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Performance Evaluation of WiMAX/IEEE 802.16 OFDM Physical Layer - page 44 / 107

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that equal average power is achieved for the symbols. The constellation mapped data are assigned to all allocated data subcarriers of the OFDM symbol in order of increasing frequency offset index.

4.2.6 IFFT

The grey mapped data are then sent to IFFT for time domain mapping. Mapping to time domain needs the application of Inverse Fast Fourier Transform (IFFT). In our case we have incorporated the MATLAB ´ifft´ function to do so. This block delivers a vector of 256 elements, where each complex number clement represents one sample of the OFDM symbol.

4.2.7 Cyclic Prefix Insertion:

A cyclic prefix is added to the time domain samples to combat the effect of multipath. Four different duration of cyclic pre ix are available in the standard. Being G the ratio of CP time to OFDM symbol time, this ratio can be equal to 1/32, 1/6, 1/8 and 1/4

4.3 Channel Model:

In order to evaluate the performance of the developed communication system, an accurate description of the wireless channel is required to address its propagation environment. The radio architecture of a communication system plays very significant role in the modeling of a channel. The wireless channel is characterized by:

Path loss (including shadowing) Multipath delay spread Fading characteristics Doppler spread Co­channel and adjacent channel interference

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