1. Chapter 9 Science and Technology Tutorials
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2. 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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6. 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: => ( )
B: => ( )
C: => ( )
D: => ( )
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7. 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 = ( ). E can convert the signal S to S·C = ( )/8 = 1
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9. 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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10. 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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11. 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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13. 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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14. 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 log 7 = = 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 of the input power survives.
40 dB attenuation means that of the input power survives.
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15. 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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16. 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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19. 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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20. 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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24. 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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25. 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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27. 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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29. 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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30. 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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31. 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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33. 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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35. Example:
V1 = R1
V2 = R1  R2  R3
V3 = R1  R3
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37. 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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38. 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 )
WHOT I XELL YOU THREE TIMEB IS VRUE
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39. 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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42. 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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44. Walsh Hadamard Matrix W 1 = | 0 | 0 0 0 1 W2 = 0 0 0 1 0 0 0 1 W3 =
1 1
1 0
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47. 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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