Mitigation of Channel Impairments and Fading

7/28/2019 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

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Mitigation of Channel Impairments and Fading