OFDM
Juan Camilo Sacanamboy

Content
1. Concepts
2. Basic idea
3. FDM: The “mother” of OFDM
4. OFDM
5. Applications

Concepts (1)
Modulation
Modulation is the process of conveying a
message signal (modulating signal)
inside another signal (carrier signal) that
can be physically transmitted.
Carrier signal
Waveform that is modulated with an
input signal for the purpose of conveying
information. Reference: Link

Concepts (2)
Broadband
Wide bandwidth of a transmission medium. Ability to transport
multiple signals and multiple traffic types simultaneously.

Concepts (3)
Subcarrier
It is an already-modulated
s i g n a l , w h i c h i s t h e n
modulated into another signal
of higher frequency and
bandwidth.
Frequency range in a given
bandwidth Reference: Link

Concepts (4)
Orthogonality
Reference: Link
The peak of one subcarrier
coincides with the null of an
adjacent subcarrier.

Concepts (5)
Intersymbol Interference
Reference: Link

Concepts (6)
Selective Fading
Reference: Link
Is a radio propagation anomaly caused by partial
cancellation of a radio signal by itself – the signal
arrives at the receiver by two different paths, and
at least one of the path is changing (lengthening
or shortening).

Basic idea
• Orthogonal Frequency Division Multiplexing • Multicarrier broadband modulation technique for
transmitting large amounts of digital data.

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 (1)
• 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

OFDM (2)
Basic OFDM System
• A very high rate data stream is divided into multiple parallel
low rate data streams. • Each smaller data stream is then mapped to individual data
subcarrier and modulated using some sorts of PSK or QAM.
i.e. BPSK, QPSK, 16-QAM, 64-QAM.

OFDM (3)
Basic OFDM System
Basic OFDM System
(Hanzo, Webb, & Keller, 2000)

OFDM (4)
Characteristics
• High Spectral Efficiency: OFDM needs less bandwidth than FDM
to carry the same amount of information.
• Resilience: More resilient in NLOS environment than FDM.
• Fault Tolerance: It can efficiently overcome interference and
frequency-selective fading caused by multipath because ecualizing
is done on a subset of subcarriers instead of a single broader
carrier.
• Supress effect of ISI (Inter Symbol Interference): Longer symbol
period of the parallel OFDM subcarriers than a single carrier
system.

OFDM (5)
Disadvantage
The main disadvantage of the OFDM system is the
complexity of implement N modulators at the transmitter and
N demodulators at the receiver.
Solution: This problem can be reduced using the discrete
Fourier transform (DFT), implemented as a fast Fourier
transform (FFT).

OFDM (6)
QAM-OFDM
The basic system has N sub-bands,
each separated from its neighbour
by a guard band.
The available spectrum can be used
much more efficiently if the spectra
of the individual sub-bands are
allowed to overlap.
Serial to parallel
convertor
Detailed OFDM System
(Hanzo, Webb, & Keller, 2000)

OFDM (7)
Detailed OFDM System
• The input serial data stream is rearranged into a sequence {
푑↓푛 } of N QAM symbol at baseband.
• At the 푛th symbol instant, the QAM symbol is represented by
an in-phase component 푎(푛) and a quadrature component
푏(푛).
▫ 푑(푛)=푎(푛)+푗푏(푛)
• A block of N QAM symbols are applied to a serial-to-parallel
convertor and the resulting in-phase symbols are applied to N
pairs of balanced modulators.

OFDM (8)
Detailed OFD System
• The quadrature components 푎(푛) and 푏(푛)
modulate the carriers cos ( 푤↓푛 푡)and sin( 푤↓푛 푡)
respectively.
• The modulated carriers 푎(푛) cos⁠( 푤↓푛 푡) and
푏(푛)sin ( 푤↓푛 푡) when added together constitute a
QAM signal. The 푛th QAM signal is given by:
▫ 푋↓푛 =푎(푛) cos⁠( 푤↓푛 푡) +푏(푛)sin ( 푤↓푛 푡)

OFDM (9)
FDM/QAM signal
퐷(푡)= Σ푛=0↑푁−1▒ 푋↓푛 (푡)

OFDM (10)
Modulation by Discrete Fourier Transform
• Large number of sub-channels modems
• Taking the DFT of the original block of N QAM symbols and
then transmitting the DFT coefficients serially is exactly
equivalent to the operations required by the OFDM
transmitter of a Detailed OFDM System.
• Simplifications can be achieved if the bank of sub-cannel
modulators/demodulators is implemented using (IFFT/FFT).

OFDM (11)
Reference: Link
FFT and IFFT are linear transformations
on signals. One is the reverse of the other
one.
It doesn’t matter the order if apply IFFT in
the transmitter or in the receiver because
IFFT and FFT are inverse.
The question becomes why use IFFT in
the transmitter. That’s because signals
need to be modulated by N-QAM of the
ortogonal subcarriers. Mathematically, the
process can be represented by IFFT.

Applications
• ADSL
• HomePlug AV
• WiMedia UWB
• Wifi (801.11 a/g/ac)
• WiMax

References
• Conniq. (s.f.). Introduction to FDM, OFDM,
OFDMA, SOFDMA.
• Hanzo, L., & Keller, T. OFDM and MC-CDMA.
• Hanzo, L., Webb, W., & Keller, T. (2000).
Single- and Multi-carrier Quadrature
Amplitude Modulation : principles and
applications for personal communications,
WLANs and broadcasting.