Page 1. Spreading and frequency selective fading. #4: Spread Spectrum Technology. How do we overcome limitations imposed by the radio channel?

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1 EEC173B/ECS15C, Spring 009 How do we overome limiaions imposed by he radio hannel? Fundamenals o Wireless Communiaions #4: Spread Sperum #5: Muliplexing #6: Frequeny euse (Cellular Conep) Case Sudy: Wireless ellular neworks #7: Hando Fla ading ounermeasures Fade Margin Diversiy Coding and Inerleaving Adapive Tehniques Delay Spread Counermeasures Equalizaion Muliarrier Spread Sperum Anenna Soluions Aknowledgmen: Seleed slides rom Pro. Shiller #4: Spread Sperum Tehnology Spreading and requeny seleive ading Problem o radio ransmission: requeny dependen ading an wipe ou narrow band s or duraion o he inererene Soluion: spread he narrow band ino a broad band using a speial ode Proeion agains narrow band inererene hannel qualiy 1 narrow band 3 4 guard spae 5 6 requeny narrowband hannels Power inererene spread Power Deeion a reeiver spread inererene hannel qualiy spread sperum 1 requeny spread sperum hannels 3 4 Page 1

2 Spread Sperum Dire Sequene Spread Sperum (DSSS) Spread sperum inreases he ransmi bandwidh o redue he ees o la ading, ISI and inererene. SS is used in all wireless LAN produs in he ISM band equired or operaion wih reasonable power Minimal perormane impa on oher sysems IEEE sandard There are wo SS mehods: dire sequene and requeny hopping Dire sequene muliplies he daa sequene by a aser hip sequene. Frequeny hopping varies he arrier requeny by he same hip sequene 5 XO o he wih pseudo random number (hipping sequene) Many hips per bi (e.g., 18) resul in higher bandwidh o he Advanages edues requeny seleive ading In ellular neworks Base saions an use he same requeny range Several base saions an dee and reover he So handover Disadvanages Preise power onrol neessary b b : bi period : hip period user daa XO hipping sequene = resuling 6 DSSS Frequeny Hopping Spread Sperum (FHSS) reeived user daa X hipping sequene radio arrier demodulaor spread sperum ransmier lowpass ilered reeiver radio arrier hipping sequene modulaor orrelaor ransmi sampled produs sums daa X inegraor deision 7 Disree hanges o arrier requeny Sequene o requeny hanges deermined via pseudo random number sequene Two versions Fas Hopping: several requenies per user bi Slow Hopping: several user bis per requeny Advanages Frequeny seleive ading and inererene limied o shor period Simple implemenaion Uses only small porion o sperum a any ime Disadvanages No as robus as DSSS Simpler o dee 8 Page

3 FHSS (Frequeny Hopping Spread Sperum) II FHSS b user daa user daa narrowband modulaor modulaor spread ransmi d slow hopping (3 bis/hop) ransmier requeny synhesizer hopping sequene 3 1 d b : bi period d : dwell ime as hopping (3 hops/bi) reeived hopping sequene demodulaor requeny synhesizer narrowband demodulaor daa reeiver 9 10 EEC173B/ECS15C, Winer 006 Fundamenals o Wireless Communiaions #4: Spread Sperum #5: Muliplexing #6: Frequeny euse (Cellular Conep) Case Sudy: Wireless ellular neworks #7: Hando Aknowledgmen: Seleed slides rom Pro. Shiller #5: Muliple Aess Single shared (broadas) wireless ommuniaion hannel Only one an send suessully a a ime Two or more simulaneous ransmissions Inererene How o share he wireless media? Design goals Fairness among users High eiieny Low delay Faul olerane Design hoie Fully enralized (onrolled) vs. deenralized 1 Page 3

4 Muliplexing Frequeny muliplex Goal: muliple use o a shared medium Muliplexing in 4 dimensions spae (s i ) ime () requeny () ode () Imporan: guard spaes needed! hannels k i s 1 k 1 k k 3 k 4 k 5 k 6 s 3 s Separaion o he whole sperum ino smaller requeny bands and alloae o dieren users A hannel ges a erain band o he sperum or he whole ime Advanages: No dynami oordinaion k 1 neessary Works also or analog s Disadvanages: Wase o bandwidh i he rai is disribued unevenly Inlexible Guard spaes k k 3 k 4 k 5 k Time muliplex Time and requeny muliplex A hannel ges he whole sperum or a erain amoun o ime Time is divided ino slos whih are alloaed o dieren users Advanages: Only one arrier in he medium a any ime Throughpu high even or many users Disadvanages: Preise synhronizaion neessary k 1 k k 3 k 4 k 5 k 6 15 Combinaion o boh mehods A hannel ges a erain requeny band or a erain amoun o ime Example: GSM Advanages: Beer proeion agains apping Proeion agains requeny seleive inererene k 1 Higher daa raes ompared o ode muliplex Bu: preise oordinaion required k k 3 k 4 k 5 k 6 16 Page 4

5 Code muliplex #6: Frequeny euse Time and bandwidh are used k 1 simulaneously by dieren users Eah hannel has a unique ode Signal modulaed by orhogonal or semiorhogonal odes (e.g., spread sperum) Advanages: Bandwidh eiien No oordinaion and synhronizaion neessary Good proeion agains inererene and apping Disadvanages: Lower user daa raes More omplex regeneraion k k 3 k 4 k 5 k 6 BASE STATION Frequenies (or ime slos or odes) are reused a spaially separaed loaions. Inrodues inererene => sysem apaiy is inererene limied. Mainly designed or irui swihed ommuniaions, e.g., ellular nework Cell sruure Implemens spae division muliplex: base saion overs a erain ransmission area (ell) Mobile saions ommuniae only via he base saion Cell sizes rom some 100 m in iies o, e.g., 35 km on he ounry side (GSM) even less or higher requenies Cellular Conep Advanages o ell sruures: Higher apaiy, higher number o users Less ransmission power needed More robus, deenralized Base saion deals wih inererene, ransmission area e, loally Base saions perorm enralized onrol unions. (all seup, hando, rouing, e.) Problems: Fixed nework needed or he base saions Handover (hanging rom one ell o anoher) neessary Inererene wih oher ells 19 0 Page 5

6 Signal Srengh Signal Srengh (In Praie) Signal srengh (in db) Signal srengh (in db) Cell i Cell j Cell i Cell j Sele ell i on le o boundary Sele ell j on righ o boundary Ideal boundary -100 Signal srengh onours indiaing aual ell iling. This happens beause o errain, presene o obsales and aenuaion in he amosphere. 1 Cell Shape Frequeny Planning (1) Eah ell size has a radius, Cell Frequeny reuse only wih a erain disane, D, beween he base saions Sandard model using 7 requenies: Cluser size, N = 7 (a) Ideal ell (b) Aual ell () Dieren ell models D r 3 4 Page 6

7 Design Consideraions euse disane (D) Disane beween wo ells using he same requeny (ime slo/ode) Smaller reuse disane paks more users ino a given area, bu also inreases heir o hannel inererene Cell adius () Dereases he ell size inreases he sysem apaiy, bu ompliaes nework unions o hando and rouing Frequeny Planning () N anno be jus any number! The luser size (or he number o ells per luser, or he oal number o requenies per luser) is given by j N i ij j where i and j are inegers. N = 1, 3, 4, 7, 9, 1, 13, 16, 19, 1, 8,, e. The popular value o N being 4 and o i 5 6 Illusraion Illusraion i and j are posiive inegers. To ind he neares o hannel neighbor o a pariular ell, you have o (1) move i ells along any hain o hexagons and hen () urn 60 degrees ouner lokwise and move j ells ell luser ell luser 7 D j 3 5 For N = 7, i=, j=1 6 1 i h h g 1 g h 1 h 1 g h 3 g h 3 g 3 g 3 g 1 3 g g 3 3 ell luser wih 3 seor anennas 7 8 Page 7

8 euse Disane Co Channel Inererene Cluser F7 F F6 F1 F3 F5 F4 F7 F For hexagonal ells, he reuse disane is given by D 3N where is ell radius and N is he reuse paern (he luser size or he number o ells per luser). Seond ier ohannel Base Saion D 6 D 5 D 1 D 4 D Firs ier ohannel Base Saion Mobile Saion euse disane D F6 F1 F3 F5 F4 euse aor is D 3N D 3 Serving Base Saion 9 30 Wors Case Co hannel inererene Esimaing Co Channel Inererene Wors ase o hannel inererene happens when a user is a a ell boundary Example: N=7 4 3 Serving Base Saion Mobile 6 Saion Co-hannel Base Saion Carrier or o inererene raio (C/I) is given by: C / I Carrier Inererene C M I k k 1 where I is o-hannel inererene and M is he maximum number o o-hannel inerering ells. For M = 6, C/I is given by C I 6 D k k 1 where is he propagaion pah loss exponen Page 8

9 C/I or SI Example (Con d) Assuming D i = D or i = 1,,.6 C I 6D D 6 General approximaion or SI (in db), onsidering only 1 s ring o inererers: D SI 10log M where M = number o inererers (depending on anenna used, e.) I a arrier o inererene raio o 15 db is required or saisaory orward hannel perormane o a ellular sysem, wha is he requeny reuse aor and luser size ha should be used or maximum apaiy i he pah loss exponen is (a) =4, and (b) =3? Assume ha here are six o hannel ells in he irs ier, and all o hem are a he same disane rom he mobile. Use suiable approximaion Soluion Cell Seoring by Anenna Design 10 o b a 10 o b a (a). Omni (b). 10 o seor (). 10 o seor (alernae) d 90 o a b e d 60 o a b (d). 90 o seor (e). 60 o seor Page 9

10 Cell Seoring by Anenna Design Wors ase or orward hannel inererene in hree seors Plaing direional ransmiers a orners where hree adjaen ells mee C B X A BS BS D D BS MS BS C I q C q Frequeny Assignmen Cell Breahing Fixed requeny (hannel) assignmen FCA Eah ell is assigned a ixed number o hannels (requenies) Channels used or boh handos and new alls Problem: dieren rai load in dieren ells eservaion hannels wih FCA Eah ell reserves some hannels or hando ells Channel borrowing A ell may borrow ree hannels rom neighboring ells Dynami requeny (hannel) assignmen DCA Base saion hooses requenies depending on he requenies already used in neighbor ells More apaiy in ells wih more rai Assignmen an also be based on inererene measuremens CDM sysems: ell size depends on urren load Addiional rai appears as noise o oher users I he noise level is oo high users drop ou o ells Page 10

11 EEC173B/ECS15C, Winer 006 Example overage o GSM neworks ( AT&T Wireless (Cingular), Norh Ameria Fundamenals o Wireless Communiaions #4: Spread Sperum #5: Muliplexing #6: Frequeny euse (Cellular Conep) Case Sudy: GSM Wireless ellular neworks #7: Hando Aknowledgmen: Seleed slides rom Pro. Shiller China Mobile Vodaone, UK 4 GSM Cellular nework Segmenaion o he area ino ells ell Possible radio overage o he ell Idealized shape o he ell Use o several arrier requenies No he same requeny in adjoining ells Cell sizes vary rom some 100 m up o 35 km depending on user densiy, geography, ranseiver power e. Hexagonal shape o ells is idealized (ells overlap, shapes depend on geography) I a mobile user hanges ells handover o he onneion o he neighbor ell #7: Handos (or Handover) equired o suppor mobiliy when he user moves ino a dieren ell Involves Ideniying a new BS in new ell Find uplink/downlink hannel pair rom new ell o arry on he all Drop he link rom he old BS Page 11

12 Handos: Design Issues (1) Opimal BS seleion: BS neares o MT may no neessarily be he bes in erms o srengh, espeially new he ell boundaries Ping pong Ee Call ges bouned bak and orh beween wo BS (a series o handos) Daa loss Inerrupion due o hando may ause a loss in daa Delay in relinquishing hannel in old ell and resume in new all may be aepable or voie, bu ause daa loss Hando: Design Issues () Deeion o hando requiremen Mobile iniiaed: MT moniors srengh rom BS and requess a hando when drops below a hreshold) Nework iniiaed: BS ores a hando when s rom an MT weaken, queries neighboring BS abou srengh rom he MT and dedue whih BS o hando oo Mobile assised sheme: ombinaion o mobile and nework iniiaed shemes. MT evaluaes srengh, bu inal hando deision is made by BS Handover Deision Hando Qualiy eeive level BTS old MS BTS old HO_MAGIN eeive level BTS old MS BTS new Hando delay: Signaling during a hando auses delay in ranser I delay is oo large, SIN may all below minimum hreshold, ausing all o be dropped Duraion o inerrupion Hard hando: hannel pair rom old BS anel and hen hannel pair rom nex BS is used o oninue he all Hando suess: probabiliy o suessul handos Depends on number o available hannel pairs, apaiy o swih beore SIN alls below hreshold Probabiliy o unneessary hando E.g., Ping Pong ee Inrease ing overhead, leading o unwaned delays and inerrupions Page 1

13 Quesions? 49 Page 13