Antenna

ANTENNASANTENNAS

ANTENNASANTENNAS

• Antennas form a essential part of any radio communication system.

• Antenna is that part of a transmitting or receiving system which is designed to radiate or to receive electromagnetic waves.

• An antenna can also be viewed as a transitional structure between free-space and a transmission line (such as a coaxial line).

• An important property of an antenna is the ability to focus and shape the radiated power in space e.g.: it enhances the power in some wanted directions and suppresses the power in other directions.

• Many different types and mechanical forms of antennas exist.

• Each type is specifically designed for special purposes.

ANTENNAS TYPESANTENNAS TYPES• In mobile communications two main categories of antennas used are

– Omni directional antennaOmni directional antenna

• These antennas are mostly used in rural areas.

• In all horizontal direction these antennas radiate with equal power.

• In the vertical plane these antennas radiate uniformly across all azimuth angles and have a main beam with upper and lower side lobes.

ANTENNAS TYPESANTENNAS TYPES– Directional antennaDirectional antenna

• These antennas are mostly used in mobile cellular systems to get higher gain compared to omnidirectional antenna and to minimise interference effects in the network.

• In the vertical plane these antennas radiate uniformly across all azimuth angles and have a main beam with upper and lower side lobes.

• In these type of antennas, the radiation is directed at a specific angle instead of uniformly across all azimuth angles in case of omni antennas.

ANTENNA ANTENNA CHARACTERISTICSCHARACTERISTICS

Radiation PatternRadiation Pattern

• The main characteristics of antenna is the radiation pattern.

• The antenna pattern is a graphical representation in three dimensions of the radiation of the antenna as a function of angular direction.

• Antenna radiation performance is usually measured and recorded in two orthogonal principal planes (E-Plane and H-plane or vertical and horizontal planes).

• The pattern of most base station antennas contains a main lobe and several minor lobes, termed side lobes.

• A side lobe occurring in space in the direction opposite to the main lobe is called back lobe.


Radiation PatternRadiation Pattern


Antenna GainAntenna Gain

• Antenna gain is a measure for antennas efficiency.

• Gain is the ratio of the maximum radiation in a given direction to that of a reference antenna for equal input power.

• Generally the reference antenna is a isotropic antenna.

• Gain is measured generally in “decibels above isotropic(dBi)” or “decibels above a dipole(dBd).

• An isotropic radiator is an ideal antenna which radiates power with unit gain uniformly in all directions. dBi = dBd + 2.15

• Antenna gain depends on the mechanical size, the effective aperature area, the frequency band and the antenna configuration.

• Antennas for GSM1800 can achieve some 5 to 6 dB more gain than antennas for GSM900 while maintaining the same mechanical size.

ANTENNA ANTENNA CHARACTCHARACTERISTICSERISTICS

Main Lobe Axis

½ Power Beamwidth

Side Lobe

Back Lobe

First Null


Front-to-back ratioFront-to-back ratio

• It is the ratio of the maximum directivity of an antenna to its directivity in a specified rearward direction.

• Generally antenna with a high front-to-back ratio should be used.

First Null BeamwidthFirst Null Beamwidth

• The first null beamwidth (FNBW) is the angular span between the first pattern nulls adjacent to the main lobe.

• This term describes the angular coverage of the downtilted cells.


Antenna LobesAntenna Lobes

• Main lobe is the radiation lobe containing the direction of maximum radiation.

• Side lobes

Half-power beamwidthHalf-power beamwidth

• The half power beamwidth (HPBW) is the angle between the points on the main lobe that are 3dB lower in gain compared to the maximum.

• Narrow angles mean good focusing of radiated power.

PolarisationPolarisation

• Polarisation is the propagation of the electric field vector .

• Antennas used in cellular communications are usually vertically polarised or cross polarised.


Frequency bandwidthFrequency bandwidth

• It is the range of frequencies within which the performance of the antenna, with respect to some characteristics, conforms to a specified standard.

• VSWR of an antenna is the main bandwidth limiting factor.

Antenna impedanceAntenna impedance

• Maximum power coupling into the antennas can be achieved when the antenna impedance matches the cables impedance.

• Typical value is 50 ohms.

Mechanical sizeMechanical size

• Mechanical size is related to achievable antenna gain.

• Large antennas provide higher gains but also need care in deployement and apply high torque to the antenna mast.

COUPLING BETWEEN COUPLING BETWEEN ANTENNASANTENNAS

• Antenna radiation pattern will become superimposed when the distance between the antennas becomes too small.

• This means the other antenna will mutually influence the individual antenna patterns.

• Generally 5 to 10 horizontal separation provides sufficient decoupling of antenna patterns.

• The vertical distance needed for decoupling is usually much smaller as the vertical beamwidth is generally less.

• A 1 separation in the vertical direction is sufficient in most cases.

ANTENNA ANTENNA INSTALLATIONINSTALLATION

• Antenna installation configurations depend on the operators preferences.

• It is important to keep sufficient decoupling distances between antennas.

• If TX and RX direction use separated antennas, it is advisable to keep a horizontal separation between the antennas in order to reduce the TX signal power at the RX input stages.

ANTENNA ANTENNA DOWNTILTINGDOWNTILTING

• Network planners often have the problem that the base station antenna provides an overcoverage.

• If the overlapping area between two cells is too large, increased switching between the base station (handover) occurs.

• There may even be interference of a neighbouring cell with the same frequency.

• If hopping is used in the network, then limiting the overlap is required to reduce the overall hit rate.

• In general, the vertical pattern of an antenna radiates the main energy towards the horizon.

• Only that part of the energy which is radiated below the horizon can be used for the coverage of the sector.

• Downtilting the antenna limits the range by reducing the field strength in the horizon.

ANTENNA ANTENNA DOWNTILTINGDOWNTILTING

• Antenna downtilting is the downward tilt of the vertical pattern towards the ground by a fixed angle measured w.r.t the horizon.

• Downtilting of the antenna changes the position of the half-power beamwidth and the first null relative to the horizon.

• Normally the maximum gain is at 0• (parallel to the horizon) and never intersects the horizon.

• A small downtilt places the beams maximum at the cell edge

• With appropriate downtilt, the received signal strength within the cell improves due to the placement of the main lobe within the cell radius and falls off in regions approaching the cell boundary and towards the reuse cell.

• There are two methods of downtilting

– Mechanical downtilting

– Electrical downtilting.

MECHANICAL MECHANICAL DOWNTILTINGDOWNTILTING

• Mechanical downtilting consists of physically rotating an antenna downward about an axis from its vertical position.

• In a mechanical downtilt as the front lobe moves downward the back lobe moves upwards.

• This is one of the potential drawback as compared to the electrical downtilt because coverage behind the antenna can be negatively affected as the back lobe rises above the horizon.

• Additionally , mechanical downtilt does not change the gain of the antenna at +/- 90deg from antenna horizon.

• As the antenna is given downtilt, the footprint starts changing with a notch being formed in the fron’t while it spreads on the sides.

• After 10 degrees downtilt the notch effect is quiet visible and the spread on the sides are high. This may lead to inteference on the sides.


Vertical antenna pattern at 0

Vertical antenna pattern at 15 downtilt

Backlobe shoots over the horizon

ELECTRICAL DOWNTILTELECTRICAL DOWNTILT

• Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards.

• This allows the the antenna to be mounted vertically.

• Electrical downtilt is the only practical way to achieve pattern downtilting with omnidirectional antennas.

• Electrical downtilt affects both front and back lobes.

• If the front lobe is downtilted the back lobe is also downtilted by equal amount.

• Electrical downtilting also reduces the gain equally at all angles on the horizon. The that adjusted downtilt angle is constant over the whole azimuth range.

• Variable electrical downtilt antennas are very costly.

ELECTRICAL DOWNTILTELECTRICAL DOWNTILT

ELECTRICAL DOWNTILTELECTRICAL DOWNTILTHorizontal and vertical pattern for allgon 7144 antenna

Horizontal Beamwidth = 90 Vertical Beamwidth = 16 Electrical Downtilt = 16

OBSTACLE OBSTACLE REQUIREMENTREQUIREMENT

• Nearby obstacles are those reflecting or shadowing materials that can obstruct the radio beam both in horizontal and vertical planes.

• When mounting the antenna on a roof top, the dominating obstacle in the vertical plane is the roof edge itself and in the horizontal plane, obstacles further away like surrounding buildings, can act as reflecting or shadowing material.

• The antenna beam will be distorted if the antenna is too close to the roof. Hence the antenna must be mounted at a minimum height above the rooftop or other obstacles.

• If antennas are wall mounted, a safety margin of 15 degrees between the reflecting surface and the 3-dB lobe should be kept.

OBSTACLE OBSTACLE REQUIREMENTREQUIREMENT

Main RadiationDirection

Half Power Beamwidth

Safety Margin15 Degrees

Building

OPTIMAL DOWNTILTOPTIMAL DOWNTILT• Although the use of downtilt can be a effective tool for controlling

interference, there is a optimum amount by which the antenna can be downtilted whereby both the coverage losses and the interference at the reuse cell can be kept at a minimum.

downtilt angle (D)

3 dB Beamwidth

Main lobe

Height (H)

Cellmax

OPTIMAL DOWNTILTOPTIMAL DOWNTILT• The figure shows a cells coverage area.

• The primary illumination area is the area on the ground that receives the signal contained within the 3dB vertical beamwidth of the antenna.

• The distance from the base station to the outer limit of the illumination area is denoted by Cellmax.

• It should be noted that the cellmax can be different from the cell boundary area which is customer defined.

• Ideally in a well planned network Cellmax should always be less than the co-channel reuse distance to minimise interference.

• We now derive the relation between height (H), downtilt angle (D), 3dB vertical beamwidth and Cellmax.

• As shown in the schematic is the angle between the upper limit of the 3dB beamwidth and the horizon.

OPTIMAL DOWNTILTOPTIMAL DOWNTILT• tan ( ) = Cellmax / H

= D - 0.5 * 3dB vertical beamwidth

Cellmax = H * tan (D - 0.5 * 3dB vertical beamwidth)

• For the Cellmax to be a positive quantity , downtilt angle must be more than half of the 3dB vertical beamwidth.

• When the downtilt angle is less than half of the 3dB beamwidth, part of the signal from the main beam shoots over the horizon .

• The signal directed towards or above the horizon can potentially cause interference at the reuse sites.

DIVERSITY DIVERSITY ANTENNA ANTENNA SYSTEMSSYSTEMS

Diversity Antenna SystemsDiversity Antenna SystemsNEED OF DIVERSITY

Building

Building

Building

Diversity Antenna SystemsDiversity Antenna SystemsNEED OF DIVERSITY

• In a typical cellular radio environment, the communication between the cell site and mobile is not by a direct radio path but via many paths.

• The direct path between the transmitter and the receiver is obstructed by buildings and other objects.

• Hence the signal that arrives at the receiver is either by reflection from the flat sides of buildings or by diffraction around man made or natural obstructions.

• When various incoming radiowaves arrive at the receiver antenna, they combine constructively or destructively, which leads to a rapid variation in signal strength.

• The signal fluctuations are known as ‘multipath fading’.

Diversity Antenna SystemsDiversity Antenna SystemsMultipath PropagationMultipath Propagation

• Multipath propagation causes large and rapid fluctuations in a signal

• These fluctuations are not the same as the propagation path loss.

Multipath causes three major thingsMultipath causes three major things

• Rapid changes in signal strength over a short distance or time.

• Random frequency modulation due to Doppler Shifts on different multipath signals.

• Time dispersion caused by multipath delays

• These are called “fading effects

• Multipath propagation results in small-scale fading.

Diversity Antenna SystemsDiversity Antenna SystemsDIVERSITY TECHNIQUE

• Diversity techniques have been recognised as an effective means which enhances the immunity of the communication system to the multipath fading. GSM therefore extensively adopts diversity techniques that include

Diversity techniquesInterleaving

In time domain

Frequency HoppingIn Frequency domain

Spatial diversityIn spatial domain

Polarisation diversityIn polarisation domain

Diversity Antenna SystemsDiversity Antenna SystemsCONCEPT OF DIVERSITY ANTENNA SYSTEMS

• Spatial and polarisation diversity techniques are realised through antenna systems.

• A diversity antenna system provides a number of receiving branches or ports from which the diversified signals are derived and fed to a receiver. The receiver then combines the incoming signals from the branches to produce a combined signal with improved quality in terms of signal strength or signal-to-noise ratio (S/N).

• The performance of a diversity antenna system primarily relies on the branch correlation and signal level difference between branches.

Diversity Antenna SystemsDiversity Antenna Systems

Transmission media 1

Transmission Tmedia 2

Peak

Fade

ReceiverInformation

CONCEPT OF DIVERSITY ANTENNA SYSTEMS

Diversity Antenna SystemsDiversity Antenna SystemsCORRELATION BETWEEN BRANCHES

• The branch correlation coefficient (r) represents the degree of similarity between the signals from two different receiving branches.

• The correlation coefficient ranges from 0 to 1.

• r=1 means the signals from two different branches behave exactly the same. In this case, the signals are coherent.

• r=0 means the signals from two different branches behave completely different. In this case, the signals are uncorrelated.

• To achieve the best performance, a diversity antenna system is required to provide uncorrelated signals.

• For r=1, the diversity antenna becomes ineffective in combating the multipath fading.

• In reality, however, it is not always practical to have a diversity antenna system which guarantees r=0. Extensive research in this field has revealed that a diversity antenna system can perform satisfactorily provided that r £0.7.


Time

Sig

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Combined signalSignal 1Signal 2

Combining

Combined signalfed to receiver Signal 2

Signal 1

Diversity Antenna SystemsDiversity Antenna SystemsSIGNAL LEVEL DIFFERENCE

• The second key parameter for a good diversity antenna system is the mean signal level difference.

• The difference is a statistical parameter which indicates the balance of the signal strengths from the two receiving branches.

• In a real system, the statistical balance can be verified by comparing the mean values of the two signals measured over a lengthy period.

• If the ratio betn the median values is 0dB, the two receiving branches are statistically balanced.

• The performance of the diversity system will deteriorate while the ratio increases or decreases from 0dB.


Signal level difference

Sig

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Time

SIGNAL LEVEL DIFFERENCE

Diversity Antenna SystemsDiversity Antenna SystemsSPATIAL DIVERSITY ANTENNA SYSTEMS

• The spatial diversity antenna system is constructed by physically separating two receiving base station antennas.

• Once they are separated far enough, both antennas receive independent fading signals. As a result, the signals captured by the antennas are most likely uncorrelated.

• The further apart are the antennas, the more likely that the signals are uncorrelated.

• The types of the configuration used in GSM networks are: horizontal separation vertical separation composite separation.

Diversity Antenna SystemsDiversity Antenna SystemsTYPICAL SPATIAL ANTENNA DIVERSITY CONFIGURATIONS

Horizontal Separation Vertical Separation


Branch correlation

• The physical limitation of the supporting structure should also be considered while selecting the spatial diversity antenna configuration. For example, if a wide framework is not permitted on top of a mounting tower, vertical separation is a alternative to be considered.

• To achieve the required correlation coefficient (r £0.7) different configurations require different separations.

• The separation indicated in Table below shows that low values of correlation are more easily obtained with horizontal rather than vertical separation.

• That is why most of the diversity antenna systems in GSM networks use horizontal separation.

CRITERIA FOR SELECTING TYPE OF SPATIAL SEPARATION

d/ 900MHZ 1800MHZ d/ 900MHZ 1800MHZSeparation 10 3.3m 1.7m 17 5.7m 2.8m

Horizontal Separation Vertical Separation

Diversity Antenna SystemsDiversity Antenna SystemsCRITERIA FOR SELECTION OF SPATIAL SEPARATION

Signal level difference• A system using horizontally separated diversity antennas has a

symmetrical configuration and is therefore able to provide balanced signal strengths.

• A system using vertically separated antennas needs large separation to meet the required correlation.

• The consequence is that the two antennas have different antenna height gains, which may result in imbalance between the two signal strengths.

Diversity Antenna SystemsDiversity Antenna SystemsCRITERIA FOR SELECTION OF SPATIAL SEPARATION

Angular dependence

• Angular dependence reflects the dependence of the performance of a diversity antenna system on the angular position of a mobile relative to the boresight of the antenna.

• Horizontally separated antenna system has high dependence on the mobile’s angular position.

• The effective separation reduces as the mobile moves away from the antenna boresight.

• As the mobile is 90° off the antenna boresight, the effective separation becomes zero.

• In such a case, the signals from two antennas are very likely coherent which will then lead to a deterioration of the diversity performance.

Diversity Antenna SystemsDiversity Antenna SystemsANGULAR DEPENDANCE

•Most of the GSM cell sites are 3 sectored cell sites.

•The maximum angular offset is therefore approximately 60°.

•Simulation shows that the performance of a horizontally separated antenna system experiences noticeable deterioration only when the angular offset exceeds 70° .

SeparationReduced

SeparationZero

Separation

View from boresight View from 45 deg off boresight View from 90 deg off boresight

Diversity Antenna SystemsDiversity Antenna SystemsPROS AND CONS OF HORIZONTAL CONFIGURATION

Advantages

• Easier to achieve low values of correlation and balance between the signals. Hence widely used.

Disadvantages

• High angular dependence. The impact is however marginal for sectorised applications.

• Require sizeable headframe on the supporting structure.

Diversity Antenna SystemsDiversity Antenna SystemsPROS AND CONS OF VERTICAL CONFIGURATION

Advantages

• Slim supporting structure.

• Angular independence

Disadvantages

• Require large separation for low values of correlation.

• May cause imbalance between the two diversity branches.

• Generally not used.

Diversity Antenna SystemsDiversity Antenna SystemsTHREE ANTENNA SPATIAL CONFIGURATION

10 Separation

Receive 1 Transmit Receive 2

Diversity Antenna SystemsDiversity Antenna SystemsTWO ANTENNA SPATIAL CONFIGURATION

10 Separation

Receive 2Tx Rx

Transmit Receive 1

Duplexer

Diversity Antenna SystemsDiversity Antenna SystemsPOLARISATION DIVERSITY ANTENNA SYSTEMS

• A single (say vertical) polarised electromagnetic wave is converted to a wave with two orthogonal polarised fields while it is propagating through scattering environment.

• It has also been found that the two fields exhibit some extent of decorrelation.

Diversity Antenna SystemsDiversity Antenna SystemsDUAL POLARISED ANTENNAS

• A dual-polarisation antenna consists of two sets of radiating elements which radiate or, in reciprocal, receive two orthogonal polarised fields.

• The antenna has two input connectors which separately connects to each set of the elements.

• The antenna has therefore the ability to simultaneously transmit and receive two orthogonally polarised fields.

H / V Slant 45

Diversity Antenna SystemsDiversity Antenna SystemsADVANTAGES OF DUAL POLARISED ANTENNAS

• The best advantage of using the dual polarisation antenna is the reduction in the number of antennas per sector.

• Reduced size of the headframe of the supporting structure

• Reduced windload and weight.

• Reduced difficulty in site acquisition and installation.

• Cost saving

– Requiring slim tower– Requiring less installation time.– Cost of one dual polarisation antenna is generally lower

than that of two – Single polarised antennas

Diversity Antenna SystemsDiversity Antenna SystemsDUAL POLARISED ANTENNA CONFIGURATIONS

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Antenna

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