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Mw presentation 1
1. MW Basic Knowledge
• Microwave
Microwave is kind of electromagnetic wave. In a broad sense, the
microwave frequency range is from 300MHz to 300GHz. But in
microwave communication, the frequency range is generally from
3GHz to 30GHz.
According to the characteristics of microwave propagation,
microwave can be considered as plane wave.
The plane wave has no electric field and magnetic field longitudinal
components along the propagation direction. The electric field and
magnetic field components are vertical to the propagation direction.
Therefore, it is called transverse electromagnetic wave and TEM wave
for short
4. MW Basic Knowledge
Microwave Radio Link
Hop Hop Hop
NO.1 NO.2 No.n
Or
TX/Rx TX/Rx TX/Rx TX/Rx
Terminal Relay station Relay station Terminal
Station (Active) (Passive) Station
Cable Radio Link Cable
Distance between the transmitter and the receiver, a few km < D < 100km
Availability and quality depends on the distance recommended by ITU-R.
5. MW Basic Knowledge
Radio Wave Propagation （ 1 ）
The radio link propagation follows the line of sight: it requires a perfect clearing between
transmitting and receiving antennas. The propagation medium is made of the lower layers
of the atmosphere (a few meters to a few hundred of meters above ground)
The non homogeneity of the atmosphere influences the waves propagation:
1. Path curvature
2. Reflecting, diverging, focusing intermittent events
Free space loss (Lfs)=92.4+20×log(f×d) (dB)
f:GHz, d:km
6. MW Basic Knowledge
Radio Wave Propagation （ 2 ）
Atmosphere influence
Refraction Partial reflection Absorption Diffusion
Diffusion
Gaz and water vapor volume
n2
n2
i.2 Freq. < 15GHz : insignificant
20 GHz : 0.1 dB / km
Rain
i.1 n1 Freq. > 10GHz
n1
500km
n1×sin i.1 = n2×sin i.2
Almost horizontal
7. MW Basic Knowledge
Radio Wave Propagation
（ 3 influence
Ground ）
Diffraction
Reflection
Diffraction on a ridge
Spherical diffraction
8. MW Basic Knowledge
Radio Wave Propagation
（ 4）
Air refraction index at sea level: n = 1.000 315
N N = 315 N units
h
Standard atmosphere ∆ N = - 39 N km
h Ro K = R/Ro = 4/3 R
Real Earth Imaginary Earth
∆ N : Gradient of air refraction index
∆ N may reach more extreme values than + 250 or - 350, during short percentage of time
An imaginary earth with the radius of 8500km, allows to simplify analysis on
Refraction propagation of radio wave.
9. MW Basic Knowledge
• Microwave Transmission Characteristic
• —Loss Characteristic
The Rules of Loss changing:
A: Higher Frequency; Longer Distance; Worse Loss.
B: Day better than Night; Winter better than Summer;
C: Cloudy, Windy more stable than Sunny day
D: Over water is worse than Over land;
E: Over Hill/Mountain is better than Over Plain;
The reason is the reflection index of electric wave
is changing with different time, location and
height
10. MW Basic Knowledge
• Microwave Transmission Characteristic
When there is loss in the link, not only with worse SNR (Signal Noise
Ratio), but also the deep loss will break off the link; To get a better
performance and reliability in MW transmission, we need to consider
some methods to avoid the loss, such as Diversity.
Diversity has the following types: Space diversity, Frequency diversity,
Polarization diversity, Angle diversity and Multiply diversity.
11. MW Basic Knowledge
• Microwave Transmission Characteristic
—Overcome Loss
1. Space Diversity
Signals have different multi-path effect over different paths and thus have
different fading characteristics. Accordingly, two or more suites of antennas at
different altitude levels to receive the signals at the same frequency which are
composed or selected. This work mode is called space diversity. If there are n
pairs of antennas, it is called n-fold diversity.
Advantages: The frequency resources are saved;
Disadvantages: The equipments is complicated, as two or more suites of
antennas are required;
Antenna distance: As per experience, the distance between the diversity
antennas is 100 to 200 times the wavelength in the frequency used frequency
bands, and normally we choose 5/10 meters, also we can calculation by;
12. MW Basic Knowledge
• Microwave Transmission Characteristic
—Overcome Loss
1. Space Diversity
d= (100~200)λ= (100~200)ν/F
Where v=3*108
13. MW Basic Knowledge
• Microwave Transmission Characteristic
—Overcome Loss
2. Frequency Diversity
Signals at different frequencies have different fading characteristics.
Accordingly two and more microwave frequencies with certain frequency
spacing to transmit and receive the same information which is then selected or
composed, to reduce the influence of fading. This work mode called frequency
diversity
Advantages: The effort is obvious. Only one antenna is required;
Disadvantages: The utilization ratio of frequency bands is low;
14. MW Basic Knowledge
• Microwave Transmission Characteristic
—Overcome Loss
3. Polarization Diversity
When using polarization diversity, utilizing the same aerial on different
polarizations and scrambling polynomials have to be different for both signals
send the same signal.
4. Angle Diversity
The efficiency of angle diversity is based on the antennas uncorrelated radiation
pattern (resultant signals are different).
One antenna and two feeders
Two antennas (are located close to each other, space diversity effect is achieved if
antennas are far a way from each other) and the other antenna has been tilted
15. MW Basic Knowledge
• Microwave Transmission Characteristic
Advantage:
1. Can be rapidly installed;
2. Can use the existing network infrastructure repeatedly (digital radio uses the
infrastructure of the analogue radio)
3. Can cross complicated terrains (rivers, lakes and mountains)
4. Can use point-to-point radio transmission structure in the remote mountains
5. Can rapidly restore the communication after the natural disasters
6. Can protect hybrid multiple transmission media
Those advantages not only apply to the fixed nodes or temporary nodes
and feeder routes in the urban areas, but also apply to very long long-
distance routes.
16. MW Basic Knowledge
• Microwave Transmission Characteristic
Disadvantage:
1. Line of sight transmission conditions should be ensured;
2. Transmission distance between two stations should be not too long;
3. Frequencies need to be applied for;
4. Communication quality is greatly affected by the environment;
5. Communication capacity is limited.
21. Difficult Areas for Microwave Links
• In areas with lots of rain, use the lowest
frequency band allowed for the project.
• Microwave hops over or in the vicinity of the
large water surfaces and flat land areas can
cause severe multipath fading. Reflections may
be avoided by selecting sites that are shielded
from the reflected rays.
• Hot and humid coastal areas
21
Notas do Editor
PDH: Plesynchronous Digital Hierarchy SDH: Synchronous Digital Hierarchy
The non homogeneity of the atmosphere influences the waves propagation 1°) Path curvature ： 2°) Reflecting, diverging, focusing intermittent events ：
Sunshine worse than sunless: The received signal on cloudy and rainy days, foggy days, and windy days is more stable than the received signal on sunny days and quiet days. Fast fading always occurs when it clear up after rain stops or fog disperses
