FDM,OFDM,OFDMA,MIMO

Group members: ( G10 )
Muhammad Salman 14-BSEE-15
Asim Hussain Farooqi 12-BSEE-15
Muhammad Nadeem 11-BSEE-15
Muhammad Shoib
Electrical Engineering department GCU Lahore 1
FDMA,OFDM And OFDMA

Contents
• Multiple Access
• Multiple Access Techniques
• FDMA
• Guard Frequency band
• Advantages and Disadvantages of FDMA
• OFDM
• Difference between OFDM And FDM
• Characterizes and drawbacks of OFDM
• OFDMA
• Orthonaglity principle
• Advantages and disadvantages

Multiple Access
• Multiple access is a technique that lets multiple mobile users share the
allotted spectrum in the most effective manner.
• This is carried out by permitting the available bandwidth to be used
simultaneously by different users.

Why Multiple Access?
• Reason 1:
• We don't need multiple access techniques in wireless
communication… if we only have one user.
• But. If we have more than one user that needs to share a wireless
communications system then we need, as the name suggests, a
mechanism to allow Multiple users to Access the same base station.

Reason 2
• Battle of Bandwidth Rates
• Since the spectrum is limited and very expensive that’s why the sharing
is necessary to improve the overall capacity over a geographical area.

Multiple Access Techniques
Multiple Access Technique
SDMACDMAFDMATDMA

FDMA
Frequency Division Multiple Access

FDMA
• In Frequency-division multiple access (FDMA), the available channel
bandwidth is divided into many non overlapping frequency bands,
where each band is dynamically assigned to a specific user to transmit
data.

Conti..

Conti..
• Every user is assigned a specific slot by using this user can transmit or
receive data
• Assigning the frequency slot to the user may be static or dynamic .
• In Static Channel Allocation frequency is pre assigned to each user
and it can not be changed. It is an old method.
• In Dynamic channels Allocation are not pre-assigned. It is assigned
on the requirement of the user.

Guard Frequency band
• A guard band is a narrow frequency range that separates two ranges
of wider frequency. This ensures that simultaneously used
communication channels do not experience interference, which
would result in decreased quality for both transmissions.

Number of Channel in FDMA

Example

Working
• At Transmitting End

Conti..
• At the Receiving end

Advantages of FDMA
1. Continuous transmission of data
2. Time delay is not present as in TDMA
3. It is very simple to implement with respect to hardware resources.
4. It reduces the cost and lowers the inter symbol interference (ISI)
5. Since the transmission is continuous, less number of bits are
required for synchronization and framing.
6. Equalization is not necessary.
7. FDMA used in both analogue and digital communication

Conti..
• Network Timing is not required.
• Simple Algorithmically
• Channel Band width is narrow relatively (30KHZ)

Disadvantages of FDMA
1. It requires RF filters to meet stringent adjacent channel rejection
specifications. This increases cost of the system.
2. Network and spectrum planning is cumbersome and time
consuming.
3. Maximum bit rate per channel is constant and hence can not be used
for varying data rate .
4. The maximum flow rate per channel is fixed and small.
5. Guard bands lead to a waste of capacity.
6. In FDMA data rate is slower than the TDMA

OFDM
(Orthogonal Frequency
Division Multiplexing )

Orthogonality:
• The main concept in OFDM is Orthogonality of the subcarriers.
• All the subcarriers used are either sine waves or cosine waves.
• OFDM transmission depends on orthogonality principle.
• If two vectors are orthogonal, angle between them =90 degree.
• Sin(x) and Cos(x) are orthogonal to each other due to 90 degree
phase difference.

Cont…
• Orthogonal signals do not interfere.
• Orthogonal channels are designed to save bandwidth(BW).
• The orthogonality allows simultanous transmission of lots of sub-
carriers without interference from each other.

Single and Multi-Carriers:
• In single-carrier communication system, data are transported over
only one carrier.
• Modulation can be AM, FM, PM in analog communication system.
• In multi-carrier communication system input data are distributed
among several carriers and simultanously transmitted.
• If we use multiple carriers to transport input data , it is called as
multi-carrier modulation scheme.
• OFDM is an example of using this technique.

Cont…
• Multi-carrier modulation techniques are used in fourth generation
(4G) Communication System.
• The carriers used in OFDM are known as sub-carriers.
• Each sub-carrier can be modulated by any kind of digital modulation
techniques such as PSK,QPSK,QAM etc.

Basic Concept of OFDM :
Send a sample using
the entire band
Send samples concurrently using
multiple orthogonal sub-channels
Wide-band channel Multiple narrow-band channels

FDM: The “mother” of OFDM
• Frequency Division Multiplexing
• Signals from multiple transmitters are transmitted
simultaneously over multiple frequencies.
• Each subcarrier is modulated separately by different data
stream and a guard band is placed between subcarriers to
avoid signal overlap.
Reference: Link

OFDM :
• Like FDM, OFDM uses multiple subcarriers BUT:
o There are closely spaces to each other without
causing interference, removing guard bands.
o Its possible because subcarriers are orthogonal.
Reference: Link

Importance of Orthogonality:
• Why not just use FDM (frequency division
multiplexing)
• „Not orthogonal
f
Individualsub-‐channel
f
• Need guard bands between adjacent frequency
bands extra overhead and lower throughput
•Leakage Interference from
Adjacent sub-carriers
•Guard band
protect leakage
interference.

Difference between FDM and OFDM:
guardband
f
Frequency division multiplexing
f
Orthogonal sub-carriers in OFDM
Don’tneedguardbands

OFDM Signal
Ch.1
Ch.2 Ch.3 Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.10
Ch.3 Ch.5 Ch.7 Ch.9
Ch.2 Ch.4 Ch.6 Ch.8 Ch.10
Ch.1
Conventional multicarrier techniques
Orthogonal multicarrier techniques
50% bandwidth saving
Frequency
Frequency

CYCLIC PREFIX :
 Guard time between adjacent symbols is inserted to
eliminate ISI.
 No ISI will occurs, if guard time is larger than delay spread.
 Guard time is a pure system overhead, contains no
information.
 CP is inserted in order to preserve Orthogonality.
 CP provides multipath immunity & synchronization
tolerance
 CP increases required transmission bandwidth, hence
lowersspectral efficiency
 Transmit power associated with CP is a waste.

Characteristics:
• High Spectral Efficiency: OFDM needs less bandwidth than FDM to carry the
same amount of information.
• Robust against intersymbol interference (ISI) and fading caused by multipath
propagation.
• OFDM has excellent robustness in multi-path environments.
• Cyclic prefix preserves orthogonality between sub-
carriers.
• Cyclic prefix allows the receiver to capture multi-
path energy more efficiently.

Draw backs:
• Loss of efficiency caused by cyclic prefix/guard interval.
• OFDM is sensitive to frequency, clock and phase offset.
• Sensitive to Doppler shift.

Applications:
• LTE(Long Term Evaluation)=4th Generation wireless cellular
standard.
• Digital audio broadcasting (DAB).
• Digital video broadcasting (DVB).
• Wireless LAN 802.11a.

Orthogonal Frequency Division
Multiple Access
(OFDMA)
Muhammad Nadeem
0011-BSEE-2015

Orthogonal Frequency Division Multiple
Access (OFDMA)
 What is OFDMA ?

OFDMA (Cont…)
• OFDMA allocates multi-user in time
domain as well in frequency domain. OFDMA
is a very popular Multiplexing method used
for many of the latest wireless and
telecommunication standards,

OFDMA (Cont…)
• Such as Wi-Fi 802.11ac, both 4G LTE
and 5G cellular technologies, WiMAX, and
many more.

Comparison between
FDMA & OFDM
FDMA
OFDM

Combination of three Signals in
OFDMA

Why they are Orthogonal?

DE multiplexing in OFMDA

Difference between
OFDM &OFDMA

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

Orthogonality Principle cont..
• Real Function space
f1 (t)  A sin(wt )
f2 (t)  B cos(wt )
fm (t) fn (t)dt  0
fm (t)  M sin(m wt)
fn (t)  N cos(nwt )
f1 (t) f2 (t)dt  0


 T
 T

Orthogonality Principle cont..
f (t)  sin(wt)sin(2wt)
m.nΝ T
 sin(mwt)sin(nwt)dt  0 where m  n
0
T
 sin(mwt)cos(nwt)dt  0
0
Here mw and nw are called
m-th and n-th harmonics of
w respectively

15
Multiplexing(OFDMA)
• 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

Example of OFDMA
• Lets we have following information bits
–1,1,-1,-1,-1 1,-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
-1
C4
-1
1 1 1 -1
1 -1 -1 -1
-1 1 -1 -1
-1 1 1 -1
-1 -1 1 1

Example of OFDMA 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

Binary Phase Shift Keying (BPSK)
• Binary Phase Shift Keying (BPSK) is a two phase
modulation scheme, where the -1's and 1's in a binary
message are represented by two different phase states
in the carrier signal: θ=0∘ for binary 1 and θ=180∘ for
binary -1

Example of OFDM cont..
• Final OFDM Signal = Sum of all signal
V(t)   I (t)sin(2nt)
N 1
n0
n
Generated OFDM signal, V(t)
V(t)

Uplink & Downlink OFDMA

Advantages of OFDMA cont..
• 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

Disadvantages of OFDMA
• Peak to average power
ratio (PAPR)
avgP
x(t)
PAPR 
2
The large amplitude variation increases in-band noise and
increases the BER when the signal has to go through amplifier
nonlinearities.

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–

Research issues and Conclusion
• Future works
Peak-to-average power reduction in OFDM
Timing and Frequency Synchronization
 Multiple input/Multiple output (MIMO) OFDM
• Conclusion
It may be concluded that OFDMA is a very
efficient technique for broadband data
transmission over radio frequency. It can be
implemented digitally with simplicity and low
cost. So it is adopted in all new kind of
technologies

Techniques used for communication are:-
SISO:
It is also named as Single-Input Single-Output which means that the
transmitter and receiver of the radio system have only one antenna.
SISO is a simple single variable control system which is less complex
than the MIMO systems. This system doesn’t require any additional
processing or diversity.
SIMO:
The SIMO or Single-Input Multiple-Outputs means the transmitter has
single antenna and the receiver has multiple antennas. This is also
known as receiving diversity, and it is applicable in many applications

MISO:
The MISO or Multiple-Inputs Single-Output is also termed as transmit
diversity. This transmitter has multiple antennas, and the receiver has
only one antenna. The advantage with this system is that it has multiple
antennas and the redundancy coding is moved from the receiver to the
transmitter. It also creates a positive impact based on the size, price and
life of the battery.
MIMO:
MIMO is also termed as Multiple-Inputs Multiple-Outputs. MIMO is
also used to provide improvement in both channel robustness and as well
as channel throughput. MIMO means both transmitter and receiver have
multiple antennas.

Function of MIMO System
MIMO can be categorized into three:
Precoding
Spatial Multiplexing
Diversity Coding

Precoding:-
Precoding is a multi-stream beam forming – the narrowest form of a
definition. In general, it is can be termed as the spatial processing that
happens at the transmitter. In beam forming, the emitted signals from the
transmitting antennas with an appropriate phase and gain weighting such
that the signal power gets maximized at the receiver input. The assistance
of this beam forming is to increase the gain of received signals, by
making the emitted signals from different antennas to add up positively,
and to decrease the multipath fading effect. When the receiver has
multiple antennas, the transmitted beam forming cannot maximize the
signal level of the receiver antennas simultaneously, and the precoded
multiple streams are used.

Spatial Multiplexing:-
Spatial multiplexing needs MIMO antenna configuration. In spatial multiplexing, a
signal placed at high rate splits into lower rate streams in multiples and each stream
is transferred from different transmitting antennas in a similar frequency channel. If
this transmitted signal reaches the receiver antenna array with different spatial
signatures, the receiver can discrete these streams parallel into channels. Spatial
multiplexing is a very influential method used for increasing channel capacity at
higher signal-to-noise ratios (SNR). The maximum number of spatial streams is
limited by the lesser number of antennas placed at both the transmitter and receiver
ends. This multiplexing technique can be used with or without any transmitting
knowledge of the channel. Spatial multiplexing can also be used for transmission of
data to multiple receivers simultaneously; this method is also known as Space
Division Multiple Accessing

Diversity Coding:-
Diversity coding technique is used to increase the link reliability in the
occurrence of fading conditions. With this technique, same data can be
encoded in multiple versions and that encoded data can be transmitted
over multiple antennas. The encoding adds sometimes diversity level.
The multiple signals that are propagated by using different paths are
affected differently with fading process. The receiver then improves the
original stream either by selecting the received signal or by merging all
the information received.

C=capacity of channel in bits per second
B=Bandwidth of channel in hertz
SNR=Signal to Noise Ratio

FDM,OFDM,OFDMA,MIMO

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