3G Optimisation Workshop

Pilot Pollution

London, 28th of April 2003

Definition

A pilot pollution area is an area where an excessive number of scrambling codes are received and lead to a degradation of quality

In other words Pilot Pollution corresponds to DL interference

Practical condition:

Best server CPICH_Ec is “good”

And

Best server CPICH EcNo is “bad”

Thresholds

A single EcNo threshold is not recommended :

>when Ec decreases, even with a single cell and no traffic, the EcNo naturally degrades, therefore at low Ec level it is less straightforward to relate a weak EcNo to pilot pollution

>When the Ec level is low, the main problem is coverage, not quality

Recommended thresholds:

Best server CPICH_Ec > -100 dBm

And

Best server CPICH EcNo < -10 dB

Caution:

for unloaded network only

EcNo: theory

Formula:

RSSI

EcEcNo Ec: Energy_chip, received level on the CPICH

RSSI: Received Signal Strength Indicator

In detail:

EcP

PFP

EcEcNo

CPICH

BSNoise )1(

EcNo: Single cell no traffic EcNo versus Ec: single cell, no traffic

-18.0

-16.0

-14.0

-12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0-120

-119

-118

-117

-116

-115

-114

-113

-112

-111

-110

-109

-108

-107

-106

-105

-104

-103

-102

-101

-100

-99 -98 -97 -96 -95 -94 -93 -92 -91 -90 -89 -88 -87 -86 -85 -84 -83 -82 -81 -80

Ec level (dBm)

EcNo: Handover, no traffic EcNo versus Ec curves

-18.0

-16.0

-14.0

-12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0-120

-119

-118

-117

-116

-115

-114

-113

-112

-111

-110

-109

-108

-107

-106

-105

-104

-103

-102

-101

-100

-99 -98 -97 -96 -95 -94 -93 -92 -91 -90 -89 -88 -87 -86 -85 -84 -83 -82 -81 -80

Ec level (dBm)

Singlecell

2 wayHO

3 wayHO

4 wayHO

EcNo: typical values

EcNo degrades with Ec level

EcNo degrades with overlaps. BLER degradation may occur only when 15 cells are received with the same level (lab test, field test to be performed). In the field it seems that the BLER starts degrading when EcNo < -10 dB.

EcNo is almost similar for all overlap situations at lowest Ec levels, meaning that a pilot pollution area has a low impact on cell edge.

Ec EcNo (1 cell) EcNo (2 cells) EcNo (3 cells) EcNo (4 cells) EcNo (15 cells)dBm dB dB dB dB dB-115 -15 -15.5 -16 -16 -18-100 -4.8 -7 -8.5 -9.5 -15> -90 -3 -6 -7.8 -9 -15

Main reasons for pilot pollution

Radio design (F factor)– High sites

– Large antenna beamwidth

– Non tilted antennas

EcP

PFP

EcEcNo

CPICH

BSNoise )1(

Traffic (P_bs)– P_bs = DL power used by the Base Station.

– The higher the number of users, the stronger P_bs

– More interferences at busy hours

A closer look at the formula gives reasons for a bad EcNo:

Solution How can we improve Pilot Pollution areas?

2 ways:

1) Improve the Ec level of the best server

2) Reduce the Ec level from some of the interferers

Either way we need to identify and sort:– the dedicated best serving cells

– the interfering cells

Generic Process: display First step is to display the pilot pollution areas

With either scanner or trace mobile data, this can be easily done using the condition:

Best server CPICH_Ec > -100 dBm

And

Best server CPICH EcNo < -10 dB

Generic Process: pre-check Before optimising a pilot pollution area, basic questions shall be

answered:– Were there maintenance problems? - check if all sites were transmitting properly

– Is there a site planned to be on air in a near future?

– Is the area worth optimising? (Secondary road, hill top…)

Generic Process: identify cells Reporting levels from cells involved in a pilot pollution area is

much easier than tracking an interferer in GSM

With a scanner, all possibly decoded SCs are reported

With a trace mobile, all SCs from the monitored set are reported… meaning that no missing neighbours shall remain before analysing pilot pollution

Serving and interfering cells

One basic difficulty when tackling a pilot pollution area is to define among all reported cell:

1) Which cells are the proper dedicated best servers of the area

2) Which cells are the interferers to optimise

Both local and large scale analysis are needed

Serving cells

The wished best serving cells are not obviously the measured best cells:

>in the area, top cells may fluctuate

>best or 2nd best measured cell may be a remote cell that we would rather optimise

A practical approach is necessary

>define as best cells those from the closest physical sites

>keep a remote cell only if absolutely necessary (bringing useful service that closer cell could not provide)

How many cells?

>3 cells is a typical number, as it is the size of the active set

Interfering cells

Basically all non best server cells are the interferers

Which cells to optimise?

>It is necessary to get the “big picture” of the area and identify the worst interferers

Worst interferers can be defined as:

>Cells involved in a high number of pilot pollution areas

>Cells received with interfering levels without being best server. The following layer shall be displayed:

dBicellEcNoserverbestEcNodB 5____0

Optimisation actions

We shall specify which levers may be used to optimise a pilot pollution area. Ideally we should be able to optimise antennas, namely:

– Tilt modification

– Azimuth change

– Change of antenna (to get a more narrow horizontal beam-width, or an electrical tilt)

We do not recommend CPICH power changes

>The best option is tilt modification

Optimisation actions

Ideally the Ec level reduction on the interferer shall be such that:

dBerfererEcNoserverbestEcNo 10int___

Caution when optimising the interferer:

>Do not reduce useful coverage

>Do not create new pilot pollution area by reducing the Ec level where this interferer is best server

Use the field measurements to assess what dB reduction is needed, but use planning tool for a full simulation of the optimisation

Summary: flow-chart Tools required:

>Agilent Scanner

>NATSuite (post-process)

>NetAct planning tool

Pilot Pollution Flow Chart

Field measurement(Agilent scanner)

Import in post-processingtool (NATSuite)

Display pilot pollution

Pre-check

Sort best servers fromworst interferers

Optimise interferers andcheck with planning tool

(NetAct)

Identify all cells

Note: A different process will be required with a loaded network (to be studied)

>Updated thresholds

>Focus on busy hours

>Traffic related optimisation

Risks specific to UMTS

In GSM:

– Frequency Planning

– High number of frequency

>Few interferences

>High sites and large antenna beam-width may not be a problem

In UMTS– No Frequency Planning

– All cell use the same frequency

– interferences depending on radio design and traffic

>same radio design as for GSM may not be suitable

Expectations

Moreover interferences will arise quickly with load and can take a few months or more to be optimised

>therefore pre-emptive actions are advised before the network gets uncontrollable

In the early days, few pilot pollution areas are expected (e.g. EN in Bristol),

Without load we should not drop calls due to pilot pollution

Yet feedback from areas with a higher site density of than EN Bristol will be useful

3G Optimisation Workshop

Pilot Pollution

London, 28th of April 2003