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Introduction to UMTS Radio Network

Planning , Design and Optimization

Senthil Kumar

Last Updated: 13/01/2008

UMTS radio network planning and optimization is altogether a vast and interesting field. It

involves the simple work of deploying the Node-B's and RNC's but, hold on...!! Complex

estimations of various radio network parameters, Dimensioning, Detailed Planning and

Optimization of radio network, Network simulation softwares and Field measurements are

involved in enforcing the simplicity mentioned earlier!! The goal is to get sufficient

coverage and capacity by maintaining the lowest possible deployment costs.

Dimensioning:

In this phase the basic general properties of the "would-be" network is investigated. This

initial phase of Network Planning includes calculating the approximate number of Node-B's

required, deciding the antenna heights and the network configuration parameters keeping in

mind the kind of mobile services the service provider advertised to offer their customers!

Detailed Planning:

Usually a site-survey is done at first to get to know about the geographical locations, traffic

volume in different areas.Topology planning and configuration planning is done in this

stage.Power budget calculation, also called as link budget is done by taking all the availableradio parameters that include information of different gains and losses in the communication

path of the radio link.

Cell breathing should be taken into account while calculating cell range in UMTS

network.When the traffic in the cell increases, the sensitivity of the Node-B decreases

because of the increased interference and so the UE will require more power to remain

connected to the cell.And when there is not much load in the cell, the interference would be

low and the UE can use the same power to connect to that particular cell even from a longer

distance.This variation in cell ranges is called as cell breathing. Simply put, cell capacity

influences cell coverage area.So one of the parameter in link budget is Interference margin.

MInterference, dB = 1...3 dB (20...50% Load).

Usually in RF Network planning, they start with uplink because the downlink is normally

adjusted to balance the uplink by selecting the appropriate power amplifier at the base

station. Near cell edges, the users transmit with full power to reach the Node-B and the inner

loop power control mechanism in uplink causes a problem, because the UE cannot respond

to power control commands due to limitations in transmission power capacity. This should

be taken into account while calculating link budget, and it is done by introducing a Power

control headroom margin also called as Fast fading margin, which guarantees continuous

service for users in cell edges.

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The Fast fading margin or Power control headroom is given by

MFastFade, dB = 2...5 dB.

In UMTS, coverage and capacity planning are interrelated and cannot be done separately

because of cell breathing phenomenon and they are done in planning phase called topologyplanning. In coverage planning accurate cell ranges are calculated from power budget

calculations provided in configuration planning phase. Normally, cell coverage's are made to

overlap excessively to give high service probability to users in cell edges, thus providing soft

handover resulting in macro-diversity gains.But more number of soft handovers implies

significant cost in the capacity. After measuring the pilot strength in the area, the size of

handover zones within the cell footprint should be decreased. Handover zones should be

shifted from high-traffic areas to low-traffic areas but at the same time should be

strategically placed to get the advantage of macro-diversity gains for UEs in cell edges.

The detailed planning phase includes usage of Network simulation soft wares for coverage

and capacity estimations. The inputs include user traffic distributions and other network

specific parameters. Simulations are mainly done to know the optimal number of Node-B's

required,UE transmit powers etc.Simulation results are used for optimizing the network.

However simulations are also done in parallel to actual planning before the network is

actually launched. Base station locations should be selected so that they are always placed on

the traffic hot spots, since this offers the best link budget for the mobile users served by

those base stations.

Field measurements are performed to study soft handover areas, network interference, and

call drops that happen in the real network.Field measurements are mostly performed when

planned coverage and propagatiion channel characteristics are examined in an alreadyoperational network.

Before the network is launched code and parameter planning are needed. Certain amount of

Channelisation codes(OVSF codes or walsh codes) are allocated for each cell to separate

users in downlink direction.

Optimization:

The actual parameters being used in the real network are fine tuned for better performance.

The power control parameters, number of channelisation codes being used in a particular cell

and other critical parameters that have a great impact on the performance are optimized fromthe network simulation software and field measurements.Use of WCDMA repeaters is one

way to increase coverage and capacity in an easy and affordable way.The article discussed

only about Interference margin, Power control headroom margin and soft handover macro

diversity gains.

The following link budget, lists the other parameters.

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Parameter Speech Data Units

DL UL DL UL

Bit rate 12.2 12.2 384 64 Kbps

Load 50 50 75 30 %

Thermal noise density -173.93 -173.93 -173.93 -173.93 dBm

Receiver noise figure 8 4 8 4 dB

Noise power at receiver -100.13 -104.13 -100.13 -104.13 dBm

Interference margin 3.01 3.01 6.02 1.55 dB

Total noise power at receiver -97.12 -101.12 -94.11 -102.58 dBm

Processing gain 24.98 24.98 10 17.78 dB

Required Eb/No 7 5 1.5 2.5 dB

Receiver sensitivity -115.10 -121.10 -102.61 -117.86 dBm

RX antenna gain 0 18 0 18 dBiCable loss/body loss 2 5 2 5 dB

Soft handover diversity gain 3 2 3 2 dB

Power control headroom 0 3 0 3 dB

Required signal level -116.10 -133.10 -103.61 -129.86 dBm

TX power per connection 33 21 37 21 dBm

Cable loss/body loss 5 2 5 2 dB

TX antenna gain 18 0 18 0 dBi

Peak EIRP 46 19 50 19 dBm

Maximum allowed path loss 162.10 152.10 153.61 148.86 dBm

References :1.3GPP TS 25.401, UTRAN Overall Description (Release 6),v6.6.0

2.3GPP TR 25.956, UTRA repeater : Planning Guidelines and System analysis(Release

6),v6.0.0