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TOBB ETU Bil557 Wireless Networks Lecture 12 November 26, 2007

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Title: TOBB ETU Bil557 Wireless Networks Lecture 12 November 26, 2007


1
TOBB ETU Bil557 Wireless Networks Lecture
12 November 26, 2007
  • Fall 2007
  • Monday 1330 1600
  • Room Number 109
  • Bülent Tavli
  • Office 169
  • btavli_at_etu.edu.tr

2
Introduction to Wireless
  • Chapter 1

3
What is wireless communication?
  • Any form of communication that does not require
    the transmitter and receiver to be in physical
    contact through guided media
  • Electromagnetic wave propagated through
    free-space
  • Radar, RF, Microwave, IR, Optical
  • Simplex one-way communication (e.g., radio, TV)
  • Half-duplex two-way communication but not
    simultaneous (e.g., push-to-talk radios)
  • Full-duplex two-way communication (e.g.,
    cellular phones)
  • Frequency-division duplex (FDD)
  • Time-division duplex (TDD) simulated full-duplex

4
Why use wireless communication?
  • Provides mobility
  • A user can send and receive messages no matter
    where he/she is located
  • Added convenience / reduced cost
  • Enables communications without adding expensive
    infrastructure
  • Can easily setup temporary wireless LANs
    (disaster situations)
  • Developing nations use cellular telephony rather
    than laying wires to each home
  • Use resources only when sending or receiving
    signal

5
Why is wireless different than wired?
  • Noisy, time-varying channel
  • BER varies by orders of magnitude
  • Enviromental conditions affect transmission
  • Shared medium
  • Other users create interference
  • Must develop ways to share the channel
  • Bandwidth is limited
  • TÜK, FCC determines the frequency allocation
  • ISM band for unlicensed spectrum (902-928 MHz,
    2.4-2.5 GHz, 5.725-5.875 GHz)
  • Requires intelligent signal processing and
    communications to make efficient use of limited
    bandwidth in error-prone environment

6
Wireless Comes of Age
  • 1893 Nikola Tesla demonstrated the first ever
    wireless information transmission in New York
    City
  • 1897 Marconi demonstrated transmission of radio
    waves to a ship at sea 29 km away
  • 1915 Wireless telephony established-- VA and
    Paris
  • 1920's Radio broadcasting became popular
  • 1930's TV broadcasting began
  • 1946 First public mobile telephone service in US
  • 1960's Bell Labs developed cellular concept--
    brought mobile telephony to masses
  • 1960s Communications satellites launched
  • Late 1970's IC technology advances enable
    affordable cellular telephony-- ushers in modern
    cellular era
  • Early 1990s Cellular telephony in Türkiye
  • 2007 GncTrkCll cellular service is introduced by
    Turkcell ?

7
Some Milestones in Wireless Communications
8
Modern Cellular Standards
  • First generation (1G) systems (analog)
  • 1979 NTT (Japan), FDMA, FM, 25 kHz channels,
    870-940 MHz)
  • 1981 NMT (Sweden and Norway), FDMA, FM, 25 kHz,
    450-470 MHz
  • 1983 AMPS (US), FDMA, FM, 30 kHz channels,
    824-894 MHz
  • 1985 TACS (Europe), FDMA, FM, 25 kHz channels,
    900 MHz
  • Second generation (2G) systems (digital)
  • Supported voice and low-rate data (up to 9.6
    kbps)
  • 1990 GSM (Europe), TDMA, GMSK, 200 kHz channels,
    890-960 MHz
  • 1991 USDC/IS-54 (US), TDMA, p/4 DQPSK, 30 kHz
    channels, 824-894 MHz
  • 1993 IS-95 (US), CDMA, BPSK/QPSK, 1.25 MHz
    channels, 824-894 MHz and 1.8-2.0 GHz
  • 1993 CDPD (US) FHSS GMSK 30 kHz channels 824-894
    Mhz
  • Enhanced 2G (2.5G) systems
  • Increased data rates
  • General Packet Radio System (GPRS) packet-based
    overlay to GSM, up to 171.2 kbps
  • Enhanced Data rates for GSM Evolution (EDGE)
    modulation enhancements to GSM to support up to
    180 kbps
  • 3rd generation (3G) systems
  • Up to 2 Mbps
  • Internet, VoIP
  • 2004-2005 IMT-2000, 2000 MHz range - W-CDMA
    (UMTS), cdma2000, TD-SCMA

9
Fast facts Cellular subscribers
10
Fast facts cellular growth
11
Protocol Stack - II
Application
Transport
Network
MAC
Physical
Channel
12
Transmission Fundamentals
  • Chapter 2

13
Electromagnetic Signal
  • Function of time
  • Can also be expressed as a function of frequency
  • Signal consists of components of different
    frequencies

14
Time-Domain Concepts
  • Analog signal - signal intensity varies in a
    smooth fashion over time
  • No breaks or discontinuities in the signal
  • Digital signal - signal intensity maintains a
    constant level for some period of time and then
    changes to another constant level
  • Periodic signal - analog or digital signal
    pattern that repeats over time
  • s(t T ) s(t ) -8lt t lt 8
  • where T is the period of the signal

15
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16
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17
Time-Domain Concepts
  • Aperiodic signal - analog or digital signal
    pattern that doesn't repeat over time
  • Peak amplitude (A) - maximum value or strength of
    the signal over time typically measured in volts
  • Frequency (f )
  • Rate, in cycles per second, or Hertz (Hz) at
    which the signal repeats

18
Time-Domain Concepts
  • Period (T ) - amount of time it takes for one
    repetition of the signal
  • T 1/f
  • Phase (?) - measure of the relative position in
    time within a single period of a signal
  • Wavelength (?) - distance occupied by a single
    cycle of the signal
  • Or, the distance between two points of
    corresponding phase of two consecutive cycles

19
Sine Wave Parameters
  • General sine wave
  • s(t ) A sin(2?ft ?)
  • Figure 2.3 shows the effect of varying each of
    the three parameters
  • (a) A 1, f 1 Hz, ? 0 thus T 1s
  • (b) Reduced peak amplitude A0.5
  • (c) Increased frequency f 2, thus T ½
  • (d) Phase shift ? ?/4 radians (45 degrees)
  • note 2? radians 360 1 period

20
Sine Wave Parameters
21
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

22
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

23
Signal-to-Noise Ratio
  • Ratio of the power in a signal to the power
    contained in the noise thats 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

24
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

25
Example of Nyquist and Shannon Formulations
  • Spectrum of a channel between 3 MHz and 4 MHz
    SNRdB 24 dB
  • Using Shannons formula

26
Example of Nyquist and Shannon Formulations
  • How many signaling levels are required?

27
Frequency-division Multiplexing
28
Time-division Multiplexing
29
Communication Networks
  • Chapter 3

30
Speed and Distance of Communications Networks
31
Switched Network
32
Techniques Used in Switched Networks
  • Circuit switching
  • Dedicated communications path between two
    stations
  • E.g., public telephone network
  • Packet switching
  • Message is broken into a series of packets
  • Each node determines next leg of transmission for
    each packet

33
Packet Switching
34
Datagram versus Virtual Circuit
35
Protocols and the TCP/IP Suite
  • Chapter 4

36
Key Features of a Protocol
  • Syntax
  • Concerns the format of the data blocks
  • Semantics
  • Includes control information for coordination and
    error handling

37
Agents Involved in Communication
  • Applications
  • Exchange data between computers (e.g., electronic
    mail)
  • Computers
  • Connected to networks
  • Networks
  • Transfers data from one computer to another

38
TCP/IP Layers
  • Physical layer
  • Network access layer
  • Internet layer
  • Host-to-host, or transport layer
  • Application layer

39
TCP/IP Physical Layer
  • Covers the physical interface between a data
    transmission device and a transmission medium or
    network
  • Physical layer specifies
  • Characteristics of the transmission medium
  • The nature of the signals
  • The data rate
  • Other related matters

40
TCP/IP Network Access Layer
  • Concerned with the exchange of data between an
    end system and the network to which it's attached
  • Software used depends on type of network
  • Circuit switching
  • Packet switching (e.g., X.25)
  • LANs (e.g., Ethernet)
  • Others

41
TTCP/IP Internet Layer
  • Uses internet protocol (IP)
  • Provides routing functions to allow data to
    traverse multiple interconnected networks
  • Implemented in end systems and routers

42
TCP/IP Host-to-Host, or Transport Layer
  • Commonly uses transmission control protocol (tcp)
  • Provides reliability during data exchange
  • Completeness
  • Order

43
TCP/IP Application Layer
  • Logic supports user applications
  • Uses separate modules that are peculiar to each
    different type of application

44
Protocol Data Units (PDUs)
45
TCP/IP Concepts
46
Common TCP/IP Applications
  • Simple mail transfer protocol (SMTP)
  • Provides a basic electronic mail facility
  • File Transfer Protocol (FTP)
  • Allows files to be sent from one system to
    another
  • TELNET
  • Provides a remote logon capability

47
Layers of the OSI Model
48
Comparison of OSI and TCP/IP
49
Elements of Standardization within OSI Framework
  • Protocol Specification
  • Format of protocol data units (PDUs) exchanged
  • Semantics of all fields
  • Allowable sequence of PDUs
  • Service Definition
  • Functional description that defines what services
    are provided, but not how the services are to be
    provided
  • Addressing
  • Entities are referenced by means of a service
    access point (SAP)

50
Network Differences Routers Must Accommodate
  • Addressing schemes
  • Different schemes for assigning addresses
  • Maximum packet sizes
  • Different maximum packet sizes requires
    segmentation
  • Interfaces
  • Differing hardware and software interfaces
  • Reliability
  • Network may provide unreliable service

51
Internetworking Example
52
Antennas and Propagation
  • Chapter 5

53
Introduction
  • An antenna is an electrical conductor or system
    of conductors
  • Transmission - radiates electromagnetic energy
    into space
  • Reception - collects electromagnetic energy from
    space
  • In two-way communication, the same antenna can be
    used for transmission and reception

54
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

55
Propagation Modes
  • Ground-wave propagation
  • Sky-wave propagation
  • Line-of-sight propagation

56
Ground Wave Propagation
  • Follows contour of the earth
  • Can Propagate considerable distances
  • Frequencies up to 2 MHz
  • Example
  • AM radio

57
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 earths surface
  • Reflection effect caused by refraction
  • Examples
  • Amateur radio
  • CB radio

58
Line-of-Sight Propagation
59
Categories of Noise
  • Thermal Noise
  • Intermodulation noise
  • Crosstalk
  • Impulse Noise

60
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

61
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 10-23 J/K
  • T temperature, in kelvins (absolute temperature)

62
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

63
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

64
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

65
Other Impairments
  • Atmospheric absorption water vapor and oxygen
    contribute to attenuation
  • Multipath obstacles reflect signals so that
    multiple copies with varying delays are received
  • Refraction bending of radio waves as they
    propagate through the atmosphere

66
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

67
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

68
Types of Fading
  • Fast fading
  • Slow fading
  • Flat fading
  • Selective fading
  • Rayleigh fading
  • Rician fading

69
Rayleigh and Rician Fading
  • K 0 Rayleigh
  • K 8 AWGN

70
Distortions
Perfect channel
White noise
Phase jitter
71
Error Compensation Mechanisms
  • Forward error correction
  • Adaptive equalization
  • Diversity techniques

72
Adaptive Equalization
  • Can be applied to transmissions that carry analog
    or digital information
  • Analog voice or video
  • Digital data, digitized voice or video
  • Used to combat intersymbol interference
  • Involves gathering dispersed symbol energy back
    into its original time interval
  • Techniques
  • Lumped analog circuits
  • Sophisticated digital signal processing algorithms

73
Signal Encoding Techniques
  • Chapter 6

74
Encoding and Modulation
75
Signal Encoding Criteria
  • What determines how successful a receiver will be
    in interpreting an incoming signal?
  • Signal-to-noise ratio
  • Data rate
  • Bandwidth
  • An increase in data rate increases bit error rate
  • An increase in SNR decreases bit error rate
  • An increase in bandwidth allows an increase in
    data rate

76
  • Sources Generate Sine Waves

Voltage
Time
Frequency
Voltage
Spectrum Analyzer
Oscilloscope
This is the ideal output most specs deal with
deviations from the ideal and adding modulation
to a sine wave
RF
Millimeter
Microwave
20-50 GHz
300 GHz
3-6 GHz
77
  • Modulation
  • ...Where the information resides

78
Multiple Frequency-Shift Keying (MFSK)
79
Phase Shift Keying (PSK)
Baseband Data
1
0
1
0
BPSK modulated signal
s0
s0
s1
s1
where s0 -Acos(?ct) and s1 Acos(?ct)
  • Major drawback rapid amplitude change between
    symbols due to phase discontinuity, which
    requires infinite bandwidth. Binary Phase Shift
    Keying (BPSK) demonstrates better performance
    than ASK and BFSK
  • BPSK can be expanded to a M-ary scheme, employing
    multiple phases and amplitudes as different states

80
Phase-Shift Keying (PSK)
  • Two-level PSK (BPSK)
  • Uses two phases to represent binary digits

81
Example BPSK Constellation Diagram
Q
I
?Eb
-?Eb
Constellation diagram
82
QPSK
  • Quadrature Phase Shift Keying (QPSK) can be
    interpreted as two independent BPSK systems (one
    on the I-channel and one on Q), and thus the same
    performance but twice the bandwidth efficiency
  • Large envelope variations occur due to abrupt
    phase transitions, thus requiring linear
    amplification

83
QPSK Constellation Diagram
Q
Q
I
I
Carrier phases 0, ?/2, ?, 3?/2
Carrier phases ?/4, 3?/4, 5?/4, 7?/4
  • Quadrature Phase Shift Keying has twice the
    bandwidth efficiency of BPSK since 2 bits are
    transmitted in a single modulation symbol

84
Types of QPSK
Q
I
Conventional QPSK
?/4 QPSK
Offset QPSK
  • Conventional QPSK has transitions through zero
    (i.e. 1800 phase transition). Highly linear
    amplifiers required.
  • In Offset QPSK, the phase transitions are limited
    to 900, the transitions on the I and Q channels
    are staggered.
  • In ?/4 QPSK the set of constellation points are
    toggled each symbol, so transitions through zero
    cannot occur. This scheme produces the lowest
    envelope variations.
  • All QPSK schemes require linear power amplifiers

85
Quadrature Amplitude Modulation
  • QAM is a combination of ASK and PSK
  • Two different signals sent simultaneously on the
    same carrier frequency

86
Multi-level (M-ary) Phase and Amplitude Modulation
16 QAM
16 APSK
16 PSK
  • Amplitude and phase shift keying can be combined
    to transmit several bits per symbol.
  • Often referred to as linear as they require
    linear amplification.
  • More bandwidth-efficient, but more susceptible to
    noise.
  • For M4, 16QAM has the largest distance between
    points, but requires very linear amplification.
    16PSK has less stringent linearity requirements,
    but has less spacing between constellation
    points, and is therefore more affected by noise.

87
M256 M128 M64 M32 M16 M4

88
Delta Modulation
89
Spread Spectrum
  • Chapter 7

90
Frequency Hoping Spread Spectrum (FHSS) - I
  • Signal is broadcast over seemingly random series
    of radio frequencies
  • A number of channels allocated for the FH signal
  • Width of each channel corresponds to bandwidth of
    input signal
  • Signal hops from frequency to frequency at fixed
    intervals
  • Transmitter operates in one channel at a time
  • Bits are transmitted using some encoding scheme
  • At each successive interval, a new carrier
    frequency is selected

91
Frequency Hoping Spread Spectrum - II
Time
400 ms
Frequency
1 MHz
92
Frequency Hoping Spread Spectrum - V
  • MFSK signal is translated to a new frequency
    every Tc seconds by modulating the MFSK signal
    with the FHSS carrier signal
  • For data rate of R
  • duration of a bit T 1/R seconds
  • duration of signal element Ts LT seconds
  • Tc ? Ts - slow-frequency-hop spread spectrum

93
Frequency Hoping Spread Spectrum - VI
  • Tc lt Ts - fast-frequency-hop spread spectrum

94
Direct Sequence Spread Spectrum - III Another
Example
95
Direct Sequence Spread Spectrum - IV Spreading
and De-spreading DSSS
96
Direct Sequence Spread Spectrum - V Yet Another
Example
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
97
Direct Sequence Spread Spectrum - VIII
98
Direct Sequence Spread Spectrum - IX
Direct Sequence (DS)
user data
data rate
Modulation (primary modulation)
Spreading (secondary modulation)
Tx
99
Direct Sequence Spread Spectrum - X
If you know the correct spreading sequence (code)
,
received signal
100
Direct Sequence Spread Spectrum - XI
If you dont know the correct spreading sequence
(code)
received signal
01010101
01010101
01010101
101
Direct Sequence Spread Spectrum - XII
Privacy, Security
Power density of SS-signals could be lower than
the noise density.


transmitted SS-signal
received signal
de-modulator
Noise
They cannot perceive the existence of
communication, because of signal behind the noise.
102
Code Division Multiple Access IV
103
Code Division Multiple Access V
104
Coding and Error Control
  • Chapter 8

105
Coding and Error Control Coping with Data
Transmission Errors
  • Error detection codes
  • Detects the presence of an error
  • Automatic repeat request (ARQ) protocols
  • Block of data with error is discarded
  • Transmitter retransmits that block of data
  • Error correction codes, or forward correction
    codes (FEC)
  • Designed to detect and correct errors

106
Error Detection I 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

107
Error Detection II 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

108
Error Detection III Parity Check
  • Parity bit appended to a block of data
  • Even parity
  • Added bit ensures an even number of 1s
  • Odd parity
  • Added bit ensures an odd number of 1s
  • Example, 7-bit character 1110001
  • Even parity 11100010
  • Odd parity 11100011

109
Error Detection IV 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

110
Error Detection V CRC using Modulo 2 Arithmetic
  • Exclusive-OR (XOR) operation
  • Parameters
  • T n-bit frame to be transmitted
  • D k-bit block of data the first k bits of T
  • F (n k)-bit FCS the last (n k) bits of T
  • P pattern of nk1 bits this is the
    predetermined divisor
  • Q Quotient
  • R Remainder

111
Error Detection VI CRC using Modulo 2 Arithmetic
  • For T/P to have no remainder, start with
  • Divide 2n-kD by P gives quotient and remainder
  • Use remainder as FCS

112
Error Detection VII CRC using Modulo 2
Arithmetic
  • Does R cause T/P have no remainder?
  • Substituting,
  • No remainder, so T is exactly divisible by P

113
Error Detection VIII CRC using Polynomials
  • All values expressed as polynomials
  • Dummy variable X with binary coefficients

114
Error Detection IX CRC using Polynomials
  • Widely used versions of P(X)
  • CRC12
  • X12 X11 X3 X2 X 1
  • CRC16
  • X16 X15 X2 1
  • CRC CCITT
  • X16 X12 X5 1
  • CRC 32
  • X32 X26 X23 X22 X16 X12 X11 X10
    X8 X7 X5 X4 X2 X 1

115
Block Error Correction Codes I Wireless
Transmission Errors
  • Error detection requires retransmission
  • Detection inadequate for wireless applications
  • Error rate on wireless link can be high, results
    in a large number of retransmissions
  • Long propagation delay compared to transmission
    time

116
Block Error Correction Codes II 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

117
Block Error Correction Codes III FEC Decoder
Outcomes
  • No errors present
  • Codeword produced by decoder matches original
    codeword
  • Decoder detects and corrects bit errors
  • Decoder detects but cannot correct bit errors
    reports uncorrectable error
  • Decoder detects no bit errors, though errors are
    present

118
Block Error Correction Codes IV Block Code
Principles
  • Hamming distance for 2 n-bit binary sequences,
    the number of different bits
  • E.g., v1011011 v2110001 d(v1, v2)3
  • Redundancy ratio of redundant bits to data bits
  • Code rate ratio of data bits to total bits
  • Coding gain the reduction in the required Eb/N0
    to achieve a specified BER of an error-correcting
    coded system

119
Block Error Correction Codes V Hamming Code
  • Designed to correct single bit errors
  • Family of (n, k) block error-correcting codes
    with parameters
  • Block length n 2m 1
  • Number of data bits k 2m m 1
  • Number of check bits n k m
  • Minimum distance dmin 3
  • Single-error-correcting (SEC) code
  • SEC double-error-detecting (SEC-DED) code

120
Block Error Correction Codes VI Hamming Code
Process
  • Encoding k data bits (n -k) check bits
  • Decoding compares received (n -k) bits with
    calculated (n -k) bits using XOR
  • Resulting (n -k) bits called syndrome word
  • Syndrome range is between 0 and 2(n-k)-1
  • Each bit of syndrome indicates a match (0) or
    conflict (1) in that bit position

121
Block Error Correction Codes VII Hamming Code
Process
122
Block Error Correction Codes VIII Hamming Code
Process
123
Block Error Correction Codes IX Cyclic Codes
  • Can be encoded and decoded using linear feedback
    shift registers (LFSRs)
  • For cyclic codes, a valid codeword (c0, c1, …,
    cn-1), shifted right one bit, is also a valid
    codeword (cn-1, c0, …, cn-2)
  • Takes fixed-length input (k) and produces
    fixed-length check code (n-k)
  • In contrast, CRC error-detecting code accepts
    arbitrary length input for fixed-length check code

124
Block Error Correction Codes X
125
Block Error Correction Codes XI BCH Codes
  • For positive pair of integers m and t, a (n, k)
    BCH code has parameters
  • Block length n 2m 1
  • Number of check bits n k lt mt
  • Minimum distancedmin gt 2t 1
  • Correct combinations of t or fewer errors
  • Flexibility in choice of parameters
  • Block length, code rate

126
Block Error Correction Codes XII
127
Block Error Correction Codes XIII 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

128
Convolutional Codes-I
  • 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

129
Convolutional Codes-II Convolutional Encoder
130
Convolutional Codes-III (Decoding)
  • Trellis diagram expanded encoder diagram
  • Viterbi code error correction algorithm
  • Compares received sequence with all possible
    transmitted sequences
  • Algorithm chooses path through trellis whose
    coded sequence differs from received sequence in
    the fewest number of places
  • Once a valid path is selected as the correct
    path, the decoder can recover the input data bits
    from the output code bits

131
Convolutional Codes-II Convolutional Encoder
132
(No Transcript)
133
Satellite Communications
  • Chapter 9

134
Introduction - II Earth Satellites
  • The Most Well-Known Types of Earth Satellites
  • International Space Station
  • Global Positioning System
  • Synchronous Satellites
  • The Moon

135
Geostationary Earth Orbit
136
Low Earth Orbit (LEO)
137
Medium Earth Orbit (MEO)
138
Cellular Wireless Networks
  • Chapter 10

139
Cellular Network Organization
  • Use multiple low-power transmitters (100 W or
    less)
  • Areas divided into cells
  • Each served by its own antenna
  • Served by base station consisting of transmitter,
    receiver, and control unit
  • Band of frequencies allocated
  • Cells set up such that antennas of all neighbors
    are equidistant (hexagonal pattern)

140
Frequency Reuse
  • Adjacent cells assigned different frequencies to
    avoid interference or crosstalk
  • Objective is to reuse frequency in nearby cells
  • 10 to 50 frequencies assigned to each cell
  • Transmission power controlled to limit power at
    that frequency escaping to adjacent cells
  • The issue is to determine how many cells must
    intervene between two cells using the same
    frequency

141
Example of Mobile Cellular Call
142
Handoff Between Two Cells
143
Types of Power Control
  • Open-loop power control
  • Depends solely on mobile unit
  • No feedback from BS
  • Not as accurate as closed-loop, but can react
    quicker to fluctuations in signal strength
  • Closed-loop power control
  • Adjusts signal strength in reverse channel based
    on metric of performance
  • BS makes power adjustment decision and
    communicates to mobile on control channel

144
Grade of Service
145
First-Generation Analog
  • Advanced Mobile Phone Service (AMPS)
  • In North America, two 25-MHz bands allocated to
    AMPS
  • One for transmission from base to mobile unit
  • One for transmission from mobile unit to base
  • Each band split in two to encourage competition
  • Frequency reuse exploited

146
AMPS Parameters
147
Differences Between First and Second Generation
Systems
  • Digital traffic channels first-generation
    systems are almost purely analog
    second-generation systems are digital
  • Encryption all second generation systems
    provide encryption to prevent eavesdropping
  • Error detection and correction
    second-generation digital traffic allows for
    detection and correction, giving clear voice
    reception
  • Channel access second-generation systems allow
    channels to be dynamically shared by a number of
    users

148
Second Generation Cellular
149
GSM Network Architecture
150
Mobile Station
  • Mobile station communicates across Um interface
    (air interface) with base station transceiver in
    same cell as mobile unit
  • Mobile equipment (ME) physical terminal, such
    as a telephone or PCS
  • ME includes radio transceiver, digital signal
    processors and subscriber identity module (SIM)
  • GSM subscriber units are generic until SIM is
    inserted
  • SIMs roam, not necessarily the subscriber devices

151
Base Station Subsystem (BSS)
  • BSS consists of base station controller and one
    or more base transceiver stations (BTS)
  • Each BTS defines a single cell
  • Includes radio antenna, radio transceiver and a
    link to a base station controller (BSC)
  • BSC reserves radio frequencies, manages handoff
    of mobile unit from one cell to another within
    BSS, and controls paging

152
Network Subsystem (NS)
  • NS provides link between cellular network and
    public switched telecommunications networks
  • Controls handoffs between cells in different BSSs
  • Authenticates users and validates accounts
  • Enables worldwide roaming of mobile users
  • Central element of NS is the mobile switching
    center (MSC)

153
Mobile Switching Center (MSC) Databases
  • Home location register (HLR) database stores
    information about each subscriber that belongs to
    it
  • Visitor location register (VLR) database
    maintains information about subscribers currently
    physically in the region
  • Authentication center database (AuC) used for
    authentication activities, holds encryption keys
  • Equipment identity register database (EIR)
    keeps track of the type of equipment that exists
    at the mobile station

154
ITUs View of Third-Generation Capabilities
  • Voice quality comparable to the public switched
    telephone network
  • 144 kbps data rate available to users in
    high-speed motor vehicles over large areas
  • 384 kbps available to pedestrians standing or
    moving slowly over small areas
  • Support for 2.048 Mbps for office use
  • Symmetrical / asymmetrical data transmission
    rates
  • Support for both packet switched and circuit
    switched data services

155
ITUs View of Third-Generation Capabilities
  • An adaptive interface to the Internet to reflect
    efficiently the common asymmetry between inbound
    and outbound traffic
  • More efficient use of the available spectrum in
    general
  • Support for a wide variety of mobile equipment
  • Flexibility to allow the introduction of new
    services and technologies

156
Cordless Systems and Wireless Local Loop
  • Chapter 11

157
Cordless System Operating Environments
  • Residential a single base station can provide
    in-house voice and data support
  • Office
  • A single base station can support a small office
  • Multiple base stations in a cellular
    configuration can support a larger office
  • Telepoint a base station set up in a public
    place, such as an airport

158
Time Division Duplex (TDD)
  • TDD also known as time-compression multiplexing
    (TCM)
  • Data transmitted in one direction at a time, with
    transmission between the two directions
  • Simple TDD
  • TDMA TDD

159
Differential Quantization
  • Speech signals tend not to change much between
    two samples
  • Transmitted PCM values contain considerable
    redundancy
  • Transmit difference value between adjacent
    samples rather than actual value
  • If difference value between two samples exceeds
    transmitted bits, receiver output will drift from
    the true value
  • Encoder could replicate receiver output and
    additionally transmit that difference

160
MOS
161
WLL Configuration
162
Advantages of WLL over Wired Approach
  • Cost wireless systems are less expensive due to
    cost of cable installation thats avoided
  • Installation time WLL systems can be installed
    in a small fraction of the time required for a
    new wired system
  • Selective installation radio units installed
    for subscribers who want service at a given time
  • With a wired system, cable is laid out in
    anticipation of serving every subscriber in a
    given area

163
Propagation Considerations for WLL
  • Most high-speed WLL schemes use millimeter wave
    frequencies (10 GHz to about 300 GHz)
  • There are wide unused frequency bands available
    above 25 GHz
  • At these high frequencies, wide channel
    bandwidths can be used, providing high data rates
  • Small size transceivers and adaptive antenna
    arrays can be used

164
Propagation Considerations for WLL
  • Millimeter wave systems have some undesirable
    propagation characteristics
  • Free space loss increases with the square of the
    frequency losses are much higher in millimeter
    wave range
  • Above 10 GHz, attenuation effects due to rainfall
    and atmospheric or gaseous absorption are large
  • Multipath losses can be quite high

165
802.16 Standards Development
  • Use wireless links with microwave or millimeter
    wave radios
  • Use licensed spectrum
  • Are metropolitan in scale
  • Provide public network service to fee-paying
    customers
  • Use point-to-multipoint architecture with
    stationary rooftop or tower-mounted antennas
  • Provide efficient transport of heterogeneous
    traffic supporting quality of service (QoS)
  • Use wireless links with microwave or millimeter
    wave radios
  • Are capable of broadband transmissions (gt2 Mbps)

166
Mobile IP and Wireless Application Protocol
  • Chapter 12

167
Mobile IP Uses
  • Enable computers to maintain Internet
    connectivity while moving from one Internet
    attachment point to another
  • Mobile user's point of attachment changes
    dynamically and all connections are automatically
    maintained despite the change
  • Nomadic - user's Internet connection is
    terminated each time the user moves and a new
    connection is initiated when the user dials back
    in
  • New, temporary IP address is assigned

168
Mobile IP Scenario
169
Wireless Application Protocol (WAP)
  • Open standard providing mobile users of wireless
    terminals access to telephony and information
    services
  • Wireless terminals include wireless phones,
    pagers and personal digital assistants (PDAs)
  • Designed to work with all wireless network
    technologies such as GSM, CDMA, and TDMA
  • Based on existing Internet standards such as IP,
    XML, HTML, and HTTP
  • Includes security facilities

170
WAP
171
WAP Protocol Stack
172
Wireless Markup Language (WML) Features
  • Text and image support formatting and layout
    commands
  • Deck/card organizational metaphor WML documents
    subdivided into cards, which specify one or more
    units of interaction
  • Support for navigation among cards and decks
    includes provisions for event handling used for
    navigation or executing scripts

173
Wireless LAN Technology
  • Chapter 13

174
Strengths of Infrared Over Microwave Radio
  • Spectrum for infrared virtually unlimited
  • Possibility of high data rates
  • Infrared spectrum unregulated
  • Equipment inexpensive and simple
  • Reflected by light-colored objects
  • Ceiling reflection for entire room coverage
  • Doesnt penetrate walls
  • More easily secured against eavesdropping
  • Less interference between different rooms

175
Drawbacks of Infrared Medium
  • Indoor environments experience infrared
    background radiation
  • Sunlight and indoor lighting
  • Ambient radiation appears as noise in an infrared
    receiver
  • Transmitters of higher power required
  • Limited by concerns of eye safety and excessive
    power consumption
  • Limits range

176
IR Data Transmission Techniques
  • Directed Beam Infrared
  • Ominidirectional
  • Diffused

177
Directed Beam Infrared
  • Used to create point-to-point links
  • Range depends on emitted power and degree of
    focusing
  • Focused IR data link can have range of kilometers
  • Cross-building interconnect between bridges or
    routers

178
Directed Beam Infrared - II
179
Omnidirectional
  • Single base station within line of sight of all
    other stations on LAN
  • Station typically mounted on ceiling
  • Base station acts as a multiport repeater
  • Ceiling transmitter broadcasts signal received by
    IR transceivers
  • IR transceivers transmit with directional beam
    aimed at ceiling base unit

180
Omnidirectional - II
181
Diffused
  • All IR transmitters focused and aimed at a point
    on diffusely reflecting ceiling
  • IR radiation strikes ceiling
  • Reradiated omnidirectionally
  • Picked up by all receivers

182
IEEE 802.11 Wireless LAN Standard
  • Chapter 14

183
IEEE 802 Protocol Layers
184
Protocol Architecture
  • Functions of physical layer
  • Encoding/decoding of signals
  • Preamble generation/removal (for synchronization)
  • Bit transmission/reception
  • Includes specification of the transmission medium
  • Functions of medium access control (MAC) layer
  • On transmission, assemble data into a frame with
    address and error detection fields
  • On reception, disassemble frame and perform
    address recognition and error detection
  • Govern access to the LAN transmission medium
  • Functions of logical link control (LLC) Layer
  • Provide an interface to higher layers and perform
    flow and error control

185
MAC Frame Format
  • MAC control
  • Contains Mac protocol information
  • Destination MAC address
  • Destination physical attachment point
  • Source MAC address
  • Source physical attachment point
  • CRC
  • Cyclic redundancy check

186
IEEE 802.11 Architecture
  • Distribution system (DS)
  • Access point (AP)
  • Basic service set (BSS)
  • Stations competing for access to shared wireless
    medium
  • Isolated or connected to backbone DS through AP
  • Extended service set (ESS)
  • Two or more basic service sets interconnected by
    DS

187
Distribution of Messages Within a DS
  • Distribution service
  • Used to exchange MAC frames from station in one
    BSS to station in another BSS
  • Integration service
  • Transfer of data between station on IEEE 802.11
    LAN and station on integrated IEEE 802.x LAN

188
Transition Types Based On Mobility
  • No transition
  • Stationary or moves only within BSS
  • BSS transition
  • Station moving from one BSS to another BSS in
    same ESS
  • ESS transition
  • Station moving from BSS in one ESS to BSS within
    another ESS

189
Association/Security/Privacy
  • Association
  • Establishes initial association between station
    and AP
  • Reassociation
  • Enables transfer of association from one AP to
    another, allowing station to move from one BSS to
    another
  • Disassociation
  • Association termination notice from station or AP
  • Authentication
  • Establishes identity of stations to each other
  • Deathentication
  • Invoked when existing authentication is
    terminated
  • Privacy
  • Prevents message contents from being read by
    unintended recipient

190
IEEE 802.11 Medium Access Control
  • MAC layer covers three functional areas
  • Reliable data delivery
  • Access control
  • Security

191
Reliable Data Delivery
  • More efficient to deal with errors at the MAC
    level than higher layer (such as TCP)
  • Frame exchange protocol
  • Source station transmits data
  • Destination responds with acknowledgment (ACK)
  • If source doesnt receive ACK, it retransmits
    frame
  • Four frame exchange
  • Source issues request to send (RTS)
  • Destination responds with clear to send (CTS)
  • Source transmits data
  • Destination responds with ACK

192
Access Control
193
RTS/CTS 4 way handshake access method
194
Interframe Space (IFS) Values
  • Short IFS (SIFS)
  • Shortest IFS
  • Used for immediate response actions
  • Point coordination function IFS (PIFS)
  • Midlength IFS
  • Used by centralized controller in PCF scheme when
    using polls
  • Distributed coordination function IFS (DIFS)
  • Longest IFS
  • Used as minimum delay of asynchronous frames
    contending for access

195
IFS Usage
  • SIFS
  • Acknowledgment (ACK)
  • Clear to send (CTS)
  • Poll response
  • PIFS
  • Used by centralized controller in issuing polls
  • Takes precedence over normal contention traffic
  • DIFS
  • Used for all ordinary asynchronous traffic

196
MAC Frame Format
197
MAC Frame Fields
  • Frame Control frame type, control information
  • Duration/connection ID channel allocation time
  • Addresses context dependant, types include
    source and destination
  • Sequence control numbering and reassembly
  • Frame body MSDU or fragment of MSDU
  • Frame check sequence 32-bit CRC

198
Bluetooth
  • Chapter 15

199
Overview
  • Universal short-range wireless capability
  • Uses 2.4-GHz band
  • Available globally for unlicensed users
  • Devices within 10 m can share up to 720 kbps of
    capacity
  • Supports open-ended list of applications
  • Data, audio, graphics, video

200
Bluetooth Standards Documents
  • Core specifications
  • Details of various layers of Bluetooth protocol
    architecture
  • Profile specifications
  • Use of Bluetooth technology to support various
    applications

201
Bluetooth Physical link
  • Point to point link
  • master - slave relationship
  • radios can function as masters or slaves

202
Physical Links between Master and Slave
  • Synchronous connection oriented (SCO)
  • Allocates fixed bandwidth between point-to-point
    connection of master and slave
  • Master maintains link using reserved slots
  • Master can support three simultaneous links
  • Asynchronous connectionless (ACL)
  • Point-to-multipoint link between master and all
    slaves
  • Only single ACL link can exist

203
Connection Setup
  • Inquiry - scan protocol
  • to learn about the clock offset and device
    address of other nodes in proximity

204
Inquiry on time axis
f1
f2
Slave1
Master
Slave2
205
Piconet formation
  • Page - scan protocol
  • to establish links with nodes in proximity

206
Addressing
  • Bluetooth device address (BD_ADDR)
  • 48 bit IEEE MAC address
  • Active Member address (AM_ADDR)
  • 3 bits active slave address
  • all zero broadcast address
  • Parked Member address (PM_ADDR)
  • 8 bit parked slave address

207
Piconet MAC protocol Polling
FH/TDD
f1
f3
f4
f5
f2
f6
m
s1
s2
625 ?sec
1600 hops/sec
208
Multi slot packets
FH/TDD
f1
f4
f5
f6
m
s1
s2
625 µsec
Data rate depends on type of packet
209
Physical Link Types
  • Synchronous Connection Oriented (SCO) Link
  • slot reservation at fixed intervals
  • Asynchronous Connection-less (ACL) Link
  • Polling access method

m
s1
s2
210
Packet Types
Data/voice packets
Control packets
Voice
data
ID Null Poll FHS DM1
HV1 HV2 HV3 DV
DH1 DH3 DH5
DM1 DM3 DM5
211
Packet Format
54 bits
72 bits
0 - 2744 bits
Access code
Header
Payload
header
Data
Voice
CRC
No CRC No retries
ARQ
FEC (optional)
FEC (optional)
625 µs
master
slave
212
Access Code
72 bits
Access code
Payload
Header
Purpose
  • Synchronization
  • DC offset compensation
  • Identification
  • Signaling

X
213
Packet Header
54 bits
Access code
Payload
Header
Purpose
  • Addressing (3)
  • Packet type (4)
  • Flow control (1)
  • 1-bit ARQ (1)
  • Sequencing (1)
  • HEC (8)

16 packet types (some unused)
Broadcast packets are not ACKed
For filtering retransmitted packets
Verify header integrity
total
18 bits
Encode with 1/3 FEC to get 54 bits
214
Voice Packets (HV1, HV2, HV3)
240 bits
54 bits
72 bits
366 bits
Access code
Header
30 bytes
Payload
HV1
10 bytes
1/3 FEC
20 bytes
HV2
2/3 FEC
30 bytes
HV3
215
Data Packet Types
Asymmetric
Symmetric
2/3 FEC
Asymmetric
Symmetric
No FEC
216
Piconets and Scatternets
  • Piconet
  • Basic unit of Bluetooth networking
  • Master and one to seven slave devices
  • Master determines channel and phase
  • Scatternet
  • Device in one piconet may exist as master or
    slave in another piconet
  • Allows many devices to share same area
  • Makes efficient use of bandwidth

217
Scatternet
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