Chapter 5 TDMA FDMA CDMA OFDMA Reading Assignment Solution
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Transcript of Chapter 5 TDMA FDMA CDMA OFDMA Reading Assignment Solution
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Orthogonal Frequency Division Multiple Access (OFDMA)
Zeeshan Asim
SSUET
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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 OFDMA
4. Conclusion
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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-based Multiple 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 terminals
initial access process IEEE 802.11 uses CSMA/CA based contention access
scheme
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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
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Non-channelization Non-contention-based MA
Terminals transmit sequentially using the same channel
Example: Polling based medium access
Standard: IEEE 802.15(WPAN) IEEE 802.11(WLAN)
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Channelization Non-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
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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
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Frequency Division Multiple Access (FDMA)
American Mobile Phone System (AMPS)
Total Bandwidth 25 MHz
Each Channel 30 KHz
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Objectives
What is OFDM
How OFDM works
Types of OFDM
Differences from other multiplexing techniques
Applications
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What is OFDM
Definition
The first OFDM schemes presented in 1966
Development of OFDM over years
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A
F
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How OFDM works
It distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other
than their own.
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Types of OFDM
C-OFDM
MIMO-OFDM
V-OFDM
W-OFDM
Flash-OFDM
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C-OFDM
Coded OFDM
Digital Audio Broadcasting (DAB)
Digital Video Broadcasting (DVB-T)
COFDM offers real benefit in the presence of isolated narrow-band interfering signals
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Multiple Input, Multiple Output OFDM (MIMO-OFDM)
Developed by Iospan Wireless
Uses multiple antennas to transmit and receive radio signals
Spatial multiplexing
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V-OFDM
Vector OFDM
Developed by CISCO
Increases subscriber coverage
Lowers the cost of provisioning and deploying infrastructure
Employs both frequency and spatial diversity
Creates a robust processing technique for multi-path fading and narrow band interference
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Subscriber
Base Station
Paths
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W-OFDM
Wideband OFDM
Invented by Wi-LAN
Large spacing between carriers
Advantages: - Optimal performance against Multi-path
- Less sensitive to carrier offset
-Optimal power efficiency of the transmitter amplifier
- More immune against fading
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Flash-OFDM
Fast-hopped OFDM
Wide-band spread-spectrum technology
Avoids the compromises inherent in other mobile data systems
Capability to work around interfering signals
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Differences from other multiplexing techniques
OFDM versus WDM
-more flexible
-higher bandwidth efficiency
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Differences from other multiplexing techniques
OFDM versus TDM
-number of carriers
-synchronization
-sensitivity
-capacity/efficiency advantages
-complexity and cost issues
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Applications
Digital Audio and Video Broadcasting
Asymmetric Digital Subscriber Line (ADSL)
Wireless Networking
Power-line Technology
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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
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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
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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
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Advantages of OFDMA cont..
Multi-user Diversity
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Advantages of OFDMA cont..
Efficient use of Spectrum
4/3 Hz per symbol
6/5 Hz per symbol
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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
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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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Thank You Questions
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