Radio Propagation

1. Radio Propagation CSCI 694 24 September 1999 Lewis Girod

2. Outline • Introduction and terminology • Propagation mechanisms • Propagation models Radio Propagation

3. What is Radio? • Radio Xmitter induces E&M fields • Electrostatic field components µ 1/d3 • Induction field components µ 1/d2 • Radiation field components µ 1/d • Radiation field has E and B component • Field strength at distance d = EB µ 1/d2 • Surface area of sphere centered at transmitter Radio Propagation

4. General Intuition • Two main factors affecting signal at receiver • Distance (or delay)  Path attenuation • Multipath  Phase differences Green signal travels 1/2 farther than Yellow to reach receiver, who sees Red. For 2.4 GHz,  (wavelength) =12.5cm. Radio Propagation

5. Objective • Invent models to predict what the field looks like at the receiver. • Attenuation, absorption, reflection, diffraction... • Motion of receiver and environment… • Natural and man-made radio interference... • What does the field look like at the receiver? Radio Propagation

6. 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 Radio Propagation

7. Outline • Introduction and some terminology • Propagation Mechanisms • Propagation models Radio Propagation

8. Radio Propagation Mechanisms • Free Space propagation • Refraction • Conductors & Dielectric materials (refraction) • Diffraction • Fresnel zones • Scattering • “Clutter” is small relative to wavelength Radio Propagation

9. Free Space • 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 Radio Propagation

10. Free Space Propagation Model • Received power at distance d is • where Pt is the transmitter power in Watts • a constant factor K depends on antenna gain, a system loss factor, and the carrier wavelength Radio Propagation

11. Refraction • Perfect conductors reflect with no attenuation • Dielectrics reflect a fraction of incident energy • “Grazing angles” reflect max* • Steep angles transmit max* q qr qt • Reflection induces 180 phase shift Radio Propagation *The exact fraction depends on the materials and frequencies involved

12. T R 1st Fresnel zone Obstruction Diffraction • Diffraction occurs when waves hit the edge of an obstacle • “Secondary” waves propagated into the shadowed region • Excess path length results in a phase shift • Fresnel zones relate phase shifts to the positions of obstacles Radio Propagation

13. 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) Path 1 Path 2 Fresnel zones are ellipses with the T&R at the foci; L1 = L2+l Radio Propagation

14. Power Propagated into Shadow • 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 Rappaport, pp. 97 Tip of Shadow 1st 2nd Obstruction of Fresnel Zones  Radio Propagation

15. Scattering • Rough surfaces • critical height for bumps is f(,incident angle) • scattering loss factor modeled with Gaussian distribution. • Nearby metal objects (street signs, etc.) • Usually modelled statistically • Large distant objects • Analytical model: Radar Cross Section (RCS) Radio Propagation

16. Outline • Introduction and some terminology • Propagation Mechanisms • Propagation models • Large scale propagation models • Small scale propagation (fading) models Radio Propagation

17. Propagation Models: Large • Large scale models predict behavior averaged over distances >>  • Function of distance & significant environmental features, roughly frequency independent • Breaks down as distance decreases • Useful for modeling the range of a radio system and rough capacity planning Radio Propagation

18. Propagation Models: Small • Small scale (fading) models describe signal variability on a scale of  • Multipath effects (phase cancellation) dominate, path attenuation considered constant • Frequency and bandwidth dependent • Focus is on modeling “Fading”: rapid change in signal over a short distance or length of time. Radio Propagation

19. Large Scale Models • Path loss models • Outdoor models • Indoor models Radio Propagation

20. Free Space Path Loss • Path Loss is a measure of attenuation based only on the distance to the transmitter • Free space model only valid in far-field; • Path loss models typically define a “close-in” point d0 and reference other points from there: What is dB? Radio Propagation

21. Log-Distance Path Loss Model • Log-distance generalizes path loss to account for other environmental factors • Choose a d0 in the far field. • Measure PL(d0) or calculate Free Space Path Loss. • Take measurements and derive  empirically. Radio Propagation

22. Log-Distance 2 • Value of  characterizes different environments Rappaport, Table 3.2, pp. 104 Radio Propagation

23. Log-Normal Shadowing Model • Shadowing occurs when objects block LOS between transmitter and receiver • A simple statistical model can account for unpredictable “shadowing” • Add a 0-mean Gaussian RV to Log-Distance PL • Markov model can be used for spatial correlation Radio Propagation

24. Outdoor Models • “2-Ray” Ground Reflection model • Diffraction model for hilly terrain Radio Propagation

25. T R ht hr Phase shift! 2-Ray Ground Reflection • For d >> hrht, • low angle of incidence allows the earth to act as a reflector • the reflected signal is 180 out of phase • Pr 1/d4 (=4) Radio Propagation

26. T R p0 ht hr p1 Ground Reflection 2 • Intuition: ground blocks 1st Fresnel zone • Reflection causes an instantaneous 180 phase shift • Additional phase offset due to excess path length • If the resulting phase is still close to 180,the gound ray will destructively interfere with the LOS ray. 180 Radio Propagation

27. Hilly Terrain • Propagation can be LOS or result of diffraction over one or more ridges • LOS propagation modelled with ground reflection: diffraction loss • But if there is no LOS, diffraction can actually help! Radio Propagation

28. Indoor Path Loss Models • Indoor models are less generalized • Environment comparatively more dynamic • Significant features are physically smaller • Shorter distances are closer to near-field • More clutter, scattering, less LOS Radio Propagation

29. Indoor Modeling Techniques • Modeling techniques and approaches: • Log-Normal, <2 for LOS down corridor • Log-Normal shadowing model if no LOS • Partition and floor attenuation factors • Computationally intensive “ray-tracing” based on 3-D model of building and attenuation factors for materials Radio Propagation

30. Outline • Introduction and some terminology • Propagation Mechanisms • Propagation models • Large scale propagation models • Small scale propagation (fading) models Radio Propagation

31. Recall: Fading Models • Small scale (fading) models describe signal variability on a scale of  • Multipath effects (phase cancellation) dominate, path attenuation considered constant • Frequency and bandwidth dependent • Focus is on modeling “Fading”: rapid change in signal over a short distance or length of time. Radio Propagation

32. Factors Influencing Fading • Motion of the receiver: Doppler shift • Transmission bandwidth of signal • Compare to BW of channel • Multipath propagation • Receiver sees multiple instances of signal when waves follow different paths • Very sensitive to configuration of environment Radio Propagation

33. Effects of Multipath Signals • Rapid change in signal strength due to phase cancellation • Frequency modulation due to Doppler shifts from movement of receiver/environment • Echoes caused by multipath propagation delay Radio Propagation

34. h(t,)   The Multipath Channel • One approach to small-scale models is to model the “Multipath Channel” • Linear time-varying function h(t,) • Basic idea: define a filter that encapsulates the effects of multipath interference • Measure or calculate the channel impulse response (response to a short pulse at fc): t Radio Propagation

35. SKIP Channel Sounding • “Channel sounding” is a way to measure the channel response • transmit impulse, and measure the response to find h(). • h() can then be used to model the channel response to an arbitrary signal: y(t) = x(t)h(). • Problem: models the channel at single point in time; can’t account for mobility or environmental changes h(t,)   Radio Propagation

36. Characterizing Fading* *Adapted from EE535 Slides, Chugg ‘99 • From the impulse response we can characterize the channel: • Characterizing distortion • Delay spread (d): how long does the channel ring from an impulse? • Coherence bandwidth (Bc): over what frequency range is the channel gain flat? • d1/Bc In time domain, roughly corresponds to the “fidelity” of the response; sharper pulse requires wider band Radio Propagation

37. Effect of Delay Spread* For a system with bw W and symbol time T... • Does the channel distort the signal? • if W << Bc: “Flat Fading” • Amplitude and phase distortion only • if W > Bc: “Frequency Selective Fading” • If T < d, inter-symbol interference (ISI) occurs • For narrowband systems (W  1/T), FSF  ISI. • Not so for wideband systems (W >> 1/T) Radio Propagation

38. RMS Delay spread () Mean excess delay Qualitative Delay Spread Typical values for  : Indoor: 10-100 ns Outdoor: 0.1-10 s Noise threshold Power(dB) Delay Radio Propagation

39. Characterizing Fading 2* • Characterizing Time-variation: How does the impulse response change with time? • Coherence time (tc): for what value of  are responses at t and t+ uncorrelated? (How quickly is the channel changing) • Doppler Spread (fd): How much will the spectrum of the input be spread in frequency? • fd1/tc Radio Propagation

40. Effect of Coherence Time* For a system with bw W and symbol time T... • Is the channel constant over many uses? • if T << tc: “Slow fading” • Slow adaptation required • if T > tc: “Fast fading” • Frequent adaptation required • For typical systems, symbol rate is high compared to channel evolution Radio Propagation

41. Statistical Fading Models • Fading models model the probability of a fade occurring at a particular location • Used to generate an impulse response • In fixed receivers, channel is slowly time-varying; the fading model is reevaluated at a rate related to motion • Simplest models are based on the WSSUS principle Radio Propagation

42. WSSUS* • Wide Sense Stationary (WSS) • Statistics are independent of small perturbations in time and position • I.e. fixed statistical parameters for stationary nodes • Uncorrelated Scatter (US) • Separate paths are not correlated in phase or attenuation • I.e. multipath components can be independent RVs • Statistics modeled as Gaussian RVs Radio Propagation

43. Common Distributions • Rayleigh fading distribution • Models a flat fading signal • Used for individual multipath components • Ricean fading distribution • Used when there is a dominant signal component, e.g. LOS + weaker multipaths • parameter K (dB) defines strength of dominant component; for K=-, equivalent to Rayleigh Radio Propagation

44.  s(t) R1 r(t)  R2 Application of WSSUS • Multi-ray Rayleigh fading: • The Rayleigh distribution does not model multipath time delay (frequency selective) • Multi-ray model is the sum of two or more independent time-delayed Rayleigh variables Rappaport, Fig. 4.24, pp. 185. Radio Propagation

45. Saleh & Valenzuela (1987) Rappaport, pp. 188 • Measured same-floor indoor characteristics • Found that, with a fixed receiver, indoor channel is very slowly time-varying • RMS delay spread: mean 25ns, max 50ns • With no LOS, path loss varied over 60dB range and obeyed log distance power law, 3 > n > 4 • Model assumes a structure and models correlated multipath components. Radio Propagation

46. Saleh & Valenzuela 2 • Multipath model • Multipath components arrive in clusters, follow Poisson distribution. Clusters relate to building structures. • Within cluster, individual components also follow Poisson distribution. Cluster components relate to reflecting objects near the TX or RX. • Amplitudes of components are independent Rayleigh variables, decay exponentially with cluster delay and with intra-cluster delay Radio Propagation

47. References • Wireless Communications: Principles and Practice, Chapters 3 and 4, T. Rappaport, Prentice Hall, 1996. • Principles of Mobile Communication, Chapter 2, G. Stüber, Kluwer Academic Publishers, 1996. • Slides for EE535, K. Chugg, 1999. • Spread Spectrum Systems, Chapter 7, R. Dixon, Wiley, 1985 (there is a newer edition). • Wideband CDMA for Third Generation Mobile Communications, Chapter 4, T. Ojanpera, R. Prasad, Artech, House 1998. • Propagation Measurements and Models for Wireless Communications Channels, Andersen, Rappaport, Yoshida, IEEE Communications, January 1995. Radio Propagation

48. The End Radio Propagation

49. Scattering 2 • hc is the critical height of a protrusion to result in scattering. • RCS: ratio of power density scattered to receiver to power density incident on the scattering object • Wave radiated through free space to scatterer and reradiated: Radio Propagation

50. Free Space 2a • Free space power flux density (W/m2) • power radiated over surface area of sphere • where Gt is transmitter antenna gain • By covering some of this area, receiver’s antenna “catches” some of this flux Radio Propagation