Wireless Networks (PHY): Design for Diversity Y. Richard Yang 9/20/2012.

Wireless Networks (PHY): Design for Diversity

Y. Richard Yang

9/20/2012

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Outline

Admin and recap

Design for diversity

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Admin

Assignment 1 questions

Assignment 1 office hours Thursday 3-4 @ AKW 307A

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Channel characteristics change over location, time, and frequency

small-scale fading

Large-scalefading

time

power

Recap: Wireless Channels

path loss

log (distance)

Received Signal Power (dB)

frequency

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Outline

Recap Wireless channels Physical layer design

design for flat fading• how bad is flat fading?• diversity to handle flat fading

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Recap: Impact of Channel on Decisions

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Recap: Impact of Channel

Averaged out over h,

at high SNR.

Assume h is Gaussian random:

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Recap: Impacts of Channel

static channel

flat fading channel

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Outline



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Main Storyline Today

Communication over a flat fading channel has poor performance due to significant probability that channel is in a deep fade

Reliability is increased by providing more resolvable signal paths that fade independently

Name of the game is how to find and efficiently exploit the paths

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Where to Find Diversity?

Time: when signal is bad at time t, it may not be bad at t+t

Space: when one position is in deep fade, another position may be not

Frequency: when one frequency is in deep fade (or has large interference), another frequency may be in good shape

2121 22

dd

c

ddf

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Outline



– time

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Time Diversity Time diversity can be obtained by interleaving

and coding over symbols across different coherent time periods

interleave

coherencetime

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1 2 3 4 5 6 7 8

935-960 MHz124 channels (200 kHz)downlink

890-915 MHz124 channels (200 kHz)uplink

frequ

ency

time

GSM TDMA frame

GSM time-slot (normal burst)

4.615 ms

546.5 µs577 µs

tail user data TrainingSguardspace S user data tail

guardspace

3 bits 57 bits 26 bits 57 bits1 1 3

Example: GSM Time Structure

S: indicates data or control

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Example: GSM Bit Assignments

Amount of time diversity limited by delay constraint and how fast channel varies

In GSM, delay constraint is 40 ms (voice) To get better diversity, needs faster moving vehicles

!

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Simplest Code: Repetition

After interleaving over L coherence time periods,

Le SNRP

1

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Performance

Le SNRP

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Beyond Repetition Coding

Repetition coding gets full diversity, but sends only one symbol every L symbol times

We can use other codes, e.g. Reed-Solomon code

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Outline



– time– space

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Space Diversity: Antenna

Receive Transmit Both

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User Diversity: Cooperative Diversity

Different users can form a distributed antenna array to help each other in increasing diversity

Interesting characteristics: users have to exchange information and

this consumes bandwidth broadcast nature of the wireless medium

can be exploited we will revisit the issue later in the course

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Outline



– time– space– frequency

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1'

dd

cff

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Discrete changes of carrier frequency sequence of frequency changes determined via pseudo

random number sequence used in 802.11, GSM, etc

Co-inventor: Hedy Lamarr patent# 2,292,387

issued on August 11, 1942 intended to make radio-guided

torpedoes harder for enemies to detect or jam

used a piano roll to change between 88 frequencies

Sequential Frequency Diversity: FHSS (Frequency Hopping Spread Spectrum)

http://en.wikipedia.org/wiki/Hedy_Lamarr

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Two versions slow hopping: several user bits per frequency fast hopping: several frequencies per user bit

Sequential Frequency Diversity: FHSS (Frequency Hopping Spread Spectrum)

user data

slowhopping(3 bits/hop)

fasthopping(3 hops/bit)

0 1

tb

0 1 1 t

f

f1

f2

f3

t

td

f

f1

f2

f3

t

td

tb: bit period td: dwell time

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Frequency selective fading and interference limited to short period

Simple implementation what is a major issue in design?

Uses only small portion of spectrum at any time explores frequency sequentially used in simple devices such Bluetooth

FHSS: Advantages

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Bluetooth Design Objective

Design objective: a cable replacement technology 1 Mb/s

range 10+ meters

single chip radio + baseband (means digital part)

• low power • low price point (target price $5 or lower)

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Bluetooth Architecture

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Bluetooth Radio Link

Bluetooth shares the same freq. range as 802.11 Radio link is the most expensive part of a

communication chip and hence chose simpler FHSS

• 2.402 GHz + k MHz, k=0, …, 78• 1,600 hops per second

A type of FSK modulation• 1 Mb/s symbol rate

transmit power: 1mW

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Bluetooth Physical Layer Nodes form piconet: one master and upto 7 slaves

Each radio can function as a master or a slave The slaves follow the pseudorandom jumping sequence of the

master

A piconet

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Piconet Formation

Master hopes at a universal frequency hopping sequence (32 frequencies) announce the master and sends Inquiry msg

Joining slave: jump at a much lower speed after receiving an Inquiry message, wait for a

random time, then send a request to the master

The master sends a paging message to the slave to join it

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Outline




» sequential » parallel

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Direct Sequence Spread Spectrum (DSSS)

Basic idea: increase signaling function alternating rate to expand frequency spectrum (explores frequency in parallel)

fc: carrier freq. Rb: freq. of data10dB = 10; 20dB =100

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Direct Sequence Spread Spectrum (DSSS)

Approach: One symbol is spread to multiple chips the number of chips is called the expansion factor examples

• 802.11: 11 Mcps; 1 Msps– how may chips per symbol?

• IS-95 CDMA: 1.25 Mcps; 4,800 sps– how may chips per symbol?

• WCDMA: 3.84 Mcps; suppose 7,500 sps – how many chips per symbol?

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dP/df

f

dP/df

f

sender

Effects of Spreading

un-spread signal

spread signal

BbBbBs Bs

Bs : num. of bits in the chip * Bb

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DSSS Encoding/Decoding: An Operating View

Xuser data

chippingsequence

modulator

radiocarrier

spreadspectrumsignal

transmitsignal

transmitter

demodulator

receivedsignal

radiocarrier

X

chippingsequence

receiver

low pass

products

decisiondata

sampledsums

correlator

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DSSS Encoding

user data d(t)

chipping sequence c(t)

resultingsignal

1 -1

-1 1 1 -1 1 -1 1 -11 -1 -1 1 11

X

=

tb

tc

tb: bit periodtc: chip period

-1 1 1 -1 -1 1 -1 11 -1 1 -1 -11

DSSS Encoding

Data: [1 -1 ]

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chip:

-1 1 1 -1 1 -1

-1 1 1 -1 1 -1

1 -1 -1 1 -1 1

DSSS Decoding

Data: [1 -1]

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1 -1 -1 1 -1 1

chip:

-1 1 1 -1 1 -1

-1 1 1 -1 1 -1

-1 1 1 -1 1 -1

Transchips

-1 1 1 -1 1 -1Chipseq:

innerproduct: 6

decision: 1

1 -1 -1 1 -1 1-1 1 1 -1 1 -1decodedchips

-6

-1

DSSS Decodingwith noise

Data: [1 -1]

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1 -1 -1 1 -1 1

chip:

-1 1 1 -1 1 -1

-1 1 1 -1 1 -1

-1 1 1 -1 1 -1

Transchips

-1 1 1 -1 1 -1Chipseq:

innerproduct: 4

decision: 1

1 -1 1 1 -1 -1-1 1 1 -1 -1 -1decodedchips

-2

-1

DSSS Decoding (BPSK): Matched Filter

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s: modulating sinoid

compute correlationfor each bit time

c: chipping seq.

y: received signaltake N samples ofa bit timesum = 0;for i =0; { sum += y[i] * c[i] * s[i] } if sum >= 0 return 1;else return -1;

bit time

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Outline




» DSSS: why it works?

Assume no DSSS

Consider narrowband interference

Consider BPSK with carrier frequency fc

A “worst-case” scenario data to be sent x(t) = 1 channel fades completely at fc (or a jam

signal at fc) then no data can be recovered

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Why Does DSSS Work:A Decoding Perspective Assume BPSK modulation using carrier frequency

f : A: amplitude of signal f: carrier frequency x(t): data [+1, -1] c(t): chipping [+1, -1]

y(t) = A x(t)c(t) cos(2 ft)

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Add Noise/Jamming/Channel Loss

Assume noise at carrier frequency f:

Received signal: y(t) + w(t)

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DSSS/BPSK Decoding

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dP/df

f

i)

dP/df

f

ii)

senderuser signalbroadband interferencenarrowband interference

dP/df

f

iii)

dP/df

f

iv)

receiver

f

v)

dP/df

Why Does DSSS Work:A Spectrum Perspective

i) → ii): multiply data x(t) by chipping sequence c(t) spreads the spectrum ii) → iii): received signal: x(t) c(t) + w(t), where w(t) is noiseiii) → iv): (x(t) c(t) + w(t)) c(t) = x(t) + w(t) c(t)iv) → v) : low pass filtering

Backup Slides

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Inquiry Hopping

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The Bluetooth Link Establishment Protocol

FS: Frequency Synchronization

DAC: Device Access Code

IAC: Inquiry Access Code

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Bluetooth Links

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Bluetooth Packet Format

Header

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Multiple-Slot Packet

Wireless Networks (PHY): Design for Diversity Y. Richard Yang 9/20/2012.

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Transcript of Wireless Networks (PHY): Design for Diversity Y. Richard Yang 9/20/2012.