Title: Orthogonal Frequency Division Multiple Access (OFDMA)
1Orthogonal Frequency Division Multiple
Access(OFDMA)
2Outline
- Background
- Multiple Access (MA) Methods
- Orthogonal Frequency Division Multiplexing (OFDM)
Based Multiple Access (OFDMA) - Orthogonality Principle
- OFDM
- OFDM-FDMA
- Advantages and Disadvantages of OFDMA
- Conclusion
3Multiple 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
4Contention-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 terminals
initial access process - IEEE 802.11 uses CSMA/CA based contention access
scheme
5Non-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
- Non-channelization
- Channelization
6Non-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)
7ChannelizationNon-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)
8Time Division Multiple Access (TDMA)
- GSM
- Time slot 0.577 ms
- Frame 4.6 ms
- 8 time slots per frame
- Frequency band 20 KHz
9Code 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
10Frequency Division Multiple Access (FDMA)
- American Mobile Phone System (AMPS)
- Total Bandwidth 25 MHz
- Each Channel 30 KHz
11Orthogonal Frequency Division Multiplexing (OFDM)
Based Multiple Access (OFDMA)
- Orthogonality Principle
- OFDM
- OFDM-FDMA
12Orthogonality Principle
- Vector space
- A, B and C vectors in space are orthogonal to
each other - A.BB.CC.A0
- (ABC).A(mod A)2
- (ABC).B(mod B)2
- (ABC).C(mod C)2
13Orthogonality Principle cont..
14Orthogonality Principle cont..
Here mw and nw are called m-th and n-th
harmonics of w respectively
15Orthogonal 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
16Example 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 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
17Example of OFDM cont..
Modulate each column with corresponding
sub-carrier using BPSK
18Example of OFDM cont..
- Final OFDM Signal Sum of all signal
V(t)
Generated OFDM signal, V(t)
19OFDM-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
20OFDM-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
21OFDM-FDMA (System View)
22Advantages 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
23Advantages of OFDMA cont..
Multi-user Diversity
24Advantages of OFDMA cont..
- Efficient use of Spectrum
4/3 Hz per symbol
6/5 Hz per symbol
25Advantages 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
26Disadvantages of OFDMA
- Peak to average power ratio (PAPR)
The large amplitude variation increases in-band
noise and increases the BER when the signal has
to go through amplifier nonlinearities.
27Disadvantages 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
28Research 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
29Thank You