Outdoor propagation models involve estimation of propagation loss over irregular terrains such as mountainous regions, simple curved earth profile, etc., with obstacles like trees and buildings. All such models predict the received signal strength at a particular distance or on a small sector. These models vary in approach, accuracy and complexity. Hata Okumura model is one such model.
In 1986, Yoshihisa Okumura traveled around Tokyo city and made measurements for the signal attenuation from base station to mobile station. He came up with a set of curves which gave the median attenuation relative to free space path loss. Okumura came up with three set of data for three scenarios: open area, urban area and sub-urban area. Since this was one of the very first model developed for cellular propagation environment, there exist other difficulties and concerns related to the applicability of the model. Okumura model can be adopted for computer simulations by digitizing those curves provided by Okumura and using them in the form of look-up-tables [1]. Since it is based on empirical studies, the validity of parameters is limited in range. The parameter values outside the range can be obtained by extrapolating the curves. There are also concerns related to the calculation of effective antenna height. Thus every RF modeling tool incorporates its own interpretations and adjustments when it comes to implementing Okumura model.
Hata, in 1980, came up with closed form expressions based on curve fitting of Okumura models. It is the most referred macroscopic propagation model. He extended the Okumura models to include effects due to diffraction, reflection and scattering of transmitted signals by the surrounding structures in a city environment.
The generic closed form expression for path loss (PL) in dB scale, is given by
where, the Tx-Rx separation distance (d) is specified in kilometers (valid range 1 km to 20 Km). The factors A,B,C depend on the frequency of transmission, antenna heights and the type of environment, as given next.
● fc = frequency of transmission in MHz, valid range – 150 MHz to 1500 MHz
● hb= effective height of transmitting base station antenna in meters, valid range 30 m to 200 m
● hm=effective receiving mobile device antenna height in meters, valid range 1m to 10 m
● a(hm) = mobile antenna height correction factor that depends on the environment (refer table below)
● C = a factor used to correct the formulas for open rural and suburban areas (refer table below)
The function to simulate Hata-Okumura model is given in the book – Wireless Communication Systems using Matlab. The simulated path loss in three types of environments are plotted in Figure 1. The simulated results are obtained over a range of distances for the following parameter values fc=1500 MHz, hb=70 m and hm=1.5 m.
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References
Topic in this chapter
- Introduction to Large scale propagation models
- Friis free space propagation model
- Log distance path loss model
- Two ray ground reflection model
- Modeling diffraction loss
- Hata Okumura model for outdoor propagation
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