Nokia Siemens Networks

Guidelines To be followed for Pre Srvey

1.1 Site Selection

Sites represent expensive long term investments for the operator. Good site selection is critical to the performance of a 3G radio network. Neither RF optimisation nor parameter optimisation can compensate for poor site selection. Site acquisition teams are often under pressure to offer large numbers of sites while radio

network planning teams are often under pressure to accept large numbers of sites. Site selection criteria are used to evaluate whether or not a site is suitable. Sites should only be evaluated and accepted subsequent to site visit and a detailed evaluation.

NSN Recommendation: Site visits should be used in addition to planning tool results to evaluate the suitability of candidate sites. Site selection criteria should be divided into two categories. The first category including criteria which determine whether or not a site should be considered for inclusion within the 3G radio network plan. The second category including criteria which can be used to prioritise those sites being considered for inclusion within the 3G radio network plan. The list of site selection criteria provided in this section should be used as a minimum set of requirements. The site selection criteria should be applied as strictly as possible when accepting new sites from the site acquisition teams. Exceptions to the criteria should only be accepted subsequent to escalation and a more detailed evaluation. During a site visit the planner should collect as much information as possible, including panoramic photographs of the surrounding terrain and neighbouring sites.

Sites are expensive long term investments for the operator. If sites are selected which have a poor location or a poor set of characteristics then system performance is likely to be poor irrespective of any subsequent RF and parameter optimisation. A good site location should maximise coverage across the intended area while limiting interference into neighbouring areas.

1.1.1 Common Practice

The majority of operators incorporate a set of criteria for site selection as part of their radio network planning process. However it is common for operators to be relatively relaxed about allowing candidate sites which do not fulfill the complete set of criteria. This results from the pressure to acquire sites and deploy the network in a short period of time. Site acquisition teams and planning teams may have personal incentives which are based upon the quantity of sites accepted rather than the quality of those sites. Sites may be rejected during the initial evaluation

but subsequently accepted if site acquisition teams fail to find any alternatives. In some cases, rejected sites may be used for other radio network solutions, e.g. repeaters or microcells. Site visits are almost always completed although they are often a source of errors, e.g. panoramic photos being taken at the wrong height, antenna heights being measured or recorded incorrectly, feeder lengths being estimated inaccurately.

1.1.2 Requirements

Site selection criteria can be divided into two categories. The first category includes criteria which determine whether or not a site should be considered for inclusion within the 3G radio network plan. The second category includes criteria which can be used to prioritise those sites being considered for inclusion within the 3G radio network plan. In general, it is difficult to acquire sites and there will not be many sites to prioritise between. If any of the criteria belonging to the first category are not satisfied then the site should be excluded from further consideration unless there are no alternatives and the benefit of introducing the site is believed to justify its cost. Table 1 presents the minimum set of criteria which should be used to determine whether or not a site should be considered for inclusion within the 3G radio network plan. These criteria should be evaluated after a site visit and not only from the information available within a radio network planning tool.

1 Does the site allow the main beam of each proposed antenna to have good visibility of the surrounding terrain without any high obstacles blocking the view?

Yes / No

2 Can the main beam of each antenna be positioned such that it does not cross the main beam of another antenna?

Yes / No

3 Can the main beam of each antenna be positioned such that they are not shadowed by the building or structure upon which they are secured?

Yes / No

4 Can each antenna be mounted above the roof-tops of the neighbouring buildings without being excessively above them? Typically < 10 m above the neighbouring roof-tops

Yes / No / Not

Applic.

5 Do neighbouring cells have antenna heights which are within 15 m of the proposed antenna heights?

Yes / No

6 Are neighbouring cells of similar size? Yes / No

7 Is the site unlikely to be very dominant and unlikely to cause significant interference to neighbouring cells?

Yes / No

8 Is the best server area of the site unlikely to be fragmented? Yes / No

9 If the proposed site is a rooft-top site, is there sufficient space for the appropriate antenna mountings to ensure that there is adequate clearance from the roof-top?

Yes / No / Not

Applic.

10 Is the site safe from new neighbouring buildings which may be constructed in the future and which may block the main beam of an antenna?

Yes / No

11 Are the cabling distances between the Node B cabinet and the antennas reasonable? Yes / No

12 Is there access to leased lines or microwave links for transmission purposes? Yes / No

13 Is there availability of the Node B power supply requirements? Yes / No

14 Is there space to accommodate the Node B equipment? Yes / No

15 Are rental costs acceptable? Yes / No

16 Is there reasonable access to the site? Yes / No

Table 1 – Site selection criteria used to determine whether or not a site should be considered for inclusion within the 3G radio network plan

Assuming that a site satisfies the criteria in Table 1 then it may be considered for inclusion within the 3G radio network plan. Table 2 presents a set of criteria which may be used to prioritise between sites being considered for inclusion within the 3G radio network plan. Similar to the first set of criteria, these should be evaluated from a site visit and not from the information available within a radio network planning tool. Sites which result in the greatest number of ‘yes’ responses should be selected to be built.

1 Is the site an existing GSM site? Yes / No

2 Do antenna locations allow for changes in azimuth? Yes / No

3 Do antenna locations allow for changes in height? Yes / No

4 Do antenna locations allow sufficient isolation from other antennas, e.g. GSM antennas?

Yes / No

5 Is the site away from environmentally protected or historic areas? Yes / No

6 Is the site unlikely to require any special permits? Yes / No

7 Is the site unlikely to cause public disapproval? Yes / No

8 Does the site form a regular pattern with its neighbours? Yes / No

9 Is the site close to where the traffic is expected? Yes / No

10 Is the site capable of accommodating potential capacity upgrades? Yes / No

Table 2 – Site selection criteria used to prioritise between sites which are being considered for inclusion within the 3G radio network plan

Site visits represent a significant part of the site selection process. They should be used to collect all of the information required to evaluate the suitability of a site as well additional information for radio and transmission planning, information for site build and installation and information for site design. Site visits are relatively expensive and time consuming and should be planned carefully. Planners completing site visits should, as a minimum take with them:a paper map of the area

a paper diagram of the building coverage plot from the planning tool best server plot from the planning tool a GPS receiver binoculars and compass a digital camera an altimeter a tape measure or other measuring device safety equipment if necessary

The paper map of the area should be used to mark the proposed antenna azimuths and general antenna locations. The paper diagram of the building should be used to make a more detailed record of the proposed antenna locations and the positions from where any photos are taken. The digital camera should be used to obtain panoramic views from the proposed antenna locations. Photos should also be taken of the neighbourhood and surrounding environment. The location and visibility of neighbouring sites should be recorded as should potential gaps in coverage. The different possibilities for antenna mounting should be noted and feeder length requirements estimated. The GPS receiver should be used to determine an exact set of co-ordinates for the site as well as the height above sea and ground level. The property address and owner should be recorded as should the possibilities for site access.

1.2 Site Design

Site design involves identifying an appropriate location for each antenna and the Node B cabinet. It also involves identifying a route to be taken by each feeder.

When there is a requirement to achieve a specific isolation from another radio system then that isolation is easier to achieve if the antennas are separated vertically rather than horizontally.

NSN Recommendation: Site visits should be used as an essential part of the site design process. Antennas should be mounted such that their main beams are not shadowed by any obstructions, e.g. roof-tops, walls, trees. Roof-top antennas should be mounted with sufficient height to avoid shadowing by the roof-top. Wall mounted antennas should be directed to avoid having their horizontal beamwidth shadowed by the wall. Whenever possible antennas should be mounted such that it is possible to adjust their azimuth and height. Antennas should be mounted such that they achieve their isolation requirements from other radio systems. Node B cabinets should be located relatively close to the antennas to avoid excessive feeder losses. On projects where the operator has completed the site design, NSN planners should gain access to the site design documentation.

Site design should be coupled with the site selection process and with the radio network planning process. If a site does not permit a good site design then it should not be selected. The site design should allow the antennas to be configured in the way that they appear within the radio network planning tool. A poor site design can have a significant impact upon the performance of a potentially good site.

1.2.1 Common Practice

Unless a project is turn-key, it is common for operators to provide the design for each site. The majority of operators have their own set of site design guidelines. These guidelines are typically similar to those which are presented in this section. It is relatively common to find sites which do not follow the site design guidelines. Site visits are often required after the site has been built to identify problems with the design. Corrections to the original site design can be relatively expensive and in the extreme case can result in a site being switched off.

1.2.2 Requirements

The most important requirement is that antennas should mounted such that their main beams are not obstructed. In the case of a roof-top site, obstructions could be other antennas or cabins located on the same or a neighbouring roof. In the case of mast or pole mounted antennas, obstructions could be trees or nearby buildings. Figure 1 illustrates examples of poor and good roof-top antenna positions.

Poor Position Good PositionPoor Position Good Position

Figure 1 – Examples of poor and good roof-top antenna locations

In the example of a poor position, the 3G antenna is located behind and slightly higher than some existing antennas. In this case the main beam of the 3G antenna is obstructed and its performance is likely to be deteriorated. In the example of a good position, the 3G antenna is located in front of and slightly lower than some existing antennas. In this case the main beam of the 3G antenna is not obstructed although care should be taken that the rear lobe of the 3G antenna does not cause interference to the other radio systems. There is also a requirement to ensure that the edge of the roof-top does not cause shadowing of the 3G antenna. If an antenna is positioned on the edge of a roof-top then it is unlikely to incur any shadowing from the roof-top itself. However as the antenna position is moved away from the edge then the antenna is more likely to incur shadowing. Antennas which are located away from the edge should be mounted with an increased height. A general rule is that if you can walk in front of the antenna then it should be mounted 3 m above the roof-top. Figure 2 illustrates the principle of shadowing from a roof-top and suggests a range of heights which could be used to avoid shadowing.

hClearance angle

d

d < 10 m h > d/2

10 < d < 20 m h > d/3

d > 20 m d > d/4

General rule

hClearance angle

d

d < 10 m h > d/2

10 < d < 20 m h > d/3

d > 20 m d > d/4

General rule

Figure 2 – Principle of avoiding shadowing from a roof-top

In most cases an antenna would be mounted less than 10 m from the edge of the building and its suggested height would be obtained by dividing the distance to the edge by 2. Wherever possible, antenna mountings should allow the height and azimuth of the antenna to be adjusted.

In the case of antennas which are mounted on walls then the azimuth should be configured to ensure that the horizontal beamwidth of the antenna is not compromised. In general a 15 degree safety margin should be added to each side the half power horizontal beamwidth and then a check made to ensure that the composite beamwidth is free from obstruction. Figure 3 illustrates the principle of avoiding shadowing from the walls upon which antennas are mounted.

Direction of main beam

Half power beam width

15° safety margin

Direction of main beam

15° safety margin

Good Position

Poor Position

Direction of main beam

Half power beam width

15° safety margin

Direction of main beam

15° safety margin

Good Position

Poor Position

Figure 3 – Principle of avoiding shadowing from walls

In the case of the poor position, the antenna horizontal beamwidth and the 15 degree safety margin are intersected by the wall. This indicates that the wall will cause significant deterioration of the antenna gain pattern. In the good position, neither the antenna horizontal beamwidth nor the safety margin are intersected by the wall and there is also some flexibility for adjusting the antenna azimuth if required to do so.

When there are other antennas on the same mast, the same roof-top or the same wall then the isolation from those antennas should be maximised without compromising the position of the 3G antennas. The isolation requirement will depend upon the systems to which the antennas belong. WCDMA 900 Planning Guide in Error: Reference source not found describes the isolation requirement between the WCDMA and GSM systems. The isolation requirement can be translated into a physical separation using curves which plot measured isolation as a function of physical separation. These curves depend upon the gain patterns of the antennas being used and whether or not the antennas are cross-polar. As an example, the GSM900 system requires 40 dB of isolation from the WCDMA system. If the antennas have a vertical separation then there should be at least 0.2 m between the base of one antenna and the top of the other antenna. If the antennas have a horizontal physical separation and a horizontal beamwidth of 65° then there should be at least 0.5 m between them. Figure 4 illustrates these physical separation requirements.

0.2 m achieves 40 dB isolation

0.5 m achieves 40 dB isolation

Vertical Separation

Horizontal Separation (65º horizontal

beamwidth antennas)0.2 m achieves 40 dB isolation

0.5 m achieves 40 dB isolation

Vertical Separation

Horizontal Separation (65º horizontal

beamwidth antennas)

Figure 4 – Minimum requirements for horizontal and vertical antenna separations

Whenever possible, a vertical separation should be combined with a horizontal separation to increase the achieved isolation. In cases where these separations cannot be achieved then the isolation requirement should be studied in greater detail.

Alternatively, the isolation requirement can be achieved using a diplexor and allowing the two radio systems to share the same feeders. A diplexor typically offers 40 dB of isolation. Radio systems may also share the same antennas. In general, this has the drawback of restricting both radio systems to using the same antenna downtilts, i.e. downtilts cannot be configured separately for each system.

Antennas which have remotely controllable tilts are generally more expensive but tilt changes can be made with relative ease and the antennas may be compatible with the NSN RealTilt solution.

The Node B cabinet and antenna locations should be selected to help minimise feeder losses. Part of the site design process should be to compute the feeder loss associated with the specific feeder type and length. If feeder losses (excluding connector and MHA insertion losses) exceed 4 dB then alternatives should be considered.

1.3 Scrambling Code Planning

The 512 downlink primary scrambling codes are organised into 64 groups of 8. Each cell within the radio network plan should be assigned a primary scrambling code. Scrambling code planning strategies can be defined that maximise the number of neighbours

belonging to the same code group or that maximise the number of neighbours belonging to different code groups. The difference between the two strategies has not been quantified in the field but is likely to be dependant upon the UE implementation.

Scrambling code planning may require co-ordination at international borders. Scrambling code planning can be completed independently for each RF carrier. Scrambling code planning can be completed using either a radio network planning tool or a

‘home-made’ tool. It can also be completed manually for small areas. Radio network planning tools are able to plan scrambling codes according to a specific strategy and exclude specific scrambling codes for future expansion. Consideration should be given to moving the scrambling code plan to and from the live network.

NSN Recommendation: The isolation between cells which are assigned the same scrambling code

should be maximised. The isolation between cells which are assigned the same scrambling code should be sufficiently great to ensure that a UE never simultaneously receives the same scrambling code from more than a single cell. The isolation between cells which are assigned the same scrambling code should be sufficiently great to ensure that a UE never receives a scrambling code from one cell while it is expecting to receive the same scrambling code from second cell. Scrambling code planning should maximise the number of neighbours belonging to the same scrambling code group. A different scrambling code strategy can be accepted as long as the operator is made aware of the theoretical trade-off between cell synchronisation reliability and UE power consumption. Specific scrambling codes should be excluded from the plan to allow for future network expansion. The same scrambling code plan should be assigned to each RF carrier. Co-ordination across international borders should be accounted for according to the recommendations of the local regulatory body. The Tcell RNC databuild parameter should be configured according the NSN default values. Scrambling code planning should be completed in conjunction with neighbour list planning. Scrambling code audits should be completed in combination with neighbour list audits. Checks should be made to ensure that no cells are neighboured to two or more cells which have neighbour lists including the same scrambling code for different target cells.

1.3.1 Common Practice

A large number of different strategies have been adopted for scrambling code planning. These include those which maximise the number of neighbours which belong to the same scrambling code group as well as those which maximise the number of neighbours which belong to different scrambling code groups. Neither approach has raised any issues and any difference in performance remains unquantified.

It is common for operators to recognise the requirement to reserve scrambling codes for future network expansion. This has been done either by excluding specific scrambling codes from each code group or by excluding all scrambling codes from specific code groups. It is common for operators to complete audits on their scrambling code plans in combination with audits upon their neighbour lists. This is often completed using a ‘home-made’ script rather than a planning tool. Operators have co-ordinated either RF carriers or scrambling code plans at international borders.

1.3.2 Tools

Scrambling code planning can be completed using a planning tool or it can be completed manually. It is not recommended to complete scrambling code planning manually for sections of network which include more than 30 Node B.

Radio network planning tools support scrambling code planning (see Error: Reference sourcenot found and Error: Reference source not found) which makes use of propagation predictions, neighbour lists and a minimum re-use distance. It is also able to account for specific scrambling code planning strategies and is able to exclude specific scrambling codes for future network expansion. It is planned for NetAct Optimizer to have a similar capability as Radio network planning tools. Both Radio network planning tools and Optimizer allow relatively straightforward movement of the scrambling code plan to and from the live network. Radio network planning tools and Optimizer are also able to audit the neighbour lists to ensure that there are no duplicate scrambling codes.

Third party radio network planning tools are also likely to have scrambling code planning functionality. If an operator uses a third party radio network planning tool then it is likely to be more appropriate to use that tool for scrambling code planning rather than introduce Radio

network planning tools. Nevertheless, the functionality of the third party tool should be compared with that of Radio network planning tools. Consideration should also be given to how the resulting scrambling code plan can be moved to and from the live network.

Scrambling code planning can also be completed using either NSN or customer ‘home-made’ tools, e.g. written in visual BASIC or C code. These tools tend to make use of distance rather than path loss but in general they are capable of generating the desired results.