1 / 18

Wireless Issues

Wireless Issues. Problems Unique to Wireless (not wired) systems: Interference from other service providers Out-of-band non-linear Tx emissions Interference from other users (same network) CCI due to frequency reuse ACI due to Tx/Rx design limitations & large # users sharing finite BW

arthur-norris
Download Presentation

Wireless Issues

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Wireless Issues • Problems Unique to Wireless (not wired) systems: • Interference from other service providers • Out-of-band non-linear Tx emissions • Interference from other users (same network) • CCI due to frequency reuse • ACI due to Tx/Rx design limitations & large # users sharing finite BW • Shadowing • Line of Sight (LOS) obstructions cause areas of weak RSS • Fading • Multipath reflections cause destructive interference @ Rx • Mobile Rx in motion influences rate of fading ECE 4730: Lecture #7

  2. Wireless Issues • Wired Channel  stationary & predictable • Wireless Channel  random & unpredictable • Must be characterized in a statistical fashion • Field measurements often needed to characterize radio channel performance ** Mobile Radio Channel (MRC) has unique problems that limit performance of wireless systems *** ECE 4730: Lecture #7

  3. Radio Signal Propagation • Two basic goals of propagation modeling: 1) Predict magnitude and rate (speed) of RSS fluctuations over short distances and/or time durations • “Short”  typically a few land 10-100 milliseconds!! • RSS can vary by 30 to 40 dB!! • Small-scale RSS fluctuations fading (next chapter) 2) Predict average RSS for given Tx/Rx separation over km-scale distances • Characterize RSS over distances from 20 m to 20 km • Large-scale models • Needed to estimate coverage area of base station • Fig. 4.1, pg. 106 ECE 4730: Lecture #7

  4. Radio Signal Propagation Small-scale RSS Large-scale RSS Indoor System ECE 4730: Lecture #7

  5. Radio Signal Propagation • Wireless Link • Line of Sight (LOS) • Obstructed (OBS) • Basic Propagation Mechanisms • Free space • Reflection • Diffraction • Scattering ECE 4730: Lecture #7

  6. Radio Signal Propagation • Free-Space Signal Propagation • Clear & unobstructed LOS path  satellite and fixed microwave • Link transmission formula  Rx power (Pr) vs. Tx-Rx separation (d) wherePt = Tx power (W) G = Tx or Rx antenna gain (unitless)  relative to isotropic source  far-field of antenna l= wavelength = c / f(m) L = system losses (antennas, T.L., atmosphere, etc.)  unitless L = 1 for zero loss ECE 4730: Lecture #7

  7. Large Scale Path Loss • Free-Space Path Loss (PL) in dB: • d2 power law relationship • Pr decreases at rate of 20 dB/decade of distance • e.g. Pr is 20 dB smaller @ 1 km vs. 100 m ECE 4730: Lecture #7

  8. Large Scale Path Loss • Close in reference point (do) used in large-scale models wheredf : far-field distance of antenna = 2D2 / l D : largest linear dimension of antenna do : typically 100 m for outdoor systems and 1 m for indoor systems ECE 4730: Lecture #7

  9. Large Scale Path Loss • Relating power to field strength: where o : free-space wave impedance = 120p = 377 W Ae : effective aperture of antenna = Gl2 / 4p ECE 4730: Lecture #7

  10. Reflection • Reflections  occur when RF energy incident upon boundary between two materials (e.g air/ground) with different electrical characteristics (m, e, s) • Reflecting surface must be large relative to l of RF energy • Reflecting surface must be smooth relative to l of RF energy • “Specular” reflection • Important reflecting surfaces for mobile radio: • Earth (ground) • Concrete streets • Buildings Walls/tinted windows/floors • Vehicles (metal surfaces) • Water (lakes, rivers, ponds, etc.) ECE 4730: Lecture #7

  11. : incident field : reflected field : transmitted field qi: angle of incidence = 90° Snell’s Law qi = qr q i m2, e2, 2 m1, e1, 1 Reflection • Fresnel reflection coefficient G • Describes magnitude of reflected RF field strength • Depends upon material properties, polarization, & angle of incidence • Consider boundary between two materials: ECE 4730: Lecture #7

  12. Reflection • Fresnel Reflection Coefficient  •  = wave impedance = • G is a measure of the discontinuity between electrical material properties • For free space : • e = eo = 8.85·10-12 F/m & m = mo = 4p·10-7 H/m •  = o = = 377 W • For most materials mmo so that G depends largely on dielectric constant e • e = eo er + jewhere • er : relative dielectric constant • e: imaginary component ECE 4730: Lecture #7

  13. Reflection • Imaginary component, e, of dielectric constant • e= 0 for “lossless” dielectric • e 0 for “lossy” dielectric  transmitted energy undergoes attenuation due to absorption • Important for quantifying signal penetration thru walls/floors and into buildings • General G formulas 4.194.20 take into account polarization and angle of incidience (qi ) • **For small qi | G | approaches 1  total reflection** • For conductors (metals)  | G | = 1 ECE 4730: Lecture #7

  14. Ground Reflection Fig. 4.7, pg. 121 ECE 4730: Lecture #7

  15. Ground Reflection Model • Ground Reflection (2-Ray) Model • results from combination of direct LOS path and ground reflected path** • Let be at reference point do then • Let direct path d=d’ and reflected path d = d” then • **assuming path to ground is LOS for both incident and reflected field ECE 4730: Lecture #7

  16. Ground Reflection Model • Let direct path d = d’ and reflected path d = d” then • For large TxRx separation : qi 0 and G = 1 • Constructive/destructive interference can occur depending on phase difference between direct and reflected E fields • Phase differenceq due to path length difference,  = d”d’, between and !! ECE 4730: Lecture #7

  17. Ground Reflection Model • For d >> • and • d4 vs. d2 for free space !! • Pr1 / d4 or 40 dB/decade ECE 4730: Lecture #7

  18. Ground Reflection • Two-ray path loss model: PL (dB)= 40 logd [10 logGt +10 logGr + 20 loght + 20 loghr] • For q = p = 180° d = 4ht hr/l • Vertical height difference between LOS & reflected path @ reflection point corresponds to first “Fresnel Zone” • Fresnel zone clearance used to determine if path is LOS or obstructed for point-to-point microwave systems ECE 4730: Lecture #7

More Related