无线通信网络第5章-天线与传播
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N 0kTW/Hz
N0 = noise power density in watts per 1 Hz of bandwidth
k = Boltzmann's constant = 1.3803 ´ 10-23 J/K T = temperature, in kelvins (absolute temperature)
Signal must maintain a level sufficiently higher than noise to be received without error
Attenuation is greater at higher frequencies, causing distortion
antenna)
Parabolic Reflective Antenna
Antenna Gain
Antenna gain
Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna)
Refraction – bending of microwaves by the atmosphere
Velocity of electromagnetic wave is a function of the density of the medium
When wave changes medium, speed changes Wave bends at the boundary between mediums
LOS Wireless Transmission Impairments
Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction Thermal noise
transmission media Cannot be eliminated Function of temperature Particularly significant for satellite
communication
Thermal Noise
Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is:
Noise Terminology
Intermodulation noise – occurs if signals with different frequencies share the same medium
Free Space Loss
Free space loss accounting for gain of other antennas
P P r t 4 G rG 2t d2 2A r d A t2fc 2A rd A 2t
Gt = gain of transmitting antenna Gr = gain of receiving antenna At = effective area of transmitting antenna Ar = effective area of receiving antenna
Attenuation
Strength of signal falls off with distance over transmission medium
Attenuation factors for unguided media:
Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal
Free Space Loss
Free space loss equation can be recast:
LdB 1l0oP P grt 2l0og 4 d
2 l o 0 2 l g d o 0 2 . 9 g d 1 8 B
2l0 o 4 fg d 2l0 o f g 2l0 o d g 1.5 4 d 6 7 B c
Line-of-Sight Equations
Optical line of sight
d3.57h
Effective, or radio, line of sight
d3.57h
d = distance between antenna and horizon (km) h = antenna height (m) K = adjustment factor to account for refraction,
Free Space Loss
Free space loss accounting for gain of other antennas can be recast as
L d 2 B l o 0 2 l g d o 0 1 lg A o t A 0 r g
2 l f o 0 2 l d g o 0 1 l A g t o A r 0 1 . 5 g d 6
Satellite communication – signal above 30 MHz not reflected by ionosphere
Ground communication – antennas within effective line of site due to refraction
Depicted as two-dimensional cross section
Beam width (or half-power beam width)
Measure of directivity of antenna
Reception pattern
Receiving antenna’s equivalent to radiation pattern
Introduction
An antenna is an electrical conductor or system of conductors
Transmission - radiates electromagnetic energy into space
Reception - collects electromagnetic energy from space
AM radio
Sky Wave Propagation
Sky Wave Propagation
Signal reflected from ionized layer of atmosphere back down to earth
Signal can travel a number of hops, back and forth between ionosphere and earth’s surface
Thermal Noise
Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz
(in watts):
NkTB
or, in decibel-watts
N 1 lk o 0 1 l g T o 0 1 l g o B 0g 2 . 6 d 2 B 1 8 lT o 0 W 1 l g B o 0
Free Space Loss
Free space loss, ideal isotropic antenna
P Prt 4d22 4cf2d2
Pt = signal power at transmitting antenna Pr = signal power at receiving antenna = carrier wavelength d = propagation distance between antennas c = speed of light (»3 ´ 10 8 m/s) where d and are in the same units (e.g., meters)
Types of Antennas
Isotropic antenna (idealized)
Radiates power equally in all directions
Dipole antennas
Half-wave dipole antenna (or Hertz antenna) Quarter-wave vertical antenna (or Marconi
Reflection effect caused by refraction Examples
Amateur radio CB radio
Line-of-Sight Propagation
Line-of-Sight Propagation
Transmitting and receiving antennas must be within line of sight
Categories of Noise
Thermal Noise Intermodulation noise Crosstalk Impulse Noise
Thermal Noise
Thermal noise due to agitation of electrons Present in all electronic devices and
In two-way communication, the same antenna can be used for transmission and reception
Radiation Patterns
Radiation pattern
Graphical representation of radiation properties of an antenna
Effective area
Related to physical size and shape of antenna
Antenna Gain
Relationship between antenna gain and effective area
G4A 2e 4cf22Ae
G = antenna gain Ae = effective area f = carrier frequency c = speed of light (»3 ´ 108 m/s) = carrier wavelength
Propagation Modes
Ground-wave propagation Sky-wave propagation Line-of-sight propagation
Ground Wave Propagation
Grouຫໍສະໝຸດ Baidud Wave Propagation
Follows contour of the earth Can Propagate considerable distances Frequencies up to 2 MHz Example
rule of thumb K = 4/3
Line-of-Sight Equations
Maximum distance between two antennas for LOS propagation:
3 .5 7 h 1 h 2
h1 = height of antenna one h2 = height of antenna two
N0 = noise power density in watts per 1 Hz of bandwidth
k = Boltzmann's constant = 1.3803 ´ 10-23 J/K T = temperature, in kelvins (absolute temperature)
Signal must maintain a level sufficiently higher than noise to be received without error
Attenuation is greater at higher frequencies, causing distortion
antenna)
Parabolic Reflective Antenna
Antenna Gain
Antenna gain
Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna)
Refraction – bending of microwaves by the atmosphere
Velocity of electromagnetic wave is a function of the density of the medium
When wave changes medium, speed changes Wave bends at the boundary between mediums
LOS Wireless Transmission Impairments
Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction Thermal noise
transmission media Cannot be eliminated Function of temperature Particularly significant for satellite
communication
Thermal Noise
Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is:
Noise Terminology
Intermodulation noise – occurs if signals with different frequencies share the same medium
Free Space Loss
Free space loss accounting for gain of other antennas
P P r t 4 G rG 2t d2 2A r d A t2fc 2A rd A 2t
Gt = gain of transmitting antenna Gr = gain of receiving antenna At = effective area of transmitting antenna Ar = effective area of receiving antenna
Attenuation
Strength of signal falls off with distance over transmission medium
Attenuation factors for unguided media:
Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal
Free Space Loss
Free space loss equation can be recast:
LdB 1l0oP P grt 2l0og 4 d
2 l o 0 2 l g d o 0 2 . 9 g d 1 8 B
2l0 o 4 fg d 2l0 o f g 2l0 o d g 1.5 4 d 6 7 B c
Line-of-Sight Equations
Optical line of sight
d3.57h
Effective, or radio, line of sight
d3.57h
d = distance between antenna and horizon (km) h = antenna height (m) K = adjustment factor to account for refraction,
Free Space Loss
Free space loss accounting for gain of other antennas can be recast as
L d 2 B l o 0 2 l g d o 0 1 lg A o t A 0 r g
2 l f o 0 2 l d g o 0 1 l A g t o A r 0 1 . 5 g d 6
Satellite communication – signal above 30 MHz not reflected by ionosphere
Ground communication – antennas within effective line of site due to refraction
Depicted as two-dimensional cross section
Beam width (or half-power beam width)
Measure of directivity of antenna
Reception pattern
Receiving antenna’s equivalent to radiation pattern
Introduction
An antenna is an electrical conductor or system of conductors
Transmission - radiates electromagnetic energy into space
Reception - collects electromagnetic energy from space
AM radio
Sky Wave Propagation
Sky Wave Propagation
Signal reflected from ionized layer of atmosphere back down to earth
Signal can travel a number of hops, back and forth between ionosphere and earth’s surface
Thermal Noise
Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz
(in watts):
NkTB
or, in decibel-watts
N 1 lk o 0 1 l g T o 0 1 l g o B 0g 2 . 6 d 2 B 1 8 lT o 0 W 1 l g B o 0
Free Space Loss
Free space loss, ideal isotropic antenna
P Prt 4d22 4cf2d2
Pt = signal power at transmitting antenna Pr = signal power at receiving antenna = carrier wavelength d = propagation distance between antennas c = speed of light (»3 ´ 10 8 m/s) where d and are in the same units (e.g., meters)
Types of Antennas
Isotropic antenna (idealized)
Radiates power equally in all directions
Dipole antennas
Half-wave dipole antenna (or Hertz antenna) Quarter-wave vertical antenna (or Marconi
Reflection effect caused by refraction Examples
Amateur radio CB radio
Line-of-Sight Propagation
Line-of-Sight Propagation
Transmitting and receiving antennas must be within line of sight
Categories of Noise
Thermal Noise Intermodulation noise Crosstalk Impulse Noise
Thermal Noise
Thermal noise due to agitation of electrons Present in all electronic devices and
In two-way communication, the same antenna can be used for transmission and reception
Radiation Patterns
Radiation pattern
Graphical representation of radiation properties of an antenna
Effective area
Related to physical size and shape of antenna
Antenna Gain
Relationship between antenna gain and effective area
G4A 2e 4cf22Ae
G = antenna gain Ae = effective area f = carrier frequency c = speed of light (»3 ´ 108 m/s) = carrier wavelength
Propagation Modes
Ground-wave propagation Sky-wave propagation Line-of-sight propagation
Ground Wave Propagation
Grouຫໍສະໝຸດ Baidud Wave Propagation
Follows contour of the earth Can Propagate considerable distances Frequencies up to 2 MHz Example
rule of thumb K = 4/3
Line-of-Sight Equations
Maximum distance between two antennas for LOS propagation:
3 .5 7 h 1 h 2
h1 = height of antenna one h2 = height of antenna two