microwave radio

1/144 D. Courivaud, Groupe ESIEE, Paris, May 2004

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Microwave radio

Microwave radio

A very brief history of microwavesIntroductionPropagation

Digital codingSystems


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1. A very brief history of microwaves


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Theoretical foundation

� Maxwell 1831-1879– light consists of transverse

undulations of the same medium which is the cause of electric and magnetic phenomena: "On aDynamical Theory of the Electromagnetic Field" (1865).

– formulation of electricity and magnetism: « A Treatise onElectricity and Magnetism » (1873),which included the formulas today known as the Maxwell equations that implicitly required the existence of electromagnetic waves traveling at the speed of light.


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First experimental proof

� Hertz 1857 – 1894– He proved experimentally the

existence of electromagnetic waves (1888)

– a wire connected to an induction coil produces the waves and induced current produced a spark across a small loop of wire

– he showed his electromagnetic waves to have analogous propertiesas light.


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First commercial exploitation

� Marconi 1874 – 1937– In 1895 he began laboratory

experiments and succeeded insending wireless signals over a distance of one and a half miles

– In 1899 he established wirelesscommunication between Franceand England across the EnglishChannel

– He obtained the first patent inthe history of radio (1900)

– On December 12, 1901, he sent a radio signal across the Atlantic (2100 miles)

– Nobel Prize in physics (1909)


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2. Introduction


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Wavelength

� Wavelength is the distance between identical points in the adjacent cycles of a waveform. In wireless systems, this length is usually specified in meters, centimeters, or millimeters

� The size of the wavelength varies depending on the frequency of the signal. Generally speaking, the higher the frequency the smaller the wavelength

� Wavelength is of great interest for antenna dimensioning and positionning

300m

MHZ

cf f

λ = =

At 10 GHz, λ = 3 cmAt 1 GHz, λ = 30 cmAt 100 MHz, λ = 3 mAt 10 MHz, λ = 30 m

…


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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.

� 103 = Kilohertz (kHz)� 106 = Megahertz (MHz)� 109 = Gigahertz (GHz)� 1012 = Terahertz (THz)


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Power units


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Path loss in dB


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dBm


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What is Spectrum ? (1/8)

� The Electromagnetic Spectrum– A continuum of all electromagnetic waves arranged according

to frequency and wavelength– Electromagnetic waves:

• Vibrations of electric and magnetic fields that travel through space.

• The electrical fields and magnetic fields are coupled together but are perpendicular to each other and to the direction of the wave.

– Same frequency and phase• Mostly invisible except for the visible spectrum • Speed of light (c = 3 x 108m/s (vacuum)) (light is EM)• Sinusoidal waves• Do not need molecules to transmit energy. Can travel through air,

solid materials and empty space.


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� Electromagnetic Wave


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� The Electromagnetic Spectrum– Can be described in terms of wavelength, frequency

and energy– These terms are all related mathematically– Use most convenient units– For example:

• Radio waves - frequency• Light - wavelength• X-rays - energy


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� Electromagnetic Spectrum – Radio spectrum (a.k.a. RF) (frequency)– Microwave (frequency)– Infrared– Visible Spectrum– Ultraviolet– X-rays – Gamma-Rays


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What is Spectrum ? (6/8)Ultra-low frequency (ULF)

Extremely Low Frequency (ELF)Voice Frequencies

Very Low Frequency (VLF)Low Frequency (LF)

Medium Frequency (MF)

High Frequency (HF)

Very High Frequency (VHF)

Ultra High Frequency (UHF)

Super High Frequency (SHF)

Extremely High Frequency (EHF)

3-30 Hz30 – 300 Hz

300 Hz – 3 kHz3 – 30 kHz

30 – 300 kHz300 kHz – 3 MHz

3 – 30 MHz

30 – 300 MHz

300 MHz – 3 GHz

3 – 30 GHz

30 – 300 GHz

AM Broadcast

Microwave RelayEarth-Satellite

RadarMobile Radio

AeronauticalNavigation

NavigationSatellite

UHF TelevisionMobile Radio

VHF Television FM BroadcastMobile Radio Aeronautical

Amateur radioInternational radio

Citizen band


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Relationship between Data Rate and Bandwidth

� The greater the bandwidth, the higher the information-carrying capacity

� Hints– Any digital waveform will have infinite bandwidth– BUT the transmission system will limit the bandwidth

that can be transmitted– AND, for any given medium, the greater the

bandwidth transmitted, the greater the cost– HOWEVER, limiting the bandwidth creates

distortions


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Reasons for Choosing Data and Signal Combinations

� Digital data, digital signal– Equipment for encoding is less expensive than digital-to-

analog equipment� Analog data, digital signal

– Conversion permits use of modern digital transmission and switching equipment

� Digital data, analog signal– Some transmission media will only propagate analog

signals– Examples include optical fiber and satellite

� Analog data, analog signal– Analog data easily converted to analog signal


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Analog Transmission

� Transmit analog signals without regard to content

� Attenuation limits length of transmission link � Cascaded amplifiers boost signal’s energy for

longer distances but cause distortion– Analog data can tolerate distortion– Introduces errors in digital data


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Digital Transmission

� Concerned with the content of the signal� Attenuation endangers integrity of data� Digital Signal

– Repeaters achieve greater distance– Repeaters recover the signal and retransmit

� Analog signal carrying digital data– Retransmission device recovers the digital data from

analog signal– Generates new, clean analog signal


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About Channel Capacity

� Impairments, such as noise, limit data rate that can be achieved

� For digital data, to what extent do impairments limit data rate?

� Channel Capacity – the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions


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Concepts Related to Channel Capacity

� Data rate - rate at which data can be communicated (bps)

� Bandwidth - the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium (Hertz)

� Noise - average level of noise over the communications path

� Error rate - rate at which errors occur– Error = transmit 1 and receive 0; transmit 0 and

receive 1


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Nyquist Bandwidth

� For binary signals (two voltage levels)– C = 2B

� With multilevel signaling– C = 2B log2 M

• M = number of discrete signal or voltage levels


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Signal-to-Noise Ratio

� Ratio of the power in a signal to the power contained in the noise that’s present at a particular point in the transmission

� Typically measured at a receiver� Signal-to-noise ratio (SNR, or S/N)

� A high SNR means a high-quality signal, low number of required intermediate repeaters

� SNR sets upper bound on achievable data rate

power noisepower signal

log10)( 10dB =SNR


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Shannon Capacity Formula

� Equation:

� Represents theoretical maximum that can be achieved

� In practice, only much lower rates achieved– Formula assumes white noise (thermal noise)– Impulse noise is not accounted for– Attenuation distortion or delay distortion not

accounted for

( )SNR1log2 += BC


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Example of Nyquist and Shannon Formulations

� Spectrum of a channel between 3 MHz and 4 MHz ; SNRdB = 24 dB

� Using Shannon’s formula

( )251SNR

SNRlog10dB 24SNRMHz 1MHz 3MHz 4

10dB

===

=−=B

( ) Mbps88102511log10 62

6 =×≈+×=C


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Example of Nyquist and Shannon Formulations

� How many signaling levels are required?

( )

16

log4

log102108

log2

2

266

2

==

××=×

=

M

M

M

MBC


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3. Propagation


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Classifications of Transmission Media

� Transmission Medium– Physical path between transmitter and receiver

� Guided Media– Waves are guided along a solid medium– E.g., copper twisted pair, copper coaxial cable,

optical fiber� Unguided Media

– Provides means of transmission but does not guide electromagnetic signals

– Usually referred to as wireless transmission– E.g., atmosphere, outer space


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Twisted Pair Wire

� Two or more pairs of single conductor wires that have been twisted around each other.

� Twisted pair wire is classified by category. Twisted pair wire is currently Category 1 through Category 5e.

� Twisting the wires helps to eliminate electromagnetic interference between the two wires.

� Shielding can further help to eliminate interference.


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Coaxial Cable

� A single wire wrapped in a foam insulation surrounded by a braided metal shield, then covered in a plastic jacket. Cable can be thick or thin.

– Baseband coaxial technology uses digital signaling (DC) in which the cable carries only one channel of digital data.

– Broadband coaxial technology transmits analog signals (RF) and is capable of supporting multiple channels of data.


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Fiber Optic Cable

� A thin glass cable approximately a little thicker than a human hair surrounded by a plastic coating and packaged into an insulated cable.

� A photo diode or laser generates pulses of light which travel down the fiber optic cable and are received by a photo receptor.


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Mixing Media


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Unguided Media

� Transmission and reception are achieved by means of an antenna

� Configurations for wireless transmission– Directional – Omnidirectional


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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


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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


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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


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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


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Broadcast

� Guided propagation– Immunity to

interferences and noise– Quality of reception– Wide frequency band– No antenna

� Free space propagation– Free space propagation is

naturally omni-directional– Infinite number of receivers– Large area coverage

Attenuation(dB)

Guided propagation

Radio propagation

Distance (m)


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Point to point

� Guided propagation– Well known medium– Easy frequency reuse

with a new medium– Length and density are

not limited by interferences

– Confidential communications

– Long life

� Free space propagation– Communications possible

with difficult access area– Temporary communications

are possible– Quasi ready-to-use– Cost in quasi independant

of bandwith


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Multiplexing

� Capacity of transmission medium usually exceeds capacity required for transmission of a single signal

� Multiplexing - carrying multiple signals on a single medium– More efficient use of transmission medium


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Multiplexing


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Reasons for Widespread Use of Multiplexing

� Cost per kbps of transmission facility declines with an increase in the data rate

� Cost of transmission and receiving equipment declines with increased data rate

� Most individual data communicating devices require relatively modest data rate support


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Multiplexing Techniques

� Frequency-division multiplexing (FDM)– Takes advantage of the fact that the useful

bandwidth of the medium exceeds the required bandwidth of a given signal

� Time-division multiplexing (TDM)– Takes advantage of the fact that the achievable bit

rate of the medium exceeds the required data rate of a digital signal

� Code-division multiplexing (CDM)– Takes advantage of the fact that each individual

communication is below the noise floor


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FDMA-TDMA-CDMA


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Free space vs transmission line propagation

Guided propagation

Free space propagation

Increase with d

Increase with f1/2

Distance (d)

Frequency (f)

Increase with d2

Increase with f2


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Wired technology

� Advantages– High-quality 2-way Hybrid Fiber Coaxial (HFC) Networks– Data Networking Infrastructure: Routers, Routing

Switches, Switches, Servers, Network Management Systems, Cable Modem Internet Connectivity

– Serve Thousands of Broadband Internet Subscribers

� Disadvantages– High Cost

Broadband Internet Connection over a Cable Network


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Wireless technology (1/2)

Broadband Fixed Wireless Systems Operate at Different Frequencies

Multichannel Multipoint Distribution System (MMDS)

� 1-way Video Broadcast Service� Operates at 2 GHz

Microwave Video Distribution System (MVDS)

Local Multipoint Distribution System (LMDS)

� Undergo Technical Trials� Operate at 12GHz and 42 GHz

� Operates at 26 - 31 GHz


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Wireless technology (2/2)

� Advantages– Lower Cost of Deployment– Lower Cost of Network Maintenance, Management

and Operating Costs– Easier to Adapt to Changing Market Conditions


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Media Selection Criteria

� Speed:– What level of data transfer do we need (10Mbps-

100Mbps+)?� Cost:

– What can we afford (cat5e relatively inexpensive)?� Distance and expandability� Environment:

– What is the noise level?� Security/encryption:

– Possibility of wiretapping/“hot spots”?


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Radio Propagation modes


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Fading

� Fade duration TD

� Fade occurrence interval TI

� Defined at the signal level Rp that is exceeded P(%) ofthe time.


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Radio path planning design (1/2)

1. Select the most direct route using survey maps. If possible inspect the terrain (by air!) for possible obstacles

2. Break route into sections approximately 40-50km long. A zigzag path may be required to avoid interference.Locate repeaters so as to use vantage points such ashills or tall buildings.

3. For each hop, plot the profile of the terrain taking elevations from a contour map. Remember to take into account unmarked obstructions such as buildings and forests.

http://ludo.ece.jcu.edu.au/subjects/ee3710/notes/Propagation.PDF


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Radio path planning design (2/2)

4. Select antenna heights and superimpose the beam profileand the 1st Fresnel zone on the terrain profile.

5. Inspect the path profile for obstructions and repeat step 4until the first Fresnel zone or at least 0.6 of it is clear. Ifthe path remains obstructed calculate additional losses expected.Ensure the selected antenna heights do notleads to severe fading under varying conditions. Forexample, could there be reflections from the earth, will trees grow so as to obstruct the path?

6. Determine total path losses and assign a fading margin.From the specified system signal-to-noise ratio Determine the antenna gains and the transmitter power.

http://ludo.ece.jcu.edu.au/subjects/ee3710/notes/Propagation.PDF


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Free space propagation (1/4)


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� Assumptions: isotropic antennas (homogeneouspropagation in all directions)

� PT = power radiated� Power density

� Field strength

Tx

Rx

( )22 W/m

4TP

PDdπ

= d

( )120 V/mFS PDπ=


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� Received power depends on effective aperture of receiving antenna

� For the hypothetical isotropic receiving antenna

� Free space loss equation

� Free space losses

( )WR effP PD A= ⋅

2

4effAλπ

=

( )2 2

2 24W

4R T TP P P f dd c

λ ππ

� � � �= =� � � ��

10 1032.44 20log ( ) 20 log ( )(dB)MHz kmFSL f d= + +


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� Free space losses increase with the distance– Radiated power is

spread on the surface of a sphere with increasing radius

� Free space losses increase with thefrequency– Receiving antenna

effective aperture decreases with frequency

� But, antenna directivity gain increases…

Loss (dB)

Distance (m)


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Free Space Losses Calculations

� Hertzian Link– 40 km @ 3.5 GHz

� Spatial Link– 36000 km @ 12 GHz

� GSM Link– 5 km @ 900 MHz

� UMTS Link– 500 m @ 2 GHz

135.4 dB

205.1 dB

105.5 dB

92.44 dB


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Propagation Mechanisms

� Reflection– Propagating wave impinges on an object which is large

compared to wavelength– E.g., the surface of the Earth, buildings, walls, etc.

� Diffraction– Radio path between transmitter and receiver obstructed by

surface with sharp irregular edges– Waves bend around the obstacle, even when LOS does not

exist� Scattering

– Objects smaller than the wavelength of the propagating– wave– E.g., foliage, street signs, lamp posts


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2nd*2nd* 1st*1st*3rd*3rd*

* * FresnelFresnelZonesZones

Fresnel Zones


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• Fresnel Zone diameter depends upon Wavelength, and Distances from the sites along axis

• For minimum Diffraction Loss, clearance of at least 0.6F1 is required

Radius of n th

Fresnel Zone given by:

21

21

dd

ddnrn+

= λ

The First Fresnel Zone

Site A

Site Bd2

d1


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The Path Profile (1/3)

Path Profile characteristics may change over time, due to vegetation, building construction, etc.


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See calculations on next slide…


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� d1 = 4 km� d2 = 33.6 km� f = 6 GHz� λ = 5 cm

1 21

1 2

13.4 md d

rd dλ= =

+

d1 d2

60% clearance of F1 = 8 m


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Path loss

Received power = K d-n

n = 2 for free space propagation2 < n < 4 for practical cases


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Long term propagation modes


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Short term propagation modes


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Line of sight : radio horizon

� Radio horizon: The locus of points at which direct raysfrom a point source of radiowaves are tangential to thesurface of the Earth.

� Radio waves go behind the geometrical horizon due to refraction in the air

Approximate distance 17 17km Tx Rxd h h= +


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Radio Horizon Calculation

� hTx = 60 m� hRx = 60 m

� Radio Horizon is about 64 km

� Geometrical Horizon is about 55 km– r = 6368 km (earth radius)– h = tower height

2 2( )GH r h r= + −


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Diffraction

� Caused by obstacles in or near path� Examples: buildings, hills� Can cause destructive interference which

weakens received signal


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Multipath


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Atmospheric Attenuation (1/2)

� attenuation caused by atmospheric gases� note molecular resonance peaks


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Atmospheric Attenuation (2/2)

� attenuation caused by rain� can increase path loss by an order of magnitude (

10 x)


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ITU North American rain zones


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ITU European rain zones


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Rainfall effects


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Parameters affecting propagation

� Free space parameters– Frequency (GHz)– Path Length (km)– Excess attenuation due to water vapor– Excess attenuation due to mist and fog– Excess attenuation due to Oxygen– Gaseous loss– Excess attenuation due to rainfall

� Others– Trees, Buildings, Terrain and other blockage


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Example of total excess attenuation (LMDS)

� a = excess attenuation in dB due to water vapor

– 0.08 dB/km at sea level and 15degrees C (7.5 g/m3)

� b = excess attenuation in dB due to mist/fog

– 0.1 dB/km

� c = excess attenuation in dB due to Oxygen

– 0.02 dB/km at sea level and 15degrees C

� d = absorption losses due toother gases

– 0.08 dB/km

� e = excess attenuation due torain

– 3.67 dB/km for Dallas-Houston, Vertical Polarization, 99.9Avail.

– = 2.0 dB/km for Chicago, Vertical Polarization, 99.9Avail.

Total = 3.95 dB/km (Dallas-Houston)

Total = 2.28 dB/km (Chicago)


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Basic link budget (1/5)

Coverage radius = 2.3 kmRain zone = KHorizontal polarization41 GHz band with a 99,985 % availability.

AP antenna gain: 18 dBi ± 1 dB(90° sector antenna)

AT antenna gain: 37 dBi ± 1 dB(30 cm diameter antenna)


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� Rain attenuation = 23 dB– ITU-R Recommendation P.530-6– ITU-R Recommendation P.838– ITU-R Recommendation P.837-1

� Free space loss = 133.1 dB� Antenna gain = 55 dB

Required system gain (@ 99,985 %)= Rain attenuation + Free space loss - Antenna gain

= 101.1 dB.


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� BER = 10-6 for link availability� BER = 10-11 for link quality.� Receiver sensitivity

– ∆loss includes all implementation losses– Rxloss is the receiver branching loss


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PHY C/N dB(BER=10-6) 10-6 10-11

Prx @ 28 GHzdBm

10-6 10-11

Prx @ 32 GHzdBm

10-6 10-11

Prx @ 42 GHzdBm

QAM4 5 -88 -87 -87 -86 -86 -85

QAM16 18 -75 -74 -74 -73 -73 -72

QAM64 25 -66 -65 -65 -64 -64 -63


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Sources of Interference

� Intra-System Interference– Multipath– Cross Polarization Component– Adjacent Channel Interference– Co-channel Interference

� Inter-System Interference– Satellite Systems– Other Systems– Out-of-Band Interference


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Interference Mitigation (Intra-System) (1/2)

� Multi-Path– Use Highly Directional Antennas– Give Careful Consideration to Placement of Antennas– Use antennas with low side lobes for CPE– Use robust modulation and error correction techniques

� Cross Polarization– Major factor for systems which exploit polarization at same

base– station for frequency reuse--need antennas with good cross-pol– Minor problem for systems which use polarization for separating– base stations– Use robust modulation and error correction techniques


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Interference Mitigation (Intra-System) (2/2)

� Adjacent Channel– Use constant envelope modulation– Use linear power amplifiers– Use robust modulation and error correction techniques

� Co-Channel Interference– Use highly directional antennas with low side lobes for CPE– Deploy base stations with maximum separation distance for

same– frequency, same polarization– Use minimum transmit power; control TX power on return path– Use robust modulation and error correction techniques– Use adaptive interference suppression techniques


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Interference Mitigation (Inter-System)

� Use highly directional antennas with low side lobes� Use high-dynamic range LNA’s and first mixers� Develop standards for coexistence� Employ low LO leakage designs in TX and RX circuits� Develop adequate image rejection receivers� Employ filtering at MMW frequencies� Use linear Power Amplifiers� Use constant envelope modulation


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Frequency Diversity (1/2)

� Use two transmitters and two receivers operating at different frequencies, preferably separated by at least 5%.

� Phase relationship between direct and reflected path will be different at the two frequencies

� Also provides a redundant system in case of equipment failure (“hot standby”)

� Disadvantage: uses twice the bandwidth


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Frequency Diversity (2/2)


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Space Diversity (1/2)

� Use two receiving antennas separated in space (preferably by 200 wavelengths or more, though less separation is often used)

� Path length will be different to the two antennas, so cancellation is unlikely over both paths

� Needs larger towers, but does not require increased bandwidth


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Space Diversity (2/2)


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4. Digital coding


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Digital Coding

� Character: A symbol that has a common, constant meaning.

� Characters in data communications, as in computer systems, are represented by groups of bits [1’s and 0’s].

� The group of bits representing the set of characters in the “alphabet” of any given system are called a coding scheme, or simply a code.


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Digital Coding

� A byte consists of 8 bits that is treated as a unit or character. (Some Asian languages use 2 bytes for each of their characters, such as Chinese.)

� (The length of a computer word could be 1, 2, 4 bytes.)� There are two predominant coding schemes in use

today:� United States of America Standard Code for Information

Interchange (USASCII or ASCII) � Extended Binary Coded Decimal Interchange Code

(EBCDIC)


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Advantages of Digital Transmission

� The signal is exact� Signals can be checked for errors� Noise/interference are easily filtered out� A variety of services can be offered over one

line� Higher bandwidth is possible with data

compression


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Why Use Analog Transmission?

� Already in place� Significantly less expensive� Lower attenuation rates� Fully sufficient for transmission of voice signals


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Analog Encoding of Digital Data

� Data encoding and decoding technique to represent data using the properties of analog waves

� Modulation: the conversion of digital signals to analog form

� Demodulation: the conversion of analog data signals back to digital form


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Methods of Modulation

� Amplitude modulation (AM) or amplitude shift keying (ASK)

� Frequency modulation (FM) or frequency shift keying (FSK)

� Phase modulation or phase shift keying (PSK) � Differential Phase Shift Keying (DPSK)


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Amplitude Shift Keying (ASK)

� In radio transmission, known as amplitude modulation (AM)

� The amplitude (or height) of the sine wave varies to transmit the ones and zeros

� Major disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude


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Amplitude Modulation and ASK


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Frequency Shift Keying (FSK)

� In radio transmission, known as frequency modulation (FM)

� Frequency of the carrier wave varies in accordance with the signal to be sent

� Signal transmitted at constant amplitude� More resistant to noise than ASK� Less attractive because it requires more

analog bandwidth than ASK


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Frequency Modulation and FSK


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Phase Modulation

� Frequency and amplitude of the carrier signal are kept constant

� The carrier signal is shifted in phase according to the input data stream

� Each phase can have a constant value, or value can be based on whether or not phase changes (differential keying)


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Phase Modulation and PSK


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Differential Phase Shift Keying (DPSK)

0 1 1 0


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PSK constellation


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Analog Channel Capacity: BPS vs. Baud

� Baud=# of signal changes per second. ITU-T now recommends the term baud rate be replaced by the term symbol rate.

� BPS=bits per second� In early modems only, baud=BPS. The bit rate and the

symbol rate (or baud rate) are the same only when one bit is sent on each symbol.

� Each signal change can represent more than one bit, through complex modulation of amplitude, frequency, and/or phase

� Increases information-carrying capacity of a channel without increasing bandwidth

� Increased combinations also leads to increased likelihood of errors


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Sending Multiple Bits Simultaneously


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� In practice, the maximum number of bits that can be sent with any one of these techniques is about five bits. The solution is to combine modulation techniques.

� One popular technique is quadrature amplitude modulation (QAM) involves splitting the signal into eight different phases, and two different amplitude for a total of 16 different possible values.


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The 4-PSK method


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The 4-PSK characteristics


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Time domain for an 8-QAM signal


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The 4-QAM and 8-QAM constellations


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� Trellis coded modulation (TCM) is an enhancement of QAM that combines phase modulation and amplitude modulation. It can transmits different numbers of bits on each symbol (6-10 bits per symbol).

� The problem with high speed modulation techniques such as TCM is that they are more sensitive to imperfections in the communications circuit.


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16-QAM constellations


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Modem

� An acronym for modulator-demodulator� Uses a constant-frequency signal known as a

carrier signal� Converts a series of binary voltage pulses into

an analog signal by modulating the carrier signal

� The receiving modem translates the analog signal back into digital data


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Digital Transmission of Analog Data

� Codec = Coder/Decoder� Converts analog signals into a digital form and

converts it back to analog signals� Where do we find codecs?

– Sound cards– Scanners– Voice mail– Video capture/conferencing


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Codec vs. Modem

� Codec is for coding analog data into digital form and decoding it back. The digital data coded by Codec are samples of analog waves.

� Modem is for modulating digital data into analog form and demodulating it back. The analog symbols carry digital data.


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Digital Encoding of Analog Data

� Primarily used in retransmission devices� The sampling theorem: If a signal is sampled at

regular intervals of time and at a rate higher than twice the significant signal frequency, the samples contain all the information of the original signal.

� Pulse-code modulation (PCM)– 8000 samples/sec sufficient for 4000hz


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5. Communication systems


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Fixed Wireless - Broadband

� MMDS: Multi-channel multi-point distribution service at 2.5 GHz

� Point-to-point wireless broadband at greater than 18 GHz

� LMDS: Local multi-point distribution service at 28 GHz

Note: Cell size for low frequency solutions are large,while cell size for high frequency solutions are small


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Frequency spectrum

1 GHz 2.5 3.5 5.8 10 24 26 28 38 40 60

LOSLOS

5000200013501000400 30020020 30

Voice, Data, Fax,

ISDN

256 Kbps256 Kbps

1 to 501 to 50MbpsMbps

+ TDMLeased Lines

Ban

dwid

th(M

Hz)

1400200

100

+ High SpeedInternet and Multimedia

+ UltraHigh-SpeedLAN/WAN

No LOSNo LOS

10 to 10010 to 100MbpsMbps

>100 Mbps>100 Mbps

LMDSMMDS UNII


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Cost per suscriber

CPE Cost

��

��

# of Subscribers

Cos

t Per

Sub

scr i

ber

This is the point at which PMP becomes more economically viable than

a PTP network, generally ~8 links.


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Reach Distance From Hub

Point-to-Point Links Point-to-Multipoint

28 GHz

38 GHz

5.34 6.92

Reach Distance, Km

3.577.5 5.0

2.5 GHz

20.050.0

23 GHz

6 GHz

28 GHz

2.5 GHz

40

24 GHz

2.5 GHz


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MMDS, MVDS (1/3)

MMDS, MVDS2,5 GHz, 40 GHz


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MMDS, MVDS (2/3)

� Multichannel Multipoint Distribution Service (MMDS)� A cost-effective, wireless alternative to digital subscriber

lines (DSL) and cable modem service for delivering broadband wireless access (BWA) to the last mile (> T1 connexion)

� MMDS uses licensed microwave frequencies in the 2.1 GHz to 2.7 GHz band

� MMDS is less affected by weather and can deliver reliable service up to a 25 mile radius under line-of-sight (LOS) conditions

� MMDS offers physical layer technologies for next generation broadband wireless access.


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MMDS, MVDS (3/3)

� Start with very large cells and expand capacity with cell splitting and antennas on CMRS towers

� Originally used for analog wireless cable– One way broadcast video of up to 28 channels– Up to 168 MHz of spectrum– Up to 25 miles coverage from one antenna

� In the USA, FCC modified rules to increase competition– Two way digital services allowed– Use for video, data, or voice telephony– Allow point-to-multipoint operation


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PMP

2 Mbit/s

broadband PMP3,5 GHz, 10,5 GHz, 26 GHz


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Point to multipoint


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LMDS

1 Mbit/s

LMDS28 GHz, 40 GHz

25-50 Mbit/s

n*2 Mbit/s


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LMDS

� Use Millimeter-wave Signals� Can be used for voice, data, and/or video� Intended for point-to-multipoint operation, but early

implementations have been PTP� Much smaller cells than MMDS (3-10 miles in diameter)� Unlikely for residential video / telephony because of high

cost & line-of-sight issues� Likely use is DS3 to business customer� Travel through Copper Wire, Co-axial Cable,

CAT5


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LMDS concept


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LMDS & MMDS COMPARISON

Wireless Systems LMDS MMDS

Frequency Range 10-43 GHz 2.5-2.7 GHz,3.4-3.7 GHz

Signal Radius Two-way Mostly One-way

Access Interface TDMA, TDMAFDMA, TDM

Bandwidth IP, ATM IP, ATMAllocation Method

Target Market Multi-dwelling Rural and Urbanhigh-rise units, Residential, SOHOSME, SOHO


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Modulations vs Data rate

0.4 MHz0.4 MHz6464--QAMQAM



0.8 MHz0.8 MHz8PSK8PSK

1.4 MHz1.4 MHzQPSKQPSK

1.4 MHz1.4 MHzDQPSKDQPSK

2.8 MHz2.8 MHzBPSKBPSK

MHz for 2Mbps CBR MHz for 2Mbps CBR ConnectionConnection

NameName


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LMDS key applications

� Broadband Access for SOHOs and SMEs� Cellular Backbone� Consumer Multimedia� Copper or Fibre Backup� LAN Interconnect� Video Conferencing� Video Monitoring


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LMDS

� An LMDS system consists of a series of cells whose centers are defined by individual base stations, and of a central control point to which all of the base stations communicate

� LMDS network uses highly directional antennas -sectorized antennas at the base station and single-beam parabolic microwave reflectors at the subscriber site


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Advantages of LMDS

� Fast to deploy, no digging roads

� Solution to lack of fiber in some rural areas

� Build-out on demand, scalable

� Multi-Gigabit capacity� Can provide integrated

services: voice/data/video

� Challenges– Requires “line of sight”

between Tx and Rx– Signal attenuation by

rain and moisture in vegetation

– Shorter range requires more hub sites for coverage


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Experience with LMDS

� Quick to deploy (enables service provider to capture customers before a competitor does)

� Build-out, or relocate, as needed� Useful for sites not readily accessible to other

broadband services� Signal loss due to rain must be compensated for � Operation, Administration, Maintenance, and

Provisioning is similar to other telecomm. equip.


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Bibliography (1/2)

� www.terena.nl/conferences/nato-anw2000/ 03_bb_fixed_wirel.ppt� Broadband Radio Access Networks (BRAN); HIPERACCESS; System

Overview - ETSI TR 102 003 V1.1.1 (2002-03)� Broadband Wireless Access - IEEE 802 Executive Committee Meeting

Albuquerque, NM November 12, 1998� Fixed Radio Systems; Digital Multipoint Radio Systems; Part 1:

Common Characteristics and Non Essential Parameters of Multipoint Radio Systems – ETSI DEN/TM-04130-1 v0.0.2 (2003-03-25)

� Fundamentals of Communications 15: Radio Channels - Professor Ian Groves - King's College London

� Local Multipoint Distribution System - Berkin Özmen. Sercan Uslu -Computer Networks.

� Local Multipoint Distribution Service Tutorial - 1999 IEEE EmergingTechnologies Symposium on Wireless Communications and Systems -Langston, Marks, Reese


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Bibliography (2/2)

� Physical Layer Chapter 8: Data communication Fundamentals –Computer Science Department – University of Geneva

� Propagation Considerations important for today’s Radiocommunication Systems - Kevin A. Hughes - ITU Radiocommunication Bureau

� Radio Propagation – Katz - University of California, Berkeley� Radio-wave Propagation Basics - Struzak – Radio Regulations Board,

ITU� Terrestrial Microwave Systems – Niagara College – Canada� Transmission fundamentals, The Media Conducted and Wireless –

Georgia State University� The last mile: wireless technologies for broadband and home networks

– Cordeiro, Gossain, Ashok, Agrawal – University of Cincinnatti� ITU-T Recommendation G.821: "Error performance of an international

digital connection operating at a bit rate below the primary rate and forming part of an integrated services digital network".

microwave radio

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