The
Rectangular Pulse
Advantages:
+ Simple to implement: there is no complex
filter system required to detect such pulses and to generate them.
+ The pulse has a clearly defined duration.
This is a major advantage in case of
multipath propagation environments as it simplifies handling of intersymbol
interference.
Disadvantage:
 it allocates a quite huge spectrum. However the
spectral power density has null points exactly at multiples of the frequency fs
= 1/Ts. This will be important in OFDM.
OFDM Basics
•
Transmits hundreds or even
thousands of separately modulated
radio signals using orthogonal
subcarriers spread across a wideband channel

Data is sent in parallel
across the set of subcarriers, each subcarrier only transports a part of the
whole transmission

The throughput is the sum of
the data rates of each individual (or used) subcarriers while the power is
distributed to all used subcarriers

FFT ( Fast Fourier Transform)
is used to create the orthogonal subcarriers. The number of subcarriers is
determined by the FFT size ( by the
bandwidth)
Multiple Access Methods Comparison
OFDM is the stateoftheart and most efficient and
robust air interface
OFDM is the stateoftheart and most efficient
and robust air interface
OFDM Signal
OFDM VS OFDMA
•
The basic idea is to assign subcarriers to users based
on their bit rate services. With this approach it is quite easy to handle high
and low bit rate users simultaneously in a single system.
•
But still it
is difficult to run highly variable traffic efficiently.
•
The solution
to this problem is to assign to a single users so called resource blocks or
scheduling blocks.
•
such block is
simply a set of some subcarriers over some time.
•
A single user
can then use 1 or more Resource
Blocks.
OFDM Weaknesses
InterCarrier Interference (ICI) in OFDM
•
The price for
the optimum subcarrier spacing is the sensitivity of OFDM to frequency errors.
•
If the receiver’s frequency slips some
fractions from the subcarriers center frequencies, then we encounter not only interference
between adjacent carriers, but in principle between all carriers.
•
This is known as InterCarrier Interference
(ICI) and sometimes also referred to as Leakage Effect in the theory of
discrete Fourier transform.
•
One possible cause that introduces frequency
errors is a fast moving Transmitter or Receiver (Doppler effect).
Leakage effect due to
Frequency Drift: ICI