RF Propagation Pathloss Model

Introduction

What is the need of RF Propagation Model?

In wireless communication the transmitted signal propagates in space i.e the radio signal. Wireless communication system is the most widely used communication system now a days and communication took place in such system with the help of radiowave in space. Thus a detailed analysis and study of radiowave propagation is required and it is a very important aspect before we finally design and implement a wireless communication system.

When we talk about the communication through a wired line medium such as RF Coaxial cable or twisted pair cable or optical fiber provides reliable communication with minimum pathloss as signal is well secured in it from the atmospheric distortions. Though the laying and maintaining of the wired medium takes time and is cost effective as well but it is reliable means of communication. on the other hand communication in such medium i.e. in wireless communication is quite unreliable and unsecured. In wireless communication a limited operating frequency bandwidth is available for communication as it has many other applications of radiowave propagation in space.

Every communication system is designed with some specific objectives of providing continuous communication services to mobile subscribers that are present in the service area i.e. the area upto which the operator will provide communication services. But as we studied earlier a good quality service can only be obtained if loss of transmitted signal will be minimum and maximum signal will reach at receiver end.

The design can be optimized only when the parameters under which a system has to operate are fully understood whether the frequency band allocated or the height of antenna, distance between transmitter and receiver, and the most important is the pathloss and an estimation of received signal level at the receiver end. If proper radio analysis is done than it will be of great use to design a network and implement it in hardware level with proper and desired results which are expected from a system.

The most important design parameter in mobile communication system is to achieve the desired signal coverage as the basic purpose of operator is to serve the subscribers in a specified service area. Whenever a system is designed it is designed for some specific aims and that is to provide a better service to subscribers with in the range for which the services are introduced but if we directly implement the system without checking it whether the system is giving desired output or not and directly implement it than if it is not performing according to ones expectation the system will be wasted and soon our efforts so it is the most important factor to check and calculate the pathloss.

Classification of RF Propagation Pathloss model

Different models were developed which are empirical in nature that means they are developed based on the collection of data in specific areas. Data must be sufficient enough so as to study the losses that may happen in any specific area, so that on the basis of that data one can come to a conclusion as propagation models does not tell the exact pathloss that may happen but it will tell the most likely behavior of the link under some defined conditions.

Mainly RF Propagation Models can be classified as below:

Basically RF Propagation models can be classified into three main categories which can be classified as above defined.

• Empirical models
• Semi-deterministic models
• Deterministic models

The Basic classification of RF Propagation Pathloss Models can be seen in the above figure 3.1.

Three kinds of models

Figure 3.1 Classification of RF Propagation Pathloss Model

Empirical models : based on measurement data, simple (few parameters),

use statistical properties, not very accurate.

Semi-deterministic models : based on empirical models + deterministic aspects

Deterministic models : site-specific, require enormous number of geometry information about the cite, very important computational effort, accurate.

Every Model is defined for a different and a specific environment and for different cells. Cells in a mobile communication is nothing but a geographical area covered by a telephone transmitter. cells can also be divided into three main types and they are as follows:

• Macrocell

• Microcell

• Picocell

Macrocell – The area of largest coverage. Macrocells are used to described the widest range of cell sizes.

Example : An Entire City.

It covers the area upto 1-5 km long.

Microcell– The area of intermediate coverage. Mainly the range of microcell is less than two kilometers.

It covers the area upto 100- 1 kilo meters.

Pico cell – A picocell is a small cellular base station which covers a small station such as in buildings, offices, shopping malls etc.

It covers the area upto 500 meters.

From the above figure we can understand the concept and coverage area covered by a macrocell, microcell and picocell.

Representation of macrocell, microcell, picocell

A table is shown where different cells and the area covered by those cells i.e Macrocell, Microcell, and picocell is shown in table 3.1.3.

Table 3.1.3 Different cell sites and area they covered.

Different RF Propagation Pathloss models are defined for different cells which can better perform on different cells and better results can be obtained. Now the cells and the models defined for it will be described.

Okumura-Hata model

It is the most popular model which is based on the empirical information obtained from measurements in japan.(okumura 1965). The okumura Hata predicts the Pathloss L between the transmitter and receiver taking into considering the frequency, the distance between transmitter and receiver, height of antenna, terrain type.this model fits best for urban and suburban areas.

General formula

The general formula for the path loss L according to the Okumura-Hata model is:

L = a0 + a4·logf – a2·loghb – a·hm + [a1 – a3·loghb]·logd – Lc,

where a0, a1, a2, a3, and a4 are adjustable parameters

Specific formulas

The path loss L can be found by the standard path loss formulas for GSM-900, GSM-1800, and GSM-1900 according to the Okumara-Hata model.

GSM–900 formula

The path loss formula for GSM-900 is:

L = 69.55 + 26.16·logf – 13.82·logh_t – a·hm + [44.9 – 6.55·logh_t]·logd – L_c.

GSM–1800/1900 formula

The path loss formula for GSM-1800/1900 is:

L = 46.3 + 33.9·logf – 13.82·logh_t – a·hm + [44.9 – 6.55·logh_t]·logd -L_c

Where,

L= Pathloss

f_c=frequency of transmission

h_t=antenna height

a_r=correction factor for effective mobile antenna height which depends on the size of the in area

Lc=correction factor for different terrains.

Correction factor for different clutter classes is determined according to okumura hata model and following is the table 3.1.4 which shows the clutter classes and correction factor for it.

Table 3.1.4 Correction factor defined according to okumura Hata Model

Cost 231 Hata Propagation Model

A Model that is widely used for predicting pathloss in wireless system operating from 500 MHz to 2000 MHz is the cost 231 Hata Model. It contains correction factor for Urban, suburban and open rural environment. The general expression for calculating pathloss in the urban area as given by COST 231 Hata model is:

L_pch(urban)db)=46.3 + 33.9logf_c – 13.82logh_t – a_r + (44.9-6.55logh_t)logr + C_m

where, f_c=frequency of transmission

h_t=antenna height

a_r=correction factor for effective mobile antenna height which depends on the size of the coverage area.

C_m =3db for urban environments and 0db for suburban or open environment

Walfisch-Ikegami model

The Walfisch-Ikegami model is more recent than the Okumura-Hata model. It is based upon the deterministic models of Walfisch, Ikegami, and Bertoni.

It works in 2 cases : LOS and NLOS

LOS (Line of sight)

LLOS [dB] = 42.6 + 26 log10 d[km] + 20 log10 f [MHz]

NLOS (Non line of sight)

LNLOS [dB] = LFS + Lrts (wr, f, ΔhMobile , Φ ) + LMSD (ΔhBase, hBase, d, f, bS )

LFS = free space path loss = 32.4 + 20 log10 d[km] + 20 log10 f [MHz]

Lrts= roof-to-street loss

LMSD= multi-diffraction loss

But it has some restrictions which are as follows:

• Frequency f between 800 MHz and 2000 Mhz
• TX height hBase between 4 and 50 m
• RX height hMobile between 1 and 3 m
• TX – RX distance d between 0.02 and 5 km

Longley – Rice Propagation Model

The Longley – Rice Propagation model also known as irregular terrain model. It basically calculates large scale median propagation loss relative to free space loss propagation loss over irregular terrain. It is mainly applicable for point to point wireless communication system.

It operates in the frequency range from 40 MHz to 100 GHz.

It is mainly used for frequency planning in television broadcasting.

The modified model can also be used for radio wave propagation in urban areas for mobile communication.

The Longley – Rice propagation models has two parts: model

Area to area prediction model.

Point to point prediction model.

The area to area is used when path profile is not given but point ot point is used when detailed path profile is given. But, the main drawback of this model is that it doesnot consider the effect of multipath, foliage, buildings and other environment factors.

Two Ray Point to Point Propagation model

The point to point propagation model is a basic system design tool that is used to generate a signal coverage map, an interference area map, or a handoff occurrence map. In many mobile communication system the maximum distance between the cellsite and the mobile is at the most only a few tens of kilometers and surface is assumed flat. A simple two ray model can be used to predict pathloss and received signal strength.

L_pch = 40 log r – 20 log h_t – 20 log h_r

Thus it implies that the propagation pathloss in a mobile communication L_pch increases by 40dB for every increase in distance by 10 times. If the cellsite antenna height is doubled, there will be reduction in pathloss by 6dB.

The two ray model is found to be reasonably accurate for predicting the large scale received signal strength over distances of several kilometers for mobile radio communication system that use tall cellsite towers and los microcell application in urban area.