ICT 5305 Mobile Communications

Transcription

1 ICT 5305 Mobile Communicaions Lecure - 2 April 2016 Dr. Hossen Asiful Musafa 2.1

2 Frequencies for communicaion VLF = Very Low Frequency LF = Low Frequency MF = Medium Frequency HF = High Frequency VHF = Very High Frequency UHF = Ulra High Frequency SHF = Super High Frequency EHF = Exra High Frequency UV = Ulraviole Ligh Frequency and wave lengh - = c/f - wave lengh, speed of ligh c 3x10 8 m/s, frequency f 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 m 3 THz 1 m 300 THz VLF LF MF HF VHF UHF SHF EHF infrared visible ligh UV 2.2

3 Example 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, beam forming - large bandwidh available Wireless LANs use frequencies in UHF o SHF range - some sysems planned up o EHF - limiaions due o absorpion by, e.g., waer (dielecric heaing, see microwave oven) - weaher dependen fading, signal loss caused by heavy rainfall ec. 2.3

4 Frequencies and regulaions Examples Europe USA Japan Cellular neworks GSM , , , UMTS , LTE , , Cordless phones CT , CT DECT AMPS, TDMA, CDMA, GSM , TDMA, CDMA, GSM, UMTS , PACS , PACS-UB PDC, FOMA , PDC , FOMA , PHS JCT Wireless LANs b/g b/g b g Oher RF sysems 27, 128, 418, 433, , , 868 In general: ITU-R holds aucions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences); 3GPP specific: see e.g. 3GPP TS V ( ) 2.4

5 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 - sine wave as special periodic signal for a carrier: s() = A sin(2 f + ) 2.5

6 Fourier represenaion of periodic signals g( ) 1 2 c n 1 a n sin(2 nf) n 1 b n cos(2 nf) ideal periodic signal 0 real composiion (based on harmonics) 2.6

7 Signals II Differen represenaions of signals - ampliude (ampliude domain) - frequency specrum (frequency domain) - consellaion diagram (ampliude M and phase in polar coordinaes) A [V] A [V] Q = M sin [s] I= M cos 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!) 2.7

8 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 2.8

9 Anennas: simple dipoles Real anennas are no isoropic radiaors bu, e.g., dipoles wih lenghs /4 on car roofs or /2 as Herzian dipole shape of anenna proporional o wavelengh /4 /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) 2.9

10 Anennas: direced and secorized Ofen used for microwave connecions or base saions for mobile phones (e.g., radio coverage of a valley) y y z x 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 2.10

11 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 /4 /2 /4 /2 /2 /2 + + ground plane 2.11

12 MIMO Muliple-Inpu Muliple-Oupu - Use of several anennas a receiver and ransmier - Increased daa raes and ransmission range wihou addiional ransmi power or bandwidh via higher specral efficiency, higher link robusness, reduced fading Examples - IEEE n, LTE, HSPA+, Funcions - Beamforming : emi he same signal from all anennas o maximize signal power a receiver anenna - Spaial muliplexing: spli high-rae signal ino muliple lower rae sreams and ransmi over differen anennas - Diversiy coding: ransmi single sream over differen anennas wih (near) orhogonal codes sender 2 Time of fligh 2 = 1 +d 2 3 = 1 +d 3 Sending ime 1: 0 2: 0 -d 2 3: 0 -d 3 receiver 2.12

13 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 Warning: figure misleading bizarre shaped, ime-varying ranges in realiy! 2.13

14 Signal propagaion Propagaion in free space always like ligh (sraigh line) Receiving power proporional o 1/d² in vacuum much more aenuaion in real environmens, e.g., d 3.5 d 4 (d = disance beween sender and receiver) Receiving power addiionally influenced by - fading (frequency dependen) - shadowing - reflecion a large obsacles - refracion depending on he densiy of a medium - scaering a small obsacles - diffracion a edges shadowing reflecion refracion scaering diffracion 2.14

15 Mulipah propagaion Signal can ake many differen pahs beween sender and receiver due o reflecion, scaering, diffracion LOS pulses mulipah pulses LOS (line-of-sigh) 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 2.15

16 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) power long erm fading Addiional changes in - disance o sender - obsacles furher away slow changes in he average power received (long erm fading) shor erm fading 2.16

17 Muliplexing Muliplexing in 4 dimensions - space (s i ) - ime () - frequency (f) - code (c) Goal: muliple use of a shared medium channels k i s 1 k 1 k 2 k 3 k 4 k 5 k 6 c f c Imporan: guard spaces needed! c s 2 f s 3 f 2.17

18 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 - works also for analog signals Disadvanages - wase of bandwidh if he raffic is disribued unevenly c k 1 k 2 k 3 k 4 k 5 k 6 f - inflexible 2.18

19 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 k 1 k 2 k 3 k 4 k 5 k 6 Disadvanages - precise synchronizaion necessary c f 2.19

20 Time and frequency muliplex Combinaion of boh mehods A channel ges a cerain frequency band for a cerain amoun of ime Example: GSM, Blueooh Advanages - beer proecion agains apping - proecion agains frequency selecive inerference bu: precise coordinaion required c k 1 k 2 k 3 k 4 k 5 k 6 f 2.20

21 Cogniive Radio Typically in he form of a specrum sensing CR - Deec unused specrum and share wih ohers avoiding inerference - Choose auomaically bes available specrum (inelligen form of ime/frequency/space muliplexing) Disinguish - Primary Users (PU): users assigned o a specific specrum by e.g. regulaion - Secondary Users (SU): users wih a CR o use unused specrum Examples - Reuse of (regionally) unused analog TV specrum (aka whie space) - Temporary reuse of unused specrum e.g. of pagers, amaeur radio ec. SU f PU SU SU PU PU SU SU PU PU SU SU PU space mux PU PU PU PU PU SU SU SU frequency/ime mux 2.21

22 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 c f Disadvanages - varying user daa raes - more complex signal regeneraion Implemened using spread specrum echnology 2.22

23 Modulaion Digial modulaion - digial daa is ranslaed ino an analog signal (baseband) - ASK, FSK, PSK - main focus in his chaper - 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., /4) - Frequency Division Muliplexing - medium characerisics - Basic schemes - Ampliude Modulaion (AM) - Frequency Modulaion (FM) - Phase Modulaion (PM) 2.23

24 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 2.24

25 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

26 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 2.26

27 Example of MSK daa even bis odd bis low frequency high frequency bi even odd signal h n n h value h: high frequency n: low frequency +: original signal -: invered signal MSK signal No phase shifs! 2.27