Fundamentals of Microwave & Satellite Technologies TEL 117 Telekomunikasi Dasar.

Fundamentals of Microwave & Satellite

Technologies

TEL 117Telekomunikasi Dasar

Historical Perspective

Founded during WWII Used for long-haul

telecommunications Displaced by fiber optic networks Still viable for right-of-way bypass and

geographic obstruction avoidance

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

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

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.

Reflections

Reflections can occur as the

microwave signal traverses a body

of water or fog bank; cause

multipath conditions

Diffraction

Diffraction is the result of

variations in the terrain the signal

crosses

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.

Line of SightFresnel 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.

LOS & FZC-cont’dFresnel Zone

D1

D2

72.2

D1 X D2

F x D

secret formula

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

Range-cont’d

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

Components of a Microwave System

Digital Modem

Radio Frequency (RF) Unit

Antenna

Digital Modem

The digital modem modulates the

information signal (intermediate

frequency or IF).

RF Unit

IF is fed to the RF unit which is

mounted as close physically to the

antenna as possible (direct

connect is optimal).

Antenna

The antenna is a passive device

that radiates the modulated signal.

It is fed by direct connect of the RF

unit, coaxial cable, or waveguides

at higher frequencies.

Waveguides

Waveguides are hollow channels of low-loss material used to direct the

signal from the RF unit to the antenna.

Modulation Methods

Primarily modulated today with

digital FM or AM signals

Digital signal remains quiet until

failure threshold bit error rate renders

it unusable

Bit Error Rate (BER) The BER is a performance measure

of microwave signaling throughput 10 or one error per million

transmitted bits of information Data fail over is at 10; voice traffic

can withstand this error rate

-6

-3

Diversity Space Diversity Frequency Diversity Hot Standby PRI

Space Diversity

Normal Signal

Faded Signal

Transmitter Receiver

Space Diversity-cont’d Space Diversity protects against

multi-path fading by automatic switch over to another antenna place below the primary antenna. This is done at the BER failure point or signal strength attenuation point to the secondary antenna that is receiving the transmitted signal at a stronger power rating.

Frequency Diversity

Receiver

Active TxFrequency #1

Protect TxFrequency #2

RxFrequency #1

RxFrequency #2

Transmitter

Frequency Diversity-cont’d Frequency Diversity uses separate

frequencies (dual transmit and receive systems); it monitors primary for fail over and switches to standby. Interference usually affects only one range of frequencies. Not allowed in non-carrier applications because of spectrum scarcity.

Hot Standby*

Receiver

System TxPrimary #1

System TxStandby #2

failure switch

Active Rx#1

Standby Rx#2

Transmitter

*Hot standby is designed for equipment failure only

PRI (Primary Rate Interface)

ReceiverTransmitter

Connect to PRI interface

& PSTN

Connect to PRI interface

& PSTN

To PSTN To PSTN

System Transmission

Facilities

System Receiver Facilities

PRI is an ISDN service providing user with 23,64 kbps for channel message and 1,64 kbps data channel for control & signalling. PRI is ISDN E1 interface.

Availability Formula

Percent Availability equals:

1 – (outage hours/8760 hours per year)

Private microwaves have 99.99% availability

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

Microwave Radio Applications

Satellite Communications

Satellite-Related Terms Earth Stations – antenna systems on or near earth Uplink – transmission from an earth station to a

satellite Downlink – transmission from a satellite to an

earth station Transponder – electronics in the satellite that

convert uplink signals to downlink signals

Ways to CategorizeCommunications Satellites

Coverage area Global, regional, national

Service type Fixed service satellite (FSS) Broadcast service satellite (BSS) Mobile service satellite (MSS)

General usage Commercial, military, amateur, experimental

Classification of Satellite Orbits Circular or elliptical orbit

Circular with center at earth’s center Elliptical with one foci at earth’s center

Orbit around earth in different planes Equatorial orbit above earth’s equator Polar orbit passes over both poles Other orbits referred to as inclined orbits

Altitude of satellites Geostationary orbit (GEO) Medium earth orbit (MEO) Low earth orbit (LEO)

Geometry Terms Elevation angle - the angle from the

horizontal to the point on the center of the main beam of the antenna when the antenna is pointed directly at the satellite

Minimum elevation angle Coverage angle - the measure of the portion

of the earth's surface visible to the satellite

Minimum Elevation Angle Reasons affecting minimum elevation angle

of earth station’s antenna (>0o) Buildings, trees, and other terrestrial objects

block the line of sight Atmospheric attenuation is greater at low

elevation angles Electrical noise generated by the earth's heat

near its surface adversely affects reception

GEO Orbit Advantages of the the GEO orbit

No problem with frequency changes Tracking of the satellite is simplified High coverage area

Disadvantages of the GEO orbit Weak signal after traveling over 35,000 km Polar regions are poorly served Signal sending delay is substantial

GEO : Geosynchronous equatorial orbit

LEO Satellite Characteristics Circular/slightly elliptical orbit under 2000 km Orbit period ranges from 1.5 to 2 hours Diameter of coverage is about 8000 km Round-trip signal propagation delay less than 20

ms Maximum satellite visible time up to 20 min System must cope with large Doppler shifts Atmospheric drag results in orbital deterioration

LEO : Low earth orbit

LEO Categories Little LEOs

Frequencies below 1 GHz 5MHz of bandwidth Data rates up to 10 kbps Aimed at paging, tracking, and low-rate messaging

Big LEOs Frequencies above 1 GHz Support data rates up to a few megabits per sec Offer same services as little LEOs in addition to voice

and positioning services

MEO Satellite Characteristics Circular orbit at an altitude in the range of 5000 to

12,000 km Orbit period of 6 hours Diameter of coverage is 10,000 to 15,000 km Round trip signal propagation delay less than 50

ms Maximum satellite visible time is a few hours

MEO : Medium Earth Orbit

Satellite Systems

SOURCE: WASHINGTON UNIV.

GEO

M EO

LEO

GEO (22,300 mi., equatorial) high bandwidth, power, latency

MEO high bandwidth, power, latency

LEO (400 mi.) low power, latency

more satellites

small footprint

V-SAT (Very Small Aperture

Terminal)

private WAN

SATELLITE MAP

Geostationary Orbit

SOURCE: BILL LUTHER, FCC

GPS Satellite Constellation

• Global Positioning System• Operated by USAF• 28 satellites• 6 orbital planes at a height of 20,200 km• Positioned so a minimum of 5 satellites are visible at all times• Receiver measures distance to satellite

SOURCE: NAVSTAR

Frequency Bands Available for Satellite Communications

Satellite Link Performance Factors

Distance between earth station antenna and satellite antenna

For downlink, terrestrial distance between earth station antenna and “aim point” of satellite Displayed as a satellite footprint (Figure 9.6)

Atmospheric attenuation Affected by oxygen, water, angle of elevation, and

higher frequencies

Satellite Footprint

Satellite Network Configurations

Capacity Allocation Strategies Frequency division multiple access

(FDMA) Time division multiple access (TDMA) Code division multiple access (CDMA)

Frequency-Division Multiplexing Alternative uses of channels in point-to-point

configuration 1200 voice-frequency (VF) voice channels One 50-Mbps data stream 16 channels of 1.544 Mbps each 400 channels of 64 kbps each 600 channels of 40 kbps each One analog video signal Six to nine digital video signals

Frequency-Division Multiple Access Factors which limit the number of

subchannels provided within a satellite channel via FDMA Thermal noise Intermodulation noise Crosstalk

Forms of FDMA Fixed-assignment multiple access (FAMA)

The assignment of capacity is distributed in a fixed manner among multiple stations

Demand may fluctuate Results in the significant underuse of capacity

Demand-assignment multiple access (DAMA) Capacity assignment is changed as needed to respond

optimally to demand changes among the multiple stations

FAMA-FDMA FAMA – logical links between stations are

preassigned FAMA – multiple stations access the

satellite by using different frequency bands Uses considerable bandwidth

DAMA-FDMA Single channel per carrier (SCPC) – bandwidth

divided into individual VF channels Attractive for remote areas with few user stations near

each site Suffers from inefficiency of fixed assignment

DAMA – set of subchannels in a channel is treated as a pool of available links For full-duplex between two earth stations, a pair of

subchannels is dynamically assigned on demand Demand assignment performed in a distributed fashion

by earth station using CSC

Reasons for Increasing Use of TDM Techniques

Cost of digital components continues to drop

Advantages of digital components Use of error correction

Increased efficiency of TDM Lack of intermodulation noise

FAMA-TDMA Operation Transmission in the form of repetitive sequence of

frames Each frame is divided into a number of time slots Each slot is dedicated to a particular transmitter

Earth stations take turns using uplink channel Sends data in assigned time slot

Satellite repeats incoming transmissions Broadcast to all stations

Stations must know which slot to use for transmission and which to use for reception

FAMA-TDMA Uplink

FAMA-TDMA Downlink