Antenna and Propagation

“EM Wave Propagation” Dr. Cahit Karakuş, 2018

How electromagnetic waves bending in the atmosphere?

Electromagnetic waves

3

Computational Electromagnetics

• Computational Electromagnetics has been successfully applied to several engineering areas, including: – Antennas

– Microwave devices and circuits

– Surveillance and intelligence gathering

– Communications

– Homeland Security

– Energy generation and conservation

– Biological electromagnetic (EM) effects

– Medical diagnosis and treatment

– Electronic packaging and high speed circuits

– Superconductivity

– Law enforcement

– Environmental issues

– Avionics

– Signal Integrity

4

Overview of Computational Electromagnetics

• Engineering Electromagnetics

– The study of electrical and magnetic fields and their interaction

– Governed by Maxwell’s Equations (Faraday’s Law, Ampère’s Circuital Law, and Gauss’ Laws)

• Maxwell’s Equations relate the following Vector and Scalar Fields

E: the Electric Field Intensity Vector (V/M)

H: the Magnetic Field Intensity Vector (A/m)

D: the Displacement Flux Density Vector (C/m2)

B: the Magnetic Flux Density Vector (T)

J: the Current Density Vector (A/m2)

r: the Volume Charge Density (C/m3)

m: is the Permeability of the medium (H/m)

e: the Permittivity of the medium (F/m)

r D

DJHt

0 B

BEt

Faraday’s Law:

Ampère’s Circuital Law:

Gauss’ Laws:

HB m ED e

Constitutive Equations:

Maxwell’s Equations

Elektromanyetik Teori

• Maxwell tarafından 1864-1865’de ortaya atılmıştır.

• 1887’de Hertz’in deneyleri ile doğrulanmıştır.

• Huygens, ışık dalgaları ile elektromanyetik dalgaların aynı karakterde olduklarını belirten teorik tahmin ve kuralları geliştirmiştir.

Electromagnetic Waves

• In a vacuum, the EM wave propagates at the speed of light. In the atmosphere, the wave propagates with a speed slower than the speed of light. The wave’s speed, direction of propagation, and amplitude are dependent upon several atmospheric variables including temperature, moisture, and pressure.

• Electromagnetic (EM) propagation effects of the lower atmosphere on the radar, electronic warfare, or communication systems. The effects are optical interference, diffraction, tropospheric scatter, refraction, evaporation and surface-based ducting, and water vapor absorption under horizontally homogeneous atmospheric conditions.

Electromagnetic waves • A wave is a disturbance that propagates in a medium

• transverse waves on a string

• longitudinal sound waves in air

• an electromagnetic wave is an electric and magnetic disturbance that propagates through space (even vacuum) at the speed of light 299,792,458 m/s or 186,000 miles per second.

• EM waves include radio, microwaves, x-rays, light waves, gamma rays, Infrared radiation

Electric Field Magnetic Field

electric field of a

positive charge magnetic field of a

current in a wire

Elektromanyetik Işıma

• Elektrik ve manyetik alanların dalgalar şeklinde yayıldığı bir ortamdan veya vakumdan yayılan enerji şeklidir.

• Dalga, bir ortamda enerji taşıyan bir uyarıcıdır.

• Dalga boyu ( λ ), birbirini izleyen iki tepeciğin en alt ya da en üst noktaları arasındaki uzaklıktır.

• Frekans ( ν ), belirli bir noktadan birim zamanda geçen max veya min sayısıdır. Birimi 1/zaman yani 1/s olup saniyedeki çevrim sayısıdır.

• Dalganın hızı, dalganın frekansı ile dalga boyunun çarpımıdır.

İyonlaştırıcı radyasyon

• İyonlaştırıcı radyasyon, çarptığı maddede yüklü parçacıklar (iyonlar) oluşturabilen radyasyon demektir.

• İyon meydana gelmesi yani iyonizasyon olayı herhangi bir maddede meydana gelebileceği gibi insanlar dahil tüm canlılarda da oluşabilir.

• İyonlaştırıcı radyasyonlar, önlem alınmadığı takdirde tüm canlılar için zararlı olabilecek radyasyon çeşitleridir.

• Başlıca beş iyonlaştırıcı radyasyon çeşidi vardır. Bunlar, Alfa parçacıkları, Beta parçacıkları, X ışınları, Gama ışınları ve Nötronlardır.

Alfa parçacığı • Alfa ışınlarının özelikleri: Fotoğraf filmlerine etki ederler. + yüklü oldukları için elektrik ve

manyetik alanda - kutup ‘ a doğru saparlar. Karşilaştiklari moleküllerden elektron kopararak , iyonlaşmaya neden olurlar.

• Alfa parçacığı iki proton ve iki nötrondan oluşmuş bir helyum (2He4) çekirdeğidir ve pozitif yüklüdür. α işaretiyle sembolize edilirler. Çekirdeğin,alfa çıkararak parçalanması olayı atom numarası büyük izotoplarda görülür ve genellikle doğal radyoaktif atomlarda rastlanır. Alfa parçacıklarını çok küçük kalınlıklardaki maddelerle (örneğin ince bir kağıt tabaka ile) durdurmak mümkündür. Bunun sebebi, diğer radyasyon çeşitlerine göre sahip oldukları nispeten büyük elektrik yükleridir. Sahip oldukları bu elektrik yükü, alfa parçacıklarının herhangi bir madde içerisinden geçerken yolları üzerinde yoğun bir iyonlaşma meydana getirmelerine ve bu yüzden de enerjilerini çabucak kaybetmelerine yol açar. Enerjilerini bu şekilde çabucak kaybeden alfa parçacıklarının erişme uzaklıkları da dolayısıyla çok kısadır. Bu yüzden de normal olarak dış radyasyon tehlikesi yaratmazlar. Ancak, mide, solunum ve yaralar vasıtasıyla vücuda girdiklerinde tehlikeli olabilirler.

Beta parçacıkları

• Çekirdekteki enerji fazlalığı çekirdek civarında, E = mc2 eşitliğiyle açıklanabilen, bir kütle oluşturur. Bu kütle çekirdekteki fazla yükü alır ve dışarıya bir beta ışını olarak çıkar. Bunlar pozitif veya negatif yüklü elektronlardır. Pozitif yüklü elektronlar “β+” ile, negatif yüklü iyonlar ise “β-" işaretiyle sembolize edilirler. Çekirdekteki enerji fazlalığı proton fazlalığından meydana geliyorsa β+, nötron fazlalığından meydana geliyorsa β- çıkar.

• Beta parçacıkları da alfa parçacıkları gibi belli bir yük ve kütleye sahip olduklarından madde içerisinden geçerken yolları üzerinde iyonlaşmaya sebep olurlar. Ancak bu iyonlaşma, alfa parçacıklarının oluşturduğu iyonlaşmadan daha azdır. Çünkü bu parçacıklar alfa parçacıklarına göre daha hafif ve yüz kere daha giricidirler. Yine de bunlardan korunmak için ince alüminyum levhadan yapılmış bir zırh malzemesi yeterlidir.

Gama ışınları

• Gama ışınlarının kaynağı atomun çekirdeğidir. Bu ışınlar atom çekirdeğinin enerji seviyelerindeki farklılıklardan meydana gelir. Çekirdek bir alfa veya bir beta parçacığı çıkarttıktan sonra genellikle kararlı bir durumda olmaz. Fazla kalan çekirdek enerjisi bir elektromanyetik radyasyon halinde yayınlanır. Gama ışınları, beta ışınlarından daha yüksek enerjili ve dolayısıyla daha girici (nüfuz edici) ışınlardır. γ sembolize edilirler.

• Gama ve x ışınlarının, alfa ve beta parçacıklarına göre madde içine nüfuz etme kabiliyetleri çok daha fazla, iyonlaşmaya sebep olma etkileri ise çok daha azdır. Ancak birkaç santimetre kalınlığındaki kurşun tuğlalarla ve sadece belli bir kısmı durdurulabilir. Madde içerisinden geçerken üstel bir fonksiyon şeklinde bir şiddet azalmasına uğrarlar. Yüksüz olduklarından elektrik ve manyetik alanda sapma göstermezler.

X ışınları • Röntgen ışınları da denilen X ışınları, görünür ışık dalgaları ve mor ötesi ışınları gibi dalga

şeklindedir. Bir atoma dışarıdan gelen veya gönderilen yüksek enerjili elektronlar o atomun ilk halkalarından elektronlar koparırlar. Atomdan kopan bu elektronun yerine daha yüksek seviyelerden (üst halkalardan) elektronlar atlayarak kopan elektronun yerindeki boşluğu doldururlar. Bu sırada ortaya çıkan enerji fazlalığı X ışını şeklinde dışarı salınır.

• Çekirdek içerisinde bulunan protonlardan bir tanesi hareketi esnasında atomun ilk halkalarındaki elektronu yakalar ve nötrleşir. Yakalanan bu elektronun halkasındaki boşalan yere diğer bir halkadan bir elektron atlamasıyla X ışını meydana gelebilir.

• Bunların dışında da X ışını yapay olarak, röntgen tüplerinde de elde edilir. Tüp içerisinde ısıtılmış katottan yayılan elektronlar, onbinlerce voltluk gerilimle hızlandırılarak karşıdaki hedef anota çarptırılır. Bu çarpışma sonucu elektronlar durdurulurken elektronların kaybettiği enerji X ışınları olarak yayınlanır. Bu olaya Bremmstrahlung (Frenleme ışını) olayı, çıkan X ışınlarının oluşturduğu sürekli spektruma da Bremmstrahlung adı verilir.

Nötronlar • Nötronlar yüksüz parçacıklardır.

• Bu özelliklerinden dolayı herhangi bir madde içerisine kolaylıkla nüfuz edebilirler.

• Doğrudan bir iyonlaşmaya sebep olmazlar.

• Ancak atomlarla etkileşmeleri, iyonlaşmaya neden olan alfa, beta, gama veya x ışınlarının ortaya çıkmasına neden olabilir.

• Nötronlar sadece kalın beton, su veya parafin kütleleriyle durdurulabilirler.

Foton • Foton, elektromanyetik alanın kuantumu, Işığın temel "birimi" ve tüm elektromanyetik ışınların

kalıbı olan temel parçacıktır. Foton ayrıca elektromanyetik kuvvet'in kuvvet taşıyıcısıdır.

• Foton hem dalga hem de parçacık özelliği gösterir.

Electromagnetic Spectrum

Frekans Planlama

Frequency

Frequency is the number of complete cycles per second in alternating current direction. The standard unit of frequency

is the hertz, abbreviated Hz. If a current completes one cycle per second, then the frequency is 1 Hz.

Kilohertz (kHz)

Megahertz (MHz)

Gigahertz (GHz)

Terahertz (THz)

• 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

Electromagnetic Spectrum

Electrical Stimulating Currents

Commercial Radio and Television

Shortwave Diathermy

Microwave Diathermy

Infrared LASER

Visible Light

Ultraviolet

Ionizing Radiation

{

Longest

Wavelength

Shortest

Wavelength

Lowest

Frequency

Highest

Frequency

The Electromagnetic Spectrum

The range of electromagnetic signals encompassing all frequencies is referred to as the electromagnetic spectrum.

– A signal is located on the frequency spectrum according to its frequency and wavelength.

Frequency is the number of cycles of a repetitive wave that occur in a given period of time.

– A cycle consists of two voltage polarity reversals, current reversals, or electromagnetic field oscillations.

– Frequency is measured in cycles per second (cps).

– The unit of frequency is the hertz (Hz).

Wavelength is the distance occupied by one cycle of a wave and is usually expressed in meters.

– Wavelength is also the distance traveled by an electromagnetic wave during the time of one cycle.

– The wavelength of a signal is represented by the Greek letter lambda (λ).

Wavelength (λ) = speed of

light ÷ frequency

Speed of light = 3 × 108

meters/second

Therefore:

λ = 3 × 108 / f

Example:

What is the wavelength if the

frequency is 4MHz?

λ = 3 × 108 / 4 MHz

= 75 meters (m)

The Electromagnetic Spectrum

Frequency Ranges from 30 Hz to 300 GHz

– The electromagnetic spectrum is divided into segments:

Extremely Low Frequencies (ELF) 30–300 Hz.

Voice Frequencies (VF) 300–3000 Hz.

Very Low Frequencies (VLF) include the higher end of the human hearing

range up to about 20 kHz.

Low Frequencies (LF) 30–300 kHz.

Medium Frequencies (MF)

300–3000 kHz

AM radio 535–1605 kHz.

The Electromagnetic Spectrum

Frequency Ranges from 30 Hz to 300 GHz

High Frequencies (HF)

(short waves; VOA, BBC broadcasts; government

and military two-way communication; amateur

radio, CB.

3–30 MHz

Very High Frequencies (VHF)

FM radio broadcasting (88–108 MHz), television

channels 2–13.

30–300 MHz

Ultra High Frequencies (UHF)

TV channels 14–67, cellular phones, military

communication.

300–3000 MHz

The Electromagnetic Spectrum

Frequency Ranges from 30 Hz to 300 GHz

Microwaves and Super High Frequencies (SHF)

Satellite communication, radar, wireless LANs,

microwave ovens

1–30 GHz

Extremely High Frequencies (EHF)

Satellite communication, computer data, radar

30–300 GHz

The Electromagnetic Spectrum

Optical Spectrum

– The optical spectrum exists directly above the millimeter wave region.

– Three types of light waves are:

• Infrared

• Visible spectrum

• Ultraviolet

The Electromagnetic Spectrum

Optical Spectrum: Ultraviolet

– Ultraviolet is not used for communication

– Its primary use is medical.

İyonize ve İyonize Olmayan Işıma

• İyonize radyasyon, Gamma ve X ışınları olarak sıralanır. İyonize radyasyon insan hücrelerinin değişimine neden oldukları, kanser oluşturdukları ve kromozomları değiştirdikleri için tehlikelidir.

• İyonize olmayan dalgalar ise Ses dalgaları, Radyo dalgaları, Mikrodalga, Kızıl ötesi ışık, Görünen ışık, ve Morötesi ışık olarak sıralanır. İyonize olmayan dalgalar girdikleri dokulara enerjilerini aktararak ısısını artırır ya da hücre zarlarının çalışma biçimini değiştirir.

The Electromagnetic Spectrum

The electromagnetic spectrum.

c f• c = velocity of light 3x108 metres/sec

• f = frequency (Hz)

= wavelength (m)

Frequencies for communication

VLF = Very Low Frequency UHF = Ultra High Frequency

LF = Low Frequency SHF = Super High Frequency

MF = Medium Frequency EHF = Extra High Frequency

HF = High Frequency UV = Ultraviolet Light

VHF = Very High Frequency

1 Mm

300 Hz

10 km

30 kHz

100 m

3 MHz

1 m

300 MHz

10 mm

30 GHz

100 mm

3 THz

1 mm

300 THz

visible light VLF LF MF HF VHF UHF SHF EHF infrared UV

optical transmission coax cable twisted pair

SPECTRUM RANGE

• General Radio Frequency Spectrum

– 30 - 300 Hz Extremely Low Frequency (ELF)

– 300 - 3 kHz Voice Frequency (VF)

– 3 - 30 kHz Very Low Frequency (VLF)

– 30 - 300 kHz Low Frequency (LF)

– 300 - 3 MHz Medium Wave Frequency (MW)

– 3 - 3 0 MHz Short Wave Frequency (SW)

– 30 - 300 MHz Very High Frequency (VHF)

– 300 - 3000 MHz Ultra High Frequency (UHF)

– 3 - 30 GHz Super High Frequency (SHF)

– 30 - 300 GHz Extremely High Frequency (EHF)

• Microwave Spectrum

– 0.3 GHz to 3000 GHz

• Depending on the frequency range bands are

designated for operating reasons

– L Band 1 to 2 GHz

– S Band 2 to 4 GHz

– C Band 4 to 8 GHz (Satellite TV channels)

– X Band 8 to 12 GHz (Lab purposes)

– Ku Band 12 to 18 GHz

– K Band 18 to 26 GHz

– Ka Band 26 to 40 GHz (Water vapour)

– Q Band 30 to 50 GHz

– U Band 40 to 60 GHz

– V Band 46 to 56 GHz

– W Band 56 to 100 GHz and even higher (Atmospheric

studies)

•AM radio: 535 - 1,700 kHz (0.535 - 1.7 MHz)

•Short wave - 5.9 - 26.1 MHz

•Citizens band (CB) radio - 27 MHz.

•FM radio: 88 - 108 MHz.

•Television - 54 - 220 MHz.

•Microwave band: 0.3 to 300 GHz

•Mobile phones: 824 - 849 MHz

•Global Positioning System: 1.2 -1.6 GHz

•Blue tooth devices: Around 2.4 GHz

•RADAR: Weather Military

•Satellite Communication

•Long distance trunk telephone

•Microwave oven: 2.45 GHz

BROAD USAGE

MICROWAVE RADIO

• 1.7 – 2.7 GHz: Personal Commn. System

• 3.8 – 4.2 GHz: Public Operator Band (TV downlink)

• 5.9 – 7.1 GHz: Public Operator Band (TV uplink)

• 7.1 – 8.5 GHz: Long distance

• 10.7 – 11.7 GHz: Public Operator Band

• 12.7 – 13.3, 14.4 – 15.4, 21.2 – 23.6, 24.5 – 26.5, 37 – 39.5 GHz: Communication channel

• 17.7 – 19.7 GHz: Public Operator Band

General Frequency Ranges

• Microwave frequency range

– 1 GHz to 40 GHz

– Directional beams possible

– Suitable for point-to-point transmission

– Used for satellite communications

• Radio frequency range

– 30 MHz to 1 GHz

– Suitable for omnidirectional applications

• Infrared frequency range

– Roughly, 3x1011 to 2x1014 Hz

– Useful in local point-to-point multipoint applications within confined areas

MICROWAVES

MICROWAVES

• Television

• Telephone

– Long distance

– Cordless and Mobile phones

– Bluetooth

Terrestrial Microwave

• Description of common microwave antenna

– Parabolic "dish", 3 m in diameter

– Fixed rigidly and focuses a narrow beam

– Achieves line-of-sight transmission to receiving antenna

– Located at substantial heights above ground level

• Applications

– Long haul telecommunications service

– Short point-to-point links between buildings

• Microwave ovens

• Weather forecasting

• Speed sensor by traffic police

• Global positioning system for cars

• RADARs for national security and safe transport for

yourself

Satellite Microwave • Description of communication satellite

– Microwave relay station

– Used to link two or more ground-based microwave transmitter/receivers

– Receives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink)

• Applications

– Television distribution

– Long-distance telephone transmission

– Private business networks

Broadcast Radio • Description of broadcast radio antennas

– Omnidirectional

– Antennas not required to be dish-shaped

– Antennas need not be rigidly mounted to a precise alignment

• Applications

– Broadcast radio

• VHF and part of the UHF band; 30 MHZ to 1GHz

• Covers FM radio and UHF and VHF television

Wireless Transmission

• Frequencies • Signals • Antenna • Signal propagation • Multiplexing • Spread spectrum • Modulation • Cellular systems

Microwave

Spectrum

Range is approximately 1 GHz to 40 GHz

Total of all usable frequencies under 1 GHz gives a

reference on the capacity of in the microwave range

Components of a Microwave System

Digital Modem

Radio Frequency (RF) Unit

Antenna

Microwave Impairments • Equipment, antenna, and waveguide failures

• Fading and distortion from multipath reflections

• Absorption from rain, fog, and other atmospheric conditions

• Interference from other frequencies

Microwave Engineering Considerations • Free space & atmospheric attenuation

• Reflections

• Diffractions

• Rain attenuation

• Skin affect

• Line of Sight (LOS)

• Fading

• Range

• Interference

Microwave Path Analysis

• Transmitter output power

• Antenna gain

– proportional to the physical characteristics of the antenna (diameter)

• Free space gain

• Antenna alignment factor

• Unfaded received signal level

Radiated Power

Rx Sensitivity

Path Loss

Tx Power

Tx is short for “Transmit”

All radios have a certain level or Tx power that the radio generates at the RF interface. This power is calculated as the amount of energy

given across a defined bandwidth and is usually measured in one of two units:

1. dBm – a relative power level referencing 1 milliwatt

2. W – a linear power level referencing Watts

dBm = 10 x log[Power in Watts / 0.001W]

W = 0.001 x 10[Power in dBm / 10 dBm]

The NCL and LMS radios have Tx power of +18dBm, which translates into .064 W or 64 mW.

Rx Sensitivity

Rx is short for “Receive”

All radios also have a certain ‘point of no return’, where if they receive a signal less than the stated Rx Sensitivity, the

radio will not be able to ‘see’ the data.

This is also stated in dBm or W.

The NCL and LMS radios have a receive sensitivity of –82 dBm. At this level, a Bit Error Rate (BER) of 10-5 (99.999%) is

seen.

The actual level received at the radio will vary depending on many factors.

Radiated Power In a wireless system, antennas are used to convert electrical waves into electromagnetic waves. The amount of energy the antenna can ‘boost’ the sent and received signal by is referred to as the antennas Gain.

Antenna gain is measured in:

1. dBi: relative to an isotropic radiator

2. dBd: relative to a dipole radiator

0 dBd = 2.15 dBi

There are certain guidelines set by the FCC that must be met in terms of the amount of energy radiated out of an antenna. This ‘energy’

is measured in one of two ways:

1. Effective Isotropic Radiated Power (EIRP)

measured in dBm = power at antenna input [dBm] + relative antenna gain [dBi]

2. Effective Radiated Power (ERP)

measured in dBm = power at antenna input [dBm] + relative antenna gain [dBd]

Energy Losses In all wireless communication systems there are several factors that contribute to the loss of signal strength. Cabling, connectors,

lightning arrestors can all impact the performance of your system if not installed properly.

In a ‘low power’ system (such as the NCL and LMS products) every dB you can save is important!! Remember the “3 dB Rule”.

For every 3 dB gain/loss you will either double your power (gain) or lose half your power (loss).

-3 dB = 1/2 power

-6 dB = 1/4 power

+3 dB = 2x power

+6 dB = 4x power

Sources of loss in a wireless system: free space, cables, connectors, jumpers, obstructions

The greatest amount of loss in your wireless system will be from Free Space Propagation. The Free Space Loss is

predictable and given by the formula:

FSL(dB) = 32.45 + 20Log10F(MHz) + 20Log10D(km)

The Free Space Loss at 1km using a 2.4 GHz system is:

FSL(dB) = 32.45 + 20Log10(2400) + 20Log10(1)

= 32.45 + 67.6 + 0

= 100.05 dB

The Free Space Loss

Understanding RF Propagation

Goals

1. Estimate radio coverage area

2. Estimate link performance

3. Estimate network design parameters

1. Transmitters and their location

2. Transmit power

3. Antenna type

Free Space Propagation Model

PT

PR

d

2/

24mW

d

PP T

Di

Isotropic power

density

24 d

GPP TT

D

Power density along

the direction of

maximum radiation

effTT

R Ad

GPP

24

4

2

G

Aeff

2

dGGPP RTTR

Power received by

Antenna effDR APP

Predict received signal

strength when the transmitter

and receiver have a clear

line-of-sight path between them

Also known

as Friis free

space formula

Path Loss (relative measure)

Pt

PR 2

dGG

P

PRT

T

R

)log20log205.32()()( 1010 fdGGP

PdBRdBT

dBT

R

2

3

)(

10*57.0

dfGG

P

PRT

T

R

f is in MHz

d is in Km

Path Loss represents signal attenuation

(measured on dB) between the effective

transmitted power and the receive power

(excluding antenna gains)

Path Loss (Example)

Pt

PR

50 W

= 47 dBm

Assume that antennas are isotropic.

Calculate receive power (in dBm) at free space distance of 100m

from the antenna. What is PR at 10Km?

dBP

P

dBT

R 5.71

)log20log205.32()()( 1010 fdGGP

PdBRdBT

dBT

R

dBP

P

dBT

R 5.111

)900log201.0log205.32(00 1010

dBT

R

P

P

-20 (for d = 0.1)

59

20 (for d = 10)

dBmP dBmR 5.245.7147)( dBmP dBmR 5.645.11147)(

Electric and Magnetic Fields

• For waves we use the following units: – Electric field strength E (V/m) Magnetic field strength H (A/m) Power density PD (W/m2) – Ohm’s law holds if characteristic impedance Z of medium is used

• For free space, Z = 377 Ohm

HEZ /

Ohm’s Law in Space Power Density

EH

H

EPD

Z

Z2

2

2

377

ES

37700 cZ m2

0 0

E B ES

cm m

Attenuation of Free Space • Power stays the same but power density is

reduced with increasing distance r • Power density is total power divided by

surface area of sphere • Unit: watts/meter

24 r

PP t

D

Free Space Electric Field

• Electric field strength is relatively easy to measure

• Often used to specify signal strength

• Unit: volts/meter r

PE t30

Power Density at distance r including antenna Gain

24 r

GPA

PAP

TTeff

DeffR

24 r

GPP TT

D

Received Power

Calculation of Effective Area

4

2

Reff

GA

Path Loss

• Friis’s Formula expresses the attenuation of free space in a convenient decibel form

• Units are typical engineering units, not basic units like Hz and meters

• Loss is in dB, distance in km, frequency in MHz, gains in dBi (decibels with respect to an isotropic radiator)

Friis’s Formula RTfs GGfdL log20log2044.32

Link Budget Calculations

To establish the viability of a link prior to installing any equipment, a Link Budget Calculation needs to be made. Performing

this calculation will give you an idea as to how much room for path loss you have, and give you an idea as to link quality.

Using the WaveRider Link Path Analysis Tool (LPA Tool), the Fade Margin and other link criteria can be mathematically

calculated to determine link quality.

Fade Margin

– Defined as the difference between the Receive Signal Level RSL, and the Rx

Threshold or other chosen reference Level.

– For path lengths of 16km or less, a minimum 10dB Fade Margin is recommended

–Path Loss (dB)

–Field Factor (dB)

–Antenna Gain

–(dBi)

–Cable Losses

–(dB)

–Connector

–Losses

–(dB)

–Connector

–Losses

–(dB) –Cable Losses

–(dB)

–A –B

–Received Signal Level – (dBm) = Tx Output (dBm) - Path

–Loss(dB) - Field Factor (dB) + Total Antenna Gains (dB) - Total

–Cable Losses (dB) - Total Connector Losses (dB)

–Antenna Gain

–(dBi)

–Tx Output (dBm) –Tx Output (dBm)

Line of Sight Attaining good Line of Sight (LOS) between the sending and receiving antenna is essential in both Point to Point and Point

to Multipoint installations.

Generally there are two types of LOS that are used discussed during installations:

1. Optical LOS - is related to the ability to see one site from the other

2. Radio LOS – related to the ability of the receiver to ‘see’ the

transmitted signal

The Earth's Atmosphere

Structure and Characteristics of the Earth's Atmosphere

• The earth's atmosphere is a collection of many gases together with suspended particles of liquid and solids. Excluding variable components such as water vapor, ozone, sulfur dioxide, and dust, the gases of nitrogen and oxygen occupy about 99 percent of the volume, with argon and carbon dioxide being the next two most abundant gases. From the earth's surface to an altitude of approximately 80 kilometers, mechanical mixing of the atmosphere by heat-driven air currents evenly distributes the components of the atmosphere. At about 80 kilometers, the mixing decreases to the point where the gases tend to stratifr, in accordance with their weights.

• Refractivity measurements from different geographic regions as well as new global maps of various meteorological parameters.

Propagation Models

• Optical Interference Region

• Optical Path Length Difference

• Reflection Coefficients

• Antenna Pattern Factors

• Diffraction

• Troposcatter

• Water Vapor Absorption

• Sea Clutter

• Raytracing

• Evaporation Duct Height

• Wind

FREE SPACE LOSS

LINK POWER & NOISE -LOS

FREE SPACE LOSS

REFRACTION BY THE LOWER ATMOSPHERE

EQUIVALENT EARTH RADIUS

DISTANCE FROM Rx ANTENNA

STANDARD REFRACTION

PRESSURE MODELLING

The Refractive Index

EFFECTIVE EARTH RADIUS

ANTENNA PATTERN FACTOR

EM waves in free space • v2 = 1/(eoµo) so v = 3 x 108 m/s

– eo = 8.855 x 10-12 Farads/m

– µo = 1.2566 x 10-6 Henrys/m

• EM waves in free space propagate freely without attenuation

• What is a plane wave?

– Example is a wave propagating along the x-direction

– Fields are constant in y and z directions, but vary with time and space along the x-direction

– Most propagating radio (EM) waves can be thought of a plane waves on the scale of the receiving antenna

E & H fields and Poynting Vector for Power Flow

• Power flow in the EM field – P = E x H (P is Poynting vector)

• In free space E and H are perpendicular

• P is perpendicular to both E and H

• Plane wave radiated by an antenna – P = E x H -> Eo Ho Sin2(t-kx)

– P = [Eo2/] Sin2(t-kx)

– Pavg = (1/2) [Eo2/] in W/m2

– = impedance of free space = 377

Electromagnetic Spectrum

After Kraus & Marhefka, 2003

Signal

propagation

Propagation Modes

• Ground-wave propagation

• Sky-wave propagation

• Line-of-sight propagation

Ground Wave Propagation

Ground Wave Propagation

• Follows contour of the earth

• Can Propagate considerable distances

• Frequencies up to 2 MHz

• Example

– AM radio

Line-of-Sight Propagation

Line-of-Sight Propagation

• Transmitting and receiving antennas must be within line of sight

– Satellite communication – signal above 30 MHz not reflected by ionosphere

– Ground communication – antennas within effective line of site due to refraction

• Refraction – bending of microwaves by the atmosphere

– Velocity of electromagnetic wave is a function of the density of the medium

– When wave changes medium, speed changes

– Wave bends at the boundary between mediums

Line-of-Sight Equations

• Optical line of sight

• Effective, or radio, line of sight

• d = distance between antenna and horizon (km)

• h = antenna height (m)

• K = adjustment factor to account for refraction, rule of thumb K = 4/3

hd 57.3

hd 57.3

Line-of-Sight Equations

• Maximum distance between two antennas for LOS propagation:

• h1 = height of antenna one

• h2 = height of antenna two

2157.3 hh

Multipath Propagation

• Reflection - occurs when signal encounters a surface that is large relative to the wavelength of the signal

• Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave

• Scattering – occurs when incoming signal hits an object whose size in the order of the wavelength of the

signal or less

The Effects of Multipath Propagation

• Multiple copies of a signal may arrive at different phases

– If phases add destructively, the signal level relative to noise declines, making detection more difficult

• Intersymbol interference (ISI)

– One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit

Types of Fading

• Fast fading

• Slow fading

• Flat fading

• Selective fading

• Rayleigh fading

• Rician fading

Signal propagation ranges

distance

sender

transmission

detection

interference

• Transmission range

– communication possible

– low error rate

• Detection range

– detection of the signal possible

– no communication possible

• Interference range

– signal may not be detected

– signal adds to the background noise

• Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction

• Time dispersion: signal is dispersed over time

• interference with “neighbor” symbols, Inter Symbol Interference (ISI)

• The signal reaches a receiver directly and phase shifted

• distorted signal depending on the phases of the different parts

Multipath propagation

signal at sender

signal at receiver

LOS pulses multipath

pulses

Modes of Propagation

• Ground wave – medium wave broadcast

• Sky wave – HF bands 3 to 30 MHz

• Line of Sight (LOS) – higher frequencies

Ground-Wave Propagation

• Happens at relatively low frequencies

– up to about 2 MHz

• Only works with vertically polarized waves

• Waves follow the curvature of earth

– range varies from worldwide at 100 kHz and less to about 100 km at AM broadcast band frequencies (approx. 1 MHz)

Sky Wave Propagation

Sky Wave Propagation

• Signal reflected from ionized layer of atmosphere back down to earth

• Signal can travel a number of hops, back and forth between ionosphere and earth’s surface

• Reflection effect caused by refraction

• Examples

– Amateur radio

– CB radio

Ionospheric Propagation • Useful mainly in HF range (3-30 MHz)

• Signals are refracted in ionosphere and returned to earth

• Worldwide communication is possible using multiple “hops”

Ionospheric Layers

• D layer: height approx. 60-90 km

• E layer: height approx. 90-150 km

• F1 layer: height approx. 150-250 km

• F2 layer: height approx. 250-400 km

• D, E layers disappear at night

• F layers combine into one at night

Ionospheric Activity

• More ionization causes signals to bend more

• Ionization caused by solar radiation

– greater during daytime

– greater during sunspot cycle peaks (we are about at a decreasing value now-2004)

• D,E layers are less highly ionized than F layer and usually just absorb signals

If we consider a wave of frequency , f incident on an ionospheric layer whose maximum density

is N then the refractive index of the layer is given by

Effects of the Ionosphere on the Sky wave

2

811

f

Nn

If the frequency of a wave transmitted vertically is increased, a point will be

reached where the wave will not be refracted sufficiently to curve back to earth and

if this frequency is high enough then the wave will penetrate the ionosphere and

continue on to outer space. The highest frequency that will be returned to earth

when transmitted vertically under given atmospheric conditions is called the critical

frequency.

Critical Frequency

Nfc

9

There is a best frequency for communication between any two points under specific

ionospheric conditions. The highest frequency that is returned to earth at a given distance is

called the Maximum Usable Frequency (MUF).

Maximum Usable Frequency

sec9 Nfmuf

This is the frequency which provides the most consistent communication and is therefore the

best to use. For transmission using the F2 layer it is defined as

Optimum Working Frequency

sec985.0 Nfowf

Skip Zone

• Region between maximum ground-wave distance and closest point where sky waves are returned from the ionosphere,

Free Space & Atmospheric Attenuation

• Free space & atmospheric attenuation is defined by the loss the

signal undergoes traveling through the atmosphere. Changes in

air density and absorption by atmospheric particles.

Rain Attenuation

• Raindrop absorption or scattering of the microwave signal can

cause signal loss in transmissions.

Skin Affect

• Skin Affect is the concept that high frequency energy travels only on the outside skin of a conductor and does not penetrate into it any great distance. Skin Affect determines the properties of microwave signals.

Microwave Fading

Normal Signal

Reflective Path

Caused by multi-path reflections and heavy rains

Range

• The distance a signal travels and its increase in frequency are inversely proportional

• Repeaters extend range

– Back-to-back antennas

– Reflectors

• High frequencies are repeated/received at or below one mile

• Lower frequencies can travel up to 100 miles but 25-30 miles is the typical placement for repeaters

Interference

• Adjacent Channel Interference

– digital not greatly affected

• Overreach

– caused by signal feeding past a repeater to the receiving antenna at the next station in the route. Eliminated by zigzag path alignment or alternate frequency use between adjacent stations

Interference Countermeasures

1. Short Paths

2. Narrow Beam Antennas (high gain)

3. Frequency Selection

4. Antenna Polarization

5. Antenna Azimuth

6. Equipment/Antenna Location

Propagation Modes

LOS Wireless Transmission Impairments

• Attenuation and attenuation distortion

• Free space loss

• Noise

• Atmospheric absorption

• Multipath

• Refraction

• Thermal noise

Signal propagation • Propagation in free space always like light (straight line)

• Receiving power proportional to 1/d² (d = distance between sender and receiver)

• Receiving power additionally influenced by

• fading (frequency dependent)

• shadowing

• reflection at large obstacles

• refraction depending on the density of a medium

• scattering at small obstacles

• diffraction at edges

reflection scattering diffraction shadowing refraction

● Atmospheric attenuation

● Reflection off of earth’s surface

● Over-the-horizon diffraction

● Atmospheric refraction

Radar beams can be attenuated, reflected and bent by the environment

Reflection:

Refraction:

EM waves propagation

Radio Channels

– Frequency & Wavelength

– Classification & Use

– Modes of Propagation

– Propagation Mechanisms

– Atmospheric Attenuation

– Propagation Models

– Fading Channels

– Multipath

– Noise

Propagation Mechanisms

• At low frequencies (long wavelengths) propagating radio waves tend to follow the earth’s surface.

• At higher frequencies they tend to travel in straight lines.

• At HF (3 – 30 MHz) radio waves are reflected by the ionosphere:

– a series of layers of charged particles, ionised by radiation from the sun, at between 30 and 250 miles about the earth’s surface and known as D, E and F layers. Coalesce at night gives longer skip distances.

Radio Propagation… 2

• Above 300 MHz propagation is by line of sight.

• Higher still, above 3 GHz say, atmospheric gases (mainly oxygen), water vapour and precipitation (rain!) absorb and scatter radio waves.

– 23 GHz – water vapour resonance

– 62 GHz – oxygen absorption

• Care needed in design of microwave links and ground to satellite links.

Atmospheric Attenuation

– a) attenuation caused by atmospheric gases – note molecular resonance peaks

– b) attenuation caused by rain – can increase path loss by an order of magnitude ( 10 x)

Reflection

• Specular reflection: smooth surface – Angle of incidence = angle of reflection

• Diffuse reflection: rough surface – Reflection in all directions because angle of incidence varies

over the surface due to its roughness

• Reflections can occur as the microwave signal traverses a body of

water or fog bank; cause multipath conditions

Refraction • Occurs when waves move from one medium to another with a different propagation

velocity

• Index of refraction n is used in refraction calculations

• Bending of signals by atmosphere decreases with increasing frequency

• Bending of signals by atmosphere increases with increasing ionization

rn e

Refraction

• Refractivity and Modified Refractivity

• Effective Earth Radius Factor

• Refractive Gradients (Standard, Subrefraction, Superrefraction, Trapping)

• Atmospheric Ducts (Surface ducts, Evaporation ducts, Elevated ducts)

Diffraction

• Occurs when radiation passes an object with dimensions small compared with wavelength

• The object appears to act as a source of radiation

• Allows radio stations to be received on the shadow side of obstacles

• Diffraction is the result of variations in the terrain the signal crosses

Polarization

• Polarization of a wave is the direction of the electric field vector

• Linearly polarized waves have the vector in the same direction at all times – Horizontal and vertical polarization are common

• Circular and elliptical polarization are also possible

Terrestrial Propagation

• Propagation over earth’s surface

• Different from free-space propagation

– Curvature of the earth

– Effects of the ground

– Obstacles in the path from transmitter to receiver

– Effects of the atmosphere, especially the ionosphere

Doppler Shift

Doppler Shifts Classical Waves

Note that the exact value of the shift depends on whether the source or the receiver is moving (though the sign depends only on relative motion).

This is because the medium in which the sound wave propagates gives a preferred frame of reference—you can tell whether you are moving through the air, or if the approaching fire truck is moving through the air.

However, according to special relativity there can be no preferred frame of reference—the speed of light is identical for all observers (c.f., the Michelson-Morley experiments). The consequence of this is that the Doppler shift for electromagnetic radiation is the same whether or not the source is moving or the receiver is moving (see appendix for derivation):

1

1

vc vcf f f

v c vc

- Top signs for decreasing separation

- Bottom signs for increasing separation

- If v << c, f΄ ≈ f (1+ v/c)

Applications: Police Radar

Here there is a double effect:

First, my car is moving toward the radar transmitter as an “observer”;

second, my car acts as a source which is moving toward the police car

receiver.

2 2(1 ) (1 ) (1 ) v v v

c c cf f f f

100 miles/hour = 44.7 m/sv

10.6 GHzf " 10.60000316 GHzf

Ex.

3160 Hz f

Appendix: Relativistic Doppler Shift

' sv TT T

c

2

2

1

1 v

c

T T

1(1 )sv

T Tf c

2

211 1

1 (1 )

s

s s

v

c

v v

c c

fT T

2

21

1 s

v

c

v

c

f

(1 )(1 )

1

s s

s

v v

c c

v

c

f

(1 )

(1 )

s

s

v

c

v

c

f f

For vs<<c

(1 )svf f

c

Moving clocks run slower (“time dilation”)

→ emit less often

→ period bigger:

sv T

vs ☻

Doppler Shift • Doppler shift causes change in measured frequency.

– When a light source moves towards an observer, the light frequency is shifted higher (i.e. blue shift).

– When a light source moves away from an observer, the light frequency is shifted lower (i.e. red shift).

– Only difference with “classical” Doppler shift for sound is the incorporation of time dilation (causes square root factor).

Approaching - blue shift

1 /

1 /obs source

v cf f

v c

Note: For a receding source, switch signs.

Doppler Shift:

The light from a nearby star is observed to be shifted toward the red by 5% (f = 0.95 f o). Is the star approaching

or receding from the earth? How fast is it moving?

2 2 2

2 2

2 2

7

1where 0.95 and solve for

1

1 1 1 1

1 1 0.95 0.0975

1.90251 1 0.95

(1.54 10.0512 0 m/s)

o

o

o o o

o o o

o o o

f vf f

f c

f f f f f f

f f f f

f f f f

The star is receding the earth because the frequency is shifted to a lower value.

Fresnel Zones

Line of Sight Fresnel Zone Clearance

• Fresnel Zone Clearance is the minimum clearance over obstacles that the signal needs to be sent over. Reflection or path bending will occur if the clearance is not sufficient.

To quantify Radio Line of Sight, the Fresnel Zone theory is applied. Think of the Fresnel Zone as a football shaped

tunnel between the two sites which provides a path for the RF signals.

At WaveRider acceptable Radio Line of Sight means that at least 60% of the first Fresnel Zone plus 3 meters is clear of

any obstructions.

LOS & FZC-cont’d Fresnel Zone

72.2

D1 X D2

F x D

secret formula

2nd* 1st* 3rd*

* Fresnel Zones

Fresnel Zones

Site A

Site B • Fresnel Zone diameter depends upon Wavelength, and

Distances from the sites along axis

• For minimum Diffraction Loss, clearance of at least 0.6F1+ 3m

is required

d2

d1

Radius of n th Fresnel Zone

given by:

2 1

2 1

d d

d d n r

n

The First Fresnel Zone

• Radius of the first Fresnel zone

R=17.32(x(d-x)/fd)1/2

where d = distance between antennas (in Km)

R= first Fresnel zone radius in meters

f= frequency in GHz

x

y

d=x+y

R

Fading and Fade margins

Microwave Link Design

Microwave Link Design is a methodical, systematic and sometimes lengthy process that includes

• Loss/attenuation Calculations

• Fading and fade margins calculations

• Frequency planning and interference calculations

• Quality and availability calculations

Microwave Link Design Process The whole process is iterative and may go through many redesign phases before the required quality and availability are achieved

Frequency

Planning

Link Budget

Quality

and

Availability

Calculations

Fading

Predictions

Interference

analysis

Propagation losses

Branching

losses

Other Losses

Rain

attenuation

Diffraction-

refraction

losses

Multipath

propagation

Loss / Attenuation Calculations

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

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.

Gas attenuation versus frequency

T=30o

RH=50%

Frequency (GHz)

0 25 50

0.4

T=40oC

RH=80%

1.0

23GHz

Total specific

gas attenuation

(dB/Km)

Link Budget

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)

Fading and Fade margins

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.They are all dependent on path length and are estimated as the probability of exceeding a given (calculated) fade margin

• Multipath fading

-Flat fading

-Frequency-selective fading

• Rain fading

• Refraction-diffraction fading (k-type fading)

• 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

Fading and Fade margins

• 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

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

Fading and Fade margins

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 margins

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 margins

• 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

Fading and Fade margins – 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.

• 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, crossband diversity is very effective

– Route diversity with paths separated by more than about 8 Km can be used successfully

Fading and Fade margins – 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 polarisation frequencies.

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

Frequency planning

– 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 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

• 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. In India the authority is WPC (Wireless Planning & Coordination Wing)

• 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

• 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.

Interference fade margin

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.

• Adjacent-channel interference fade margin (AIFM) (in decibels) accounts for receiver threshold degradation due to interference from adjacent channel transmitters

• Interference fade margin (IFM) 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.

Interference fade margin

• There are two widely used methods. The C/I (carrier to interference) and T/I (threshold to interference) methods. C/I method is the older method developed to analyse interference cases into analog radios. In the new T/I method, 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

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 errored second

Availability is expressed as a percentage as : -

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

For each interfering transmitter, the receive power level in dBm is compared to the maximum power level to determine whether the interference is acceptable. The T/I curves are based on the actual lab measurements of the radio.

Composite Fade Margin (CFM) is the fade margin applied to multipath fade outage equations for a digital microwave radio

CFM = TFM + DFM + IFM + AIFM

= -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

Space Diversity

Normal Signal

Transmitter Receiver

Frequency Diversity

Receiver

Active XTMR

Frequency #1

Protect XTMR

Frequency #2

RCVR

Frequency #1

RCVR

Frequency #2

Transmitter

Improving the Microwave System

• Hardware Redundancy

– Hot standby protection

– Multichannel and multiline protection • Diversity Improvement

– Space Diversity

– Angle Diversity

– Frequency Diversity

– Crossband Diversity

– Route Diversity

– Hybrid Diversity

– Media Diversity

NOISE

Categories of Noise

• Thermal Noise

• Intermodulation noise

• Crosstalk

• Impulse Noise

Thermal Noise

• Thermal noise due to agitation of electrons

• Present in all electronic devices and transmission media

• Cannot be eliminated

• Function of temperature

• Particularly significant for satellite communication

Thermal Noise

• Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is:

• N0 = noise power density in watts per 1 Hz of bandwidth

• k = Boltzmann's constant = 1.3803 x 10-23 J/K

• T = temperature, in kelvins (absolute temperature)

W/Hz k0 TN

Thermal Noise

• Noise is assumed to be independent of frequency

• Thermal noise present in a bandwidth of B Hertz (in watts):

or, in decibel-watts

TBN k

BTN log10 log 10k log10

BT log10 log 10dBW 6.228

Noise Terminology • Intermodulation noise – occurs if signals with different frequencies share the same medium

– Interference caused by a signal produced at a frequency that is the sum or difference of original frequencies

• Crosstalk – unwanted coupling between signal paths

• Impulse noise – irregular pulses or noise spikes – Short duration and of relatively high amplitude

– Caused by external electromagnetic disturbances, or faults and flaws in the communications system

– Primary source of error for digital data transmission

Expression Eb/N0

• Ratio of signal energy per bit to noise power density per Hertz

• The bit error rate for digital data is a function of Eb/N0 – Given a value for Eb/N0 to achieve a desired error rate, parameters of this formula can be selected

– As bit rate R increases, transmitted signal power must increase to maintain required Eb/N0

TR

S

N

RS

N

Eb

k

/

00

Kaynaklar

• Antennas from Theory to Practice, Yi Huang, University of Liverpool UK, Kevin Boyle NXP Semiconductors UK, Wiley, 2008.

• Antenna Theory Analysis And Desıgn, Third Edition, Constantine A. Balanis, Wiley, 2005

• Antennas and Wave Propagation, By: Harish, A.R.; Sachidananda, M. Oxford University Press, 2007.

• Navy Electricity and Electronics Training Series Module 10—Introduction to Wave Propagation, Transmission Lines, and Antennas NAVEDTRA 14182, 1998 Edition Prepared by FCC(SW) R. Stephen Howard and CWO3 Harvey D. Vaughan.

• Lecture notes from internet.

Usage Notes

• These slides were gathered from the presentations published on the internet. I would like to thank who prepared slides and documents.

• Also, these slides are made publicly available on the web for anyone to use

• If you choose to use them, I ask that you alert me of any mistakes which were made and allow me the option of incorporating such changes (with an acknowledgment) in my set of slides.

Sincerely,

Dr. Cahit Karakuş

cahitkarakus@gmail.com