Measurements are almost always superior to computer modeling. If the budget permits and transmitter locations are available, measurements should be collected. Typically, the coverage survey is accomplished after the land mobile radio system is constructed as part of the final acceptance test. Because coverage testing is labor intensive and expensive, it should not be done haphazardly. One must employ accurate, efficient and thorough collection methods to ensure the results are unambiguous.
The fundamental elements of the drive-test survey are the receiver system, the drive routes, and methods to ensure reproducibility. The receiver should employ an omnidirectional antenna; its sensitivity should be equal to or better than the user radio; it must be accurate (+/- 1.5 dB); and it should have high dynamic range. The receiver should also be computer-controlled and include GPS data-logging. The service area boundaries should be well defined and the drive routes should be developed prior to conducting the survey. Ideally, measurements should be collected on a uniform grid, but alternatively, randomly distributed data can be plotted to a uniform grid during post-processing. One must drive a dense grid that includes both thoroughfares and side streets. If possible, coverage surveys for acceptance testing should be accomplished when the leaves are out because foliage will attenuate the radio signal.
Pericle generally follows the guidance found in TIA-TSB-88, ensuring that each measurement is a linear average of at least 50 subsamples collected over at least 40 wavelengths of distance. Over several years we have developed custom software to analyze the drive test data, grid it, compute time delay interference (for simulcast systems), and plot the data on digital maps. As a minimum, the coverage survey should collect signal strength measurements, but signal strength alone does not guarantee intelligible voice at the receiver, especially for digital voice systems. Pericle collects signal strength, bit-error rate and voice recordings during its coverage surveys. The voice recordings can be automated and machine scored to greatly reduce the labor required and increase the reliability of the results. Coverage measurements for the old City of Denver System (pre-2007) are plotted in the map below.
Pericle also collects indoor measurements. Because GPS is not reliable indoors, we have developed subroutines that operate within AutoCAD. Each measurement is recorded with an x-y coordinate tied to the AutoCAD drawing coordinate system. Each grid tile is assigned a signal strength (or BER or DAQ) value which is color-coded. For example, the AutoCAD drawing below shows 800 MHz public safety radio coverage on the third floor of the Mall of America in Bloomington, Minnesota (a suburb of Minneapolis-St. Paul).
Our experience conducting coverage surveys translates into measurable, enforceable standards when we are asked to prepare RFPs for our public safety clients.
For more information on conducting coverage surveys, see our paper in the Technical Papers section, “Guidelines for Conducting Drive Test Surveys for 800 MHz Rebanding,” and “How to Conduct a Drive Test Survey.”