Performance Analysis of CIR and Path Loss Propagation Models in the Downlink of 3G Systems

This paper analyses the Carrier to Interference Ratio (CIR) and path Loss (PL) variation in downlink 3G FDD-UMTS mobile system. The evaluation was taken in urban, suburban and rural environments. Also, frequency band of 2110 Hz is used in this work. The received CIR analysis is based on comparative study of seven Path Loss propagation models: COST-231 Hata, COST-231 WIM (Walfisch-Ikegami Model), SUI (Stanford University Interim), FSM (Free Space Model), PSM (Standard propagation model), Ecricsson and ECC33 (Electronic Communication Committee). Simulation results show that SUI and SPM models showed the lowest Path Loss for all environments. Also, we can show that received CIR is affected not only by the geometry of the UMTS base station location but also by the number of users presented in each cell.


INTRODUCTION
In the recent years, the using of mobile communication systems has become more and more popular. More than one billion users are expected to use high data rate multimedia services through advanced mobile systems [1]. UMTS (universal Mobile Telecommunication System) technology can support a variety of services and applications with different bit rate and quality of service necessities [2]. Consequently, an efficient management of wireless spectrum is becoming more and more important. In this context, radio wave propagation modeling has been used extensively for an efficient planning and optimization of the downlink UMTS systems.
Path Loss models define the signal attenuation between the transmitting and the receiving antenna. So, path Loss is related to many parameters as the operating frequency, the distance between the sender and the receiver and different blocking obstacles [3]. The propagation environment is an important factor that must be studied in the planning and exploitation of base stations [4]. The propagation Loss is considerably related to the type of covered area (urban, suburban, rural, etc.). So, many types of Path Loss propagation models have been predicted for different area of a Mobile system. Besides, the selection of the suitable radio propagation model can reduce the maximum interference sources, achieve a high throughput and optimize the received Carrier to Interference (CIR) at the UMTS user equipment.
This paper analyses a collection of Path Loss models through a comparative study for the downlink UMTS system. The received CIR parameter is analyzed with the suitable Path Loss propagation model. Two parameters are used in this study: the geometry of the UMTS base station location and the number of users presented in inside each cell.
The paper is organized as follows; the Path Loss propagation models used in downlink UMTS Mobile system is described in Section A. Section B details with the calculation of the carrier to Interference Ratio (CIR) at the receiver UMTS system. The scenario and the downlink UMS model used in this work is defined in Section C. Simulation results and discussion are given in the section D. In Section E, we conclude the paper.

A. Signal propagation and Path Loss models
First, if a signal is transmitted through space between two base stations, it diminishes with the distance. Consequently, the received power being considerably less than the transmitted power. This phenomenon is defined as a propagation loss. The Path Loss propagation diagram between the sender and the receiver is shown in the figure 1.The propagation path between the sender and the receiver can give different scenarios includes line-of-sight (LOS) and Non Line of Sight (NLOS) caused by to diffraction (1), reflecting (2) and scattering (3). The Path Loss is defined by the ratio between transmitted and received power and it is expressed by equation (1) [5]: Where: d = distance between the sender and the transmitter; d0 = the reference point at 1 Km; n = the Path Loss exponent; The base station height 'hb ' range is (30 m -200 m); The receiver antenna height 'hm 'is (1m-10 m); The distance between two antennas 'd ' is from 1km -2 km. The path Loss PL in dB is equal to [7]:

Cost-231 WIM Model
This propagation model describes various areas with different parameters. The cost-231 model has two separate equations for NLOS and LOS line [8].
For Urban environment, the path loss is expressed as [9]: Where: For suburban environment, the path loss is given by [9]: For Rural environment, the path loss is equal to:

SUI (Stanford University Interim) Propagation Model
The propagation model is derived from the HATA model with frequency greater than 1900 MHz. The transmitter antenna height is between 10 m and 80 m and the receiver antenna height is between 2m and 10m. The model defines three terrain categories of terrain, namely A, B and C. The path Loss propagation model is given by [10]: Where: Xf : The correction for frequency in MHz Xh : The correction for receiving the antenna height in meters S: The correction for shadowing in dB;  : The path loss exponent; d0=100 m; a, b and c are constants and they are related to the type of the terrain as defined in the table 1.

SPM (Standard Propagation Model)
The standard propagation model is derived from the formula Hata. It is suitable for frequencies between 150 MHz and 3500 MHz and for long distance between 1 km and 20 km. The Path Loss equation in dB in equal to [12]: Where: A1, A2, A3, B1, B2, and B3: Hata parameters hb defines the transmitter antenna height; Ccluster is the cluster correction function; hm is the receiver antenna height; d is the distance in km; fc is the carrier frequency in MHz.
The Hata parameters of different terrain for SPM models are detailed in the table 2.

ECC33 (Electronic Communication Committee) Model
The ECC-33 propagation model is used for the high frequencies. The path Loss equation for the ECC-33 model is defined as [14]: Where: Afs is the free space attenuation; Ahm is the basic median Path Loss; Gb is the base station height gain; Gr is the received antenna height gain. Then, Afs , Ahm , Gb and Gr are, respectively, defined as: Where: hb defines the transmitter antenna height; hm defines the receiver antenna height; fc is the carrier frequency in MHz; d is the distance in km.

A. CIR and Path Loss propagation
The Carrier To Interference Ratio, CIR, is the most important parameter to evaluate the performance of mobile communication systems.
A sufficient CIR must be guaranteed at the receiver in order to diminish the signal attenuation that occurs with radio propagation. CIR is defined as the ratio between the received wanted carrier signal power and the sum of all received interference power. The CIR is calculated as: Where: C is the carrier power happening in the Mobile receiver; I is the total interference In that takes place in the receiver from others base stations; PN represents the total thermal noise power in the receiver and n is the number of interfering base stations.
In our present work, we will study the downlink UMTS -FDD system (base station sends and mobile station receives) as described in the figure 2. Then, the source of interference will be the other base stations that transmit on the same frequency and the radio signal of which will be received by the studied mobile station.

Fig.2: Intra and Inter cell interferences
The total interference I is composed by two parts: intra-cell interference and inter-cell interference. Intra-cell interference is caused the partial loss of orthogonality between the different codes attributed by the Base station to all users in the same cell. However, the interference inter-cell that is the power received by mobile station from Base station in adjacent cells.
The mathematical model to calculate the intra-cell interference on mobile station, MSi served by the base station, BSj is given by [15]: It is clear that the CIR at the mobile receiver is inversely proportional to the path Loss.

B. Downlink UMTS model
In this paper, the study should focus on the downlink UMTS/FDD system with multi-cell environment. The present model is shown in figure 3. Simulation parameters used in this scenario are given in table 4.
Where: R is the radius of each cell;  is the angle between axis formed by BSj and BSj with MSi.

C. Simulation results and discussion
The measured data was taken in urban, suburban and rural environments at 2100 MHz all propagation models are calculated using MATLAB Software. Figures 5-7 shows the variation in path loss with the distance d respectively for Urban, suburban and rural areas. All studied propagation models are simulated.
So, by observing the path loss variation for urban area in figure 5, it is clear that SPM propagation model have the least path loss and ECC33 propagation model have the highest value in different distances compared with all others propagation models. The simulation results of reviewed models for rural environments are shown in figure 7. IT can be observed that SPM model in rural have the smallest path loss at the same distance compared with results obtained in urban and suburban area. The SPM model is used in figure 8 in order to simulate the variation of CIR with distance for different angles  . It can be observed that the With the smallest distance d, the variation of angle has not effect on the CIR at received signal; however, for long distance, a small angle can increase the interference and will degrade CIR. The best CIR value is taken with an angle of 120° in this case. Figure 9 shows the variation of CIR with the number of user in the same cell. An angle of 120° has been chosen for this simulation The SPM model is used in figure 8 in order to simulate the variation of : CIR variation with the number of user. If we increase the number of user in the same cell, the total interference will be increase. Consequently, the CIR at received system is inversely proportional to the number of user. The interference level is directly related to the user's density in the same cell.

D. Conclusion
The main objective of this research is to analyze the path loss propagation models and CIR effects for urban, suburban and rural environments in FDD/UMTS system. The path loss has been simulated using seven models, COST-231 Hata, COST-231 WIM (Walfisch-Ikegami Model), SUI (Stanford University Interim),FSM (Free Space Model), PSM (Standard propagation model), Ericson and ECC33 (Electronic Communication Committee) models. Path loss values of different models are analyzed and compared in urban, suburban and rural environments at 2110 MHz It can be concluded that SPM model gives better results for all environments. The CIR calculation and simulation with SPM model is detailed for a FDD/UMTS scenario. Simulation results show that the CIR at received system is inversely proportional to the number of user in the cell and it is affected by the position of mobile.