4215L3-Signal propagation basics
- 1. 9/21/2023 CSE 4215, Winter 2011 1 CSE 4215/5431: Mobile Communications Winter 2011 Suprakash Datta datta@cs.yorku.ca Office: CSEB 3043 Phone: 416-736-2100 ext 77875 Course page: http://www.cs.yorku.ca/course/4215 Some slides are adapted from the book website
- 2. 9/21/2023 CSE 4215, Winter 2011 2 Signal propagation basics Many different effects have to be considered
- 3. 9/21/2023 CSE 4215, Winter 2011 3 Signal propagation ranges • Transmission range – communication possible – low error rate • Detection range – detection of the signal possible – no communication possible • Interference range – signal may not be detected – signal adds to the background noise distance sender transmission detection interference
- 4. 9/21/2023 CSE 4215, Winter 2011 4 Signal propagation • Propagation in free space always like light (straight line) • Receiving power proportional to 1/d² in vacuum – much more in real environments (d = distance between sender and receiver) • Receiving power additionally influenced by • fading (frequency dependent) • shadowing • reflection at large obstacles • refraction depending on the density of a medium • scattering at small obstacles • diffraction at edges reflection scattering diffraction shadowing refraction
- 5. 9/21/2023 CSE 4215, Winter 2011 5 Real world example
- 6. 9/21/2023 CSE 4215, Winter 2011 6 Propagation Modes • Ground-wave propagation • Sky-wave propagation • Line-of-sight propagation
- 7. 9/21/2023 CSE 4215, Winter 2011 7 Ground Wave Propagation
- 8. 9/21/2023 CSE 4215, Winter 2011 8 Ground Wave Propagation • Follows contour of the earth • Can Propagate considerable distances • Frequencies up to 2 MHz • Example – AM radio
- 9. 9/21/2023 CSE 4215, Winter 2011 9 Sky Wave Propagation
- 10. 9/21/2023 CSE 4215, Winter 2011 10 Sky Wave Propagation • Signal reflected from ionized layer of atmosphere back down to earth • Signal can travel a number of hops, back and forth between ionosphere and earth’s surface • Reflection effect caused by refraction • Examples – Amateur radio – CB radio
- 11. 9/21/2023 CSE 4215, Winter 2011 11 Line-of-Sight Propagation
- 12. 9/21/2023 CSE 4215, Winter 2011 12 Line-of-Sight Propagation • Transmitting and receiving antennas must be within line of sight – Satellite communication – signal above 30 MHz not reflected by ionosphere – Ground communication – antennas within effective line of site due to refraction • Refraction – bending of microwaves by the atmosphere – Velocity of electromagnetic wave is a function of the density of the medium – When wave changes medium, speed changes – Wave bends at the boundary between mediums
- 13. 9/21/2023 CSE 4215, Winter 2011 13 Line-of-Sight Equations • Optical line of sight • Effective, or radio, line of sight • d = distance between antenna and horizon (km) • h = antenna height (m) • K = adjustment factor to account for refraction, rule of thumb K = 4/3 h d 57 . 3 h d 57 . 3
- 14. 9/21/2023 CSE 4215, Winter 2011 14 Line-of-Sight Equations • Maximum distance between two antennas for LOS propagation: • h1 = height of antenna one • h2 = height of antenna two 2 1 57 . 3 h h
- 15. 9/21/2023 CSE 4215, Winter 2011 15 LOS Wireless Transmission Impairments • Attenuation and attenuation distortion • Free space loss • Atmospheric absorption • Multipath (diffraction, reflection, refraction…) • Noise • Thermal noise
- 16. 9/21/2023 CSE 4215, Winter 2011 16 Attenuation • Strength of signal falls off with distance over transmission medium • Attenuation factors for unguided media: – Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal – Signal must maintain a level sufficiently higher than noise to be received without error – Attenuation is greater at higher frequencies, causing distortion
- 17. 9/21/2023 CSE 4215, Winter 2011 17 Free Space Loss • Free space loss, ideal isotropic antenna • Pt = signal power at transmitting antenna • Pr = signal power at receiving antenna • = carrier wavelength • d = propagation distance between antennas • c = speed of light ( 3 10 8 m/s) where d and are in the same units (e.g., meters) 2 2 2 2 4 4 c fd d P P r t
- 18. 9/21/2023 CSE 4215, Winter 2011 18 Free Space Loss • Free space loss equation can be recast: d P P L r t dB 4 log 20 log 10 dB 98 . 21 log 20 log 20 d dB 56 . 147 log 20 log 20 4 log 20 d f c fd
- 19. 9/21/2023 CSE 4215, Winter 2011 19 Free Space Loss • Free space loss accounting for gain of other antennas • Gt = gain of transmitting antenna • Gr = gain of receiving antenna • At = effective area of transmitting antenna • Ar = effective area of receiving antenna t r t r t r r t A A f cd A A d G G d P P 2 2 2 2 2 2 4
- 20. 9/21/2023 CSE 4215, Winter 2011 20 Free Space Loss • Free space loss accounting for gain of other antennas can be recast as r t dB A A d L log 10 log 20 log 20 dB 54 . 169 log 10 log 20 log 20 r t A A d f
- 22. 9/21/2023 CSE 4215, Winter 2011 22 Multipath propagation • Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction • Time dispersion: signal is dispersed over time – interference with “neighbor” symbols, Inter Symbol Interference (ISI) • The signal reaches a receiver directly and phase shifted – distorted signal depending on the phases of the different parts signal at sender signal at receiver LOS pulses multipath pulses
- 23. 9/21/2023 CSE 4215, Winter 2011 23 Atmospheric absorption • Water vapor and oxygen contribute most • Water vapor: peak attenuation near 22GHz, low below 15Ghz • Oxygen: absorption peak near 60GHz, lower below 30 GHz. • Rain and fog may scatter (thus attenuate) radio waves. • Low frequency band usage helps…
- 24. 9/21/2023 CSE 4215, Winter 2011 24 Effects of mobility • Channel characteristics change over time and location – signal paths change – different delay variations of different signal parts – different phases of signal parts – quick changes in the power received (short term fading) • Additional changes in – distance to sender – obstacles further away – slow changes in the average power received (long term fading) short term fading long term fading t power
- 25. 9/21/2023 CSE 4215, Winter 2011 25 Fading channels • Fading: Time variation of received signal power • Mobility makes the problem of modeling fading difficult • Multipath propagation is a key reason • Most challenging technical problem for Mobile Communications
- 26. 9/21/2023 CSE 4215, Winter 2011 26 Types of Fading • Short term (fast) fading • Long term (slow) fading • Flat fading – across all frequencies • Selective fading – only in some frequencies • Rayleigh fading – no LOS path, many other paths • Rician fading – LOS path plus many other paths
- 27. 9/21/2023 CSE 4215, Winter 2011 27 Fading models
- 28. 9/21/2023 CSE 4215, Winter 2011 28 Dealing with fading channels • Error correction • Adaptive equalization – attempts to increase signal power as needed – can be done with analog circuits or DSP