UMTS Capacity Planning
The number of installed transceivers limits the mobile network theoretical capacity. In cdma systems interference, accepted and planned quality and grade of service will determine the system capacity. Cdma systems also have soft capacity, which complicates the network area capacity estimations. The link budget is used to calculate the maximum allowed path loss and the maximum range for cell. The link budget includes the interference margin, which is the increased noise level caused by greater load in a cell. So by increasing the cell load, cell coverage area becomes smaller. That's how cell coverage and capacity dimensioning are interlinked.
System capacity planning is divided to two parts:
The first thing is to estimate a single transceiver and site capacity. Calculations how the noise raises as the cell load increases is out of the scope of this page, but in-cell noise, Eb/No requirements, planned data rates, coverage probability, air resources usage activity factor, target interference margin and processing gains are needed to approximate the transceiver and site capacity. Depending on the parameter values, planned transceiver capacity is typically from 400 kbits/s to 700 kbits/s per transceiver.
The second part of the process is to estimate how many mobile users each cell can serve. Once the cell capacity and subscriber traffic profiles are known, network area base station requirements can be calculated. Estimations can be done in Erlangs per subscriber or kilobits per subscriber. Network vendor normally has simulation tools to test system parameters and verify rough estimations. A lot of data is required for comprehensive network dimensioning; number of subscribers and growth estimations, traffic / user / busy hour / geographic segment and required throughput including service mixes in geographic segments for example.
Each type of traffic has to be estimated for capacity calculations.
Here is a rough downlink capacity calculation example:
During a busy hour an average user downloads 10 Mbits with 384 kbits/s, 2 Mbits with 144 kbits/s and makes one 60-second voice call. Data has to be retransmitted 1.1 times because of network conditions.
Used kbits/s per user per busy hour downlink only are:
|(10000 kbits / 3600 sec) x 1.1*)
|(2000 kbits / 3600 sec) x 1.1*)
|(60sec x 12.2 kbits/s) / 3600 sec
||3.87 kbits/s / user / busy hour
If a cell capacity is estimated to be 500 kbits/s, each cell can be dimensioned for about 129 users.
This example was simplified, but please remember when you see capacity estimations with various traffic mixes, that those are just estimations. Notice how sensitive the capacity is to variations of download amount, retransmit rate and estimated cell capacity values. Before UMTS networks are on air and customers start to use high speed services, network capacity calculations are anybody's guess!
Even when the capacity calculations are done in a very beginning, normally the mobile networks are initially planned to meet the coverage objectives. Capacity sites and transceiver upgrades are installed later, once the real traffic load is known. In the early 1990s most of capacity requirements of new 2G networks were initially over-estimated, partly because operators needed to present (over-) optimistic business plans to secure the funding and partly because nobody knew how much people would use their phone on certain price levels. Generally busy hour Erlangs per subscribers were not what was originally anticipated, and this tendency is likely to continue in 3G.