Adding capacity

Date: 2015-10-07; view: 444.

Practical radio planning

Armed with a propagation model it is possible to calculate both the wanted signal strength and the interference level for all locations in a cell. Generally this is done using a computer based tool which can draw upon a database of cell site information and terrain data. Some advanced tools can also take account of diffraction losses. For practical purposes a planner will aim to achieve the required signal strength and C/I ratio over 90% of the cell coverage area, by varying antenna heights, transmitter powers, frequency allocations and other factors as appropriate.

To simplify calculations, an allowance for Rayleigh fading and shadow fading is usually made within the system power budget. A typical power budget is shown in Table 47.1.

Once a cellular network has been planned to provide overall cover­age, there are a' number of ways of adding additional capacity. A simple and cost effective option is to allocate further radio channels to existing cells. However, this can only be done by an extension band, for example the ETACS allocation in the UK. Other alterna­tives involve rearranging the cellular plan, either by cell splitting or by sectorisation.

Cell splitting is achieved by dividing an existing cell up into a number of smaller cells, by adding additional base stations as shown in Figure 47.7; it is then necessary to reallocate the radio channels. By repeatedly splitting cells; the cell size, and hence the system capacity, can be tailored to meet the traffic capacity requirements demanded by customer behaviour in all areas.

Table 47.1 Typical power budget (TACS) (1 = Key planning parameters)

Figure 47.7Cell splitting

In rural areas, cells may be 20km to 30km in radius. In practice, as cell sizes decrease, propagation effects, particularly in city areas, cause an increase in co-channel interference, even if the repeat pattern is maintained. Also, as cell sizes decrease, it becomes increasingly difficult to find suitable base station sites, which need to be accurately positioned in order to keep to a regular pattern.

The cost of providing and maintaining a large number of individ­ual base stations is also a factor, such that in addition to cell splitting, sectorisation of cells is commonly used in urban areas.

In a regular cellular layout, co-channel interference will be re­ceived from six surrounding cells which all use the same channel set. One way of cutting significantly the level of interference is to use several directional antennas at the base stations, with each antenna illuminating a sector of the cell, and with a separate channel set allocated to each sector.

There are two commonly used methods of sectorisation, using three 120 degree sectors or six 60 degree sectors as shown in Figure 47.8, both of which reduce the number of prime interference sources to one. This is because, of the six surrounding co-channel cells, only one will be directed at the wanted cell.

A disadvantage of sectorisation is that the channel sets are divided between the sectors such that there are fewer channels per sector, and thus a reduction in trunking efficiency. This means that the total traffic which can be carried for a given level of blocking is reduced. However, this effect is offset by the ability to use smaller cells, such that the end result is a significant increase in total capacity.

Figure 47.8Sectorisation

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