MICROWAVE LINK - FUNDAMENTALS

MICROWAVE LINK - FUNDAMENTALS

INTRODUCTIONDefinition of MicrowaveMicrowaves 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 INDUSTRY1. BTS connectivity2. STM 1 (63 E1) ring closure3. BTS on spur4. 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 LINKSFrequency used in MW LinksMicrowave 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 types1. SDH2. PDHFrequency 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)

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SOME PARAMETERSFor 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 FieldIn 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

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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 Polarization1. Linear - can be sub-divided into Vertical and Horizontal2. CircularVERTICAL POLARIZATIONAn 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 POLARIZATIONAn 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 POLARIZATIONAn 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)

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.

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

FACTORS AFFECTING MW LINK

Following major phenomenon affect MW Link1. REFLECTION2. REFRACTION3. DIFFRACTION4. SCATTERING5. ABSORPTION

Factors affecting MW link - REFLECTION

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

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.

Fig MW.7.1 Reflection in MW Link

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 is REFRACTION. 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 informationK-Factor = R / R`whereR = Radius of ray beam curvatureR` = Radius of earthK=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

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

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

ABSORPTIONAbove 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 links1. Frequency Diversity2. Space DiversityFig MW.10.1 and MW.10.2 shows these diversities respectively.

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 asLfs = 92.45 + 20 log (dist * freq)wheredist = MW hop length in Kms.freq = Frequency of MW link in GHz.EXAMPLEFor 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 roughlyAntenna Gain = 17.8 + 20 log (f * dia)wheref = Frequency in GHzdia = Diameter of MW antenna.EXAMPLEFor 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

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.

wherern = radius of fresnel zone. Generally we consider n=1 i.e. first fresnel zone clearance.d1 = distance of point from Point Ad2 = distance of point from Point BLambda = Wavelength

Fig MW.12.1 MW Communication

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 asRxA = TxA + GA - Lfs - Arain + GB

whereTxA = Transmit PowerGA = Gain of Antenna ALfs = Free Space LossArain = Attenuation due to rainGB = Gain of Antenna B

EXAMPLESuppose 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.SOLUTIONFirst 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 dBiThen, we will calculate rough free space loss as= 98.45 + 20 log (dist * freq)= 98.45 + 20 log (20 * 6.2)= 140.318 dBmFinally 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 AnswerNOTEReceiver 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.