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 for known 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 and type 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 as a 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 - ETACS transmitter 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 above specification, 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
line of greatest diversity gain (ie diversity direction)
GRX1
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°.
250o
River
190o
City
220o
Motorw ay
City
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 the antenna labeled GRx0 (see figure below).
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3 me ters m
inim
um d
ivers
ity sp acing
Diversity Direction GRXO
GRX1
Diversity Direction 20o R0 R1 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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D iversity D irection
3 m eters m inim um (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 0 -1 m 1 - 10 m 10 - 30 m > 30m
Height (h) above roof (obstacle) 0.5 m * 2m 3m 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 axis would 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 L.A. GTX
3000
2700
1500 * 300 3000
GRX0
GRX1 * 300mm for clamping -ALL MEASUREMENTS IN mm (NOT TO
JBEAM 7540 Template
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L.A.
GTX
3800 3500
1500 GRX1
3000 GRXO
* 300 500
1500
* 300mm for clamping -ALL MEASUREMENTS IN mm (NOT TO SCALE)
8.0
Frequency Planning
8.1 Manual Frequency Planning For manual frequency planning in a large system, a frequency management chart is usually required to do a systematic and easy to view frequency plan. The challenge in manual frequency planning is to tune it in such a manner that most of the interference would fall in areas where there is not much traffic. Below is a chart for a n=4 (4 reuse frequency pattern) with two transceivers per-cell :A1 51 63
B1 52 64
C1 53 65
D1 54 66
A2 55 67
B2 56 68
C2 57 69
D2 58 70
A3 59 71
B3 60 72
C3 61 73
D3 62 74
B2 44 56 68
C2 45 57 69
D2 46 58 70
A3 47 59 71
B3 48 60 72
C3 49 61 73
D3 50 62 74
A n=4 chart with three transceivers :A1 39 51 63
B1 40 52 64
C1 41 53 65
D1 42 54 66
A2 43 55 67
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A pattern/color is given to each group for easy visualization as below :-
A!
A1
Group A
Group B
Group C
Group D
A3 A2
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 of this 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. PApwr - 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 * PApwr -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. PApwr 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
MS/BS transmit pwr MS/BS transmit ERP BS comb. loss BS cable loss
Uplink Downlink 33 42 dBm before combiner 33 48.9 dBm -2.1 dB -3 -3 dB
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BS connector loss BS Antenna gain MS Antenna gain MS cable loss BS diversity gain Fade Margin Body/polarization loss BS/MS Recvr. sens. Max. Path loss Path imbalance
-1 13 0 0 4 -6 -4 -104 140 -0.9
-1 dB 13 dBd 0 dB 0 dB dB -6 dB -4 dB -102 dBm 140.9 dB 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 dB MS 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 level is 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 1 2 3
No. of TCH Channels Traffic in Erlang/hour 7 2.936 14 8.201 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 : f1m = + mf1 + nf2 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 any sharp 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 have the 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
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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 interference control 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 fulfill the requirement for the antenna spacing, polarised diversity is the better option.
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