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REDES INALÁMBRICAS Máster de Ingeniería de Computadores-DISCA Redes Inalámbricas – Tema 2.A The radio channel  Antennas  Bands  Characteristics of the.

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Presentation on theme: "REDES INALÁMBRICAS Máster de Ingeniería de Computadores-DISCA Redes Inalámbricas – Tema 2.A The radio channel  Antennas  Bands  Characteristics of the."— Presentation transcript:

1 REDES INALÁMBRICAS Máster de Ingeniería de Computadores-DISCA Redes Inalámbricas – Tema 2.A The radio channel  Antennas  Bands  Characteristics of the wireless channel Fading  Propagation models  Power budget

2 REDES INALÁMBRICAS Máster de Ingeniería de Computadores-DISCA Redes Inalámbricas – Tema 2.A The radio channel  Antennas  Bands  Characteristics of the wireless channel Fading  Propagation models  Power budget

3 REDES INALÁMBRICAS MIC 2010/2011 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 3 TX RX Width of band denotes current magnitude Zero current at each end Maximum current at the middle Current induced in receiving antenna is vector sum of contribution of every tiny “slice” of radiating antenna each tiny imaginary “slice” of the antenna does its share of radiating

4 REDES INALÁMBRICAS MIC 2010/2011 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….. 4 TX Electromagnetic Field current almost no current Antenna 1 Vertically Polarized Antenna 2 Horizontally Polarized RX

5 REDES INALÁMBRICAS MIC 2010/2011 Antenna Polarization (continued) 5

6 REDES INALÁMBRICAS MIC 2010/2011 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) 6 YAGI Directional Antenna

7 REDES INALÁMBRICAS MIC 2010/2011 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  P T =100, P R =10  [P T /P R ] dB = 10 log(P T /P R ) = 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] dB Ej.: [1mW] dBm = 0 dBm  [n W] dBW = [n/W] dB Ej.: [1 mW] dBW = -30 dBW  From decibels to power: P = 10 dB/10  An interesting web page:  7 log 10 2 ~ 0,3

8 REDES INALÁMBRICAS MIC 2010/2011 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) 8 Typical Example Horizontal Plane Pattern 0 (N) 90 (E) 180 (S) 270 (W) dB Notice -3 dB points Front-to-back Ratio 10 dB points Main Lobe a Minor Lobe nulls or minima

9 REDES INALÁMBRICAS MIC 2010/2011 Long reach antenas 9 Yagi antenna (13,5 dBi) Reach: 6 Km at 2 Mb/s 2 Km at 11 Mb/s Parabolic Antenna (20 dBi) Reach: 10 Km at 2 Mb/s 4,5 Km at 11 Mb/s

10 REDES INALÁMBRICAS MIC 2010/2011 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 10 In phase Out of phase

11 REDES INALÁMBRICAS MIC 2010/2011 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 11 RF power Collinear Vertical Array Yagi Log-Periodic

12 REDES INALÁMBRICAS MIC 2010/2011 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 12 Vertical Plane Pattern Up Down Horizontal Plane Pattern N E S W

13 REDES INALÁMBRICAS MIC 2010/2011 An example: SECTOR VP Micro Strip (1/2) 13

14 REDES INALÁMBRICAS MIC 2010/2011 An example: SECTOR VP Micro Strip (2/2) 14

15 REDES INALÁMBRICAS MIC 2010/2011 Wall mounted antennas 15 For walls (8,5 dBi) Reach: 3 Km at 2 Mb/s 1 Km at 11 Mb/s

16 REDES INALÁMBRICAS MIC 2010/2011 Antennas? 16

17 REDES INALÁMBRICAS Máster de Ingeniería de Computadores-DISCA Redes Inalámbricas – Tema 2.A The radio channel  Antennas  Bands  Characteristics of the wireless channel Fading  Propagation models  Power budget

18 REDES INALÁMBRICAS MIC 2010/2011 Bands without license in USA  Industrial, Scientific, and Medical (ISM)  902 – 928 MHz band. Currently not being used for WLAN  2400 – MHz ISM band.  Unlicensed National Information Infrastructure (UNII):  5.15 – 5.25 GHz.  5.25 – 5.35 GHz.  – GHz ISM band. 18

19 REDES INALÁMBRICAS MIC 2010/2011 Bands without license in Europa  Bands approved by the CEPT (European Conference of Postal and Telecommunications Administrations)  2400 – MHz, based on ISM.  5.15 – 5.35 GHz.  – GHz. 19 Extremely Low Very Low MediumHighVery High Ultra High Super High InfraredVisible Light Ultra- violet X-Rays Audio AM Broadcast Short Wave RadioFM Broadcast Television Infrared wireless LAN Cellular (840MHz) NPCS (1.9GHz) GHz 83.5 MHz (IEEE ) 5 GHz (IEEE ) HyperLAN HyperLAN2 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) 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.

20 REDES INALÁMBRICAS MIC 2010/2011 Details about the 5 GHz band 20 Europe 19 Channels (*assumes no antenna gain) 1W200mW GHz UNII Band 5.25 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) UNII-1 40mW (22 dBm EIRP) UNII-2 200mW (29 dBm EIRP) US (FCC) 12 Channels (*can use up to 6dBi gain antenna) UNII-3 800mW (35 dBm EIRP) 4 Channels *if you use a higher gain antenna, you must reduce the transmit power accordingly 4 Channels 11 Channels

21 REDES INALÁMBRICAS Máster de Ingeniería de Computadores-DISCA Redes Inalámbricas – Tema 2.A The radio channel  Antennas  Bands  Characteristics of the wireless channel Fading  Propagation models  Power budget

22 REDES INALÁMBRICAS MIC 2010/2011 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. 22 The green signal travels 1/2 more than the yellow line. The receiver receives the red line. For f = 2,4 GHz, = c/f = 12.5cm T R

23 REDES INALÁMBRICAS MIC 2010/2011 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 phase of 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 23

24 REDES INALÁMBRICAS MIC 2010/2011 Fading effects comparison 24 Distance Power m (1-10 secs) m ( msecs) Exponencial Slow Fading Fast Fading

25 REDES INALÁMBRICAS MIC 2010/2011 Delay Spread  The difference between the first wave's arrival and the last arrival is indicated as the delay spread. Receivers can pick through the noise to find the signal, but only if the delay spread is not excessive. Some vendors also quote the maximum delay spread on their data sheets. Table below reports the delay spread for three of the cards listed above.  Cards rated for higher delay spreads are capable of dealing with worse multipath interference. The Cisco Aironet 350 was an extremely capable card for its day, capable of dealing with over twice the time- smearing as the Hermes-based card. 25 Ref.: “ Wireless Networks: The Definitive Guide,” Matthew Gast

26 REDES INALÁMBRICAS Máster de Ingeniería de Computadores-DISCA Module 2. The radio channel  Antennas  Bands  Characteristics of the wireless channel Fading  Propagation models  Power budget

27 REDES INALÁMBRICAS MIC 2010/2011 Free space propagation  Computing the received power when LOS between T and R  “signal attenuation without considering all the effects of diffraction, obstruction, reflection, scattering.”  Friis formula: 27

28 REDES INALÁMBRICAS MIC 2010/2011 Path loss  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 path loss: PL(d) = PL (d 0 )+10nlog(d/d 0 ) (dB)  PL(d 0 ) is obtained from Friis formula considering G t =G r =L=1: 28 TR d d0d0 dfdf

29 REDES INALÁMBRICAS MIC 2010/2011 Path loss: a few examples  Given: d=10km, f=900MHz,  =c/f = 3*10 8 /9*10 8 = 1/3m  d 0 =1km  PL(d 0 ) = 20log(4  1000/ ) = 91,5 dB  free space n=2 PL(d) = PL (d 0 )+10nlog(d/ d 0 ) = 91,5 + 10*2*log(10000/1000) = 111,5 dB  Urban area n=3.5 PL(d) = PL (d 0 )+10nlog(d/ d 0 ) = 91,5 + 10*3.5*log(10000/1000) = 126,5 dB 29 Environmentn Free space2 Urban area Shadowed urban area3-5 Indoor LOS Indoor no LOS4-6 TR d d0d0 dfdf

30 REDES INALÁMBRICAS Máster de Ingeniería de Computadores-DISCA Module 2. The radio channel  Antennas  Bands  Characteristics of the wireless channel Fading  Propagation models  Power budget

31 REDES INALÁMBRICAS MIC 2010/2011 Power Budget  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 31 EIRP (Effective Isotropically Radiated Power) = P tx +G pa +G txa -G txl TX PAPA RX RARA P tx G pa G TXA L path G rxa G ra SrSr G txl G rxl

32 REDES INALÁMBRICAS MIC 2010/2011 Power budget: graphic representation 32


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