p6.More_about_Electromagnetic_wave - bxyics More About g...

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Unformatted text preview: bxyics More About g Electromagnetic Wave 40504050-403 Concepts of Wireless Networks Antenna Gain Antenna alone is a passive device Signal is amplified after passing through an antenna Mainly due to the antenna design. Determine factors are the size of the antenna and the radiation patterns of the antenna. It can increase the energy by tightening beams bxyics 2 Intentional Radiator An RF device specifically designed to generate and radiate RF signals An intentional radiator includes the RF device and all cabling and connectors up to, but not including, the antenna. cables AP connectors Intentional Radiator bxyics 3 1 bxyics EIRP Equivalent Isotropically Radiated Power The power actually radiated by the antenna element Regulated by FCC R l db Determines the viability of a link Includes the antenna gain bxyics 4 dBi "i" stands for "isotropic radiator" A theoretically ideal transmitter that produces electromagnetic field output in all directions with equal intensity. eq al intensity It is a relative measure in the same manner of dB. bxyics 5 Example A device generates a 20 dBm signal passing through three connectors with 3dB loss each and two cables with also 3dB loss each. The antenna gain is 18 dBi. What is the EIRP? dBi bxyics 6 2 bxyics E-Plane and H-Plane HE-plane is the plane containing the electric field and the direction of the maximum radiation H-plane is the plane containing the magnetic field and the direction of the maximum radiation bxyics 7 Polarization It is the physical orientation of the antenna Vertical polarization The electrical field is perpendicular to the ground Horizontal polarization The electrical field is parallel to the ground bxyics 8 Beamwidth We can narrowing or focusing antenna beams to increase the antenna gain Measured in degree Vertical Beamwidth Horizontal Beamwidth bxyics 9 3 bxyics Beamwidth Horizontal Beamwidth OmniOmni-directional 360 Patch/Panel Yagi Parabolic Dish bxyics Antenna Type Vertical Beamwidth 7-80 6-90 14-64 144-21 10 30-180 3030-78 304-25 VSWR Voltage Standing Wave Ratio The ratio of forward power over reflected power is called the Reflection Coefficient () ( VSWR = 1+ 1- = bxyics V reflected V forward 11 VSWR Occurs when impedance mismatches Causes a part of signal reflected back at a point of connection Causes signal return loss It is a ratio indicating the degree of mismatching relative to a perfect match Effects of VSWR bxyics 12 4 bxyics Line of Sight (LOS) Visual LOS is the apparently straight line from the transmitter and the receiver It is apparent because light waves are also subject to refraction, diffraction and reflection. RF behave differently Frequencies of Radio wave is much lower than th t of visual light Fr q n i f R di i m hl r th n that f i l li ht RF LOS bigger than the Visual LOS Foggy day bxyics 13 Fresnel zone A series of concentric ellipsoid-shaped areas around ellipsoidthe LOS path that is important to the integrity of the RF link bxyics 14 Fresnel zone a d1 r1 0.6r1 b d2 a + b = d1 + d 2 + r1 bxyics 2 d1d 2 d1 + d 2 15 5 bxyics Fresnel zone bxyics 16 Free space path loss Incurred by a RF signal due largely to "signal dispersion" The wider the wave front, the less power can be induced into the receiving antenna bxyics 17 Free space path loss PathLoss (dB) = 20 log10 4 d bxyics 18 6 bxyics Example A cell phone sends a signal 900 MHz to its nearest tower. Cell phone's input power to its antenna is at 19 dBm. Cell phone's antenna gain is 16 dBi The receiving signal power at the dBi. tower is -80 dBm. How far is the cell phone from the tower? bxyics 19 The 6dB rule Each 6dB increase in EIRP equates to a doubling of range Rough estimates of Path g Loss for 2.4 GHz radios. 40+20log10d Distance (m) 100 200 500 1000 2000 bxyics Loss (dB) 80.23 86.25 94.21 100.23 106.25 20 Link Budgets A calculation of signal powers, noise powers and/or signal to noise ratios for a complete communication link bxyics 21 7 bxyics Link Budgets bxyics 22 Link Budgets Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) - Losses (dB) Pr = Pt+Gt+Gr LfsArAt other loss Lfs: f space loss free l Other loss: scattering, fading, polarization mismatch, etc. bxyics 23 Link Budgets Receive Sensitivity Fade margin Known antenna gains Known attenuations Calculate the maximum range from a given Tx power and vice versa bxyics 24 8 bxyics Example 1 Receive sensitivity: is -80dBm Gt: 6 dBi Gr: 0 Lfs: 80 dB Fade Margin: 10 dB What is the Tx power ? bxyics 25 Example 2 Tx power: 13 dBm Tx antenna gain: 8.5 dBi Rx antenna gain: 0 dBi Rx sensitivity: -89 dBm Fade margin: 20 dB What is the maximum range? bxyics 26 Problems A 802.11b device transmits at 17dBm. The receive sensibility is at 11Mbps is -84dBm. Both antennas have 18 dBi gain. Each cable connector introduces 3 dB signal loss. 10 dB fade margin is needed for this wireless link link. What is the maximum distance between the transmitter and the receiver in order to have a robust wireless link between the two? If there is a 16 dB loss due to the environment, what is the maximum distance between Tx and Rx? bxyics 27 9 ...
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This note was uploaded on 04/21/2008 for the course VNSA 403 taught by Professor Yuan during the Winter '07 term at RIT.

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