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Performance Evaluation of WiMAX/IEEE 802.16 OFDM Physical Layer - page 36 / 107

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Doppler values: The effect of Doppler shi t due to user movement should be taken into account.

3.3.4.2 SYSTEM DESIGN PARAMETERS: The design parameters are derived according to the system requirements. The design parameters for an OFDM system are as follows [24]

Number of subcarriers: We stated earlier that the selection of large number of subcarriers will help to combat multipath effects. But, at the same time, this will increase the synchronization complexity at the receiver side. Symbol duration and CP length: A perfect choice of ratio between the CP length and symbol duration should be selected, so that multipath effects are combated and not signi icant amount bandwidth is lost due to CP. Subcarrier spacing: Subcarrier spacing will be depend on available bandwidth and

number of subcarriers used. synchronization is achievable.

But,

this

must

be

chosen

at

a

level

so

Modulation type per subcarrier: The performance requirement will decide the selection of modulation scheme. Adaptive modulation can be used to support the performance requirements in changing environment. FEC coding: A suitable selection of FEC coding will make sure the robustness of the channel to the random errors.

3.3.5 BENEFITS AND DRAWBACKS of OFDM:

In the earlier section, we have stated that how an OFDM system combats the ISI and reduces the ICI. Besides those benefits, there are some other benefits as follows:

High spectral efficiency because of overlapping spectra Simple implementation by fast fourier transform

25

that

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