8/27/08. Wireless Transmission Frequencies Signals, antennas, signal propagation Multiplexing Spread spectrum, modulation Cellular systems

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1 Wireless Transmission Frequencies Signals, anennas, propagaion Muliplexing Spread specrum, modulaion Cellular sysems Frequencies or Communicaion VLF = Very Low Frequency UHF = Ulra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Exra High Frequency HF = High Frequency UV = Ulraviole Ligh VHF = Very High Frequency Frequency and wave lengh λ = c/ wave lengh λ, speed o ligh c 3x10 8 m/s, requency wised pair coax cable opical ransmission 1 Mm 300 H 10 km 30 kh 100 m 3 MH 1 m 300 MH 10 mm 30 GH 100 µm 3 TH 1 µm 300 TH VLF LF MF HF VHF UHF SHF EHF inrared visible ligh UV Frequencies or Mobile Communicaion VHF-/UHF-ranges or mobile radio simple, small anenna or cars deerminisic propagaion characerisics, reliable connecions SHF and higher or direced radio links, saellie communicaion small anenna, beam orming large bandwidh available Wireless LANs use requencies in UHF o SHF range some sysems planned up o EHF limiaions due o absorpion by waer and oxygen molecules (resonance requencies) weaher dependen ading, loss caused by heavy rainall, ec. 1

2 8/27/08 Frequencies and Regulaions ITU-R holds aucions or new requencies, manages requency bands worldwide (WRC, World Radio Conerences) Examples Europe USA Japan Cellular phones GSM , , , UMTS , AMPS, TDMA, CDMA, GSM , TDMA, CDMA, GSM, UMTS , PDC, FOMA , PDC , FOMA , Cordless phones CT , CT DECT PACS , PACS-UB PHS JCT Wireless LANs b/g b/g b g Oher RF sysems 27, 128, 418, 433, , , 868 Radio Specrum Signals I Physical represenaion o daa Funcion o ime and locaion Signal parameers: parameers represening he value o daa Classiicaion coninuous ime/discree ime coninuous values/discree values analog = coninuous ime and coninuous values digial = discree ime and discree values Signal parameers o periodic s: period T, requency =1/T, ampliude A, phase shi ϕ sine wave as special periodic or a carrier: s() = A sin(2 π + ϕ) A [s] ϕ 2

3 Fourier Represenaion o Periodic Signals g() = 1 2 c + a n sin(2πn) + b n cos(2πn) n=1 n= ideal periodic 0 real composiion (based on harmonics) Signals II Dieren represenaions o s ampliude (ampliude domain) requency specrum (requency domain) phase sae diagram (ampliude M and phase ϕ in polar coordinaes) A [V] A [V] Q = M sin ϕ [s] ϕ I= M cos ϕ ϕ [H] Composed s ranserred ino requency domain using Fourier ransormaion Digial s need ininie requencies or perec ransmission modulaion wih a carrier requency or ransmission (analog!) Anennas: Isoropic Radiaors Radiaion and recepion o elecromagneic waves, coupling o wires o space or radio ransmission Isoropic radiaor: equal radiaion in all direcions (hree dimensional) - only a heoreical reerence anenna Real anennas always have direcive eecs (verically and/or horionally) Radiaion paern: measuremen o radiaion around an anenna y x y x ideal isoropic radiaor 3

4 Anennas: simple Dipoles Real anennas are no isoropic radiaors bu, e.g., dipoles wih lenghs λ/4 on car roos or λ/2 as Herian dipole shape o anenna proporional o wavelengh λ/4 λ/2 Example: Radiaion paern o a simple Herian dipole y y simple x x dipole side view (xy-plane) side view (y-plane) op view (x-plane) Gain: maximum power in he direcion o he main lobe compared o he power o an isoropic radiaor (wih he same average power) Anennas: direced and secoried Oen used or microwave connecions or base saions or mobile phones (e.g., radio coverage o a valley) y x y x direced anenna side view (xy-plane) side view (y-plane) op view (x-plane) x x secoried anenna op view, 3 secor op view, 6 secor Anennas: diversiy Grouping o 2 or more anennas muli-elemen anenna arrays Anenna diversiy swiched diversiy, selecion diversiy receiver chooses anenna wih larges oupu diversiy combining combine oupu power o produce gain cophasing needed o avoid cancellaion λ/4 λ/2 λ/4 λ/2 λ/2 λ/2 + + ground plane 4

5 Signal propagaion ranges Transmission range communicaion possible low error rae Deecion range deecion o he possible no communicaion possible Inererence range may no be deeced adds o he background noise sender ransmission deecion inererence disance Signal propagaion Propagaion in ree space always like ligh (sraigh line) Receiving power proporional o 1/d² in vacuum much more in real environmens (d = disance beween sender and receiver) Pah loss (aenuaion) Fundamenal propagaion behaviors: ground wave (<2MH): ollow earh s surace, long disances (submarine communicaion, AM radio) sky wave (2-30MH): releced a ionosphere, around he world (inl. broadcass, amaeur radio) line-o-sigh (>30MH): LOS, sraigh line, waves are ben by amosphere due o reracion (mobile phones, saellie, cordless) Mos sysems we will discuss work wih >100MH: LOS (quesion: so how do mobile phones work hen???) Oher propagaion eecs Receiving power addiionally inluenced by ading (requency dependen) shadowing relecion a large obsacles reracion depending on he densiy o a medium scaering a small obsacles diracion a edges shadowing relecion reracion scaering diracion 5

6 Real world example Mulipah propagaion Signal can ake many dieren pahs beween sender and receiver due o relecion, scaering, diracion Delay Spread mulipah LOS pulses pulses a sender a receiver Time dispersion: is dispersed over ime inererence wih neighbor symbols, Iner Symbol Inererence (ISI) The reaches a receiver direcly and phase shied disored depending on he phases o he dieren pars Eecs o mobiliy Channel characerisics change over ime and locaion pahs change dieren delay variaions o dieren pars dieren phases o pars quick changes in he power received (shor erm ading) Addiional changes in disance o sender obsacles urher away slow changes in he average power received (long erm ading) power shor erm ading long erm ading 6

7 Muliplexing Muliplexing in 4 dimensions space (s i ) ime () requency () code (c) channels k i k 1 c k 2 k 3 k 4 k 5 k 6 c Goal: muliple use o a shared medium s 1 c s 2 Imporan: guard spaces needed! SDM s 3 Frequency division muliplexing (FDM) Separaion o he whole specrum ino smaller requency bands A channel ges a cerain band o he specrum or he whole ime Advanages k 1 no dynamic coordinaion c necessary works also or analog s k 2 k 3 k 4 k 5 k 6 Disadvanages wase o bandwidh i he raic is disribued unevenly inlexible Time division muliplexing (TDM) A channel ges he whole specrum or a cerain amoun o ime Advanages only one carrier in he medium a any ime hroughpu high even or many users c k 1 k 2 k 3 k 4 k 5 k 6 Disadvanages precise synchroniaion necessary 7

8 Time and requency muliplex Combinaion o boh mehods A channel ges a cerain requency band or a cerain amoun o ime Example: GSM Advanages k 1 beer proecion agains c apping proecion agains requency selecive inererence bu: precise coordinaion required k 2 k 3 k 4 k 5 k 6 Code division muliplexing (CDM) Each channel has unique code k 1 All channels use he same specrum a he same ime Advanages bandwidh eicien no coordinaion and synchroniaion necessary good proecion agains inererence and apping Disadvanages varying user daa raes more complex regeneraion Implemened using spread specrum echnology k 2 k 3 k 4 k 5 k 6 c Modulaion Digial modulaion digial daa is ranslaed ino an analog (baseband) ASK, FSK, PSK - main ocus here dierences in specral eiciency, power eiciency, robusness Analog modulaion shis cener requency o baseband up o he radio carrier Moivaion smaller anennas (e.g., λ/4) Frequency Division Muliplexing medium characerisics Basic schemes Ampliude Modulaion (AM) Frequency Modulaion (FM) Phase Modulaion (PM) 8

9 Modulaion and demodulaion analog digial daa digial baseband analog modulaion modulaion radio ransmier radio carrier analog demodulaion radio carrier analog baseband synchroniaion decision digial daa radio receiver Digial modulaion Modulaion o digial s known as Shi Keying Ampliude Shi Keying (ASK): very simple low bandwidh requiremens very suscepible o inererence Frequency Shi Keying (FSK): needs larger bandwidh Phase Shi Keying (PSK): more complex robus agains inererence Advanced Frequency Shi Keying FSK can cause sudden changes in phase (which can cause high requencies) Special pre-compuaion avoids sudden phase shis MSK (Minimum Shi Keying) bi separaed ino even and odd bis, he duraion o each bi is doubled depending on he bi values (even, odd) he higher or lower requency, original or invered is chosen he requency o one carrier is wice he requency o he oher 9

10 Example o MSK daa even bis odd bis bi even odd h n n h value low requency high requency h: high requency n: low requency +: original -: invered MSK No phase shis! Advanced Phase Shi Keying BPSK (Binary Phase Shi Keying): bi value 0: sine wave bi value 1: invered sine wave very simple PSK low specral eiciency robus, used e.g. in saellie sysems QPSK (Quadraure Phase Shi Keying): 2 bis coded as one symbol symbol deermines shi o sine wave needs less bandwidh compared o BPSK more complex Oen also ransmission o relaive, no absolue phase shi: DQPSK - Dierenial QPSK A Q I Q 11 I Quadraure Ampliude Modulaion Quadraure Ampliude Modulaion (QAM) combines ampliude and phase modulaion i is possible o code n bis using one symbol 2 n discree levels, n=2 idenical o QPSK Bi error rae increases wih n, bu less errors compared o comparable PSK schemes Example: 16-QAM (4 bis = 1 symbol) Q 0010 Symbols 0011 and 0001 have 0001 he same phase φ, bu dieren ampliude a and 1000 have 0011 φ 0000 dieren phase, bu same ampliude. a I

11 Spread specrum echnology Problem o radio ransmission: requency dependen ading can wipe ou narrow band s or duraion o he inererence Soluion: spread he narrow band ino a broad band using a special code proecion agains narrow band inererence power inererence spread power deecion a receiver Side eecs: coexisence o several s wihou dynamic coordinaion ap-proo Alernaives: Direc Sequence, Frequency Hopping spread inererence Eecs o spreading and inererence dp/d dp/d i) dp/d ii) sender dp/d user broadband inererence narrowband inererence dp/d iii) iv) receiver v) Spreading and requency selecive ading channel qualiy 1 narrow band requency guard space narrowband channels channel qualiy spread specrum channels spread specrum requency 11

12 DSSS (Direc Sequence Spread Specrum) I XOR o he wih pseudo-random number (chipping sequence) many chips per bi (e.g., 128) resul in higher bandwidh o he b Advanages user daa reduces requency selecive ading 0 1 XOR in cellular neworks base saions can use he c chipping sequence same requency range several base saions can deec and recover he so handover = resuling Disadvanages precise power conrol necessary b : bi period c : chip period DSSS (Direc Sequence Spread Specrum) II user daa X spread specrum modulaor ransmi chipping sequence radio carrier ransmier received demodulaor lowpass ilered correlaor sampled producs sums daa X inegraor decision radio carrier chipping sequence receiver FHSS (Frequency Hopping Spread Specrum) I Discree changes o carrier requency sequence o requency changes deermined via pseudo random number sequence Two versions Fas Hopping: several requencies per user bi Slow Hopping: several user bis per requency Advanages requency selecive ading and inererence limied o shor period simple implemenaion uses only small porion o specrum a any ime Disadvanages no as robus as DSSS simpler o deec 12

13 FHSS (Frequency Hopping Spread Specrum) II b d d user daa slow hopping (3 bis/hop) as hopping (3 hops/bi) b : bi period d : dwell ime FHSS (Frequency Hopping Spread Specrum) III user daa modulaor narrowband modulaor spread ransmi ransmier requency synhesier hopping sequence received demodulaor narrowband demodulaor daa hopping sequence requency synhesier receiver Cell srucure Implemens space division muliplex base saion covers a cerain ransmission area (cell) Mobile saions communicae only via he base saion Advanages o cell srucures higher capaciy, higher number o users less ransmission power needed more robus, decenralied base saion deals wih inererence, ransmission area ec. locally Problems ixed nework needed or he base saions handover (changing rom one cell o anoher) necessary inererence wih oher cells Cell sies rom some 100 m in ciies o, e.g., 35 km on he counry side (GSM) - even less or higher requencies 13

14 Frequency planning I Frequency reuse only wih a cerain disance beween he base saions Sandard model using 7 requencies: Fixed requency assignmen: cerain requencies are assigned o a cerain cell problem: dieren raic load in dieren cells Dynamic requency assignmen: base saion chooses requencies depending on he requencies already used in neighbor cells more capaciy in cells wih more raic assignmen can also be based on inererence measuremens 1 Frequency planning II cell cluser cell cluser h h 2 h h 2 1 h 3 1 h 3 g 1 g 2 g 3 g 1 g 2 g g 1 g 2 g 3 3 cell cluser wih 3 secor anennas Cell breahing CDM sysems: cell sie depends on curren load Addiional raic appears as noise o oher users I he noise level is oo high users drop ou o cells 14

15 Key Poins o Take Away Regulaion and harmoniaion o requencies is a big challenge Many hings can happen o elecromagneic waves: aecs sysem design, requency choice, modulaion choice, user experience, Muliplexing is key o being eicien and minimie inererence (SDM, FDM, TDM, CDM) Spread specrum allows us o implemen several eaures, e.g., securiy, robusness Cellular sysems implemen SDM o raise overall capaciy o mobile phone sysems 15