3G RF Optimisation Basics
WCDMA Basics & Generic 3G
Optimisation Process
1-WCDMA Basics
Frequency Reuse
Scrambling Codes
Signal Strength/Quality Measurements in
WCDMA
Pilot Pollution
Soft/Softer Handover
Missing Neighbours
2-Generic 3G Optimisation Process
Process overview
RF KPIs
In Call KPIs
Tools for the job
Frequency Reuse
•Initially operators have been launching with just a single
carrier
•Network will initially be FDD only
e.g. Operator X will launch using only the FDD carrier (F1
=10588)
All launch cells will use F1 & therefore Operator X’s 3G
network will have a frequency reuse of 1
Scrambling Codes & CPICH
• It carries no information and can be thought of as a
“beacon” constantly transmitting the Scrambling Code of
the cell
• The Common Pilot Indication Channel (CPICH) is
broadcast from every cell
• It is this “beacon” that is used by the phone for its cell
measurements for network acquisition and handover
purposes (Ec, Ec/Io).
3G Coverage Measurements
The majority of 3G coverage measurements are based upon
measurements of the CPICH
Golden Rule: If the UE can’t see the CPICH the UE can’t
see the cell.
Initial 3G network optimisation will be 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)
Total Received Power Io
In a WCDMA network the User Equipment (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
Using the properties of SCs 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)
From the previous two measures we can calculate a signal quality for each CPICH
(SC) received
Ec/Io = Ec - Io (dB)
*Note: Sometimes Ec/Io is referred to as Ec/No
Example
From the above three measurements we can calculate for each pilot the Ec level
for that particular pilot
Ec1 = -80 - 5 = -85dBm
Ec2 = -80 - 10 = -90dBm
Ec, Io and Ec/Io Measurement
All commercial scanners and test UEs are capable of
making Ec, Io and Ec/Io measurements
It is these measurements that are used for cover analysis
and basic optimisation
Handovers in WCDMA
Various handover types exist in WCDMA
Those between WCDMA sites (intra-system HO)
Those between WCDMA and GSM (inter-system HO)
Handovers in WCDMA - Softer HO
Softer handover occurs between sectors of the same site
Handovers in WCDMA - Soft HO
Soft handover occurs between sectors of the different sites
For both softer and soft it is the Ec/Io levels used to determine
whether a cell should be added or removed from the active set
Handovers - Inter frequency HO
Inter frequency handover occurs between two WCDMA
carriers
Will be used once operator deploys its second carrier, for
microcell layer or capacity purposes
Handovers - Inter system HO
Inter system handover occurs between 3G and 2G sites
As with all handovers, accurate adjacencies will be required
Optimisation Basics
1-Coverage Optimisation
2-Neighbour Optimisation
3-Pilot Pollution Optimisation
4-SHO Optimisation
Coverage Optimisation
As with 2G if the coverage is inadequate then the call quality
will be inadequate
Therefore this first stage in 3G optimisation is ensuring
adequate coverage
Link budgets should be used to determine both the planning
targets and the drive survey level targets
Note that the planning and drive survey level targets will be
different
Neighbour Optimisation
•Missing Neighbours was and still is the biggest cause of poor
performance in most commercial 3G networks.
•The majority of call drops in early cluster are due to missing neighbours.
It is therefore essential that the initial neighbour list is thorough, ideally
with neighbours ranked in order of importance.
•In 3G the UE must be on the best cell at all times … otherwise it will drop
the call – this is not the case in 2G.
•3G/2G neighbour lists must also be comprehensive, incorrect 3G/2G
neighbour lists will lead to dropped calls when moving from 3G to 2G and
poor 3G re-selection performance when on 2G
3G/3G Neighbour Optimisation
•We declare a 3G missing neighbour as a a cell not declared as
neighbour of the best active cell although it is eligible to be added
in the active set.
EcNo
MISSING _ N
≥ EcNo
BEST
− M arg in MISSING
_N
•A practical margin of 5dB is recommended as an initial window …
anything bigger and the neighbour lists become too large
•Solution: 3G Scanner Survey, Actix Analysis, Neighbours
Added
2G/3G Neighbour Optimisation
•2G/3G neighbour optimisation will require the use of both 3G
and 2G drive survey equipment
•Dualband scanner files can quickly be processed to pair up
the best 3G cells with the best 2G cells for every sample
along the drive survey.
Pilot Pollution Optimisation
•As more and more 3G sites are integrated, the pilot pollution
within the network will increase
•With >4 pilots of a similar level it can be very difficult to
establish even a voice call
•The higher the data rate the more susceptible the service is to
pilot pollution (bad Ec/Io).
•Whilst 3G voice requires an Ec/Io of above -15dB, 384kbps will
required Ec/Io levels of -9dB or higher !!
•Pilot pollution should not be confused with poor coverage,
where there is poor coverage, there is generally pilot
pollution/bad Ec/Io
Pilot Pollution Optimisation
To avoid confusing areas of poor coverage with areas of pilot pollution
we can define an area of pilot pollution as somewhere where
Best server CPICH_Ec >= Ec target
(dBm)
And
Best server CPICH EcNo < Ec/Io
target (dB)
•The Ec target is the target coverage = -100dBm
•The current recommended Ec/Io target = -10dB
Pilot Pollution Optimisation
There are various means of reducing pilot pollution
New Sites: Additional sites may reduce pilot pollution by bringing dominance to
an area (before optimising for pilot pollution, always check that a new site is not
about to be brought on air in the locality)
Antenna Downtilts: By adjusting tilts on the best servers, or worst interferers,
pilot pollution can be removed
Solution: 3G Scanner Survey, Analysis, Downtilts, New Sites?
Handover Optimisation
•Softer, Soft, Inter-frequency & system HOs have all been optimised on
Nokia’s trial and other customers networks
•Therefore initial HO Optimisation should not require any changes to the
UTRAN parameters ….
•Instead initial HO optimisation will involve studying how much HO occurs
in the network and where this HO occurs.
•For 3G/3G HO, Actix is able to display areas where Softer & Soft HO is
occurring and calculate the percentage of a drive route in SHO
•Typically we should be aiming for a SHO area of less that 40%
•Optimisation of SHO area should be performed using downtilt and azimuth
changes
•Solution: 3G Scanner Survey, Analysis, Downtilt and Azimuth
changes
Optimisation Basics
•Coverage Optimisation
•Neighbour Optimisation
•Pilot Pollution Optimisation
•SHO Optimisation
•Everything is based around Ec/Io, to improve this we need
to
• Increase Ec (improve the best server(s))
•
•
Reduce Io (reduce pilots not eligible for the active set)
Get all neighbours optimised
•In call drive runs should only take place once Radio
Optimisation is complete