Wireless Transmission
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- Frederick Brown
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1 Wireless Transmission Arjan Durresi These slides are available a: hp:// Louisiana Sae Universiy 2 Wireless Transmission - 1 Overview Frequencies Signals Anenna Signal propagaion Muliplexing Spread specrum Modulaion Cellular sysems Louisiana Sae Universiy 2 Wireless Transmission - 2
2 Why Wireless Mobile Neworking? Negropone s Swich : Personal mobile communicaions go on Eher, Broadcas communicaions on cable Frequency Specrum probably he mos valuable naural resource Progress in microelecronic - very smar mobile erminals More open for business opporuniies Mobile phone he only echnology wih a growh rae higher han Inerne. By he year 2003: 700 millions Inerne users and 830 millions mobile phone users Louisiana Sae Universiy 2 Wireless Transmission - 3 Compuers for he nex decades? Compuers are inegraed small, cheap, porable, replaceable - no more separae devices Technology is in he background compuer are aware of heir environmen and adap ( locaion awareness ) compuer recognize he locaion of he user and reac appropriaely (e.g., call forwarding, fax forwarding, conex awareness )) Advances in echnology more compuing power in smaller devices fla, lighweigh displays wih low power consumpion new user inerfaces due o small dimensions more bandwidh per cubic meer muliple wireless inerfaces: wireless LANs, wireless WANs, regional wireless elecommunicaion neworks ec. ( overlay neworks ) Louisiana Sae Universiy 2 Wireless Transmission - 4
3 Mobile communicaion Two aspecs of mobiliy: user mobiliy: users communicae (wireless) anyime, anywhere, wih anyone device porabiliy: devices can be conneced anyime, anywhere o he nework Wireless vs. mobile Examples saionary compuer noebook in a hoel wireless LANs in hisoric buildings Personal Digial Assisan (PDA) The demand for mobile communicaion creaes he need for inegraion of wireless neworks ino exising fixed neworks: local area neworks: sandardizaion of IEEE , ETSI (HIPERLAN) Inerne: Mobile IP exension of he inerne proocol IP wide area neworks: e.g., inerneworking of GSM and ISDN Louisiana Sae Universiy 2 Wireless Transmission - 5 Mobile vs Wireless Mobile Wireless Mobile vs Saionary Wireless vs Wired Wireless media sharing issues Mobile rouing, addressing issues Louisiana Sae Universiy 2 Wireless Transmission - 6
4 Applicaions Vehicles ransmission of news, road condiion, weaher, music personal communicaion using GSM posiion via GPS local ad-hoc nework wih vehicles close-by o preven accidens, guidance sysem, redundancy vehicle daa (e.g., from busses, high-speed rains) can be ransmied in advance for mainenance Emergencies early ransmission of paien daa o he hospial, curren saus, firs diagnosis replacemen of a fixed infrasrucure in case of earhquakes, hurricanes, fire ec. crisis, war,... Louisiana Sae Universiy 2 Wireless Transmission - 7 Typical applicaion: road raffic UMTS, WLAN, DAB, GSM, TETRA,... ad hoc Personal Travel Assisan, DAB, PDA, lapop, GSM, UMTS, WLAN, Blueooh,... Louisiana Sae Universiy 2 Wireless Transmission - 8
5 Applicaions Traveling salesmen direc access o cusomer files sored in a cenral locaion consisen daabases for all agens mobile office Replacemen of fixed neworks remoe sensors, e.g., weaher, earh aciviies flexibiliy for rade shows LANs in hisoric buildings Enerainmen, educaion,... oudoor Inerne access inelligen ravel guide wih up-o-dae locaion dependen informaion ad-hoc neworks for muli user games Buil 150BC Louisiana Sae Universiy 2 Wireless Transmission - 9 Locaion dependen services Locaion aware services wha services, e.g., priner, fax, phone, server ec. exis in he local environmen Follow-on services auomaic call-forwarding, ransmission of he acual workspace o he curren locaion Informaion services push : e.g., curren special offers in he supermarke pull : e.g., where is he Black Forres Cherry Cake? Suppor services caches, inermediae resuls, sae informaion ec. follow he mobile device hrough he fixed nework Privacy who should gain knowledge abou he locaion Louisiana Sae Universiy 2 Wireless Transmission - 10
6 Pager receive only iny displays simple ex messages Mobile devices PDA simple graphical displays characer recogniion simplified WWW Lapop fully funcional sandard applicaions Sensors, embedded conrollers Mobile phones voice, daa simple ex displays Palmop iny keyboard simple versions of sandard applicaions performance Louisiana Sae Universiy 2 Wireless Transmission - 11 Effecs of device porabiliy Power consumpion limied compuing power, low qualiy displays, small disks due o limied baery capaciy CPU: power consumpion ~ CV 2 f C: inernal capaciy, reduced by inegraion V: supply volage, can be reduced o a cerain limi f: clock frequency, can be reduced emporally Loss of daa higher probabiliy, has o be included in advance ino he design (e.g., defecs, hef) Limied user inerfaces compromise beween size of fingers and porabiliy inegraion of characer/voice recogniion, absrac symbols Limied memory limied value of mass memories wih moving pars flash-memory or? as alernaive Louisiana Sae Universiy 2 Wireless Transmission - 12
7 Wireless neworks in comparison o fixed neworks Higher loss-raes due o inerference emissions of, e.g., engines, lighning Resricive regulaions of frequencies frequencies have o be coordinaed, useful frequencies are almos all occupied Low ransmission raes local some Mbi/s, regional currenly, e.g., 9.6kbi/s wih GSM Higher delays, higher jier connecion seup ime wih GSM in he second range, several hundred milliseconds for oher wireless sysems Lower securiy, simpler acive aacking radio inerface accessible for everyone, base saion can be simulaed, hus aracing calls from mobile phones Always shared medium secure access mechanisms imporan Louisiana Sae Universiy 2 Wireless Transmission - 13 Wireless vs. fixed Louisiana Sae Universiy 2 Wireless Transmission - 14
8 Early hisory of wireless communicaion Many people in hisory used ligh for communicaion heliographs, flags ( semaphore ), BC smoke signals for communicaion; (Polybius, Greece) 1794, opical elegraph, Claude Chappe Here elecromagneic waves are of special imporance: 1831 Faraday demonsraes elecromagneic inducion J. Maxwell ( ): heory of elecromagneic Fields, wave equaions (1864) H. Herz ( ): demonsraes wih an experimen he wave characer of elecrical ransmission hrough space Louisiana Sae Universiy 2 Wireless Transmission - 15 Hisory of wireless communicaion 1895 Guglielmo Marconi firs demonsraion of wireless elegraphy (digial!) long wave ransmission, high ransmission power necessary (> 200kw) 1907 Commercial ransalanic connecions huge base saions (30 100m high anennas) 1915 Wireless voice ransmission New York - San Francisco 1920 Discovery of shor waves by Marconi reflecion a he ionosphere smaller sender and receiver, possible due o he invenion of he vacuum ube Louisiana Sae Universiy 2 Wireless Transmission - 16
9 Hisory of wireless communicaion 1928 many TV broadcas rials (across Alanic, color TV, TV news) 1933 Frequency modulaion (E. H. Armsrong) 1958 A-Nez in Germany analog, 160MHz, connecion seup only from he mobile saion, no handover, 80% coverage, cusomers 1979 NMT a 450MHz (Scandinavian counries) 1982 Sar of GSM-specificaion goal: pan-european digial mobile phone sysem wih roaming 1983 Sar of he American AMPS (Advanced Mobile Phone Sysem, analog) 1984 CT-1 sandard (Europe) for cordless elephones Louisiana Sae Universiy 2 Wireless Transmission - 17 Hisory of wireless communicaion 1991 Specificaion of DECT Digial European Cordless Telephone (oday: Digial Enhanced Cordless Telecommunicaions) MHz, ~ m range, 120 duplex channels, 1.2Mbi/s daa ransmission, voice encrypion, auhenicaion, up o several user/km 2, used in more han 50 counries 1992 Sar of GSM in D as D1 and D2, fully digial, 900MHz, 124 channels auomaic locaion, hand-over, cellular roaming in Europe - now worldwide in more han 170 counries services: daa wih 9.6kbi/s, FAX, voice,... Louisiana Sae Universiy 2 Wireless Transmission - 18
10 Hisory of wireless communicaion 1996 HiperLAN (High Performance Radio Local Area Nework) ETSI, sandardizaion of ype 1: GHz, 23.5Mbi/s recommendaions for ype 2 and 3 (boh 5GHz) and 4 (17GHz) as wireless ATM-neworks (up o 155Mbi/s) 1997 Wireless LAN - IEEE IEEE sandard, GHz and infrared, 2Mbi/s already many (proprieary) producs available in he beginning 1998 Specificaion of GSM successors for UMTS (Universal Mobile Telecommunicaion Sysem) as European proposals for IMT-2000 Iridium 66 saellies (+6 spare), 1.6GHz o he mobile phone Louisiana Sae Universiy 2 Wireless Transmission - 19 Hisory of wireless communicaion 1999 Sandardizaion of addiional wireless LANs IEEE sandard b, GHz, 11Mbi/s Blueooh for picones, 2.4Ghz, <1Mbi/s Decision abou IMT-2000 Several members of a family : UMTS, cdma2000, DECT, Sar of WAP (Wireless Applicaion Proocol) and i-mode Firs sep owards a unified Inerne/mobile communicaion sysem Access o many services via he mobile phone 2000 GSM wih higher daa raes HSCSD offers up o 57.6kbi/s Firs GPRS rials wih up o 50 kbi/s (packe oriened!) UMTS aucions/beauy coness Hype followed by disillusionmen (approx. 50 B$ payed in Germany for 6 UMTS licences!) 2001 Sar of 3G sysems Cdma2000 in Korea, UMTS in Europe, Foma (almos UMTS) in Japan Louisiana Sae Universiy 2 Wireless Transmission - 20
11 Wireless sysems: overview of he developmen cellular phones 1981: NMT : NMT : GSM 1994: DCS 1800 analog digial 1991: CDMA 1983: AMPS 1991: D-AMPS 1993: PDC 2001: UMTS/IMT-2000 saellies 1982: Inmarsa-A 1988: Inmarsa-C 1992: Inmarsa-B Inmarsa-M 1998: Iridium Louisiana Sae Universiy 2 Wireless Transmission - 21 cordless phones 1980: CT0 1984: CT1 1987: CT : CT : DECT 4G fourh generaion? wireless LAN 199x: proprieary 1995/96/97: IEEE , HIPERLAN 2005?: MBS, WATM Evoluion of Applicaions Nework Mobile 2G E v o l u e d 2G 3G E v o l u e d 3G 3G applicaions more of he same 4G and on Fixed nework Broadband applicaions go wireless/mobile Wireless/mobile Specific broadband applicaions Fixed NB WB BB Broadband exreme bi raes Broadband Mbps Louisiana Sae Universiy 2 Wireless Transmission - 22
12 Generaions Mobile Sysems Louisiana Sae Universiy 2 Wireless Transmission - 23 Cellular Subscribers Louisiana Sae Universiy 2 Wireless Transmission - 24
13 Growh of Cellular Marke Fixed Louisiana Sae Universiy 2 Wireless Transmission - 25 Worldwide wireless subscribers Americas Europe Japan ohers oal Louisiana Sae Universiy 2 Wireless Transmission - 26
14 Mobile + Inerne [IPWRLSS] Louisiana Sae Universiy 2 Wireless Transmission - 27 Growh of Wireless Louisiana Sae Universiy 2 Wireless Transmission - 28
15 Personal Area Nework Louisiana Sae Universiy 2 Wireless Transmission - 29 Wireless Applicaions, Sysems, and coverage regions Louisiana Sae Universiy 2 Wireless Transmission - 30
16 Overlay Neworks - he global goal Inegraion of heerogeneous fixed and mobile neworks wih varying ransmission characerisics verical hand-over meropolian area regional campus-based horizonal hand-over in-house Louisiana Sae Universiy 2 Wireless Transmission - 31 Simple reference model used here Applicaion Applicaion Transpor Transpor Nework Nework Nework Nework Daa Link Daa Link Daa Link Daa Link Physical Physical Physical Physical Radio Medium Louisiana Sae Universiy 2 Wireless Transmission - 32
17 Influence of mobile communicaion o he layer model Applicaion layer Transpor layer Nework layer Daa link layer Physical layer service locaion new applicaions, mulimedia adapive applicaions congesion and flow conrol qualiy of service addressing, rouing, device locaion hand-over auhenicaion media access muliplexing media access conrol encrypion modulaion inerference aenuaion frequency Louisiana Sae Universiy 2 Wireless Transmission - 33 wised pair Frequencies for communicaion coax cable opical ransmission 1 Mm 300 Hz 10 km 30 khz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 µm 3 THz 1 µm 300 THz VLF LF MF HF VHF UHF SHF EHF infrared visible ligh UV 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/f wave lengh λ, speed of ligh c 3x10 8 m/s, frequency f Louisiana Sae Universiy 2 Wireless Transmission - 34
18 Frequencies for mobile communicaion VHF-/UHF-ranges for mobile radio simple, small anenna for cars deerminisic propagaion characerisics, reliable connecions SHF and higher for direced radio links, saellie communicaion small anenna, focusing large bandwidh available Wireless LANs use frequencies in UHF o SHF specrum some sysems planned up o EHF limiaions due o absorpion by waer and oxygen molecules (resonance frequencies) weaher dependen fading, signal loss caused by heavy rainfall ec. Louisiana Sae Universiy 2 Wireless Transmission - 35 Frequencies and regulaions ITU-R holds aucions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences) Cellular Phones Cordless Phones Wireless LANs Ohers Europe USA Japan GSM , / , , / , / UMTS (FDD) , UMTS (TDD) , CT , CT DECT IEEE HIPERLAN , RF-Conrol 27, 128, 418, 433, 868 AMPS, TDMA, CDMA , TDMA, CDMA, GSM , PACS , PACS-UB IEEE , RF-Conrol 315, 915 PDC , , , PHS JCT IEEE RF-Conrol 426, 868 Louisiana Sae Universiy 2 Wireless Transmission - 36
19 Signals I physical represenaion of daa funcion of ime and locaion signal parameers: parameers represening he value of daa classificaion coninuous ime/discree ime coninuous values/discree values analog signal = coninuous ime and coninuous values digial signal = discree ime and discree values signal parameers of periodic signals: period T, frequency f=1/t, ampliude A, phase shif ϕ sine wave as special periodic signal for a carrier: s() = A sin(2 π f + ϕ ) Louisiana Sae Universiy 2 Wireless Transmission - 37 Fourier represenaion of periodic signals 1 g( ) = c + 2 n= 1 a sin(2πnf) + n n n= 1 b cos(2πnf) ideal periodic signal 0 real composiion (based on harmonics) Louisiana Sae Universiy 2 Wireless Transmission - 38
20 Differen represenaions of signals ampliude (ampliude domain) frequency specrum (frequency domain) Signals II 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 f [Hz] Composed signals ransferred ino frequency domain using Fourier ransformaion Digial signals need infinie frequencies for perfec ransmission modulaion wih a carrier frequency for ransmission (analog signal!) Louisiana Sae Universiy 2 Wireless Transmission - 39 Anennas: isoropic radiaor Radiaion and recepion of elecromagneic waves, coupling of wires o space for radio ransmission Isoropic radiaor: equal radiaion in all direcions (hree dimensional) - only a heoreical reference anenna Real anennas always have direcive effecs (verically and/or horizonally) Radiaion paern: measuremen of radiaion around an anenna y z x z y x ideal isoropic radiaor Louisiana Sae Universiy 2 Wireless Transmission - 40
21 Anennas: simple dipoles Real anennas are no isoropic radiaors bu, e.g., dipoles wih lenghs λ/4 on car roofs or λ/2 as Herzian dipole shape of anenna proporional o wavelengh λ/4 λ/2 Example: Radiaion paern of a simple Herzian dipole y y z x z x simple dipole side view (xy-plane) side view (yz-plane) op view (xz-plane) Gain: maximum power in he direcion of he main lobe compared o he power of an isoropic radiaor (wih he same average power) Louisiana Sae Universiy 2 Wireless Transmission - 41 Anennas: direced and secorized Ofen used for microwave connecions or base saions for mobile phones (e.g., radio coverage of a valley) y x y z z x direced anenna side view (xy-plane) side view (yz-plane) op view (xz-plane) z z x x secorized anenna op view, 3 secor op view, 6 secor Louisiana Sae Universiy 2 Wireless Transmission - 42
22 Anennas: diversiy Grouping of 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 Louisiana Sae Universiy 2 Wireless Transmission - 43 Signal propagaion ranges Transmission range communicaion possible low error rae Deecion range deecion of he signal possible no communicaion possible Inerference range signal may no be deeced signal adds o he background noise sender ransmission deecion inerference disance Louisiana Sae Universiy 2 Wireless Transmission - 44
23 Propagaion Scenario Louisiana Sae Universiy 2 Wireless Transmission - 45 Signal propagaion Propagaion in free space always like ligh (sraigh line) Receiving power proporional o 1/d² (d = disance beween sender and receiver) Receiving power addiionally influenced by fading (frequency dependen) shadowing reflecion a large obsacles scaering a small obsacles diffracion a edges shadowing reflecion scaering diffracion Louisiana Sae Universiy 2 Wireless Transmission - 46
24 Mulipah propagaion Signal can ake many differen pahs beween sender and receiver due o reflecion, scaering, diffracion signal a sender signal a receiver Time dispersion: signal is dispersed over ime inerference wih neighbor symbols, Iner Symbol Inerference (ISI) The signal reaches a receiver direcly and phase shifed disored signal depending on he phases of he differen pars Louisiana Sae Universiy 2 Wireless Transmission - 47 Fading Louisiana Sae Universiy 2 Wireless Transmission - 48
25 Fading + loss Louisiana Sae Universiy 2 Wireless Transmission - 49 Effecs of mobiliy Channel characerisics change over ime and locaion signal pahs change differen delay variaions of differen signal pars differen phases of signal pars quick changes in he power received (shor erm fading) Addiional changes in disance o sender obsacles furher away slow changes in he average power received (long erm fading) power shor erm fading long erm fading Louisiana Sae Universiy 2 Wireless Transmission - 50
26 Real world example Louisiana Sae Universiy 2 Wireless Transmission - 51 Access mehods SDMA/FDMA/TDMA SDMA (Space Division Muliple Access) segmen space ino secors, use direced anennas cell srucure FDMA (Frequency Division Muliple Access) assign a cerain frequency o a ransmission channel beween a sender and a receiver permanen (e.g., radio broadcas), slow hopping (e.g., GSM), fas hopping (FHSS, Frequency Hopping Spread Specrum) TDMA (Time Division Muliple Access) assign he fixed sending frequency o a ransmission channel beween a sender and a receiver for a cerain amoun of ime Louisiana Sae Universiy 2 Wireless Transmission - 52
27 Muliplexing channels k i Muliplexing in 4 dimensions space (s i ) ime () frequency (f) code (c) s 1 k 1 c k 2 k 3 k 4 k 5 k 6 f c Goal: muliple use of a shared medium c s 2 f Imporan: guard spaces needed! s 3 f Louisiana Sae Universiy 2 Wireless Transmission - 53 Frequency muliplex Separaion of he whole specrum ino smaller frequency bands A channel ges a cerain band of he specrum for he whole ime Advanages: no dynamic coordinaion necessary k 1 works also for analog signals Disadvanages: wase of bandwidh if he raffic is disribued unevenly inflexible guard spaces c k 2 k 3 k 4 k 5 k 6 f Louisiana Sae Universiy 2 Wireless Transmission - 54
28 Time muliplex A channel ges he whole specrum for a cerain amoun of ime Advanages: only one carrier in he medium a any ime hroughpu high even for many users c k 1 k 2 k 3 k 4 k 5 k 6 Disadvanages: precise synchronizaion necessary f Louisiana Sae Universiy 2 Wireless Transmission - 55 Time and frequency muliplex Combinaion of boh mehods A channel ges a cerain frequency band for a cerain amoun of ime Example: GSM Advanages: beer proecion agains k apping 1 proecion agains frequency c selecive inerference higher daa raes compared o code muliplex bu: precise coordinaion required k 2 k 3 k 4 k 5 k 6 f Louisiana Sae Universiy 2 Wireless Transmission - 56
29 FDD/FDMA - general scheme, example GSM f 960 MHz MHz 915 MHz MHz 200 khz MHz 1 Louisiana Sae Universiy 2 Wireless Transmission - 57 TDD/TDMA - general scheme, example DECT 417 µs downlink uplink Louisiana Sae Universiy 2 Wireless Transmission - 58
30 Aloha/sloed aloha Mechanism random, disribued (no cenral arbier), ime-muliplex Sloed Aloha addiionally uses ime-slos, sending mus always sar a slo boundaries collision Aloha sender A sender B sender C Sloed Aloha collision sender A sender B sender C Louisiana Sae Universiy 2 Wireless Transmission - 59 DAMA - Demand Assigned Muliple Access Channel efficiency only 18% for Aloha, 36% for Sloed Aloha (assuming Poisson disribuion for packe arrival and packe lengh) Reservaion can increase efficiency o 80% a sender reserves a fuure ime-slo sending wihin his reserved ime-slo is possible wihou collision reservaion also causes higher delays ypical scheme for saellie links Examples for reservaion algorihms: Explici Reservaion according o Robers (Reservaion-ALOHA) Implici Reservaion (PRMA) Reservaion-TDMA Louisiana Sae Universiy 2 Wireless Transmission - 60
31 Code muliplex Each channel has a unique code k 1 k 2 k 3 k 4 k 5 k 6 All channels use he same specrum a he same ime Advanages: bandwidh efficien no coordinaion and synchronizaion necessary good proecion agains inerference and apping Disadvanages: lower user daa raes more complex signal regeneraion Implemened using spread specrum echnology c f Louisiana Sae Universiy 2 Wireless Transmission - 61 Modulaion Digial modulaion digial daa is ranslaed ino an analog signal (baseband) ASK, FSK, PSK differences in specral efficiency, power efficiency, robusness Analog modulaion shifs cener frequency of baseband signal 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) Louisiana Sae Universiy 2 Wireless Transmission - 62
32 Modulaion and demodulaion analog baseband digial signal daa digial analog modulaion modulaion radio ransmier radio carrier analog demodulaion analog baseband signal synchronizaion decision digial daa radio receiver radio carrier Louisiana Sae Universiy 2 Wireless Transmission - 63 Digial modulaion Modulaion of digial signals known as Shif Keying Ampliude Shif Keying (ASK): very simple low bandwidh requiremens very suscepible o inerference Frequency Shif Keying (FSK): needs larger bandwidh Phase Shif Keying (PSK): more complex robus agains inerference Louisiana Sae Universiy 2 Wireless Transmission - 64
33 Advanced Frequency Shif Keying bandwidh needed for FSK depends on he disance beween he carrier frequencies special pre-compuaion avoids sudden phase shifs MSK (Minimum Shif Keying) bi separaed ino even and odd bis, he duraion of each bi is doubled depending on he bi values (even, odd) he higher or lower frequency, original or invered is chosen he frequency of one carrier is wice he frequency of he oher Equivalen o offse QPSK even higher bandwidh efficiency using a Gaussian low-pass filer GMSK (Gaussian MSK), used in GSM Louisiana Sae Universiy 2 Wireless Transmission - 65 daa even bis odd bis Example of MSK bi even odd signal h n n h value low frequency high frequency h: high frequency n: low frequency +: original signal -: invered signal MSK signal No phase shifs! Louisiana Sae Universiy 2 Wireless Transmission - 66
34 Advanced Phase Shif Keying BPSK (Binary Phase Shif Keying): bi value 0: sine wave bi value 1: invered sine wave very simple PSK low specral efficiency robus, used e.g. in saellie sysems QPSK (Quadraure Phase Shif Keying): 2 bis coded as one symbol symbol deermines shif of sine wave needs less bandwidh compared o BPSK more complex Ofen also ransmission of relaive, no absolue phase shif: DQPSK - Differenial QPSK (IS-136, PHS) A Q Q 0 I 11 I 01 Louisiana Sae Universiy 2 Wireless Transmission - 67 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 Q Example: 16-QAM (4 bis = 1 symbol) Symbols 0011 and 0001 have he same phase φ, φ bu differen ampliude a and 1000 have a I differen phase, bu same ampliude used in sandard 9600 bi/s modems Louisiana Sae Universiy 2 Wireless Transmission - 68
35 Spread specrum echnology Problem of radio ransmission: frequency dependen fading can wipe ou narrow band signals for duraion of he inerference Soluion: spread he narrow band signal ino a broad band signal using a special code proecion agains narrow band inerference power inerference spread signal power deecion a receiver signal spread inerference f proecion agains narrowband inerference Side effecs: coexisence of several signals wihou dynamic coordinaion ap-proof Alernaives: Direc Sequence, Frequency Hopping f Louisiana Sae Universiy 2 Wireless Transmission - 69 Effecs of spreading and inerference dp/df dp/df i) dp/df ii) f sender dp/df f user signal broadband inerference narrowband inerference dp/df iii) iv) v) f receiver f f Louisiana Sae Universiy 2 Wireless Transmission - 70
36 Spreading and frequency selecive fading channel qualiy 1 narrow band signal guard space 5 6 frequency narrowband channels channel qualiy spread specrum channels spread specrum frequency Louisiana Sae Universiy 2 Wireless Transmission - 71 DSSS (Direc Sequence Spread XOR of he signal wih Specrum) pseudo-random number I (chipping sequence) many chips per bi (e.g., 128) resul in higher bandwidh of he signal Advanages b reduces frequency selecive fading 0 1 in cellular neworks base saions can use he same frequency range several base saions can deec and recover he signal sof handover Disadvanages precise power conrol necessary c b : bi period c : chip period user daa XOR chipping sequence = resuling signal Louisiana Sae Universiy 2 Wireless Transmission - 72
37 Direc-Sequence Spread Specrum Daa Frequency 5µs Time Spreading facor = Code bis/daa bi, commercial (Min 10 by FCC), 10,000 for miliary Signal bandwidh >10 daa bandwidh Code sequence synchronizaion Correlaion beween codes Inerference Orhogonal Louisiana Sae Universiy 2 Wireless Transmission - 73 DS Specrum Time Domain Frequency Domain Time (a) Daa Frequency (b) Code Frequency Louisiana Sae Universiy 2 Wireless Transmission - 74
38 DSSS (Direc Sequence Spread Specrum) II user daa X spread specrum signal modulaor ransmi signal chipping sequence radio carrier ransmier correlaor received signal demodulaor lowpass filered signal X producs inegraor sampled sums decision daa radio carrier chipping sequence receiver Louisiana Sae Universiy 2 Wireless Transmission - 75 FHSS (Frequency Hopping Spread Specrum) I Discree changes of carrier frequency sequence of frequency changes deermined via pseudo random number sequence Two versions Fas Hopping: several frequencies per user bi Slow Hopping: several user bis per frequency Advanages frequency selecive fading and inerference limied o shor period simple implemenaion uses only small porion of specrum a any ime Disadvanages no as robus as DSSS simpler o deec Louisiana Sae Universiy 2 Wireless Transmission - 76
39 Frequency Hopping Spread Specrum Frequency 50 ms Time Pseudo-random frequency hopping Spreads he power over a wide specrum Spread Specrum Developed iniially for miliary Paened by acress Hedy Lamarr Narrowband inerference can' jam Louisiana Sae Universiy 2 Wireless Transmission - 77 FHSS (Frequency Hopping Spread Specrum) II b user daa f f 3 f 2 f 1 f f 3 f 2 f d d b : bi period d : dwell ime slow hopping (3 bis/hop) fas hopping (3 hops/bi) Louisiana Sae Universiy 2 Wireless Transmission - 78
40 Specrum Signal Noise Noise Signal (a) Normal (b) Frequency Hopping Louisiana Sae Universiy 2 Wireless Transmission - 79 FHSS (Frequency Hopping Spread Specrum) III user daa narrowband signal modulaor modulaor spread ransmi signal ransmier frequency synhesizer hopping sequence received signal demodulaor narrowband signal demodulaor daa hopping sequence frequency synhesizer receiver Louisiana Sae Universiy 2 Wireless Transmission - 80
41 TDMA vs. CDMA Specrum Efficiency: Which muliple access scheme has beer bps/hz.cell? Flexibiliy: Which access scheme offers beer flexibiliy o handle muli-rae, -cell, -load, and - services? TDMA: some flexibiliy advanages, bu has a specrum efficiency disadvanage CDMA: Less flexibiliy bu has beer specrum efficiency Has Acual resuls depend on sandards deails Louisiana Sae Universiy 2 Wireless Transmission - 81 Orogonal Frequency Division Muliplexing - OFDM OFDM is designed o reduce mulipah effecs The mulipah effec is increased wih he frequency OFDM reduces he bi rae by spliing high speed daa ino several lower speed sreams OFDM shares a user beween many frequencies Some wireless LANs spli a signal ino 104 channels The wase of bandwidh is obained by using orhogonal carrier waves Is OFDM really spread specrum? They share some characerisics: Boh use orhogonal frequencies and use wide bandwidh specrum Louisiana Sae Universiy 2 Wireless Transmission - 82
42 OFDM How 6 Mbps a uses OFDM and BPSK o ransmi daa Louisiana Sae Universiy 2 Wireless Transmission - 83 Comparison SDMA/TDMA/FDMA/CDMA Approach SDMA TDMA FDMA CDMA Idea segmen space ino spread he specrum cells/secors using orhogonal codes Terminals Signal separaion Advanages Disadvanages Commen only one erminal can be acive in one cell/one secor cell srucure, direced anennas very simple, increases capaciy per km² inflexible, anennas ypically fixed only in combinaion wih TDMA, FDMA or CDMA useful segmen sending ime ino disjoin ime-slos, demand driven or fixed paerns all erminals are acive for shor periods of ime on he same frequency synchronizaion in he ime domain esablished, fully digial, flexible guard space needed (mulipah propagaion), synchronizaion difficul sandard in fixed neworks, ogeher wih FDMA/SDMA used in many mobile neworks segmen he frequency band ino disjoin sub-bands every erminal has is own frequency, uninerruped filering in he frequency domain simple, esablished, robus inflexible, frequencies are a scarce resource ypically combined wih TDMA (frequency hopping paerns) and SDMA (frequency reuse) all erminals can be acive a he same place a he same momen, uninerruped code plus special receivers flexible, less frequency planning needed, sof handover complex receivers, needs more complicaed power conrol for senders sill faces some problems, higher complexiy, lowered expecaions; will be inegraed wih TDMA/FDMA Louisiana Sae Universiy 2 Wireless Transmission - 84
43 Cell srucure Implemens space division muliplex: base saion covers a cerain ransmission area (cell) Mobile saions communicae only via he base saion Advanages of cell srucures: higher capaciy, higher number of users less ransmission power needed more robus, decenralized base saion deals wih inerference, ransmission area ec. locally Problems: fixed nework needed for he base saions handover (changing from one cell o anoher) necessary inerference wih oher cells Cell sizes from some 100 m in ciies o, e.g., 35 km on he counry side (GSM) - even less for higher frequencies Louisiana Sae Universiy 2 Wireless Transmission - 85 Frequency planning I Frequency reuse only wih a cerain disance beween he base saions Sandard model using 7 frequencies: f 4 f 5 Fixed frequency assignmen: cerain frequencies are assigned o a cerain cell problem: differen raffic load in differen cells Dynamic frequency assignmen: base saion chooses frequencies depending on he frequencies already used in neighbor cells more capaciy in cells wih more raffic assignmen can also be based on inerference measuremens f 1 f 3 f 2 f 3 f 2 f 6 f 7 f 4 f 5 f 1 Louisiana Sae Universiy 2 Wireless Transmission - 86
44 f 3 f 1 f 2 f 3 f 2 f 1 f 3 f 3 f 2 f 1 f 2 f 3 Frequency planning II f 3 f 1 f 1 f 3 f 2 3 cell cluser f 3 f 4 f 2 f 5 f 1 f 3 f 2 f 3 f 2 f 6 f 7 f 7 f 4 f 5 f 1 f 3 f 6 f 5 f 2 7 cell cluser f 2 f 2 f 2 f 1 f f 1 f f 1 3 h 3 h f 3 h 2 h 2 g 1 g 1 2 h 3 2 h g 3 g g 2 1 g 1 3 g g 1 3 g 3 3 cell cluser wih 3 secor anennas Louisiana Sae Universiy 2 Wireless Transmission - 87 Cell breahing CDM sysems: cell size depends on curren load Addiional raffic appears as noise o oher users If he noise level is oo high users drop ou of cells Louisiana Sae Universiy 2 Wireless Transmission - 88
45 Summary Applicaions Modulaion TDMA, FDMA, CDMA Cellular sysems Louisiana Sae Universiy 2 Wireless Transmission - 89
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