Transcript Orthogonal Frequency Division Multiple Access ( )

Orthogonal Frequency Division
Multiple Access
(OFDMA)
Khaja Mohammad Shazzad
Outline
1.
Background

2.
Orthogonal Frequency Division Multiplexing
(OFDM) Based Multiple Access (OFDMA)
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3.
4.
Multiple Access (MA) Methods
Orthogonality Principle
OFDM
OFDM-FDMA
Advantages and Disadvantages of OFDMA
Conclusion
2
Multiple Access (MA)
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General wireless cellular systems are multi-users
systems
Radio resource are limited
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Limited Bandwidth
Limited number of channels
The radio resource must be shared among multiple
users
Multiple Access Control (MAC) needed
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Contention-based
Non-contention-based
Contention-based
Multiple Access(MA)
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Contention-based
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Each terminal transmits in a decentralized way
No central controller (Base stations or access points)
Example:
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ALOHA
Carrier Sensing Multiple Access (CSMA)
Standard:
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GSM [l] uses the slotted ALOHA in the terminal’s initial
access process
IEEE 802.11 uses CSMA/CA based contention access
scheme
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Non-contention-based
Multiple Access (MA)
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A logic controller (BS or AP) is needed to
coordinate the transmissions of all the terminals
The controller informs each device when and on
which channel it can transmit
Collisions can be avoided entirely
Two Subdivisions
1. Non-channelization
2. Channelization
Non-channelization
Non-contention-based MA
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Terminals transmit sequentially using the same
channel
Example:
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Polling based medium access
Standard:
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IEEE 802.15(WPAN)
IEEE 802.11(WLAN)
Channelization
Non-contention-based MA
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Terminals transmit simultaneously using different channels
Most commonly used protocols in cellular systems
Example:
– 1. Time Division Multiple Access (TDMA)
– 2. Code Division Multiple Access( CDMA)
– 1. Frequency Division Multiple Access (FDMA)
Standard
– 1. GSM (TDMA)
– 2. IS-95 (CDMA)
– 3. American Mobile Phone System, AMPS (FDMA)
Time Division Multiple Access
(TDMA)
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GSM
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Time slot 0.577 ms
Frame 4.6 ms
8 time slots per frame
Frequency band 20 KHz
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Code Division Multiple Access
(CDMA)
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IS-95
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Orthogonal Walsh codes
64 codes (channels)
One pilot channel
Seven paging channels
55 traffic channels
Each carrier 1.25 MHz
Frequency Division Multiple Access
(FDMA)
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American Mobile Phone
System (AMPS)
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Total Bandwidth 25 MHz
Each Channel 30 KHz
Orthogonal Frequency Division
Multiplexing (OFDM) Based Multiple
Access (OFDMA)
Orthogonality Principle
 OFDM
 OFDM-FDMA

Orthogonality Principle
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Vector space
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A, B and C vectors in
space are orthogonal to
each other
A.B=B.C=C.A=0
(A+B+C).A=(mod A)^2
(A+B+C).B=(mod B)^2
(A+B+C).C=(mod C)^2
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B
A
C
Orthogonality Principle cont..

Real Function space
f1 (t )  A sin( wt )
f 2 (t )  B cos( wt )
 T

f1 (t ) f 2 (t ) dt  0
f m (t )  M sin( mwt)
f n (t )  N cos( nwt )
 T

f m (t ) f n (t ) dt  0
Orthogonality Principle cont..
f (t )  sin( wt ) sin(2wt )
m.n Ν
T
 sin(mwt)sin(nwt)dt  0 where m  n
0
T
 sin(mwt)cos(nwt)dt  0
0
Here mw and nw are called
m-th and n-th harmonics of
w respectively
Orthogonal Frequency Division
Multiplexing(OFDM)
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It is a special kind of FDM
The spacing between carriers are such that
they are orthogonal to one another
Therefore no need of guard band between
carriers.
One example makes the thing clear
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Example of OFDM
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Lets we have following information bits
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1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, …
Just converts the serials bits to parallel bits
C1
C2
C3
C4
1
1
-1
-1
1
1
1
-1
1
-1
-1
-1
-1
1
-1
-1
-1
1
1
-1
-1
-1
1
1
Example of OFDM cont..
Modulate each column with corresponding sub-carrier using BPSK
Modulated signal for C1
Modulated signal for C2
Modulated signal for C3
Modulated signal for C4
Example of OFDM cont..
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Final OFDM Signal = Sum of all signal
V (t )   I (t ) sin( 2nt )
N 1
n 0
n
Generated OFDM signal, V(t)
V(t)
OFDM-FDMA (OFDMA)
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Each terminal occupies
a subset of sub-carriers
Subset is called an
OFDMA traffic channel
Each traffic channel is
assigned exclusively to
one user at any time
user4
user3
user2
user1
OFDM-FDMA (OFDMA)
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The IEEE 802.16e/ WiMax use OFDMA as
Multiple access technique
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Bandwidth options 1.25, 5, 10, or 20 MHz
Entire bandwidth divided into 128, 512, 1024 or
2048 sub carriers
20 MHz bandwidth with 2048 sub carriers has 9.8
KHz spacing between sub carriers
OFDM-FDMA (System View)
Advantages of OFDMA
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Multi-user Diversity
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broadband signals experience frequency selective
fading
OFDMA allows different users to transmit over
different portions of the broadband spectrum (traffic
channel)
Different users perceive different channel qualities,
a deep faded channel for one user may still be
favorable to others
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Advantages of OFDMA cont..
Multi-user Diversity
Advantages of OFDMA cont..
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Efficient use of Spectrum
4/3 Hz per symbol
6/5 Hz per symbol
Advantages of OFDMA cont..
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Receiver Simplicity
– It eliminates the intra-cell interference avoiding
CDMA type of multi-user detection
– Orthogonality of code destroyed by selective
fading
– Only FFT processor is required
Bit Error Rate performance is better only in Fading
environment
Disadvantages of OFDMA
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Peak to average power
ratio (PAPR)
x(t )
PAPR 
P
2
avg
The large amplitude variation increases in-band noise and
increases the BER when the signal has to go through amplifier
nonlinearities.
Disadvantages of OFDMA cont..
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Synchronization
– Tight Synchronization between users are
required for FFT in receiver
– Pilot signals are used for synchronizations
Co-channel interference
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Dealing with this is more complex in OFDM than in CDMA
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Dynamic channel allocation with advanced
coordination among adjacent base stations
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Research issues and Conclusion
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Future works
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Peak-to-average power reduction in OFDM
Timing and Frequency Synchronization
Efficient digital signal processing Implementation of OFDM
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Multiple input/Multiple output (MIMO) OFDM
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Conclusion
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Different variations of OFDMA are proposed and
have different pros and cons
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Thank You
Questions
or
Comments?