Coverage Modeling

In the absence of coverage measurements, one of the first steps in designing a new land mobile radio system is to model coverage from prospective sites and through trial-and-error, find the smallest number of sites that meets the coverage requirement. Alternatively, one may start with a fixed budget and design for the best overall coverage the budget allows.

The land mobile radio channel is rarely line-of-sight and the received signal is the sum of many reflected and diffracted replicas of the original. The term multipath fading is used to describe the time-varying amplitude and phase that characterize the composite signal at the receiver. Because mobile radio receivers are designed to operate in multipath fading with a minimum mean amplitude, we are more interested in modeling the mean signal, not the rapid fluctuations caused by fading.

The mean signal amplitude is a function of many factors, including free space loss, terrain loss, and clutter loss. At the frequencies used for land mobile radio, we can usually ignore losses due to precipitation and atmospheric absorption.

Free Space Loss

Most propagation models assume that the minimum loss is free space loss, given by 22 + 20 log₁₀(d/λ) dB where d is the path distance and λ is the wavelength of the radio carrier (both in the same units). Other losses are added to the free space loss to estimate the total path loss. Free space loss is easy to compute, so the real problem is to predict the losses due to terrain and clutter.

Terrain Loss and Digital Terrain Databases

Terrain loss is primarily diffraction loss and most models use principles of ray optics to estimate diffraction loss. A popular method for sorting out the best way to treat multiple obstacles is the Epstein-Peterson method. A computerized diffraction model is of little use without a digital terrain database and there are several to choose from, some very coarse and others very fine. In the United States, the earliest digital terrain databases were the National Geophysical Data Center (NGDC) 30 arc second and 3 arc second databases. One pitfall of these databases is that both are taken from the same coarse maps. In other words, the 3 second database is simply a more finely sampled version of the 30 second database. In mountainous terrain, large elevation errors from these databases are likely to occur. Modern propagation studies should be done with the 30 meter (1 second) database or its equivalent.

Clutter Loss

Clutter loss falls into two categories: foliage and man-made. Foliage loss is computed from a database of loss factors that are a function of both radio frequency and the type of foliage. Man-made clutter includes buildings, vehicles, bridges, etc. Man-made clutter loss is usually calculated from a clutter database which applies a clutter category to individual tiles (cells) in the geographical area under study. Typical clutter categories include dense urban, urban, suburban, industrial, agricultural, and rural. A common approach is to apply a single clutter loss factor corresponding to the tile of interest, regardless of the antenna height of the base station/repeater site. This relatively crude model can result in inaccuracies because it is not a function of antenna look angle. The steeper the look angle, the smaller the clutter loss and the shallower the look angle, the greater the clutter loss.

Pericle uses EDX Signal™ or Harris RAPTR software programs to predict land mobile radio coverage. To improve graphics quality, we often export the data from Signal™ and plot it in ESRI Arcview™. Arcview allows us to apply a variety of GIS tools and databases to the plot, including census data. Signal offers several propagation models, including line-of-sight, reflection plus multiple diffraction, Epstein-Peterson, Longley-Rice, TIREM and TIA-TSB-88.

Each model will give a different answer, so it is important that the user be well-trained on the software and knowledgeable of the underlying physics of the problem. Even the best models used by the most knowledgeable engineers result in errors of 6-8 dB (compared to measurements), so one must be careful to not attribute high precision to coverage models, especially when purchasing a new radio system.