Redes Inalámbricas – Tema 2.A The radio channel

1. Redes Inalámbricas – Tema 2.AThe radio channel Antennas Bands Characteristics of the wireless channel Fading Propagation models Power budget

2. Redes Inalámbricas – Tema 2.AThe radio channel Antennas Bands Characteristics of the wireless channel Fading Propagation models Power budget

3. Zero current at each end each tiny imaginary “slice” of the antenna does its share of radiating RX TX Maximum current at the middle Current induced in receiving antenna is vector sum of contribution of every tiny “slice” of radiating antenna Width of band denotes current magnitude Antenna Radiation • An antenna is just a passive conductor carrying RF current • RF power causes the current flow • Current flowing radiates electromagnetic fields • Electromagnetic fields cause current in receiving antennas

4. Antenna Polarization • The orientation of the antenna is called its polarization. • RF current in a conductor causes electromagnetic fields that seek to induce current flowing in the same direction in other conductors. • Coupling between two antennas is proportional to the cosine of the angle of their relative orientation • To intercept significant energy, a receiving antenna must be oriented parallel to the transmitting antenna • A receiving antenna oriented at right angles to the transmitting antenna is “cross-polarized”; will have very little current induced • Vertical polarization is the default convention in wireless telephony • In the cluttered urban environment, energy becomes scattered and “de-polarized” during propagation, so polarization is not as critical • Handset users hold the antennas at seemingly random angles….. Antenna 1 Vertically Polarized Antenna 2 Horizontally Polarized Electromagnetic Field TX RX current almost no current

5. Antenna Polarization (continued)

6. Antennas basic types • Isotropic Radiator • Truly non-directional -- in 3 dimensions • Difficult to build or approximate physically, but mathematically very simple to describe • Provides a reference point for representing the gain of an antenna • Usually expressed in dB isotropic (dBi) • Dipole Antenna • Non-directional in 2-dimensional plane only • The smallest, simplest, most practical type of antenna that can be made • But that also exhibits the least amount of gain • Has a fixed gain over that of an isotropic radiator of 2.15 dB • For microwave and higher frequency antennas • Gain is usually expressed in dB dipole (dBd) YAGI Directional Antenna

7. Decibels • The decibel (dB) is a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power or intensity) relative to a specified or implied reference level. Since it expresses a ratio of two quantities with the same unit, it is a dimensionless unit. • Gains adds instead of multiply • Example: computing the T-R attenuation • PT=100, PR=10 • [PT/PR]dB = 10 log(PT/PR) = 10 log(10) = 10 dB • Useful values: • [2/1]dB~ 3 dB • [1000/1]dB=30 dB • Expressing absolute values: • [n mW]dBm = [n/mW]dBEj.: [1mW]dBm = 0 dBm • [n W]dBW = [n/W]dBEj.: [1 mW]dBW= -30 dBW • From decibels to power: P = 10dB/10 • An interesting web page: • http://www.phys.unsw.edu.au/jw/dB.html log102 ~ 0,3

8. Radiation Patterns • An antenna’s directivity is expressed as a series of patterns • The Horizontal Plane Pattern graphs the radiation as a function of azimuth (i.e..,direction N-E-S-W) • The Vertical Plane Pattern graphs the radiation as a function of elevation (i.e.., up, down, horizontal) Typical Example Horizontal Plane Pattern Notice -3 dB points 0 (N) 0 10 dB points -10 -20 Main Lobe -30 dB 90 (E) 270 (W) nulls or minima a Minor Lobe Front-to-back Ratio 180 (S)

9. Long reach antenas Parabolic Antenna (20 dBi) Reach: 10 Km at 2 Mb/s 4,5 Km at 11 Mb/s Yagi antenna (13,5 dBi) Reach: 6 Km at 2 Mb/s 2 Km at 11 Mb/s

10. In phase Out of phase How Antennas Achieve Their Gain • Quasi-Optical Techniques (reflection, focusing) • Reflectors can be used to concentrate radiation • technique works best at microwave frequencies, where reflectors are small • Examples: • corner reflector used at cellular or higher frequencies • parabolic reflector used at microwave frequencies • grid or single pipe reflector for cellular • Array techniques (discrete elements) • Power is fed or coupled to multiple antenna elements; each element radiates • Elements’ radiation in phase in some directions • In other directions, a phase delay for each element creates pattern lobes and nulls

11. RF power RF power Types Of Arrays • Collinear vertical arrays • Essentially omnidirectional in horizontal plane • Power gain approximately equal to the number of elements • Nulls exist in vertical pattern, unless deliberately filled • Arrays in horizontal plane • Directional in horizontal plane: useful for sectorization • Yagi • one driven element, parasitic coupling to others • Log-periodic • all elements driven • wide bandwidth • All of these types of antennas are used in wireless Collinear Vertical Array Yagi Log-Periodic

12. Sector Antennas • Typical commercial sector antennas are vertical combinations of dipoles, yagis, or log-periodic elements with reflector (panel or grid) backing • Vertical plane pattern is determined by number of vertically-separated elements • varies from 1 to 8, affecting mainly gain and vertical plane beamwidth • Horizontal plane pattern is determined by: • number of horizontally-spaced elements Vertical Plane Pattern Up Down Horizontal Plane Pattern N W E S

13. Anexample: SECTOR VP Micro Strip (1/2)

14. Anexample: SECTOR VP Micro Strip (2/2)

15. Wall mounted antennas Forwalls (8,5 dBi) Reach: 3 Km at 2 Mb/s 1 Km at 11 Mb/s

16. Antennas?

17. Redes Inalámbricas – Tema 2.AThe radio channel Antennas Bands Characteristics of the wireless channel Fading Propagation models Power budget

18. Bandswithoutlicense in USA • Industrial, Scientific, and Medical (ISM) • 902 – 928 MHz band. • Currently not being used for WLAN • 2400 – 2483.5 MHz ISM band. • Unlicensed National Information Infrastructure (UNII): • 5.15 – 5.25 GHz. • 5.25 – 5.35 GHz. • 5.725 – 5.850 GHz ISM band.

19. Bandswithoutlicense in Europa • Bandsapprovedbythe CEPT (EuropeanConference of Postal and TelecommunicationsAdministrations) • 2400 – 2483.5 MHz, basedon ISM. • 5.15 – 5.35 GHz. • 5.470 – 5.725 GHz. • U N - 51 Aplicaciones ICM por encima de 2,4 GHz • Bandas de frecuencias designadas para aplicaciones industriales, científicas, y médicas (Aplicaciones ICM, no servicios de radiocomunicaciones). • 2400 a 2500 MHz (frecuencia central 2450 MHz) • 5725 a 5875 MHz (frecuencia central 5800 MHz) • 24,00 a 24,25 GHz (frecuencia central 24,125 GHz) • 61,00 a 61,50 GHz (frecuencia central 61,250 GHz) • Los servicios de radiocomunicaciones (notas UN-85, 86, 130 y 133) que funcionen en las citadas bandas deberán aceptar la interferencia perjudicial resultante de estas aplicaciones. • La utilización de estas frecuencias para las aplicaciones indicadas se considera uso común. • http://www.mityc.es/telecomunicaciones/Espectro/Paginas/CNAF.aspx Short Wave Radio FM Broadcast Infrared wireless LAN AM Broadcast Television Audio Cellular (840MHz) NPCS (1.9GHz) Low Medium High Infrared X-Rays Extremely Low Very Low Very High Ultra High Super High Visible Light Ultra- violet 2.4 - 2.4835 GHz 83.5 MHz (IEEE 802.11) 5 GHz (IEEE 802.11) HyperLAN HyperLAN2

20. 5.15 5.25 5.35 5.470 5.725 5.825 5 GHz UNII Band Detailsaboutthe 5 GHz band 4 Channels 4 Channels 11 Channels 4 Channels US (FCC) 12 Channels (*can use up to 6dBi gain antenna) UNII-1 40mW (22 dBm EIRP) UNII-2 200mW (29 dBm EIRP) UNII-3 800mW (35 dBm EIRP) Europe 19 Channels (*assumes no antenna gain) 200mW 1W UNII-1: Indoor Use, antenna must be fixed to the radio UNII-2: Indoor/Outdoor Use, fixed or remote antenna UNII-3: Outdoor Bridging Only (EIRP limit is 52 dBm if PtP) *if you use a higher gain antenna, you must reduce the transmit power accordingly

21. Redes Inalámbricas – Tema 2.AThe radio channel Antennas Bands Characteristics of the wireless channel Fading Propagation models Power budget

22. Characteristics of the wireless channel • The wireless channel suffers basically from the effects of the following two phenomena: • Distance  Path attenuation • Multipath or scattering over time due to the differing paths of the signal • Other effects: diffraction, obstruction, reflection Ref.: “Wireless Communications : Principles and Practice”, Theodore S. Rappaport. T R The green signal travels 1/2 more than the yellow line. The receiver receives the red line.Forf = 2,4 GHz,  = c/f = 12.5cm

23. More technically: Fading • “Path attenuation” and “Multipath” are also referred to using the terms slow and fast fading. • They refer to the rate at which the magnitude and phaseof the signal change due to the channel. • Slow (large-scale) fading arises when the coherence time of the channel is large relative to the delay constraint of the channel. • In this regime, the amplitude and phase change imposed by the channel can be considered roughly constant over the period of use. • Example: a large obstruction such as a hill or large building obscures the main signal path between the transmitter and the receiver. • Fast (small-scale) fading occurs when the coherence time of the channel is small relative to the delay constraint of the channel. • In this regime, the amplitude and phase change imposed by the channel varies considerably over the period of use. • Examples: • Multipath: Multiple copies of the signal arrive at destination • Doppler shift of the carrier frequency: relative motion of the receiver and transmitter causes Doppler shifts

24. Fading effectscomparison Exponencial 0.1 -1 m (10-100 msecs) Power Fast Fading Slow Fading 10-100 m (1-10 secs) Distance

25. Delay Spread • Thedifferencebetweenthefirstwave'sarrival and thelastarrivalisindicated as thedelay spread. Receivers can pick throughthenoisetofindthesignal, butonlyifthedelay spread isnotexcessive. Somevendorsalsoquotethemaximumdelay spread ontheir data sheets. Tablebelowreportsthedelay spread forthree of thecardslistedabove. • Cardsratedforhigherdelay spreads are capable of dealingwithworsemultipathinterference. The Cisco Aironet 350 wasanextremelycapablecardforitsday, capable of dealingwithovertwicethe time-smearing as the Hermes-basedcard. Ref.: “802.11 Wireless Networks: The Definitive Guide,” Matthew Gast

26. Module 2.The radio channel Antennas Bands Characteristics of the wireless channel Fading Propagation models Power budget

27. Free spacepropagation • Computing the received power when LOS between T and R • “signal attenuation without considering all the effects of diffraction, obstruction, reflection, scattering.” • Friis formula:

28. T R d0 d df Pathloss • Path loss (or path attenuation) is the reduction in power density (attenuation) of an electromagnetic wave as it propagates through space. Path loss is a major component in the analysis and design of the link budget of a telecommunication system. • Computing pathloss: PL(d) = PL (d0)+10nlog(d/d0) (dB) • PL(d0) isobtainedfromFriis formula consideringGt=Gr=L=1:

29. Pathloss: a fewexamples • Given: d=10km, f=900MHz, • l=c/f = 3*108/9*108 = 1/3m • d0=1km PL(d0) = 20log(4p1000/l) = 91,5 dB • free space n=2 • PL(d) = PL (d0)+10nlog(d/ d0) = 91,5 + 10*2*log(10000/1000) = 111,5 dB • Urban area n=3.5 • PL(d) = PL (d0)+10nlog(d/ d0) = 91,5 + 10*3.5*log(10000/1000) = 126,5 dB T R d0 d df

30. Module 2.The radio channel Antennas Bands Characteristics of the wireless channel Fading Propagation models Power budget

31. PA RA Ptx Gpa GTXA Lpath Grxa Gra Sr RX TX Gtxl Grxl PowerBudget • Prx = Ptx+Gpa-Gtxl+Gtxa-Lpath+Grxa+Gra-Grxl • Ptx[dBm]=Power generated by TX • Gpa[dB]=Gain of the Power Amplifier • Gtxa[dBi]=Gain of TX antenna • Gtxl[dB]=Gain (loss) of transmission line • Lpth[dB]=Loss of the transmission medium • Grxa[dBi]=Gain of RX antenna • Gra[dB]=Gain of the Receive Amplifier • Grxl[dB]=Gain (loss) of receiving line • Prx[dBm]=Power received • Sr[dBm]=Sensivity of receiver Gtxl • Must hold the condition Prx > Sr EIRP(Effective Isotropically Radiated Power)= Ptx+Gpa+Gtxa-Gtxl

32. Power budget: graphic representation