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Performance Evaluation of WiMAX/IEEE 802.16 OFDM Physical Layer - page 41 / 107

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MSB

LSB

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Data Out

Data In

Figure 4.4: PRBS generator for randomization.

4.2.2 Reed­Solomon Encoder

The randomized data are arranged in block format before passing through the encoder and a single 0X00 tail byte is appended to the end of each burst. The implemented RS e n c o d e r i s d e r i v e d f r o m a s y s t e m a t i c R S ( N = 2 5 5 , K = 2 3 9 , T = 8 ) c o d e u s i n g G F ( 2 8 ) . T h following polynomials are used for code generator and field generator: e

G(x) = (x+ 0)(x+ 0)… (x+ p(x) = x8 + x4 + x3 + x2 + 1

2T­1),

= 02HEX

(4.1) (4.2)

The encoder support shortened and punctured code to facilitate variable block sizes and variable error­correction capability. A shortened block of k´ bytes is obtained through adding 239­k´ zero bytes before the data block and after encoding, these 239­k´ zero bytes are discarded. To obtain the punctured pattern to permit T´ bytes to be corrected, the first 2T´ of the 16 parity bytes has been retained.

4.2.3 Convolutional Encoder

The

outer

RS

encoded

block

is

fed

to

inner binary

convolutional

encoder.

implemented encoder has native rate of

polynomial in Equation

(4.3)

and

(4.4)

1/2, a constraint length of 7 and the generator to produce its two code bits. The generator is

shown in Figure 4.5.

G1 = 171OCT

For X

G2 = 133OCT

For Y

(4.3) (4.4)

30

The

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