cellular and mobile communications · diversity receiver in co-channel interference –different...

CELLULAR AND MOBILE COMMUNICATIONS

by

VIDYA SAGAR POTHARAJU

Associate Professor,

Dept of ECE,

VBIT.

August 3, 2018

UNIT II Syllabus

CO-CHANNEL INTERFERENCE

Measurement of real time Co-channel interference, Design of antenna system, Antenna

parameters and their effects, Diversity technique- Space diversity, Polarization diversity,

Frequency diversity, Time diversity.

NON CO-CHANNEL INTERFERENCE

Adjacent channel interference, Near end far end interference, cross talk, Effects on coverage

and interference by power decrease, Antenna height decrease, Effects of cell site

components.

August 3, 2018 VIDYA SAGAR P 2

Co-Channel Interference

•Frequency reuse - there are several cells that use the same set of frequencies

co-channel cells

co-channel interference

•To reduce co-channel interference, co-channel cell must be separated by a minimum distance.

•When the size of the cell is approximately the same

co-channel interference is independent of the transmitted power

co-channel interference is a function of

R: Radius of the cell

D: distance to the center of the nearest co-channel cell

•Increasing the ratio Q=D/R, the interference is reduced.

•Q (co-channel interference reduction method) is called the co-channel reuse ratio


Co-channel Interference Reduction Factor

Q= D/R

D = f(KI, C/I)

where KI is the number of co channel interfering cells in the first tier and

C/I is the received carrier‐to‐interference ratio at the desired mobile receiver


Real time Co-Channel Interference

Signal is

Interference is

The received signal is

Where

And


• The average processes on X and Y are

• The signal‐to‐interference ratio


Co-channel measurement design of antenna system

Design of an Omni-directional Antenna System

in the Worst Case

The worst case is at the location where the

mobile unit would receive the weakest signal

from its own cell site but strong interferences

from all interfering cell sites.

To prove that a K = 7 cell pattern does not

provide a sufficient frequency‐reuse distance


where q = 4.6, C/I = 17 dB, which is lower than 18 dB.

If we use the shortest distance D − R, then


Therefore, in an omni-directional-cell system, K = 9 or K = 12

would be a correct choice. Then the values of q are


Design of a Directional Antenna System

Call traffic begins to increase

Use the frequency spectrum efficiently

Avoid increasing the number of cells

When K increases, the number of frequency channels assigned in a cell must become smaller

The efficiency of applying the frequency‐reuse scheme decreases

Instead of increasing k ,we use directional antennas to reduce co channel interference


Three sector case

Let q = 4.6; then


Six sector case

For q = 4.6, the equation becomes


Diversity Receiver In Co-Channel Interference –Different Types

Diversity: It is the technique used to compensate for fading channel

impairments. It is implemented by using two or more receiving antennas.

Diversity is usually employed to reduce the depth and duration of the fades

experienced by a receiver in a flat fading channel.

These techniques can be employed at both base station and mobile receivers.

Types of Diversity

Spatial or antenna diversity → most common

•Use multiple Rx antennas in mobile or base station

•Even small antenna separation (∝ λ ) changes phase of signal → constructive /destructive

nature is changed

Other diversity types → polarization, frequency, & time diversity


Diversity arrangements Illustration of interference pattern from above

Received power [log scale]

Movement

Position

A B

A B

Transmitter

Reflector

The principle of diversity is to transmit the same information on M statistically independent channels.

By doing this, we increase the chance that the information will be received properly.


Macroscopic diversity Microscopic diversity

Diversity


Types Of DiversityMACROSCOPIC DIVERSITY

Prevents Large Scale fading.

Large Scale fading is caused by shadowing due to

variation in both the terrain profile and the nature of

the surroundings.

This fading is prevented by selecting an antenna which

is not shadowed when others are, this allows increase

in the signal-to-noise ratio.

Frequency diversity/Polarization diversity/Spatial

Diversity/ Temporal Diversity are not suitable here.

To solve the problem use a separate BS

Large distance between BS1 and BS2 gives rise to

macro diversity.

Use on-frequency repeaters (receive the signal and

retransmit the amplified version). It is simpler as

synchronization is not necessary but delay and

dispersion are larger. Simulcast (same signal

transmitted simultaneously from different BSs.)

MICROSCOPIC DIVERSITY

Prevents Small Scale fading.(fading created by

interference effects) fading is caused by multiple

reflections from the surroundings. It is characterized by

deep and rapid amplitude fluctuations which occur as

the mobile moves over distances of a few wavelength.

This fading is prevented by selecting an antenna which

gives a strong signal that mitigates this small signal

fading effect.

Use multiple antennas separated in space


Microscopic diversity Techniques

Spatial Diversity (several antenna elements separated in space)

Temporal Diversity (repetition of the transmit signal at different times)

Frequency Diversity (transmission of the signal on different frequencies)

Angular Diversity (multiple antennas with different antenna patterns)

Polarization Diversity (multiple antennas receiving different polarizations)


Polorization Diversity

Selection Diversity Scanning Diversity Maximal Ratio Combining Equal Gain Combining

Space Diversity Frequency Diversity Time Diversity

Diversity


Polarization Diversity

Principle :

Polarization diversity relies on the decorrelation of the two receive ports to achieve diversity gain. The two receiver ports must remain cross-polarized.


Selection Diversity Scanning Diversity Maximal Ratio Combining Equal Gain Combining

Space Diversity


Signals received from spatially separated antennas on the mobile would have

essentially uncorrelated envelopes for antenna separations of one half wavelength or

more.

Space Diversity

Principle :A method of transmission or reception, or both, in which the effects of fading are

minimized by the simultaneous use of two or more physically separated antennas, ideally

separated by one half or more wavelengths.


Use more than one antenna to receive the signal.

Space Diversity


•Selection diversity offers an average improvement in the link margin without requiring additional transmitter power or sophisticated receiver circuitry.

•Selection diversity is easy to implement because all that is needed is a side monitoring station and an antenna switch at the receiver.

•However it is not an optimal diversity technique because it does not use all of the possible branches simultaneously.

•In practice the SNR is measured as (S+N)/N, since it is difficult to measure SNR.

Selection Diversity Technique :

Principle :Selecting the best signal among all the signals received from different

braches at the receiving end.


Feedback or Scanning Diversity

Principle : Scanning all the signals in a fixed sequence until the one with SNR more than

a predetermined threshold is identified.

This method is very simple to implement, requiring only one receiver.

The resulting fading statistics are somewhat inferior to those obtained by the other methods.


Maximal Ratio Combining

Principle :Combining all the signals in a co-phased and weighted

manner so as to have the highest achievable SNR at the receiver at all times.

The voltage signals from each of the M

diversity branches are co-phased to provide

coherent voltage addition and are

individually weighted to provide optimal

SNR

( is maximized when )Mr NrG ii /

The SNR out of the diversity combiner is the sum of the SNRs in each branch.


Equal Gain Combining

Principle :Combining all the signals in a co-phased manner with

unity weights for all signal levels so as to have the highest achievable SNR at the receiver at all times.

Combine multiple signals into one

G = 1, but the phase is adjusted for each received signal.

The signal from each branch are co-phased vectors add in-phase.

Better performance than selection diversity


•The rational behind this technique is that frequencies separated by more than the coherence

bandwidth of the channel will not experience the same fade.

•The probability of simultaneous fade will be the product of the individual fading probabilities.

•This is often employed in microwave LOS links which carry several channels in a frequency division

multiplex mode(FDM).

Frequency Diversity

Principle :The same information signal is transmitted and received

simultaneously on two or more independent fading carrier frequencies.


Time Diversity

Principle :The signals representing the same information are sent over the

same channel at different times.

•Time Diversity repeatedly transmits information at time spacing that exceeds the coherence

time of the channel.

• Multiple repetitions of the signal will be received with multiple fading conditions, thereby

providing for diversity.

•A modern implementation of time diversity involves the use of RAKE receiver for spread

spectrum CDMA, where multipath channel provides redundancy in the transmitted message.

•Multiple antennas installed at just one link (usually at BS)

•Uplink transmission from MS to BS - multiple antennas act as Rx diversity branches

•For downlink diversity branches originate at Txr.- Transmit Diversity with channel-state

information

- Transmit Diversity without channel-state information


The RAKE Rx is a time diversity Rx that collects time-shifted versions of

the original Tx signal


Antenna Parameters

August 3, 2018VIDYA SAGAR P 34

Radiation Pattern

• The radiation pattern of an antenna is a plot of the far-field radiation

from the antenna. More specifically, it is a plot of the power radiated

from an antenna per unit solid angle, or its radiation intensity U [watts

per unit solid angle]. This is arrived at by simply multiplying the power

density at a given distance by the square of the distance r, where the

power density S [watts per square meter] is given by the magnitude of

the time-averaged Pointing vector:

• U=r^²S


Directivity

• The directivity D of an antenna, a function of direction

• is defined by the ratio of radiation intensity of antenna in direction to

the mean radiation intensity in all directions.


Radiation Resistance and Efficiency

• The resistive part of the antenna impedance is split into two parts, a

radiation resistance Rr and a loss resistance Rl. The power dissipated in

the radiation resistance is the power actually radiated by the antenna, and

the loss resistance is power lost within the antenna itself. This may be

due to losses in either the conducting or the dielectric parts of the

antenna. Radiation efficiency e of the antenna as e is the ratio of power

radiated to the power accepted by antenna

• antenna with high radiation efficiency therefore has high associated

radiation resistance compared with the losses. The antenna is said to be

resonant if its input reactance Xa =0.


Power Gain

• The power gain G, or simply the gain, of an antenna is the ratio of its

radiation intensity to that

• of an isotropic antenna radiating the same total power as accepted by

the real antenna. When

• antenna manufacturers specify simply the gain of an antenna they are

usually referring to the

• maximum value of G.


Bandwidth

• The bandwidth of an antenna expresses its ability to operate over a

wide frequency range. It is often defined as the range over which the

power gain is maintained to within 3dB of its maximum value, or the

range over which the VSWR is no greater than 2:1, whichever is smaller.

The bandwidth is usually given as a percentage of the nominal

operating frequency. The radiation

• pattern of an antenna may change dramatically outside its specified

operating bandwidth.


Reciprocity

• Reciprocity theorem:

• If a voltage is applied to the terminals of an antenna A and the current measuredat the terminals of another antenna B then an equal current will be obtained at

the terminals of antenna A if the same voltage is applied to the terminals of

antenna B.


Effective Aperture

• If an antenna is used to receive a wave with a power density S [W m2], it will produce

a power in its terminating impedance (usually a receiver input impedance) of Pr watts.

The constant of proportionality between Pr and S is Ae, the effective aperture of the

antenna in

• square metres: Pr = AeS

• The antenna gain G is related to the effective aperture as follows G=4pi/ (lambda)2Ae


Thank you………………

August 3, 2018VIDYA SAGAR P

44

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