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Performance Evaluation of WiMAX/IEEE 802.16 OFDM Physical Layer - page 40 / 107

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Random data generation

Channel Encoding

Mapping

IFFT

Cyclic Prefix insertion

Output Data

Channel

De­

FFT

Cyclic Prefix

decoding

mapping

removal

Figure 4.1: Simulation Setup

Data Randomization

Reed­Solomon Encoding

Convolutional Encoding

Interleaving

FEC

Figure 4.2: Channel Encoding Setup

De­Interleaving

Convolutional Decoding

Reed­Solomon Decoding

De­Randomization

FEC

Figure 4.3: Channel Decoding Setup

4.2.1 Scrambler

The scrambler performs randomization of input data on each burst on each allocation to avoid long sequence of continuous ones and zeros. This is implemented with a Pseudo Random Binary Sequence (PRBS) generator which uses a 15­stage shift register with a generator polynomial of 1+x14+x15 with XOR gates in feedback con iguration as shown in figure 4.4. The implemented scrambler complies with the initialization process as specified in section 8.3.3.1 of the standard [1].

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