Coverage considerations will dictate the number and locations of your radio sites (also referred to as towers). Together with frequency availability and traffic patterns, they will determine whether the system should be simulcast, multicast or a hybrid of the two.
Coverage engineering may be the most complex area of the radio system specification and design process and is one critical area where your investment in a competent consultant will be well justified.
There are many ways to describe coverage performance. It is typically done in several different ways, which together describe what your user can reasonably expect when the new system is implemented. Delivered Audio Quality (DAQ) is the most common signal quality measure in P25 for Public Safety.
|Unusable. Speech present but not understandable.
|Speech understandable with considerable effort. Requires frequent repetition due to noise or distortion.
|Speech understandable with slight effort. Requires occasional repetition due to noise or distortion.
|Speech understandable without repetition. Some noise or distortion present.
|Speech easily understandable. Little noise or distortion.
|Speech easily understandable. Rare noise or distortion.
|Perfect. No distortion or noise discernible.
For Public Safety, the accepted objective is to provide DAQ 3.4 over the service area. DAQ 3.4 is defined as “speech understandable with repetition only rarely required, and with some noise and/or distortion.”
A lower DAQ (for example 3.0) may require excessive speech repetition while a higher value (for example 4.0) may require a prohibitively high level of infrastructure investment.
Predicted reliability is the other important measure of coverage performance. For Public Safety, the industry standard for coverage design is to provide 95% reliability.
This is a simplified statement, which in reality means that you can expect a signal of the requested quality (for example, DAQ 3.4) 95% of the time across 95% of your coverage area.
Defining coverage needs
For most systems, it is insufficient to define uniform coverage requirements across your entire service area, for example 95/95 @3.4 DAQ. Typically, there are areas of special consideration:
- critical use – prisons, courthouses, hospitals,
- high population density – urban areas, challenging terrain – mountains, canyons,
- challenging construction – significant buildings with “dead spots”.
Map your geographical area to identify the geographical features, buildings and other locations with specific coverage level requirements.
Identify where in-building coverage is needed. Include schools and colleges, hospitals, prisons, airports and any other critical areas. There are several approaches to define in-building coverage requirements.
- Some projects demand a uniform signal level in critical areas, assuming it will be sufficient in the relevant buildings. For example, general residential areas can be identified on a map and marked up for signal strength 10 dB above what is required in open spaces. It is advisable to characterize the most critical buildings up-front to determine the appropriate signal strength margin. Take signal strength measurements from inside the buildings, using the signal from desired existing sites or temporary reference transmitters.
- You may identify specific “must cover” buildings and place the burden of engineering on the vendors. For example, you may decide that interiors of all schools must provide firstname.lastname@example.orgDAQ and it is up to the vendors to engineer the system to provide this performance.
The first method requires more work up-front, but results in lower cost and better quality proposals as vendors can apply a more uniform approach to coverage design.
The second method shifts the burden of coverage performance to the vendors. This may be easier for the agency, but requires more time to engineer the response and is more expensive.
While this may sound straightforward and intuitive, it is only the tip of the iceberg. The coverage requirements and, above all, coverage acceptance test plans need to be approached with great care and respect.
Most vendors maintain separate, specialized coverage engineering units. The processes, tests, parameters and vocabulary are well defined in the TSB 88 standards, maintained and updated by Telecommunications Industry Association (TIA). Your specifications should refer to this standard to minimize the risk of misinterpretation of either your RFP or vendors’ responses.
This article is taken from Specifying your P25 System guide which is a part of P25 Best Practice. P25 Best Practice is a comprehensive and authoritative set of guides to P25 radio which distills P25 radio knowledge and wisdom of 30 experts with over 450 combined years of experience.
You can download the guide, absolutely free, from the P25 Best Practice website.