lte uplink power control and base station antenna

2010 European Wireless Conference

LTE UPLINK POWER CONTROL AND BASE STATION ANTENNA DOWN TILT IN A 3D CHANNEL MODEL Xiaojia Lu1 , Esa Kunnari1, Jouko Leinonen1, Olli Piirainen2 , Markku Vainikka2 , Markku Juntti1 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 A BSTRACT 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 channel 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 performance, 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 Advanced (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 exists. 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 ratio (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 reduction 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 omnidirectional antennas. By tilting the antenna array plane down by a certain angle towards the terrain, the served cell can be

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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 dimensions, 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 extra spatial degree of freedom in the vertical domain, which means that the users are not only horizontally but also vertically 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 realistic 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) radio propagation on the power control and the BS down tilt angle will be studied. In comparison with an isotropic elevation pattern, an elevation beam gain is attained. Furthermore, 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 C HANNEL M ODEL

The 3D channel model is based on the WIM II channel model which is a geometry based stochastic model [8]. The

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MIMO channel matrix is given by

beam attenuation pattern 120

H(t, τ ) =

N 

−2

Hn (t; τ ),

(1)

−4

110

n=1

−6 100 Elevation angle

where n is the path index, N is the total number of paths τ is the delay time, t is the time index. Hn (t, τ ) is the channel matrix for cluster n which is expressed as [8]  Hn (t; τ ) = FRx (φ)h(t; τ, φ, ϕ)FT Tx (ϕ)d(φ)d(ϕ), (2)

−8 −10

90

−12 80

−14 −16

70 −18

where FRx (φ) and FTx (ϕ) are the beam gain matrix for received antenna (Rx) and transmit antenna (Tx) on direction φ and ϕ respectively. h(t; τ, φ, ϕ) is the dual-polarized channel response matrix. The channel coefficient from Tx s to Rx element u for cluster n is as follows [8] Hu,s,n (t; τ ) = M   m=1

FRx,u,V (ϕn,m ) FRx,u,H (ϕn,m )



V αV n,m αHV n,m

H αV n,m αHH n,m



FTx,u,V (φn,m ) FTx,u,H (φn,m )



¯ × exp(j2πλ−1 ¯n,m · r¯Rx,u )) exp(j2πλ−1 ¯Tx,s )) 0 (ϕ 0 (φn,m · r × exp(j2πvn,m t)δ(τ − τn,m ),

(3)

where FRx,u,V and FRx,u,H are the field patterns for vertical and horizontal polarizations of antenna element u respecV H and αVn,m are the complex gains of vertical-totively, αVn,m vertical and horizontal-to-vertical polarizations of ray n, m respectively. Parameter λ0 is the wave length of the carrier frequency, ϕ¯n,m is the angle of arrival (AoA) unit vector, φ¯n,m is the angle of departure (AoD) unit vector, r¯Tx,s and r¯Rx,u are the location vectors of elements s and u respectively, and vn,m is the Doppler frequency of ray n, m. If polarization is not considered, the central matrix in the second line of (3) is replaced by a scalar αn,m and only vertically polarized field pattern is considered. More detailed description of the channel model can be found in [8]. The beam pattern gains were adopted from [14]. The ITU-specified azimuth antenna gain (dB) is [14]   θ AA (θ) = − min 12( ), Am , −180◦ ≤ θ ≤ 180◦, θ3dB (4) where θ3dB is the 3dB beamwidth which is 70◦ for 3-sector cell and Am = 20dB is the maximum attenuation. The elevation antenna gain (dB) is given by [14]   ϑ − ϑtilt AE (ϑ) = − min 12( ), Am , −90◦ ≤ ϑ ≤ 90◦ , ϑ3dB (5) where ϑ3dB is the elevation 3dB beamwidth, which may be assumed to be 15◦ , and ϑtilt is the tilt angle. The combined antenna pattern is computed as [14] A = − min [−(AA (θ) + AE (ϑ)), Am ] . The antenna attenuation gain A is plotted in Fig. 1.

60 50

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150 200 Azimuth angle

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−20

Figure 1: Antenna pattern in elevation and azimuth. III.

O PEN LOOP POWER CONTROL IN LTE U PLINK

The UE transmit power (dBm) is given by [1] P = min{Pmax , P0 +10·log10 M +α·P L+Δmcs +f (Δi )}, (7) where Pmax is the maximum UE transmit power, M is the number of assigned resource blocks (RBs), P0 is a cell specific parameter (default value), α is a cell specific path loss compensation factor, P L is the downlink path loss calculated at the UE. Furthermore, the two closed-loop related parameters are Δmcs , which is assigned by the upper layer and Δi , which is a UE-specific closed-loop correction value with an accumulated or absolute value depending on function f (). For OLPC, f (Δi ) is excluded and Δmcs is set to zero to every modulation and coding scheme (MCS). Thus the UE transmit power (dBm) can simplified as P = min{Pmax , P0 + 10 · log10 M + α · P L}. IV.

(8)

J OINT A NALYSIS OF THE OPLC, 3D BEAM GAIN AND BS DOWN TILT ANGLE

The term P L in (8) is replaced with the link loss L = ¯ i.e., the sum of the path loss, shadow P L + σSF − FRx (ϕ), fading and antenna gain, where σSF is the shadow fading attenuation (dB) with zero mean Log-Normal distribution, ϕ¯ is the line of sight (LoS) BS-MS arrival direction. The received signal strength can be expressed as ¯ PRx = P − L = P − P L − σSF + FRx (ϕ).

(9)

If we calculate the mean PRx , σSF can be excluded. F (ϕ) ¯ is the antenna beam attenuation. For WIM II urban macro cell scenario, the LoS path loss can be calculated as [8] P L = 40 log10 (d) + 13.47 − 14 log10 (hBS ) −14 log10 (hMS ) + 6 log10 (fc /5),

(6)

(10)

where hBS and hMS are the BS and MS antenna heights with default values 25 m and 1.5 m respectively, fc is the

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carrier frequency and d is the distance between BS and UE. The LoS direction ϕ¯ = ϕ(ϕ ¯ A , ϕE ) from BS to MS in azimuth with respect to BS antenna broad side can be calculated as

OPLC transmit power (dBm) of different P0 values vs distance with no Rx SINR constraint, alpha = 0.4. 30

20 BS transmit power(dBm)

−yBS − arctan( xyMS ) + 90◦ − ΩBS , xMS ≥ xBS MS −xBS (11) yMS −yBS − arctan( xMS −xBS ) − 90◦ − ΩBS , xMS < xBS

ϕA = {

and LoS direction from BS to MS in elevation direction is (hMS − hBS )

. ϕE = arctan  (xBS − xMS )2 + (yBS − yMS )2

10

0 P = −20 dBm

(12)

0

−20

P0 = −50 dBm P0 = −60 dBm 0

50

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150 Distance (m)

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Figure 3: Link loss (path loss + beam gain) vs. BS-UE distance for different Po (assuming UE is at the peak of the azimuth main lobe). OPLC transmit power (dBm) of different alpha values vs distance with no Rx SINR constraint 30

20

BS transmit power(dBm)

Combining (4), (5), (6), (12) and (13), the azimuth, elevation, and total beam pattern attenuation for the LoS BS-MS link can be calculated. By knowing the positions of BSs and UEs in the layout, the path loss can be calculated by (10). If a certain shadow fading value is known, we can calculate the receive signal strength by (9) and do the OLPC based on (8). To study the tilt angle effects on elevation beam gain, assuming UEs have isotropic beam pattern gain, link losses L vs. d for different tilt angles are plotted in Fig. 2 (σSF = 0). It is clear that a larger tilt angle is better for the UEs that are near to BS and provides lower antenna gain for far away UEs. Total link loss (path loss + beam gain) vs. Distansce for different tilt angles −60 Tilt angle 0 degs Tilt angle 4 degs Tilt angle 8 degs Tilt angle 12 degs Tilt angle 16 degs Tilt angle 20 degs

10

0

−10

−20 alpha = 0 alpha = 0.4 alpha = 0.6 alpha = 0.8

−30

−40

−100 Link loss (dB)

P = −30 dBm P0 = −40 dBm

−30

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0

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60 80 Distance (m)

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−120

Figure 4: Link loss (path loss + beam gain) vs. BS-UE distance for different alpha values (assuming UE is at the peak of the azimuth main lobe).

−140

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V. 0

100

200 300 Distance (m)

400

500

Figure 2: Link loss (path loss + beam gain) vs. BS-UE distance for different BS down tilt angles (assuming UE is at the peak of the azimuth main lobe). To analyze the parameters in OLPC, tilt angle is set to 8◦ . UE transmit power P in (8) is plotted vs. distance in Fig. 3. Different P0 values are studied and α is fixed to 0.4. In Fig. 4, P0 is fixed to −40 dBm, we evaluate how P is affected by different α values. α = 0 corresponds to the case that the path loss is not compensated at all, so the transmit power does not depend on the BS-MS distance. As α increases, the UE uses more power to compensate a larger fraction of the path loss until it reaches the maximum power limit. In both figures, the receive SINR constraint is not considered.

S YSTEM LEVEL A SSUMPTIONS AND S IMULATION R ESULTS

The system contains 19 BSs and a wrap-around model with 3 sectors per BS ending up with 57 sectors in total. The network layout is shown in Fig. 5. The lines are illustrations of the modeled links (one desired and 56 interfering links) of a UE. The central 57 sectors are the original sectors and the outer sectors are the copies of the central sectors. User locations are uniformly distributed within the central 57 sectors. The users are connected to the BS with which they have the smallest link loss. Once a user is selected, the RBs are random allocated to that user. If there are not enough free RBs available, the user is dropped. In the Table 1, simulation parameters are listed. In Fig. 6, we compare the average SINR of the UEs between normalized isotropic beam pattern and the ITU elevation beam pattern. The gain form the elevation beam is

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Comparison of SINR with and without elevation beam pattern

Illustraion of the network layout of 19 BS (3 sectors/cell) with one user as an example 6000

1 0.9

4000 0.8 0.7

C.D.F (SINR)

Distance

2000

0

−2000

0.6 0.5 0.4 0.3

−4000

0.2 ITU elevation beam pattern (3D) Isotropic elevation beam pattern (2D)

0.1

−6000 −6000

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SINR(dB)

Figure 5: Illustration of the network layout.

Figure 6: Comparison of isotropic elevation beam pattern and ITU elevation beam pattern.

Table 1: Simulation parameters.

Propagation scenario Cell radius Maximum UE transmit power Maximum antenna gain Thermal noise density Number of users BS receiver antenna UE antenna UE speed Shadow fading Shadowing correlation Down tilt angle Power control parameters P0 α

SINR CDF of different alpha values

Value 19cells, 3 sectors/cell - wrap around Base coverage urban 300 m 23 dBm 17 dBi −174 dBm/Hz 50 in 19 cells 2 1 3km/h Log-Normal, 8 dB standard deviation independent 0, 4, 8, 12, 15 degree -30, -50, -70, -90 dBm 0, 0.4, 0.6, 0.8, 1

about 13 dB according to the cumulative distribution function (CDF) at 0.5. In Fig. 7, we investigated in the fractional OLPC. SINR performance with different values of α are compared. α = 0 means the link loss is not compensated at all and α = 1 is the situation that link loss is fully compensated. From the figure we can see that when α = 0, the OLPC gives the worse SINR performance among all the values. α = 1 and α = 0.8 give almost the same performance and better than the others values do. α = 0.4 gives about 2dB better SINR than α = 0.6. We studied fractional OLPC performance with different P0 values in Fig. 8. The value is selected from -50, -70, -90 dBm. We can see from the figure that P0 = −50 dBm gives the best SINR performance. P0 = −70 dBm is about 3 dB worse than P0 = −50 dBm at CDF = 0.5 and about 1 dB better than P0 = −90 dBm. Fig. 9 shows the SINR performance for different down tilt angles. When antenna is down tilted by 4◦ , the system

1 0.9 0.8 0.7

C.D.F (SINR)

Parameters Layout

alpha = 0 alpha = 0.4 alpha = 0.6 alpha = 0.8 alpha = 1

0.6 0.5 0.4 0.3 0.2 0.1 0 −20

−10

0

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SINR(dB)

Figure 7: Performance comparison of different alpha values. performs best below SINR of 23 dB. The 8◦ tilt angle performs close to 4◦ and has the best performance when SINR is between 23 and 30 dBm. If the BS antenna is down tilted by a large angle, for example 15◦ , the system performs well at high SINR values but poor at low SINR values. This is because the near-BS users benefit more if the peak of the beam tilts towards the center of the cell and cell-edge users are not covered by the main lobe and experience a low SINR. Similar analysis was already presented in Fig. 2. At CDF of 0.5, the 4◦ tilt is about 4 dB better than the 15◦ tilt. VI.

C ONCLUSION

We studied the LTE UL OLPC jointly with the BS down tilt in a realistic 3D channel model. We presented expressions to calculate link losses in 3D channel based on the parameters of OLPC, down tilt angle and BS-UE direction. We have shown that the gain of elevation beam gain over an isotropic elevation pattern is significant. Based on the urban

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macro cell path loss model given by WIM II, we studied the effect of different BS down tilt angles. Tilt angles between 4◦ and 8◦ are found to be good for cells with radius in the range of 300 m. With the propagation scenarios and simulation parameters in this paper, power control parameters P0 with value of 50 gives best performance. SINR CDF comparison of different values of Po 1 Po = −50, alpha = 0.6 Po = −70, alpha = 0.6 Po = −90, alpha = 0.6

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C.D.F (SINR)

[5] G. Wilson, “Electrical downtilt through beam-steering versus mechanical downtilt [base station antennas],” in Proceedings of the IEEE Vehicular Technology Conference, May 1992, pp. 1–4 vol.1. [6] F. Gunnarsson, M. Johansson, A. Furuskar, M. Lundevall, A. Simonsson, C. Tidestav, and M. Blomgren, “Downtilted base station antennas - a simulation model proposal and impact on HSPA and LTE performance,” in Proceedings of the IEEE Vehicular Technology Conference, Sept. 2008. [7] 3GPP-TR25-996, “Spatial channel model for multiple input multiple output MIMO simulations, (3GPP TR 25.996 v6.1.0,” 3rd Generation Partnership Project (3GPP), Tech. Rep., Sept.

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[8] P. Ky¨osti, “Winner II channel models, IST-4-027756 WINNER II, D1.1.2 V1.1,” WINNER Project, Tech. Rep., Sept. 2007.

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[9] M. Narandzic, M. Kaske, C. Schneider, M. Milojevic, M. Landmann, G. Sommerkorn, and R. Thoma, “3D-antenna array model for ISTWINNER channel simulations,” in Proceedings of the IEEE Vehicular Technology Conference, Apr. 2007, pp. 319–323.

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Figure 8: Performance comparison of different Po values.

Comparison of antenna down tilt angles.

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[11] A. Poon, R. Brodersen, and D. Tse, “Degrees of freedom in multipleantenna channels: a signal space approach,” IEEE Transactions on Information Theory, vol. 51, no. 2, pp. 523–536, Feb. 2005.

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[10] M. Shafi, M. Zhang, A. L. Moustakas, P. J. Smith, A. F. Molisch, F. Tufvesson, and S. H. Simon, “Polarized MIMO channels in 3D: Models, measurements and mutual information,” IEEE Journal on Selected Areas in Communications, vol. 24, pp. 514–527, 2006.

[12] C. Castellanos, D. Villa, C. Rosa, K. Pedersen, F. Calabrese, P.-H. Michaelsen, and J. Michel, “Performance of uplink fractional power control in UTRAN LTE,” in Proceedings of the IEEE Vehicular Technology Conference, May 2008, pp. 2517–2521.

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control in UTRAN LTE networks,” in Proceedings of the IEEE International Symposium on Wireless Communication Systems. New York, NY, USA: ACM, 2009, pp. 1410–1416.

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[14] “Guidelines for evaluation of radio interface technologies for IMT-Advanced,” International Telecommunication Union, Tech. Rep. ITU-R M.2135, Nov. 2008. [Online]. Available: http://www.itu.int/publ/R-REP-M.2135-2008/en

0.5 0.4 0.3 0.2 0.1 0 −5

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Figure 9: Performance comparison of different down tilt angle values.

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