 dB

• Local-mean BER for BPSK, versus antenna speed.

• Local mean SNR of 10, 20 and 30 dB.

• Comparison between MC-CDMA and uncoded OFDM for fc = 4 GHz

• Frame durationTs= 896s

• FFT size: N = 8192.

• Sub. spacing fs = 1.17 kHz

• Data rate 9.14 Msymbol/s

Antenna Speed [m/s]

ECE537/5 #46

802 Family

ECE537/5 #47

802.11 Infrastructure Mode

ECE537/5 #48

Basic MAC Operation

• CSMA/CA - Carrier Sense Multiple Access with Collision Avoidance– Also called DCF - Distributed Coordination Function– Listen for a current transmission– After transmissions stop, wait for the DIFS (DCF Inter-

Frame Spacing) plus a random additional time– First transmitter “wins”– Next frame in a sequence is sent after a shorter SIFS

(Short Inter-Frame Spacing), locking out other transmitters

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SIFS

Standard SIFS (µs)

IEEE 802.11a 16

IEEE 802.11b 10

IEEE 802.11g 10

ECE537/5 #50

DIFS

• DIFS = SIFS + 2 x (slot time)

Standard Slot Time DIFS (µs)

IEEE 802.11a 9 34

IEEE 802.11b 20 50

IEEE 802.11g 9 or 20 28 or 50

ECE537/5 #51

CSMA: (Carrier Sense Multiple Access)

CSMA: listen before transmit:• If channel sensed idle: transmit entire frame• If channel sensed busy, defer transmission

• Human analogy: don’t interrupt others!

ECE537/5 #52

CSMA collisions

collisions can still occur:propagation delay means two nodes may not heareach other’s transmissioncollision:entire packet transmission time wasted

spatial layout of nodes

note:role of distance & propagation delay in determining collision probability

ECE537/5 #53

CSMA/CD (Collision Detection)

CSMA/CD: carrier sensing, deferral as in CSMA– collisions detected within short time

– colliding transmissions aborted, reducing channel wastage

• collision detection: – easy in wired LANs: measure signal strengths, compare

transmitted, received signals

– difficult in wireless LANs: receiver shut off while transmitting

• human analogy: the polite conversationalist

ECE537/5 #54

CSMA/CD collision detection

ECE537/5 #55

IEEE 802.11: multiple access• Collision if 2 or more nodes transmit at same time

• CSMA makes sense:– get all the bandwidth if you’re the only one transmitting

– shouldn’t cause a collision if you sense another transmission

• Collision detection doesn’t work: hidden terminal problem

ECE537/5 #56

IEEE 802.11 MAC Protocol: CSMA/CA

802.11 CSMA: sender

- if sense channel idle for DIFS sec.

then transmit entire frame

(no collision detection)

-if sense channel busy, then binary backoff

802.11 CSMA receiver

- if received OK, return ACK after SIFS delay

(ACK is needed due to hidden terminal problem)

ECE537/5 #57

Collision avoidance mechanisms

• Problem: – two nodes, hidden from each other,

transmit complete frames to base station– wasted bandwidth for long duration !

• Solution: – small reservation packets– nodes track reservation interval with

internal “network allocation vector” (NAV)

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Propagation Time is Important

ECE537/5 #59

CSMA/CA + ACK

ECE537/5 #60

Hidden Node Problem

ECE537/5 #61

Exposed Node Problem

ECE537/5 #62

MAC Timing

ECE537/5 #63

What About Collisions?

• Non-broadcast packets are acknowledged

• Un-acknowledged frames are retransmitted– Retransmissions wait for a longer than normal

back-off period– A configurable counter limits the number of

retransmissions for a frame

ECE537/5 #64

RTS/CTS and the NAV

• If hidden nodes exist, transmissions will collide– Both frames are lost– Random back-off before retransmit should fix the

problem– Expensive if this happens a lot with large frames

• Stations can send an RTS frame– Include the Network Allocation Vector (NAV),

essentially “I need the network for NAV amount of time”

– AP responds with CTS including a NAV• The hidden station uses this as a “virtual carrier sense”

ECE537/5 #65

Collision Avoidance: RTS-CTS exchange

• sender transmits short RTS (request to send) packet: indicates duration of transmission

• receiver replies with short CTS (clear to send) packet– notifying (possibly hidden)

nodes

• hidden nodes will not transmit for specified duration: NAV

ECE537/5 #66

Collision Avoidance: RTS-CTS exchange

• RTS and CTS short:

– collisions less likely, of shorter duration

– end result similar to collision detection

• IEEE 802.11 allows:

– CSMA

– CSMA/CA: reservations

– polling from AP

ECE537/5 #67

RTS/CTS: Another View

ECE537/5 #68

ECE537/5 #69

ECE537/5 #70

IEEE 802.16 Wireless MAN Standard for Broadband Wireless

Metropolitan Area Networks

• Broad bandwidth– Up to 134 Mbps in 10-66 GHz band

• Comprehensive and modern security– Packet data encryption

• DES and AES used

– Key management protocol • Use RSA to set up a shared secret between subscriber station and

base station

• Use the secret for subsequent exchange of traffic encryption keys (TEK)

ECE537/5 #71

Summary

• Wireless networking adds many demands to both the design of network physical layer elements and to protocols

• Increasing demand for wireless networking will likely stretch our ability to provide robust networking that compares favorably with wired systems

ECE537/5 #72Spring 2009© 2000-2009, Richard A. Stanley

Homework

• Consider that you have built a wired network operating at 100 Mbps over 802.3. It has become necessary to extend service to users who must connect to the base network wirelessly. Choose a solution for this problem and describe how you will implement it. What problems must be dealt with? You need not limit your solutions to those that have been discussed in class to this point. Prepare a paper of approximately 1100 words describing your findings.

• Be prepared to discuss your findings with the class for 5-10 minutes next week. You may use slides if you desire.

ECE537/5 #73Spring 2009© 2000-2009, Richard A. Stanley

Disclaimer

• Parts of the lecture slides contain original work of Henrik Schober, Friedrich Jondral, Richard A. Stirling-Gallacher, Zhaocheng Wang, Mike Willis, R. J. Mohr Associates, Inc., Jean-Paul M.G. Linnartz, Bhaskaran Raman, Hans Kruse, Carl Bruggeman, and Y. Chen and remain copyrighted materials by the original owner(s). The slides are intended for the sole purpose of instruction in computer networks at Worcester Polytechnic Institute.