Advance Communication System Lectures Part 8

8/2/2019 Advance Communication System Lectures Part 8

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Multiple Access Techniques

& Spread Spectrum

Preston UniversityKashif Shahzad07-1


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Todays Class

Multiple Access Techniques

Spread Spectrum in detail



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MULTIPLE

ACCESSTECHNIQUES

FDMAFrequency

Time

TDMA

Frequency

Time

CDMA

Frequency

Time


4/27

Question

The EM spectrum is a limited resource

How can we share it? Time

Space

Frequency

Polarization

Spread Spectrum - use a wider bandwidth?



5/27

Multiple Access techniques

Goal

Time Division Multiple Access (TDMA)

Space Division Multiple Access (SDMA)

Frequency Division Multiple Access (FDMA) Polarization Division Multiple Access (PDMA)

Code Division Multiple Access (CDMA) aka Spread Spectrum

allow many users tosimultaneously share acommunications resource



6/27

Key Issue separate the signals at the receiver to extractyourinformation

Two methods

Do not mix the signals in the first place can use space or time (SDMA or TDMA)

Use distinctive properties of each signal as a means

to identify Frequency spectrum (FDMA)

Polarization of waves (PDMA)

code sequence attached to each message (CDMA)



7/27

TDMA Illustration

Divides the radio spectrum into time slots,and in each slot only one user is allowed toeither transmit or receive



8/27

TDMA Details

The incoming data from each source are brieflybuffered and scanned to to form a composite

digital data stream mc( t) .


Buffer

Buffer

Buffer

1 2 N

U1

U2

UN

m1( t)

m2( t)

mN

( t)

mc( t)

Scan operation

prea

mble 1 2 N

prea

mble...

information

Each slot may be empty or occupied.

+ has preamble & guard bits

Frame Frame

Time slot


9/27

TDMA Pros and Cons

Advantages

flexible bit rate

channels may havevarying data rates

efficient use ofchannels

Disadvantages

Synchronization

must lock on to your timeslot (signal processing)

Overhead processing required for

buffering...



10/27

SDMA Space Division Multiple Access Use highly directional Ae

The receiver selects the beam that provides the greatestsignal enhancement and interference reduction

Smart antenna systemscan adjust their antennapattern to enhance thedesired signal, null orreduce interference.


DesiredSignal

Direction


11/27

SDMA Pros and Cons

Advantages

BW increases with km2

Simple system

Disadvantages

Restricted Geometry

terminals in same directioncannot share

May have unused BW if no terminals in given

zone, bw not used



12/27

FDMA Illustration

assigns individual channels or bands toindividual users, which can only be used bythat user for the period of the call



13/27

FDMA Pros and ConsAdvantages

Simple to implement

Continuous txminimal overhead bits

Disadvantages

Nonlinear Effects

sharing of Ae causesspreading at saturation

Inefficient once assigned, cannot be

reassigned even if not inuse



14/27

SPREAD SPECTRUM

CDMA - FHMA - DSMA - SSMA


15/27

Definition - Spread Spectrum The transmission bandwidth must be much

larger than the information bandwidth

The resulting RF bandwidth is determined by a

function other than the information being sent



16/27

f

Power

Density

PDi

PDSS

ConventionalTransmission

Spread SpectrumTransmission

BSS

Bi

same total power

Spread Spectrum - illustrated



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How Two main methods

Frequency Hopped Multiple Access (FHMA)

Direct Sequence Multiple Access (DSMA)

THMA does exist, but not common

Both depend on pseudo random orthogonalcodes remember part 2 of definition...

often called pseudo noise



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FHSS Frequency Hopping Multiple Access

message is "cut" into small "chunks"

Each chunk is modulated by a different fc(determined by pseudo-random code)

A band pass filter accepts the signals that followthe hopping sequence and rejects all other requiressynchronization

note - some early systems used shortpredictable patterns



19/27

FHSS - illustrated


Frequency

Time

TuneTime

Frequency

Hop

Dwell

Time


20/27

DSMA Direct Sequence Multiple Access

Each bit is chipped

Example - time domain


Data

Chips

0.1 ms

1 bit

0.1 s

1000 chips

Requires much wider bandwidth


21/27

Cross Correlation

to determine start of code

to lock onto correct code

Mathematical process used to determine thesimilarity between two signals


111101011001000

011110101100100

100011110101100Modulo-2 sum

15-bit Code

Received Signal

Correlation = -1/15 (very poor)

Used for despreading


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Pseudo Random Orthogonal... Different sequences are said to be orthogonal

if they do not interfere with one another (ie havelow cross correlation)

A sequence is pseudo random if it isorthogonal with a time shifted version of itself

note - this significantly reduces the number of

codes available


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Spreading Process


Transmitted(Coded) Signal

BasebandSignal

Noise

Info Signal

Before spreading After spreading

Info

How can you recover signal < noise


24/27

SNR gain of spread spectrum The ratio of the SNR out to the SNR into the

demodulator ( spreading factor).


SNRout BWRF

SNRin RinfoGP = =


25/27

CDMA Pros and Cons

Advantages

security

channel capacity

4xTDMA, 20xFDMA One channel for all

< fading in wideband

LPE, LPI, LPD low probability of

exploitation

interception

detection

Disadvantages

self jamming

near far problem

power issues



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Final

Most modern systems use a combination ofTDMA, SDMA, FDMA, CDMA

Take advantage of best features of each



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Comparison SDMA/TDMA/FDMA/CDMA

P t U i itK hif Sh h d07-27

Approach SDMA TDMA FDMA CDMA

Idea segment space into

cells/sectorstime

segment sending

time into disjointslots, demand

driven or fixedpatterns

Terminals onlyone terminal canbe active in onecell/one sector

all terminals areactive for shortperiods of time onthe same frequency

bands

segment the

frequency band intodisjoint sub

every terminal has itsown frequency,uninterrupted

spread the spectrum

using orthogonal codes

all terminals can be activeat the same place at thesame moment,uninterrupted

Signalseparation

cell structure, directedantennas

synchronization inthe time domain

filtering in thefrequency domain

code plus specialreceivers

Advantages very simple, increasescapacity per km

established, fullydigital, flexible

simple, established,robust

Dis-advantages

inflexible, antennastypically fixed

guard spaceneeded (multipathpropagation),synchronizationdifficult

inflexible,frequencies are ascarce resource

flexible, less frequencyplanning needed, softhandover

complex receivers, needsmore complicated powercontrol for senders

Comment only in combinationwith TDMA, FDMA orCDMA useful

standard in fixednetworks, togetherwith FDMA/SDMAused in manymobile networks

typically combinedwith TDMA(frequency hoppingpatterns) and SDMA(frequency reuse)

still faces some problems,higher complexity,lowered expectations; willbe integrated withTDMA/FDMA

From: Prof Dr -Ing Jochen Schiller http://www jochenschiller de/ MC SS02

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Advance Communication System Lectures Part 8