“Radio Frequency Communications Slides” – Dr. Tim Pratt

Report
REU
June 2013
Radio Frequency Communications
Tim Pratt Instructor
[email protected]
June 2013
 Tim Pratt 2013
1
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
2
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
3
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
June 2013
 Tim Pratt 2013
4
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
5
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
6
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
7
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
8
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
9
June 2013
 Tim Pratt 2013
10
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
11
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
12
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
13
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
14
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
June 2013
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
 Tim Pratt 2013
15
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
16
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
17
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
18
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
June 2013
 Tim Pratt 2013
19
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
20
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
21
Ionospheric layers
multipath
Tx
Rx
Earth
Fig. 9.2 HF Radio Communication
June 2013
 Tim Pratt 2013
22
Tx
Rx
Earth
Fig. 9.3 LOS Microwave Communications
June 2013
 Tim Pratt 2013
23
GEO satellite
Altitude 35,680 km
Tx
Rx
Earth
Fig. 9.4 Satellite Communications
June 2013
 Tim Pratt 2013
24
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
25
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
26
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
27
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
28
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
29
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
30
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
31
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
32
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
33
June 2013
 Tim Pratt 2013
34
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
35
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
36
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
37
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
38
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
39
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
40
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
41
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
42
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
43
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
44
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
45
Waveguide
(loss Lra)
Waveguide
(loss Lta)
Atmospheric loss
Tx
shelter
Rx
shelter
Reflection point
Fig. 9.10 LOS Link Losses
June 2013
 Tim Pratt 2013
46
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
47
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
48
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
49
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
50
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
51
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
June 2013
 Tim Pratt 2013
52
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
53
Link
Key
Key
Fig. 9.13 Automatic Telegraph Repeater Station
(~1860)
June 2013
 Tim Pratt 2013
54
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
June 2013
 Tim Pratt 2013
55
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
56
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
57

similar documents