2. Wireless Transmission. Frequencies for Communication (1)

Transcription

1 2. Wireless Transmission Frequencies and Signals Muliplexing Modulaion and Spread Specrum 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 2 Frequencies or Communicaion (1) wised pair 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 = 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 VLF LF MF HF VHF UHF SHF EHF inrared visible ligh UV VHF = Very High Frequency Frequency and wave lengh: λ = c/ wave lengh λ, speed o ligh c 3x10 8 m/s, requency 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 3 1

2 Frequencies or Mobile Communicaion (2) 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, ocusing Large bandwidh available Wireless LANs use requencies in UHF o SHF specrum Some sysems planned up o EHF Limiaions due o absorpion by waer and oxygen molecules (resonance requencies) Weaher dependen ading, signal loss caused by heavy rainall ec Burkhard Siller and Jochen Schiller FU Berlin M2 4 Frequencies and Regulaions ITU-R holds aucions or new requencies and manages requency bands worldwide (WRC, World Radio Conerences) Europe USA Japan Cellular Phones Cordless Phones Wireless LANs Ohers 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, Burkhard Siller and Jochen Schiller FU Berlin M2 5 PDC , , , PHS JCT IEEE RF-Conrol 426, 868 2

3 Signals (1) 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 signal = coninuous ime and coninuous values Digial signal = discree ime and discree values Signal parameers o periodic signals: Period T, requency =1/T, ampliude A, and phase shi ϕ Sine wave as special periodic signal or a carrier: s() = A sin(2 π + ϕ ) 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 6 Signals (2) Dieren represenaions o signals: Ampliude (ampliude domain) Frequency 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 [Hz] Composed signals ranserred ino requency domain using Fourier ransormaion Digial signals need: Ininie requencies or perec ransmission Modulaion wih a carrier requency or ransmission (analog signal!) 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 7 3

4 Fourier Represenaion o Periodic Signals 1 g( ) = c + 2 n= 1 a sin(2πn) + n n n= 1 b cos(2πn) Ideal periodic signal 0 Real composiion (based on harmonics) 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 8 Anennas: Isoropic Radiaor 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 horizonally Radiaion paern: Measuremen o radiaion around an anenna y z z x y x Ideal isoropic radiaor 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 9 4

5 Signal Propagaion Ranges Transmission range: Communicaion possible Low error rae Deecion range: Deecion o he signal possible No communicaion possible Inererence range: Signal may no be deeced Signal adds o he background noise Sender Transmission Deecion Inererence Disance 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 10 Signal Propagaion Propagaion in ree space always like ligh (sraigh line) Receiving power proporional o 1/d² d = disance beween sender and receiver Receiving power addiionally inluenced by: Fading (requency dependen) Shadowing (Abschaung) Relecion (Relexion) a large obsacles Reracion (Brechung) depending on he densiy o a medium Scaering (Sreuung) a small obsacles Diracion (Beugung) a edges shadowing relecion reracion scaering diracion 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 11 5

6 Real World Examples 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 12 Muli-pah Propagaion Signal can ake many dieren pahs beween sender and receiver due o: Relecion, scaering, and diracion LOS pulses Muli-pah pulses Signal a sender Signal a receiver Time dispersion: signal is dispersed over ime Inererence wih neighbor symbols, Iner Symbol Inererence (ISI) The signal reaches a receiver direcly and phase shied Disored signal depending on he phases o he dieren pars 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 13 6

7 Eecs o Mobiliy Channel characerisics change over ime and locaion: Signal pahs change permanenly Dieren delay variaions o dieren signal pars Dieren phases o signal pars Quick changes in he power received (shor erm ading) Power Addiional changes in: Disance o sender Obsacles urher away Slow changes in he average power received (long erm ading) Shor erm ading Long erm ading 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 14 Muliplexing Muliplexing in 4 dimensions: Space (s i ) Time () Frequency () Code (c) Channels k i k 1 k 2 k 3 k 4 k 5 k 6 c c Goal: Muliple use o a shared medium s 1 c s 2 Imporan: guard spaces needed! s Burkhard Siller and Jochen Schiller FU Berlin M2 15 7

8 Frequency Muliplexing Separaion o he enire specrum ino smaller requency bands A channel ges a cerain band o he specrum or he ull ime Advanages: No dynamic coordinaion necessary Works also or analog signals c k 1 k 2 k 3 k 4 k 5 k 6 Drawbacks: Wase o bandwidh i he raic is disribued unevenly Inlexible Guard spaces 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 16 Time Muliplexing A channel ges he whole specrum or a cerain amoun o ime Advanages: Only one carrier in he medium a any ime Throughpu high even or many users c k 1 k 2 k 3 k 4 k 5 k 6 Drawbacks: Precise synchronizaion necessary Common clock 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 17 8

9 Time and Frequency Muliplexing Combinaion o boh mehods A channel ges a cerain requency band or a cerain amoun o ime Example: GSM Advanages: Beer proecion agains apping Proecion agains requency selecive inererence Higher daa raes compared o code muliplex c k 1 k 2 k 3 k 4 k 5 k 6 Bu: Precise coordinaion required 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 18 Code Muliplexing 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 eicien No coordinaion and synchronizaion necessary Good proecion agains inererence and apping Drawbacks: Lower user daa raes More complex signal regeneraion Implemened using spread specrum echnology c 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 19 9

10 Modulaion Digial modulaion: Digial daa is ranslaed ino an analog signal (baseband) ASK, FSK, PSK - main ocus in his chaper Dierences in specral eiciency, power eiciency, robusness Analog modulaion: Shis cener requency o baseband signal up o he radio carrier Moivaion or he necessiy o analog modulaion in wireless neworks: Smaller anennas (e.g., λ/4) Frequency Division Muliplexing Medium characerisics Basic analog modulaion schemes: Ampliude Modulaion (AM) Frequency Modulaion (FM) Phase Modulaion (PM) 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 20 Modulaion and Demodulaion Analog Base-band Digial Daa Signal Digial Analog Modulaion Modulaion Radio Transmier Radio Carrier Analog Demodulaion radio carrier Analog Base-band Signal Synchronizaion Decision Digial Daa Radio Receiver 2005 Burkhard Siller and Jochen Schiller FU Berlin M

11 Digial Modulaion Modulaion o digial signals 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 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 22 Advanced Frequency Shi Keying Bandwidh needed or FSK depends on he disance beween he carrier 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 The requency o one carrier is wice he requency o he oher Equivalen o ose QPSK Even higher bandwidh eiciency using a Gaussian low-pass iler: GMSK (Gaussian MSK), used in GSM 2005 Burkhard Siller and Jochen Schiller FU Berlin M

12 Advanced Phase Shi Keying BPSK (Binary Phase Shi Keying): Bi value 0: sine wave (0 shi) Bi value 1: invered sine wave (180 shi) Very simple PSK Low specral eiciency Robus, e.g., used 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 (regular synchronizaion, pilo signal) Oen also ransmission o relaive, no absolue phase shi: DQPSK - Dierenial QPSK (American IS-136, Japanese PHS) 1 Q BPSK A 0 I QPSK Q 11 I Burkhard Siller and Jochen Schiller FU Berlin M2 24 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 I 1000 Example: 16-QAM (4 bis = 1 symbol) Symbols 0011 and 0001 have he same phase, bu dieren ampliude and 1000 have dieren phase, bu same ampliude. used in sandard 9600 bi/s modems 2005 Burkhard Siller and Jochen Schiller FU Berlin M

13 Hierarchical Modulaion DVB-T (Digial Video Broadcas Terresrial) modulaes wo separae daa sreams ono a single DVB-T sream High Prioriy (HP) embedded wihin a Low Prioriy (LP) sream Muli-carrier sysem, abou 2000 or 8000 carriers QPSK (2 irs bis), 16 QAM, 64 QAM Q Example: 64 QAM Good recepion: resolve he enire 64 QAM consellaion Poor recepion, mobile recepion: 10 resolve only QPSK porion 6 bi per QAM symbol, 2 mos signiican deermine QPSK 00 HP service coded in QPSK (2 bi), LP uses remaining 4 bi I 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 26 Spread Specrum Technology Problem o radio ransmission: Frequency-dependen ading can wipe ou narrow band signals or duraion o he inererence Soluion: Spread he narrow band signal ino a broad band signal using a special code Inererence Power Spread Power Signal Signal Deecion a Receiver Proecion agains narrow-band inererence Side eecs: Coexisence o several signals wihou dynamic coordinaion Tap-proo Alernaives: Direc Sequence, Frequency Hopping Spread Inererence 2005 Burkhard Siller and Jochen Schiller FU Berlin M

14 Spreading and Frequency Selecive Fading Channel Qualiy Narrow-band Channels 1 2 Narrow-band Signal Guard Space 1. 6 signals in FDM 2. Qualiy changes over ime 3. Spo view 4. 1, 2, 5, 6 ok, 3, 4 no ok 5. Apply SS ono 6 signals! Channel Qualiy Spread Specrum Spread Specrum Channels 1. 6 signals uilize ull 2. No planning required 3. Separaion by codes per signals 4. CDM, unique per signal 5. Exension o 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 28 DSSS (Direc Sequence Spread Specrum) (1) XOR o he signal wih pseudo-random number (chipping sequence): Many chips per bi (e.g., 128) resul in higher bandwidh o he signal Advanages: Reduces requency selecive ading In cellular neworks: Base saions can use he same requency range Several base saions can deec and recover he signal So hand-over Drawbacks: Precise power conrol necessary Spreading acor s = b / c -> s*w bandwidh requiremens 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 29 b 0 1 c User Daa XOR Chipping Sequence = Resuling Signal b : Bi Period c : Chip Period 14

15 DSSS (Direc Sequence Spread Specrum) (2) User Daa X Chipping Sequence C s Spread Specrum Signal Modulaor Radio Carrier r Transmier Transmi Signal Example: IEEE Chip: (Barker Code) 1 MHz user daa generae 11 MHz signal in 2.4 GHz requency Received Signal Low-pass Correlaor Filered Signal Producs Demodulaor X Inegraor Sampled Sums Daa Decision Radio Carrier r Receiver Chipping Sequence C s 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 30 FHSS (Frequency Hopping Spread Specrum) (1) Discree changes o carrier requency: Available bandwidh o small channels separaed as in FDM Sequence o requency changes deermined via pseudo random number (TDM) Two versions: Fas Hopping: several requencies per user bi Slow Hopping: several user bis per requency Advanages: Frequency selecive ading and inererence limied o shor period Simple implemenaion Uses only small porion o specrum a any ime (dwell ime: Verweildauer) Drawbacks: No as robus as DSSS Simpler o deec (no knowledge o a chipping sequence as in DSSS required!) 2005 Burkhard Siller and Jochen Schiller FU Berlin M

16 FHSS (Frequency Hopping Spread Specrum) (2) b User Daa d d Slow Hopping (3 bis/hop), e.g., GSM 2 1 b : Bi Period d : Dwell Time Fas Hopping (3 hops/bi), e.g., Blueooh, 1600 changes/s ino 79 carrier 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 32 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, decenralized Base saion deals wih inererence, ransmission area ec. locally Problems: Fixed nework needed or he base saions Hand-over (changing rom one cell o anoher) necessary Inererence wih oher cells Cell sizes rom some 100 m in ciies o, e.g., 35 km on he counry side (GSM): Even less or higher requencies 2005 Burkhard Siller and Jochen Schiller FU Berlin M

17 Frequency Planning (1) 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 2005 Burkhard Siller and Jochen Schiller FU Berlin M2 34 Frequency Planning (2) Cell Cluser Cell Cluser h 3 h h 2 h 2 g 1 g 1 2 h 3 2 h g 3 g g 2 1 g 1 g 3 g 1 3 g 3 3 Cell Cluser wih 3 Secor Anennas 2005 Burkhard Siller and Jochen Schiller FU Berlin M

18 Cell Breahing CDM (Code Division Muliplexing) sysems: Cell size depends on curren load Low load: larger areas High load: smaller areas Reason: Addiional raic appears as noise o oher users: Increasing noise due o increasing number o paricipans I he noise level is oo high users, locaed a cell boundaries, drop ou o cells Noisy area 2005 Burkhard Siller and Jochen Schiller FU Berlin M