OFDM is a digital multi-carrier modulation scheme that divides the available bandwidth into multiple orthogonal subcarriers. It converts a high-rate data stream into multiple lower-rate streams that are transmitted in parallel over narrowband channels. OFDM has advantages for high data rate transmission in delay dispersive environments and is used in technologies like DAB, DVB, wireless LANs, and 4G cellular systems. It works by splitting information into parallel streams that modulate distinct subcarriers. The subcarriers are chosen to be orthogonal to avoid interference between signals. OFDM can be implemented digitally using the IDFT/DFT, which makes it simpler and more efficient than analog implementations.

1. OFDM – ORTHOGONAL FREQUENCY
DIVISION MULTIPLEXING
R.Ramalakshmi
Assistant Professor
Ramco Institute of Technology
Rajapalayam

2. • It is a modulation scheme. It is especially suitable
for higher data rate transmission in delay
dispersive environments.
• It converts a high rate data stream into a number
of low rate streams that are transmitted over
parallel, narrowband channels that can be easily
equalized.
• OFDM is a special case of multicarrier
transmission, where a single data stream is
transmitted over a number of lower rate
subscribers.

3. Advantages of OFDM
• DAB – Digital Audio Broadcasting
• DVB – Digital Video Broadcasting
• Wireless LANs
• Fourth generation cellular systems, 3GPP-LTE
and WiMAX.

4. Principles of OFDM
• OFDM splits the information into N parallel
streams which are then transmitted by
modulating N distinct carriers called sub-carriers
or tones.
• Symbol duration Ts on each subcarrier thus
becomes larger by a factor of N.
• To separate signals carried by different
subcarriers by the receiver, they have to be
orthogonal.

5. • In FDMA, large frequency spacing between
carriers is used. Precious spectrum are wasted
by the carriers.
• In OFDM, a narrower spacing of subcarriers
can be achieved. The subcarriers frequencies
is given by,
fn = nW/N
• Where, n-integer
N-Different carriers
W-Total available bandwidth = N/Ts

7. • The modulation scheme on each of the
subcarriers is PAM – Pulse Amplitude Modulation
with rectangular basis pulses, and then the
subcarriers are mutually orthogonal,
• In time domain, due to rectangular shape of
pulses, the spectrum of each modulated carrier
has a sin(x)/x shape.

8. • The spectra of different modulated carriers overlap,
but each carrier is in the spectral nulls of all other
carriers.
• Therefore, the receiver does the appropriate
demodulation and integrating over symbol duration,
the data streams of any two subcarriers will not
interfere.

9. Implementation of Transceivers
• OFDM can be implemented in two ways,
(i) Analog Interpretation
(ii) Digital Interpretation

11. Transceiver structures for OFDM in
purely analogy technology
• Original data stream is first split into N parallel data
streams by S/P conversion, each of which has a lower
data rate.
• Then we have a number of local oscillators. It oscillates
at a frequency, fn = nW/N
• Each of the parallel data streams then modulates one
of the carrier.
• Actual implementation hardware effort of multiple
local oscillators is too high.

12. • Let the complex transmit symbol at time
instant i on the nth carrier be Cni , the transmit
signal is then,
• gn(t) – basis pulse, Cni – nth carrier, TS – symbol
period

13. • The transmitter can be realized by performing an IDFT on the
block of transmit symbols.
• Data source is connected with the serial to parallel converter.
• It is connected with the transmitter section.
• Its output is connected with the channel.
• The other end of the channel is connected with the receiver
section.
• The outputs from various receivers are given to the P/S block.
• The output of the P/S block is connected with the data sink.
This is the final output.

14. Transceiver structures for OFDM in
digital technology
• The received signal is demodulated by using
locally generated subcarriers.
• Then each signal is serially converted by P/S
conversion. The result is an estimate of the
original data Cn.
• Digital interpretation method - the data
streams are divided into blocks of N symbols.

15. • The block of N data symbols are subjected to an IFFT and
then transmitted.
• The input of IFFT is made up of N samples and therefore
the output from the IFFT also consists of N values.
• These N temporal sample values to be transmitted using
parallel to serial converter, one after the other.
• We can reverse the process the received is to parallel
signal is converted to serial using S/P.
• Then perform FFT on this vector, this resulted signal is an
estimate of original data Cn.

16. Advantages of Digital Interpretation
• It is much simpler
• Highly efficient structures exist for the
implementation of an FFT so called butterfly
structures.

17. REFERENCES
• 1. Rappaport,T.S., “Wireless communications”,
Second Edition, Pearson Education, 2010.
• 2. Andreas.F. Molisch, “Wireless
Communications”, John Wiley – India, 2006.

18. Thank You