Why OFDM ?(cont.)
OFDM overcomes most of the problems with both FDMA and TDMA. OFDM splits the
available bandwidth into many narrow band channels (typically 100-8000). The carriers
for each channel are made orthogonal to one another, allowing them to be spaced very
close together, with no overhead as in the FDMA example. Because of this there is no
great need for users to be time multiplex as in TDMA, thus there is no over head
associated with switching between users.
The orthogonality of the carriers means that each carrier has an integer number of
cycles over a symbol period. Due to this, the spectrum of each carrier has a null at the
centre frequency of each of the other carriers in the system. This results in no
interference between the carriers, allowing then to be spaced as close as theoretically
possible. This overcomes the problem of overhead carrier spacing required in FDMA.
Each carrier in an OFDM signal has a very narrow bandwidth (i.e. 1kHz), thus the
resulting symbol rate is low. This results in the signal having a high tolerance to
multipath delay spread, as the delay spread must be very long to cause significant
inter-symbol interference (e.g > 500usec).