MICROWAVE COMMUNICATION SYSTEM ALMA CHRISTINE C. DANZALAN, ECE
Introduction Microwaves are the ultrahigh, superhigh, and
extremely high frequencies directly above the lower frequency ranges where most radio communication now takes place and below the optical frequencies that cover infrared, visible, and ultraviolet light.
Microwave frequency bands
Microwave MicrowaveRadio-Frequency Radio-Frequency Assignments Assignments Band
Frequency (GHz)
Application
L
1–2
S
2–4
Marine radar
C
4–8
Commercial use, satellites
X
8 – 12
Military
Ku
12 – 18
Commercial use, satellites
K
18 – 27
Commercial use, satellites
Ka
27 – 40
Military
U
60 – 80
W
80 – 100
Microwave Communication Set-up
DOMSAT ANTIPOLO (TO BE TRANSFERRED TO NBC TOWER)
PLDT SAMPALOC
OB VAN ENG VAN
GMA EDSA COMPLEX
TANDANG SORA TRANSMIITTER
Applications: A microwave oven for the purpose of cooking food. Microwaves are used in broadcasting transmissions Radar also uses microwave radiation Wireless LAN protocols, such as Bluetooth and the IEEE 802.11g and b
specifications Metropolitan Area Networks - MAN protocols, such as WiMAX based in the IEEE 802.16 specification Cable TV and Internet access Mobile phone networks
Many semiconductor processing techniques use
microwaves to generate plasma for such purposes as reactive ion etching and PECVD.
Important considerations Signals follow a straight line or Line of Sight (LOS)
path. Signals are affected by obstructions (Man-made objects, earth’s terrain) and weather/atmospheric conditions (free space attenuation); Installed in tall towers, buildings or mountains Relay can extend across country and continents.
Important considerations Tight beam, high frequency; Frequencies used are greater than 300 MHZ
covering UHF, SHF and EHF range (actually between 1 to 100 GHz). Microwave system capacities vary form 12 to 22000 voice channels. Presently, microwave systems carry pulse code modulated time division multiplexed voice band circuits and use more modern digital modulation techniques, such as phase shift keying and quadrature amplitude modulation.
Types of Microwave Transmission Link:
Short Haul usually at lower capacity ranging from 64
kbps up to 2Mbps. Long Haul used for backbone route applications at
34 Mbps to 620 Mbps capacity.
FM MW Radio Transmitter
Baseband (audio,data,video)
Preemphasis Network
FM Deviator
Mixer
Microwave Generator
BPF
C h a n n e l c o m b i n i n g N e t w o r k
(
RF out
FM MW Receiver
Baseband out (audio,data,video)
De-emphasis Network
FM Detector
Mixer
Microwave Generator
BPF
C h a n n e l S e p a r a t i o n n e t w o r k
RF in
System variables for permissible distances between the transmitter and its associated receiver: 1. Transmitter output power 2. Receiver Noise threshold 3. Terrain 4. Atmospheric condition 5. System Capacity 6. Reliability objectives 7. Performance expectation
CLASSIFICATIONS OF MICROWAVE COMMUNICATIONS SYSTEMS
Terrestrial Microwave
Communications
Satellite
Communications
Categories of Microwave 1. Line of Sight Communication Use low transmit power and highly directional antennas Subject to earth bulge and other obstructions Maximum distance is 30 to 50 statute miles
Categories of Microwave 2. Tropospheric Scatter Communications Requires very high powered beam of energy Beyond LOS Uses refractive properties Provides reliable communications up to 400 statute mile
Terrestrial Microwave Basically there are 2 types of microwave stations.
Terminals – are points in the system where the baseband signals either originate or terminate
Repeaters – are points in the system where the baseband signals maybe reconfigured or simply repeated or amplified.
Types of passive repeaters
1.
Two identical parabolic antennas placed back to back
Types of passive repeaters 2. A flat “billboard type” reflecting surface. - flat metal type used to reflect microwave signals
Types of FM Active microwave Repeater
1. IF repeater– with an IF repeater, the received IF carrier is down converted to an IF frequency, amplified and then retransmitted.
Types of FM Active microwave Repeater 2. Baseband Repeater - with the baseband repeater, the RF carrier is downconverted to an IF frequency, amplified, filtered and then further demodulated to baseband.
Microwave and HF/ VHF
BAND
TRANSMISSIO N MEDIUM
ANTENN A
Tx to Ant Signal Form
Operating Frequency
HF/VHF
Wire
Dipole
Voltage /current
30 – 300 MHz
Microwave
Waveguide
Parabolic Electrom /Horn agnetic waves
1 – 30 GHz
TYPES OF MICROWAVE PATH
1. Line-of-Sight Path 2. Grazing Path 3. Obstructed Path
CHARACTERISTICS OF MICROWAVE
1. Microwaves behave similarly as light waves. 2. It can be reflected, refracted and diffracted.
3. It can be absorbed by some of the particles in the atmosphere. 4. It can be scattered. 5. It can be polarized.
Diversity It suggests that there are more than one
transmission path or method of transmission between a transmitter and the receiver. It can select the path or method that gives the
highest quality of received signal.
Diversity Techniques Space diversity Frequency diversity Polarization diversity Crossband diversity Angle diversity Hybrid diversity Path diversity
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
Space diversity
Space diversity scheme: Antenna Separation (S)
S = 3λRe / L
Where: path length
L=
Re = effective earth radius (m)
Space diversity
Frequency Diversity Microwave transmitters operating on two
frequencies (with a typical in-band diversity spacing of about 2% –3% 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
Frequency Diversity
Other Diversity Techniques: Crossband diversity –the signals are transmitted using
entirely different allocations. Hybrid Diversity – combination of space and frequency diversity Polarization Diversity – a single RF carrier is propagated with 2 different electromagnetic polarizations (Vertical and horizontal) Angle diversity – transmission of information at two or more slightly different angles resulting to two or more paths based on illuminating different scatter volume in troposcater system.
Other Diversity Techniques Path Diversity – in this method, the signal is sent to
a link which connects to the original destination, but is not as severely affected by fade.
Degree of Diversity It describes the number of signals carrying the same
information available at the receiver. Degree of diversity are: 1. Dual diversity 2. Triple diversity 3. Quad diversity
Protection Switching 2 Types of protection switching equipment
1. Hot Standby 2. Diversity
Hot Standby* Active RCVR #1
System XTMR Primary #1
Standby RCVR #2
System XTMR Standby #2
failure switch Transmitter
Receiver
*Hot standby is designed for equipment failure only
Earth Búlge Earth’s curvature presents LOS obstruction and must be compensated using 4/3 earth radius for atmospheric bending of waves.
EFFECTIVE EARTH’S RADIUS FACTOR (K) 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
Atmospheric Conditions 1. 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” 2. Standard atmosphere (K=4/3) - the microwave beam is slightly bent towards the surface of the earth - normal atmospheric condition
Atmospheric Conditions 3. 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. Homogenous atmosphere (K=1) - no refractive effect on the microwave signal - no density gradient
Atmospheric Conditions 5. Infinity condition (K=∞) - the microwave beam tend to follow the curvature of the earth - results to zero curvature or “Flat Earth Condition”
Atmospheric Conditions
Propagation Condition Propagation Condition Perfect – temperature zone, no fog, no ducting, good atmospheric condition day and night Ideal – dry, mountainous, no fog Average – Flat, temperate, some fog Difficult – coastal Bad – coastal, water, tropical
K – factor 4/3
1 – 4/3 2/3 –1 2/3 – ½ ½ - 2/5
Earth Bulge - change in vertical height of the earth’s surface from a horizontal reference line with respect to the distance.
Earth Bulge Computations
where: eb=earth bulge d1=distance from site 1 d2=distance from site 2 K=effective earth’s radius factor
Path Calculations
where: d = total distance in mi dt = radio horizon for transmit antenna (mi) dr = radio horizon for receive antenna (mi) ht = transmit antenna height (feet) hr = receive antenna height (feet)
Path Calculations Terrestrial microwave links
generally use line-of-sight propagation The maximum distance between two stations depends upon the height of the transmitting and receiving antennas, as well as the terrain between them Line-of-sight propagation is defined by: For accurate calculations, it is
necessary to use topographic maps and plot the path on graph paper
d 17hT 17hR
Power Budget Calculations 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.
Link Budget Transmitter Power
Transmission Line Losses
Transmission Losses a. WAVEGUIDE LOSS -Taken from the specs of the waveguide used. This is the amount of loss, usually expressed in dB per unit length (dB/ft or dB/m) of signal as it travels in the waveguide. b. CONNECTOR LOSS - taken from specs (0.5 dB) c. COUPLING LOSS - taken form specs (coax to waveguide to air) d. HYBRID LOSS – taken from specs, AKA circulator loss (1dB) e. RADOME LOSS – taken from the specs (0.5 dB)
3. Atmospheric Absorption Loss (AAL)
a. OXYGEN ABSORPTION LOSS - attenuation due to the absorption of radio frequency energy by oxygen molecules in the atmosphere. b. WATER VAPOR LOSS - attenuation due to the absorption of radio frequency energy by water vapor in the atmosphere.
4. Miscellaneous Path Loss (MPL)
a. DIFFRACTION LOSSES - Defined as the modulation or redistribution of energy within a wave front when it passes near the edge of an opaque object.
- path is blocked by an obstruction i. DLP – Diffraction Loss due to Path ii. DLS - Diffraction Loss due to Shielding b. REFLECTION LOSS (RL)
5. OTHER LOSSES a. RAIN LOSSES - attenuation due to the effects of
rain
b. CLUTTER LOSSES - attenuation due to trees and buildings in the front of the antenna
c. ANTENNA MISALIGNMENT - human factor error. This loss comes from the condition of the antenna when being installed. The value of this loss is assumed never to go above 0.25dB per antenna or 0.5 dB for the link.
Parabolic Antenna Gain English system:
GdB = 7.5 + 20logf GHz + 20logD ft Metric system:
GdB = 17.8 + 20logf GHz + 20logD m
Free Space Loss
English system:
FSL dB = 96.6 + 20logf GHz + 20logD miles
Metric system:
FSL dB = 92.4 + 20logf GHz + 20logD km
Net Path Loss
Received Signal Level
1. Effective Isotropically Radiated Power (EIRP) the amount of power that would have to be emitted by an isotropic antenna to produce the peak power density observed in the direction of maximum antenna gain.
EIRP = Pt + Gant – TLL where: Pt = RF power output (dBm) Gant = transmit antenna gain (dB) TLL = total transmission line loss at transmitter (taken from specs, in dB)
2. Antenna Gain Formula
Gant = η (πd/λ)2 where: η – antenna efficiency (typical value = 0.55) d = diameter of antenna in meters
Gant = 20 log f (GHz) + 20 log d (m) + 17.8 where: f = frequency in GHz d = diameter of antenna in meters *
3. Isotropic Receive Level (IRL)
IRL = EIRP – FSL where: EIRP = Effective Isotropically Radiated Power in dBm FSL = free space loss in dB
5. Received Signal Level (RSL) – unfaded
RSL = IRL + Gant – TLL RSL = Pt + Gant(Tx) – TLL(Tx) – FSL + Gant(Rx) – TLL(Rx) where: IRL = in dBm Gant(Rx) = receive antenna gain (dB) TLL(Rx) = transmission line loss at receiver
6. Receiver Threshold (C/N)
the minimum wideband carrier power (Cmin) at the input to a receiver that will provide a usable baseband output; sometimes called receiver sensitivity
Receiver Threshold (C/N) C/N(dB) = RSL(dBm) - Pn(dBm) where: Pn = thermal noise threshold of system
the receiving
7. Thermal Noise Threshold (Pn) Pn = 174 + 10 log B + NF where: NF = receiver noise figure B = Bandwidth (hertz)
8. Fade Margin (FM) equation considers the non-ideal and less predictable characteristics of radio wave propagation such as multi-path loss and terrain sensitivity
Using Barnett-Vignant Equation: FM = RSL – Receiver Threshold Power Level FM = 30 log D + 10 log (6ABf) – 10 log (1 –R) – 70 where: 30 log D = multi-path effect 10 log (6ABf) = terrain sensitivity 10 log (1 –R) = reliability objectiveness
where: FM - Fade Margin D - Distance (km) f - Frequency (GHz) R - Reliability (1 – R) – Reliability objective A – roughness factor B – factor to convert a worst month probability to an annual probability
A Values 4
smooth terrain, over water, or flat desert
1
average terrain
0.25 mountains, very rough or very dry terrain
B Values 0.5 0.25
hot humid areas average inland areas, normal, interior temperature or sub-arctic areas
0.125 mountainous or very dry but nonreflective areas
System Gain - It is the difference between the nominal output power of a transmitter and the minimum input power required by a receiver. - It must be greater than or equal to the sum of all gains and losses incurred by a signal as it propagates from a transmitter to a receiver. - It represents the net loss of a radio system.
GS = Pt - Cmin Pt - Cmin > Losses – Gains where: GS – System Gain (dB) Pt – transmitter output power (dBm) Cmin – minimum receiver input power for a given quality objective (dBm)
GS = FM + FSL + Lf + Lb – At - Ar Gains At – transmit antenna gain (dB) Ar – receive antenna gain (dB) Losses FSL – free space path loss between antennas Lf – waveguide feeder loss between distribution network and antenna Lb – branching and coupling losses FM – Fade Margin for a given reliability objective
Sample Problems 1. For a carrier frequency of 6 GHz and a distance of
50 km, determine the free-space path loss. 2. An FM LOS microwave link operates at 6.15 GHz. The transmitter output power is 1 watt. The path length is 34 km; the antennas at each end have a 35-dB gain and the transmission line losses at each end are 3 dB. Find the received signal level (RSL).
3. Consider a space-diversity microwave radio system operating at an RF carrier frequency of 1.8 GHz. Each station has a 2.4-meter diameter parabolic antenna that is fed by a 100m of air-filled coaxial cable. The terrain is smooth and the area has a humid climate. The distance between stations is 40 km. A reliability objective of 99.99% is desired. Determine the system gain. The air-filled coaxial cable has a feeder loss of 5.4 dB/100m and branching loss of 2 dB.
RECEIVER SENSITIVITY
the minimum wideband carrier power of the input of
the reciever that will provide a usable baseband output. Receiver Thermal Noise - sometimes called “Detection Threshold” or “Absolute Noise Threshold”
Pn = - 174 + 10 log B + NF
Improvement Threshold
this is the point at which the RF carrier-to noise ratio
is equal to10 dB
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.
FADE MARGIN
A “safety margin” of excess signal that the path can
fade before the receiver becomes unusable due to noise.
Fade Margin
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.
Fade Margin Interference Fade Margin External Interference Fade Margin - is receiver threshold degradation due to interference from external systems Adjacent Channel Interference Fade Margin - accounts for receiver threshold degradation due to interference from adjacent channel transmitters in one’s own system
Fade Margin
Rayleigh Distribution of Fading
Sample Problem 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 Fade margin?
Sample Problem 2 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 considering -92.5 dBm practical threshold.
SYSTEM PERFORMANCE
System Gain System Reliability System Unavailability Path Reliability
System Reliability the percentage of time the system or link meets
performance requirements
System Unavailability
Path Reliability It represents the percentage of time the link is expected to operate without an outage caused by propagation conditions
Sample Problems 1. If the MTBF of a communications circuit is 20,000 hours and its MTTR is 3 hours, what is its availability? 2. 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 MTTR and reliability of the route? Assume that the failure occur at different periods of time.
