Transcript COSC 393: Lecture 2

COSC 393: Lecture 2
Radio Fundamentals
Radio Communication
• Radio signals
• Spectrum
• Transmitter
• Signal propagation
• Modulation
Radio Wave
s(t) = At sin(2  ft t + t)
Frequency and Wave length
• Relationship:

 = c/f
• wave length ,
• speed of light c  3x108m/s,
• frequency f
Radio Spectrum
twisted
pair
coax cable
1 Mm
300 Hz
10 km
30 kHz
VLF
•
•
•
•
•
•
•
•
•
LF
optical transmission
100 m
3 MHz
MF
HF
1m
300 MHz
VHF
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
UHF
10 mm
30 GHz
SHF
EHF
100 m
3 THz
infrared
1 m
300 THz
visible light UV
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.
• Dipoles with lengths /4 or Hertzian dipole with
length /2 (length proportional to wavelength)
/4
/2
• Example: Radiation pattern of a simple Hertzian
dipole
y
y
x
side view (xy-plane)
z
z
side view (yz-plane)
x
simple
dipole
top view (xz-plane)
• Gain: maximum power in the direction of the main
lobe compared to the power of an isotropic
radiator (with the same average power)
• Often used for base stations in a cellular
system (e.g., covering a valley)
y
y
z
x
z
side view (xy-plane)
x
side view (yz-plane)
top view (xz-plane)
z
z
x
x
top view, 3 sector
directed
antenna
top view, 6 sector
sectorized
antenna
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
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?
Three means of propagation
• Ground wave
• Tropospheric wave
• Ionospheric or sky wave
Ground Wave
• travels in contact with earth’s 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
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)
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
4 possible events
Radio wave
shadowing
Radio wave
scattering
Radio wave
reflection
diffraction
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
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
Free Space Propagation
• Suppose we have unobstructed line-of-sight
Pr(d) = (Pt Gt Gr ^2)/(4)^2 d^2 L)
-Pt transmitted power
-Gt, Gr Antenna gain
- wavelength in meters
- d distance in meters
- L (>= 1) system loss factor (not related to
propagation.
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)
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)
Radio Signal Fading
Short term fading
Long term fading
T
Time
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
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) 2) e^[-(log m – E(m))^2/(2 s(m)^2)]
Delay Spread
• Signal follows different paths to reach same
destination.
• So same signal may arrive many times at
different time intervals.
t
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)
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. Shannon’s work suggest that signaling
scheme exists for error-free transmission if
the rate of transmission is lower than the
channel capacity.
Shannon’s 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)]
Shannon’s work - continued
• Solving for (E/N) (aka. signal to noise ratio)
(E/N) = (2^a –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.
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!
Empirical models to predict
propagation losses
• Okumura’s model
- based on free space path loss + correction
factors for suburban and rural areas, irregular
terrain, street orientations
• Sakagmi and Kuboi model
- extend Okumura’s model using regression
analysis of data.
• Hata’s model
- empirical formula to describe Okumura’s
data
More models
• Ibrahim and Parsons model
- equations developed to best fit data
observed at London. (freq. 168-900 MHz)
• Lee’s model
– Use at 900MHZ
– 3 parameters (median trasmission loss, slope of
the path loss curve and adjustment factor)
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 by2.2.1
heavy
rainfall etc.
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)
Types of Modulation
•
•
•
•
Amplitude modulation
Frequency modulation
Phase modulation
Combination modulation
digital
data
101101001
digital
modulation
analog
baseband
signal
analog
modulation
radio transmitter
radio
carrier
analog
demodulation
radio
carrier
analog
baseband
signal
synchronization
decision
digital
data
101101001
radio receiver
Amplitude Modulation
Frequency Modulation
Phase Modulation
Digital modulation
• Amplitude Shift Keying (ASK):
– simple
– low bandwidth
– susceptible to interference
1
0
1
t
1
0
1
• Frequency Shift Keying (FSK):
t
– somewhat larger bandwidth
1
• Phase Shift Keying (PSK):
– more complex (both ends)
– robust against interference
0
1
t