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8/2/2019 Advance Communication System Lectures Part 8
1/27
Multiple Access Techniques
& Spread Spectrum
Preston UniversityKashif Shahzad07-1
8/2/2019 Advance Communication System Lectures Part 8
2/27
Todays Class
Multiple Access Techniques
Spread Spectrum in detail
Preston UniversityKashif Shahzad07-2
8/2/2019 Advance Communication System Lectures Part 8
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MULTIPLE
ACCESSTECHNIQUES
FDMAFrequency
Time
TDMA
Frequency
Time
CDMA
Frequency
Time
8/2/2019 Advance Communication System Lectures Part 8
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?
Preston UniversityKashif Shahzad07-4
8/2/2019 Advance Communication System Lectures Part 8
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
Preston UniversityKashif Shahzad07-5
8/2/2019 Advance Communication System Lectures Part 8
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)
Preston UniversityKashif Shahzad07-6
8/2/2019 Advance Communication System Lectures Part 8
7/27
TDMA Illustration
Divides the radio spectrum into time slots,and in each slot only one user is allowed toeither transmit or receive
Preston UniversityKashif Shahzad07-7
8/2/2019 Advance Communication System Lectures Part 8
8/27
TDMA Details
The incoming data from each source are brieflybuffered and scanned to to form a composite
digital data stream mc( t) .
Preston UniversityKashif Shahzad07-8
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
8/2/2019 Advance Communication System Lectures Part 8
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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...
Preston UniversityKashif Shahzad07-9
8/2/2019 Advance Communication System Lectures Part 8
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.
Preston UniversityKashif Shahzad07-10
DesiredSignal
Direction
8/2/2019 Advance Communication System Lectures Part 8
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
Preston UniversityKashif Shahzad07-11
8/2/2019 Advance Communication System Lectures Part 8
12/27
FDMA Illustration
assigns individual channels or bands toindividual users, which can only be used bythat user for the period of the call
Preston UniversityKashif Shahzad07-12
8/2/2019 Advance Communication System Lectures Part 8
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
Preston UniversityKashif Shahzad07-13
8/2/2019 Advance Communication System Lectures Part 8
14/27
SPREAD SPECTRUM
CDMA - FHMA - DSMA - SSMA
8/2/2019 Advance Communication System Lectures Part 8
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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
Preston UniversityKashif Shahzad07-15
8/2/2019 Advance Communication System Lectures Part 8
16/27
f
Power
Density
PDi
PDSS
ConventionalTransmission
Spread SpectrumTransmission
BSS
Bi
same total power
Spread Spectrum - illustrated
Preston UniversityKashif Shahzad07-16
8/2/2019 Advance Communication System Lectures Part 8
17/27
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
Preston UniversityKashif Shahzad07-17
8/2/2019 Advance Communication System Lectures Part 8
18/27
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
Preston UniversityKashif Shahzad07-18
8/2/2019 Advance Communication System Lectures Part 8
19/27
FHSS - illustrated
Preston UniversityKashif Shahzad07-19
Frequency
Time
TuneTime
Frequency
Hop
Dwell
Time
8/2/2019 Advance Communication System Lectures Part 8
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DSMA Direct Sequence Multiple Access
Each bit is chipped
Example - time domain
Preston UniversityKashif Shahzad07-20
Data
Chips
0.1 ms
1 bit
0.1 s
1000 chips
Requires much wider bandwidth
8/2/2019 Advance Communication System Lectures Part 8
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Cross Correlation
to determine start of code
to lock onto correct code
Mathematical process used to determine thesimilarity between two signals
Preston UniversityKashif Shahzad07-21
111101011001000
011110101100100
100011110101100Modulo-2 sum
15-bit Code
Received Signal
Correlation = -1/15 (very poor)
Used for despreading
8/2/2019 Advance Communication System Lectures Part 8
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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
8/2/2019 Advance Communication System Lectures Part 8
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Spreading Process
Preston UniversityKashif Shahzad07-23
Transmitted(Coded) Signal
BasebandSignal
Noise
Info Signal
Before spreading After spreading
Info
How can you recover signal < noise
8/2/2019 Advance Communication System Lectures Part 8
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SNR gain of spread spectrum The ratio of the SNR out to the SNR into the
demodulator ( spreading factor).
Preston UniversityKashif Shahzad07-24
SNRout BWRF
SNRin RinfoGP = =
8/2/2019 Advance Communication System Lectures Part 8
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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
Preston UniversityKashif Shahzad07-25
8/2/2019 Advance Communication System Lectures Part 8
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Final
Most modern systems use a combination ofTDMA, SDMA, FDMA, CDMA
Take advantage of best features of each
Preston UniversityKashif Shahzad07-26
8/2/2019 Advance Communication System Lectures Part 8
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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