MULTIPLEXING. Multiplexing. Space, Frequency, Time, Code 8/30/17. Jasleen Kaur Multiplexing: Multiplexing in 4 dimensions: Using guard spaces

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1 MULTIPLEXING Space, Freuency, Time, Code 22 Muliplexing Muliplexing: Ø Goal: muliple use o a shared medium Muliplexing in 4 dimensions: Ø Space (s), Freuency (), Time (), Code (c) Task: Ø Assign space, ime, reuency, code o each communicaion channel Ø Minimize inererence Using guard spaces Ø Maximize medium uilizaion 23 Jasleen Kaur

2 Space Division Muliplex A separae sender or each communicaion channel channels k i k 1 k 2 k 3 k 4 k 5 k 6 Ø Wih wide enough disance beween senders Guard spaces Used by FM radio saions Ø Many saions around he world use same reuency wihou inererence Wha i several radio saions wan o broadcas in same ciy? s 1 c c s 3 s 2 c 24 Freuency Division Muliplex Subdivide he reuency dimension ino several nonoverlapping reuency bands Ø Senders can use heir assigned band coninuously Used or radio saions wihin same region Pros: Ø No dynamic coordinaion k 1 k 2 k 3 k 4 k 5 k 6 Cons: c Ø Wase o bandwidh i usage is non-uniorm Ø Limis # o senders Ø Inlexible assignmen 25 Jasleen Kaur

3 Time Division Muliplex A sender ges he whole bandwidh or a cerain amoun o ime Advanages Ø Only one carrier in he medium a any ime Ø Throughpu high even or many users Disadvanages Ø Precise synchronizaion necessary c k 1 k 2 k 3 k 4 k 5 k 6 26 Freuency and Time Muliplex Combinaion: Ø A sender can use a reuency band or a cerain amoun o ime Ø Used by GSM Pros: Ø (weak) proecion agains apping Ø proecion agains reuencyselecive inererence k 1 k 2 k 3 k 4 k 5 k 6 Cons: Ø Precise coordinaion c reuired 27 Jasleen Kaur

4 Code Division Muliplex Each channel has a uniue code Ø All channels use same specrum a same ime Ø guard spaces realized by using orhogonal code Pros: Ø Good proecion agains inererence and apping Ø Bandwidh eicien Cons: Ø Relaively complex receiver Ø Precise synchronizaion reuired Ø Precise power conrol reuired All s should reach receiver wih nearly eual srengh k 1 k 2 k 3 k 4 k 5 k 6 c 28 DIGITAL MODULATION Ampliude, Freuency, Phase 29 Jasleen Kaur

5 Modulaion digial daa digial modulaion analog baseband analog modulaion Radio Transmier radio carrier analog demodulaion analog baseband synchronizaion decision digial daa radio carrier Radio Receiver Digial modulaion: Ø Digial daa is ranslaed ino an analog (baseband) Analog modulaion: Ø Shis cener reuency up o ha o radio carrier 30 digial daa Analog Modulaion digial modulaion Radio Transmier analog baseband analog modulaion radio carrier Why analog modulaion? Ø Ø Ø Smaller carrier wavelengh or smaller anennas To enable FDM To exploi/avoid reuency-dependen medium characerisics 3 basic schemes: AM (ampliude), FM (reuency), PM (phase) 31 Jasleen Kaur

6 Digial Modulaion Modulaion o digial s known as Shi Keying Issues: Ø Specral eiciency: How eicienly is he reuency specrum uilized Ø Power eiciency: How much power is needed o ranser bis Very imporan or porable devices Ø Robusness o muli-pah propagaion, noise, inererence 32 Digial Modulaion Mehods Ampliude Shi Keying Ø Binary values are represened by dieren ampliudes Ø Low bandwidh reuiremens Ø Very suscepible o inererence Used only in direced inra-red beams Freuency Shi Keying Ø Needs larger bandwidh Ø Less suscepible o errors Phase Shi Keying Ø Uses shis in he phase 180 o, i change in daa Ø Synchronizaion reuired (re, phase) Ø Mos resisan o inererence Ø Complex receiver/ransmier Jasleen Kaur

7 Advanced Freuency Shi Keying Minimum Shi Keying (MSK): avoids abrup phase changes Ø Bis duraions doubled and divided ino even, odd bis Depending on he bi values (even, odd) he higher or lower reuency, original or invered is chosen Ø Higher reuency is wice he lower reuency daa even bis odd bis low reuency high reuency bi even odd h n n h value h: high reuency n: low reuency +: original -: invered MSK 34 Advanced Phase Shi Keying Binary PSK: phase shi o 180 o Ø Very simple, bu low eiciency Quadraure PSK: phase shi o 45 o Ø Codes wo bis ino one phase shi Relaive o phase o a reerence Reuires reuen synchronizaion Relaive o phase o previous wo bis More eicien and less complex Ø Achieves high bi-raes or same bandwidh Can be exended o: Ø More angles Ø Combine wih ASK Ø Leads o more receiver complexiy A Q 0 Q I 11 I Jasleen Kaur

8 Muli-carrier modulaion Splis high bi-rae sream ino many wih lower bi-raes Ø Each is sen using an independen carrier reuency Pros: Ø Independen reuencies do no inerere wih each oher Ø Freuency-selecive ading doesn inluence whole Ø Guard spaces used beween symbol (groups) Helps handle muli-pah propagaion, ISI miigaion Ø Compuaionally eicien based on FFT DAB uses sub-carriers 36 SPREAD SPECTRUM 37 Jasleen Kaur

9 Spread Specrum Freuency-dependen ading bad or narrowband s Ø Narrowband inererence can wipe ou s Spread he narrowband ino a broadband Ø Receiver de-spreads ( spreads narrowband inererence) dp/d dp/d dp/d dp/d (i) (ii) (iii) (iv) sender user broadband inererence narrowband inererence receiver dp/d (v) 38 Spread Specrum: Muliple Channels Resisance o narrowband inererence Coexisence o muliple s wihou coordinaion Ø No need or reuency planning Ø Resisance o reuency-selecive ading Ø Tap-proo (wih secre code and CDM) Ø Characerisics like background noise channel ualiy 1 narrow band guard space 5 6 reuency narrowband channels channel ualiy spread specrum reuency spread specrum channels 39 Jasleen Kaur

10 Direc Seuence Spread Specrum XOR o he wih chipping seuence Ø Chipping seuence is a pseudo-random number Many chips per bi è higher bandwidh Ø By he acor, s = b / c Ø Civil applicaions, s o Ø Miliary applicaions, up o 10,000 IEEE uses Barker codes Ø Good robusness agains inererence Ø Insensiiviy o muli-pah propagaion b 0 1 c b : bi period c : chip period user daa XOR chipping seuence = resuling 40 Direc Seuence Spread Specrum Receiver has o perorm correlaion: Ø Synchronize o ideniy bi boundaries Ø Do XOR wih he chipping seuence Complicaion: muli-pah propagaion Ø Dieren delays, disorion Rake receiver: Ø Uses n correlaor or he n sronges pahs Ø Oupu o correlaors are combined and ed ino decision uni received demodulaor user daa X chipping seuence lowpass ilered X spread specrum correlaor producs radio carrier inegraor modulaor sampled sums ransmi ransmier decision daa radio carrier chipping seuence receiver 41 Jasleen Kaur

11 DSSS Advanages Ø Reduces reuency selecive ading Ø In cellular neworks Base saions can use he same reuency range Several base saions can deec and recover he So handover Challenges Ø Precise power conrol necessary Ø Precise synchronizaion necessary 42 Freuency Hopping Spread Specrum Discree changes o carrier reuency Ø Seuence o reuency changes is pseudo-random Ø Slow Hopping: several user bis per reuency Ø Fas Hopping: several reuencies per user bi b user daa 0 1 d b : bi period d : dwell ime slow hopping (3 bis/hop) d as hopping (3 hops/bi) 43 Jasleen Kaur

12 FHSS Implemenaion user daa modulaor narrowband modulaor spread ransmi ransmier reuency synhesizer hopping seuence received demodulaor narrowband demodulaor daa hopping seuence reuency synhesizer receiver 44 DSSS vs. FHSS FHSS: Ø Simpler o implemen Ø Uses only small porion o he specrum a any ime Ø Used by Blueooh, GSM DSSS: Ø Always uses he oal bandwidh available Ø More resisan o ading and muli-pah eecs Ø Signals are much harder o deec Virually impossible wihou knowing he spreading code Ø Used by IEEE a 45 Jasleen Kaur

13 CELLULAR SYSTEMS Freuency Planning 46 Cell Srucure Cellular sysems implemen space division muliplexing Ø Each ransmier (base saion) covers a cerain area (cell) Ø Mobile saions communicae only via he base saion Cell characerisics: Ø Cell radii can vary Tens o meers (buildings) Hundreds o meers (ciies) (say) ens o kilomeers in counry-side (GSM) Ø Cells are never perec circles or hexagons, depend on: Environmen (buildings, mounains, valleys), Weaher condiions, And even sysem load 47 Jasleen Kaur

14 Small Cells vs. Huge Cells Advanages o small cells: Ø Higher capaciy: SDM allows re-use o (scarce) reuency Allows higher users per km 2, very small cells used in ciies Ø Less ransmission power needed Energy is a serious problem or mobile handheld devices Ø Local inererence only Base saion deals wih inererence or only local saions Ø More robus o ailure o single componens I one anenna ails, i only inluences local communicaion Disadvanages: Ø Huge inrasrucure needed or connecing base saions Ø Handover (changing rom one cell o anoher) needed Ø Careul reuency planning needed 48 Freuency Planning Goal: never use same reuency a same ime wihin he inererence range Ø Freuency re-used only wih cerain disan base saions Sandard models: Ø All cells wihin a cluser use disjoin reuencies Ø Limied ransmission power used Dynamic reuency assignmen Ø Base saion chooses reuencies Depending on reuencies already used in neighbor cells Based on inererence measuremens Ø More capaciy in cells wih more raic cell cluser cell cluser Jasleen Kaur

15 Cell Breahing CDM sysems do no need elaborae reuency planning or channel allocaion However, cell size depends on curren load Ø Cells are said o breahe Ø Why? Addiional raic appears as noise o oher users I noise level is oo high, arher away users drop ou o cells 50 Jasleen Kaur