Comparison of Remote Electrical and Mechanical Antenna DownTilt Performance for 3gpp LTE

Comparison of Remote Electrical and Mechanical Antenna Downtilt Performance for 3GPP LTE Osman N. C. Yilmaz, Seppo Hämäläinen

Jyri Hämäläinen

Research, Technology and Platforms Nokia Siemens Networks Espoo, Finland [email protected], [email protected]

Department of Communications and Networking Helsinki University of Technology Espoo, Finland [email protected]

Abstract—The aim of this paper is to evaluate and compare impacts of mechanical and electrical downtilt angles of antenna systems to the downlink performance of the 3GPP Long Term Evolution (LTE) networks. Cell coverage and capacity in different macro-cellular network scenarios for various combinations of antenna parameter configurations have been studied in terms of signal to interference plus noise ratio (SINR) performance. Simulation studies are performed for uniform UE distribution and full-buffer traffic by using LTE snap-shot simulator modeling electrical and mechanical antenna tilt in details and propagation in three dimensions. Keywords; LTE, Remote Electrical Antenna Tilt, Mechanical Antenna Downtilt, Antenna Modeling, Adaptive Antenna Solutions.

I.

INTRODUCTION

The Long Term Evolution (LTE) is a new air interface designed by the Third Generation Partnership Project (3GPP). [1]. 3GPP work on the evolution of the 3G mobile system is aimed to achieve additional substantial leaps in terms of service provisioning and cost reduction. As a basis for this work, 3GPP has concluded a set of targets and requirements for this longterm evolution: LTE will admit higher peak data rates and more users per cell as well as lower control plane latency than currently employed 3G technologies. Also network architecture will be simplified so that networks can meet requirements set by anticipated explosion of data traffic volume, quest for low latency and hopes of simple, cost effective operation. Radio technology is based on Orthogonal Frequency Division Multiple Access (OFDMA) and it applies sophisticated scheduling and multi-antenna methods. 3G LTE is further developed to meet requirements set for IMT-Advanced technologies. Currently discussion on the role of antenna parameter selection in evolution of 3G LTE (LTEAdvanced) is going on. Due to that reason we have adopted 3G LTE framework for simulations and antenna parameter investigations [4]. From network operator’s point of view optimized antenna parameter selection is an economical and attractive way to increase the network performance in terms of coverage and capacity. In this paper, 3G LTE network performance impacts of electrical and mechanical tilt techniques are evaluated. Antenna tilt is defined as the angle of the main beam of the

antenna below the horizontal plane. Positive and negative angles are also referred to as downtilt and uptilt respectively [2]. Antenna downtilt can be adjusted mechanically and/or electrically. There are different existing techniques for electrical tilt such as remote electrical tilt (RET), variable electrical-tilt (VET) and fixed electrical tilt. Usage of RET antennas removes the need for tower climb and base station site visits by controlling tilt angle via network management system (NMS) so that operational cost is saved. Hence, remote electrical tilt has become more popular for network’s operators e.g. adjusting tilt angle when eNB insertion or deletion occurs. On the other hand, mechanical downtilt is also needed because electrical tilt range is limited compared to mechanical tilt’s. In earlier work, network implications due to remote electrical antenna downtilt [6, 7] and mechanical antenna downtilt [6, 8] have been investigated. In [6], capacity gain due to electrical antenna downtilt was shown to be up to 48.4% in scenario with 1.5 km inter-site distance and 0° tilt angle. As discussed in [6, 8] mechanical antenna downtilt also improves network performance and can provide capacity gain up to 45.6% for 1.5 km inter-site distance and 0° tilt angle scenario. It is noteworthy that previous studies have not examined gains due to electrical downtilt and mechanical downtilt comprehensively for 3G LTE. Here we fill this gap and compare the performance impacts of electrical and mechanical antenna downtilts for 3G LTE by means of network simulation. II.

PERFORMANCE EVALUATION METHODOLOGY

The proposed evaluation methodology aims to identify scenarios where antenna parameters lead to show a remarkable SINR increase with respect to other antenna parameter adjustments. A. Simulation Assumptions Evaluations are performed by using a static snap-shot network level LTE simulator that models antennas in details and propagation in three dimensions. Parameters and assumptions follow those selected by 3GPP for performance evaluations of LTE-Advanced technologies [5]. We note that the impact of antenna parameters can be investigated either from network capacity or coverage perspective. Simulations are carried out for two different inter-

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site distance (ISD) scenarios based on 3GPP simulation assumptions as shown in Table 1. TABLE I.

3GPP Case 1

Network layout System frequency System bandwidth Number of PRBs Frequency reuse factor Inter-site distance BTS height UE height Shadowing STD Shadowing Correlation Shadowing Corr. Distance Propagation loss model

3GPP Case 3

Total network elements: 91 eNBs, 273 cells Monitored nework elements: 19 eNBs, 57 cells 2000 MHz 10 MHz 50 1 500 m 1732 m 32 m 1.5 m 8 dB 0.5 (sites), 1 (sectors) 50 m L = 128.1 + 37.6 log10 ( R ) , R in kilometers

Penetration loss TX power Antenna Techniques Horizontal HPBW

20 dB 46 dBm SISO, 1x2 MRC, 2x2 MIMO SM & STTD

Vertical HPBW

θ 3dB = 10 o Calculated based on antenna characteristics 2 dB 0 dB 0 dB -121.4 dBm 9 dB Uniform Full-buffer Round-Robin

TX antenna gain TX cable loss RX antenna gain RX body loss Thermal Noise per PRB RX noise figure Traffic Distribution Traffic Model Scheduling

U (ϕ ,θ ) Pin

ϕ3 dB = 70o

Extrapolation of the 3D pattern from two perpendicular cross-sections azimuth and elevation patterns is defined in [5] as shown below: A(ϕ , θ ) = − min{− [AH (ϕ ) + AV (θ )], Am }

The 50%-tile and 5%-tile of the SINR CDF are assumed as performance comparison criterions. These values have been chosen as capacity and coverage indications respectively because 50%-tile represents group of UEs which indicate the average performance of the cell and 5%-tile represents group of UEs with worst channel conditions and located near the cell edge. Furthermore, cell throughput performance is given for some specific antenna parameter configurations to evaluate capacity optimization in details for different network deployments.

Figure 1. Modeling of Horizontal Pattern

B. Modeling of Antenna Radiation Pattern In 3GPP LTE simulations, we apply two formulas below for horizontal and vertical radiation patterns [5], and antenna gain [9] respectively:

⎤ ⎞ ⎟, Am ⎥ , Am = 25dB ⎟ ⎠ ⎦⎥

(1)

⎡ ⎛ θ A(θ ) = − min ⎢12⎜⎜ ⎣⎢ ⎝ θ 3dB

⎤ ⎞ ⎟, SLAv ⎥ , SLAv = 20dB ⎟ ⎠ ⎦⎥

(2)

where Am is the front-to-back attenuation and SLAv is side lobe attenuation .

(4)

What is more, it needs to be considered that typically antenna characteristics are measured in anechoic chambers as shown with blue curves in Figure 1 and Figure 2 [10], whereas in real-world deployment there are significant impacts of scattering in the near field of the antenna (mast, mountings, other objects in the vicinity, such as roof-top, etc.) and diffraction. These near-field scatterers and diffractions are not accounted for by the propagation models; therefore they need conceptually be included in an effective antenna pattern. A basic property of such an effective antenna pattern would be that attenuation of nulls and front-to-back attenuation Am will be reduced as shown in Figure 1 and side lobe attenuation SLAv in Figure 2. It is visible from Figures 1 and 2 that the creation of narrow vertical beam in a practical antenna leads to more severe and strong side lobes than we face in case of more wide horizontal pattern.

a. Simulations assumptions are based on [4, 5].

⎡ ⎛ ϕ A(ϕ ) = − min ⎢12⎜⎜ ⎣⎢ ⎝ ϕ 3dB

(3)

where U is the radiation intensity and total input power Pin .

LTE DL SIMULATION ASSUMPTIONS Simulation Cases

Parameter

G = 4π

Figure 2. Modeling of Vertical Pattern

In the model applied for the simulations, the antenna effects are combined as a sum of antenna gain, horizontal pattern and elevation pattern. The sum of horizontal and vertical patterns is limited for a common front-to-back attenuation Am because it takes inaccuracies of the real world implementation into account and gives more an environmental view of the effect of the antenna pattern and corresponds better to e.g. the limited isolations of co-sited sectors typically found in field measurements. C. Modeling of Mechanical Tilt and Electrical Tilt The angle of the main beam of the antenna below the horizontal plane is called antenna tilt. Positive and negative angles are also referred to as downtilt and up-tilt respectively [2] as shown in Figure 3.

III.

PERFORMANCE EVALUATION RESULTS

3G LTE Network performance is simulated to investigate the effects of different antenna tilt approaches. Various tilt angles for 3GPP case 1 and case 3 are considered. Figure 4 illustrates the difference between mechanical and electrical downtilt in terms of SINR for 3GPP case 1. To emphasize the difference we have set shadow fading to 0dB. Figure 4 reflects the fact that the shape of the antenna gain pattern may admit remarkable change when the mechanical antenna downtilt is applied while the antenna gain pattern keep well its original shape in case of electrical downtilt. On the other hand, since electrical tilt range is limited in practice due to changing sidelobe level, the impact of hybrid approach which includes both mechanical and electrical downtilts with different proportions is investigated to find optimal solution in terms of network performance. Simulation results indicate that optimum downtilt angle depends on the network environment and different environments may lead to different optimization results in terms of capacity and coverage performance.

Figure 3. Mechanical downtilt vs. Electrical downtilt

In electrical downtilt, main, side and back lobes are tilted uniformly by adjusting phases of antenna elements [3]. However, in mechanical downtilt, antenna main lobe is lowered on one side and the antenna back lobe is raised on the other side because antenna elements are physically directed towards ground in mechanical downtilt [2]. Mechanical downtilt angle can be expressed in terms of electrical down-tilt according to [5] as shown in (5):

ζ (i, k ) = arctan(cosθ (i, k ) tan δ (i ))

(5)

where ζ (i, k ) is the corrected downtilt angle in for mobile user k which is served or interfered by cell i in radians with the mechanical downtilt angle δ (i ) in radians, and the horizontal offset angle θ (i, k ) in radians.

Figure 4. Mechanical tilt vs. Electrical tilt in 3GPP case 1 for 0 dB shadow fading

For interference limited 3GPP case 1 where inter-site distance is 500m the electrical downtilt provides better performance both in terms of coverage and capacity as shown in Figures 5 and 6. In this case electrical downtilt can offer around 1 dB better SINR performance if compared to mechanical downtilt.

Angle between main lobe center and line connecting cell i and mobile terminal k in horizontal plane in radians in vertical plane φ (i, k ) is obtained by (6-7) [5]: ⎛ h(i) ⎞ ⎟⎟ ⎝ d (i, k ) ⎠

ω (i, k ) = arctan⎜⎜

φ (i, k ) = ω (i, k ) − ϕ (i )

(6) (7)

where d (i, k ) is distance between mobile user k and cell i , h(i, k ) is the height difference between the antenna of mobile user k and the antenna of cell i , ω (i, k ) is the antennamobile line of sight angle in radians, and ϕ (i ) is the electrical downtilt angle of cell i in radians.

Figure 5. Coverage (5%-tile SINR) performance of mechanical and electrical downtilt for 3GPP case 1.

Figure 6. Capacity (50%-tile SINR) performance of mechanical and electrical downtilt for 3GPP case 1

Figure 8. Coverage (5%-tile SINR) performance of mechanical and electrical downtilt for 3GPP case 3

There is a system throughput difference between mechanical and electrical tilts in 3GPP case 1 as observed from Figure 7. We note that performance difference in Figure 7 is observed when using optimal tilt angles. According to Figures 5 and 6 we can assess that throughput penalty from nonoptimal tilt angles could be large. We also emphasize that gains from downtilt angle optimization are smaller in synthetic test network with uniform inter-site distances than they would be in practical networks where inter-site distances and antenna elevations vary from site to site. Yet, gains in practical networks are always case dependent and therefore investigation of synthetic 3GPP test cases is justified. We note that selfoptimization of remote electrical tilt is needed to reach similar gain in practical networks.

Figure 9. Capacity (50%-tile SINR) performance of mechanical and electrical downtilt for 3GPP case 3

In case of long inter-site distances as in 3GPP Case 3, when system becomes noise limited, the impact of electrical or mechanical downtilt is insignificant in terms of cell throughput as shown in Figure 10.

Figure 7. Cell throughput for 3GPP case 1 when using optimized mechanical and electrical downtilt angles.

In 3GPP case 3, neither electrical downtilt nor mechanical downtilt bring any significant gain in terms of coverage as shown in Figure 8. On the other hand, it is sill possible to provide capacity with notable gain by using either mechanical downtilt or electrical downtilt as observed from Figure 9. Figure 10. Cell throughput for 3GPP case 3 when using optimized mechanical and electrical downtilt angles

IV.

CONCLUSIONS

In this paper performance difference electrical and mechanical antenna tilt in 3G LTE was discussed. System performance was investigated using snapshot simulator with 3D antenna modeling. As a measure the 5%-tile and 50%-tile SINR was used to statistically describe the coverage and capacity performance respectively. System performance results in presence of both mechanical and electrical downtilt were simulated for different downtilt angles. According to the results, electrical downtilt provides better performance in case of interference limited system, while performance difference is insignificant for noise limited cases. Furthermore, optimal downtilt angles in mechanical and electrical tilt techniques are slightly different from each other. It is also worth of noticing that coverage and capacity criteria may lead to slightly different optimal tilt angles in interference limited system with short inter-site distance. V.

FUTURE WORK

Future work includes corresponding simulations for LTE uplink transmission as well as optimization of azimuth as a part of studies for Adaptive Antenna Solutions (AAS). Research area shall further evolve towards autonomous antenna parameter optimization as a part of self-organized network (SON) frame work.

REFERENCES [1] [2]

http://www.3gpp.org/Highlights/LTE/LTE.htm. Iana Siomina, Peter Värbrand, Di Yuan, ‘Automated Optimization of Service Coverage and Base Station Antenna Configuration in UMTS Networks’, Linköping University, Linköping, Sweden. [3] William C. Y. Lee, Mobile Communications Engineering, McGraw-Hill, 1998. [4] 3GPP TR 25.814 V7.1.0, Physical layer aspect for evolved Universal Terrestrial Radio Access (UTRA). [5] 3GPP TS 36.814 V0.4.1, Evolved Universal Terrestrial Radio Access (E-UTRA); Further advancements for E-UTRA Physical layer aspects. [6] Jarno Niemelä, Tero Isotalo, Jukka Lempiäinen, “Optimum Antenna Downtilt Angles for Macrocellular WCDMA Network”, EURASIP Journal on Wireless Communications and Networking 2005:5, 816–827. [7] T. Isotalo, J. Niemelä, and J. Lempiäinen, “Electrical antenna downtilt in UMTS network,” in Proc. 5th European Wireless Conference (EW’04), pp. 265–271, Barcelona, Spain, February 2004. [8] J. Niemelä and J. Lempiäinen, “Impact of mechanical antenna downtilt on performance of WCDMA cellular network,” in Proc. IEEE 59th Vehicular Technology Conference (VTC ’04), vol. 4, pp. 2091–2095, Milan, Italy, May 2004. [9] Constantine A. Balanis, “Antenna Theory”, John Wiley & Sons, 2005. [10] http://www.powerwave.com/antennaproducts.asp.