tranasmission

Network Services :Transmission Planning

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TRANSMISSION…

• Transportation of information (Data) from Source (Tx) to Destination (Rx).

Tx Rx

Media:• Copper• OFC• Satellite

• Microwave (Radio)

Media


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• The fundamental aim of a radio link is to deliver sufficient signal power to the receiver at the far end of the link to achieve some performance objective.

•For a data transmission system, this objective is usually specified as a minimum bit error rate (BER).

•In the receiver demodulator, the BER is a function of the signal to noise ratio (SNR).

•At the frequencies under consideration here, the noise power is often

dominated by the internal receiver noise; however, this is not always the case, especially at the lower (VHF) end of the range.

A Radio Link …


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•In addition, the “noise” may also include significant power from interfering signals, necessitating the delivery of higher signal power to the receiver than would be the case under more ideal circumstances (i.e., back-to-back through an attenuator).

•If the channel contains multipath, this may also have a major impact on the BER.

… Let us focus on predicting the signal power which will be available to the receiver…


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RADIO RELAY LINK…

Tx/Rx

(IDU)

Tx/Rx

(IDU)

+ Gt + Gr

- Lt- Lr

Pt Rx

Branching LossBranching Loss

Atmospheric Loss

FSL

ODU ODU


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The benchmark by which we measure the loss in a transmission link is the loss that would be expected in free space - in other words, the loss that would occur in a region which is free of all objects that might absorb or reflect radio energy.

And mathematically, FREE SPACE LOSS is expressed as:

FSL (dB) = 32.4 + 20*LOG(f) + 20*LOG(D)

Where, f : Frequency of Operation, in MHz

D: Hop length, in Km

OR,

FSL (dB) = 92.45 + 20*LOG(f) + 20*LOG(D)

Where, f : Frequency of Operation, in GHz

D: Hop length, in Km


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A Real Radio Relay System…

Of course, in looking at a real system, we must consider the actual antenna gains and cable losses in calculating the signal power Pr which is available at the receiver input:


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Path Loss on Line of Sight Links…

Line of Sight (LOS):the antennas at the ends of the link can “see” each other, at least in a radio sense.

In many cases, radio LOS equates to optical LOS: you’re at the location of the antenna at one end of the link, and with the unaided eye or binoculars, you can see the antenna (or its future site) at the other end of the link. In other cases, we may still have an LOS path even though we can’t see the other end visually: This is because the radio horizon extends beyond the optical horizon. Radio waves follow slightly curved paths in the atmosphere, but if there is a direct path between the antennas which doesn’t pass through any obstacles, then we still have radio LOS.

Optical LOS Radio LOS Optical LOS

Does having LOS mean that the path loss will be equal to the free space case which we have just considered? In some cases, the answer is yes, but it is definitely not a sure thing. There are three mechanisms which may cause the path loss to differ from the free space case:• Refraction in the earth’s atmosphere, which alters the trajectory of radio waves, and which can change with time.


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• Diffraction effects resulting from objects near the direct path.• Reflections from objects, which may be either near or far from the direct path.

Atmospheric Refraction :

Under normal circumstances, the index of refraction decreases monotonically with increasing height (gaseous density decreases with increasing altitude), which causes the radio waves emanating from the transmitter to bend slightly downwards towards the earth’s surface instead of following a straight line.

Under normal conditions, the gradient in refractivity index is such that real world propagation is equivalent to straight-line propagation over an

earth whose radius is greater than the real one by a factor of 4/3 - thus the often-heard term “4/3 earth radius” in discussions of terrestrial propagation.


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In superrefraction, the rays bend more than normal and the radio horizon is extended – Ducting.

In subrefraction, in which the bending of the rays is less than normal, thus shortening the radio horizon and reducing the clearance over obstacles

along the path. This may lead to increased path loss, and possibly even an outage.

Super-refraction

( Temp Inversion)


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Sub-refraction K < 4/3)

Hyper-refraction( K> 4/3)

Normal-refraction ( K= 4/3)

•DIFFRACTION

PowerDensity

dB

Object PositionApprox. 6 dBLoss at the grazing

O

B

S

T

A

C

L

E

P

0

0-10-20-3

10203


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A Fresnel zone : it is the volume of space enclosed by an ellipsoid which has the two antennas at the ends of a radio link at its foci.

Direct Path : AB, the actual Hop Length d

Diffracted Path : ACB = d1 + d2

(ACB) – (AB ) = Path Difference = D= n/2,For the first Fresnel zone, n = 1 and the path length differs by /2 (i.e., a 180phase reversal with respect to the direct path). For most practical purposes, only the first Fresnel zone need be considered. A radio path has first Fresnel zone clearance if, as shown in Fig. objects capable of causing significant diffraction can not penetrate the corresponding ellipsoid.

d1d2

d

The two-dimensional representation of a Fresnel zone :


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First Fresnel Radius, F1 = in mtr.

where d1 and d2 are the distances ( in mtr) from the tip of the obstacle to the two ends of the radio link and f is frquency in GHz.

REFLECTIONS :• Ground reflections:smooth ground or bodies of water.

Ground reflections can be good news or bad news, but are more often the latter. In a radio path consisting of a direct path plus a ground-reflected path, the path loss depends on the relative amplitude and phase relationship of the signals propagated by the two paths. In extreme cases, where the ground reflected path has Fresnel clearance and suffers little loss from the reflection itself (or attenuation from trees, etc.), then its amplitude may approach that of the direct path. Then, depending on the relative phase shift of the two paths, we may have an enhancement of up to 6 dB over the direct path alone, or cancellation resulting in additional path loss of 20 dB or more.


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Effects of Rain, Snow and Fog

The loss of LOS paths may sometimes be affected by weather conditions (other than the refraction effects which have already been mentioned). Rain and fog (clouds) become a significant source of attenuation only when we get well into the microwave region. Attenuation from fog only becomes noticeable (i.e., attenuation of the order of 1 dB or more) above about 30 GHz. Snow is in this category as well. Rain attenuation becomes significant for > 10 GHz, where a heavy rainfall may cause additional path loss.


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Microwave Link’s Parameters …• Free Space Loss FSL (db)= 92.45 + 20*LOG( fGHz*dKm )

• Propagation Loss The propagation loss relative to the free-space loss is the sum the following contributions:• attenuation due to atmospheric gases ( Considearble for > 15 Ghz)

• multipath fading • attenuation due to precipitationNORMAL RADIO PATH :In a normal Radio Path, Net Path Loss = FSL + Aa + Lf1 + Lf2 + Lb + Lo

Where, Aa = attenuation due to atmospheric gases,

Lf1 & Lf2 = feeder loss in dB

Lb = branching loss in dB (circulators, filters)

Lo = other loss in dB (e.g. attenuators , degradation of threshold)


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Signal strength received at Receiver is given byRSL(dBm) = Pt(dBm) – Net Path Loss(dB) + Gt (dBi) + Gr (dBi)

Where Pt = Transmissted Power; Gt = Tx Ant Gain; Gr = Rx Ant Gain

FADING MARGIN :The difference between normal received signal level and the receiver threshold level is called the fading margin:

FM (dB) = RSL (dBm) – PTh (dBm) { - PThdeg }


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PARAMETERS Which Affect the Link Reliability…

• FADINGFor small percentages of the time a path may experience that the signal level decreases or that the signal gets distorted. This signal fading is mainly due to two mechanisms called multipath fading and fading due to precipitation (rain).

Outages due to multipath are normally of short duration, less than 10 seconds. The sum of these outages gives the error performance of the radio relay system and should be compared with the ITU-R objectives given in ITU-R recommendations.

Signature Information and Selective (Dispersive) Fade Margin is calculated using ITU P.530-6 and ITU-R P.530-7. A default value of 40 db is used only when no signature information has been defined in the radio equipment database.

On the other hand, outages due to precipitation last longer than 10 consecutive seconds and are therefore termed unavailability and are added to the total unavailability of the radio-relay system. This total unavailability should be compared with the unavailability objectives given in ITU-R recommendations.


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•UNAVAILABILITY DUE TO RAIN

On any path there is a possibility of additional attenuation of the radio signal due to absorption and scattering by rain and sleet. This can be ignored at frequencies below 5 GHz. At higher frequencies, in particular above 10 GHz, it can be quite significant.

The model described in ITU-R Rec. P.530 [5.] is used to calculate the unavailability due to rain. The rainfall contour maps may be used if specific rainfall data for the region of interest is not available.


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Unavailability due to rain is given by :

Where,

Effective Path Length, d = actual path legth

= Specific Rain Attenuation

k and are regression coefficients that have been calculated for oblate spheroid raindrops for a range of frequencies. These parameters are appropriate to the polarization. These regression coefficients are given in ITU-R Rec. 838 [2.]. It should be noted that the specific attenuation is lowest for the vertical polarization.

R - the rain intensity in mm/h not exceeded for more than 0.01% of the worst month.


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Radio Threshold :It is the minimum RSL which a radio Receiver can receive with aspecified BER ( Generally: 10e-6 as Low BER & 10e-3 as High BER).It is sometimes termed as Radio Sensitivity.Threshold Degradation :It describes how much the cumulative interference will decrease theradio’s sensitivity.It is based on the equation where interference is considered to cause extra internal noise for radio equipment.


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Unreliability Equations…

Unreliability caused by Rain : It is given by Prain ( < 0.005 %)

Unreliability caused by Equipment : Pe ( <0.0022831 % single - end)

Total Annual Outage :PT = Pns + Ps + Pxp

Where, Pns – outage due to flat fading

Ps – outage due to selective fading

Pxp – reduction of the cross polar discrimination

Total Annual Unavailability :PURe = PT + PE + PR

Total Annual Availability :PRe = 100 - PURe


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FRQUENCY PLANNING…

• Connectivity Terminology• Chain• Star• Mixed • Ring / Loop

• Hi / Lo Concept

•Frequency selection / Polarization selection Done by Tool


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1 2 3 4 5 6 7 8

0 Link management 123456789

10111213141516171819202122232425262728293031

E1/PCM : 2048 kbps

For a 4+4+4 BTS:

Total TRX = 12

One TRX = 8 Ch. Of 16 Kbps each

One TRX = 2 Ch. Of 64 kbps in E1

12 TRX = 24 Ch. in E1

TRXSIG (LAPD-Sig) : 16 Kbps for each TRX

12 TRX => 16*12/64 = 3 Ch. in E1

BCFSIG (OMU) : 16 Kbps

Total Requirement : 24+3 = 27 Ch. of E1 plus 16 Kbps (2 bits) for OMU.

Rest will carry no traffic.

16 kbps Channel

64 kbps Channel


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RADIO RELAY LINK…

Branching LossBranching Loss

Tx/Rx Tx/Rx

FSL

+ Gt + Gr

- Lt - Lr

Pt Rx

Atmospheric Loss

MUX MUX16 E1s / 63 E1s

34.4 Mbps/155.52 Mbps

Access / Backbone


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A Typical Radio relay System…

Figure : Simplified block diagram of the transmitter (a) and receiver (b) of a wireless communication system.


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PDH MUX


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PDH Hierarchy


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SDH MUX


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SDH Hierarchy


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Figure : Atmosphere surrounding Earth.

Atmosphere surrounding Earth


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Frequency Bands…


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