External factors
Engineers cannot do much about two external factors – they have to consider these when planning.
The physics of radio wave propagation, or how far a signal will travel, depend first on the frequency of the signal. A 400-MHz signal propagates 50% further than an 800-MHz signal of the same power, for example. But the allotted frequency will generally be decided by regulators, not network builders.
Geography and buildings can get in the way of the radio signal. If a radio wave encounters a mountain or a skyscraper, for instance, or the base station is in a forest, the signal can be significantly attenuated.
Equipment matters
Another major factor that determines the maximum distance between the base station and the user device – and hence the network cell radius - is the transmission power and sensitivity of the base stations and radio devices.
Roughly speaking, doubling the range requires around 10 times more power.
The transmission power of a base station in a typical radio communications network is between 20 and 50 Watts, while the output power of a radio device is usually between 1 and 3 Watts.
In fact, the sensitivity of the base stations is crucial, since the signal coming back from the terminal will be much weaker than the signal transmitted from the network to the end user.
Let’s say the radio device has -105 dBm sensitivity and the base station has -112 dBm sensitivity. The engineer can calculate that a 25 W base station, with diversity and panel antennas for uplink, requires the device to transmit at 1 W. This will in fact guarantee maximum coverage.
Any more power in the radio device brings no benefit (except if the radio is used without the base station, in Direct Mode, DMO).
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Handpicked content -
Take a look at this Secure Comms blog post which explains the power relationship between the radio devices and base stations. Read "Do you know the secret about Watts in your PMR radio? (Nobody else will tell you)"
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Engineering the network
Once the technology and the network products have been chosen, it is the radio network engineer’s job to optimize coverage by juggling customer requirements, cost and other constraints. The customer requirements are usually expressed in terms of the areas that need coverage and the Quality of Service, QoS.
QoS will depend on the kind of devices being used and the voice quality expected. Constraints include the environment, legislation regarding any power and frequency restrictions and the availability of base station sites.
Taking all this into account, the engineer carries out a network study to maximise the possible coverage.
The first job is to identify the best locations for base station sites and decide on the antenna height. The antenna is always a compromise between coverage and cost: a taller antenna is more expensive but gives more coverage.
The antenna configuration can then be designed to maximise the coverage. This is actually the point at which radio engineers can have the biggest impact. Airbus products include many special features to help them gain the best possible coverage from every investment.
Network planning is a skilled, iterative process, but the engineer should end up with a study that covers the entire network. The resulting network should provide full coverage using the minimum number of sites in the most cost-effective way.
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