LTE UPLINK POWER CONTROL AND BASE STATION ANTENNA DOWN TILT IN A 3D CHANNEL MODEL Xiaojia Lu 1 , Esa Kunnari 1 , Jouko Leinonen 1 , Olli Piirainen 2 , Markku Vainikka 2 , Markku Juntti 1 1 Centre for Wireless Communications, 2 Nokia Siemens Networks 1 P.O. Box 4500 FI-90014 University of Oulu, 2 Kaapelitie 4, 90630 Oulu Finland {xiaojia.lu, esa.kunnari, jouko.leinonen, markku.juntti}@ee.oulu.fi, {olli.piirainen, markku.j.vainikka}@nsn.com ABSTRACT This paper examines the impact of the uplink power control and base station (BS) antenna down tilt on the system level performance in a realistic multicell three dimensional chan- nel model. The 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) adopts single carrier frequency division multiple access (SC-FDMA) as an air interface. The intra-cell interference is thus avoided but the inter-cell interference is still an issue. The system level performance depends not only on the transmitted power but also on the inter-cell interference level. Uplink power control scheme is used to limit the mobile station (MS) transmit power in order to reduce the interference. To evaluate the system per- formance, a realistic propagation environment is essential. We extend the IST WINNER II channel model (WIM II) in the elevation domain and includ the elevation beam pattern. We show mathematical expressions of link loss related to the BS-MS distance, down tilt angle, link loss, beam gain, and the effect of MS open-loop-controlled power. Based on the 3D channel model, the impact of BS antenna down tilt angle effect along with open loop power control scheme is studied. I. I NTRODUCTION In Long Term Evolution (LTE) Release 8 and LTE Ad- vanced (LTE-A), single carrier frequency division multiple access (SC-FDMA) with frequency reuse factor 1 is chosen in air interface. The data will be transmitted and received in frequency/time blocks. Though the scheduling makes sure no intra-cell interference, the inter-cell interference still ex- ists. The uplink transmit power control (PC) [1, 2, 3, 4] is used to limit the maximum transmission power of a user equipment (UE) if the signal to interference-plus-noise ra- tio (SINR) of UE is large. The open loop fractional power control (OLPC) is already standardized in the 3GPP [2]. The OLPC compensates the fractional path loss that a user experiences until a preferred receive SINR value is reached. Antenna tilt is another effective inter-cell interference re- duction method [5, 6]. The basic idea is to mechanically tilt the antenna plane or shift the phases of a baseband signal over vertical antenna elements. The former one is normally used for directional antennas and the latter one for omnidi- rectional antennas. By tilting the antenna array plane down by a certain angle towards the terrain, the served cell can be covered by the peak of the main beam while neighbor cells are interfered by the side lobes. This not only increases the link power but also decreases the interference to other cells. The system performance with different down tilt angles is studied in this paper with the OLPC scheme. The channel modelings have mainly focused on two di- mensions, meaning that BSs and UEs are assumed to be in a horizontal plane. The recent 3GPP spatial channel model (SCM) [7] considered a cross-polarized two-dimensional (2D) model and WINNER II channel model (WIM II) [8] took an elevation beam pattern into account. Narandzic et al. [9] considered 3D antenna array modeling based on SCM/WIM channel models. The 3D channel gives an ex- tra spatial degree of freedom in the vertical domain, which means that the users are not only horizontally but also verti- cally separated. In [10, 11], the impact of an elevation angle on MIMO capacity is studied. In [12], the performance of OLPC was studied but the elevation gain was not considered. The performance of OLPC and closed loop power control are compared in a 2D network in [4]. In [6], the impact of a down tilted BS antenna was evaluated in the SCM channel model. In [13], the OLPC along with the down tile angle for LTE UL was studied, but the path loss model used therein was not real- istic enough to evaluate the large multicell scenario. The scheduling scheme therein was also different to ours. In this paper, the impact of three-dimensional (3D) ra- dio propagation on the power control and the BS down tilt angle will be studied. In comparison with an isotropic ele- vation pattern, an elevation beam gain is attained. Further- more, this paper also evaluates the LTE UL system level performance in the 3D channel model. The OLPC for UL is evaluated jointly with the BS antenna down tilting in a 3D channel model. The remainder of the paper is organized as follows. In Section II, we present the 3D channel model. In Section III, the OLPC scheme is introduced. The joint consideration of OLPC, BS antenna down tilt and 3D beam gain is analyzed in Section IV. The simulation assumptions and results are presented in Section V, and, finally, in Section VI, the paper is concluded. II. 3D CHANNEL MODEL The 3D channel model is based on the WIM II channel model which is a geometry based stochastic model [8]. The 2010 European Wireless Conference 978-1-4244-6001-4/10/$26.00 ©2010 IEEE 377