microwave transmission basics

April 11, 2023, Slide No 1

Transmission System

Introduction. Low capacity transmission. High capacity transmission. Optical Fibre. Network configuration.


The Transport Network is divided into two parts:Access network

Users are grouped together by being connected to the same switching node; the local exchange or access node (AN).

Trunk network Trunk network connects ANs and network nodes (NN) with other ANs and NNs.

Transmission Media’sCopper CableRadio linkOptical fibre

Access and Trunk Network


MS Mobile Station = phone + SIM card

BTS Base Transceiver Station

MSCVLR

PSTNFixed network

BSC

SMSC

EIR

INTERNET

PABXPSTNInternational

BSC

Telenor Nett

IN

PrePaidNode

VMS

HLR

MSC

AUC


Media

Copper cable Low bandwidth (-) Sensitive to crosstalk and noise(-) Attenuation per Km depends on wire diameter (0.4-1.1mm) and frequency.Reliable(+)

*** Attenuation is the loss of signal energy is measured in decibel,db.

Attenuation = 10log(P1/P2) P1=transmitted signal power P2=received signal power.

***Noise is the random variation in current and voltage produced by all electrical circuits.

***Crosstalk is the interference from another conversation/line.


Media

Radio Flexibility +

Quick installation +

Sensitive to ambient disturbance. (Rain and Multipath fading).-

Time and frequency dependent.+

Modulation /Demodulation.+

Low bit error (BER) in bursts.+

QAM (Quadrature Amplitude Modulation) [PSK+AM] .

Radio license fees.-

***Non modulated signal is called Baseband signal


Media

Optical Fibre

High transmission quality. (minimum attenuation). + High transmission capacity. + Resistance to ambient disturbance. + Long Implementation time. -

.


Plesiochronous Digital Hierarchy (PDH)

As the semiconductor development proceeded successfully higher hierarchic levels have been standardized, each level handling timing transparency.

PDH is a standardized bit interleaved multiplexing hierarchy for digital plesiochronous signals, using bit justification to achieve timing transparency.

Actually there are three PDH hierarchies- two are based on 1.5 Mbits and used in North America & Japan. The European hierarchy is 2Mbits .


140Mbps

34Mbps

8Mbps

2Mbps

x4

x4

x4

x31

64 Kbps

2Mbps

8Mbps

64Kbps

8Kbps

x31

x120

x4

400Mbps

100 Mbps

32Mbps

6Mbps

1.5Mbps

nx45Mbps

45mbps

6Mbps

x4

x3

x5

x4 x4

x24

64Kbps

Europe and Most other countries

Japan North America

Synchronous

Plesiochronous


Radio Path Calculation

Radio wave propagationPropagation mechanismEarth Properties

Fading free conditionFree space lossGas absorbtionObstacle loss

Fading ConditionFade MarginFading MechanismLink Budget

RepeatersActive or Passive


• Free space• Absorption• Diffraction• Reflection/scattering• Refraction• Multipath propagation

• Atmosphere• Earth topography• Electrical properties of earth

surface• Earth curvature

Earth properties Propagation mechanisms

Attenuation of the radio wave


Fading free condition

Pr(dbm)=Pt(dbm)+Gt(db)+Gr(db)-20logf(ghz)-20logd(km)-92.4

Free space loss= 20logf(ghz)+20logd(km)+92.4

Fading Free Condition Free space loss Gas absorbtion Obstacle loss


The Fresnel zone

ReflectedhA hB

The radiated power is distributed in a zone surrounding the direct line-of-sight

Direct

Refracted

d

da db

1st fresnel zone : da+db-d=λ/2


Ground clearance and k factor

ReflectedhA hB

Radio optical line of sight

Direct

Refracted

d

da db

Due to refraction in the atmosphere the radio beam is bent normally slightly downwardsThe bending effect is described by the earth factor kK =157/157+dn/dh=4/3 ; where dn/dh=- 40, N= densityH=height

K=α K=4/3

K=1

K=2/3Ground clearance

Geometrical line of sight


Trans-mitter

Receiver

OutputPower(dBm)

Effective radiated power(dBW)

Path loss

Cable loss

Antenna gainAntenna gain

Cable loss

Wave propagation loss•Free-space loss•Gas absorption•Obstacle loss

ReceivedPower(dBm)

Terminology


When the size of an obstacle is large compared with the wavelength,diffraction may occur giving obstacle loss

20 dB16 dB12 dB6 dB0 dB0 dB

Obstacle loss


Fading Mechanisms

Fading Mechanisms

Rain Fading Multipath Fading K- fading

Flat fading Frequency selective fadingReflections

Atmospheric fading ( Absorbtion, refraction)Ground based fading( Tidesand other variations)Man made fading( temporary constructions, boats , aircrafts)

Atmospheric fading ( Absorption, refraction) Ground based fading( Tides and other variations)

Man made fading( temporary constructions, boats , aircrafts)

Variation in signal strength in time, phase or polarization Attenuation that is changing temporarily.


antenna gain

feeder lossreceived power

fade margin

receiver thresholdvalue

wave propagation losses

antennagain

outputpower

feeder loss

POWER

Free space loss +gas absorption +

obstacle loss

Link budget & Fade Margin

Free space loss +gas absorption +

obstacle loss

Fade marginShould be large enough toguarantee that quality and availability objectives are metduring fading conditionsFM=P (n) – P (th)Maximum attenuation before giving out of service

Fade Margin is required for proper performanceIf Rx power is high from the level of the fade margin then

Interference will occur.


• Free space• Absorption• Diffraction• Reflection/scattering• Refraction• Multipath propagation

• Atmosphere• Earth topography• Electrical properties of earth

surface• Earth curvature

Earth properties Propagation mechanisms

Attenuation of the radio wave


Obstacle lossReflection loss

Predictableif present

Link budget

Quality & availability

Fading prediction

Free-space attenuationGas attenuation Rain fading

Multipath fading

Always present andpredictable

Not always presentbut statistically pre-dictable

Maximum values

Free space 130 dB

Gas 5 dB

Obstacle < 20 dB

Reflection 15 dB

Loss and fading


Recommendations from ITU-T

G.821

Error performance of a digital connection below the primary rate (commonly used in PDH Network)

G.826

Error performance of a digital connection at or above the primary rate (Commonly used in SDH network)

Quality and availability Targets


Errored second (ES) A one-second period in which one or more bits are in error.

Severely errored second (SES) A one-second period which has a bit error ratio higher than 10

-3.

Definitions of bit errors

Errored blocks (EB)A block in which one or more bits are in error.

Errored second (ES) A one-second period in which one or more blocks are in error.

Severely errored second (SES) A one-second period which contains >30% errored blocks.

Background block error (BBE)An errored block not occurring as part of an SES

G.821

G.826


Definitions of availability (G.826)

T1306430-95

Time

10 secsec< 10 10 sec

Unavailability detected Availability detected

Unavailable period Available period

Severely Errored Second

Errored Second (non-SES)

Error-free Second


Thank you!

microwave transmission basics

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