ofdma.ppt
DESCRIPTION
OFDM-TDMA==OFDMATRANSCRIPT
Orthogonal Frequency Division Multiple Access
(OFDMA)
Khaja Mohammad Shazzad
2
Outline
1. Background Multiple Access (MA) Methods
2. Orthogonal Frequency Division Multiplexing (OFDM) Based Multiple Access (OFDMA) Orthogonality Principle OFDM OFDM-FDMA
3. Advantages and Disadvantages of OFDMA4. Conclusion
Multiple Access (MA)
General wireless cellular systems are multi-users systems
Radio resource are limited– Limited Bandwidth– Limited number of channels
The radio resource must be shared among multiple users
Multiple Access Control (MAC) needed– Contention-based– Non-contention-based
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Contention-basedMultiple Access(MA)
Contention-based– Each terminal transmits in a decentralized way– No central controller (Base stations or access points)– Example:
ALOHA Carrier Sensing Multiple Access (CSMA)
Standard:– GSM [l] uses the slotted ALOHA in the terminal’s initial
access process– IEEE 802.11 uses CSMA/CA based contention access
scheme
Non-contention-based Multiple Access (MA)
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-channelizationNon-contention-based MA
Terminals transmit sequentially using the same channel
Example:– Polling based medium access
Standard:– IEEE 802.15(WPAN)– IEEE 802.11(WLAN)
ChannelizationNon-contention-based MA
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)
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Time Division Multiple Access (TDMA)
GSM– Time slot 0.577 ms– Frame 4.6 ms– 8 time slots per frame– Frequency band 20 KHz
Code Division Multiple Access (CDMA)
IS-95– 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)
American Mobile Phone System (AMPS)
– Total Bandwidth 25 MHz– Each Channel 30 KHz
Orthogonal Frequency Division Multiplexing (OFDM) Based Multiple Access (OFDMA)
Orthogonality PrincipleOFDMOFDM-FDMA
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Orthogonality Principle
Vector space– 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
A
B
C
Orthogonality Principle cont..
Real Function space
0)()(
)cos()(
)sin()(
0)()(
)cos()(
)sin()(
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2
1
dttftf
nwtNtf
mwtMtf
dttftf
wtBtf
wtAtf
n
T
m
n
m
T
Orthogonality Principle cont..
)2sin()sin()( wtwttf
T
00s(nwt)dtsin(mwt)co
nmwhereT
00n(nwt)dtsin(mwt)si
Νm.n
Here mw and nw are called m-th and n-th harmonics of w respectively
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Orthogonal Frequency Division Multiplexing(OFDM)
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
Example of OFDM
Lets we have following information bits– 1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, …
Just converts the serials bits to parallel bitsC1 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..
Modulated signal for C1 Modulated signal for C2
Modulated signal for C3 Modulated signal for C4
Modulate each column with corresponding sub-carrier using BPSK
Example of OFDM cont..
Final OFDM Signal = Sum of all signal
)2sin()()(1
0nttItV
N
nn
Generated OFDM signal, V(t)
V(t)
OFDM-FDMA (OFDMA)
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 user1
user2
user3
user4
OFDM-FDMA (OFDMA)
The IEEE 802.16e/ WiMax use OFDMA as Multiple access technique– 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)
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Advantages of OFDMA
Multi-user Diversity– 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
Advantages of OFDMA cont..
Multi-user Diversity
Advantages of OFDMA cont..
Efficient use of Spectrum
4/3 Hz per symbol
6/5 Hz per symbol
Advantages of OFDMA cont..
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
Peak to average power ratio (PAPR)
avgP
txPAPR
2
)(
The large amplitude variation increases in-band noise and increases the BER when the signal has to go through amplifier nonlinearities.
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Disadvantages of OFDMA cont..
Synchronization– Tight Synchronization between users are
required for FFT in receiver– Pilot signals are used for synchronizations
Co-channel interference – Dealing with this is more complex in OFDM than in CDMA
– Dynamic channel allocation with advanced coordination among adjacent base stations
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Research issues and Conclusion
Future works– Peak-to-average power reduction in OFDM – Timing and Frequency Synchronization– Efficient digital signal processing Implementation of OFDM
– Multiple input/Multiple output (MIMO) OFDM
Conclusion– Different variations of OFDMA are proposed and
have different pros and cons
Thank You
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