PROPAGATION MODELING ledoyle@tcd.ie 4C4 http://ledoyle.wordpress.com/temp/
Classification Band Initials Frequency Range Characteristics Extremely low ELF < 300 Hz Infra low ILF 300 Hz - 3 khz Ground wave Very low VLF 3 khz - 30 khz Low LF 30 khz - 300 khz Medium MF 300 khz - 3 MHz Ground/Sky wave High HF 3 MHz - 30 MHz Sky wave Very high VHF 30 MHz - 300 MHz Ultra high UHF 300 MHz - 3 GHz Super high SHF 3 GHz - 30 GHz Space wave Extremely high EHF 30 GHz - 300 GHz Tremendously high THF 300 GHz - 3000 GHz
SIMPLISTIC VIEW OF SITUATION WITH REGARD TO SPACE WAVES
MORE COMPLEX VIEW MANY MORE PATHS MOVING OBJECTS
MORE COMPLEX STILL NOISE GETS ADDED TO THE SYSTEM [from a variety of sources] INTERFERENCE FROM OTHER SIGNALS THAT ARE BEING TRANSMITTED WE WILL NOT BE ABLE TO TAKE EVERYTHING INTO ACCOUNT IN THE COURSE
models
what is a model Relation between the signal radiated and signal received as a function of distance and other variables Different models Various dominating propagation mechanisms different environments (indoor-outdoor; land-sea-space; ) different applications (point-to-point, point-to-area, ) different frequency ranges Some models include random variability
There are tonnes of different propagation models out there!
http://people.seas.harvard.edu/~jones/es151/prop_models/propagation.html location 1, free space loss is likely to give an accurate estimate of path loss. location 2, a strong line-of-sight is present, but ground reflections can significantly influence path loss. The plane earth loss model appears appropriate. location 3, plane earth loss needs to be corrected for significant diffraction losses, caused by trees cutting into the direct line of sight. location 4, a simple diffraction model is likely to give an accurate estimate of path loss. location 5, loss prediction fairly difficult and unreliable since multiple diffraction is involved.
LARGE AND SMALL SCALE MODELS RF propagation models generally characterize two aspects of RF propagation: large scale and small scale fading Large Scale propagation models predict the mean signal strength for a given transmitter and receiver separation distance and are used to predict RF coverage Friis Free Space Path Loss Model Two Ray Ground Reflection Model Log Distance Path Loss Model with Shadowing Small scale propagation models characterize the rapid fluctuations of received signal strength over short distances or a short time duration Small-scale models are generally associated with predicting multipath fading
Fading Fast Fading (Shortterm fading) Slow Fading (Longterm fading) Signal Strength (db) Path Loss Distance 15
MOVING OBJECT WILL EXPERIENCE FAST FADING MULTIPATH FADING
some terminology As we are mainly interested in the more complex environments of mobile communications we talk about the BS = basestation, the MS = mobile station. Sometimes the mobile station is referred to as the UE = user equipment or just the user. Sometimes the basestation might be called an access point!
WE ARE NOT GOING TO GET TO LOOK AT ALL THE ISSUES AND MODELS BUT IT IS GOOD TO REALISE THAT MANY EXIST!
Fading Fast Fading (Shortterm fading) Slow Fading (Longterm fading) Signal Strength (db) Path Loss Distance 19
PATH LOSS MODELS
Fading Fast Fading (Shortterm fading) Slow Fading (Longterm fading) Signal Strength (db) Path Loss Distance 21
path loss Path loss is the phenomenon which occurs when the received signal becomes weaker and weaker due to increasing distance between mobile and base station. A transmission via a radio channel will be affected by path loss (average signal power attenuation), which is largely depending on the distance between the transmitting and receiving radio antennas. Further, characteristics of objects in the radio channel, particularly in the vicinity of the receiving MS, such as terrain, buildings and vegetation may also have a significant impact on the path loss.
path loss PATH loss in decreasing order: Urban area (large city) Urban area (medium and small city) Suburban area Open area
FOLLOWING SLIDES TAKEN FROM GREAT THESIS
BASICS
An antenna typically has a gain. ANTENNA GAIN In laymen s terms, antenna gain refers to the ability of the antenna to focus scattered RF waves into a narrower, useful plane, thereby increasing signal strength. Normally antenna gain is expressed in db The letter G is used to denote it
this world is obsessed with dbs
going beyond free space
plane earth model
Okumura s Model Okumura s model is one of the most frequently used macroscopic propagation models. It was developed during the mid 1960's as the result of large-scale studies conducted in and around Tokyo. The model was designed for use in the frequency range 200 up to 1920 MHz and mostly in an urban propagation environment.
additional notes in the following slides taken from: http://my.fit.edu/~kostanic/rf%20propagation /Old%20Notes/RF%20Propagation%20-%2007- Okumura%20and%20Hata%20Macroscopic%2 0Propagation%20Models.pdf
A mu (f,d)
antenna gain factors to take into account the antennas are not at the heights for the previous A mu (f,d) curves. Figure 2
antenna gain factors to take into account the antennas are not at the heights for the previous A mu (f,d) curves. Figure 3
Okumura s model has a 10-14 db empirical standard deviation between the path loss predicted by the model and the path loss associated with one of the measurements used to develop the model
advancing on Okumura The so-called Okumura-Hata curve is an approximation of radio wave propagation characteristics based on aggregated data obtained in actual tests of propagation characteristics between a base station and mobile stations in various areas, such as open land, suburbs, a medium city, and a large city. The electric field intensity for each area is calculated by applying a correction value to the standard field intensity, which is that of an urban area in quasi-smooth terrain.
Okumura Hata
exercise Plot a series of curves comparing the functionality of the free space and plane earth model for different distances and frequencies. Assume isotropic antennas
some calculations for you to do... what happens to the free space loss every time the carrier frequency is doubled? every time the separation between antennas is doubled? what does this tell you about how you might structure a network??