reu june 2013

June 2013 Tim Pratt 2013 1

REUJune 2013

Radio Frequency CommunicationsTim Pratt Instructor

[email protected]


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

• 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)


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

H

z

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


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 …


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


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


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



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


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


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


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


• 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

Widely Used Radio Frequency Bands


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


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


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


100

A dB

10

1.0

0.1

3 10 100 GHzFig 9.1 Zenith Attenuation in Clear Air

50%RH

Dry air

50%RH

O2 resonance


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


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.


Earth

Ionospheric layers

Tx Rx

multipath

Fig. 9.2 HF Radio Communication


Earth

Tx Rx

Fig. 9.3 LOS Microwave Communications


Earth

Tx Rx

GEO satelliteAltitude 35,680 km

Fig. 9.4 Satellite Communications


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

h

d

Clearance over buildings and trees is needed – towers must be higher


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


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


Fig 9.6 Microwave Link Multipath

hReflection point

Tx RxLOS path

Inversion layer

multipath

multipath

Vertical scale is exaggerated. Grazing angle is << 1o


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


Fig. 9.7 Antenna Diversity in LOS Link

Reflection point

TxRx

LOS path

multipath

Vertical scale is exaggerated. Grazing angle is << 1o


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


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 %


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


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


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


Isotropic sourceEIRP = Pt W Area

A m2

Part of sphereradius Rsurface area As

R

Incident flux densityF W / m2

Fig. 9.8 Flux density from an isotropic source


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


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)


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)


SourceEIRP = Pt W

Incident fluxdensity F W/m2

Pr

Receiver

Fig 9.9 Radio Link

Receiving antennaArea Ae m2


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


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


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


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


Fig. 9.10 LOS Link Losses

Reflection point

Txshelter

Rxshelter

Atmospheric loss

Waveguide (loss Lta)

Waveguide(loss Lra)


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


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 dBLp – 148.0 dB

La -5.0 dB Lwg -12.0 dB Pr -90.0 dBW


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)


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

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



LNA BPF Mixer1 GHz

IF amp

23 GHz Local oscillator

BPF

24 GHzAntenna

PrGm GIF

D

Demodulator

Fig 9.12 Simplified Superhet Receiver for LoS link

Narrow band pass filteris last filter in IF stage

BPF


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


Fig. 9.13 Automatic Telegraph Repeater Station (~1860)

Key

Key

Link


LNA

Image rejectBPF Mixer

700 MHz BPF

700 MHz IF amplifier

First L.O.5300 MHz

Second L.O.5200 MHz

Mixer

6.1 GHz BPF

6 GHz

6.1 GHz LPA HPA

Fig. 9.17 Linear Repeater for 6 GHz LOS Link


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


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

reu june 2013

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