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EE 6331, Spring, 2009 Advanced Telecommunication

EE 6331, Spring, 2009 Advanced Telecommunication. Zhu Han Department of Electrical and Computer Engineering Class 5 Feb. 3 th , 2009. Outline. Review Trunking and Grade of Service Technique to improve the capacity Call procedure Free Space Propagation Model Reflection Diffraction

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EE 6331, Spring, 2009 Advanced Telecommunication

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  1. EE 6331, Spring, 2009Advanced Telecommunication Zhu Han Department of Electrical and Computer Engineering Class 5 Feb. 3th, 2009

  2. Outline • Review • Trunking and Grade of Service • Technique to improve the capacity • Call procedure • Free Space Propagation Model • Reflection • Diffraction • Scattering ECE6331 Spring 2009

  3. Trunking • Trunking: the channel is allocated on demand and recycle after usage • Tradeoff between the number of channels and blocking probability • Grade of service • Likelihood of a call is blocked or the delay greater than a threshold during the busiest time. • Trunking theory • Erlang, a Danish Mathematician studied how a large population could be accommodated by a limited number of servers. • Erlang capacity: the percentage of line/channel occupied over time ECE6331 Spring 2009

  4. Trunking Theory • Each user generates a traffic intensity of Au Erlangs. • Au=H, where H is the average duration of a call and  is the average number of call requests per unit time for each user • A=UAu, where U no. of users and A is the total offered traffic intensity • Ac=UAu/C, where C is the number of channels • Offered traffic >= the traffic carried by the trunked system • First type of trunked system • Blocked calls cleared: no queuing for call requests, no setup time • M/M/m/m: memory-less, Poisson arrival, exponential service, finite channels. • Erland B formula ECE6331 Spring 2009

  5. Blocked Calls Delayed • Blocking calls are delayed until channels are available, queuing • Erlang C • Probability of delay lager than t • Exponential service distributions • Pr[delay>t]=Pr[delay>0]Pr[delay>t|delay>0] =Pr[delay>0]exp(-(C-A)t/H) • The average delay D • D=Pr[delay>0]H/(C-A) ECE6331 Spring 2009

  6. Approaches to Increasing Capacity • Frequency borrowing • frequencies are taken from adjacent cells by congested cells • Cell splitting • cells in areas of high usage can be split into smaller cells • Cell sectoring • cells are divided into a number of wedge-shaped sectors, each with their own set of channels • Microcells • antennas move to buildings, hills, and lamp posts ECE6331 Spring 2009

  7. Speed, Wavelength, Frequency • Light speed = Wavelength x Frequency = 3 x 108 m/s = 300,000 km/s ECE6331 Spring 2009

  8. Type of waves ECE6331 Spring 2009

  9. Radio Frequency Bands ECE6331 Spring 2009

  10. Large-scale small-scale propagation ECE6331 Spring 2009

  11. Models are Specialized • Different scales • Large scale (averaged over meters) • Small scale (order of wavelength) • Different environmental characteristics • Outdoor, indoor, land, sea, space, etc. • Different application areas • macrocell (2km), microcell(500m), picocell ECE6331 Spring 2009

  12. Free space propagation model • Assumes far-field (Fraunhofer region) • d >> D and d >>  , where • D is the largest linear dimension of antenna •  is the carrier wavelength • No interference, no obstructions • Black board 4.1 • Effective isotropic radiated power • Effective radiated power • Path loss • Fraunhofer region/far field • In log scale • Example 4.1 and 4.2 ECE6331 Spring 2009

  13. Relating power to Electric Field • Blackboard equations ECE6331 Spring 2009

  14. Antenna Model • Example 4.3 ECE6331 Spring 2009

  15. Radio Propagation Mechanisms • Refraction • Conductors & Dielectric materials (refraction) • Propagation wave impinges on an object which is large as compared to wavelength - e.g., the surface of the Earth, buildings, walls, etc. • Diffraction • Fresnel zones • Radio path between transmitter and receiver obstructed by surface with sharp irregular edges • Waves bend around the obstacle, even when LOS (line of sight) does not exist • Scattering • Objects smaller than the wavelength of the propagation wave - e.g. foliage, street signs, lamp posts • “Clutter” is small relative to wavelength ECE6331 Spring 2009

  16. Refraction • Perfect conductors reflect with no attenuation • Like light to the mirror • Dielectrics reflect a fraction of incident energy • “Grazing angles” reflect max* • Steep angles transmit max* • Like light to the water • Reflection induces 180 phase shift • Why? See yourself in the mirror q qr qt ECE6331 Spring 2009

  17. Reflection from smooth surface ECE6331 Spring 2009

  18. Typical electromagnetic properties ECE6331 Spring 2009

  19. Superposition for polarization • Equation 4.19-4.25 ECE6331 Spring 2009

  20. Reflection coefficients • Equation 4.26, example 4.4, Brewster angle, perfect conductors ECE6331 Spring 2009

  21. Classical 2-ray ground bounce model • One line of sight and one ground bound ECE6331 Spring 2009

  22. Method of image ECE6331 Spring 2009

  23. Vector addition of 2 rays ECE6331 Spring 2009

  24. Simplified model • Far field simplified model • Example 4.6 ECE6331 Spring 2009

  25. Diffraction Diffraction occurs when waves hit the edge of an obstacle “Secondary” waves propagated into the shadowed region Water wave example Diffraction is caused by the propagation of secondary wavelets into a shadowed region. Excess path length results in a phase shift The field strength of a diffracted wave in the shadowed region is the vector sum of the electric field components of all the secondary wavelets in the space around the obstacle. Huygen’s principle: all points on a wavefront can be considered as point sources for the production of secondary wavelets, and that these wavelets combine to produce a new wavefront in the direction of propagation. ECE6331 Spring 2009

  26. Diffraction geometry Derive of equation 4.54-4.57 ECE6331 Spring 2009

  27. Diffraction geometry ECE6331 Spring 2009

  28. Diffraction geometry Fresnel-Kirchoff distraction parameters, 4.56 ECE6331 Spring 2009

  29. Fresnel Screens Fresnel zones relate phase shifts to the positions of obstacles Equation 4.58 A rule of thumb used for line-of-sight microwave links 55% of the first Fresnel zone is kept clear. ECE6331 Spring 2009

  30. Fresnel Zones Bounded by elliptical loci of constant delay Alternate zones differ in phase by 180 Line of sight (LOS) corresponds to 1st zone If LOS is partially blocked, 2nd zone can destructively interfere (diffraction loss) How much power is propagated this way? 1st FZ: 5 to 25 dB below free space prop. LOS 0 -10 -20 -30 -40 -50 -60 0o 90 180o dB Obstruction Tip of Shadow 1st 2nd Obstruction of Fresnel Zones  ECE6331 Spring 2009

  31. Fresnel diffraction geometry ECE6331 Spring 2009

  32. Knife-edge diffraction Fresnel integral, 4.59 ECE6331 Spring 2009

  33. Knife-edge diffraction loss Gain Exam. 4.7 Exam. 4.8 ECE6331 Spring 2009

  34. Multiple knife-edge diffraction ECE6331 Spring 2009

  35. Scattering Rough surfaces Lamp posts and trees, scatter all directions Critical height for bumps is f(,incident angle), 4.62 Smooth if its minimum to maximum protuberance h is less than critical height. Scattering loss factor modeled with Gaussian distribution, 4.63, 4.64. Nearby metal objects (street signs, etc.) Usually modeled statistically Large distant objects Analytical model: Radar Cross Section (RCS) Bistatic radar equation, 4.66 ECE6331 Spring 2009

  36. Measured results ECE6331 Spring 2009

  37. Measured results ECE6331 Spring 2009

  38. Questions? ECE6331 Spring 2009

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