Radio Wave Propagation Models Used in RF Coverage Analysis
Radio wave propagation can be categorized as LOS (Line Of Sight) and non-LOS modes. LOS is direct point-to-point propagation with no obstructions in between. Non-LOS is indirect propagation in the absence of LOS path which consists of diffraction, reflection and scattering. In the HF band (3 – 30 MHz), propagation is primarily using sky wave for long distance communications. VHF and UHF (30 MHz – 3 GHz) waves travel by LOS and ground bounce propagation. The SHF (3 to 30 GHz) wave uses strictly LOS propagation.
The goal of propagation modeling is to determine the probability of satisfactory performance of a wireless system that depends on radio wave propagation. For RF systems planning, the modeling of propagation is for the purpose of RF coverage analysis. This analysis uses the propagation model and terrain data to predict the RF coverage area of a transmitter, the received signal strength at the end of a wireless link, the path loss from the transmitter to a distance receiver, the antenna tilt angle of the transmitter, the minimum antenna height to establish Line of Sight communication path and channel impairment such as delay spread due to multi-path fading.
Propagation models for different applications, environments and terrains had been developed by the US government, private organizations and standard body such as International Telecommunications Union (ITU). These models are based on large amount of empirical data collected for the purpose of characterizing propagation for that application. Since propagation models are created using statistical methods, no single model will exactly fit any particular application. It is a good idea to employ two or more independent models and use the results as bounds on the expected performance. The following are a list of most commonly used near-earth propagation models.
The Longley-Rice model predicts long term median transmission loss over irregular terrain. It is designed for frequency from 20 MHz to 20 GHz and path length from 1 to 2000 Km. The model accounts for terrain, climate, subsoil conditions and ground curvature. Longley-Rice model has two modes, point-to-point and area. The point-to-point mode uses detail terrain data and characteristics to predict path loss, whereas the area mode uses general information about the terrain characteristics to predict path loss.
The Okumura model is based on the measurements made in Tokyo in 1960, between 200 to 1920 MHz. The measured values are used to determine the median field strength and numerous correction factors. The correction factors include adjustment to the degree of urbanization, terrain roughness, base station antenna height, mobile antenna height and localized obstruction. The Okumura model is especially applicable in urban area for general coverage calculation where numerous obstructions and buildings exist.
The Cost 231 Model, also called the Hata model PCS extension, is used in most commercial RF planning tools for mobile telephony. The coverage of the Cost 231 model is frequency between 1500 to 2000 MHz, transmitter effective antenna height between 30 to 200 m, receiver effective antenna height between 1 to 10 m and link distance from 1 to 20 km. The Cost 231 model is restricted to application where the base station antenna is above adjacent roof tops.
The Egli model is a simplified model based on empirical match of measured data to mathematical formula. Its ease of implementation makes it a popular choice for use in the first analysis. It assumes gentle rolling hill height of approximately 50 feet and no terrain elevation data between the transmitter and receiver is needed for the model. The median path loss is adjusted for the height of transmit and receive antenna above ground. The model consists of a single equation for the propagation loss.
ITU terrain model is based on diffraction theory that provides a method to predict median path loss. The model predicts path loss as a function of the height of path blockage and the first Fresnel zone for the transmission link. The model is ideal for modeling line-of-sight link in any terrain and is good for any frequency and path length. The model accounts for obstructions in the middle of the communication link, hence it is suitable to be used both inside cities and open fields. The model is considered valid for losses above 15 dB.