Why OFDM ?
In FDMA each user is typically allocated a single channel, which is used to transmit all
the user information. The bandwidth of each channel is typically 10kHz-30kHz for voice
communications. However, the minimum required bandwidth for speech is only 3kHz.
The allocated bandwidth is made wider then the minimum amount required to prevent
channels from interfering with one another. This extra bandwidth is to allow for signals
from neighboring channels to be filtered out, and to allow for any drift in the center
frequency of the transmitter or receiver. In a typical system up to 50% of the total
spectrum is wasted due to the extra spacing between channels. This problem becomes
worse as the channel bandwidth becomes narrower, and the frequency band increases.
TDMA partly overcomes this problem by using wider bandwidth channels, which are
used by several users. Multiple users access the same channel by transmitting in their
data in time slots. Thus, many low data rate users can be combined together to transmit
in a single channel which has a bandwidth sufficient so that the spectrum can be used
There are however, two main problems with TDMA. There is an overhead associated
with the change over between users due to time slotting on the channel. A change over
time must be allocated to allow for any tolerance in the start time of each user, due to
propagation delay variations and synchronization errors. This limits the number of users
that can be sent efficiently in each channel. In addition, the symbol rate of each channel
is high (as the channel handles the information from multiple users) resulting in
problems with multipath delay spread.