In this section, we will discuss about the OFDM method and its design consideration.
3.3.1 OFDM BASIC:
The idea of OFDM comes from MultiCarrier Modulation (MCM) transmission technique. The principle of MCM describes the division of input bit stream into several parallel bit streams and then they are used to modulate several sub carriers as shown in Figure 3.1. Each subcarrier is separated by a guard band to ensure that they do not overlap with each other. In the receiver side, bandpass filters are used to separate the spectrum of individual subcarriers. OFDM is a special form of spectrally efficient MCM technique, which employs densely spaced orthogonal subcarriers and overlapping spectrums. The use of bandpass ilters are not required in OFDM because of the orthogonality nature of the subcarriers. Hence, the available bandwidth is used very efficiently without causing the InterCarrier Interference (ICI). In Figure 3.1, the effect of this is seen as the required bandwidth is greatly reduced by removing guard band and allowing subcarrier to overlap. It is still possible to recover the individual subcarrier despite their overlapping spectrum provided that the orthogonality is maintained. The Orthogonality is achieved by performing Fast Fourier Transform (FFT) on the input stream. Because of the combination of multiple low data rate subcarriers, OFDM provides a composite high data rate with long symbol duration. Depending on the channel
coherence time, this reduces or completely Interference (ISI), which is a common phenomenon with short symbol duration. The use of Cyclic Prefix the effect of ISI even more , but it also introduces a
eliminates the risk of InterSymbol in multipath channel environment (CP) in OFDM symbol can reduce loss in SNR and data rate.