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TOBB ETU Bil557 Wireless Networks Lecture 06 February 14, 2007

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Geostationary orbit (GEO) Medium earth orbit (MEO) Low earth orbit (LEO) Geometry Terms ... Geostationary Earth Orbit. LEO Satellite Characteristics ... – PowerPoint PPT presentation

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Title: TOBB ETU Bil557 Wireless Networks Lecture 06 February 14, 2007


1
TOBB ETU Bil557 Wireless Networks Lecture
06 February 14, 2007
  • Spring 2007
  • Wednesday 0830 1200
  • Room Number 212
  • Bülent Tavli
  • Office 169
  • btavli_at_etu.edu.tr

2
Reminders
  • Homework III
  • Due February 21, 2007 (one week from now)
  • Following questions from the text book 7.4, 7.5,
    7.7, 8.2, 8.6, 8.9, 8.16, 8.20, 8.25
  • Midterm Examination
  • February 17, 2007 Saturday (0900 AM)
  • (Chapters 1-8)

3
Highlights of The Previous Lectures
  • Chapters 1 - 8
  • (Part One Technical Background
  • Part Two Wireless Communication Technology)

4
Protocol Stack
Application
Transport
Network
MAC
Physical
Channel
5
Radiation Patterns
  • Radiation pattern
  • Graphical representation of radiation properties
    of an antenna
  • Depicted as two-dimensional cross section
  • Beam width (or half-power beam width)
  • Measure of directivity of antenna
  • Reception pattern
  • Receiving antennas equivalent to radiation
    pattern

6
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

7
  • Modulation
  • ...Where the information resides

8
Frequency Hoping Spread Spectrum
Time
400 ms
Frequency
1 MHz
9
Direct Sequence Spread Spectrum
channel output Zi,m
Zi,m di.cm
data bits
sender
slot 0 channel output
slot 1 channel output
code
slot 1
slot 0
received input
slot 0 channel output
slot 1 channel output
code
receiver
slot 1
slot 0
10
Direct Sequence Spread Spectrum
11
Code Division Multiple Access
12
Code Division Multiple Access
13
Error Detection Probabilities
  • Definitions
  • Pb Probability of single bit error (BER)
  • P1 Probability that a frame arrives with no bit
    errors
  • P2 The probability that a frame arrives with
    one or more undetected errors
  • F Number of bits per frame

14
Error Detection Process
  • Transmitter
  • For a given frame, an error-detecting code (check
    bits) is calculated from data bits
  • Check bits are appended to data bits
  • Receiver
  • Separates incoming frame into data bits and check
    bits
  • Calculates check bits from received data bits
  • Compares calculated check bits against received
    check bits
  • Detected error occurs if mismatch

15
Cyclic Redundancy Check (CRC)
  • Transmitter
  • For a k-bit block, transmitter generates an
    (n-k)-bit frame check sequence (FCS)
  • Resulting frame of n bits is exactly divisible by
    predetermined number
  • Receiver
  • Divides incoming frame by predetermined number
  • If no remainder, assumes no error

16
Forward Error Correction Process
  • Transmitter
  • Forward error correction (FEC) encoder maps each
    k-bit block into an n-bit block codeword
  • Codeword is transmitted analog for wireless
    transmission
  • Receiver
  • Incoming signal is demodulated
  • Block passed through an FEC decoder

17
Hamming Code Process
18
Block Interleaving
  • Data written to and read from memory in different
    orders
  • Data bits and corresponding check bits are
    interspersed with bits from other blocks
  • At receiver, data are deinterleaved to recover
    original order
  • A burst error that may occur is spread out over a
    number of blocks, making error correction possible

19
Convolutional Codes
  • Generates redundant bits continuously
  • Error checking and correcting carried out
    continuously
  • (n, k, K) code
  • Input processes k bits at a time
  • Output produces n bits for every k input bits
  • K constraint factor
  • k and n generally very small
  • n-bit output of (n, k, K) code depends on
  • Current block of k input bits
  • Previous K-1 blocks of k input bits

20
Convolutional Encoder
21
Convolutional Decoder
22
Automatic Repeat Request
23
Automatic Repeat Request
24
Satellite Communications
  • Chapter 9

25
Introduction - I
26
Introduction - II Earth Satellites
  • The Most Well-Known Types of Earth Satellites
  • International Space Station
  • Global Positioning System
  • Synchronous Satellites
  • The Moon

27
Introduction III International Space Station
250 miles above the earth. Goes around the
earth every 90 minutes.
28
Introduction IV Global Positioning Satellites
(GPS)
There are 24 of these satellites 6 of them are
always above the horizon at any one time. These
are what your GPS receiver uses to determine
where you are located. They are 12,000 miles
above the earth, and go around every 12 hours.
29
Introduction V Synchronous Satellites
30
Introduction VI The Moon
240,000 miles away from the Earth, Goes around
once every moonth.
31
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

32
Classification of Satellite Orbits
  • Circular or elliptical orbit
  • Circular with center at earths center
  • Elliptical with one foci at earths center
  • Orbit around earth in different planes
  • Equatorial orbit above earths 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)

33
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
  • Reasons affecting minimum elevation angle of
    earth stations antenna (gt0o)
  • 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

34
Satellite Parameters vs. Orbital Height
35
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
  • Example Turksat

36
Geostationary Earth Orbit
37
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
  • Example Iridium

38
Low Earth Orbit (LEO)
39
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

40
Iridium Commercial Status Constellation and
Gateway
  • One Commercial Gateway Provides Global
    Connectivity
  • Tempe, Arizona
  • DoD Gateway in Hawaii Supports US
  • Government Traffic
  • Satellite Constellation
  • 66 Fully Operational Satellites
  • 13 In-Orbit Spares
  • Constellation Life to 2013/2014
  • Satellite Operations
  • Main Facility in Leesburg, VA
  • Back-up Facility in Chandler, AZ
  • All Gateways Support Voice and Data
  • Services
  • Dial-up
  • Direct Internet Access
  • Short Message Service
  • Short Burst Messaging
  • Paging
  • RUDICS

41
Dial-Up, Direct Internet and RUDICS
Direct Internet/RUDICS Connectivity
Mobile Switching Center
RA
RA
Direct Internet Server
Leased Line OR VPN OR Internet
Iridium Gateway
RUDICS Server
Application Host Server
  • Faster connection time
  • 15 seconds as compared to 40 seconds for dial-up
    PPP
  • Transparent compression for improved throughput
  • Smart Connect seamless connect/disconnect
  • Reduces on-air charges
  • Maximizes ISU battery life
  • Reconnects automatically if connection dropped
  • No tail-end charges incurred at Gateway

42
Iridium Data Overview
43
Global Commercial Usage
Telephony Traffic, November 2003
44
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
  • Example GPS satellites

45
Medium Earth Orbit (MEO)
46
Orbital Comparison
47
Frequency Bands Available for Satellite
Communications
48
Satellite Link Performance Factors Distance
between earth station antenna and satellite
antenna
49
Satellite Link Performance Factors Satellite
Footprint
50
Satellite Link Performance Factors Atmospheric
Attenuation
51
Satellite Network Configurations
52
VSAT (Very Small Aperture Terminal)
53
VSAT Platforms
  • Mobile (Emergency Communications Response
    Vehicles)
  • Quick Deploy
  • Fixed

54
Capacity Allocation Strategies
  • Frequency division multiple access (FDMA)
  • Time division multiple access (TDMA)
  • Code division multiple access (CDMA)

55
Frequency-Division Multiplexing
  • Galaxy satellites (PanAmSat)
  • 500 MHz total BW
  • 24 40 MHz channels
  • Frequency reuse through polarization
  • 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

56
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

57
FAMA-FDMA
  • FAMA logical links between stations are
    preassigned
  • FAMA multiple stations access the satellite by
    using different frequency bands
  • Uses considerable bandwidth

58
FAMA/FDMA - Example
59
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

60
DAMA/FDMA - Example
61
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

62
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

63
FAMA-TDMA Frame Format
64
FAMA-TDMA Example
65
DAMA-TDMA Example
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