Orthogonal Frequency Division Multiplexing(OFDM) is a multi-carrier transmission technique that has been recently recognized
as an excellent method for high-speed bi-directional wireless data communications. Fundamentally, Frequency Division
Multiplexing(FDM) uses multiples frequencies to simultaneously transmit multiple signals in parallel. While each sub-carrier is
separated by a guard band to ensure that they do not overlap in the ordinary FDM, the sub-carriers in the OFDM are squeezed
tightly together in order to reduce the required bandwidth. In fact the neighboring sub-channels are overlapped in OFDM. However,
the sub-carriers are orthogonal to each other such that there is no inter-carrier interference (ICI).
In mobile communication environment with the moving vehicle, there is a Doppler shift effect which causes each sub-channel
to be shifted and orthogonality to be broken. To prevent this phenomena, cyclic prefix to each symbol is added. The cyclic prefix
is for implementing the FFT when one needs to use circular convolution to emulate linear convolution. Instead of just sending
blank during guard time, repeating the end of the symbol provides ”guard time” (instead of the ”guard band”) to make sure all
the sub-carriers are orthogonal. If the end interferes with the next symbol, we can subtract it out because we already received the
prefix, which is the same.
There is another benefit of OFDM. Since each subcarrier of OFDM uses a narrow bandwidth, in the time domain each symbol
is transmitted during a relatively long symbol time. This guarantees all the significant multipaths arrive within a symbol time
minimizing Inter-symbol Interference(ISI). This is good news. Because it removes the need for an equalizer at a transmitter and
a receiver side, reducing complexity and the cost.