MICROWAVE COMMUNICATIONS
SYSTEMS
-
2.
-
radiowaves whose spectrum is considered to start
at about 1 GHz and extending up to 30 GHz
also known as Line-of-Sight Communications
CLASSIFICATIONS OF MICROWAVE
COMMUNICATIONS SYSTEMS
1.
EFFECTS OF REFRACTION ON MICROWAVE
SIGNAL
-
EFFECTIVE EARTH’S RADIUS FACTOR (K)
Terrestrial Microwave Communications
- earth station to another earth station
Satellite Communications
- earth station to space station
-
MICROWAVE FREQUENCY BANDS
Wave
Region
Millimeter
Microwave
Region
(30 cm to 8 mm)
Region
Band
Designation
L
S
C
X
Ku
K
Ka
U
V
W
Mm
Frequency (GHz)
1-2
2-4
4-8
8-12
12-18
18-27
27-40
40-60
60-80
80-110
110-300
2.
3.
Line-of-Sight Path
- the straight path between the transmitting and
receiving antenna unobstructed by the horizon
Grazing Path
- the microwave beam barely touches the obstruction
Obstructed Path
- the microwave beam is hindered by an obstruction
the ratio of fictitious earth’s radius to the
actual radius of the earth
a numerical figure that considers the nonideal condition of the atmosphere resulting to
atmospheric refraction that causes the
microwave beam to bend toward the earth or
away from the earth
K
EffectiveE arth' sRadius re

TrueEarth' sRadius
ro
re ( km) 
ro
1  0.04665e ( 0.005577 N s )
where:
Ns=surface refractivity
ro=true earth radus (6370 km)
N s  N o xe 0.1057 hs
TYPES OF MICROWAVE PATH
1.
wave changes velocity as it travels from one
medium of certain density to another medium
of different density
in a normal atmosphere, the density decreases
with height (density gradient)
where:
No=sea level refractivity
hs=height of potential site in km
Sample Problem:
Determine the surface refractivity for a potential
microwave site 250 m above sea level with a sea level
refractivity of 312 and also calculate the effective earth radius.
CHARACTERISTICS OF MICROWAVE
ATMOSPHERIC CONDITIONS
1.
2.
3.
4.
5.
microwaves behave similarly as light waves
- it travels with the velocity of light
it can be reflected, refracted and diffracted
it can be absorbed by some of the particles in the
atmosphere
- due to the presence of molecules in the O2
layer
- due to the presence of uncondensed water
vapor
- due to rain ( above 10 GHz )
it can be scattered
- produces free space loss
it can be polarized
1.
2.
3.
Sub-standard atmosphere (K<1)
- the microwave beam is bent away from the
surface of the earth
- produces a phenomenon known as “Earth
Bulging Effect”
Standard atmosphere (K=4/3)
- the microwave beam is slightly bent towards
the surface of the earth
- normal atmospheric condition
Super-standard atmosphere (K>4/3)
- the microwave beam is bent towards the
surface of the earth
- produces a phenomenon known as ‘Earth
Flattening Effect”
4.
5.
Homogenous atmosphere (K=1)
- no refractive effect on the microwave signal
- no density gradient
Infinity condition (K=)
- the microwave beam tend to follow the
curvature of the earth
- results to zero curvature or “Flat Earth
Condition”
K = infinity
K = 4/3
K=1
K = 1/2
true earth
Site A
Site B
EARTH BULGE
-
change in vertical height of the earth’s
surface from a horizontal reference line with
respect to the distance
eb
d1
Site A
d2
data point
eb ( m) 
Site B
d1( k m) d 2( k m)
12.75K
where:
eb=earth bulge in meters
d1=distance from site 1 in km
d2=distance from site 2 in km
K=effective earth’s radius factor
Sample Problems:
1. Calculate the earth bulge 8 km, 16 km, and 29 km
away from a transmitter for a 32 km terrestrial
microwave link,
2. Calculate the effective height of a 100 ft obstruction
situated 10 mi from the receiving end of a 25 mi radio
link for the following values of K; (a) 4/3 (b) ½ (c) 5/2.
3. Calculate the value of K-factor that will, as if
effectively give an earth bulge of 200 ft for a 25 mi
radio link system.
4. A microwave station has a transmitting antenna
located 50 m above average terrain. Considering lineof-sight propagation, how far away could the signal be
received if the receiving antenna is 12 m above the
ground?
5. A boat is equipped with a line-of-sight communication
system which it uses to contact nearby boats and shore
stations. If the antenna on the boat is 2.3 m above the
water, calculate the maximum distance for
communication with:
a. another similar boat
b. a shore station with an antenna on a tower
22 m above the water level
c. another boat, but using the shore station as
a repeater
6. A taxi company uses a central dispatcher, with an
antenna at the top of a 15 meter tower, to communicate
with taxicabs. The taxi antennas are on the roof of the
cars, approximately 1.5 meters above the ground.
Calculate the maximum communication distance:
a. between the dispatcher and a taxi
b. between two taxis
7. An FM broadcast station has a transmitting antenna
located 50 m above the average terrain. How far away
could the signal be received:
a. by a car radio with an antenna 1.5 m above
the ground
b. by a rooftop antenna 12 m above the ground
8. Suppose that the transmitter and the receiver towers
have equal height. How high would they have to be to
communicate over a distance of 40 km?
FRESNEL ZONE
-series of concentric ellipsoids surrounding the lineof-sight path.
LINE-OF-SIGHT PROPAGATION
The maximum distance between two stations depends
on the height of the transmitting antennas as well as on the
nature of the terrain between them.
Fresnel Zone
𝑑 = √17ℎ𝑡 +√17ℎ𝑟
Site A
where:
d = maximum distance in kilometers
ht = height of the transmitting antenna in meters
hr = height of the receiving antenna in meters
Site B
First Fresnel Zone
- a surface containing every point for which the
sum of the distances from the point on the
surface to both ends of the path is exactly ½
wavelength longer than the direct path
F1  17.3
d1( km) d 2( km)
f (GHz ) D( km)
meters
where:
F1=First Fresnel Zone radius in m
d1=distance from site 1 in km
d2=distance from site 2 in km
f=operating frequency in GHz
D=path length in km
Higher Fresnel Zones
Second Fresnel Zone - a surface containing every
point for which the sum of the distances from the point on the
surface to both ends of the path is exactly 1 wavelength longer
than the direct path
Third Fresnel Zone - a surface containing every point
for which the sum of the distances from the point on the surface
to both ends of the path is exactly 1½ wavelengths longer than
the direct path
Fourth Fresnel Zone - a surface containing every point
for which the sum of the distances from the point on the surface
to both ends of the path is exactly 2 wavelengths longer than the
direct path
Fn  F1 n
where:
n= nth Fresnel zone
Fresnel Clearance
-
-
the amount of additional clearance that must
be allowed to avoid the degrading effects of
diffraction
it is the clearance or gap from the center of
the beam to the tip of the considered
obstruction
1st Fresnel Zone
60% of 1st Fresnel Zone
Site A
60 of the 1st Fresnel Radius
Site B
- a situation where there is no net change in attenuation
or “no gain, no loss” condition occurs when 60% of the 1 st
Fresnel radius clears a path obstruction
Fresnel Ratio
F
H
F1
where:
F = Fresnel ratio
H = Fresnel clearance
for line-of-sight condition
F0.6 (no gain, no loss)
Sample Problems
1. If the first Fresnel zone radius was computed to be
equal to 25 meters, what should be the additional
clearance, in meters, over an obstacle in a microwave
radio path to eliminate the degrading effects of
diffraction?
2. Solve for the total height in feet for an obstacle situated
27-mi away from a 35-mi microwave system.
Assuming if tree growth exists, add 40 ft for the trees
and 10 ft for additional growth. Use an operating
frequency of 6 GHz and a Fresnel Ratio of 0.6.
3. Calculate the 5th Fresnel zone radius to clear a 35 mi
radio link operating at 12 GHz if the 1 st Fresnel zone
radius is 61.57 ft.
4. What is the first Fresnel zone radius of a 40-km
microwave link operating at 10 GHz?
5. A system is 50 km in distance from Site A to Site B.
An obstruction is sighted 20 km away from Site A. The
obstruction was assumed to have a tree growth of 15
m. Other data is given below:
Site A elevation
100 m
Site A antenna height
20 m
Site B elevation
90 m
Site B antenna height
20 m
obstruction elevation
40 m
effective earth’s radius factor
1
Solve for the Fresnel clearance
6. A line-of-sight radio link operating at a frequency of 6
GHz has a separation of 40 km between antennas. A
obstacle in the path is located 10 km from the
transmitting antenna. By how much must the beam
clear the obstacle?
7. If the first Fresnel zone radius was computed to be
equal to 25 meters, what should be the additional
clearance, in meters, over an obstacle in a microwave
radio path to eliminate the degrading effects of
diffraction?
8. A microwave link between site A, 23 m ASL, and site
B, 45 mASL, uses an 10-meter tall flat billboard type
metal reflector, 80 m ASL, located 24 km away from
site A and 18 km away from site B. A possible
obstruction between site A and the billboard was
sighted 7.5 km away from site A and has an elevation
of 40 m ASL while another obstruction was sighted
between the billboard and site B which is 11 km away
from the billboard and has an elevation of 53 m ASL.
Considering a 15 m tree growth for all obstructions
and a Fresnel ratio of 0.6 in a homogenous
atmosphere, find the height of the antennas on both
sites if the system is operating at 8 GHz.
9. A microwave communication link between site A, 20
m ASL, and site B, 50 m ASL, uses a 15 meter tall
back-to-back passive repeater, 80 m ASL, located 23
km away from site A and 17 km away from site B. A
possible obstruction between site A and the Repeater
was sighted 8 km away from site A and is 40 m ASL,
while another obstruction was sighted between
Repeater and site B which is 12 km away from the
Repeater and is 55 m ASL. Considering a 15 m tree
growth for all obstructions and a Fresnel ratio of 1,
find the height of the antenna on both sites. Assume
K=1 and operating frequency of 6 GHz.
10. A microwave communications system operating at 6
GHz is to be set up to link a 55 km stretch from a town
in Bataan up to Magalang in Pampanga. The particular
town in Bataan is 65 m ASL while Magalang is 30 m
ASL. It was found out that there are two possible
obstructions upon inspecting the line of sight path. The
first obstruction was sighted 10 km away from
Magalang and is 40 m ASL while the other one is 30
km away from Bataan station and is 42 m ALS.
Considering a tree growth of 15 m on each obstruction
site and a homogenous atmosphere, compute for the
antenna height of the two sites.
direct pa
th
refle
cted
path
Site A
-
Site B
in some cases, the reflection point will be
blocked naturally by terrain features
direct signal
refle
l
signa
cted
Site B
Site A
-
if the reflecting surface falls on an odd
Fresnel zone, signal addition occurs
odd Fresnel zone
signals will arrive in-phase
direct path
refle
cted
path
Site A
CONSIDERING PATH REFLECTIONS
-
-
Site B
180 degrees phase
inversion
a point of reflection will exist somewhere
along the length of the path where (with
regard to the reflecting plane and antennas at
both ends) the angle of incidence equals the
angle of reflection
if antennas at each end of the path are at the
same height above a flat, reflecting surface,
the reflection point will be located halfway
between the two
- if the reflecting surface falls on an even Fresnel zone,
signal cancellation occurs
even Fresnel zone
signals will arrive out of
phase
direct path
refle
cted
path
Site A
direct path
refle
cted
Site B
180 degrees phase
inversion
path
POWER BUDGET CALCULATIONS
Site A
Site B
-
if the antenna elevations are different, the
reflection point will be closer to the lower
antenna
a path power budget is nothing but an
itemized list of all system losses and gains (in
decibels) from the transmitter on one end of
the path to the receiver on the other end, and
everything in between
Transmitter
2.
Receiver
Gt
Gr
FSL
Atmospheric Losses
TLL
TLL
Pt
Pr
where:
Pt = transmitter output power
TLL = transmission line loss
Gt = gain of the transmitting antenna
FSL = free space loss
Gr = gain of the receiving antenna
RSL = receive signal level (received power)
Transmitted Power
Pt  10 log
Po ( watts)
Pref
Transmission Line Loss
3.
TLL = length x attenuation factor
Antenna Gain
for Parabolic Antenna
G  7.5  20 log f GHZ  20 log D ft
for Flat Billboard
G  22.2  40 log FGHz  20 log A ft 2  20 log cos 
Free-Space Loss
FSL  92.44  20 log f GHz  20 log Dkm
Net Path Loss
4.
NPL = total Losses – total Gains
Received Signal Level
RSL = Pt - NPL
Sample Problems
1. A microwave system is given with the following
specifications:
Transmitter output
2 watts
Operating frequency
1.86 GHz
Path length
30 miles
transmitter
receiver
waveguide
150 ft.
200 ft.
length
waveguide
3 dB/ 100 ft.
3 dB/ 100 ft.
attenuation
antenna diameter 6 ft.
6 ft.
Find the signal strength at the receiver (RSL).
5.
A certain cellular network intends to put-up a basetransceiver station in town A to improve their system
capacity. The base-transceiver station is located 25 km
away from the base station and the most economical
mode of communication between the two stations is
through the application of microwave link. Given an
operating frequency of 7 GHz and
Base-transceiver station antenna 18 m
Base station antenna
21 m
Elevation ASL of BTS
42 m
Elevation ASL of BS
73 m
Total fixed losses and cable
attenuation at BTS
3.5 dB
Total fixed losses and cable
attenuation at BS
4.5 dB
Parabolic dish at BTS
10 feet
Transmitter output power
25 dBm
Assuming that the received signal level of the system
is -31.5 dBm, what is the required antenna diameter at
the Base Station ?
When a direct microwave path cannot be established
between two points because of some geographical or
man-made obstacles, it is sometimes possible to
establish a path by way of passive repeater. The most
common type of a passive repeater is a flat billboard
type metal reflector which acts as a microwave mirror.
Given the following data below:
 6 GHz band
 20 ft x 30 ft passive with an included
angle of 102˚
 10 ft dishes at its end
 short leg of 0.5 mile
 long leg of 25 miles
 omit waveguide loss
 correction factor of -1.8 dB
Find the Net Path Loss.
A transmitter and a receiver operating at 6 GHz are
separated by 40 km. How much power is delivered to
the receiver if the transmitter has an output power of
2W, the transmitting antenna has a gain of 20 dBi, and
the receiving antenna has a gain of 25 dBi?
A microwave link operating at 7.5 GHz uses a flat
billboard located 20 km from Site A and 1 km from
Site B with an included angle of 105º.Transmission
line and connector losses are set to 5 dB/site. If the
transmitter output power is 2 watts and the antenna
diameter on both sites is 8 ft., determine the required
size of the flat billboard when the receive signal level
is at -58 dBm.
RECEIVER SENSITIVITY OR THRESHOLD
-
weakest signal the receiver can detect
Receiver Thermal Noise
- sometimes called “Detection Threshold” or
“Absolute Noise Threshold”
NT( dBm)  114  10 log BWMHz  NFdB
where:
NT = receiver thermal noise
BW = bandwidth
Adjacent Channel Interference Fade Margin
- accounts for receiver threshold degradation
due to interference from adjacent channel
transmitters in one’s own system
Improvement Threshold
Flat Fade Margin
-
this is the point at which the RF carrier-tonoise ratio is equal to10 dB
the point at which the “capture effect” takes
place
at this point, the peaks of the signal begin to
exceed the peaks of the noise and quieting
begins
IT( dBm)  104  10 log BWMHz  NFdB
Carrier-to-Noise Ratio
- the ratio of the minimum wideband carrier
power at the input of a receiver that will
provide a usable baseband output to the
wideband noise power present at the input of
a receiver and the noise introduced within the
receiver
C
 RSLdBm  NTdBm
N dB
FADE MARGIN
-
-
the difference in dB between the normal
unfaded signal and the Improvement
Threshold
a “safety margin” of excess signal that the
path can fade before the receiver becomes
unusable due to noise
FM dB  RSLdBm  ITdBm
Thermal Fade Margin
- the difference between the receiver threshold
value and the receive signal level (RSL)
being applied to the receiver under normal
path conditions
Dispersive Fade Margin
- defined by the radio manufacturer, and is
determined by the type of modulation,
effectiveness of any equalization in the
receive path, and the multipath signal’s delay
time
Interference Fade Margin
External Interference Fade Margin
- is receiver threshold degradation due to
interference from external systems
FM FLAT  10 log[10
(
 FM THERMAL
)
10
 10
(
 FM AIFM
)
10
 10
(
 FM EIFM
)
10
]
Composite Fade Margin
FM COMPOSITE  10 log[10
(
 FM FLAT
)
10
 RD 10
(
 FM DISPERSIVE
)
10
]
where:
RD = Fade Occurrence Factor
Rayleigh Distribution of Fading
Propagation
Reliability (%)
90
99
99.9
99.99
99.999
99.9999
Required Fade
Margin (dB)
8
18
28
38
48
58
Sample Problems
1. An FM LOS microwave link operates at 6.15 GHz.
The required receiver IF bandwidth is 20 MHz. The
transmitter output power is 30 dBm. The receiver’s
front end active stage is a mixer with a noise figure of
9 dB. The path length is 21 mi, the antennas at each
end have a 35 dB gain and the transmission line losses
at each end are 3 dB. If the FM Improvement threshold
is used as the unfaded reference, what is the reliability
of the radio link?
2. Compute for the required output power of the
transmitter in a 99.999% reliable point-to-point
communications system using a 10-feet parabolic dish
on both ends with a transmission line and fixed losses
of 3.5 dB per site. The system is operating at 8GHz
with site separation of 45 km and using a receiver with
an absolute noise threshold of -103 dBm.
3. Two towns 25 miles apart will be provided with a
point-to-point microwave system operating at 7.5
GHz. The system will be used by a telephone company
and it will be designed to carry 500 voice channels
with a reliability of 99.99%. Considering a bandwidth
of 4 KHz per voice channel, find the size of the
antenna that will meet the required availability if:
Transmitter output power
7.06 dBm
Receiver noise figure
12 dB
Connector loss per site
0.5 dB
Radome loss per site
1 dB
Waveguide attenuation
1.5 dB/100 feet
Waveguide length per site `
100 feet
4.
5.
6.
7.
8.
9.
A single hop microwave relay system has the
following specifications:
Operating frequency
4 GHz
Rx/Tx antenna diameter
3 ft.
Hop distance
20 miles
Total waveguide loss
5 dB
Transmitter output power
1 watt
Receiver threshold
-78 dBm
Determine the fade margin and the estimated
reliability.
A certain radio station is transmitting at 0 dBW. The
transmission line losses at the transmitting and
receiving ends are both 2 dB. Total propagation loss is
138 dB and the receiver noise threshold is -124 dBW.
Determine the gain of the receiving and transmitting
antennas to obtain a carrier-to-noise ratio of 10 dB at
the receiver front end.
A microwave system operating at 6 GHz uses a
transmitter with an output power of 1 W. Both sites
uses a 6 ft parabolic dish antenna with a waveguide
loss of 5 dB per site. If the distance between the two
sites is 30 miles, determine the reliability of the system
using Rayleigh Distribution of Fading.
A microwave link operating at 7 GHz uses a back-toback repeater located 18 km from Site A and 17 km
from Site B. Transmission line and connector losses
are set to 4 dB/ site for Site A and Site B and 1.5 dB
for the Repeater. The link uses 8-feet parabolic dishes
on all sites. If the transmitter output power is at 4 watts
and the Thermal Noise at the receiver is -126 dBm,
determine the estimated reliability of the system.
Compute the Noise Figure of the receiver in a 99.999%
reliable point-to-point communications system using a
10-feet parabolic dish on both ends with transmission
line and fixed losses of 3.5 dB per site. The system is
operating at 6 GHz with site separation of 45 km using
a transmitter with an output power of 15 dBm. The
channel is carrying a signal having a bandwidth of 2.5
MHz.
Given the following equipment and operating
specifications:
Transmitter power output
2 watts
Operating frequency
2 GHz
Attenuation factor
3 dB/ 100 ft.
Antenna gain
18 dB
Minimum receiver input
116 dB below the
transmitter output
Path A-B loss
142 dB
Path A-C loss
124 dB
A
B
C
Antenna
100 ft.
100 ft.
100 ft.
height
Transmission
150 ft.
200 ft.
125 ft.
line
Determine which path is acceptable, path A-B or path
A-C ? Support your answer.
SYSTEM PERFORMANCE
System Gain
-
the difference between the nominal output
power of a transmitter and the minimum
input power required by the receiver.
GSYSTEM( dB)  Po  IT
System Reliability
-
the percentage of time the system or link
meets performance requirements
R  (1  Outage) x100%
for milti-hop link
R  R1 xR2 xR3 ........ xRn
where:
Outage = probability that the system will not meet the
requirements
R1, R2, R3….Rn = individual reliability
System Unavailability
U
U
downtime
total _ time
MTTR
x100%
MTBF  MTTR
A  (1  U ) x100%
where:
U = unavailability
A = availability (reliability)
MTTR = mean time to restore
MTBF = mean time between failure
Path Reliability
-
it represents the percentage of time the link is
expected to operate without an outage caused
by propagation conditions
using the Vigants-Barnett Model
5
U NDP  a * b * 6 *10 * f * D *10
3
R  (1  U NDP ) x100%

FM
10
where:
UNDP = non-diversity annual outage probability
due to multipath fading
D = path length km
f = frequency in GHz
FM = fade margin in dB
a = terrain factors
4 : very smooth terrain (
1 : average terrain, with some roughness
¼ : mountainous, very rough, or very dry
b = climatic factor
½ : hot, humid
¼ : normal, temperate
1/8 : mountainous or very dry
Sample Problems
1. If the MTBF of a communication circuit is 20,000
hours and its MTTR is 3 hours, what is the
availability?
2. What is the reliability of the equipment with a total
downtime of 16 hours during the whole year?
3. A long distance telephone company employs five
microwave radio hops over a single route to link two
important cities. If each hop has an MTBF of 10,000
hours and an MTTR of 3 hours, what is the outage ratio
of the entire system and the reliability of the system?
4. Determine the fade margin for a 60-km microwave
hop. The RF carrier is 8 GHz, the terrain is very
smooth and dry and the reliability objective is 99.95%.
5. Given a 25-mile path with average terrain but with
some roughness in an inland temperate climate, and a
link operating at a frequency of 6.7 GHz with a
desired propagation reliability of 99.95%, what fade
margin should be assigned to the link?
6. A point-to-point communications system operating at
7.5 GHz uses a transmitter with an output power of 0
dBW and a receiver with a Noise Figure of 5 dB at a
bandwidth of 5 MHz. If the target reliability for the
system is 99.995%, determine the necessary antenna
diameter needed for a 61.67 km link considering a
transmission line and connector loss of 3 dB per site.
7. A microwave communications system is being
designed to operate at 7 GHz with a transmitter output
power of 1W and a receiver Practical Noise Threshold
of -127 dBm. The link uses 4 feet parabolic dishes with
2 dB transmission line and connector loss per site. An
insurmountable obstruction was sighted between the
sites and the only possible repeater site is located 15
km from Site A and 10 km from Site B. If the
minimum required reliability for the system is 99.95%,
kindly help the designer to decide on whether to use a
back-to-back passive repeater with 4 feet parabolic
dishes and transmission line and connector loss of 1
dB or an 8’x12’ flat billboard reflector with an
included angle of 100º. Support your answer.
8. A point-to-point microwave system is to be
established between two sites that is 30 miles apart.
The system is operating at 5.9452 GHz and is capable
of carrying 960 voice channels and to meet a
propagation reliability of 99.9%. Consider a 52.2 KHz
9.
bandwidth per voice channel. A waveguide length of
350 ft at both sites is needed. The microwave radio
being considered has a transmitter output power of 562
mW with a receiver noise figure of 12 dB. Consider a
connector loss of 1.5 dB per site and waveguide
attenuation of 1.4 dB per 100 ft. Find the size of the
antenna that will meet the required reliability.
A full-duplex, 1+0 (non-redundant) microwave radio
system comprises of three (3) hops.
a. How many transmitters are used?
b. How many receivers are used?
c. How many antennas are used?
d. What is the minimum number of
frequencies used (or re-used) by the
system?
e. How many repeaters are required?
INCREASING PATH RELIABILITY
Space Diversity
- addition of another receive antenna,
separated in distance from the first
- improvement in reliability comes from the
reduced probability that both paths will be
adversely affected by fading at the same time
- more vertical spacing between antennas
offers less path correlation and better path
reliability
Rx1
Tx
Rx2
FM
I SD
(1.2 *10 3 * f * S 2 *10 10 )

D
where:
ISD = space diversity improvement factor
f = frequency in GHz
S = vertical antenna spacing, in meters,
between centers
D = path length in km
FM = path fade margin (if one antenna's path
has a smaller fade margin, use that figure).
Frequency Diversity
- microwave transmitters operating on two
frequencies (with a typical in-band diversity
spacing of about 2%), and sometimes in two
frequency bands (called crossband diversity),
-
-
are used to transmit the same information to
separate receivers at the other end of the path
reliability improvement comes from the
reduced chances of fading occurring on both
frequencies (or frequency bands) at the same
time
requires the use of more spectrum because it
uses two sets of frequencies
Tx1
f1
Rx1
f1
f1
X
Tx2
how many receivers are used
how many antennas are used
how many frequencies are required
REPEATERS FOR MICROWAVE SYSTEMS
-
used to extend a line-of-sight microwave
system for several additional kilometers
Active Repeaters
- repeater receives a signal at frequency F1,
amplifies it, translates the frequency to F2,
and amplifies and radiates that signal
X
f2
f2
b.
c.
d.
f2
Rx2
Baseband Repeater
- fully demodulates the incoming RF signal to
baseband
- the demodulated baseband is used to
modulate the transmitter used in the next
section
f
10 10
f 2D
where:
IFD = frequency diversity improvement factor
f/f = frequency diversity spacing
FM = fade margin
MIXER
70 MHz
Fin
P
AM
DISCRIMINATOR
P
AM
BASEBAND
BASEBAND
FM
I FD  80.5
Fout
TRANSMITTER
Flo
LOCAL
OSCILLATOR
Flo = Fin +/- 70 MHz
1.
2.
3.
4.
Consider a 30-mile path with average terrain, with
some roughness, in an inland temperate climate,
operating at a frequency of 6.7 GHz with fade margin
of 40 dB. Compute for the unavailability and
reliability for:
a. non-diversity path
b. path with 2% frequency diversity
c. path with 40-foot vertical space diversity
Compute for the reliability for (a) non-diversity path
(b) path with 5% frequency diversity (c) path with 28foot vertical space diversity of a microwave
communication system with a 35-km path having a
very smooth terrain in a hot and humid climate
operating at 7 GHz with fade margin of 35 dB.
In a one-hop, full-duplex microwave radio system,
frequency diversity arrangement, determine:
a. how many transmitters are used
b. how many receivers are used
c. how many antennas are used
d. how many frequencies are required
In a one-hop, full-duplex microwave radio system,
space diversity arrangement, determine:
a. how many transmitters are used
Fin
Fout
MIXER
Flor
LOCAL
OSCILLATOR
Flor = Fin +/- 70 MHz
P
AM
UPCONVERTER
(MIXER)
Fout
Sample Problems
IF Heterodyne Repeater
- simply translates the incoming signal to IF
with the appropriate local oscillator and a
mixer, amplifies the derived IF, and then upconverts it to a new RF frequency
70 MHz
U NDP
I
70 MHz
U DIV 
P
AM
Flot
UPCONVERTER
LOCAL
OSCILLATOR
Flot = Fout +/- 70 MHz
RF Heterodyne Repeater
- amplification is carried out directly at RF
frequencies
- incoming signal is amplified, up- or donconverted, and amplified again, and then
reradiated
P
AM
Fin
Fin
Fout
Fout
MIXER
P
AM
P
AM
-
the efficiency of back-to-back passive is
approximately 30% compared to a 98%
efficiency rating for the billboard
Fco
CONVERTER
OSCILLATOR
Fco = Fout - Fin
or
Fco = Fin - Fout
Reflectors and Passive Repeaters
-
-
-
because radio waves bounce off reflective
surfaces in much the same way light is
reflected by a mirror, radio reflectors can be
thought of as radio mirrors
the use of radio mirrors is dictated essentially
by topographical conditions where the
ruggedness of intervening terrain either
makes a direct path impossible or requires
that the antenna towers be extremely high
Generally speaking, radio mirrors fall into
two categories – reflectors and passive
repeaters. Those used in periscope antenna
applications are referred to as reflectors. The
large “billboards”, usually found on isolated
hilltops, and certain “back-to-back” parabolic
reflector are both classified as passive
repeaters.
Periscope Arrangement
- economic studies reveal that when the
waveguide run to the parabolic antenna
approaches distances of 150 feet and beyond,
it is usually less expensive to use the
periscope arrangement and beam the signal
from the ground to the reflector atop the
tower
Passive Repeaters
- the two general types of passive repeaters in
common use are the billboard and the backto-back passive which uses two standard
antenna dishes directly joined by a short
length of waveguide
OLIVER R. MARIANO, MS ECE, PECE, ASEAN Engr.
Electronics Engineering
Bulacan State University