1 WCDMA BASICS

Scrambling Codes in WCDMA

Since every cell in a WCDMA network can transmit on the same frequency some means of differentiation is required. This differentiation is achieved through the use of Scrambling Codes. In the downlink there are 512 Scrambling Codes which are grouped into 64 groups of 8 codes.

Table 1 First three downlink code groups of WCDMA.

Scrambling Code Planning

For example a site assigned a code group of 2 will have the following scrambling codes.

Sector A

16

Sector B

17

Sector C

18

Sector E

19

Sector F

20

Sector G

21

This scrambling code group planning approach by site rather than cell has many advantages.

It simplifies the planning without degrading performance (a reuse of 64 provides the same co-code protection as a reuse of 512). In WCDMA every code has the same ability to interfere with every other code therefore there is no concept of adjacent code interference.

It means a field op. Engineer only needs to know the code group for the site and from this they can derive the scrambling codes of each sector of the site

It increases the probability of a UE camping on to the network.

Scrambling Code Measurements

The Common Pilot Indication Channel (CPICH) is a common channel broadcast from each and every cell within a WCDMA network. It carries no information and can be thought of as a beacon constantly transmitting the Scrambling Code of the cell. It is this beacon that is used by the phone for its cell measurements for network acquisition and handover purposes.

Figure 1 Each cell broadcasts a CPICH (Common Pilot Indication Channel)

The majority of 3G coverage measurements are based upon measurements of the CPICH.

Golden Rule: If the UE cant decode the CPICH the UE cant see the cell.

Initial 3G network optimisation is performed purely from CPICH measurements. Three key related measurements for 3G optimisation are;

Ec

- The Received Signal Level of a particular CPICH (dBm)

Io

- The Total Received Power (dBm)

Ec/Io

- The CPICH Quality (The ratio of the above two values) (dB)

Total Received Power Io

Figure 2 UE receives its total power from a number of sources

In a WCDMA network the UE may receive signals from many cells whether in handover or not

Io = The sum total of all of these signals + any background noise (dBm)*

*Note: Sometimes Io is referred to as No, RSSI or ISSI.

Received Power of a CPICH Ec

Figure 3 UE can differentiate between signals from different cells using the property of scrambling codes.

Using the properties of the WCDMA downlink scrambling codes the UE is able to extract the respective CPICH levels from the sites received.

Ec = The Received Power of a Particular CPICH (dBm)*

*Note: Sometimes Ec is referred to as RSCP

The CPICH Quality Ec/Io

Figure 4 For each received CPICH Scrambling Code the UE is able to make an Ec/Io quality measurement.

From the previous two measures we can calculate a signal quality for each CPICH (Scrambling Code) received

Ec/Io = Ec - Io (dB)*

*Note: Sometimes Ec/Io is referred to as Ec/No

Ec/Io Calculation Example

Figure 5 Ec/Io Calculation Example

From the above three measurements we can calculate the Ec/Io for that particular pilot:

(Ec/Io)1 = -95 - -80 = -15dB

(Ec/Io)2 = -90 - -80 = -10dB

Clearly in this example the second pilot is seen by the UE as the stronger.

Ec, Io and Ec/Io Measurement

All commercial scanners and test UEs are capable of Ec, Io and Ec/Io measurements. It is these measurements that are used for cover analysis and basic optimisation. A screen shot from the Anritsu scanner is shown below:

Figure 6 Anritsu scanner screen shot showing a number of pilots being measured.

Handovers in WCDMA

Various types of handover (HO) exist in WCDMA,

Those between WCDMA sites (intra-system HO)

Those between WCDMA and GSM (inter-system HO)

Considering further the interaction between WCDMA sites, WCDMA intra-system HO can be broken down into the following groups:

Those between cells transmitting on the same frequency (intra-frequency handover)

Those between cells transmitting different frequencies (inter-frequency handover)

The dynamics of inter-system HO and inter-frequency HO are largely controlled by parameters and therefore fall outside of the scope of this document. For the purposes of this document only intra-system (intra-frequency) HO will be considered.Softer Handover

Figure 7 Softer Handover in WCDMA

Softer handover occurs between sectors of the same site. The UE will therefore have at least two SCs from the SC group in its active set and will be communicating to the Node B via two sectors.

Soft Handover

Figure 8 Soft Handover in WCDMA

Soft handover occurs between sectors of the different sites. Again the UE will have at least two SCs in its active set but this time the SCs will belong to different SC groups.

For both softer and soft it is the Ec/Io levels that are used to determine whether a cell should be added or removed from the active set.

Basic WCDMA Optimisation

The remainder of this document will now show that using the basics introduced in this section it is possible to optimise a WCDMA network using mainly Ec and Ec/Io measurements.

2 COVERAGE OPTIMISATION

The aim of this section is to provide details on how to analyse 3G coverage from drive test measurements to determine whether or not sufficient coverage - and quality of coverage - has been provided to a particular location.

The planning levels have been derived using appropriate link budgets with appropriate shadow fading margins. However when actually measuring coverage these shadow fading margins are no longer required in the target measured coverage levels since the levels are actually measured after some or all of the shadow fading (depends on environment and where measurement is made indoor/outdoor) .

Therefore using this approach it is possible to derive target measured Ec levels to guarantee a particular service.

Target Measured Coverage Levels

Based on the WCDMA link budgets the following target measured Ec levels have been derived. All values assume CPICH transmit power of +33dBm for outdoor environment categories and +29dBm for indoor.

Table 2 Target Measured Ec Levels for Nokia (dBm).

Table 3 Target Measured Ec Levels for Alcatel (dBm).

All levels in the above table are specified for outdoor measurement (except indoor). It is expected that survey vehicles with car kit measurements will be used however care should be taken to ensure such equipment is calibrated such that the feeder and the antenna system has a gain of 0dB or any such gain/loss is removed during analysis.

The most onerous Ec level is used. For example if a voice and 64 PS data were the target services then clearly in this case voice has the higher signal level requirement because of its associated body losses.

The correct type of environment is assumed for the drive run. Obviously drive routes may cross many types of environments, therefore it is important that during the analysis the drive route is classified into the different types of environments so that the correct measured Ec levels can be used.

It is possible to make voice calls below the values given in the table. However below these levels it is impossible to offer any guaranteed level of service quality.

Target Measured Ec Levels In Train

It is now common practice to take rail coverage measurements from within trains. If this is the case the following target Ec levels should be used.

Table 4 In Train Target Measured Ec Levels for Nokia (dBm).

Table 5 In Train Target Measured Ec Levels for Alcatel (dBm).

Link Between Coverage and Capacity

In UMTS coverage and capacity are closely related. When the load of the network increases, the cell will shrink and its coverage will be reduced.

A full representation of the coverage with load from drive test would be impossible, therefore we shall not expect to get a cell capacity figure out of a drive test.

A simplified approach has been taken, basically the Ec coverage levels given above are defined for a loaded network and simply include a fixed margin to represent the shrunken cell. A margin of 4dB (60% UL load) is taken for Dense Urban, Urban/Suburban and Indoor, whereas a margin of 3dB (50% UL load) is applied for In Car, In Train and Rural.

Whilst the network is unloaded coverage will be provided at locations with signal levels a few dBs below these targets, however this coverage will disappear as the network becomes loaded. Therefore be warned of using the load margin to achieve coverage one day your coverage will disappear!!

If more coverage is required then just as with GSM either downtilts or new sites are the answer. Notes On Usable Coverage

As previously stated, quality of service in a WCDMA network is determined by both Ec and Ec/Io levels. In order to understand this better, Table 7 shows the definition of usable coverage.

Table 6 Only coverage exceeding the both the Ec and the Ec/Io targets can be guaranteed to be usable in a loaded network. Due to the nature of WCDMA, quality of service is determined not only by received signal strength, but by the quality of the received signal as well. An area with a suitable coverage level in terms of received signal strength could suffer severe interference problems in a non-optimised network, thus degrading the quality of service for a user. In this case the coverage becomes unusable.

The Ec/Io threshold value is set at -10dB according to the conditions specified for pilot pollution and the requirements to achieve and maintain a 384k radio bearer. The Ec threshold is set at -100dBm as this is the noise floor of a typical UE; at Ec levels below this the internal noise from the UE will cause further interference and degrade the received Ec/Io level.

Coverage Optimisation Methods

The methods below should be considered, when performing optimisation of usable coverage.

Antenna tilt optimisation - useful for creating dominance in an area or removing an interferer.

Antenna azimuth optimisation - to shift the coverage footprint of a site. All sectors should be transferred to the alternate configuration, i.e. sectors A,B,C become sectors E,F,G.

Raising or lowering the height of an antenna.

Changing the beamwidth of an antenna.

Installing a new site.3 Missing Neighbours OPTIMISATION

The aim of this section is to give details on how to analyse and solve missing neighbour issues using drive test measurements.

Definition of Missing Neighbours in WCDMA

By a missing neighbour we mean a cell (MISSING_N) not declared as neighbour of the best active cell (BEST) although it could be included in the active set.

In other words:

However for drive test analysis a larger margin is recommend such that:

Where MarginMissing_N = 5 dB.

Problems of Missing Neighbours

In WCDMA a missing neighbour leads to both downlink and uplink quality problems.

Downlink Quality

In the downlink two effects can occur

1: Increased DL interference

2: Call drop because of excessive DL interference

Uplink Quality

In the uplink the missing neighbour cell can experience an UL noise rise, hence cell shrinkage can occur leading to the possible deterioration of all calls on the cell.

Solution

The way to solve missing neighbour is straightforward. Any cells found to be missing neighbours need to be added in the neighbouring list of the source cell at the OMC.

The difficult part is to identify the missing neighbour. RF scanners are the best tool for this job and therefore it is recommended that they are actively used for WCDMA missing neighbour optimisation.

Missing Neighbour Detection with a Scanner

When used with Top N functionality a scanner will report all decoded scrambling codes seen within the band.

With a post-processing tool we can list all required neighbour pairs fulfilling the condition:

A comparison can then be made between this list and the declared neighbour list and this will give the missing neighbour list for the corresponding drive.

Missing Neighbour Detection with a Trace Mobile

When only a trace mobile is available it is far more difficult to detect missing neighbours. This is why scanners are really required for this purpose. The method with a trace mobile is similar to that used to detect missing neighbours in GSM.

The trace mobile will obviously not report the missing neighbour in its monitored set because this neighbour is not declared. The problem is identical in both connected and idle mode. But a few events may help identifying missing neighbours:

If the missing neighbour causes the call to drop, the UE will lose the network and revert to scanning mode in order to select a new suitable cell. In this case the UE will most probably select the missing neighbour cell. Correlating the last connected cell and the newly selected cell is likely to give a missing neighbour pair.

If cell_C is a missing neighbour to cell_A but declared as neighbour of cell_B, a handover from cell_A to cell_B will suddenly trigger the reporting of cell_Cs level. If when first reported, cell_C appears to be the best server then it was most probably a missing neighbour of cell_A.

Missing Neighbour Detection with a Scanner and a Trace Mobile

A comparison between neighbours detected by the trace mobile and those seen by the scanner will immediately point out any missing neighbours.

The process is as follows:

Step 1: Merge two sources on a time basis

Step 2: For each timestamp, edit all neighbours reported within MarginMissing_N (5dB) for both sources. The scanner source will give all necessary neighbour declarations, whereas the trace mobile source will only reports neighbours if declared.

Step 3: The comparison gives the list of missing neighbours

Note that this method does not require any comparison with a list of previously declared neighbours although this can also be done for completeness.

As can be seen from the above method, without a WCDMA scanner missing neighbour detection is very difficult. Missing neighbours are a major cause of call drop and quality degradation in a WCDMA network it is therefore highly recommended to use WCDMA scanners and either method 1 or 3 above to detect missing neighbours within its WCDMA network.

4 pilot pollution analysis

The aim of this section is to provide details of how to analyse and optimise pilot pollution from drive test measurements.

Definition of Pilot Pollution

Confusion is often made between pilot pollution and missing neighbours or number of cells in the active set.

Generally the meaning of pilot pollution is that an excessive number of scrambling codes are received at a particular location leading to degradation of downlink quality on the best serving cell.

Pilot pollution is a form of downlink interference and we shall define pilot pollution using the following conditions:

1. Best server CPICH_Ec is good and Best server CPICH_EcIo is bad

2. Best server CPICH_Ec is good and Count of good servers > Active Set Size

Caution: these formula can be applied as they stand for measurements from a scanner, however if measurements from a trace mobile are used, first make sure that no neighbours are missing otherwise a bad CPICH EcIo value could be due to a missing neighbour rather than pilot pollution.

Problems Caused by Pilot Pollution

Pilot pollution leads mainly to downlink quality problems, similar to those experienced from missing neighbours but not for the same reason.

In the downlink two effects can occur

1: Increased DL interference and cell capacity loss

2: Call drop because of excessive DL interference

Note however that in the case of an unloaded network, only level 1 should be experienced.

Call drops should only occur when the load increases.

Distinction Between Coverage and Interference Problems

Before trying to optimise areas with poor quality (Ec/Io) it is important to clearly distinguish problems of coverage from problems of interference.

Bad quality can be directly related to CPICH Ec/Io levels. However when the coverage (Ec) is poor the CPICH Ec/Io value naturally decreases, even for the single cell case. The reason being, that at cell edge the CPICH_Ec level is less than the noise floor of the UE. In this case coverage rather than interference may be blamed for the poor quality.

This is why a single Ec/Io threshold is not enough to identify the problem as being interference. An extra Ec threshold is added so that we focus only on areas where coverage is not the major issue.

Practical Thresholds for Pilot Pollution

For an unloaded network, the definitions of pilot pollution area are:

1. Best server CPICH_Ec > -100 dBm and Best server CPICH Ec/Io < -10 dB

2. Best server CPICH_Ec > -100dBm and Count of cells in SHO_Window > 3

Below a CPICH Ec/Io of -10dB call quality cannot be guaranteed, even in good coverage, so an area meeting these conditions is termed to have pilot pollution. An area can also be classed as having pilot pollution if there are too many pilots being received, causing a lack of dominance. For example, if 4 pilots are received within the SHO_Window, the best two will enter the active set and the UE will be in 3-way SHO. The two remaining pilots will only serve to increase interference in the area as they cannot be included in the active set.

Note that these levels are defined for an unloaded network and therefore include a load factor so that the network is optimised for load.

Pilot Pollution Optimisation

Pilot Pollution Optimisation Target

In order to tackle pilot pollution, we need to improve the CPICH Ec/Io of the best server to 10dB or above and also ensure that there are no more than 2 pilots (not including the best server) in the SHO_Window.

This can be done by either:

Improving the Ec level of the best server and therefore its dominance,

OR

Reducing the Ec level (a fraction of Io) of some or all of the interferers.

Generic Method

Pre-check

Before any kind of optimisation on pilot pollution areas, it should be checked that:

1. All sites in the area are up and running properly, in other words discard any maintenance issue from fine tuning problems.

2. There are no extra sites planned to come on air in the near future.

3. The area is worth optimising - if the potential traffic is very low, no optimisation may be required.

Improvement of Dominant Ec level

It may be surprising to recommend signal enhancement when tackling interference, but in some cases it is the most straightforward and easiest solution to implement. This method is recommended when the best server cell has an excessive tilt - if reducing the tilt does not create other new interference areas.

Due to the risk of creating interference areas elsewhere, this solution should be handled with care and simulation should first be performed using a planning tool.

An example where this method might be useful is the case where an antenna is pointing towards a hill with a downtilt of 6 degrees.

Hilly areas have a tendency for interference since line of sight could be possible with many sites. In these areas pilot pollution is quite likely. Trying to tilt the antennas of all of the interferers received on the hill can be an impossible task since too many cells may be involved. In this case it is much easier to clear the problem by increasing the Ec level from a specific site close to the hill by uptilting the antennas towards the hill.

Reduction of Interfering Ec levels

The first difficulty when trying to reduce interferers is to identify the interferers in an area of pilot pollution.

A few questions need to be asked:

1. Which cells are received in the area?

2. Which cells are the dedicated serving cells for the area?

3. Which cells are the worst interferers?

The answer is not straightforward. Details on this analysis will be found in the following sections.

Optimisation Actions

When the worst interferers are identified, optimisation action should be carried out in order to reduce their interference.

In order to achieve an increase or a reduction of Ec level, clarification of what changes are allowed on the network is required:

Ideally, the allowed changes should be:

1. Modify the tilt of the antennas, preferably the electrical tilt if adjustable, otherwise the mechanical tilt.

2. Modify the azimuth of the antenna.

3. Change the antenna, in order to get a narrower beamwidth (H65 instead of H85 for instance), to increase the fixed electrical tilt (T6 antenna instead of T0), or to get an adjustable tilt device.

4. Switching off a cell - for consideration in extreme circumstances when a cell is causing widespread pilot pollution over the coverage footprint of a number of other cells.

5. Installing a new site to provide dominance in an area.

The modification of the CPICH power is not recommended at this stage.

It could be argued that reducing the CPICH power helps to reduce the interference generated by a cell, however reducing the CPICH power creates other issues: a straight power reduction means that the coverage of the whole cell is reduced (including indoor areas close to the site), which is not advisable when coverage is an essential target.

The preferred option is to tilt the interfering antenna(s), such that the interference area from the remote site can be greatly reduced while the main coverage area of that site can remain unspoilt.

Practical Pilot Pollution Analysis

Analysis with a scanner

Identify pilot pollution areas

When used with Top N functionality a scanner will report all decoded scrambling codes on the carrier.

Pilot pollution areas can be displayed by means of a post-processing tool based on the conditions described in 6.4.

List all cells involved in the pilot pollution area

All decoded scrambling codes in the pilot pollution area should be listed.

Beware however that not all of the interferers will be decoded. Since all cells are transmitting on the same frequency, the interferers themselves are subject to interference until their own Ec/Io gets so weak that they will not be decoded and will not be reported by the scanner.

How can the interferers then be identified?

In normal conditions the most significant interferers should be decoded, even for a short period of time.

An optimisation drive survey dedicated to the pre-identified pilot pollution area is also an option to gain more information about the interferer list. Indeed when pilot pollution is experienced on a short stretch of road leading a UE to drop when the network is loaded, very few samples may be recorded. Performing drive surveys of side streets or repeating the drive at slower speeds will help decode more scrambling codes.

Support from a planning tool is also an option, however caution should be given on the reliability of the simulation, especially on Ec/Io values, which involve differences between signals. Be careful of optimising a site that is not actually interfering in the field.

Determine the dedicated best serving cells

From the list of decoded scrambling codes, cells which are natural best servers in the area should be identified from other cells. It might seem obvious but the issue is real.

Firstly clarification of how many best servers are relevant is needed. Bearing in mind that the maximum number of cells in the active set (in soft(er) handover) is 3, generally the 3 best serving cells should be considered as not being interferers. At a certain point between 2 sites it is natural to receive 3 cells at an equal level, meaning that the CPICH Ec/Io will at best be 9 dB. This is not pilot pollution in itself.

Secondly within an area Ec levels will fluctuate and the 3 best cells will obviously not remain the same.

The determination of the best servers then requires a pragmatic approach when there is a doubt on the best servers, namely:

1. To achieve dominance, 1 cell should be chosen as the dedicated server. Other cells shall be optimised out of the area.

2. If such optimisation is not feasible, it is possible to have up to 3 cells providing usable coverage in the area. All other cells that are good enough for the active set must be optimised out of the area.

3. If a remote cell appears consistently as best server, ask yourself whether this cell is absolutely necessary in the area. If not it shall be considered as an interferer rather than as a desired best server.

Determine the worst interferers

When the best server(s) have been identified, the remaining cells on the decoded list are the interferers. Not all of them should be optimised.

Priorities can be given to the degradation they induce. Sorting the list by descending Ec/Io will give a priority order.

Then when selecting cells to optimise, the broader picture should be considered rather than a local analysis.

Worst cells will be those involved with a high priority on the highest number of pilot pollution areas. A global analysis of a city may show that a few cells pollute large areas and create numerous pilot pollution areas. Those will be the worst interferers.

A useful condition will help display the interference effect of a single cell (cell_i) on a whole area:

If this condition is fulfilled it means that cell_i is not the best server but is potentially interfering the best server. The larger the surface the worst the interference generated by cell_i.

Finally when the worst interferers of an area have been identified, their Ec level should be reduced as explained in 6.5.3. The main optimisation action is to perform a tilt of the interfering antenna.

The most important issue when tilting antennas is to not reduce the useful coverage. A peculiarity of UMTS is that by tilting cell_i we may paradoxically end up creating new pilot pollution areas. This is why special care should be given when tilting antennas to not reduce usable coverage elsewhere.

Analysis with a trace mobile

The general method with a trace mobile is the same as with a scanner, yet some further issues will arise.

The main difference is that the trace mobile is affected by missing neighbours. A degraded Ec/Io value may be caused by a missing neighbour rather than pilot pollution. It is therefore necessary to eradicate any missing neighbour problems before studying pilot pollution effects.

Note that with a scanner such a doubt will not occur: when used in Top N mode the scanner decodes all possible scrambling codes regardless of neighbour declarations.

The trace mobile only reports Ec/Io levels for cells within the monitored set. This set is the list of declared neighbours for the cells within the active set. If the strongest cell has been omitted in the neighbour declaration, the best reported cell will not be the strongest. For more details refer to the missing neighbour analysis in section 5.

We can stress then that the dedicated tool for pilot pollution tracking is the scanner, not the trace mobile. If only the latter is available though, ensure that before analysis no missing neighbours remain.

Pilot Pollution Optimisation Summary

The scanner is the required tool for pilot pollution analysis.

Pilot pollution areas can be easily displayed but may be difficult to optimise, and will require antenna modifications - mainly tilts.

Whilst the load on the network is low, the main source of downlink interference will be missing neighbours and few quality problems are expected due to pilot pollution. However, interferences will arise quickly with load, therefore pilot pollution areas will require pre-emptive actions before the network quality rapidly degrades.

5 Soft Handover area optimisation

The aim of this section is to give details on how to analyse and optimise soft and softer HO areas from drive survey measurements.

Definition of Soft Handover Area

By SHO area we mean the area where a UE has several cells in its active set. For convenience of analysis we will not distinguish soft from softer HO, that is to say SHO in this section refers both soft and softer HO.

As a reminder soft HO means HO between 2 cells from different Node Bs, whereas softer HO means HO between 2 cells from the same NodeB.

The definition of SHO is slightly different depending on the type of measurements used:

1) Trace mobile in connected mode (call up):

SHO nb_cells_in_active_set > 1

2) Trace mobile in idle mode or scanner:In this case, we need to calculate the SHO area from Ec/Io measurements. Based upon the parameter settings for SHO. The soft HO area is defined by the equation:

Where

SHO_window is the average of Addition_window and Drop_window (Recommended SHO_Window = 5dB)

Ec/Io_best

is the CPICH Ec/Io value of the best serving cell

Ec/Io_2nd_best is the CPICH Ec/Io value of the 2nd best serving cell

Aim of SHO Area Optimisation

The aim of SHO area optimisation is to limit the SHO area within the network to a reasonable area. An excessively large SHO area generates a loss of capacity on the network since each UE in SHO uses 2 links or more for its connection.

Note that there is no quality problem linked to large SHO areas when the network is unloaded. Only when the load increases will excessive SHO areas generate measurable quality degradation, and not only for the users in SHO but also other users of the network who may not be in SHO.

Soft HO

A UE in Soft HO means:-

Twice as many channel elements in the NodeB (hard resources, in the WSP Card) are required for the call

Twice as many backhaul resources are required to communicate to the RNC.

2 cells transmitting dedicated DL power, yet slightly less power per cell than if only 1 cell was serving the UE thanks to the combination gain in DL at the UE.

No change in UL (no combination in UL for soft HO, the RNC selects the best signal received from the UE by the cells in soft HO)

Globally if a Soft HO area increases, it means a loss of hard NodeB processing capacity, backhaul capacity and DL capacity, and an unchanged UL capacity.

Softer HO

A UE in Softer HO means:-

The same number of channel elements in the NodeB (hard resources)

2 cells transmitting dedicated DL power, yet slightly less power per cell than if only 1 cell was serving the UE thanks to combination gain in DL at the UE.

UE is transmitting less UL power thanks to the combination at the NodeB for softer HO.

Globally if a Softer HO area increases, it means a loss of DL capacity, an unchanged hard capacity and a gain of UL capacity. However as it is expected that in downlink will be the capacity limiting link, softer HO reduces overall system capacity.

SHO Area Target

The target for geographical SHO area is:SHO_area < 30%

SHO In An Unloaded Network

As load increases on the network, the effects of cell breathing can be observed. The higher number of users causes an increase in uplink interference, which each UE has to overcome to communicate with the Node B. As a result the distance from the cell at which a call can be supported will reduce as load increases. Similarly, for a DL limited service, available node B power for DCHs will reduce as the load increases, causing the distance from the cell at which a call can be made to reduce. As more users are likely to be in dominant coverage, the time that users spend in SHO will reduce as load increases.

In an unloaded network, SHO areas will naturally be larger since users can make calls at greater distances from cells. As the network becomes more loaded SHO areas will reduce.

Mechanisms to Achieve SHO Area Target

The reduction of SHO areas is achieved through the reduction of the overlaps between cells. See also Pilot pollution module. In this section we shall focus purely on SHO area optimisation.

Aim of SHO Area Optimisation

The aim is to achieve a network where the overall percentage of the area in SHO is below the SHO area target of 30%. In a non-optimised network the SHO areas will be much higher than this. In practice, the way to achieve this target is to reduce the Ec level from some of the cells in SHO which are not the best server. More precisely we should reduce the Ec level of the 2nd best server (or up to nth best if there are n cells in the active set) so that the optimised cell is no longer eligible for the active set, i.e.,

SHO Area Optimisation

After having viewed the SHO area and sorted the cells in terms of priority, optimisation shall be carried out (refer to the later subsections of this section for details).

SHO area optimisation will be achieved by either;

1) Modifying the tilt of the antenna, preferably the electrical tilt if adjustable, otherwise the mechanical tilt.

2) Changing the antenna, in order to obtain a narrower beamwidth.

3) Changing the azimuth of the antenna, preferably of all sectors on the site, to direct the boresight of the antenna away from the area of SHO.

4) Switching off a site that is causing large areas of SHO.

5) Installing a new site to provide a dominant SC in the area.

Note that adding tilt to an antenna will not help reducing the Soft HO area if the SHO area is located close to the site. What will most certainly happen is just a shift of the Soft HO area, but in terms of surface area the area will not be smaller.

Tilt will be most useful in the case when the Soft HO area in which a cell is involved is remote from the site.

The beamwidth of the antenna also has a great effect on the SHO area.

If we simplify the analysis by focusing on softer HO between 2 sectors of a same site, an H65 antenna will give better results (less SHO area) than an H85 antenna.

The following table is a theoretical calculations of the softer handover areas for three of the 3G antenna types.Antenna TypeTheoretical SHO area (SHO_window=3dB)

H65 Cross Polar7%

H85 Cross Polar13%

H85 Plane Polar11%

Table 7 Softer handover area

Caution: above values are with only 2 cells and are not comparable to the SHO figure of 30%.

We can see that a H65 antenna gives better results than a H85. Therefore replacing a H85 antenna by a H65 can be a useful optimisation technique to reduce the SHO area.

RAN Parameter Changes

No parameter changes at the time are being recommended.

For the same reason detailed in the section on pilot pollution, we do not recommend any change in CPICH power.

Apart from the physical antenna downtilts, two main RAN parameters will impact the size of the SHO area, these are the Addition Window and Drop Window.

Methods of SHO Analysis

SHO Analysing with a Scanner

Display of SHO Area

With a scanner, no call can be made so no HO will be performed. Therefore the area where a UE would be in SHO needs to be extrapolated from the scrambling code measurements.

Quite simply the Ec/Io levels of the best and the 2nd best cells are compared, based on the following formula:

Note that the timing issues are not considered with this expression, but they would not really matter to get a global figure of SHO area.

Identification of Cells to Optimise

Unlike in the case of pilot pollution we cannot define an area as being of excessive SHO at a bin level. The issue requires a broader picture where percentage of SHO is assessed on a scale larger than bin level.

Therefore as it is difficult to decide that a particular area requires optimisation, it will be handy to calculate figures on a cell basis, in order to assess the cells performance.

Ideally a cell should have a large best server area and a small 2nd or 3rd best server area. If all cells follow that rule we will indirectly get good SHO percentage figures for the network.

We shall then introduce some cell based statistics:

Defining the expression In_SHO_not_best_server(i) as the area where the cell (cell_i) is not best server but may be eligible for inclusion in the active set:

A performance indicator will be the ratio of this area over its best server coverage area:

This and similar metrics are available in the post-processing tool.

The worst cells are those to be found to have the highest SHO_perf(i) and will be the top priority cells to be investigated. After double-checking on a map display where the cell is best server and In_SHO_not_best_server, we can trigger optimisation action.

Method with a Trace Mobile

A good analysis of SHO areas with a trace mobile first requires that all missing neighbour issues have been cleared.

Then displaying all areas in SHO is straightforward, simply logging in-call where

SHO count_cells_in_active_set > 1

Further analysis should use the same method as for the scanner.

High Traffic Areas

It should be made clear that the SHO area target specified in section 7.3 is for a geographical area only. As a consequence the methods of optimisation detailed so far in this section are aimed at the optimisation of the geographical SHO area.

If an area of SHO coincides with a high traffic area then this will dramatically impact on capacity and network statistics will indicate that a high percentage of call time for the local cells is in SHO.

The methods of SHO optimisation detailed in section 7.5.2 will have the effect of changing the geographical footprint of the SHO area, so these methods could also be used to direct areas of SHO away from high traffic areas. However, shifting the areas of SHO could have detrimental effects on the surrounding radio network and extreme care should be taken when considering these options. Any potential changes to the radio network should always first be modelled in a planning tool.

SHO optimisation for high traffic areas will be achieved by either:

Modifying the tilt of an antenna to either improve dominance in the high traffic area or remove a cell from the active set.

Changing the antenna in order to obtain a narrower beamwidth and remove a cell from the high traffic area.

Changing the azimuth of an antenna, again preferably for all sectors on the site, to either improve dominance or remove a cell from the high traffic area.

Switching off a site that is causing large areas of SHO.

Installing a new site to provide a dominant cell in the high traffic area.

The modification of certain parameters could also help alleviate this problem, but until a full analysis of these parameters has been completed by RSE it is not recommended that any changes are made to SHO parameters. Potential high traffic areas should always be considered carefully at the planning stage.

SHO Area Optimisation Summary

Either a scanner or a trace mobile can provide relevant results in terms of SHO areas. Note however that a planning tool will be required for simulating possible optimisation actions.

Optimised SHO areas will provide a higher capacity from a WCDMA network. SHO area optimisation should not be a serious issue before the network reaches a significant load. However SHO optimisation may require antenna changes and should therefore be considered before quality degradation is experienced on the network.

EMBED Equation.3

EMBED Equation.3

EMBED Equation.3

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