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REU June 2013

REU June 2013. Radio Frequency Communications Tim Pratt Instructor tipratt@vt.edu. Topics. Radio waves Frequency bands Atmospheric effects Link equation CNR ratio on radio links Analyzing radio links with link budgets Designing radio links. Unit 1 Radio Waves.

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REU June 2013

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  1. REU June 2013 Radio Frequency Communications Tim Pratt Instructor tipratt@vt.edu  Tim Pratt 2013

  2. Topics • Radio waves • Frequency bands • Atmospheric effects • Link equation • CNR ratio on radio links • Analyzing radio links with link budgets • Designing radio links  Tim Pratt 2013

  3. Unit 1 Radio Waves • Radio waves are electromagnetic waves (EM waves) • Radio, infra-red, light, ultra violet, X rays, alpha, beta, gamma rays are all forms of EM waves • Radio waves have wavelengths from hundreds of kilometers (ELF) to millimeters (mm waves) • Infra red, light and ultra violet have wavelengths from 20 microns to 0.2 microns (1 micron = 10-6 m)  Tim Pratt 2013

  4. EM Waves • EM waves have electric fields and magnetic fields • E field defines polarization of wave - vertical here • H field is orthogonal in space (at right angles to E) • Diagram is a snapshot at t = 0 E z H  Tim Pratt 2013

  5. EM Waves • EM waves travel at the velocity of light • c  3 x 108 m/s • Actual value is 2.99792458 x 108 m/s • Actual value is important in GPS • Position location depends on time of flight of radio waves from four GPS satellites to a GPS receiver • Wavelength  = c / f where f is frequency • Example: f = 2 GHz = 2 x 109 Hz •  = 3 x 108 / 2 x 109 = 0.15 m = 15 cm  Tim Pratt 2013

  6. Radio Waves • Maxwell’s Equations define the behavior of EM waves • Rarely used directly, but boundary conditions are important • EM waves are reflected by conducting surfaces • E field cannot be parallel to a conducting surface • Must terminate at right angles to the surface • Conducting surfaces are metal, water …  Tim Pratt 2013

  7. Polarization • All EM waves are polarized • Polarization is defined by direction of E field vector • Vertical and Horizontal polarizations are widely used in radio systems • Radio waves can be circularly polarized (LHCP and RHCP) • CP waves have E field that rotates through 360 degrees in each wavelength of travel  Tim Pratt 2013

  8. Polarization and Antennas • Transmitting antenna defines polarization of wave • Receive antenna must have same polarization • Cross polarized antenna does not pick up signal • E.g. transmit V polarization, receive antenna has H polarization; no received signal • Same applies to LHCP and RHCP • Most cell phone systems use vertical polarization  Tim Pratt 2013

  9. Radio Waves • Humans cannot sense radio waves except by heating • If you go out on a summer’s day, you can get hot by absorbing infra-red waves from the sun • Otherwise we cannot sense EM waves • They have no taste, no feel, no smell and cannot be seen • But we know they are there! • We can transfer signal power from a transmitter to a receiver  Tim Pratt 2013

  10.  Tim Pratt 2013

  11. Unit 2 Radio Frequencies and Propagation • In this unit you will learn about • Frequencies and frequency bands • Letter designations • Propagation around the earth’s curvature • Propagation in the earth’s atmosphere • Multipath in LOS and cellular phone links  Tim Pratt 2013

  12. Frequency Bands • Radio communication systems must operate in allocated frequency bands • The International Telecommunications Union (ITU) Radio group (ITU-R) allocates frequencies at World Radio Conferences (WRCs) • In the US, the Federal Communications Commission manages use of the (civil) radio spectrum  Tim Pratt 2013

  13. Widely Used Radio Frequency Bands • 500 kHz to 1550 kHz AM broadcasting • 2 MHz – 30 MHz HF band (short wave) • 30 MHz – 88 MHz Mobile radio systems, • 88 MHz – 108 MHz VHF FM broadcast band • 108 MHz – 118 MHz Aircraft navigation • 118 MHz – 136 MHz Air-ground links for ATC • 150 MHz – 155 MHz Public service radio (fire, etc) • 184 MHz – 244 MHz VHF TV Channels 3 – 13 • 450 MHz – 750 MHz UHF TV channels 14 - 64  Tim Pratt 2013

  14. Widely Used Radio Frequency Bands • 850 – 899 MHz Analog FM cellular telephones • 1030 and 1090 MHz Secondary radar for ATC • 1100 – 1200 MHz Primary radar for ATC • 1227 MHz GPS code for military navigation • 1575.5 MHz GPS code for civil navigation • 1800 – 2000 MHz Digital cellular telephones • 2430 – 2445 MHz Satellite radio broadcasting • 2445 – 2485 MHz Unlicensed band for wireless LANs, Bluetooth, WiFi, Internet access  Tim Pratt 2013

  15. Widely Used Radio Frequency Bands • 2.6 – 3.4 GHz S-band radar • 3.5 – 4.5 GHz Satellite communications downlinks • 5.7 – 6.4 GHz Satellite communications uplinks • 6.4 – 6.7 GHz C-band radars • 7 – 8 GHz Military satellite communications • 9.5 – 9.9 GHz X-band radars, airborne, ship radar • 10.0 – 12.2 GHz Satellite downlinks • 12.2 – 12.7 GHz Satellite TV broadcasting  Tim Pratt 2013

  16. RF Frequency Band Names • Above 1 GHz: • ITU designations are VHF - 30 MHz to 300 MHz UHF - 300 MHz to 3 GHz SHF - 3 GHz to 30 GHz EHF - 30 GHz to 300 GHz SHF and EHF are used mainly by US government Others use letter bands  Tim Pratt 2013

  17. Microwave Frequency Letter Bands • Letter designations (Communications) L band - 1 – 2 GHz S band - 2 – 4 GHz C band - 4 – 8 GHz Ku band - 10 – 14 GHz K band - 14 – 24 GHz Ka band - 24 – 40 GHz V band - 40 – 50 GHz  Tim Pratt 2013

  18. Propagation in Earth’s Atmosphere • Attenuation in clear air • Atmospheric gases cause attenuation • Oxygen, water vapor, are important • Oxygen resonance 55 – 60 GHz • Water vapor absorption 22 – 23 GHz • Clear air attenuation is low below 10 GHz  Tim Pratt 2013

  19. A dB O2 resonance 100 10 50%RH 50%RH 1.0 0.1 Dry air • 10 100 GHz Fig 9.1 Zenith Attenuation in Clear Air  Tim Pratt 2013

  20. Propagation in Rain • Attenuation in rain • Not very significant below 10 GHz • Increases approximately as frequency squared • Attenuation in dB  (RF frequency)2 • Rain attenuation is a major factor in design of radio communications links operating above 10 GHz • Particularly important for satellite communication • Satcom links have small margins – spare CNR dBs  Tim Pratt 2013

  21. The Earth is Curved • Radio waves above 30 MHz travel in straight lines • Ways must be found to get signals beyond horizon • Ionospheric reflection uses hf band, 2 – 30 MHz • Microwave link uses line of sight between towers • Chain of repeaters can take the signal thousands of miles • Satellite communications uses a repeater in the sky • Single link via GEO satellite can reach round one third of the earth’s surface.  Tim Pratt 2013

  22. Ionospheric layers multipath Earth Tx Rx Fig. 9.2 HF Radio Communication  Tim Pratt 2013

  23. Earth Tx Rx Fig. 9.3 LOS Microwave Communications  Tim Pratt 2013

  24. GEO satellite Altitude 35,680 km Tx Earth Rx Fig. 9.4 Satellite Communications  Tim Pratt 2013

  25. Fig. 9.5 Horizon Distance • d in km = (2 k a h) = 4.12  (h in meters) • E.g. h = 30 m (about 100 ft) • d = 22.6 km, link distance < 45 km d h Clearance over buildings and trees is needed – towers must be higher  Tim Pratt 2013

  26. Data Rate • High data rates require large transmission bandwidth • HF radio links using ionospheric reflection cannot support wide bandwidth signals • Satellite and microwave links can support bandwidths in excess of 10 GHz • Data rates up to 100 Gbps are possible • Optical fiber bandwidths exceed 30 GHz • Data rates to 100 Gbps per fiber  Tim Pratt 2013

  27. Multipath in LOS links • Line of sight (LOS) microwave links operate over land and water • When signal reflects from ground or inversion layer in air we get Multipath - two paths from the transmitter to receiver • If received signals are equal in magnitude and opposite in phase, cancellation can occur • Called multipath fading • May cause 40 dB reduction in received signal  Tim Pratt 2013

  28. Fig 9.6 Microwave Link Multipath Inversion layer multipath Rx Tx LOS path multipath h Reflection point Vertical scale is exaggerated. Grazing angle is << 1o  Tim Pratt 2013

  29. Combating Multipath in LOS Links • Antenna Diversity makes use of more than one receiving antenna, or two receiving and two transmitting antennas • Concept: If a multiple path exists from the transmit antenna to the receive antenna resulting in a deep fade, excess path length is a multiple of /2 • Create a second path to a different antenna • That path will have a different length • With paths over water – especially a tidal estuary – more paths may been needed  Tim Pratt 2013

  30. Fig. 9.7 Antenna Diversity in LOS Link LOS path Tx Rx multipath Reflection point Vertical scale is exaggerated. Grazing angle is << 1o  Tim Pratt 2013

  31. Multipath in Cellular Phone Links • Cellular phones typically do not have line of sight to a base station • Received signal consists of many components from different paths – by refection, diffraction, and attenuation of direct path • Causes near continuous multipath fading • Design of cell phone receiver and radio transmissions is dominated by multipath problem • Causes high BER on link most of time  Tim Pratt 2013

  32. Link Margin • Each radio link is designed to withstand a specific level of rain or multipath attenuation • Maximum permitted attenuation is called a link margin • If attenuation exceeds the link margin, the link will fail - link suffers an outage • Design must be based on rainfall statistics and knowledge of multipath conditions • Aim is to achieve a high percentage availability Availability = 100% - outage %  Tim Pratt 2013

  33. Summary of Unit 2 • In this unit you have learned about • Radio frequencies and letter bands • How to get radio signals past the horizon • Line of sight links and multipath propagation  Tim Pratt 2013

  34.  Tim Pratt 2013

  35. Unit 3 Link Equation • In this unit you will learn how • To calculate received power in a radio link • The calculate noise power in a receiver • To calculate carrier to noise ratio (CNR) at receiver • A superhet receiver is configured • Link margin is used in a radio communication system  Tim Pratt 2013

  36. Link Equation • The link equation is used to calculate received power in a radio link • Parameters are: • Transmitted power • Antenna gains • Distance between transmitter and receiver • Radio frequency  Tim Pratt 2013

  37. Incident flux density F W / m2 Isotropic source EIRP = Pt W Area A m2 R Part of sphere radius R surface area As Fig. 9.8 Flux density from an isotropic source  Tim Pratt 2013

  38. Flux Density • Isotropic source with power Pt watts radiates equally in all directions • Flux density at distance R meters is F Watts / m2 • F is radiated power divided by surface area of sphere • F = Pt / As = Pt / [4  R2 ] Watts /m2 (Eqn 9.1) • Flux density is independent of frequency • We often need directive antennas • Antenna has narrow beam, gain G (a ratio) • Gain describes the ability of an antenna to increase power transmitted in a particular direction  Tim Pratt 2013

  39. Antennas Definition of antenna gain: The increase in received power at a given point with the test antenna relative to the power received from an isotropic antenna Definition of an isotropic antenna: An antenna that radiates equally in all directions (does not exist)  Tim Pratt 2013

  40. Received Power • We can combine gain and transmitted power: EIRP = PtGt watts (Eqn 9.2) • EIRP = Effective Isotropically RadiatedPower • For a source with EIRP = PtGt watts • Flux density at a distance R meters is F • F = PtGt / [4  R2 ] W/m2 (Eqn 9.-3) • Power received by an aperture with area Ae m2 is • Pr = F x Ae watts (Eqn 9.4)  Tim Pratt 2013

  41. Incident flux density F W/m2 Source EIRP = PtW Receiver Pr Receiving antenna Area Ae m2 Fig 9.9 Radio Link  Tim Pratt 2013

  42. Received Power • From antenna theory, the gain of an antenna is related to its effective aperture by • G = 4  Ae / 2 (Eqn 9.5) • Hence • Ae = Gr2 /4  • Received power is Pr • Pr = F x Ae = PtGtGr2 / [ 4  R ]2 watts (Eqn. 9.6) • This is the basic link equation  Tim Pratt 2013

  43. Path Loss • The term [4  R ]2 / 2 is called free space path loss • Lp = [4  R / ]2 • It is not a loss in the sense of power being absorbed • Describes how power spreads out with distance • Loss is proportional to 1/R2 • Link Equation: • Pr = EIRP x Receive antenna gain watts Path loss The link equation is usually evaluated in decibels: Pr = Pt + Gt+Gr- 10 log [ / ( 4  R )]2dBW  Tim Pratt 2013

  44. Received Power • Additional losses must be included in the Link Equation: • Pr = Pt + Gt+Gr - Lp - La - Lta – LradBW where all parameters are in dB units and Lp = [4  R / ]2 = 20 log [4  R /  ] dB La = loss in atmosphere Lta = losses in transmitting antenna and waveguide Lra = losses in receiving antenna and waveguide  Tim Pratt 2013

  45. Link Budgets • Link budgets are used to find the power at the receiver – calculated at the input to the receiver • A link budget is called a budget because it is tabulated just like a financial budget • Parameters go on the left • Numbers go on the right in a column • Bottom line is received power Pr watts for a power budget • N watts for a noise budget • Keep power and noise budgets separate • Then calculate CNR = Pr - N in dB units  Tim Pratt 2013

  46. Waveguide (loss Lra) Waveguide (loss Lta) Atmospheric loss Tx shelter Rx shelter Reflection point Fig. 9.10 LOS Link Losses  Tim Pratt 2013

  47. Link Budget for line of Sight (LOS) link • Example of Link Budget for 24 GHz LOS link • Distance R = 25 km • Transmit power = 2 W • Antenna gain 36 dB at each end of link • Wavelength at 24.0 GHz = 0.05 m • Atmospheric loss = 5.0 dB • Waveguide loss (at each end) = 6.0 dB • Path Loss = Lp = 10 log [ 4  R /  ] 2 dB • = 20 log [ 4  x 25 x 103 / 0.0125] • = 148.0 dB  Tim Pratt 2013

  48. Link Budget for LOS link • The received power is tabulated using dB units • Example: • Pt = 2.0 W 3.0 dBW • Gt= 36.0 dB • Gr = 36.0 dB • Lp – 148.0 dB • La -5.0 dB • Lwg -12.0 dB • Pr-90.0 dBW  Tim Pratt 2013

  49. CNR at Receiver • The performance of any radio link is determined by the carrier to noise ratio (CNR) at the receiver • Carrier (C watts or dBW) is equal to Pr dBW calculated in the link budget • CNR = Pr / N as a ratio or in dB • Noise power is thermal or AWGN noise power • N = k Ts BN where k is Boltzmann’s constant k = 1.38 x 10-23 J/K = -228.6 dBW / K / Hz Tsis system noise temperature BN is noise bandwidth of the receiver (IF filter)  Tim Pratt 2013

  50. CNR at Receiver • Example: 24 GHz Line of Sight Link • Receivers have low noise amplifiers (LNAs) to keep system noise temperature Ts low • Antenna contributes noise radiated by atmosphere • Typical Ts at 24 GHz is 1000 K = 30.0 dBK • Let’s make BN = 36 MHz = 75.6 dBHz • Then N = -228.6 + 30 + 75.6 = -123.0 dBW • Pr = -90.0 dBW • CNR = Pr– N = -90.0 + 123.0 = 33.0 dB • This is the link margin above 0 dB CNR  Tim Pratt 2013

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