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

2. 2
Outline
1. Background
 Multiple Access (MA) Methods
1. Orthogonal Frequency Division Multiplexing
(OFDM) Based Multiple Access (OFDMA)
 Orthogonality Principle
 OFDM
 OFDM-FDMA
1. Advantages and Disadvantages of OFDMA
2. 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. 4
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 terminal’s initial
access process
– IEEE 802.11 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
1. Non-channelization
2. Channelization

6. 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)

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. 8
Time Division Multiple Access
(TDMA)
 GSM
– Time slot 0.577 ms
– Frame 4.6 ms
– 8 time slots per frame
– Frequency band 20 KHz

9. 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

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

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

12. 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
0)()(
)cos()(
)sin()(
0)()(
)cos()(
)sin()(
21
2
1
=
=
=
=
=
=
∫
∫
+
+
dttftf
nwtNtf
mwtMtf
dttftf
wtBtf
wtAtf
n
T
m
n
m
T
τ
τ
τ
τ

14. Orthogonality Principle cont..
)2sin()sin()( wtwttf =
∫ =
≠∫ =
∈∀
T
0
0s(nwt)dtsin(mwt)co
nmwhere
T
0
0n(nwt)dtsin(mwt)si
Νm.n
Here mw and nw are called
m-th and n-th harmonics of
w respectively

15. 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 -1 -1
1 1 1 -1
1 -1 -1 -1
-1 1 -1 -1
-1 1 1 -1
-1 -1 1 1

17. 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

18. Example of OFDM cont..
 Final OFDM Signal = Sum of all signal
)2sin()()(
1
0
nttItV
N
n
n
π∑
−
=
=
Generated OFDM signal, V(t)
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 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

21. OFDM-FDMA (System View)

22. 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)
avg
P
tx
PAPR
2
)(
=
The large amplitude variation increases in-band noise and
increases the BER when the signal has to go through amplifier
nonlinearities.

27. 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. 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
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or
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