Propagation Path loss Models for LTE-Advanced Urban Relaying Systems

Chu Quang Hien*, Jean-Marc Conrat Wireless Engineering Propagation

Orange Labs Belfort, France

{quanghien.chu, jeanmarc.conrat}@orange-ftgroup.com

Jean-Christophe Cousin Communication and Electronics Department

Télécom ParisTech Paris, France

cousin@telecom-paristech.fr

Abstract—Relaying technology - a key technical enhancement in 3GPP LTE-Advanced (3rd Generation Partnership Project Long Term Evolution - Advanced) - has attracted much research interest over the last few years. However, propagation path loss models dedicated to relaying systems are yet to be properly validated. Moreover, the impact of relay antenna height on propagation channel has not yet been adequately studied. This paper presents a measurement-based study which aims to analyze the impact of relay antenna height on path loss channel models. The analysis results demonstrate that BS-RS (Base Station – Relay Station) link path loss decreases with the increase in relay antenna height. Moreover, different propagation models which could be applied for BS-RS link are verified. The analyses show that these models are not completely adequate for relaying deployment. Finally, a new propagation model for relaying deployment which takes into account the impact of relay antenna height is proposed.

Keywords- relay; LTE-Advanced; propagation; channel model; pathloss; antenna height

I. INTRODUCTION Enhanced capacity and extended coverage with increased

cell edge bit rates are among the desired requirements for 4G wireless communication systems. Many technology components are being developed in order to meet these requirements. Among these, relay appears as a promising solution.

Relays are expected to be deployed in various scenarios from improving the reliability of cellular networks to achieving high throughput in cell-edge area or to providing coverage extension [1-3]. Reliable propagation channel models are necessary in order to design such systems. To this end, a number of propagation models have recently been suggested by 3GPP [3], WINNER (Wireless World Initiative New Radio) project [4] and IEEE 802.16j task group [5]. However, these models are basically derived from previous work and their validation in a realistic environment is yet to be fully examined with focus on relay antenna height.

The antenna height is a practical deployment parameter which has been actively studied for many years. However, these studies concentrate principally on the impact of BS antenna height on BS-MS (Base Station – Mobile Station) link [6, 7]. To the author's knowledge, very few studies concern the

link BS-RS. Webb et al. [8] presents a measurement-based study related to BS-RS link. This study proposes an antenna height correction factor modelled by the log-linear function. Nevertheless, the relay antenna height varies only in the range from 2 m to 5 m with 1 m steps. These heights are considered to be low compared to proposed relaying scenarios [3, 4]. Reference [9] demonstrates a related work in which the path loss predicted by different models with two levels of receive antenna height is examined. The analysis result of this paper shows a 10 – 20 dB path loss reduction when the receive antenna is raised from 6 m to 10 m. However, this study misses some path loss models designed for relaying systems such as those proposed by 3GPP and WINNER. Another research work concerning the impact of receive antenna height on path loss is to be found in [10]. However, this study concentrates principally on the problem of outdoor-to-indoor penetration.

The objectives of this paper are to study the impact of RS antenna height on propagation channels and to examine the validation of different existing channel models in a realistic relaying deployment. Only the BS-RS link is focused within the scope of this paper. To this end, a measurement campaign with relay was conducted in an urban macro-cellular environment. The dependence of BS-RS link path loss on relay antenna height is analysed. The validation of WINNER model [4], IEEE 802.16j [5] model, 3GPP Relay model [3] and COST-231 Walfish-Ikegami model [11] is examined by comparing their path loss predictions with measurements. Finally, a new simple propagation channel model defined for BS-RS link which takes into account the relay antenna height is proposed.

II. MEASUREMENT CAMPAIGN The measurements were performed in the city centre of

Belfort, France. The measurement environment is a typical urban medium city with 3-5 story-buildings and almost grid-like street layout. The average building height and street width are about 15-20 meters and 7-12 meters, respectively.

The BS was mounted on the roof of a 20-meter high building (Fig. 1). The BS transmitted a narrow band signal at frequency 2.1 GHz. The BS utilized a sectorial antenna with a beamwidth of 90° in azimuth and 10° in elevation. The maximum antenna gain is 12 dBi. The transmitted power was set up at 43 dBm.

2797978-1-4244-9561-0/11/$26.00 ©2011 IEEE AP-S/URSI 2011

Figure 1. Photo of BS (left) and RS with 3 antenna heights (right)

BS locationRS locations

1000 m

BS locationRS locations

1000 m

Figure 2. RS locations in the measurement area

An equipment set was installed in a van to simulate the Relay Station (RS). An omnidirectional dipole antenna was employed at RS. The RS antenna mast can be raised so that the height varies between 4-13 meters (Fig. 1).

For each RS location, the received power was measured when RS antenna mast was set at 4.7 m, 8.8 m and 12.7 m respectively (Fig. 1). These heights are below the roof-top compared to surrounding buildings. Therefore, the BS-RS link was in Non Line-of-sigh (NLOS) condition at most of the RS locations. During the measurement, the RS antenna mast was rotated around its vertical axis. This rotation was to permit measuring the local variation of received signal.

The measurements were performed at a total number of 77 RS locations distributed mainly within the BS-RS radius from 200 m to 1000 m (Fig. 2). The BS antenna orientation in the measurement area is indicated by the yellow arrow.

III. IMPACT OF RS ANTENNA HEIGHT ON BS-RS PATH LOSS The BS-RS link path loss with each RS antenna height is

calculated by:

)()()()()( dBLdBiGdBmPdBmEIRPdBPL cablerr −+−= (1)

where EIRP(dBm) represents the effective radiated transmitted power, Pr(dBm) and Gr(dBi) are respectively the RS received power and the RS antenna gain, Lcable(dB) denotes the cable losses.

For each RS location, the calculated path losses with 3 antenna heights were compared to each other. The Empirical Cumulative Distribution Function (CDF) of path loss calculated with all RS locations and RS antenna heights are presented in Fig. 3. This figure clearly shows the dependence of path loss on relay antenna height, in which the higher antenna height provides the smaller path loss (observed in 70/77 RS locations). This trend is explained by the measurement environment in which the transmitted electromagnetic waves propagate over the roof-top. When the RS antenna height is high and approaches the roof level i.e. LOS condition, the received signal is less attenuated by the diffraction or reflection caused by surrounding buildings.

The path loss differences between different antenna heights are described quantitatively by the histograms in Fig. 4. The analysis shows that the BS-RS link path loss decreases with an average value of 3.5 dB when the RS antenna is raised from 4.7 m to 8.8 m (Fig. 4a). Similarly, an average path loss reduction of 4.6 dB is observed when the antenna height is changed from 8.8 m to 12.7 m (Fig. 4b). These measured path loss differences are in agreement with the results found in previous studies [12]. Moreover, Fig. 4c confirms that path loss is always smaller in every RS locations when RS antenna height is raised from the lowest position (4.7 m) to the highest position (12.7 m). The average reduction is about 8 dB.

80 90 100 110 120 130 140 1500

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0.6

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1

x = pathloss (dB)

F(x

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RS height = 4.7 mRS height = 8.8 mRS height = 12.7 m

Figure 3. The dependence of BS-RS path loss on RS antenna height.

-10 -5 0 50

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c)-20 -15 -10 -5 00

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hS(12.7m)-RS(4.7m)

Figure 4. Distribution of Path loss difference between RS antenna heights

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IV. BS-RS LINK PATH LOSS MODELS This section describes briefly a number of existing

propagation channel models which could be applied for relaying scenarios. Their validation will be then examined with measurement data.

A. WINNER B5f model The WINNER project developed a new scalable and

flexible radio interface for beyond 3G broadband communication systems based on ITU-R requirements. Many technical components of WINNER have been contributed to LTE-Advanced and WiMAX. Among the proposed path loss models for relaying systems, the B5f model is appropriate to the measurement environment presented in this study. The B5f models the BS-RS link in NLOS condition and for urban areas. This path loss model is given by [4]:

)5(log2357)(log5.23 1010fdPLWINNER ++= (2)

where f [GHz] is the system frequency (2 GHz <f<5 GHz), d [m] is the distance between Tx and Rx (30m< d < 1.5km).

B. 3GPP models In [3], 3GPP proposes a path loss model which is dedicated

to NLOS urban BS-RS link in frequency band around 2 GHz. This model is described by:

)(log3.362.125 10Re3 dPL layGPP += (3)

Here, d [Km] is the Tx-Rx distance.

C. IEEE 802.16j model IEEE Relay Task Group 802.16j is working actively to

develop the technical standards for fixed wireless access systems. A path loss model has been proposed for BS-RS link in NLOS urban environment. The basic path loss equation with correction factors is given by [5]:

sPLPLddAPL hfj +Δ+Δ++=

016.802 10γ (4)

where d0 = 100 m and d [m] is the BS-S distance (d>d0). The other parameters A, , PLh, s are respectively the propagation loss at d0, correction factor for BS antenna height, correction factor for operating frequency and log-normal shadow fading component in dB.

Particularly, this model provides a correction factor for RS antenna height which is given by:

)3(log20 10RS

hhPL −=Δ (5)

where hRS [m] is the RS antenna height (hRS>3m).

D. COST-231 Walfish-Ikegami model The COST-231 Walfisch-Ikegami (WI) model [11] was not

initially designed for relaying systems. However, this model is known for its coherent prediction with extensive experimental

data for flat urban areas with uniform building height. In the NLOS case, the basic transmission loss is composed of the free space loss L0, the multiple screen diffraction loss Lmsd and the roof-top-to-street diffraction and scatter loss Lrts.

≤+>+++

=00,

0

0

msdrts

msdrtsmsdrtsb LLL

LLLLLL (6)

In (6), Lmsd depends on propagation distance d [Km], frequency f [MHz], average building separation b [m] and the relation hBS= hBS – hRoof, where hBS [m] is the base station antenna height and hRoof [m] is the average building height.

The term Lrts is defined as a function of street width w [m], the loss related to the road orientation LOri and the relation

hMS= hRoof - hMS where hMS [m] is the mobile antenna height. The correction factor for receive antenna height is given by log10(hRoof – hMS).

V. PATH LOSS COMPARISON BETWEEN MODEL PREDICTIONS AND MEASUREMENTS

The measured path loss will be compared against those predicted by aforementioned path loss models. The comparison results presented in Fig. 5 show that all of three models WINNER B5f, IEEE 802.16j and 3GPP Relay obviously underestimate path loss with all RS locations and RS antenna heights. Especially, the mismatch between 802.16j estimation and the measured data is very pronounced and approximately 20 dB. This result is unexpected since the 802.16j model was designed for urban relaying cellular deployment.

In the case of WINNER B5f, its non-compatibility is explained by the analytical method used to develop it. Reference [4] shows that it is derived from the model B5a designed for BS-RS link in LOS condition by artificially attenuating the correction factor by 15 dB. However, the distance dependence parameter which should not be identical in LOS and NLOS conditions is unchanged in the two models. Moreover, it is reported from [12] that B5f model is more appropriate to a mixed condition between LOS and NLOS rather than a complete NLOS condition as in the measurement environment presented in this study.

0 200 400 600 800 1000 1200 140050

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Pat

h lo

ss (

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hRS = 4.7 m - MeasurementhRS = 8.8 m - MeasurementhRS = 12.7 m - MeasurementhRS = 4.7 m - 802.16jhRS = 8.8 m - 802.16jhRS = 12.7 m - 802.16jWINNER B5f3GPP RelayhRS = 4.7 m - COST231 WIhRS = 8.8 m - COST231 WIhRS = 12.7 m - COST231 WI

Figure 5. Comparison between model predictions and measurements.

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Path loss prediction of COST-231 WI model is given with street width w = 10 m, average building height hRoof = 15 m and average building separation b = 20 m. From Fig. 5, it is observed that COST-231 WI prediction provides the closest agreement with the measurement although this model is neither designed for relaying systems nor formally specified to use at frequencies beyond 2 GHz. Moreover, the analysis shows that correction factor for different antenna heights provided by COST-231 WI is coherent with the analysis result found in Section III. The disadvantage of COST-231 WI model is that it requires detailed knowledge of propagation environment. Besides, its performance becomes poor if the terrain is not flat or the land cover is inhomogeneous [11].

COST-231 WI model defines many parameters and the relation among them. This may make it complex to be applied in certain practical cases, especially for engineering work. Therefore, in order to provide a simple propagation model which is capable of modeling the BS-RS link path loss in 2.1 GHz frequency band while taking into account the impact of relay antenna height, a new channel model has been proposed.

This model is given by:

)20(log265)(log34)( 1010 RShddBPL −++= (7)

where d [m] is distance between BS and RS, hRS [m] is the RS antenna height. The model is valid for the BS-RS link in NLOS condition and with d in the range from 20m to 1000 m. The RS antenna height is limited up to 15 m which is suitable for typical relaying deployments.

The model performance is illustrated in Fig. 6 and prediction error statistics are present in Table I. The standard deviations are from 6 to 7 dB for 3 RS antenna heights.

TABLE I. PREDICTION ERROR STATISTICS WITH PROPOSED MODEL

4.7 m 8.8 m 12.7 m Average 0.33 dB 0.17 dB 0.30 dB

Standard deviation 6.76 dB 7.01 dB 7.06 dB

VI. CONCLUSIONS This study analyzed the impact of relay antenna height on

LTE-Advanced channel modeling and focused on the BS-RS path loss models. The BS-RS link path loss was measured with 3 RS antenna heights at different RS locations in an urban area. The propagation condition is NLOS in most considered cases. The analysis results exhibit the dependence of path loss on RS antenna height in which higher RS antenna height provides smaller path loss. Moreover, various existing propagation models for relaying systems were validated with measurement data. The analysis results show that the COST-231 Walfish-Ikegami presents the closest prediction in comparison with other models. A simple empirical propagation model which considers the RS antenna height impact was also proposed for BS-RS link operating in 2.1 GHz frequency band. The analysis result indicates that the prediction of this model matches the measurement data. The error standard deviations are in the range of 6-7 dB.

0 200 400 600 800 1000 1200 140070

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hRS = 4.7 m - measurementhRS = 8.8 m - measurementhRS = 12.7 m - measurementhRS = 4.7 m - new modelhRS = 8.8 m - new modelhRS = 12.7 m - new model

Figure 6. Comparison between the prediction of proposed model and measurements

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