1. Orthogonal Frequency Division Multiple Access (OFDMA)
Khaja Mohammad Shazzad

2. Outline
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

3. 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

4. Contention-based Multiple Access(MA)
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 uses CSMA/CA based contention access scheme

5. 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
Non-channelization
Channelization

6. Non-channelization Non-contention-based MA
Terminals transmit sequentially using the same channel
Example:
Polling based medium access
Standard:
IEEE (WPAN)
IEEE (WLAN)

7. 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)

8. Time Division Multiple Access (TDMA)
GSM
Time slot ms
Frame 4.6 ms
8 time slots per frame
Frequency band 20 KHz

9. Code Division Multiple Access (CDMA)
Orthogonal Walsh codes
64 codes (channels)
One pilot channel
Seven paging channels
55 traffic channels
Each carrier 1.25 MHz

10. Frequency Division Multiple Access (FDMA)
American Mobile Phone System (AMPS)
Total Bandwidth 25 MHz
Each Channel 30 KHz

11. Orthogonality Principle OFDM OFDM-FDMA
Orthogonal Frequency Division Multiplexing (OFDM) Based Multiple Access (OFDMA)
Orthogonality Principle
OFDM
OFDM-FDMA

12. 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

13. Orthogonality Principle cont..
Real Function space

14. Orthogonality Principle cont..
Here mw and nw are called
m-th and n-th harmonics of w respectively

15. 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

16. 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 bits C1 C2 C3 C4 1 -1

17. 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

18. Example of OFDM cont.. Final OFDM Signal = Sum of all signal V(t)
Generated OFDM signal, V(t)

19. 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

20. OFDM-FDMA (OFDMA)
The IEEE e/ 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

21. OFDM-FDMA (System View)

22. 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

23. Advantages of OFDMA cont..
Multi-user Diversity

24. Advantages of OFDMA cont..
Efficient use of Spectrum
4/3 Hz per symbol
6/5 Hz per symbol

25. 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

26. Disadvantages 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.

27. 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

28. 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

29. Thank You
Questions or
Comments?