GUIDED BY :
[Lecturer of ECE Dept.]
PREPARED BY :
ORTHOGONAL FREQUENCY DIVISION
Orthogonal frequency-division multiplexing (OFDM), essentially
identical to coded OFDM (COFDM) and discrete multi-tone
modulation (DMT), is a frequency-division multiplexing (FDM)
scheme utilized as a digital multi-carrier modulation method. A
large number of closely-spaced orthogonal sub-carriers are used
to carry data. The data is divided into several parallel data streams
or channels, one for each sub-carrier. Each sub-carrier is
modulated with a conventional modulation scheme (such as
quadrature amplitude modulation or phase-shift keying) at a low
symbol rate, maintaining total data rates similar to conventional
single-carrier modulation schemes in the same bandwidth.
In terms of real functions, the integral of the product of any two different
functions is zero.
OFDM Principles :
1. The concept of multicarrier (parallel) transmission
In a mobile radio environment, the signal is carried by a
large number of paths with different strength and delays.
Such multipath dispersion of the signal is commonly
referred as„ channel-induced ISI”and yields the same
kind of ISI distortion caused by an electronic filter
2. Necessity of PAPR:
These large peaks cause saturation in power amplifiers, leading to intermodulation
products among the subcarriers and disturbing out of band energy. Therefore, it is desirable to
reduce the PAPR. · To reduce the PAPR, several techniques have been proposed such as clipping,
coding , peak windowing, Tone Reservation and Tone Injection. But, most of these methods are
unable to achieve simultaneously a large reduction in PAPR with low complexity, with low
coding overhead, without performance degradation and without transmitter receiver symbol
•There are some obstacles in using OFDM in transmission system in contrast to its advantages.
A major obstacle is that the OFDM signal exhibits a very high Peak to Average Power Ratio
7. Implementation using the FFT algorithm
8. Guard interval for elimination of inter symbol interference
5. Simplified equalization
4.Channel coding and interleaving
6. Adaptive transmission
9. OFDM extended with multiple access
OFDM FOR OPTICAL COMMUNICATIONS :
Despite the many advantages of OFDM, and its widespread
use in wireless communications, OFDM has only recently been
applied to optical communications. This is partly because of the
recent demand for increased data rates across dispersive optical
media and partly because developments in digital signal processing
(DSP) technology make processing at optical data rates
feasible. However another important obstacle has been the
differences between conventional OFDM systems and
conventional optical systems.
A. Optical OFDM Using Intensity Modulation
B. Optical OFDM Using Linear Field Modulation
C. MIMO-OFDM for Optical Communications
Types of OPTICAL COMMUNICATION
APPLICATION OF OFDM SYSTEM
• ADSL and VDSL broadband access via POTS copper wiring.
• DVB-C2, an enhanced version of the DVB-C digital cable TV standard.
• Power line communication (PLC).
• ITU-T G.hn, a standard which provides high-speed local area networking over
existing home wiring (p
• Multimedia over Coax Alliance (MoCA) home networking.
• The wireless LAN (WLAN) radio interfaces IEEE 802.11a, g, n and HIPERLAN/2.
• The terrestrial digital TV systems DVB-T and ISDB-T.
• The terrestrial mobile TV systems DVB-H, T-DMB, ISDB-T and Media FLO forward
• The wireless personal area network (PAN) ultra-wideband (UWB) IEEE 802.15.3a
The OFDM based multiple access technology OFDMA is also used in several 4G
and pre-4G cellular networks and mobile broadband standards:
• The mobility mode of the wireless MAN/broadband wireless access (BWA)
standard IEEE 802.16e (or Mobile-WiMAX).
• The mobile broadband wireless access (MBWA) standard IEEE 802.20.
•Can easily adapt to severe channel conditions without
complex time-domain equalization.
•Robust against narrow-band co-channel interference.
•Robust against intersymbol interference (ISI) and fading
caused by multipath propagation.
•High spectral efficiency as compared to conventional
modulation schemes, spread spectrum, etc.
•Efficient implementation using Fast Fourier Transform (FFT).
•Low sensitivity to time synchronization errors.
•Tuned sub-channel receiver filters are not required (unlike
•Facilitates single frequency networks (SFNs); i.e., transmitter
•Sensitive to Doppler shift.
•Sensitive to frequency synchronization problems.
•High peak-to-average-power ratio (PAPR), requiring
linear transmitter circuitry, which suffers from poor
•Loss of efficiency caused by cyclic prefix/guard interval.