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Performance Evaluation of WiMAX/IEEE 802.16 OFDM Physical Layer - page 31 / 107





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3.3 OFDM

In this section, we will discuss about the OFDM method and its design consideration.


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 [24], 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.


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