REU
June 2013
Radio Frequency Communications
Tim Pratt Instructor
tipratt@vt.edu
June 2013
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Topics
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Radio waves
Frequency bands
Atmospheric effects
Link equation
CNR ratio on radio links
Analyzing radio links with link budgets
Designing radio links
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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)
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EM Waves
• EM waves have electric fields and magnetic fields
E
z
H
• 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
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EM Waves
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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
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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 …
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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
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Polarization and Antennas
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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
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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
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Unit 2 Radio Frequencies and
Propagation
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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
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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
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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
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Widely Used Radio Frequency Bands
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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
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Widely Used Radio Frequency Bands
• 2.6 – 3.4 GHz
• 3.5 – 4.5 GHz
• 5.7 – 6.4 GHz
• 6.4 – 6.7 GHz
• 7 – 8 GHz
• 9.5 – 9.9 GHz
• 10.0 – 12.2 GHz
• 12.2 – 12.7 GHz
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S-band radar
Satellite communications downlinks
Satellite communications uplinks
C-band radars
Military satellite communications
X-band radars, airborne, ship radar
Satellite downlinks
Satellite TV broadcasting
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RF Frequency Band Names
• Above 1 GHz:
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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
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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
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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
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A dB
O2
resonance
100
10
50%RH
50%RH
1.0
0.1
Dry air
3
10
100
GHz
Fig 9.1 Zenith Attenuation in Clear Air
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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
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The Earth is Curved
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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.
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Ionospheric layers
multipath
Tx
Rx
Earth
Fig. 9.2 HF Radio Communication
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Tx
Rx
Earth
Fig. 9.3 LOS Microwave Communications
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GEO satellite
Altitude 35,680 km
Tx
Rx
Earth
Fig. 9.4 Satellite Communications
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Fig. 9.5 Horizon Distance
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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
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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
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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
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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
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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
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Fig. 9.7 Antenna Diversity in LOS Link
LOS path
Tx
Rx
multipath
Reflection point
Vertical scale is exaggerated. Grazing angle is << 1o
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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
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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 %
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Summary of Unit 2
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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
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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
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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
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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
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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
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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
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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)
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Received Power
• We can combine gain and transmitted power:
EIRP = Pt Gt watts
(Eqn 9.2)
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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)
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Incident flux
density F W/m2
Source
EIRP = Pt W
Receiver
Pr
Receiving antenna
Area Ae m2
Fig 9.9 Radio Link
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Received Power
• From antenna theory, the gain of an antenna is
related to its effective aperture by
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G = 4  Ae / 2
(Eqn 9.5)
• Hence
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Ae = Gr 2 / 4 
• Received power is Pr
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Pr = F x Ae = Pt Gt Gr 2 / [ 4  R ]2 watts
(Eqn. 9.6)
• This is the basic link equation
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Path Loss
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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
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Received Power
• Additional losses must be included in the
Link Equation:
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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
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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
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Waveguide
(loss Lra)
Waveguide
(loss Lta)
Atmospheric loss
Tx
shelter
Rx
shelter
Reflection point
Fig. 9.10 LOS Link Losses
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Link Budget for line of Sight (LOS) link
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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
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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
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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
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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)
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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
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Pr = -90.0 dBW
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CNR = Pr – N = -90.0 + 123.0 = 33.0 dB
• This is the link margin above 0 dB CNR
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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
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Narrow band pass filter
is last filter in IF stage
24 GHz
Antenna
LNA
BPF
Mixer
1 GHz
BPF IF amp BPF
D
Pr
Gm
23 GHz Local
oscillator
GIF
Demodulator
Fig 9.12 Simplified Superhet Receiver for LoS link
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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
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Link
Key
Key
Fig. 9.13 Automatic Telegraph Repeater Station
(~1860)
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LNA
Image reject
BPF
Mixer
700 MHz
BPF
700 MHz
IF amplifier
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
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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
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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
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