introduction - why modulation
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What enables wireless transmission?
Transmitter
(Electronic Equipment)
Sound waves
Microphone
time
current
frequency 4000 Hz
Audio signal
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What enables wireless transmission? (2)
• The information signal (e.g. audio) is in the form of a current (or voltage) waveform flowing through a conductor
• How do we get this information signal across a medium that employs NO conductors?
– Efficiently?
– Over reasonable distances?
• Answer: Electromagnetic waves!
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Electromagnetic spectrum
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Radio spectrum
VHF TV and FM broadcast Mobile radio
Millimeter waves (EHF)
Super high frequency
(SHF)
Ultra high frequency
(UHF)
Very high frequency
(VHF)
High frequency
(HF)
Medium frequency
(MF)
Low frequency
(LF)
Very low frequency
(VLF)
Audio band
Aeronautical Navigation
Radio teletype
AM broadcast
Business Amateur radio
International radio Citizen’s band
Mobile, aeronautical
UHF TV and mobile radio
Experimental
Navigation Satellite to Satellite
Microwave relay Earth-satellite
Radar Mobile Radio
Microwave Radio
Shortwave Radio
Longwave Radio
100 km
10 km
1 km
100 m
10 m
1 m
10 cm
1 cm
1 kHz
10 kHz
100 kHz
1 MHz
10 MHz
100 MHz
1 GHz
10 GHz
100 GHz
Ground wave propagation
Sky wave propagation
Line-of-sight (LOS) propagation
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Radio Spectrum (2) • Mode of propagation is dependent on frequency
– Ground wave – Sky wave (ionospheric propagation) – Line-of-sight propagation
• Propagation conditions are heavily dependent on frequency – Free space path loss (inverse square law) – Absorption – atmosphere (e.g. water vapour, atmosphere) – Absorption - vegetation – Reflection, refraction, diffraction – Environmental clutter
• Bandwidth
Fading due to multipath
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Spectrum allocation and assignment
• International Telecommunication Union – Radiocommunication sector (ITU-RS)
– Regional radio conference (RRC)
– World radio conference (WRC)
• Table of Frequency Allocations
• National spectrum management authorities
– Assign spectrum to service providers, network operators
– Police the use of the spectrum
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RF channels
• An example: Television broadcasting
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Modulation
• It is the process by which a signal is translated to another part of the spectrum
• As seen before, the actual spectrum location is dependent on – Nature of the application
– Propagation characteristics of that part of the spectrum
– Spectrum allocation/management authorities
– Antenna size
• Analog modulation techniques – Amplitude modulation
– Angle modulation (i.e. frequency modulation (FM), phase modulation (PM)
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Modulation • Information signal
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Modulation • Example: amplitude modulated signal
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Antennas • An alternating current in a conductor produces
– An alternating electric field
– An alternating magnetic field
Carr, Joseph J. 2001. Antenna Toolkit. 2nd ed. Newnes
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Antennas (2) • The antenna converts electrical energy into electromagnetic energy (
it is a transducer)
• Energy is radiated in all directions – Depending on the antenna design, more energy may be radiated in
some directions than in others
• For efficient radiation antennas should be at least 1/10 of the wavelength of the e.m. wave
• Baseband signals are low frequency (slow varying) signals. They will require very long (impractical) antennas
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Antennas (3)
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Antennas (5)
60 cm DirecTV
26 × 24 cm NetGear ANT24D18 ProSafe Wireless LAN Antenna
1.8 m Log Periodic Yagi Antenna,
http://www.zcg.com.au/log-periodic-yagi-base-station-antennas.htm
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Antennas (6)
http://www.raymaps.com/index.php/antennas-on-samsung-galaxy-s/
•2.6 GHz WiMAX Tx/Rx Antenna •2.6 GHz WiMAX Antenna Rx Only (as a diversity antenna) •WiFi/Bluetooth Tx/Rx Antenna •Cell/PCS CDMA/EVDO Tx/Rx Antenna •Cell/PCS CDMA/EVDO Rx Only (as a diversity antenna) •GPS Antenna Rx Only
Samsung Galaxy S
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Electromagnetic waves
mAcos
mVcos
max
max
xtHH
xtEE
z
y
z
y
p
H
E
fv1
22
T
Polarization (Vertical, Horizontal)
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Electromagnetic waves(2)
• Spatial domain • Time domain
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Propagation
http://www.nonstopsystems.com/radio/hellschreiber-performance.htm 18
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Propagation (2)
http://mysite.verizon.net/k3nco/prop.htm 19