Split the transmission bandwidth into many
sub channels which are transmitted in parallel
(Ideally) Each sub channel is narrow enough so that
it experiences a flat fading although the overall
radio propagation environment is frequency-
The time dispersion effects are less significant as the
symbol duration increases
In data communication it is common to allow
several transmitters to send information over a
single communication channel, allowing several
users to share a band of frequencies.
This is the concept of Multiple access.
CDMA uses spread spectrum technology and a
special coding scheme where the transmitter is
assigned a code for multiple users to transmit over
the same physical channel.
Advantage: (mobile) transmission power is very
small, better call quality.
Disadvantage: As the number of users increases, the
overall quality of service decreases
Several users share the same frequency channel by
dividing the signal into different time slots.
Users transmit in rapid succession, one after the
other each using its own time slots.
Multiple station share same transmission medium
Non continuous transmission
Interference with other devices at frequency directly
connected to the time slot length.
Users share the frequency channel simultaneously but
each user transmits at single frequency.
Uses both analog and digital signals.
Uses high performing filters in the radio hardware in
contrast to TDMA & FDMA.
Non vulnerable to timing problem.
Max bit rate /channel is fixed and small.
Guard bands results in wastage of capacity.
Orthogonal frequency-division multiplexing
(OFDM) is a method of digital modulation in which
a signal is split into several narrowband channels at
OFDM is similar to conventional frequency-
division multiplexing (FDM). The difference lies in
the way in which the signals are modulated and
demodulated. Priority is given to minimizing the
interference, or crosstalk , among the channels and
symbols comprising the data stream. Less
importance is placed on perfecting individual
In geometry, orthogonal means "involving right
angles" (from Greek ortho, meaning right,
and gon meaning angled). The term has been
extended to general use, meaning the characteristic
of being independent (relative to something else). It
also can mean: non-redundant, non-overlapping, or
Orthogonality between the sub-carriers allows their
overlapping while disabling the occurrence of
The sub-carriers are orthogonal to each other,
meaning that cross-talk between the sub-channels
is eliminated and inter-carrier guard bands are not
Simplifies the design of both the transmitter and
the receiver, filter for each sub-channel is not
Allows high spectral efficiency, near the Nyquist
An echo is a copy of the original signal delayed in
ISI takes place when echoes on different-length
propagation paths result in overlapping received
Thus a disturbed signal is generated.
Multipath Radio Channel
The symbols add up on the
It is advantageous to transmit a number of low-rate
streams in parallel instead of a single high-rate
Since the duration of each symbol is long, it is
feasible to insert a guard interval between the
Thus eliminating the intersymbol interference.
Insertion of guard interval (cyclic prefix):
Channel impulse response (shorter than GI):
1 OFDM symbol
Cyclic convolution of transmitted signal
with channel impulse response
multiplication in frequency-domain
Basically OFDM is very similar to that of FDM the
main difference being orthogonality between
OFDM techniques can be achieved by various
methods . However , the advent of discrete fourier
transform (DFT) made this transmission more
The fast fourier transform and the inverse fast
fourier transform are among the most efficient
implementations of the DFT technique, hence they
are normally used for the baseband OFDM
modulation and demodulation process
Makes efficient use of the spectrum by allowing
By dividing the channel into narrowband flat fading
sub channels, OFDM is more resistant to frequency
selective fading's than single carrier systems
Eliminates ISI and IFI through use of a cyclic
Using adequate channel coding and interleaving one
can recover symbols lost due to the frequency
selectivity of the channel.
The OFDM signal has a noise like amplitude with a
very large dynamic range, therefore it requires RF
power amplifiers with a high peak to average power
It is more sensitive to carrier frequency offset and
drift than single carrier systems are due to leakage
of the DFT
ADSL & HDSL
WLANs (IEEE 802.11 & Hiper LAN II)
This algorithm is based on the divide and conquer
approach for efficient computation of DFT .
This approach involves decomposition of a N-point
DFT into successfully smaller DFT’s . Specific
attention is given to the decimation in frequency
algorithm which has been used to calculate the
outputs of both .
In radix-2 FFT decimation in frequency the inputs
can be grouped into odd and even number .
The equations of FFT/IFFT functions can be derived
from the general DFT equation
Thus from the above equation the equation for
Inverse FFT can also be derived which is given by
the following equation
The main reason for opting FFT techniques over
other conventional methods is because of the speed
of computation .
This can be easily understood by just making a
simple comparison between two methods