2. 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
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
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
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(
)
(
2
1
2
1
dt
t
f
t
f
nwt
N
t
f
mwt
M
t
f
dt
t
f
t
f
wt
B
t
f
wt
A
t
f
n
T
m
n
m
T
14. Orthogonality Principle cont..
)
2
sin(
)
sin(
)
( wt
wt
t
f
T
0
0
s(nwt)dt
sin(mwt)co
n
m
where
T
0
0
n(nwt)dt
sin(mwt)si
Ν
m.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
)
2
sin(
)
(
)
(
1
0
nt
t
I
t
V
N
n
n
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
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
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
t
x
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
