Transcript Chapter 9 Science and Technology Tutorials

Chapter 9
Science and Technology
Tutorials
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Multiple Access
 Frequency
Division Multiple Access
(FDMA)
 AMPS
and CT2
 Time
Division Multiple Access (TDMA)
 Hybrid FDMA/TDMA
 Code Division Multiple Access
a
physical channel corresponds to a binary
code
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CDMA

Each station has its own unique chip
sequence (CS)
 All CS are pairwise orthogonal
 For example :(codes A, B, C and D are
pairwise orthogonal)
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A: 00011011 => (-1-1-1+1+1-1+1+1)
B: 00101110 => (-1-1+1-1+1+1+1-1)
C: 01011100 => (-1+1-1+1+1+1-1-1)
D: 01000010 => (-1+1-1-1-1-1+1-1)
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CDMA

A·B = (1+1-1-1+1-1+1-1) = 0
 B·C = (1-1-1-1+1+1-1+1) = 0
 EX: if station C transmits 1 to station E, but
station B transmits 0 and station A transmits 1
simultaneously then the signal received by
station E will become S = (-1+1-3+3-1-1-1+1).
E can convert the signal S to S·C =
(1+1+3+3+1-1+1-1)/8 = 1
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Mobile Radio Signals
 Four
Main Effects of Signals
 Attenuation
that increase with distance
 Random variation due to environmental
features
 Signal fluctuations due to the motion of a
terminal
 Distortion due to the signal travel along
different path from a transmitter to a
receiver
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Attenuation Due to Distance
 the
signal strength decreases with
distance according to the relationship
Preceive = Ptransmit const/x^
(In general,  = 2, 3 or 4)
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Slow (Shadow) Fading
 Random
Environmental Effects
 As
a terminal moves, the signal strength
gradually rises and falls with significant
changes occurring over tens of meters
 Sreceive = 10 log10(1000Preceive) dBm = Stransmit +
const -10  log10(x) dBm
 The
standard deviation of Sreceive is a
quantity  dB (4 dB <=  <= 10 dB)
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What is a Decibel- dB

Decibel is the unit used to express relative
differences in signal strength.
 It is expressed as the base 10 logarithm of the
ratio of the powers of two signals:
 dB = 10 log (P1/P2)
 Logarithms are useful as the unit of measurement
because


signal power tends to span several orders of
magnitude
signal attenuation losses and gains can be
expressed in terms of subtraction and addition
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
For example: Suppose that a signal passes
through two channels is first attenuated in the
ratio of 20 and 7 on the second. The total signal
degradation is the ratio of 140 to 1. Expressed in
dB, this become
10 log 20 + 10 log 7 = 13.01 + 8.45 = 21.46 dB
 The following table helps to indicate the order
of magnitude associated with dB:

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1 dB attenuation means that 0.79 of the input power survives.
3 dB attenuation means that 0.5 of the input power survives.
10 dB attenuation means that 0.1 of the input power survives.
20 dB attenuation means that 0.01 of the input power
survives.
30 dB attenuation means that 0.001 of the input power
survives.
40 dB attenuation means that 0.0001 of the input power
survives.
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Fast (Rayleigh) Fading

Fast (Rayleigh) Fading Due to Motion of
Terminals
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As the terminal moves, each ray undergoes a
Doppler shift, causing the wavelength of the signal
to either increase or decrease
Doppler shifts in many rays arriving at the receiver
cause the rays to arrive with different relative
phase shifts
At some locations, the rays reinforce each other.
At other locations, the ray cancel each other
These fluctuations occur much faster than the
changes due to environmental effects
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Multipath Propagation
 There
are many ways for a signal to
travel from a transmitter to a receiver
(see Fig 9.5)
 Multiple path propagation is referred to
as intersymbol interference (see Fig. 9.6)
 Path delay = the maximum delay
difference between all the paths
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Technology Implications

Systems employ power control to overcome
the effects of slow fading
 Systems use a large array of techniques to
overcome the effects of fast fading and multipath propagation
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Channel coding (Section 9.4)
Interleaving (Section 9.5)
Equalization (Section 9.6)
PAKE receivers (Section 6.3)
Slow frequency hopping (Section 7.3.3)
Antenna diversity
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Channel Reuse
 Reuse
Planning
A
channel plan is a method of assigning
channels to cells in a way that guarantees
a minimum reuse distance between cells
using the same channel
 N > = 1/3(D/R)^2 where D is the distance
between a BS and the nearest BS that use
the same channel and R is radius of a cell
 Practical value of N range from 3 to 21
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Spectrum Efficiency

Compression Efficiency and Reuse Factor

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Compression Efficiency = C conversations/per
MHz (one-cell system)
If N is the number of reuse factor, spectrum
efficiency E = C/N conversations per base station
per MHz
A measure of this tolerance is the signal-tointerference ratio S/I
A high tolerance to interference promotes cellular
efficiency
S/I is an increasing function of the reuse factor N
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Slow Frequency Hopping
 The
signal moves from one frequency to
another in every frame
 The purpose of FH is to reduce the
transmission impairments
 Without FH, the entire signal is subject
to distortion whenever the assigned
carrier is impaired
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RAKE Receiver
 Synchronization
is a major task of a SS
receiver
 Difficulty:
multi-path propagation
 Solution:
Multiple correlator
(demodulator) in each receiver
 Each
correlator operates with a digital
carrier synchronized to one propagation
path
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Channel Coding

Channel codes protect information signals
against the effects of interference and fading
 Channel coding decrease the required signalto-interference ratio (S/I)req and the reuse factor
N
 Channel coding will decrease the compression
efficiency C
 The net effect is to increase the overall
spectrum efficiency
 Channel codes can serve two purposes:

error detection and forward error correction
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Block Codes
 Block
code (n, k, dmin)
 Used
to Protect The Control Information
 n is the total number of transmitted bits
per code word
 k is the number of information bits carried
by each code word
 dmin the minimum distance between all
pairs of code word

ex: n = 3, k = 2, dmin = 2 (000, 011, 101, 110)
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Block Codes
 When
dmin = 5, there are three possible
decoder actions
 The
decoder can correct no errors and
detect up to four errors
 It can correct one error and detect two or
three errors
 It can correct two errors, three or more bit
errors in a block produce a code word error
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Convolutional Codes
 Each
time a new input bit arrives at the
encoder, the encoder produces m new
output bits
 the
encoder obtains m output bits by
performing m binary logic operations on
the k bits in the shift register
 The code rate is r = 1/m
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Example:
V1 = R1
V2 = R1  R2  R3
V3 = R1  R3
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Interleaving
 Most
error-correcting codes are
effective only when transmission error
occur randomly in time
 To prevent errors from clustering,
cellular systems permute the order of
bits generated by a channel coder
 Receivers perform the inverse
permutation
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Interleaving
 Example:

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WHAT I TELL YOU THREE TIMES IS TRUE
If there are four consecutive errors in the middle,
the result is
WHAT I TELL YBVOXHREE TIMES IS TRUE
Alternatively, it is possible to interleave the symbol
using a 5 x 7 interleaving matrix (See pp. 364-365)
WHOT I XELL YOU THREE TIMEB IS VRUE
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Adaptive Equalization

An adaptive equalizer operates in two modes
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
Training mode: Modem transmits a signal, referred
to as a training sequence, that is known to
receiver. The receiving modem process the
distorted version of training sequence to obtain a
channel estimate
Tracking mode: The equalizer uses the channel
estimate to compensate for distortions in the
unknown information sequence
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Walsh Hadamard Matrix
 The
CDMA system uses a 64 x 64
WHM in two ways:
 In
down-link transmissions, it used as an
orthogonal code, which is equivalent to an
error-correcting block code with (n, k; dmin)
= (64, 6; 32)
 In up-link transmissions, the matrix serve
as a digital carrier due to its orthogonal
property
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Walsh Hadamard Matrix
W1=|0|
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W2 =
00
01
W3 =
00
01
00
01
00
01
11
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Review Exercises
 How
does the code rate r of a channel
code influence compression efficiency C
and tolerance of interference (S/I)req in
personal communications systems?
 How can soft capacity benefit a
personal communications system? Is it
possible for TDMA or FDMA system to
operate with soft capacity?
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