Chapter 9 Science and Technology Tutorials

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Chapter 9Science 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)
• A 00011011 gt (-1-1-111-111)
• B 00101110 gt (-1-11-1111-1)
• C 01011100 gt (-11-1111-1-1)
• D 01000010 gt (-11-1-1-1-11-1)

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CDMA

• AB (11-1-11-11-1) 0
• BC (1-1-1-111-11) 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 (-11-33-1-1-11). E
  can convert the signal S to SC
  (11331-11-1)/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 lt ? lt 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
• 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
• 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
  multi-path propagation
• 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 gt 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
• 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-to-interference 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
  signal-to-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
• 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
• 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

• W 1 0

0 0 0 1
W2
0 0 0 1
0 0 0 1
W3
0 0 0 1
1 1 1 0
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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?