COSC 393: Lecture 2

1
COSC 393 Lecture 2

• Radio Fundamentals

2
Radio Communication

• Radio signals
• Spectrum
• Transmitter
• Signal propagation
• Modulation

3
Radio Wave

• s(t) At sin(2 ? ft t ?t)

4
Frequency and Wave length

• Relationship
• ? c/f
• wave length ?,
• speed of light c ? 3x108m/s,
• frequency f

5
Radio Spectrum
6
coax cable
twisted pair
optical transmission
1 Mm 300 Hz
10 km 30 kHz
100 m 3 MHz
1 m 300 MHz
10 mm 30 GHz
100 ?m 3 THz
1 ?m 300 THz
VLF
LF
MF
HF
VHF
UHF
SHF
EHF
infrared
UV
visible light

• 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

7
Antennas

• Isotropic radiator Equal radiation in all
  directions (3D) - theoretical antenna
• Real antennas always have directive effects
  (vertically and/or horizontally)
• Different antennas have different radiation
  pattern.

8

• Dipoles with lengths ?/4 or Hertzian dipole with
  length ?/2 (length proportional to wavelength)
• Example Radiation pattern of a simple Hertzian
  dipole
• Gain maximum power in the direction of the main
  lobe compared to the power of an isotropic
  radiator (with the same average power)

?/4
y
y
z
simple dipole
x
z
x
side view (xy-plane)
side view (yz-plane)
top view (xz-plane)
9

• Often used for base stations in a cellular system
  (e.g., covering a valley)

y
y
z
directed antenna
x
z
x
side view (xy-plane)
side view (yz-plane)
top view (xz-plane)
z
z
sectorized antenna
x
x
top view, 3 sector
top view, 6 sector
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Effect of a transmission

• 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
distance
detection
interference
No effect
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Signal propagation property

• Radio signal behaves like light in free space
  (straight line)
• Receiving power proportional to 1/d² (d
  distance between sender and receiver)
• So ideally, the transmitter and a receiver must
  see each other!
• Really?

12
Three means of propagation

• Ground wave
• Tropospheric wave
• Ionospheric or sky wave

13
Ground Wave

• travels in contact with earths surface
• reflection, refraction and scattering by objects
  on the ground
• transmitter and receiver need NOT see each other
• affects all frequencies
• at VHF or higher, provides more reliable
  propagation means
• signal dies off rapidly as distance increases

14
Tropospheric Wave

• bending(refraction) of wave in the lower
  atmosphere
• VHF communication possible over a long distance
• bending increases with frequency so higher
  frequency more chance of propagation
• More of an annoyance for VHF or UHF (cellular)

15
Ionospheric or Sky Wave

• Reflected back to earth by ionospheric layer of
  the earth atmosphere
• By repeated reflection, communication can be
  established over 1000s of miles
• Mainly at frequencies below 30MHz
• More effective at times of high sunspot activity

16
4 possible events
Radio wave
Radio wave
scattering
shadowing
Radio wave
reflection
diffraction
17
Multipath Characteristics

• A signal may arrive at a receiver
• - many different times
• - many different directions
• - due to vector addition
• . Reinforce
• . Cancel
• - signal strength differs from place to place

18
Mobile System

• Usually Base Station is not mobile
• Receiver could be moving (65mph!)
• Whenever relative motion exists
• - Doppler shift
• - Fading
• Even the motion of scatterers cause fading

19
Free Space Propagation

• Suppose we have unobstructed line-of-sight
• Pr(d) (Pt Gt Gr l2)/(4p)2 d2 L)
• -Pt transmitted power
• -Gt, Gr Antenna gain
• -l wavelength in meters
• - d distance in meters
• - L (gt 1) system loss factor (not related to
  propagation.

20
Propagation Losses

• Two major components
• - Long term fading m(t)
• - Short term fading r(t)
• Received signal s(t)
• s(t) m(t) r(t)

21
dB - decibel

• Decibel, a logarithmic unit of intensity used to
  indicated power lost or gained between two
  signals.  Named after Alexander Graham Bell.
• 10 log (P1/P2)

22
Radio Signal Fading
Short term fading

• Signal strength (dB)

Long term fading
T
Time
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Short term fading

• Also known as fast fading caused by local multi
  paths.
• Observed over distance ½ wave length
• 30mph will experience several fast fades in a
  sec.
• Given by Rayleigh Distribution
• This is nothing but the square root of sum of the
  square of two Gaussian functions.
• r square root ( Ac Ac As As)
• Ac and As are two amplitude components of the
  field intensity of the signal

24
Long term fading

• Long term variation in mean signal level is also
  known as slow fading
• Caused by movement over large distances.
• The probability density function is given by a
  log-normal distribution
• - normal distribution on a log scale
• P(m) (1/m s(m) 2p) e-(log m E(m))2/(2
  s(m)2)

25
Delay Spread

• Signal follows different paths to reach same
  destination.
• So same signal may arrive many times at different
  time intervals.

t
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Delay Spread

• In digital system, delay spread causes
  intersymbol interference.
• Therefore, there is a limit on the maximum symbol
  rate of a digital multipath channel.
• Obviously, delay spreads are different in
  different environment.
• (roughly between 0.2 to 3 microseconds)

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Capacity of Channel

• What is the maximum transmission rate so that the
  channel has very high reliability?
• - error free capacity of a channel
• C.E. Shannons work suggest that signaling scheme
  exists for error-free transmission if the rate
  of transmission is lower than the channel
  capacity.

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Shannons work

• C - channel capacity (bits/s)
• B transmission bandwidth (Hz)
• E energy per bit of received signal (Joule)
• R information rate (bits/s)
• S E R signal power
• N single-sided noise power spectral density
  (W/Hz)
• (C/B) log 1(S/(NB)) log 1(E/N)(R/B)
• Suppose R C we have
• (C/B) log 1(E/N)(C/B)

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Shannons work - continued

• Solving for (E/N) (aka. signal to noise ratio)
• (E/N) (2a 1)/a
• where a (C/B).
• So given C 19.2kb/s and bandwidth 30kHz
• What is E/N required for error-free transmission?
• R/B 19.2/30 0.64
• Substituting we get E/N 0.8724 -0.593dB
• So control transmission power to obtain this E/N.

30
Propagation models in built-up areas

• Propagation is strongly influenced by the
  environment
• - building characteristics
• - vegetation density
• - terrain variation
• Perfect conductors reflect the wave where as
  nonconductors absorb some energy!

31
Empirical models to predict propagation losses

• Okumuras model
• - based on free space path loss correction
  factors for suburban and rural areas, irregular
  terrain, street orientations
• Sakagmi and Kuboi model
• - extend Okumuras model using regression
  analysis of data.
• Hatas model
• - empirical formula to describe Okumuras data

32
More models

• Ibrahim and Parsons model
• - equations developed to best fit data observed
  at London. (freq. 168-900 MHz)
• Lees model
• Use at 900MHZ
• 3 parameters (median trasmission loss, slope of
  the path loss curve and adjustment factor)

33
Freq. for mobile communication

• VHF-/UHF-ranges for mobile radio
• simple, small antenna
• SHF and higher for directed radio links,
  satellite communication
• small antenna, focusing
• large bandwidth available
• Wireless LANs use frequencies in UHF to SHF
  spectrum
• limitations due to absorption by water and oxygen
  
• weather dependent fading, signal loss due to by
  heavy rainfall etc.

2.2.1
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Modulation

• Digital modulation
• digital data is translated into an analog signal
• ASK, FSK, PSK ( Shift Keying)
• differences in spectral efficiency, power
  efficiency, robustness
• Analog modulation
• shifts center frequency of baseband signal up to
  the radio carrier
• Motivation
• smaller antennas (e.g., ?/4)

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Types of Modulation

• Amplitude modulation
• Frequency modulation
• Phase modulation
• Combination modulation

36
analog baseband signal
digital data
digital modulation
analog modulation
radio transmitter
101101001
radio carrier
analog baseband signal
digital data
synchronization decision
analog demodulation
radio receiver
101101001
radio carrier
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Amplitude Modulation
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Frequency Modulation
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Phase Modulation
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Digital modulation
1
0
1

• Amplitude Shift Keying (ASK)
• simple
• low bandwidth
• susceptible to interference
• Frequency Shift Keying (FSK)
• somewhat larger bandwidth
• Phase Shift Keying (PSK)
• more complex (both ends)
• robust against interference

t
1
0
1
t
1
0
1
t