Performance Evaluation of WiMAX (802.16) Using Different Encoding Schemes

Perf or m a n c e Ev al u atio n o f W i M A X (802.16) Us i ng Di f f er e n t E n codi ng Sc h e m e s Waqar A si f, M u h am m a d B il al Qa si m , Sy e d Mu sa Ra z a Ti rm zi, Usm an Mu ham m a d Khan D ep a rt m e n t of El ec t ri ca l E ng i n ee r i ng COM SAT S Ins ti tu t e o f I n f o r m a t i o n Tech n o l og y , I s l a m a b ad , Pa k i s t a n w aq a r. as if @g m a i l . co m Ab s t r ac t — Thi s p ap e r d e al s w i th th e p e r formance of W orld w i d e I n te rop e rab i l i ty for Microw a v e A cces s (W i MAX ) , w h e n w e e nh an c e i ts p h ys i cal l ay e r attr i b ute s w i th h e l p of d i ffe re n t e ncod i n g te ch n i q u e s . For th i s e v al u at i on S p ac e Ti m e Bl ock Cod e s ( S TBC ) a nd Tu rb o cod e s are se p ar ate l y i n tr od u ce d i n to th e arc hi te ctur e of W i MAX th at w orks on a d ap ti v e mod u l ati on t e chn i q ue . Ke ywor d s - W i MAX , A d a p ti v e Mod u l ati on , O rthogon a l F re q u e n c y D i v i s i o n Mul t i p l e xing, G u ar d Ban d , I nv e rse Fouri e r Tr an sform, S p ace T i me B l ock C od e s, M axi mu m L i k e l ihood De t e ctor , Turb o C od e s , I nte rl e av e r, L og M ap De cod e r. І. IN T R O DU C T I O N W iMA X s t a n d s for W o rl dwi d e In te rop erab i lity fo r M i crowav e A c ce s s . W i MA X i s b a s e d o n wi re l es s M et ro p o l i t a n A rea N e t work ( MA N ) s ta n d a r d s d e v el o p e d b y I EEE 802. 1 6 g ro u p . It o p e r at e s i n th e l i cen s e d ex e m p t a n d l i c en s ed s p ec t rum b et ween 2-1 1 GH z an d 1 0 - 66 GH z fr eq u en cy ran g e s res p e ct i v el y [1 ]. It w as d e v e l o p ed to c on n e c t t h e In t ern et an d t o p ro v i de a las t mi l e wirel es s e x t e n s i o n t o ca b l e an d D SL b ro ad b an d ac ce ss . IEEE 8 0 2 .16 p rov ide s u p t o 5 0 k m o f l i n ea r s e rvi ce a r ea ran g e an d al l o ws u s ers co n n e c t i v i ty w i th o u t a d ir ect Li n e o f S i g h t ( L O S) t o a B a s e Sta tion (BS). Th e t ec h n o l o g y al s o p ro vide s s h ared d a t a rat e s u p t o 7 0 M b p s , which is e n o u gh b a n d w i dt h t o s i m u l ta n e o u s ly s u ppo rt m an y us ers . In S p ac e Ti m e Bl oc k C o d es (S TBC ), d at a s t ream i s en c o d e d in b l o ck s a n d d i stri b u t e d am o n g t i m e an d s p ac ed an t e n n as . A t t he t ran s mi t t e r s i d e t h i s en c o d e d d at a i s t r an s m i t t ed al on g m u l tipl e an te n n as whe reas , a t t h e r ece i v er s i d e m u l t i pl e an t en n a s r ec eive m u l t i p l e c o p i es o f t h e s a m e s i g n al an d th e n ex t ract t h e b e s t p o ss i b l e o u t o f i t . It p rov i d e s s i g n i f i c an t i nc reas e i n t h ro u g h p u t an d ran g e wi t h o u t a n y i nc reas e i n t h e o v eral l b an d w i dt h an d t ran s m i ts p o wer e x p en d i tu re. It al s o i n crea s es t he s p e ct ral e ff i c i en c y (nu m b e r o f i nfo r m a t i on b i ts p e r h ert z o f b a n d wi d t h ) o f w i r e l e s s s y s t em b y u s i n g m u l tipl e an t en n a s t h a t a re s e p a r at e d i n s p ac e a n d t i m e [ 2 ]. Tu rbo co d e s are t h e b e s t a pp r o x i m a t i on o f t h e Sh an n o n l imi t . In Tu rb o c o d e s at t h e t ran s m i tt e r s i d e a s i n g l e b it i s en c o d e d i nt o a c o m b i na t i o n o f bits d ep e n d i n g o n t he arch i te c t u re o f t h e en c o d er. W h e n th i s en co d ed da t a i s t r an s m i t t ed o v er t h e c h an n el t h e p ro b a bi l i ty o f e rr or i s red u c ed t o a g r ea t ex t e n t . W h e n t h e d at a rea ch e s t h e rec ei v er s i d e t h e d at a i s d ec o d ed b a ck i n t o t h e o ri g ina l b i ts t h at are u n d e rs t an dab l e b y th e r ec ei v er. П . W iMA X A n u m b e r o f i n d u s t ry s ta nd ard s gov ern t h e d e s i gn a n d p erf o r m a n ce o f w i r e l e s s b roa d b a n d eq u i pm e n t . Th e s t an d ard s t h at ch i ef l y c o n c ern w i rel es s b roa db and a re 8 0 2. 1 6 an d i ts d eri va t i ve 8 0 2 . 1 6 a , b o t h o f wh i ch we re d ev el o p e d b y t h e Ins t itu t e o f El ect ri c a l an d El ec t r o n i c E n g i ne e r s (I EEE), a m a j o r i n d u s try s t a n d ard s bo d y h ea dq u arte red i n t he U n i te d Sta te s . A. P h ysi c a l L a ye rs of W iM AX Th e I EEE 802 . 1 6 s t a n d a r d su p p o rts m u l t i pl e p h y s ical s p e ci fi ca t i on s d u e t o i ts m o d u l ar n a tu re. Th e f i r s t v e r s i o n o f t h e s t a nd a rd o n l y sup porte d s i ng l e carr i e r m o d u l at i o n an d afte r t h e p a s sag e o f t i m e a n d a s t e ch nol o g y g rew; OF D M an d s c al ab l e OF DMA w ere a l s o us ed but onl y fo r o p e r at i n g i n t h e N o n L i n e o f Si g h t (N LOS ) en v ir on m e n t a n d w ere m e an t to p rov ide m o b i l i t y . T h e s ta n d ards were t h e n furt h er e nh anced t o w o rk i n t h e l o wer f req u e n c y ran g e o f 2-11 GH z a l o n g w i t h t h e p rev i ou s 1 0 - 6 6 GHz ba n d . 1 ) Orth ogo n a l Div i si on Mu l t i p l ex in g ( O FD M) : Th e i d ea o f O F DM co m es f ro m M u l ti C a rr i er M o d u l at i o n (M C M ) t r an s m i s s i o n te c h n i qu e. MC M i s ba s e d on th e d i v i s i o n of i n p u t b i t s tream i n t o s e v e r al p aral l e l b it s t ream s a n d t h en u s i n g t h e m t o m o d u late s ev e ral s u b c arr i ers as s hown i n F i g 1 . G u ard b an d i s int rod u ce d i n b et ween e ac h s u b c arr i er s o t h a t t h e y d o n o t ov erl a p wi t h e ac h o t h e r. Th i s gua rd b a nd al so s upp o rt s t h e b an d p ass fi l t er o n t h e r ec eive r s i d e in i d en t i fyi n g i n d i vi d u a l s u bcarr i e rs . O F D M i s a s pe cial f o r m o f s p e c tral l y ef fi c i en t M C M t e ch n iqu e . It d i f f ers fr o m its pred e ce ss o rs i n t h e w ay t h at i t u s e s o rt ho gon a l s u b ca rr i e rs whi ch a l s o e l imi n at e t h e us e o f a b a n d p a s s f i l te r f ro m t h e rec ei v er s i d e. Th e orth o g o n al n at u re o f t he su b c arr i e rs al s o rem o v es t h e In te r C a rrier Int e r f eren c e ( ICI) w h i ch was a g reat p rob l e m p rev i o u s l y . Th e g u a r d b a nd p rev iou s l y us e d i s a l s o r em o v e d i n OF DM h en c e co n cl u d i n g i n t h e red u c t i o n o f b a n d wi d t h us a g e a s s hown in t h e Fi g . 2. (IJCSIS) International Journal of Computer Science and Information Security, Vol. 6, No. 2, 2009 332 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 Fig . 1 . OFD M T r a n s mi tt er Fig . 2 . OFD M S y m b o ls 2) Ad a p t i ve Mo dul a t i o n a n d C o d i n g : A M C a llows W iMA X s ys t e m s t o s e l ect t h e m o s t a p p ro p ri at e M o d u l at i on an d C o d i ng Sch em e ( M C S) d ep e n d i n g o n t h e p r o p a ga t i on co n d i ti o n s o f th e c o mm u n i cat i o n c h a n n el , e.g ., i f t h e p rop a g a t i o n c o n d i ti o n s are g o o d , a hi g h er o rd er m o d u l at i o n s c h em e w i t h a l o wer c o d i ng red u n d a n cy i s us e d whi ch al s o i nc reas e t h e d at a rate o f t h e t rans m i s s i o n , wh i le o n t h e o t h er h an d duri n g a s i g n al fad e , t h e s ys t e m s el ect s a m o d u l at i o n s c h em e o f a l o w er o r d er t o m ai n ta i n b o t h t he c o n n ec t i on q u a l ity a n d t h e l i n k s ta b i li t y w i th o u t an inc reas e i n t h e s i g n a l p o wer [3 ] . Fo r th i s pu rp o s e W i MA X u s es f ou r m o d u l at i on s c h em e s th a t are: Bina r y P h as e S h i f t Key i n g ( B PS K) Q u ad ratu re Pha s e S h i f t Key i ng ( Q P SK ) 16 - Qu ad rat u re A m p l i tu d e Mo d u l at i o n ( 1 6 - Q A M ) 64 - Qu ad rat u re A m p l i tu d e Mo d u l at i o n ( 6 4 - Q A M ) Ш. S YS T E M MO D EL Th e fi r s t s te p o n t he p hys ical l a y e r o f W iMA X i s t h e d ec i s i on o f t h e m o d u l at i o n s c he m e t h a t h a s t o be u s e d a s s h o wn i n Fi g 3 . Ini t i al l y t h e t ran s mi tt e r tran s mi ts t h e b i ts u s i ng 16 -Q A M m o d u l at i o n . Th e m o d u l at e d d at a is t h e n m a p p e d o n t o o rth o g o n al c h a n n els u s i n g IF F T. I n t h is m ap p ed d a t a, t o av o i d i n t e r- car ri e r int e r fer en c e a g u ard b a n d ( o f 1/8 o f t h e t o t al n u m b e r o f b i ts ) i s adde d . Th e d at a i s t h e n tran s m i t t e d o v e r t h e c h an n el a n d at t h e rece i v er en d t h e g u a r d b a n d re m o v e d an d th e da t a d e c o d e d ba ck i n t o th e ori g i na l f or m. Th e rece i v er ca l c u late s t h e err o r i n t h e rece i ve d d a t a a n d i n f o rm s t h e t ran s m i tt e r o f t h i s . Th e t ran s mi tt e r u s e s t hi s i n form a t i on t o ch a n g e i t s m o d u l at i on s c h e m e as p er th e r eq u i r e m e n t . Modulation I F F T Guard Band Insertion Parallel to Serial Noise Serial to Parallel F F T Guard Band Removal De- Modulation Fig . 3 . W iM AX Mo du le A. Int ro d u ct i o n o f S TBC i n W i MAX Sy ste m Fat t h e t ran s m i tt e r t h e STB C E n c o d e r t a k es t h e d a t a from t h e g u a r d ba n d i ns erti o n bl o ck as s h o wn i n Fi g 4 an d tran s m i ts t h at da t a o v e r t w o s pa ce d anten n a s . Di ff e r en t sy m b o l s are s i m u l ta n eo us l y t r an s m i t t ed o v e r th e s e an t e n n a s to red u ce n oi s e i nt e rf er en c e . Th e rec ei v er aft er rece i v i n g t h e s i g n al retri ev es t h e b its us ing M a x im u m L i k e l i h o o d d e c o d i ng al g o ri t h m a n d p ass e s t h e d a t a t o t he g u ard b an d rem o v al b l o ck. A s s h o w n i n Fi g 4. Modulation I F F T Guard Band Insertion STBC Encoder STBC Decoder F F T Guard Band Removal De- Modulation Fig . 4. W iM AX M o d u le w i t h S T B C 1 ) En c o d i ng a n d T ra n sm i ssio n seque n ce : For ac h i ev i n g d i ve rs ity i n t i m e a n d s pa ce sy m b o l s are t ran s m i tt e d acc o rding t o th e f oll o w i ng m a tr i x Ta b le I . T H E E NCODI NG A ND T RA NSMIS S IO N S EQ UE NCE F OR T HE T WO - B R AN C H T RAN S MIT D I VERS ITY S CHEME A n t e n n a 0 A n t e n n a 1 Ti m e t s 0 s 1 Ti m e t + T -s 1 * s 0 * For A n t e n n a 0 a t t i m e t t h e co m p l e x m u l t i p lican d d i s t o rti o n c a n b e m o d eled as h 0 (t) an d for A n t en n a1 i t c an b e (IJCSIS) International Journal of Computer Science and Information Security, Vol. 6, No. 2, 2009 333 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 m o d eled a s h 1 (t). Now i f w e ass u m e t h at f ad i ng i s c o n s t a n t ac ross t wo s y m b o l s th e n w e ca n w r i te [ 4] . h 0 (t) = h 0 (t + T ) = α 0 e j θ h 1 (t) = h 1 (t + T ) = α 1 e j θ h 2 (t) = h 2 (t + T ) = α 2 e j θ h 3 (t) = h 3 (t + T ) = α 3 e j θ (1) Th i s can b e ex p res s e d i n m at r i x f or m as Ta b le II . T H E D E F IN ITI O N OF C HA NN ELS B ETW E E N T HE T R AN S MIT A ND R ECEIVE A NT ENN AS Rx A n t e n n a 0 Rx A n t e n n a 1 T x A n t e n n a 0 h 0 h 2 T x A n t e n n a 1 h 1 h 3 Th e rece i ve d s i gn al s ca n be ex p res s e d as r 0 = h 0 s 0 + h 1 s 1 + n 0 r 1 = -h 0 s 1 * + h 1 s 0 * + n 1 r 2 = h 2 s 0 + h 3 s 1 + n 2 r 3 = -h 2 s 1 * + h 3 s 0 * + n 3 ( 2 ) h ere n 0 , n 1 , n 2 a nd n 3 a r e c o m p l e x r an d o m v a r i ab l e rep r es en t i n g r ec eive r th e r m al no i s e an d i nt e rf er en c e [ 4 ]. tx 0 tx 1 rx 0 rx 1 h0 h3 h2 h1 Channel estimator combiner Channel estimator h3 h2 h0 h1 Maximum likelihood detector h0 h1 h3 h2 s1 s0 s0 s1 s0 -s1* s1 s0* Fig . 5 . T wo - bra n c h t r a n s m i t d i v er si ty sc h e m e w ith t w o rec eiv e r s 2) Th e c o mbi n i n g S c h e me: Th e co m b i n er o f F i g 6 b u il d s t h e f oll o w ing two c o m b i n ed s i g n a l s an d s e n ds th e m to t h e m a ximu m l i k el i h o o d d et e ct o r s 0 = h 0 *r 0 + h 1 r 1 * + h 2 *r 2 + h 3 r 3 * s 1 = h 1 *r 0 – h 0 r 1 * + h 3 *r 2 – h 2 r 3 * ( 3) 3 ) Th e M a xi mum Li k e l i h o o d de c isi on Ru l e : T h e m a x im u m l ik e l iho o d u s e s t h e d e cis i o n rul e g ive n be l o w to d et e r m i n e t h e o ri g i n al t r an s mi t t ed s ign a l s . I t ch o o ses s i i f: ( α 0 2 + α 1 2 + α 2 2 + α 3 2 - 1 ) | s i | 2 + d 2 (s 0 , s i ) ≤ (α 0 2 + α 1 2 + α 2 2 + α 3 2 - 1) | s k | 2 + d 2 (s 0 , s k ) (4 ) A fte r t h i s th e s y m b o ls ob t a i ne d a re s e n t t o t h e g u a rd ba nd re m o v a l s ec t i o n o f O F D M an d t h e n o rm a l pro ce d u re o f W iMA X m o d u le co n t i nu e s i ts w o rk . B. Int ro d u c t i o n of T u rb o Co d e s in W i MAX S yst e m Tu rbo E n co d er u s e s th e b its p a ss ed o n t o i t b y t h e M A C l ay er o f W i M A X an d wi t h h el p o f R e cu rs i ve Sy s t em a t i c C o n v o l u t i on E n co d er en c o d ed t h e b i ts and p a ss e s t h e m o n t o t h e m o d u lation s ch e m e b l o ck . A t t h e rec eive r t h e L o g M A P d ec o d e r t a k es b its f r o m th e d e m o d u l at i o n b l oc k a n d p ass e s t h i s d at a o n t o t h e M A C l ay e r afte r d ec o d i n g i t [ 5] . Th i s i s s h o wn i n Fi g 6 . Turbo Encoder I F F T Modulation Guard Band Insertion Noise Guard Band Removal F F T De- Modulation Turbo Decoder Parallel to Serial Serial to parallel Fig . 6 . W iM AX Mo du le w i t h T u rb o C o d es 1) R ec u rsiv e S yst e m a t i c C on v o l u tio nal En c o d er ( R S C ) : Th i s t y p e o f e n co d er wo rk s o n a b it b y b it b a s i s . Fo r e v e r y i np u t b i t i t g en e rat e s a p a ri ty b i t d ep e n d i ng o n t h e struc t u re o f t h e en c o d er an d ou t p uts t h e s a m e i n p u t bi t at th e ou t p ut k no wn as t h e syst e m a t i c b i t . T h e en c o d e r i s i m p le men t e d u s i n g L i n ea r Feed b ac k Sh i ft R eg i s t er ( L F S R ) . Th e s e reg i s t e rs a re t h e m ai n reas o n why we ca l l i t R ec u rs i v e p r o ce s s . Th e o u t p u t i s fee d b ac k t o t h e inp u t a n d e v ery n ew o u t pu t i s d e p e n d e n t o n t h e p rev i ou s i n p u t t o t h e en c o d er. In Fi g 6 t h e D d e n o t e s t h e reg i sters whi ch a re ini t i al l y a t a k n o w n s t a t e o f 00. A fte r a p a c k et i s e n c o d ed t he reg i s t e rs c an b e in a n y o n e o f t h e f o u r p o s s i b l e st at e s 0 0 , 0 1 , 10,1 1 d ep en d i ng o n t h e p rev i o u s i np u t . Th e s e s ta te s ca u s e p rob l em for t h e n e x t p ac k et . T o rem o v e t h i s p rob lem a nd t o b ri n g t h e reg i s t e rs back i n to a kno wn s ta te o f 00 , M emory Fl u sh i n g o r Trell i s Termin a t i o n i s d o n e . Fo r m e m o ry f l us h i ng we p a d Trai l Bi t s at t h e en d o f ea c h p ac k et d e p endi n g o n t h e s t a t e o f t h e reg i s t e rs . 2) In t erl e a ve r: I n t erl e av er i s an e ss e nt i al p art o f t h e en c o d e en c od er. I t i s us ed to ch an g e th e s e q u en c e o f t h e b i t s i n (IJCSIS) International Journal of Computer Science and Information Security, Vol. 6, No. 2, 2009 334 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 a fr a m e . I n t h e e n co d i ng st ag e t h e d at a i s i n t erl e av ed b efo r e t h e d at a i s f ee d to t h e s e cond en c o d er. D D + + + Interleaver D D + + + Systeamtic Bit First Parity Second Parity Information Block Fig . 7 . Recu rsi v e S y s t e m a t i c Co nvo lu t i onal Enco de r I n p u t Bi ts Fix ed R a n d o m Per m u t a tion Ou t p u t B i ts Th e i nt e rl ea v e r o f Fi g 8 u s e s a fix e d ran d o m p er m u t a tion an d m a p s t h e i np u t a c co rd i ng t o t h a t p erm u t at i o n o rde r. Here t h e l en g t h o f t h e inp u t i s d enot e d b y L t h e grea te r t h e s i z e o f L t h e b e t te r w i l l b e t h e p e r f o r m a n c e Fi g 8 s h o w s t h at i f we t ake L=8 t h e n t h e i n te rl e av e r i np u t s a s eq u e n ce [0 1 1 0 1 0 0 1 ] an d ou tpu t s [ 0 1 0 1 1 0 0 1]. 3) Lo g M a p D ec o d er : To a c h iev e a s o ft d e cis i o n t h rough t u rbo d e c o d i n g , t wo L o g M a p Deco der s a r e us e d to get h er w h i ch wor k i te r at ive l y o n a s y m b o l b y s y m b o l d ec o d i n g m e t h o d . a ) S y m b o l By S ymbo l d ec o d i ng : F or s y m b o l b y s y m b o l d ec o d i n g p roc ed u re we b reak o u r o b s e r v at i o n s i n t o t h ree m ai n p art s . Th e firs t p a rt d e n o t e s t h e p a s t obse r v at i o n s b efo r e t i m e k , t h e s ec o nd p art d en ote s t h e p res e n t o bse rva t i ons a t t i m e k an d t h e t h i r d an d t h e l as t p a rt d e n o te s t h e fu t u re o bs e rva t i ons afte r ti m e k . i. e. [ 6] k (s ) is t h e j o i n t c o n d i ti o n a l p rob a b i l i ty o f t h e p a s t o b serv at i o n b efore t im e k an d s s k k s k k k s s s s s s s ' 1 ' 1 ) , ' ( ) ' ( ~ ) , ' ( ) ' ( ~ ) ( ~ (5) Decoder 1 Interleaver Decoder 2 De- Interleaver Interleaver Parity 1 Systematic bits Parity 2 Fig . 9 . T u r b o dec o d er H ere s s s k / ' ) ( g i v es u s t h e o v e r a l l s u mm a t i on o f t h e p o ss i b l e s t at e s s k - 1 w h o s e e n d a t a s t a t e whe re s k = s . Th e ini tial co n d i ti o n th a t i s d efi n e d as 0 1 s if 1 ) ( ~ 0 other wis e s (6) m e a n s t h a t t h e s ta rt o f t h e t rel li s s ta t e s fo r t h e en c o d e r i s at s t a t e 1 . k -1 (s ) i s t h e j o i nt c o n d i ti o n al p ro b ab i li ty o f t h e fut u re o b serv at i o n s a f t er t i m e k t h a t i s at k- 1. th i s i s a l s o co m p u t ed recu rs i ve l y as f o llows s s k k s k k k s s s s s s s ' 1 2 1 ) , ' ( ) ' ( ~ ) , ' ( ) ( ~ ) ' ( ~ (7) H ere s s s k / ' ) ( i s t h e s u mm at i o n o v e r al l t h e p o ss i bl e s ta te s s k -1 w h o s e en d i n g s ta t e i s s k = s . Th e i n i t i a l co n d i tion i s 0 1 s if 1 ) ( ~ other wis e s N (8) w h i ch m ea n s th a t t h e e n c o d er Trel l i s s t a te s al w ay s s ta rt a t s t a t e 1 . γ k (s ) i s t h e joi n t c o n d i ti o n a l p rob a b i l ity o f th e p r es e n t o b serv at i o n at t i m e k an d q i i k p i k k s k k k e c y Lc c y Lc c c L s s 2 , 1 , 1 1 1 2 1 exp 2 1 ) ( 2 1 exp ) , ' ( (9) Fig . 8 . Ra n d o m I n t e r lea v e r (IJCSIS) International Journal of Computer Science and Information Security, Vol. 6, No. 2, 2009 335 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 Th es e t h ree p ro b a bi l i t i es a re co m b i ne d t o c o m p u t e t h e s y m b o l us i n g . ] ) ( ~ ) , ' ( ) ' ( ~ ) ( ~ ) , ' ( ) ' ( ~ l og [ ) ( 1 1 u k k k u k k k k s s s s s s s s u L (10) B o t h t h e d e code rs c om p u t e t h e s y m b o l u s i n g t h e s y m b o l b y s y m b o l d e co d i ng m et h o d a n d co n ti n u e t h i s p r o ce ss o n a n i te r at i v e b a s is t i l l t h e t im e t h ey g et t o a s i mi l a r r es ult. W h e n t h ey both r eac h a t t h e s a m e r es u l t t h e d e co d i ng p ro ce ss i s s t o p p e d a n d t h e resu l t e x t ract ed . Th i s p ro ce ss rep ea t s i t s e l f for ev e ry u p c o m ing p ac k et . Ш . S IMU L A T IO N R ESU LTS Th e Sim u l at i on m o d e l was i m p l e m e n t ed i n M a tlab 7. Th e s i m u l at i on w as r un f or a p ac k et s i z e o f 1 0 6 , an d fo r b e t t er ap p rox i m at i on th i s p ac k et w as t r an s m itt ed 1 0 2 ti m es . F i g . 1 0 . s ho w s t h e result o btai n e d for t he p e rf orm a n c e o f W iMA X, u s i n g ad a p t i v e m o d u l at i o n t e ch n i qu e . Fig . 10 . Per f orm an ce G ra ph of W i MAX Ini tia l l y t h e p a cket was s e n t o v er s im p l e W i M A X arch i te c tu re a n d t h e g rap h m ark ed w i t h „ * ’ ac h i ev e d . It s h o w s i ni t i a l l y Q PS K m o d u l at i o n i s u s e d t i ll an e n erg y n u m b e r p e r b i t o f f i ve an d a f t er t h a t i t s ta rt s wo r ki ng at QA M . It m e an s t h at t h e a t m o s p h ere i s too n ois y t h a t‟s w h y i n i ti a l l y Q PS K i s u s e d t o s u c c e ss fu l l y t ran s m i t s y m b o l s b u t a s s o on a s t h e B ER reac h es a v al u e b el o w o u r d efi n ed t h re s ho l d i t s h i f ts t o a h i gh er m o d u lation s c h em e i .e. QAM . In t h e s i mil ar co n d i tion wh en we int rod u ce ST B C i n t h e arch i te c tu re o f W i M A X we h av e a g rap h m ar k ed wi t h „ . ‟ . I n t h i s g rap h t h e m o d u l at i on s c h em e s h i ft s fr o m Q PS K t o Q A M at an en e rgy nu m b e r pe r bi t of t h ree . Th i s s h o ws th a t fo r l o w er v alue s o f e n ergy num b e r p er b i t t h e o b s erv ed B ER is h i g h b u t as t h e e n erg y nu m b er i n crea s e s t h e B E R s t a rts d e crea s i n g a t a i nc reas ed r at e . Th e t h ir d g rap h m a r k ed wi t h „ + ‟ i s a l s o o f t he s i mi l ar co n d i ti o n a s were for s i m p l e W i MA X arch i t ec t u re. Th e d i f f e r en c e h e re is th at t h ere i s n o n e e d t o t r ans m i t o u r s y m b o l s o v e r QP S K. Th e wh o l e d a ta i s t ran s m i tt e d u s ing QA M m o d u l at i o n . Th i s d en o t e s th a t wh en we i nt r o d u ce T u rb o C o d i n g i nt o o u r a r ch itec t u re t h e e ff ect o f n o i s e i s g reat l y red u ce d . ІV . C O NCLU SI ON It i s c o n c l ud ed th a t w h en we i n tro d u c e T u rb o C o d e s i n t o W iMA X w e h a v e a v e ry i m p rov e d B ER , b u t t h e Tr el li s Ter mi na t i on b i ts , t h e t a i l b i t s a n d t h e p a ri t y b it s m a k e u p a n o v e r h ea d th a t i s n o t fea s i b l e for a w i r e l e s s li n k l ike W iMA X. O n t h e o t h er h an d b y t h e i n t rod uc t i o n o f STBC we h a v e a n i m p rov e d B ER c o m p ared t o s i m p l e W iMA X a rch i te ct u re w i th a v e ry l o w o v e rhe ad . So we s u g g e s t tha t to i m p rov e t h e p erf o r m a n ce o f W i M A X, ST B C s h o u l d b e i nt ro d u c ed i n i t. V . R E F R EN C E [1] S y ed Ah so n a nd Moha mmad I l y as, W iM AX Tec hnol o gies , P e r f o r m a nc e A n a l y si s a nd Q oS , CR C P r ess , T a y lo r & F r a n cis G r o u p , 2 008. [2 ]Ha mid Jaf a r k ha ni, “ A Quas i - O r t h ogo n a l S p ac e time Bl o c k C o d e, ” I E EE Tr a n s C o m mun, v o l 49, pp 1 -4, Ja n 200 1 [3 ] Da n ia Ma r ab is si, Da n ie le Tar ch i , Ro m a no Fa nt a cci a n d Fr a n c esc o b alleri , “ E f f ici en t A d a p t iv e m o d u la t i o n a n d Codi n g t ec hniq ues f or W iM AX s y s t e m s ,” I CC p r o ce e d ings 2 0 0 8 . [4 ] S ia v ash M . A la m ou t i , “ A S im p le Tra n s m i t D i v e r si ty T ec h n iq u e for W i r eless Co mmu n ic a t io n s, ” I E E E j o u rnals , vol 16, n o . 8 , Oc t o b e r 1 99 8 . [5] C . B err o u , A. G lav ie u x , a n d P . T h i t i m a js h i m a, “ N ear S h a n n on limi t e r r o r -c o r r e c t i n g c o d in g a nd dec o d in g : T u r bo -c odes ,: i n Pr o c. ICC ,pp. 1 064 - 1070, 1 9 9 3. [6 ] S . Ben edetto, D . Div sal ar , G . M o n to r si, a nd F. Po lla r a,” S o f t- Ou tp u t D ec oding A lg o r i t hms in I t e r a t i v e Dec oding o f T u r b o C o d es ” , TDA p ro g re ss r ep or t 42 - 124, Febu r a r y 15,1 9 96 [7] C h ie n - Mi n g W u . M i n g-D er S h ieh , Chie n - Hs in g W u , yin g - T su ng Hw an g, Ju n -H on g C h e n , a n d Hsin - Fu L o , ” V L S I Arch i t ec t u re Expl o r a t i on f o r S lif t i n g W i n d o w L o g- M a p De c o d e r s ”, IS ACS 2 0 0 4 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 6, No. 2, 2009 336 http://sites.google.com/site/ijcsis/ ISSN 1947-5500