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IEEE802.16dIEEE802.16dSimulator

WirelessMAN-OFDM-PHY layer

Mohamad Charafeddine

Rev-s3

24 Sept 2004



2

Introduction

� The Matlab¶s Simulink simulation work is for the

broadband wireless standard IEEE802.16d [and

subsequently for the IEEE802.16-2004 once it is

published]

� Currently the mandatory channel coding scheme is used

(Reed-Solomon/ convolutional code), 16-QAM

modulation, RS(64,48,8), CC rate=2/3

� A state machine can be implemented on top of thecurrent model to take into account adaptive rate

modulation.



3

IEEE802.16d, WirelessMAN-OFDM

256-FFT, 16-QAM



4

Randomization

�PRBS generator: 1 + x14 + x15

�On DL: the scrambler is re-initialized at

start of each frame with the vector:

100101010000000



5

Concatenated Reed-Solomon /

convolutional code (RS-CC)



6

Reed-Solomon coding

� Tail byte appended at end of the scrambled data

� For the 16-QAM mode selected, RS(64,48,8) is used, with:

� A primitive GF polynomial of: p(x) = x8 + x4 + x3 + x2 + 1

� And a code generator polynomial of: g(x)=(x+0 ) (x+1 ) (x+2 ).. (x+2T-1 ), =02 Hex



7

Convolutional Coding &

Puncturing

� Convolutional coding with a native rate of ½

� Constraint length of 7

� Generated with the following 2 generator polynomials:

G1=171oct=1111001 For X

G2=133oct=1011011 For Y

� Puncturing of 4/3: X1Y1Y2; thus overall CC rate of ½*4/3 = 2/3



8

2-steps Interleaver (I)

Define:

� N cpc the number of coded bits per carrier (i.e. 2,4,6 for QPSK,

16QAM, 64QAM respectively)

� s = N cpc/2

�k: the index of the coded bit before the first implementation

�m: the index after first permutation

� j: the index after the second permutation

----------------------------------------------------------------------------

� 1st permutation [for carriers]

m = (Ncbps/16)*mod(k,16)+floor(k/16);

� 2nd permutation [for bits constellation mapping on carriers]

j=s*floor(m/s)+mod((m+Ncbps-floor(16*m/Ncbps)),s);



9

2-steps Interleaver (II)

0 100 200 300 400 500 600 700 8000

10 0

20 0

30 0

40 0

50 0

60 0

70 0

80 0illustrating the interlea ving p roce ss

input indexing

o

u t p u t i n d e x i n g

after 1st permutation

after 2nd permutation



10

16-QAM modulation

� 16-QAM modulation

� Gray mapped

� Normalized constellation average power



11

OFDM Transmitter: Data&Pilots,

Zero Padding, Shaping, Cyclic Prefix (I)

� 192 Data carriers

� 8 Pilot carries

� 0 DC carrier

� 55 zero carriers added



12

OFDM Transmitter: Pilots (II)

� Pilots BPSK modulated

� Uses a PRBS generator of x11 + x9 + 1

� Initialized in DL with the vector: [1 1 1 1 1 1 1 1 1 1 1]

Why can¶t we always transmit all 1s on the pilots

without the need of the PRBS generator?

84 36 60 84: 1 2 ;k DL c c c c w

! ! ! ! 60 12 12 36: 1 2 k DL c c c c w

! ! ! !

84 36 12 36 60 84: 1 2 ;

k U c c c c c c

! ! ! ! ! ! 60 12: 1 2 k U c c

! !

Sample of a DL

sequence:

Corresponding sample of

an UL sequence:

PRBS for pilot modulation



13

OFDM Transmitter:

Zero Padding (II)

� 55 zeros carriers are padded.

� They will take the guard bands role.

� Reshaping is done to ensure the spectrum falls

off on both sides when plotted from ±Fs/2 to Fs/2

Freq axis: 0 Fs

Freq axis: -Fs/2 Fs/2



14

OFDM Transmitter:

IFFT & Cyclic Prefix (III)

After IFFT

before CP

After CP

Received

Spectrum

after the

AWGN

channel

� Tg/T b ratio of ¼ used. Thus CP

is 1/fourth the length of data time� Tg/Tb ratios specified by the protocol

are: ¼, 1/8, 1/16, 1/32. Question: it

does not specify when to use them.



15

Receiver side



16

OFDM receiver

� Reciprocal work of the OFDM transmitter:1. Remove Cyclic Prefix (assuming synchronization)

2. Perform the FFT

3. Remove the zero padding and reorder

4. Separate the data carriers from the pilot carriers



17

Received signal

� AWGN channel is used

� If a Rayleigh channel is to be used, the receiver side would need a channel equalization

section with the usage of the pilots in the channel estimation (currently pilots are ignored

after being removed from the received OFDM frame).



18

QAM demodulation &

2-stepsDe-interleaving

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 00

10 0

20 0

30 0

40 0

50 0

60 0

70 0

80 0

i

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i

i

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'

( 1s t

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( 0 1 tat i

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after 2n d )

ermutati2

n



19

Viterbi decoding

� Using a trellis generated from the polynomial of constraint length 7,

and the polynomials G1=171oct, and G2=133oct.

� Traceback depth specified to be equal to 34.

� It can be possible if the future to use soft decisions, LLRs, in decoding.



20

Reed Solomon decoding

� Some work is needed to compensate for the delay introduced by the Viterbi decoder.

� Reed Solomon decoding is done, and tail byte removed.

� Num of corrected is also outputted, with a -1 when the errors exceeding the max

allowed bit errors; in this case equals to T =8.



21

De-randomization

� Same operation as in the Randomization

� XORing with the same PRBS generator will

retrieve back the data



22

Bit Error Rate

� Bit Error Rate is displayed, along with,

� Number of bits in error, and� The total number of bits compared with the

original stream of raw data.



23

Generated BER curve for the IEEE 802.16d,

256-FFT, 16QAM, RS(64,48,8) & CC rate of 2/3

0 2 4 6 8 10 12 14 16 1810

4 5

104 4

104 3

104 2

104 1

100

SNR ( 5 B)

B E R

IEEE802.16a BER c6

rves, 2567 FFT 16

7 QAM , RS(64,48,8), CC rate=2/3

IEEE802.16a 2567 FFT 16

7 QAM , RS(64,48,8), CC rate=2/3



24

BER curves over AWGN channel;

incremental effects of channel coding

0 5 10 15 20 2510

8 5

108 4

108 3

108 2

108 1

100

S NR (9

B)

B E R

IEEE802.16a BER c@

rves, 256 A FFT 16 A Q AM ,B

ver AW GN cC

aD D

el, iD

cre m eD

tal effectsB

f cC

aD D

el cB

9 i

D E

Ra D 9

B

m izatiB

D

A RS A CC A I D terleavi D E

RS A CC A ID terleavi D E

CC A ID

terleaviD E

ID

terleaviD E

D

B

Ra D 9 z , D

B

RS , D

B

CC, D

B

I D terlv

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