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.
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!
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 available radio 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.
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 topology planning. 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 already operational 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.
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 from the 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.