Mitigation of Channel Impairments and Fading
Transcript of Mitigation of Channel Impairments and Fading
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Mitigation of Channel
Impairments and Fading
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FREQUENCY CONCEPTS
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CHANNEL BANDWIDTH
Bandwidth is described as the amount of frequency range
allocated to one application.
The amount of bandwidth available is an important factor in
determining the capacity of a mobile system, i.e. the number of
calls which can be handled.
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DUPLEX DISTANCE
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CARRIER SEPARATION
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CHANNEL CODING
Channel coding is used to detect and correct errors in a received
bit stream.
It adds bits to a message.
These bits enable a channel decoder to determine whether the
message has faulty bits, and to potentially correct the faulty bits.
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Convolutional code
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INTERLEAVING In reality, bit errors often occur in sequence, as caused by
long fading dips affecting several consecutive bits. Channel coding is not suitable for handling bursty errors.
Interleaving is used to separate consecutive bits of amessage so that these are transmitted in a non-
consecutive way.
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DIVERSITY COMBINING TECHNIQUES
It increases the received signal strength by taking
advantage of the natural properties of radio waves.
Two primary diversity methods:
Space diversity & Polarization diversity
Frequency diversity
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Diversity Techniques Space diversity
Signals at the same frequency using two or three antennas locatedseveral wavelengths a part.
Antennas are connected to two or three radio receivers.
The receiver will the strongest signal is elected
Disadvantage: Uses two or more antennas, therefore the need for
a large site. Frequency diversity
Signals at different frequencies received by the same antenna veryrarely fade simultaneously. Thus the use of several carrierfrequencies or the use of a wideband signal to combat fading.
A single aerial connected to a number receiver, each tuned to adifferent frequency, whose outputs are connected in parallel. Thereceiver with the strongest instantaneous signal will provide theoutput.
Disadvantage: Uses two or more frequencies to transmit thesame signal.
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Time diversity
Spread out the effects of errors throughinterleaving and coding
Multipath diversity Consider the tapped delay line model of a channel
shown previously
If multipaths can be put together coherently atthe receiver, diversity improvement results
This is what the RAKE receiver does (see nextslides)
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Antenna Diversity
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Antenna Diversity
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Polarization Diversity
This antenna has normal size but contains two differently polarized
antenna arrays.
The most common types are vertical/horizontal arrays and arrays in
45 degree slant orientation.
Two arrays are connected to the respective Rx branches in the BTS.
Polarization diversity reduces the space required for antennae.
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ADAPTIVE EQUALIZATION
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RAKE RECEPTION
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FREQUENCY HOPPING
Fading dips occur at different places for different frequencies.
To benefit from this fact, it is possible for the BTS and MS to
hop from frequency to frequency during a call.
The frequency hopping of the BTS and MS is synchronized.
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TIMING ADVANCE
Timing advance is a solution specifically designed to counteract the problem of
time alignment.
It works by instructing the misaligned MS to transmit its burst earlier than it
normally would.
In GSM, the timing advance information relates to bit times. Thus, an MS may be
instructed to advance its transmission by a certain number of bit times.
The maximum in GSM is 63 bit times.This is one of the parameters that limits the GSM cell size to a maximum of 35 km
radius.
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Quiz No 1
Q no 1: The transmitter and receiver are at a height of 15 m and areseparated from each other by 500m.
If = 0.25m at 900MHz, then
(i) Calculate radius of First Fresnel zone
(ii) Calculate the effective height of the transmitter and receiver in order to
have the appropriate
Q. no 2:A certain area is covered by a cellular radio system with 84 cells and acluster size N. 300 voice channels are available for the system. Users areuniformly distributed over the area covered by the cellular system, andthe offered traffic per user is 0.04 Erlang.
i. Determine the maximum carried traffic per cell for Blocking ProbabilityPb=1% if cluster size N = 4 is used.
ii. Determine the maximum number of users that can be served by the systemfor a blocking probability of 3% and cluster size N = 7.
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Receiver Modulator
De-Ciphering
De Interleaving
Speech coding
Viterbi Coding
D/A -conversion
Viterbi equalizer
Transmitter Modulator
Ciphering
Interleaving
Speech coding
Channel Coding
A/D -conversion
Burst formatting
Microphone
Speaker
MobileEqu
ipment
Tx Portion
Rx Portion
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Receiver Modulator
De-Ciphering
De Interleaving
Speech coding
Viterbi Coding
Viterbi equalizer
Transmitter Modulator
Ciphering
Interleaving
Speech coding
Channel Coding
D/D -conversion
Burst formatting
Basestation
Tx Portion
Rx Portion
GSM Network