Lecture 4: Propagation
Models, Antennas and Link
Budget
Anders Västberg
vastberg@kth.se
08-790 44 55
Digital Communication
System
Source of
Information
Source
Encoder
Channel
Encoder
Digital
Modulator
Modulator
RF-Stage
Channel
Information
Sink
Source
Decoder
Channel
Decoder
Digital
Demodulator
Demodulator
RF-Stage
[Slimane]
Propagation between two
antennas (not to scale)
Sky Wave
Direct Wave
Ground Wave
Ground Reflected
Wave
No Ground Wave for Frequencies > ~2 MHz
No Ionospheric Wave for Frequencies > ~30 Mhz
The Radio Link
• Design considerations
– The distance over which the system meets
the performance objectives
– The capacity of the link.
• Performance determined by
– Frequency
– Transmitted Power
– Antennas
– Technology used
[Black et. al]
Performance of Radio
Systems
• Signal attenuation
– path loss
– multipath fading
• Additive noise
– Thermal noise
– Atmospheric noise
– Cosmic Noise
– Man-made noise
Noise
• Thermal noise – White Noise
• Spectral density: N 0  kT (W/Hz)
k Boltzmann’s constant (1.38 10-23 J/K)
T Absolute temperature (in Kelvins)
• Noise power (in W)
N  kTB
B
Bandwidth (Hz)
Signal to noise ratio
(SNR)
• Ratio between signal power and noise
power
𝑆
𝑆
𝑆𝑁𝑅 = =
𝑁 𝑘𝑇𝐵
𝐸𝑏 𝑆/𝑅
𝑆
=
=
𝑁0
𝑘𝑇
𝑘𝑇𝑅
𝑆𝑁𝑅
𝑘𝑇𝑅 𝑅
=
=
𝐸𝑏 /𝑁0 𝑘𝑇𝐵 𝐵
Link Budget
𝑃𝑡 ∙ 𝐺𝑡 ∙ 𝐺𝑟
𝑃𝑟 =
𝐿𝑏
Pt Gt
Lb
𝑃𝐸𝐼𝑅𝑃 = 𝑃𝑡 ∙ 𝐺𝑡
EIRP=Effective Isotropic Radiated Power
r
𝑃𝑟 𝑃𝑡 ∙ 𝐺𝑡 ∙ 𝐺𝑟 𝑃𝑡 ∙ 𝐺𝑡 ∙ 𝐺𝑟
SNR = =
=
𝑁
𝑁𝐿𝑏
𝑘𝑇0 𝐵𝐿𝑏
SNR 𝑑𝐵 = (𝑃𝑡 )𝑑𝐵 + (𝐺𝑡 )𝑑𝐵 − (𝐿𝑏 )𝑑𝐵 + (𝐺𝑟 )𝑑𝐵 − (𝑘𝑇0 𝐵)𝑑𝐵
Gr
Pr
Propagations Models
Free Space Model
Plane Earth Model
Power Law Model
𝐿𝐹𝑆
4𝜋 𝑟
=
𝜆
𝐿𝑃𝐸
𝑟4
=
ℎ1 ℎ2
𝑟𝑛
𝐿=
𝑘
2
2
4𝜋 𝑓𝑟
=
𝑐
2
Dipole antenna
Omnidirectional
L=l/2
• Half-wave dipole
– Gain 1,64 = 2.15 dBi
– Linear Polarisation
I
I
• Quarter-wave dipole
– Conducting plane below a
single quarter wave
antenna. Acts like a halfwave dipole
L=l/4
I
Yagi-antenna
Directional
3-30 element and a gain of 8-20 dBi
http://www.urel.feec.vutbr.cz/~raida/multimedia_en/chapter-4/4_3A.html
Parabolic antenna
Directional
• Effective area
Ae = hp d2/4
h0.56
[Stallings, 2005]
Corner Reflectors
• Multiple images
results in
increased gain
• Example:
G=12 dBi
Images
l/2
Driven
Element
Loop-antenna - Directional
• Linear
Polarisation
• Gain 1,76 dBi
http://www.ycars.org/EFRA/Module%20C/AntLoop.htm
Helical antenna
Directional
• Normal mode
• Axial mode
http://hastingswireless.homeip.net/index.php?page=antennas&type=helical
Microwave Communication
[Slimane]
Propagation in the
Atmosphere
• The atmosphere around the earth contains a lot
of gases (1044 molecules)
• It is most dense at the earth surface (90% of
molecules below a height of 20 km).
• It gets thinner as we reach higher and higher
attitudes.
• The refractive index of the air in the atmosphere
changes with the Height
• This affects the propagation of radio waves.
• The straight line propagation assumption may
not be valid especially for long distances.
[Slimane]
Effective Earth Radius
[Slimane]
Line-of-Sight Range
[Slimane]
Ray Paths and Wave
fronts
Fresnel Zone
[Slimane]
Ionospheric Communication
[Davies, 1993]
Indoor models
Slow and Fast Fading
Homework before F6
• Describe the following modulation
methods:
– AM, FSK, BPSK, QAM
• Order ASK, FSK, PSK and DPSK in order
of least efficient to most efficient.