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CIS 632 / EEC 687

Mobile Computing

Mobile Communications (for Dummies)

Chansu Yu

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Contents

Modulation

Propagation

Spread spectrum

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1

0

t

digital signal

Digital Communication

Want to transform to since waves:

Why?

• Natural phenomena produce sine

waves

• When a microphone picks up an

audible tone, the output is a sine

• Electromagnetic radiation can be

represented as a sine wave

4

)2cos()2sin(2

1)(

11

nftbnftactgn

n

n

n

1

0

t

digital signal

Digital Communication - Fourier Transform

Want to send a variety of signals (FM, Wifi, Satellite,

Bluetooth, Remote control, etc.) concurrently.

This is why we need modulation!

1

0

t

decomposition

f

Frequency

domain100KHz

25KHz

50KHz 25k 100k

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5

Digital Communication - Modulation

f

Frequency

domain

25k 100k f2.4G 2.4G

+25k +100k

90M 90M

+5k +40k

FM WiFi

Modulation allows us to send a signal over a bandpass

frequency range. If every signal gets its own frequency range,

then we can transmit multiple signals simultaneously over a

single channel, all using different frequency ranges. Another

reason to modulate a signal is to allow the use of a smaller

antenna.* What 90M or 2.4G is called?

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Amplitude Shift Keying (ASK):

Frequency Shift Keying (FSK):

Phase Shift Keying (PSK):

1 0 1

t

1 0 1

t

1 0 1

t

Digital Communication – “Shift Keying”

: Digital 0/1Sine Waves

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4-ASK?

Binary Amplitude Shift Keying (ASK)

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4-FSK?

Frequency Shift Keying (FSK)

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0 180 0 0 180

(Phase changes are overlaid over the carrier signal)

Phase Shift Keying (PSK)

Constellation

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Quadrature PSK (QPSK)

Higher throughput: Encode 2 or more bits onto one signal element.

More fragile: Noise makes it more difficult to distinguish between, for example, 11 and 10.

11 10 00 01

A

t

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11

Two carriers

are 90 degree

shifted

Quadrature PSK (QPSK)

Constellation

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Quadrature PSK (QPSK)

QPSK

OffsetQPSK

Which is more robust to channel error?

8QPSK

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QAM (Quadrature Amplitude

Modulation)

In QAM, a finite number of at least two phases,

and at least two amplitudes are used.

In-phase signal (for example a cosine waveform)

Quadrature phase signal (for example a sine wave)

They are amplitude modulated with a finite

number of amplitudes, and summed.

The resulting signal is equivalent to a

combination of PSK and ASK.

Constellationfor 16 QAM

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802.11/a/b

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Bluetooth and Zigbee

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Contents

Modulation

Propagation

Spread spectrum

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Free-Space Propagation

In free space, receiving power proportional to 1/d² (d = distance between transmitter and receiver)

Suppose transmitted signal is x,received signal y = h x, where h is proportional to 1/d²

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4

dGG

P

Ptr

t

r

Pr: received power

Pt: transmitted power

Gr, Gt: receiver and transmitter antenna gain

(=c/f): wave length

Sometime we write path loss in log scale:

Lp = 10 log(Pt) – 10log(Pr)

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Let’s Do the Math

Wifi radio

Transmit power (Pt) = 28 dBm = ??? Watt

Distance (d) = 1km (0.6 mile)

Pr = Pt*GtGrht2hr2/Ld4

Gt=Gr=1, ht=hr=1.5, L=1

Pr = 3.2 x 10-12 Watt

dB = 10 log (-------)P1

P2

dBm = 10 log (-------)P1

1mW

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1x10-12 Watt

2.5x10-12 Watt

2.5x10-12 Watt

4.0x10-12 Watt

What does

Pr = 3.2 x 10-12 Watt

mean?

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Real Antennas

Real antennas are not isotropic radiators

Some simple antennas: quarter wave /4 on car roofs or half wave dipole /2 size of antenna proportional to wavelength for better transmission/receiving

/4/2

Q: Assume frequency 1 Ghz, = ?

Real Measurements

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Receiving power additionally influenced by shadowing (e.g. through a wall or a door)

refraction depending on the density of a medium

reflection at large obstacles

scattering at small obstacles

diffraction at edges

reflection

scattering

diffraction

shadow fadingrefraction

Signal Propagation

Shadowing

Signal strength loss after passing through obstacles Some sample numbers

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Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction

Multipath

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Multiple Rates in

IEEE 802.11/a/b/g

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Trace from 2004 SIGCOMM

(CRAWDAD)

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802.11 PHY Packet Formats

synchronization SFD PLW PSF HEC Payload (MPDU)

PLCP preamble PLCP header

80 16 12 4 16 Variable (<4KB)

synchronization SFD signal service HEC Payload (MPDU)

PLCP preamble PLCP header

128 16 8 8 16

length

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DSSS

FHSS

* MPDU: MAC Protocol Data Units

Always 1Mbps with BPSK modulation

Always 1Mbps with GFSK modulation

Data rate for MPDU:

Up to 11Mbps (802.11b, DSSS) in steps of 100kbps

Up to 2Mbps (802.11, DSSS) in steps of 100kbps

Up to 2Mbps (802.11, FHSS) in steps of 0.5Mbps

Variable (<4KB)

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Multi-rate Control in WLANs

Access point

E.g., increase the rate upon consecutive successes; decrease the rate upon consecutive failures

ARF (Autorate Fallback)

• Relatively simple

• A client node has only

one communication

partner (access point)

• Each node can

individually optimize its

data rate for the link to

AP

• Already being used in

commercial products

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Auto-Rate Fallback (ARF)

The first multi-rate algorithm for 1&2Mbps WaveLAN

Many commercially available 802.11 cards support some sort of ARF for automatic rate selection

Use a higher rate upon 10 consecutive successful transmissionsWhen the rate is increased, the first transmission

must succeed or the rate is immediately decreased

Fall back to a lower rate after 2 consecutive transmission failures

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Receiver-Based Auto Rate (RBAR)

Upon receiving an RTS frame,

The receiver estimates the channel quality based on SINR of

the received RTS frame and then

Determines the best data rate that the transmitter must use

The estimated optimal rate is then sent back to the sender

piggybacking in the CTS packet

A major drawback: Requires

incompatible changes to the

IEEE 802.11 standard

RTS@1

CTS+Opt.rate

Data@5.5

ACK

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OAR & MAD

Opportunistic Auto Rate (OAR) protocol Built on a multi-rate algorithm such as ARF or RBAR

Although the channel condition fluctuates, it is consistent for the time duration, which is enough to transmit more than one packet

When a high-quality channel condition is observed, send multiple back-to-back data packets

Medium Access Diversity (MAD) Looks for the receiver whose channel condition is near its

peak based on channel probing

Group RTS (GRTS) for query (with list of receivers) and CTS’s for replies (with channel condition information)

The sender then chooses the receiver that can maximally utilize the channel

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Other Multi-rate Adaptation Methods

Sender-based Auto-Rate Fallback (ARF, 97)

Adaptive ARF (AARF, 04)

Estimated Rate Fallback (ERF, 05)

MADWIFI (04)

Adaptive Multi Rate Retry (AMRR, 05)

Collision-Aware Rate Adaptation (CARA, 06)

Receiver-based Receiver-Based Auto Rate (RBAR, 01)

Opportunistic Auto Rate (OAR, 02)

Medium Access Diversity (MAD, 05)

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Contents

Modulation

Propagation

Spread spectrum

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Why Spread Spectrum?

Initially adopted in military applications, for its resistance to jamming and difficulty of interception (seemingly a noise)

Adopted recently in CDMA, WiFi & Bluetooth & Zigbee

Power

Interference

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FHSS (Frequency Hopping SS)

User data

3 hops/bit,

in general, a very

short packet/hop or

even a bit/multiple hops

as in this example

0 1

tb

0 1 1 tf

t

f

f1

f2

f3

t

td

With a binary frequency-modulation scheme in which the carrier simply shifts

up or down in frequency by about 150 kHz to represent, respectively, a 1 or a 0

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OFDM (Orthogonal frequency

division multiplexing )

In OFDM technology, the bit string to be transmitted

is broken down into N (N>1) bit strings. The N bit

strings are then transmitted in parallel through N

orthogonal sub-channels.

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IEEE 802.11/a/b

Physical layer

802.11a/g

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Advanced Topics

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*

Physical Layer Watermarking

Adjusting phase difference in M-PSK

modulation

BPSK

modulation

8PSK

modulation: add

π/4 to embed

watermark bit 1

0 (0) π (1)

π/4 (?)

0 (0) π (1)

0 (0)

Watermarked DSSS (WDSSS)

Spreading

Correlation

Original PN sequence:

00010

Two watermark values

are embedded

Aware receiver

records the flipped

positions, interprets

them into watermark

values

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802.11 MAC and Collision

Alice Bob

AP

CSMA is not perfect…

Collision

Repeatedly collide… with some random jitter

“ZigZag Decoding: Combating Hidden Terminals in Wireless Networks”, S. Gollakota and D. Katabi (MIT), SIGCOMM 2008.

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ZigZag Decoding

AP

Alice Bob

Pa1 3 Pa1 3

Pb2 4Pb42∆1 ∆2

∆1- ∆2

1st collision 2nd collision

0

Can reconstruct both packets Pa and Pb!!