1. Radio Frequency Communications
REU
June 2013
Radio Frequency Communications
Tim Pratt Instructor
June 2013
 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
June 2013
 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 = m)
June 2013
 Tim Pratt 2013

4. 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

5. EM Waves EM waves travel at the velocity of light c  3 x 108 m/s
Actual value is 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 = m = 15 cm
June 2013
 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 …
June 2013
 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
June 2013
 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
June 2013
 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
June 2013
 Tim Pratt 2013

10. June 2013
 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
June 2013
 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
June 2013
 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
June 2013
 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
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

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
June 2013
 Tim Pratt 2013

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

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

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

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

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

24. Fig. 9.4 Satellite Communications
GEO satellite
Altitude 35,680 km Tx
Earth Rx
Fig Satellite Communications
June 2013
 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 = km, link distance < 45 km d h
Clearance over buildings and trees
is needed – towers must be higher
June 2013
 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
June 2013
 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
June 2013
 Tim Pratt 2013

28. 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

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
June 2013
 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
June 2013
 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
June 2013
 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 %
June 2013
 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
June 2013
 Tim Pratt 2013

34. June 2013
 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
June 2013
 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
June 2013
 Tim Pratt 2013

37. 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 Flux density from an isotropic source
June 2013
 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
June 2013
 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)
June 2013
 Tim Pratt 2013

40. 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

41. 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

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

44. 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

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
June 2013
 Tim Pratt 2013

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

48. Link Budget for LOS link
The received power is tabulated using dB units
Example:
Pt = W dBW
Gt = dB
Gr = dB
Lp – dB
La dB
Lwg dB
Pr dBW
June 2013
 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 = x J/K = dBW / K / Hz
Ts is system noise temperature
BN is noise bandwidth of the receiver (IF filter)
June 2013
 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 = dBK
Let’s make BN = 36 MHz = dBHz
Then N = = dBW
Pr = dBW
CNR = Pr – N = = dB
This is the link margin above 0 dB CNR
June 2013
 Tim Pratt 2013

51. 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

52. 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 Simplified Superhet Receiver for LoS link
June 2013
 Tim Pratt 2013

53. 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

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

55. 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 Linear Repeater for 6 GHz LOS Link
June 2013
 Tim Pratt 2013

56. 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

57. 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