Microwave Link

MICROWAVE LINK - FUNDAMENTALS

A TUTORIAL

This tutorial aims at providing broad information about MW Links. This tutorial as such do not assume any special background, however basic knowledge of communication is necessary. It is recommended that before starting this tutorial you know about dB or have read a tutorial on it found (if not! it is coming soon) elsewhere on this website.

Ionosphere do not reflect electromagnetic waves of frequency greater than 30 MHz and hence MW waves has its own significance. It is because above this frequency we can have only Line of Sight (LOS) communication or satellite communication as far as wireless technology is concerned.

START TUTORIAL

This tutorial includes

1. Introduction

2. Frequency - MW Links (includes type of MW Link)

3. Practical View - MW Links

4. Polarization

5. Factors Affecting MW Links

6. Factors Affecting MW Links - REFLECTION

7. Factors Affecting MW Links - REFRACTION

http://www.bhupeshbatra.com/tutorials/mw_fund/mwlink_pg8.htm








8. Factors Affecting MW Links - Diffraction, Scattering & Absorption

9. Diversity in MW Links

10. Free Space Loss

11. Antenna Gain

12. Fresnel Zone

13. Link Budget


INTRODUCTION

Definition of Microwave

Microwaves are electromagnetic radiations in the frequency range 1 GHz to 30 GHz (generally for Telecom).

Various books uses various frequency ranges for identifying microwaves. Radio Frequency or Microwaves are two different terms used to break monotony. This means both terms convey similar meaning. Frequency from 300 MHz to 300 GHz are used in various ranges to define range of RF / Microwaves.

It is to be noted that higher the frequency, higher the bandwidth. Thus using high frequency gives us facility of transferring more data. However, everything comes with a price. High frequency means high processing capabilities are required and thus higher the cost. But use of frequency spectrum is very high and thus latter (i.e. high cost for high capabilities) is generally adapted now a days.

MICROWAVE APPLICATIONS FOR TELECOM INDUSTRY

1. BTS connectivity




http://www.bhupeshbatra.com/tutorials/mw_fund/mwlink_pg10-2.htm




2. STM 1 (63 E1) ring closure

3. BTS on spur

4. Point of Interconnect (POI) connectivity.

(If you are not familiar with above telecommunication terms, refer tutorial on "Introduction to basic fundamentals in telecom industry")


FREQUENCY - MW LINKS

Frequency used in MW Links

Microwave links of short distances are generally allocated with higher frequencies, because high frequency means high losses in air and thus it is good to have short distances in these cases. While for distances like 20-35 Kms or so we use lower frequencies. Please note that the terms high and low used for frequencies are relative and the values for these terms can be 15/18 GHz or 6/7 GHz say.

Microwave Links can be of two types

1. SDH

2. PDH

Frequency allocated to MW link does not depend on the type of MW link. If the type of MW link is to be explained in easiest possible manner, it may be as follows.

SDH link can carry optical signals i.e. each BTS falling in this MW link will have to have transport equipment to convert optical signal into electrical signal. This is good if we wish to have MW links of large no

of hops and wish to use it for ring closure. In this case only what is required will be dropped without disturbing the whole link. SDH link can carry maximum of STM 1 i.e. 64 E1s as a whole for one MW ring.

PDH link can carry electrical signals i.e. all 16E1s (capacity of PDH link) will have to be dropped in site falling in this link. Remaining E1s can then be retransmitted for next hop. (Hop means single MW link)

SOME PARAMETERS

For 15 GHz link, Tx and Rx bandwidth is 28 MHz. Tx and Rx separation is 420 MHz. This separation is defined by ITU and is there to avoid interference.

For 6 GHz link Tx and Rx separation is 152 MHz.

For 7 GHz link Tx and Rx separation is 154 MHz.


PRACTICAL VIEW - MW LINKS

If we wish to look at practical implementation of MW links in telecom industry, we can start from Fig MW.4.1

Fig MW.4.1 General MW Link Setup in Field

In Door Unit (IDU) which resides in Shelter, acts as Modem i.e. Modulator and Demodulator. It takes electrical / optical signal and convert it into analog (electromagnetic) which is sent to ODU (Out Door Unit).

IF cable is a co-axial cable which carries Intermediate Frequency. Details of IF cable can be seen in Fig MW.4.2. You can feel free to ignore this figure and continue. Generally, maximum permissible length of IF cable from IDU to ODU is 300m and frequency do not exceed 2 GHz.

Fig MW.4.2 IF Cable

ODU is present just near MW antenna at height in tower. ODU performs upconversion (acts as Mixer) to convert signal into required frequency allocated. For doing this ODU also have high power amplifiers and filters. Since ODU output is high frequency cable connecting ODU to antenna is "RF Low Loss Cable". Generally, for 6/7 GHz link this low loss cable is used and for 15/18 GHz link waveguide is used to connect ODU to antenna.


POLARIZATION

Polarization defines the way of movement of MW waves in air. It can be either Linear or Circular.

Type of Polarization

1. Linear - can be sub-divided into Vertical and Horizontal

2. Circular

VERTICAL POLARIZATION

An electromagnetic wave is said to be following Vertical Polarization if its electrical component is perpendicular to the horizon of earth as shown in Fig MW.5.1

Fig MW.5.1 Vertical Polarization

HORIZONTAL POLARIZATION

An electromagnetic wave is said to be following Horizontal Polarization if its electrical component is parallel to the horizon of earth as shown in Fig MW.5.2

Fig MW.5.2 Horizontal Polarization

CIRCULAR POLARIZATION

An electromagnetic wave is said to be following Circular Polarization if it radiates electric and magnetic field in all directions i.e. they keep on rotating. Phase is the deciding factor here. Don't worry about this... We generally do not use this in MW links.

WHICH POLARIZATION IS BETTER FOR MW LINKS?

There is no straight forward answer for this question. Definitely one can point out Vertical Polarization as the best in first view because it is more prone to rain fading. Rain droplets are generally flattened with increase in size (See Fig MW.5.3) and thus Vertical polarization is more prone and less affected. However, horizontal polarization is very much used to avoid interference, in case nearby areas are using Vertical Polarization. (See Fig MW.5.4)

Fig MW.5.3 Rain Droplets Fig MW.5.4 Use of V and H Polarization to avoid interference

So, vertical polarization is generally used for high frequency links, because high frequencies are more prone to rain fading and horizontal polarization is generally used to avoid interference. However, this cannot be treated as rule. Each operator is free to decide.


FACTORS AFFECTING MW LINK

Following major phenomenon affect MW Link

1. REFLECTION

2. REFRACTION

3. DIFFRACTION

4. SCATTERING

5. ABSORPTION


Factors affecting MW link - REFLECTION

REFLECTION

Reflection is one of the major factors that affect MW link. Fig MW.7.1 explains this phenomenon.

Fig MW.7.1 Reflection in MW Link

Water is good reflector. Reflected Wave can have different phase and amplitude as compared to LOS wave. Thus, this causes Fading of signal at receiver and this fading is called Multi Path Fading.

To overcome this problem, we either adjust antenna heights at two ends to avoid major source of reflection or to reduce its intensity. Another solution is to use Space Diversity, about which we will study later in this tutorial.

NOTE:

Trees are good absorbers. So, if trees are present in between MW link, chances of reflection reduces drastically.


Factors affecting MW link - REFRACTION

DO YOU KNOW THIS ?

Theory says that MW / electromagnetic waves travel in a straight line and yes, they do so in vacuum. But when it comes to atmosphere, it may come as surprise to most of us that MW waves do not travel in a straight line. Phenomenon responsible for this isREFRACTION. Density in atmosphere is not uniform. It varies from one place to another. As we all know that light ray bends towards or away from normal as it moves from higher density medium to lower or vice versa, we can easily understand why MW waves deviate from straight line path in atmosphere.

In homogeneous atmosphere vertical change in dielectric constant is gradual and hence bending or refraction is continuous. Ray is bent from thinner density air towards thicker making it follow earth curvature. This can be related with radii of spheres. First radius is of earth (6370 Km approx) and second is formed by curvature of beam of ray with its center coinciding center of earth.

We can define K Factor using above information

K-Factor = R / R`

where

R = Radius of ray beam curvature

R` = Radius of earth

K=4/3 for earth's atmosphere.

Fig MW.8.1 shows value of K according to path traveled by MW wave.

Fig MW.8.1 K-Factor in MW Link


Factors affecting MW link - Diffraction, Scattering & Absorption

DIFFRACTION

Diffraction of wave occurs when bending takes place at sharp irregular edges. This diffracted wave can interfere very much with desired signal.

SCATTERING

Scattering of ray of light occurs when object it strikes is of smaller size that its own wavelength.

ABSORPTION

Above 10 GHz, absorption in atmosphere becomes dominant. Rain droplets become comparable to wavelength.

This absorption can be 2 dB/Km or can be as high as 3 dB/Km in case of rain.


DIVERSITY IN MW LINKS

Diversity in MW Links is a sort of redundancy in network. They also help overcome various factors which affect MW links.

Two types of Diversity in MW links

1. Frequency Diversity

2. Space Diversity

Fig MW.10.1 and MW.10.2 shows these diversities respectively.

Fig MW. 10.1 Frequency Diversity Fig MW.10.2 Space Diversity

Frequency Diversity calls for use of two different frequencies for same MW link. This is normally avoided because two frequency allocation means double the annual fee payable for frequency. Frequency diversity is generally meant to overcome frequency interferences and various other factors.

Space Diversity uses two MW antennas at each side and is best suited to overcome Reflection of MW waves. Signal is received by both antennas called Main Antenna and Diversity Antenna and it is IDU to decide which signal to receive. Generally IDU receives best possible signal. This diversity also helps a lot in areas of high wind

because if one antenna gets misaligned network can function without fail from another. Thus this provides a sort of redundancy to our network.


FREE SPACE LOSS

Free Space Loss is defined as minimum loss an electromagnetic wave experiences if it travels in atmosphere. It depends from place to place. Its value for Kerela and Rajasthan will be different due to various factors one of which can be humidity. However, we may roughly define free space loss for MW link as

Lfs = 92.45 + 20 log (dist * freq)

where

dist = MW hop length in Kms.

freq = Frequency of MW link in GHz.

EXAMPLE

For MW link of 15 GHz and hop length 10 Kms free space loss can roughly be calculated as

= 92.45 + 20 log ( 10 * 15)

= 135.97 dB


ANTENNA GAIN

Antenna Gain is the gain antenna provides to the signal before transmitting it into air. For parabolic antennas used for MW link, this gain is roughly

Antenna Gain = 17.8 + 20 log (f * dia)

where

f = Frequency in GHz

dia = Diameter of MW antenna.

EXAMPLE

For 18 GHz MW link and 0.3 m size MW antenna, Antenna Gain will be approx

= 17.8 + 20 log (18*0.3)

= 32.44 dBi

(Don't worry about unit dBi, refer tutorial "Introduction to dB" elsewhere on this website. To learn more about antennas refer tutorial on it.)


FRESNEL ZONE

To understand Fresnel zone we need to first refer Fig MW.12.1

Fig MW.12.1 MW Communication

From the figure above we can see that apart from direct line of sight (LOS) we need to leave some space above and below it to allow deviation of MW wave from its original path. This deviation, as already studied, is due to refraction. Fresnel zone is nothing but distance below and above a point which should be clear for LOS communication.

where

rn = radius of fresnel zone. Generally we consider n=1 i.e. first fresnel zone clearance.

d1 = distance of point from Point A

d2 = distance of point from Point B

Lambda = Wavelength


LINK BUDGET

Now we will see link budget of MW link i.e. we will analyze gains and losses and calculate received power at other end.

Refer Fig MW.13.1 before moving further.

Fig MW.13.1 Link Budget for MW Link

From Fig MW.13.1 it can be seen clearly that received power at Point B can be calculated as

RxA = TxA + GA - Lfs - Arain + GB

where

TxA = Transmit Power

GA = Gain of Antenna A

Lfs = Free Space Loss

Arain = Attenuation due to rain

GB = Gain of Antenna B

EXAMPLE

Suppose we have 6.2 GHz MW link. Diameter of antenna at both sides is 1.8 m. Distance is 20 Kms. Calculate approx received power at point B, if transmitted power at point A is 25 dBm.

SOLUTION

First we will calculate Gain of two antennas. Since diameter is same, both antennas will roughly have gain of

= 17.8 + 20 log (freq * dia)

= 17.8 + 20 log (6.2 * 1.8)

= 38.753 dBi

Then, we will calculate rough free space loss as

= 98.45 + 20 log (dist * freq)

= 98.45 + 20 log (20 * 6.2)

= 140.318 dBm

Finally we will calculate received power at Point B from above given formula. We are assuming rain attenuation as zero.

RxB = 25 + 38.753 - 140.318 - 0 + 38.753

= - 37.812 dBm Answer

NOTE

Receiver sensitivity is generally around -65 dBm and hence the receive power we are getting is good and also take care of rain attenuation margin during rainy season. It is good practice to leave around 30 dB as rain margin.

Microwave Link Planning – An Introduction

Hey Reader!!! You are now very well aware of fundamentals of Microwave Link (refer tutorial in this website, if you are not!!!) We will now move on to planning a Microwave Link. Basically there are two approaches to planning / feasibility study of Microwave Link depending upon the location where it is suggested. If, for the time being, we ignore the fact that why a Link is required at the given lat longs (this we will discuss in our forthcoming tutorial – “Planning a Network”) and concentrate on its feasibility, antenna heights required, antenna sizes required, frequency allocations, polarization etc, this all needs in-depth fundamental knowledge. Two approaches may be summarized as

1) Microwave Link Planning for rural areas.2) Microwave Link Planning for urban areas.

In forthcoming pages, we will discuss about these with co-relation to what we read in fundamentals. Microwave Link Planning – Rural Areas

Microwave Link Planning for rural areas

If we wish to plan a microwave link for rural areas then we need a topographic map to start with. As per lat longs given to us for site A and site B we plot this on topographic map. Based on MSLs given in the map and terrain, we take various points on Line of Sight of these points with obstacle height (generally trees or terrain) with MSL at each location. These readings are then plotted on to Pathloss or any other tool which we are using for planning. This tool is supposed to provide us with antenna heights at which first Fresnel zone is clear. Based on our inputs, this tool will guide us regarding height of tower, expected Receive Signal Level (RSL) and antenna sizes. After we get clearance on this tool, it is generally recommended to have route walk in field to collect actual points and again we give these points to our tool to get clearances. Now, the point arises is that if we are having field route walk for every topographic study, why are we doing topographic study?It is because, if a link is directly sent for route walk, cost incurred is much higher than sitting and planning in office. Hence, only those links which clear our first criteria of clearance on topographic map are send to vendor / other teams for field route walk.(Don’t worry about route walk term, this will be explained in next type of approach i.e. planning in urban area) Microwave Link Planning – Urban Areas

Microwave Link Planning for urban areas For urban areas, feasibility of suggested microwave link starts directly in field. Why?* Let’s say we have a network of A, B, C… upto Z NEs and we want to add Network Element A1. As per geography our nearest neighbors are say M and Q. We will now start with route walk of A1 from M and Q and analyze which is a better solution. We have read this term route walk in last page also, and now its time to understand what it means.

Route WalkFor conducting a route walk of Microwave Link we require following

a) GPSb) Altimeterc) Clock

We start with Railway station (because it has Mean Sea Level already measured and written there) and set this MSL in our altimeter. Next we move on to our site M and return back to railway station. By this we calculate MSL of Site M. After this

we go to Site M and set calculated MSL to our altimeter and start route walk towards site A1. We try to move near LOS link which is now being displayed in GPS. En-route we try to find various possible obstacles in our LOS and note down height of obstacle (building, tree, water tank etc), lat long, time and MSL at each entry we make. Using this statistics we use Pathloss or similar planning tool to find out clearances and other parameters of Microwave Link (same as we did for rural areas) Same step is repeated to calculate feasibility from Q to A1. *Now, what is the difference here --- As compared to rural areas where we could use topographic maps, here we can’t because topographic maps are old and they are useful outside city areas wherein development is slow and new obstacles and changes are less expected. While in urban areas we have new building, new mall etc ready within 6 months or so. Thus, changes in urban areas are fast and here we rely only on route walks.

Digital Microwave Communication Equipment

Microwave Equipment Category

Trunk Microwave Equipment

High cost, large transmission capacity, more stable performance, applicable to long haul and trunk transmission

RF, IF, signal processing, and MUX/DEMUX units are all indoor. Only the antenna system is outdoor.

All Outdoor Microwave Equipment

• All the units are outdoor.• Installation is easy.• The equipment room can be saved.

Split-Mount Microwave Equipment (1)

The RF unit is an outdoor unit (ODU). The IF, signal processing, and MUX/DEMUX units are integrated in the indoor unit (IDU). The ODU and IDU areconnected through an IF cable.

The ODU can either be directly mounted onto the antenna or connected to the antenna through a short soft waveguide. Although the capacity is smaller than the trunk, due to the easy installation and maintenance, fast network construction, it's the most widely used microwave equipment.

Split-Mount Microwave Equipment (2)

Unit Functionso Antenna: Focuses the RF signals transmitted by ODUs and increases

the signal gain.o ODU: RF processing, conversion of IF/RF signals.o IF cable: Transmitting of IF signal, management signal and power

supply of ODU.o IDU: Performs access, dispatch, multiplex/demultiplex, and

modulation/demodulation for services.

Split-Mount Microwave Equipment – Installation

Microwave Antenna (1)

Antennas are used to send and receive microwave signals. Parabolic antennas and cassegrainian antennas are two common types of

microwave antennas. Microwave antenna diameters includes: 0.3m, 0.6m, 1.2m, 1.8m,2.0m,

2.4m, 3.0m, 3.2metc.

Microwave Antenna (2)

Different frequency channels in same frequency band can share one antenna.

Antenna Adjustment (1)


During antenna adjustment, change the direction vertically or horizontally. Meanwhile, use a multimeter to test the RSSI at the receiving end. Usually, the voltage wave will be displayed as shown in the lower right corner. The peak point of the voltage wave indicates the main lobe position in the vertical or horizontal direction. Large-scope adjustment is unnecessary. Perform fine adjustment on the antenna to the peak voltage point.

When antennas are poorly aligned, a small voltage may be detected in one direction. In this case, perform coarse adjustment on the antennas at both ends, so that the antennas are roughly aligned.

The antennas at both ends that are well aligned face a little bit upward. Though 1–2 dB is lost, reflection interference will be avoided.


During antenna adjustment, the two wrong adjustment cases are show here. One antenna is aligned to another antenna through the side lobe. As a result, the RSSI cannot meet the requirements.

Split-Mount Microwave Equipment – Antenna (1)

Antenna gain

Definition: Ratio of the input power of an isotropic antenna Pio to the input power of a parabolic antenna Pi when the electric field at a point is the same for the isotropic antenna and the parabolic antenna.

Calculating formula of antenna gain:�

G = Pio/Pi = (πD/λ)2 * η

Half-power angle

Usually, the given antenna specifications contain the gain in the largest radiation (main lobe) direction, denoted by dBi. The half-power point, or the –3 dB point is the point which is deviated from the central line of the main lobe and where the power is decreased by half. The angle between the two half-power points is called the half-power angle.�

Calculating formula of half-power angle:

θ0.5 = (65o ~ 70o ) λ/D

Split-Mount Microwave Equipment – Antenna (2)

Cross polarization discrimination

Suppression ratio of the antenna receiving heteropolarizing waves, usually, larger than 30 dB.

XdB� ＝10lgPo/Px

Po: Receiving power of normal polarized wave� Px: Receiving power of abnormal polarized wave�

Antenna protection ratio

Attenuation degree of the receiving capability in a direction of an antenna �compared with that in the main lobe direction. An antenna protection ratio of 180° is called front-to-back ratio.

Split-Mount Microwave Equipment –ODU (1)


Specifications of Transmittero Working frequency band

Generally, trunk radios use 6, 7, and 8 GHz frequency bands. 11, 13 GHz andhigher frequency bands are used in the access layer (e.g. BTS access).

o Output powerThe power at the output port of a transmitter. Generally, the output power is 15 to 30 dBm.


Local frequency stabilityIf the working frequency of the transmitter is unstable, the demodulated effectived signal ratio will be decreased and the bit error ratio will be increased. The value range of the local frequency stability is 3 to 10 ppm.

Transmit Frequency Spectrum FrameThe frequency spectrum of the transmitted signal must meet specified requirements, to avoid occupying too much bandwidth and thus causing too much interference to adjacent channels. The limitations to frequency spectrum iscalled transmit frequency spectrum frame.


Specifications of Receivero Working frequency band

Receivers work together with transmitters. The receiving frequency

on the local station is the transmitting frequency of the same channel on the opposite station.

o Local frequency stabilityThe same as that of transmitters: 3 to 10 ppm

o Noise figureThe noise figure of digital microwave receivers is 2.5 dB to 5 dB.


� PassbandTo effectively suppress interference and achieve the best transmission quality, the passband and amplitude frequency characteristics should be properly chosen. The receiver passband characteristics depend on the IF filter.

SelectivityAbility of receivers of suppressing the various interferences outside the passband, especially the interference from adjacent channels, image interference and the interference between transmitted and received signals.

� Automatic gain control (AGC) rangeAutomatic control of receiver gain. With this function, input RF signals change within a certain range and the IF signal level remains unchanges.


Split-Mount Microwave Equipment –IDU

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