![Long term fading
• Long term variation in mean signal level is
also known as slow fading
• Caused by movement over large distances.
• The probability density function is given by
a log-normal distribution
- normal distribution on a log scale
P(m) = (1/m (m) 2) e^[-(log m – E(m))^2/(2 (m)^2)]](https://image.slidesharecdn.com/lecture2-250727043931-62b4b14b/85/Radio-communication-and-Radio-spectrum-ppt-24-320.jpg)
![Shannon’s work
• C - channel capacity (bits/s)
• B – transmission bandwidth (Hz)
• E – energy per bit of received signal (Joule)
• R – information rate (bits/s)
• S = E R – signal power
• N – single-sided noise power spectral density (W/Hz)
(C/B) = log [1+(S/(NB))] = log [1+(E/N)(R/B)]
Suppose R = C we have
(C/B) = log [1+(E/N)(C/B)]](https://image.slidesharecdn.com/lecture2-250727043931-62b4b14b/85/Radio-communication-and-Radio-spectrum-ppt-28-320.jpg)
Radio communication and Radio spectrum .ppt
- 1. COSC 393: Lecture 2 Radio Fundamentals
- 2. Radio Communication • Radio signals • Spectrum • Transmitter • Signal propagation • Modulation
- 3. Radio Wave s(t) = At sin(2 ft t + t)
- 4. Frequency and Wave length • Relationship: = c/f • wave length , • speed of light c 3x108 m/s, • frequency f
- 6. • VLF = Very Low Frequency • LF = Low Frequency • MF = Medium Frequency • HF = High Frequency • VHF = Very High Frequency • UHF = Ultra High Frequency • SHF = Super High Frequency • EHF = Extra High Frequency • UV = Ultraviolet Light 1 Mm 300 Hz 10 km 30 kHz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 m 3 THz 1 m 300 THz visible light VLF LF MF HF VHF UHF SHF EHF infrared UV optical transmission coax cable twisted pair
- 7. • Isotropic radiator: Equal radiation in all directions (3D) - theoretical antenna • Real antennas always have directive effects (vertically and/or horizontally) • Different antennas have different radiation pattern. Antennas
- 8. • Dipoles with lengths /4 or Hertzian dipole with length /2 (length proportional to wavelength) • Example: Radiation pattern of a simple Hertzian dipole • Gain: maximum power in the direction of the main lobe compared to the power of an isotropic radiator (with the same average power) side view (xy-plane) x y side view (yz-plane) z y top view (xz-plane) x z simple dipole /4 /2
- 9. side view (xy-plane) x y side view (yz-plane) z y top view (xz-plane) x z top view, 3 sector x z top view, 6 sector x z • Often used for base stations in a cellular system (e.g., covering a valley) directed antenna sectorized antenna
- 10. Effect of a transmission distance sender transmission detection interference • 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 No effect
- 11. Signal propagation property • Radio signal behaves like light in free space (straight line) • Receiving power proportional to 1/d² (d = distance between sender and receiver) • So ideally, the transmitter and a receiver must see each other! Really?
- 12. Three means of propagation • Ground wave • Tropospheric wave • Ionospheric or sky wave
- 13. Ground Wave • travels in contact with earth’s surface • reflection, refraction and scattering by objects on the ground • transmitter and receiver need NOT see each other • affects all frequencies • at VHF or higher, provides more reliable propagation means • signal dies off rapidly as distance increases
- 14. Tropospheric Wave • bending(refraction) of wave in the lower atmosphere • VHF communication possible over a long distance • bending increases with frequency – so higher frequency more chance of propagation • More of an annoyance for VHF or UHF (cellular)
- 15. Ionospheric or Sky Wave • Reflected back to earth by ionospheric layer of the earth atmosphere • By repeated reflection, communication can be established over 1000s of miles • Mainly at frequencies below 30MHz • More effective at times of high sunspot activity
- 16. reflection scattering diffraction shadowing 4 possible events Radio wave Radio wave Radio wave
- 17. Multipath Characteristics • A signal may arrive at a receiver - many different times - many different directions - due to vector addition . Reinforce . Cancel - signal strength differs from place to place
- 18. Mobile System • Usually Base Station is not mobile • Receiver could be moving (65mph!) • Whenever relative motion exists - Doppler shift - Fading • Even the motion of scatterers cause fading
- 19. Free Space Propagation • Suppose we have unobstructed line-of-sight Pr(d) = (Pt Gt Gr ^2)/(4)^2 d^2 L) -Pt transmitted power -Gt, Gr Antenna gain -wavelength in meters - d distance in meters - L (>= 1) system loss factor (not related to propagation.
- 20. Propagation Losses • Two major components - Long term fading m(t) - Short term fading r(t) Received signal s(t) s(t) = m(t) r(t)
- 21. dB - decibel • Decibel, a logarithmic unit of intensity used to indicated power lost or gained between two signals. Named after Alexander Graham Bell. 10 log (P1/P2)
- 22. Radio Signal Fading T Signal strength (dB) Time Long term fading Short term fading
- 23. Short term fading • Also known as fast fading – caused by local multi paths. • Observed over distance = ½ wave length • 30mph will experience several fast fades in a sec. • Given by Rayleigh Distribution • This is nothing but the square root of sum of the square of two Gaussian functions. r = square root ( Ac * Ac + As * As) Ac and As are two amplitude components of the field intensity of the signal
- 24. Long term fading • Long term variation in mean signal level is also known as slow fading • Caused by movement over large distances. • The probability density function is given by a log-normal distribution - normal distribution on a log scale P(m) = (1/m (m) 2) e^[-(log m – E(m))^2/(2 (m)^2)]
- 25. Delay Spread • Signal follows different paths to reach same destination. • So same signal may arrive many times at different time intervals.
- 26. Delay Spread • In digital system, delay spread causes intersymbol interference. • Therefore, there is a limit on the maximum symbol rate of a digital multipath channel. • Obviously, delay spreads are different in different environment. • (roughly between 0.2 to 3 microseconds)
- 27. Capacity of Channel • What is the maximum transmission rate so that the channel has very high reliability? - error free capacity of a channel • C.E. Shannon’s work suggest that signaling scheme exists for error-free transmission if the rate of transmission is lower than the channel capacity.
- 28. Shannon’s work • C - channel capacity (bits/s) • B – transmission bandwidth (Hz) • E – energy per bit of received signal (Joule) • R – information rate (bits/s) • S = E R – signal power • N – single-sided noise power spectral density (W/Hz) (C/B) = log [1+(S/(NB))] = log [1+(E/N)(R/B)] Suppose R = C we have (C/B) = log [1+(E/N)(C/B)]
- 29. Shannon’s work - continued • Solving for (E/N) (aka. signal to noise ratio) (E/N) = (2^a –1)/a where a = (C/B). So given C= 19.2kb/s and bandwidth = 30kHz What is E/N required for error-free transmission? R/B = 19.2/30 = 0.64 Substituting we get E/N = 0.8724 = -0.593dB So control transmission power to obtain this E/N.
- 30. Propagation models in built-up areas • Propagation is strongly influenced by the environment - building characteristics - vegetation density - terrain variation • Perfect conductors reflect the wave where as nonconductors absorb some energy!
- 31. Empirical models to predict propagation losses • Okumura’s model - based on free space path loss + correction factors for suburban and rural areas, irregular terrain, street orientations • Sakagmi and Kuboi model - extend Okumura’s model using regression analysis of data. • Hata’s model - empirical formula to describe Okumura’s data
- 32. More models • Ibrahim and Parsons model - equations developed to best fit data observed at London. (freq. 168-900 MHz) • Lee’s model – Use at 900MHZ – 3 parameters (median trasmission loss, slope of the path loss curve and adjustment factor)
- 33. Freq. for mobile communication 2.2.1 • VHF-/UHF-ranges for mobile radio – simple, small antenna • SHF and higher for directed radio links, satellite communication – small antenna, focusing – large bandwidth available • Wireless LANs use frequencies in UHF to SHF spectrum – limitations due to absorption by water and oxygen • weather dependent fading, signal loss due to by heavy rainfall etc.
- 34. Modulation • Digital modulation – digital data is translated into an analog signal – ASK, FSK, PSK (… Shift Keying) – differences in spectral efficiency, power efficiency, robustness • Analog modulation – shifts center frequency of baseband signal up to the radio carrier • Motivation – smaller antennas (e.g., /4)
- 35. Types of Modulation • Amplitude modulation • Frequency modulation • Phase modulation • Combination modulation
- 36. synchronization decision digital data analog demodulation radio carrier analog baseband signal 101101001 radio receiver digital modulation digital data analog modulation radio carrier analog baseband signal 101101001 radio transmitter
- 39. Phase Modulation
- 40. Digital modulation • Amplitude Shift Keying (ASK): – simple – low bandwidth – susceptible to interference • Frequency Shift Keying (FSK): – somewhat larger bandwidth • Phase Shift Keying (PSK): – more complex (both ends) – robust against interference 1 0 1 t 1 0 1 t 1 0 1 t