Advanced Topics in NextGeneration Wireless
Networks
Wireless Radio
Qian Zhang
Department of Computer Science
HKUST
Characteristics of Wireless Medium
• Comparison to wired media
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–
–
–
–
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Unreliable
Low bandwidth
Untethered: supports mobility
Broadcast nature
Shared medium
Capacity limitation
• Frequency of operation and legality of access
differentiates a variety of alternatives for
wireless networking
Frequencies for Communication
The Radio Spectrum
(300Hz – 300GHz)
VLF = Very Low Frequency
LF = Low Frequency
MF = Medium Frequency
HF = High Frequency
VHF = Very High Frequency
UHF = Ultra High Frequency
SHF = Super High Frequency
EHF = Extra High Frequency
UV = Ultraviolet Light
Frequencies for Mobile
Communication
• VHF-/UHF-ranges for mobile radio
– Simple, small antenna for cars
– Deterministic propagation characteristics, reliable connections
• SHF and higher for directed radio links, satellite
communication
– Small antenna, focusing
– Large bandwidth available
• Wireless LANs use frequencies in UHF to SHF
spectrum
– Some systems planned up to EHF
– Limitations due to absorption by water and oxygen molecules
(resonance frequencies)
• Weather dependent fading, signal loss caused by heavy
rainfall etc.
Licensed and Unlicensed Bands
• Licensed
– Cellular/PCS
– Expensive (PCS bands in US were sold for around $20B)
– Time consuming to deploy new applications rapidly at low
costs
• Unlicensed
– Industrial, Medical, and Scientific (ISM) Bands
– Free, component costs are also low
– New applications such as WLAN, Bluetooth are easily
developed
• With the increase in frequency and data rate, the
hardware cost increases, and the ability to penetrate
walls also decreases
Bandwidth Allocation
• In the U.S., the FCC is responsible for allocating radio
frequencies
• Why allocate the radio spectrum?
– Prevent interference between different devices
– It would be unfortunate if the local TV station interfered with
police radio
• Generally, any transmitter is limited to a certain bandwidth
– E.g., a single 802.11 channel is 30MHz “wide”
• FCC also regulates the power and placement of transmitters
– Consumer devices generally limited to transmitting < 1W of
power
– Can’t have two TV stations on channel 5 next to each other
ISM Band
• ISM: Industrial, Scientific, and Medical
2450 ± 50MHz
• Interference in unlicensed
bands
Radio Propagation
• Three most important radio propagation
characteristics used in the design,
analysis, and installation of wireless
networks are:
– Achievable signal coverage
– Maximum data rate that can be supported by
the channel
– Rate of fluctuations in the channel
Signal
• Signal - physical representation of data
– Function of time and location
– Signal parameters to represent the value of data
• Frequency, amplitude, phase shift
• Noise
– Thermal noise
– Other transmission (e.g., microwaves, cordless phones)
• SNR – Signal to Noise Ratio
– Needs to be high enough for a receiver to correctly
receive the information
Signal Propagation Ranges
• Transmission range
– Communication possible
– Low error rate
• Detection range
– Detection of the signal
possible
– No communication possible
• Interference range
– Signal may not be detected
– Signal adds to the
background noise
sender
transmission
detection
interference
distance
Radio Environment
Shadowing
Path Loss
(clear, unobstructed LOS path)
Multi-path Fading
A. Path Loss Model
• Different, often complicated, models are used for different
environments
• A simple model for path loss, L, is
where Pr is the local mean received signal power
Pt is the transmitted power
d is the transmitter-receiver distance,
f is frequency
K is a transmission constant
(transmitter and receiver antennae gain)
The path loss exponent a = 2 in free space
B. Shadow Fading
• Received signal is shadowed by obstructions
– Such as hills and buildings
• The received signal strength for the same
distance from the transmitter will be different
– Depending on the environment and the surroundings,
and the location of objects
– This variation of signal strength due to location is
referred to as shadow fading
• This results in variations in the local mean
received signal power
C. Multipath Propagation
• Signal can take many different paths between sender
and receiver due to reflection, scattering, diffraction
Signal at Sender
Signal at Receiver
Reflection
Scattering
Diffraction
(surface of
the earth,
building,
wall)
(foliage,
street sign,
lamp posts,
etc.)
(shape
edge,
towers,
peak)
Multipath Fading
• Multipath fading
– Fluctuations of the signal amplitude because of
the addition of signals arriving in different
phases (paths)
• Multipath fading results in high BER
– Can be mitigated by FEC, diversity schemes,
and using directional antennae
Effects of Mobility
• Channel characteristics change over time and
location
• Radio propagation is very complex
– Multipath scattering from nearby objects
– Shadowing from dominant objects
– Attenuation effects
• Results in rapid fluctuations of received power
Less variation the slower
you move
What is an antenna?
• Is an electrical conductor used either for radiating
or collecting electromagnetic (EM) energy
– Antennas generally designed for a certain range of
frequencies
– Lots of types...
Antenna Technology
• Omnidirectional antenna
– Having an essentially nondirectional pattern in a given
plane
• Directional antenna
– Having the property of radiating or receiving EM energy
more effectively in some directions other than others
• Smart antenna
– An array of antenna elements connected to DSP
– Pros: enhance wireless link capacity using antenna
diversity and interference suppression
– Cons: more expensive and standardization takes time