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Microwave Link Design

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Saifullah Khan

on 19 September 2012

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Transcript of Microwave Link Design

A communication system that utilizes the radio frequency band spanning 2 to 60 GHz.
As per IEEE, electromagnetic waves between 30 and 300 GHz are called millimeter waves (MMW) instead of microwaves as their wavelengths are about 1 to 10mm.
Small capacity systems generally employ the frequencies less than 3 GHz while medium and large capacity systems utilize frequencies ranging from 3 to 15 GHz.
Frequencies > 15 GHz are essentially used for short-haul transmission. What is microwave communication? Less affected by natural calamities
Less prone to accidental damage
Links across mountains and rivers are more economically feasible
Single point installation and maintenance
Single point security
They are quickly deployed Advantages of Microwave Radio: Microwave radio communication requires a clear line-of-sight (LOS) condition
Under normal atmospheric conditions, the radio horizon is around 30 percent beyond the optical horizon
Radio LOS takes into account the concept of Fresnel ellipsoids and their clearance criteria
Fresnel Zone:
Areas of constructive and destructive interference created when electromagnetic wave propagation in free space is reflected (multipath) or diffracted as the wave intersects obstacles
Fresnel zones are specified employing ordinal numbers that correspond to the number of half wavelength multiples that represent the difference in radio wave propagation path from the direct path
The Fresnel Zone must be clear of all obstructions. Line-of-Sight Consideration: Typically the first Fresnel zone (N=1) is used to determine obstruction loss
The direct path between the transmitter and the receiver needs a clearance above ground of at least 60% of the radius of the first Fresnel zone to achieve free space propagation conditions
Earth-radius factor k compensates the refraction in the atmosphere
Clearance is described as any criterion to ensure sufficient antenna heights so that, in the worst case of refraction (for which k is minimum) the receiver antenna is not placed in the diffraction region Line-of-Sight Consideration: Clearance criteria to be satisfied under normal propagation conditions
Clearance of 60% or greater at the minimum k suggested for the certain path
Clearance of 100% or greater at k=4/3
In case of space diversity, the antenna can have a 60% clearance at k=4/3 plus allowance for tree growth, buildings (usually 3 meter) Line-of-Sight Consideration: Microwave Link Design is a methodical and systematic and sometimes lengthy process that include:
Loss/attenuation calculation
Fading and fade margin calculation
Frequency planning and interference calculation
Quality an availability calculation
The whole process is iterative and may go through many redesign phases before the required quality and availability are achieved Microwave link design: The loss/attenuation calculations are composed of three main contributions
Propagation losses:
Due to Earth’s atmosphere and terrain
Branching losses:
comes from the hardware used to deliver the transmitter/receiver output to/from the antenna
Miscellaneous (other) losses:
unpredictable and sporadic in character like fog, moving objects crossing the path, poor equipment installation and less than perfect antenna alignment etc

This contribution is not calculated but is considered in the planning process as an additional loss Loss/Attenuation Calculations: Free-space loss:
when the transmitter and receiver have a clear, unobstructed line-of-sight
Lfsl=92.45+20log(f)+20log(d) [dB]
where f = frequency (GHz)
d = LOS range between antennas (km)
Vegetation attenuation:
provision should be taken for 5 years of vegetation growth
L=0.2f 0.3R0.6(dB)
f=frequency (MHz)
R=depth of vegetation in meter’s (for R<400m)
Obstacle Loss:
also called Diffraction Loss or Diffraction Attenuation. One method of calculation is based on knife edge approximation. Propagation losses: Gas absorption:
Primarily due to the water vapor and oxygen in the atmosphere in the radio relay region
The absorption peaks are located around 23GHz for water molecules and 50 to 70 GHz for oxygen molecules
The specific attenuation (dB/Km)is strongly dependent on frequency, temperature and the absolute or relative humidity of the atmosphere
Attenuation due to precipitation:
Rain attenuation is the main contributor in the frequency range used by commercial radio links
Rain attenuation increases exponentially with rain intensity
The percentage of time for which a given rain intensity is attained or exceeded is available for 15 different rain zones covering the entire earth’s surface
The specific attenuation of rain is dependent on many parameters such as the form and size of distribution of the raindrops, polarization, rain intensity and frequency
Horizontal polarization gives more rain attenuation than vertical polarization
Rain attenuation increases with frequency and becomes a major contributor in the frequency bands above 10 GHz
The contribution due to rain attenuation is not included in the link budget and is used only in the calculation of rain fading Propagation losses: The specific attenuation of rain is dependent on many parameters such as the form and size of distribution of the raindrops, polarization, rain intensity and frequency
Horizontal polarization gives more rain attenuation than vertical polarization
Rain attenuation increases with frequency and becomes a major contributor in the frequency bands above 10 GHz
The contribution due to rain attenuation is not included in the link budget and is used only in the calculation of rain fading
The loss due to reflection on the ground is dependent on the total reflection coefficient of the ground and the phase shift
The highest value of signal strength is obtained for a phase angle of 0o and the lowest value is for a phase angle of 180o Ground Reflection: The reflection coefficient is dependent on the frequency, grazing angle (angle between the ray beam and the horizontal plane), polarization and ground properties
The grazing angle of radio-relay paths is very small – usually less than 1o
It is recommended to avoid ground reflection by shielding the path against the indirect ray
The contribution resulting from reflection loss is not automatically included in the link budget
When reflection cannot be avoided, the fade margin may be adjusted by including this contribution as “additional loss” in the link budget Ground Reflection: The free space loss of the radio signal is subtracted. The longer the link the higher the loss
These calculations give the fade margin
In most cases since the same duplex radio setup is applied to both stations the calculation of the received signal level is independent of direction
Receive Signal Level (RSL):
RSL = Po – Lctx + Gatx – Lcrx + Gatx – FSL
Link feasibility formula:
RSL  Rx (receiver sensitivity threshold)

Po = output power of the transmitter (dbm)
Lctx, Lcrx = Loss (cable, connectors, branching unit) between transmitter/receiver and antenna(dB)
Gatx = gain of transmitter/receiver antenna (dBi)
FSL = free space loss (dB) Link Budget: The fade margin is calculated with respect to the receiver threshold level for a given bit-error rate (BER)
The radio can handle anything that affects the radio signal within the fade margin but if it is exceeded, then the link could go down and therefore become unavailable
The threshold level for BER=10-6 for microwave equipment used to be about 3dB higher than for BER=10-3
Consequently the fade margin was 3 dB larger for BER=10-6 than BER=10-3
In new generation microwave radios with power forward error correction schemes this difference is 0.5 to 1.5 dB Link budget: Fading is defined as the variation of the strength of a received radio carrier signal due to atmospheric changes and/or ground and water reflections in the propagation path
Four fading types are considered while planning links
Multipath fading
Flat fading
Frequency-selective fading
Rain fading
Refraction-diffraction fading (k-type fading)
They are all dependent on path length and are estimated as the probability of exceeding a given (calculated) fade margin Fading And Fade Margin: Multipath Fading is the dominant fading mechanism for frequencies lower than 10GHz
A reflected wave causes a multipath, i.e.when a reflected wave reaches the receiver as the direct wave that travels in a straight line from the transmitter
If the two signals reach in phase then the signal amplifies. This is called upfade
Upfademax=10 log d – 0.03d (dB)
d is path length in Km
If the two waves reach the receiver out of phase they weaken the overall signal
A location where a signal is canceled out by multipath is called null or downfade
As a thumb rule, multipath fading, for radio links having bandwidths less than 40MHz and path lengths less than 30Km is described as flat instead of frequency selective Fading And Fade Margin: Flat fading:
A fade where all frequencies in the channel are equally affected
There is barely noticeable variation of the amplitude of the signal across the channel bandwidth
If necessary flat fade margin of a link can be improved by using larger antennas, a higher-power microwave transmitter, lower –loss feed line and splitting a longer path into two shorter hops
On water paths at frequencies above 3 GHz, it is advantageous to choose vertical polarization
Frequency-selective fading:
There are amplitude and group delay distortions across the channel bandwidth
It affects medium and high capacity radio links (>32 Mbps)
The sensitivity of digital radio equipment to frequency-selective fading can be described by the signature curve of the equipment
This curve can be used to calculate the Dispersive Fade Margin (DFM) Fading And Fade Margin: The link budget is a calculation involving the gain and loss factors associated with the antennas, transmitters, transmission lines and propagation environment, to determine the maximum distance at which a transmitter and receiver can successfully operate
Receiver sensitivity threshold is the signal level at which the radio runs continuous errors at a specified bit rate
System gain depends on the modulation used (2PSK, 4PSK, 8PSK, 16QAM, 32QAM, 64QAM,128QAM,256QAM) and on the design of the radio
The gains from the antenna at each end are added to the system gain (larger antennas provide a higher gain) Link Budget: DFM = 17.6 – 10log[2(f)e-B/3.8/158.4] dB
f = signature width of the equipment
B = notch depth of the equipment

Modern digital radios are very robust and immune to spectrum- distorting fade activity
Only a major error in path engineering (wrong antenna or misalignment) over the high-clearance path could cause dispersive fading problems Fading And Fade Margin: Rain Fading:
Rain attenuates the signal caused by the scattering and absorption of electromagnetic waves by rain drops
It is significant for long paths (>10Km)
It starts increasing at about 10GHz and for frequencies above 15 GHz, rain fading is the dominant fading mechanism
Rain outage increases dramatically with frequency and then with path length
Microwave path lengths must be reduced in areas where rain outages are severe
The available rainfall data is usually in the form of a statistical description of the amount of rain that falls at a given measurement point over a period of time
The total annual rainfall in an area has little relation to the rain attenuation for the area
Hence a margin is included to compensate for the effects of rain at a given level of availability
Increased fade margin (margins as high as 45 to 60dB) is of some help in rainfall attenuation fading Fading And Fade Margin: Reducing the Effects of Rain:
Multipath fading is at its minimum during periods of heavy rainfall with well aligned dishes, so entire path fade margin is available to combat the rain attenuation (wet-radome loss effects are minimum with shrouded antennas)
When permitted, cross-band diversity is very effective
Route diversity with paths separated by more than about 8 Km can be used successfully
Radios with Automatic Transmitter Power Control have been used in some highly vulnerable links
Vertical polarization is far less susceptible to rainfall attenuation (40 to 60%) than are horizontal polarization frequencies Fading And Fade Margin: Refraction – Diffraction Fading
Also known as k-type fading
For low k values, the Earth’s surface becomes curved and terrain irregularities, man-made structures and other objects may intercept the Fresnel Zone
For high k values, the Earth’s surface gets close to a plane surface and better LOS(lower antenna height) is obtained
The probability of refraction-diffraction fading is therefore indirectly connected to obstruction attenuation for a given value of Earth –radius factor
Since the Earth-radius factor is not constant, the probability of refraction-diffraction fading is calculated based on cumulative distributions of the Earth-radius factor Fading And Fade Margin: The objective of frequency planning is to assign frequencies to a network using as few frequencies as possible and in a manner such that the quality and availability of the radio link path is minimally affected by interference
The following aspects are the basic considerations involved in the assignment of radio frequencies:
Determining a frequency band that is suitable for the specific link (path length, site location, terrain topography and atmospheric effects)
Prevention of mutual interference such as interference among radio frequency channels in the actual path, interference to and from other radio paths, interference to and from satellite communication systems
Correct selection of a frequency band allows the required transmission capacity while efficiently utilizing the available radio frequency spectrum Frequency planning: Assignment of a radio frequency or radio frequency channel is the authorization given by an administration for a radio station to use a radio frequency or radio frequency channel under specified conditions
It is created in accordance with the Series F recommendations given by the ITU-R
Frequency channel arrangements:
The available frequency band is subdivided into two halves, a lower (go) and an upper (return) duplex half
The duplex spacing is always sufficiently large so that the radio equipment can operate interference free under duplex operation
The width of each channel depends on the capacity of the radio link and the type of modulation used Frequency planning: The most important goal of frequency planning is to allocate available channels to the different links in the network without exceeding the quality and availability objectives of the individual links because of radio interference
Frequency planning of a few paths can be carried out manually but, for larger networks, it is highly recommended to employ a software transmission design tool
One such vendor independent tool is Pathloss 4.0. This tool is probably one of the best tools for complex microwave design
It includes North American and ITU standards, different diversity schemes, diffraction and reflection (multipath) analysis, rain effects, interference analysis etc Frequency planning: To accurately predict the performance of a digital radio path, the effect of interference must be considered
Interference in microwave systems is caused by the presence of an undesired signal in a receiver
When this undesired signal exceeds certain limiting values, the quality of the desired received signal is affected
To maintain reliable service, the ratio of the desired received signal to the (undesired) interfering signal should always be larger than the threshold value
In normal unfaded conditions the digital signal can tolerate high levels of interference but in deep fades it is critical to control interference Interference fade margin: Adjacent-channel interference fade margin (AIFM):
(in decibels) accounts for receiver threshold degradation due to interference from adjacent channel transmitters
Interference fade margin (IFM):
It is the depth of fade to the point at which RF interference degrades the BER to 1x 10-3 . The actual IFM value used in a path calculation depends on the method of frequency coordination being used
There are two widely used methods
The C/I (carrier to interference):
C/I method is the older method developed to analyse interference cases into analog radios
T/I (threshold to interference):
threshold-to-interference (T/I) curves are used to define a curve of maximum interfering power levels for various frequency separations between interfering transmitter and victim receivers as follows
I = T- (T/I)
Where, I = maximum interfering power level (dBm)
T= radio threshold for a 10-6 BER (dBm)
T/I = threshold-to-interference value (dB) from the T/I curve for the particular radio Interference fade margin: For each interfering transmitter, the receive power level in dBm is compared to the maximum power level to determine whether the interference is acceptable
Composite Fade Margin (CFM):
It is the fade margin applied to multipath fade outage equations for a digital microwave radio
CFM = TFM + DFM + IFM + AIFM
CFM = -10 log (10-TFM/10 + 10 – DFM/10 + 10-IFM/10 + 10-AIFM/10 )
Where,
TFM = Flat fade margin (the difference between the normal (unfaded) RSL and the BER=1 x 10-3 digital signal loss-of frame point)
DFM = Dispersive fade margin
IFM = Interference fade margin
AIFM =Adjacent-channel interference fade margin Interference fade margin: The main purpose of the quality and availability calculations is to set up reasonable quality and availability objectives for the microwave path
The ITU-T recommendations G.801, G.821 and G.826 define error performance and availability objectives
The objectives of digital links are divided into separate grades:
High
Medium
Local
The following grades are usually used in wireless networks:-
Medium grade Class 3 for the access network
High grade for the backbone network Quality and Availability: Microwave Link Multipath Outage Models:
A major concern for microwave system users is how often and for how long a system might be out of service
An outage in a digital microwave link occurs with a loss of Digital Signal frame sync for more than 10 sec
Digital signal frame loss typically occurs when the BER increases beyond 1 x 10-3 
Outage (Unavailability) (%) = (SES/t) x 100
Where,
t = time period (expressed in seconds)
SES = severely error second
Availability is expressed as a percentage
A = 100 - Outage (Unavailability)
A digital link is unavailable for service or performance prediction/verification after a ten consecutive BER> 1 x 10-3 SES outage period Interference fade margin: Hardware Redundancy:
Hot standby protection
Multichannel and multiline protection
Diversity Improvement:
Space Diversity
Angle Diversity
Frequency Diversity
Cross-band Diversity
Route Diversity
Hybrid Diversity
Media Diversity
Antireflective Systems
Repeaters:
Active repeaters
Passive repeaters Improving Microwave System: Use higher frequency bands for shorter hops and lower frequency bands for longer hops
Avoid lower frequency bands in urban areas
Use star and hub configurations for smaller networks and ring configuration for larger networks
In areas with heavy precipitation , if possible, use frequency bands below 10 GHz
Use protected systems (1+1) for all important and/or high-capacity links
Leave enough spare capacity for future expansion of the system
Space diversity is a very expensive way of improving the performance of the microwave link and it should be used carefully and as a last resort Basic Recommendation: The activities of microwave path planning and frequency planning preferably should be performed in parallel with line of sight activities and other network design activities for best efficiency
Use updated maps that are not more than a year old
The terrain itself can change drastically in a very short time period
Make sure everyone on the project is using the same maps, datums and coordinate systems
Perform detailed path surveys on ALL microwave hops
Maps are used only for initial planning, as a first approximation.
Below 10 GHz , multipath outage increases rapidly with path length
It also increases with frequency , climatic factors and average annual temperature
Multipath effect can be reduced with higher fade margin
If the path has excessive path outage the performance can be improved by using one of the diversity methods. Basic Recommendation: 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 Difficult areas of microwave link: Thank You…! By: SAIF ULLAH KHAN Radius of the first Fresnel zone
R=17.32(x(d-x)/fd)1/2
Where
d=distance between antenna (Km)
R=first Fresnel zone radius (in m)
f=frequency (in GHz) Line-of-Sight Consideration: Effective Earth’s Radius = k * True Earth’s Radius
True Earth’s radius= 6371 Km
k=4/3=1.33
Standard atmosphere with normally refracted path (this value should be used whenever local value is not provided)
Variations of the ray curvature as a
function of k Line-of-Sight Consideration: Microwave Link Design Process: Having an obstacle free 60% of the Fresnel zone gives 0 dB loss Propagation losses: Radio Path Link Budget: Chain/cascade configuration: FREQUENCY PLANNING AND DIFFERENT NETWORK TOPOLOGIES: Ring configuration:
If the ring consisted of an odd number of sites there would be a conflict of duplex halves and changing the frequency band would be a reliable alternative FREQUENCY PLANNING AND DIFFERENT NETWORK TOPOLOGIES: Star configuration:
The link carrying the traffic out of the hub should use a frequency band other than the one employed inside the cluster FREQUENCY PLANNING AND DIFFERENT NETWORK TOPOLOGIES:
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