Rf Design Guide

8/11/2019 Rf Design Guide

http://slidepdf.com/reader/full/rf-design-guide 1/20

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1.0 RF Design Guidelines

1.1 Introduction

This document is intended to guide RF Design engineers during Site Surveying, RF Planning

and Design. It is also intended to help optimisation engineers understand the process and

methodology of the RF planning group.

1.2 Purpose

The purpose and objectives of these guidelines are to: -

• Standardize the way RF design is implemented irrespective of region or area.

• Provide assistance to RF engineers in ensuring RF design is done in a deterministic

manner.

These guidelines are not intended to detail advanced RF or GSM concepts but rather to look

into all the processes and variables of RF design in practical situations.

2.0 Overall Objectives of RF Planning

The overall objectives of any RF Design depend on a number of factors that are determined

by the needs and expectations of the customer; the resources made available by the customer;

any service levels determined by the contract with the customer but only insofar as they affect

the RF Design; and the resources that are available at the Technical Centre or Business Unit

that is responsible for the RF Design. Generally speaking the RF Design should satisfy the

following criteria :-

a) Maximising coverage

b) Providing sufficient capacity

c) Providing an acceptable quality of service

d) Minimising cost

2.1 Maximising Coverage

Although coverage can be measured or predicted in different ways, it is usually classified

according to the service level provided, such as in-building, in-car or on -street. Coverage

needs to be tailored according to the type of subscriber targeted (such as business or

residential); where that subscriber is likely to use their mobile telephone; and what

probability of coverage is being designed. For example, city-centre coverage targeting

business subscribers would require a design that provides a high probability of in-building

service. The design in rural areas would be based on providing a good probability of in-car

service.

Coverage is then achieved by the careful positioning of base stations and adjusting the

orientation of their antennas and their transmit powers. At all times coverage should be

sufficient to satisfy the service level and capacity required (see section 2.2 also).



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2.2 Providing sufficient capacity

Initial network design provides a baseline capacity, usually calculated from preliminary

market research or estimates of subscriber count. This baseline capacity may account forknown traffic hot spots, such as in business area, busy road intersections or transport hubs,

and provide a very good grade of service. Upon turn-on, real-life traffic may exceed the

network’s baseline capacity in some areas and the RF Design would then need to increase the

capacity, by installing more base stations or channels, or improve the service level, usually by

installing more base stations. The overall measure of the network’s ability to carry its

subscriber load during peak hours is the grade of service, usually based on a maximum of 2%

or 5%.

2.3 Providing Acceptable Quality Service

Beside coverage and capacity, proper RF planning involves giving due consideration to providing good quality service in term of voice quality, access time, etc. This can be done by

reducing interference - co-channel, adjacent channel and intermodulation, through proper

frequency planning, optimising and antenna/site location. Access time and other variables

like control channel congestion can be reduced through proper LAC design and control

channel configuration.

2.4 Minimising Cost

All the above three should be done in the most cost-effective way, while keeping within the

budgetary constraints. These constraints may be defined either by a cap on the number andtype of base stations; or by ensuring a particular service level over as large a geographical

area as possible; or by ensuring that the combined equipment, transmission and spectrum

costs of the design do not exceed a fixed amount.

3.7 RF Design

The RF design engineer for that particular region would then decide from the information

that he has the type of antenna to be used, the antenna height above ground level, the antenna

orientation, the antenna mechanical or electrical downtilt if required and the base station

maximum transmit power. The comments and coverage weaknesses is also decided upon. ‘

Analysis Report’ as shown in figure 3.8 is then filled up. For analysis purpose also added

along are the terrain height and morphology of the area.



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6.0 Grid Design

The grid is a graphical way of representing the equal signal contours of the cell sites within a

given system so that the selection of such sites can be controlled and optimized. From the

graphical relationship between the sites and the way signals attenuate as they travel from the

site, relationships can be drawn which describe the theoretical limits in system performance if

the sites were placed exactly as intended.

An example of grid design is indicated below :-

urban sub-urban

D R

Rural

Normal practice in network planning is to choose one point of a well known re-use model asa starting point. From this cell, other cells are created around it. It becomes necessary to use

cells of varying size. As one move from high density area like urban environment to low

density area like rural environment, the cell size should increase as shown above.

The distance between co-channel sites using 4 reuse pattern is 3*R (D/R=3) with a carrier to

interference ratio of 13.6dB.



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7.0 Antenna Configuration

7.1 RX and TX Antenna Separation

There should be a minimum separation (isolation) between transmitter and receiver antennas

to avoid receiver desensitization which is a reduction in receiver sensitivity. The further apart

they are kept the better the isolation would be, but due to space constraint and to avoid

imbalance in the uplink and downlink path, it is best to have a compromise.

The horizontal or vertical separation for side by side sectored antenna with a horizontal

beamwidth of 60 degrees should be at least 0.5 to 1 meters. The isolation for this distance is

more than 54dB.

1 meter

Tx

Rx Tx 0.5 meter

Rx

The separation for sectored antenna with a bigger horizontal beamwidth (> 60 degrees)

should be 1 meter or more.

The horizontal and vertical separation for omni antennas where one is above and the other is

below, the same horizontal plane should be 0 meters or more - refer to the diagram below:-

Tx

0 meter 0 meter



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Rx

For omni antenna, vertical isolation is much more effective than horizontal isolation. If

possible, putting both the receiver and transmitter omni antennas on the same horizontal

plane should be avoided.

The above contention is also applicable for antennas of different systems - ETACStransmitter and GSM receiver.

The isolation A between antennas in the 900MHz band is given by the formula :-

Horizontal spacing - A = 31.6 + 20log(d) - (Gt + Gr) dB

Vertical spacing - A = 47.3 + 40log(d) dB

where

d - distance

Gt - transmitter gain

Gr - receiver gain

Duplexer can be used if there is a space constraint. Both the receiver and transmitter path

would be through the same antenna.

In cases where the antennas have already been installed and does not confine to the abovespecification, GSM receive bandpass filter with better bandpass rejection would be able to

overcome the desensitization problem.

7.2 Receive Diversity

Receive diversity is used on the air interface of GSM systems to partially overcome the

effects of fading. It is implemented on the receiving end of the uplink by the use of two

horizontally-separated receive antennas and a diversity receiver, which when correctly

installed, cause a reduction in the link BER and hence an improvement in the speech quality

which results in an apparent gain in the link, called ‘diversity gain’.

The actual value of the diversity gain depends on propagation conditions and the performance

of the diversity receiver; however, its variation with the mobile’s direction from the receive

antennas can be characterized. The diversity gain is greatest for calls made by mobiles

located equidistant from the two horizontally-separated receive antennas, as shown below.

GRX1

GRX1

line of greatest

diversity gain

(ie diversity direction)

This line of greatest diversity gain is referred to as the diversity direction and the two receive

antennas should be orientated so as to provide the greatest diversity gain to areas that would



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benefit most from it. In the following picture the coverage should be along the road and

towards the city, so the diversity direction is a compromise between covering the road and

covering the city, and in this example would be approximately 220°.

220o

M o t o r w

a y

City

City

190o

250o

River

7.2.1 Diversity Spacing and Direction

A horizontal spacing of 3 meters (10 wavelength) should be the minimum for diversity

spacing (as a rule of thumb). For the same distance, horizontal spacing is much more

effective than vertical spacing. To obtain the same improvement as for the horizontal

separation, the vertical separation should be approximately 5 times the horizontal value.Diversity spacing is also dependent on the antenna height. The relationship between the

desirable diversity separation and the antenna height is given by:-

a >= (H/10)

H = height of mast plus building (Effective antenna height)

a = distance between Rx antennas.

Omni Antennas

In the case of omni directional antennas the diversity direction is defined as the direction that

is perpendicular to the line joining the two receive antennas and 90° clockwise from theantenna labeled GRx0 (see figure below).



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GRX1

GRXO

Diversity Direction

3 m e t e r s m i n i m u m d i v e r s i t y s p a c i n g

R 0

R 1

Diversity Direction

20o

R1

R0

Diversity Direction

200o

Sectorised Antennas

The spacing between the antennas, as shown below, needs to be maintained at a minimum of

three meters while allowing for some reorientation of the antennas during optimization. The

diversity separation is defined as the perpendicular distance between the axes of the two

receive antennas, and, of course, coincides with the sector orientation.



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Diversity Direction

3 meters minimum(allow for reorientation)

7.3 Antenna and Tower/Building/Object Separation

Always keep the antennas at least 10 - 20 wavelengths (3 - 6 meters) away from any obstacles

along the propagation paths. If possible try to achieve propagation over the top of them by at

least 5 - 10 wavelengths (1.5 - 3 meters). The first Fresnel zone must always be kept free.

For rooftop sites, it is always preferable to install sectored antenna at the edge of the rooftop.This would help avoid obstacles like guide wires, TV antennas, air-conditioning hardware

and people walking in-front of them.

Refer to below:-

h

D

Height of antenna above roof (for both omni and sector)



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Distance (D) to obstacle edge Height (h) above roof (obstacle)

0 -1 m 0.5 m *

1 - 10 m 2 m

10 - 30 m 3 m

> 30m 3.5 m

* If possible, use 2m as the minimum height if there is a risk that people can walk

close to the antenna.

The separation between an omni antenna and tower structure should be at least 3 meters.

7.4 Antenna Downtilting

Antenna downtilting can be accomplished either mechanically or electrically and is usually

done to either minimize interference or in cases where large amount of coverage overlap

between neighbouring cells exist and the majority of the cell’s service area is below the

horizon. The transmission range of the horizontal coverage mainly around the main lobe axiswould be reduced.

With the exception of sites that are situated at very high location - a few thousand meters,

downtilting does not improve the coverage. It got to be noted, up to a certain extent

diffraction does assist in bringing radio waves down. Though downtilting for sites that are

more than fifty meters high improves the signal strength around the sites (< 400 meters), it is

seldom required because the signal strength close to the sites should have been sufficient in

the first place. The exception to this case is for sites on tall buildings, where coverage is

required in the basement or the lower floors of the same building or buildings adjacent to it.

7.5 Omni vs Sectored Antenna

One of the essential RF design criteria is deciding whether a site needs to be sectorised.

7.5.1 Advantages of sectored sites

The main reason sites are sectorised is to fight against interference. By having the RF

propagation confined in a certain direction, a more efficient use of the RF bandwidth can be

utilised. Sites can be designed much closer and as such a higher traffic capacity can be

catered for. Sites are usually sectorised in urban areas. The other advantages of sectored sites

are 5-6dB more gain, the ability to customise a particular area in terms of power and

optimisation parameters more efficiently, and better suited for roof-top sites where if the

antennas were situated at the side of a roof-top, with the same ERP, the coverage would

better than that of omni antenna situated at the center of a roof-top.

7.5.2 Advantages of omni sites

Omni antennas are more cost effective where interference is not a problem. Cost of additional

antennas, RF cables (9 for sectored in comparison with 3 for omni), polyphasors, additional



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cards for BTS, etc. can be avoided. Also for the same number of transceivers, omni sites

requires less control channels and has a higher Erlang value. Refer to table in section 12.0.

For three transceivers and assuming the same number of control channels is required, the

Erlang value for omni site is 14.897 in comparison with the Erlang value of 3*2.936=8.808

for 3 sectored site with one transceiver each.

7.6 Antenna Templates

The TX antenna should be placed above the RX antenna(s) to ensure optimal performance.

Allgon 4148 Template

2700

3000

* 300

* 300mm for clamping

-ALL MEASUREMENTS IN mm (NOT TO

3000

GRX0

GTX

GRX1

1500

L.A.

JBEAM 7540 Template





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A pattern/color is given to each group for easy visualization as below :-

A!

A1 Group A Group B

A3

A2

Group C Group D

In an omni configuration system, the above table would be for a n=12 (12 reuse pattern).

Since the frequency carrier for BCCH is continuously transmitted at maximum power, to

reduce interference, the BCCH carrier should be alternated between two or more frequency,

example for A1 group it should be alternated between 39, 51 and 63. Also, cells with the

same BCCH and BSIC should be kept far apart.

8.2 Auto Frequency Plan

For auto frequency plan - using ,etc. to do the frequency calculation, a specific frequency

management chart is not required. Nonetheless the below specification for minimum

frequency spacing needs to be adhered to:-

a) In the Cell 600 kHz (example - 51,54)

b) Between 2 co-site cells 400 kHz (example - 51,53)

c) Between 2 neighbouring cells 200 kHz (example - 51,52)



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Alternate use of BCCH frequency would definitely assist in reducing interference.

9.0 Linkbudget

Linkbudget is a calculation to balance the uplink and downlink signal strength. The effect ofthis calculation is basically applicable only in places where the signal level is very low

(below -95dbm) - usually at the fringe of a cell.

In mobile communication environment the mobile ERP is the limiting factor, i.e. Up link

limited. The losses/gain due to the following components equally affect both up & down

links, so these components have negligible effect on the path balance equation. The common

components are BS (Base station) cable loss, BS connector loss, BS antenna gain, MS

(Mobile station) antenna gain, MS cable loss, Body/polarization loss.

Down Link equ. PA pwr - Comb. Loss- Other losses = -102 dBm (mobile recv. Sens.)

Up Link equ. Mob. ERP- Div. Gain- Other losses = -104 dBm (Base recv. Sens.)

Combining the above equations

PA pwr -Comb. Loss = Mob. ERP + Div. Gain*- 102+104

= 33 dBm + 4 dB + 2 dB

= 39 dBm

* RBS 918 uses Max ratio combining scheme (MRCB) for which 4 dB Diversity gain is

conservative.

Since PA output power is adjusted insteps of 2 dB by BSTPWRRED parameter, 40 dBm at

the output of the combiner results in a balanced path.

Accompanying table is provided to illustrate above calculations. PA pwr in the table is before

the combiner. Attenuation factor for Filter combiner = 2.1 dB, for Hybrid combiner = 4.8 dB.

R.F Link Budget for FILTER combiner

Note: Enter all losses as negative values

Uplink Downlink

MS/BS transmit pwr 33 42 dBm before combiner

MS/BS transmit ERP 33 48.9 dBm

BS comb. loss -2.1 dB

BS cable loss -3 -3 dB



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BS connector loss -1 -1 dB

BS Antenna gain 13 13 dBd

MS Antenna gain 0 0 dB

MS cable loss 0 0 dB

BS diversity gain 4 dBFade Margin -6 -6 dB

Body/polarization loss -4 -4 dB

BS/MS Recvr. sens. -104 -102 dBm

Max. Path loss 140 140.9 dB

Path imbalance -0.9 0.9

R.F Link Budget for HYBRID combiner

Note: Enter all losses as negative values

Uplink Downlink

MS/BS transmit pwr 33 44 dBm before combiner

MS/BS transmit ERP 33 48.2 dBm

BS comb. loss -4.8 dB

BS cable loss -3 -3 dB

BS connector loss -1 -1 dB

BS Antenna gain 13 13 dBd

MS Antenna gain 0 0 dBMS cable loss 0 0 dB

BS diversity gain 4 dB

Fade Margin -6 -6 dB

Body/polarization loss -4 -4 dB

BS/MS Recvr. sens. -104 -102 dBm

Max. Path loss 140 140.2 dB

Path imbalance -0.2 0.2

The PA power setting of 40 dBm (10 Watts) will result in a balanced up & downlink.

An alternate method is to increase the PA power setting to its maximum and adjust the

minimum access threshold of the BTS. In such a situation though the BTS might be

transmitting at a higher power level, the MS would be able to access the system only if the

uplink is strong enough. The advantage of using it is two fold - the coverage would increase

by a few dB (though not much by itself, it is being obtained at no additional cost) and not

allowing the MS to access the system until a good quality call is able to be supported.



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10.0 Interference

There are three types of interference:-

a) co-channel interference

b) adjacent channel interference

c) intermodulation/noise interference

The carrier to interference ratios for co- and adjacent channels are specified as :

C/I = 9dB minimum (co-channel)

C/I = -9dB minimum (adjacent channel) The definition for co-channel interference in GSM system is that a cell on the same channel

can cause interference, if the serving cells signal level is only 9dB higher than the interfering

signal. For adjacent channels, interference can be caused if the neighbouring cells signal levelis 9dB higher than the serving cell.

Due to fading an additional 12 dB margin has been added to support good quality call:-

C/I = 21 dB (co-channel)

C/I = 3 dB (adjacent channel)

There are various methods of combating interference :-

a) Downtilting Antenna

b) Reducing Antenna height c) Reducing power of BTS/MS d) Using uplink/downlink adaptive power control

e) Using uplink/downlink DTX (Discontinuous transmit)f) Frequency hopping g) Sectorising sites h) Using smaller beamwidth antenna

i) Improving on the frequency plan

j) Optimising the various handover parameters

k) Microcells

For intermodulation interference, please refer to section 13.0.

11.0 Traffic

Traffic intensity is usually measured through the Erlang formula. There are two variables

which determines the traffic intensity, total attempts and average holding time:-

Erlang = (average holding time * total attempts)

where the unit for average holding time is in hours.



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For design purposes only busy hour Erlang is put into consideration. The GOS (Grade of

Service) is usually set to either 2% or 5%.

Below is an Erlang-B table with 2% GOS for 7, 14 and 22 traffic channels :-

No. of Transceivers No. of TCH Channels Traffic in Erlang/hour1 7 2.936

2 14 8.201

3 22 14.897

The above table indicates for 2 transceivers configured with 14 traffic channel, a 2% blocking

would occur during busy hour when traffic reaches 8.2 Erlang/hour.

Erlang/Subscriber for cells is obtained by adding all the cells busy-hour Erlang value and

dividing it with the number of subscribers. The average Erlang/Subscribers in this region is

approximately 35 mE.

12.0 Intermodulation

For any two signals, f1 and f2 applied to a non-linear device, other signals will be produced

at :

f 1m = + mf 1 + nf 2

where m = 0, 1, 2, …

n = 0, 1, 2, …

Order of IM = m + n

The third order IM usually causes the most interference.

Intermodulation interference can be categorised under three categories :-

a) Receiver Intermodulation

b) Transmitter Intermodulation c) Site Intermodulation

12.1 Receiver Intermodulation

If the non-linearity is located inside a receiver system, it is classified as receiver

intermodulation. Receiver intermodulation for GSM mobile occurs when the signal strength

is 70dB higher than the receiver sensitivity (12 dB SINAD level) of the receiver. An example

of receiver intermodulation occurs when a mobile is in close proximity to a transmitter

antenna. Desensitization of the receiver would occur if the signal strength is higher than -32dBm.

12.2 Transmitter Intermodulation

Intermodulation can be generated in the PA of one transmitter when another signal from a

nearby transmitter enters the PA via the antenna lines and mixes with the internally generated

signal.



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12.3 Site Intermodulation

Site intermodulation occurs if the non-linearity was caused by a device that one would

normally consider to be passive. This would include things like rusty or corroded hardware,

air-conditioners, ventilation duct, etc. Care should be taken to point antenna away from any

of this.

13.0 Lightning Protection System

To protect any equipment a good grounding and lightning protection system need to be

implemented. The essential issues that need to be adhered to are :-

a) All metal objects and major elements of the system such as booms, cable ladders, tower,

transmission cables, all equipment in the room/cabin need to be grounded to a single

point. This is done to create an equal potential area so that arcing doesn’t occur between

two metal object.

b) The grounding cable path to earth should be the shortest and most direct avoiding anysharp turn. This would ensure lightning would reach earth as soon as possible and at the

same time inductance which is created by a sudden bend of the cable would be avoided

c) A lightning arrestor should be installed at the highest location and the rest of the

equipment such as antennas should fall within the zone of protection as shown below:-

cone of protection

100 meters

450

100 meters

Though lightning may breakthrough this cone of protection, chances of it occurring would be

reduced.

14.0 LAC Design

The basic function of Location Area Code (LAC) is to indicate to MSC which area a

particular mobile is in. The system need to know this for paging purposes especially for

incoming calls for the mobile. If the whereabouts of the mobile was unknown then system

wide paging would have to take place which is inefficient. LAC design should be based on

two criteria:-



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a) The LAC design should be done in such a way that MSC would be able to locate a MS as

quickly and with as little processing as possible. This is dependent on the geographical

area. As remote area have less traffic, its LAC area should be bigger in comparison with

urban area. The main aim should be to have an approximate equal distribution of traffic

between the different LAC.

b) There should be as few LAC updates as possible. Since LAC updates is done on SDCCH

channel in idle mode and is processed in MSC, too many LAC updates would cause

congestion on SDCCH channel and take up the processing capacity of MSC. LAC design

on a single high traffic highway as shown below where many LAC updates would occur,

should be avoided.

LAC 1

high traffic highway

LAC 2 LAC 3 LAC 4

A compromise should be reached between the first and second criteria.

16.0 Real Life Scenario

Q : Why can’t statistical propagation models such as CellCAD, PLANET, , etc. give a highly

accurate prediction model for urban areas taking into account individual buildings?Statistical prediction model based their calculation on median signal for each pixel. An

average building height, building spacing, rooftop diffraction, etc. is used for urban

morphology. The prediction characteristics would be the same across the board. Urban areas

for example in Ipoh and Penang would have the same morphology values. Assuming the

same terrain height, a higher resolution map for urban morphology would not improve the

prediction because individual buildings height and spacing is not defined in it.



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Ray tracer technique which is usually used for microcell design takes into account individual

buildings. However Ray tracer technique would require higher resolution maps (<10m) with

individual buildings dimensions.

Q : Can neighbouring sites have adjacent channels?Though there would be a slight degradation in quality, neighbouring sites can use adjacent

channels. It should be avoided if possible, otherwise optimisation can resolve the issue by

changing the handover parameters to have a quicker or quality based handover between the

sites. It got to be noted that the difference between co-channel and adjacent channel

interference threshold is 18dB. Refer to section 11.0. Priority should always be given in

avoiding co-channel interference.

Q : Why is drop call encountered close-by to ’s ETACS sites?

Refer to section 13.0. A transmitter or site intermodulation causes interference in the GSM

band.

Q : Why do analog systems like ETACS able to provide better coverage than digital systems

like GSM even if both of them are transmitting at the same ERP?

Though modulation does not have any effect on propagation characteristics, analog system is

able to operate at lower receive level in comparison with digital system. The receiver

sensitivity of analog system is in the region of -117dBm, where else for digital system it is -

104dBm.

Q : Why don’t most operator utilise frequency hopping?

Though theoretically, frequency hopping is able to over-come fading and average out

interference across the board, its effectiveness in GSM context is limited. For frequency

hopping to be effective, it got to hop over more than 6 frequencies. Most operator do not havethe required bandwidth to hop over that many frequencies. Also the greater the spacing

between the frequency hopped over the better it would be able to overcome fading. The

spacing used for frequency hopping within the confines of the bandwidth given is not good

enough. Furthermore GSM system employs slow frequency hopping, which is not completely

effective in overcoming fading.

Q : Since diversity improves with distance between the receiver antennas, why shouldn’t the

antennas be kept as far apart as possible ( >15 meters)?

Beside space constraint and losses due to the length of RF cable, by keeping the receiver

antenna further apart than necessary especially in urban areas, the uplink coverage would be

better than the downlink in areas that are closer to the receiver antennas.

Q : How do we determine which antenna beamwidth to use?

For cells in places like dense urban and urban environment, where frequency reuse is a

problem and if most of the cell sites fall in grid, 600 antenna beamwidth is preferable. For

cells in sub-urban and rural area where frequency reuse is no longer as big a problem, 900 or



1200 antenna might be the better choice. It would definitely provide better coverage.

Remember there is no additional gain, for smaller beamwidth antenna.

Q : Why should we use cell grid for planning?

Idealistically if all the cells were to fall in grid, the antenna direction (when using standard

antenna orientation) would be pointing to the null of the adjacent cell. The interferencecontrol for such a situation would be much better. Practically it is not always possible to get

exactly the site that we want. Nonetheless cell grid do provides one of the better option for

viewing where to situate a site in term of coverage and interference control especially for

urban area.

Q : Which is more effective - space diversity or polarised diversity?

It depends on the height of the antennas. Refer to section 8.2.1. To obtain effective space

diversity, the receiver antennas spacing should be more than the antenna height divided by

ten. For low sites, where the required antennas spacing can be achieved easily space diversity

is more effective. For high sites, on top of a hill or mountain, where it is not feasible to fulfillthe requirement for the antenna spacing, polarised diversity is the better option.

Documents

Rf Design Guide