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

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

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) June 2013  Tim Pratt 2013

EM Waves EM waves have electric fields and magnetic fields E z 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 June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

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 … June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

Fig 9.1 Zenith Attenuation in Clear Air 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 June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

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. June 2013  Tim Pratt 2013

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

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

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

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 June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

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

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 June 2013  Tim Pratt 2013

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

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 June 2013  Tim Pratt 2013

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 % June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

Fig. 9.8 Flux density from an isotropic source 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 June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

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) June 2013  Tim Pratt 2013

Received Power We can combine gain and transmitted power: EIRP = Pt Gt watts (Eqn 9.2) EIRP = Effective Isotropically Radiated Power For a source with EIRP = Pt Gt watts Flux density at a distance R meters is F F = Pt Gt / [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) June 2013  Tim Pratt 2013

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

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 = Pt Gt Gr 2 / [ 4  R ]2 watts (Eqn. 9.6) This is the basic link equation June 2013  Tim Pratt 2013

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 )]2 dBW June 2013  Tim Pratt 2013

Received Power Additional losses must be included in the Link Equation: Pr = Pt + Gt + Gr - Lp - La - Lta – Lra dBW 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 June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

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

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 June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

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 Ts is system noise temperature BN is noise bandwidth of the receiver (IF filter) June 2013  Tim Pratt 2013

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 June 2013  Tim Pratt 2013

Radio Receivers Virtually all radio receivers use the superhet design Developed by Edwin Armstrong (of FM fame) in 1917 Idea: It’s difficult to work with signals at microwave frequencies – but we can amplify them Reduce frequency using a frequency converter to an intermediate frequency that is easier to work with A frequency converter needs a local oscillator and a multiplier (mixer) IF = RF signal frequency – local oscillator frequency June 2013  Tim Pratt 2013

Fig 9.12 Simplified Superhet Receiver for LoS link Narrow band pass filter is last filter in IF stage 24 GHz Antenna 1 GHz IF amp LNA BPF Mixer BPF BPF D Pr Gm GIF 23 GHz Local oscillator Demodulator Fig 9.12 Simplified Superhet Receiver for LoS link June 2013  Tim Pratt 2013

Multi-hop LOS Links Microwave LOS can be built with multiple hops A hop is a single section Each section is joined by a repeater A repeater consists of a receiver, an IF amplifier, a frequency conversion stage and a transmitter Repeaters have high gain and cannot transmit at the same frequency as they receive June 2013  Tim Pratt 2013

Fig. 9.13 Automatic Telegraph Repeater Station (~1860) Link Key Key Fig. 9.13 Automatic Telegraph Repeater Station (~1860) June 2013  Tim Pratt 2013

Fig. 9.17 Linear Repeater for 6 GHz LOS Link Image reject BPF 700 MHz BPF 700 MHz IF amplifier Mixer LNA 6 GHz First L.O. 5300 MHz Mixer Second L.O. 5200 MHz 6.1 GHz BPF LPA HPA 6.1 GHz Fig. 9.17 Linear Repeater for 6 GHz LOS Link June 2013  Tim Pratt 2013

Digital Repeaters Digital repeaters are also called regenerative repeaters Received signals are converted to bits Bits are remodulated onto transmitter Noise does not add up along chain of repeaters Bit errors add up So long as BER on any hop is not large, link is good Not many satellites use digital repeaters – mainly restricted to military and Internet access satellites June 2013  Tim Pratt 2013

Summary of Unit 3 In this unit you have learned 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 and the significance of link margin June 2013  Tim Pratt 2013