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IEICE Communications Express, Vol.6, No.6, 405–410 Performance enhancement by beam tilting in SD transmission utilizing two-ray fading

Tomohiro Seki1a), Ken Hiraga2, Kazumitsu Sakamoto2, and Maki Arai2 1 College of Industrial Technology, Department of Electrical and Electronic Engineering, Nihon University, 1–2–1 Izumicho, Narashino 275–8575, Japan 2 NTT Network Innovation Laboratories, NTT Corporation, 1–1 Hikarinooka, Yokosuka 239–0847, Japan a) [email protected]

Abstract: A method is proposed for enhancing the transmission performance in a spatial division transmission system that utilizes the fading characteristics of two-ray ground reflection propagation. The method is tilting the elevation angle of antenna beams purposely towards out of the communicating peer. Using ray-tracing simulation, it is shown that the performance of the system is significantly improved when high-gain antennas with narrow beamwidth are used. Keywords: two-ray fading, parallel transmission Classification: Antennas and Propagation

References

[1] K. Hiraga, K. Sakamoto, M. Arai, T. Seki, T. Nakagawa, and K. Uehara, “Spatial division transmission without signal processing for MIMO detection utilizing two-ray fading,” IEICE Trans. Commun., vol. E97.B, no. 11, pp. 2491–2501, 2014. DOI:10.1587/transcom.E97.B.2491 [2] C. Cordeiro, “IEEE doc.:802.11-09/1153r2,” p. 4, 2009. [3] Wilocity: Wil6200 Chipset Datasheet. [Online]. http://wilocity.com/resources/ Wil6200-Brief.pdf. [4] W. L. Stutzman, “Estimating directivity and gain of antennas,” IEEE Antennas Propag. Mag., vol. 40, no. 4, pp. 7–11, Aug. 1998. DOI:10.1109/74.730532 [5] D. Parsons, The Mobile Radio Propagation Channel, ch. 2.3, pp. 24–26, Pentech Press, 1992. [6] K. Uehara, T. Seki, and K. Kagoshima, “Indoor propagation calculation considering antenna patterns using geometrical optics method,” IEICE Trans., vol. J78-B-2, no. 9, pp. 593–601, Sep. 1995.

405 IEICE Communications Express, Vol.6, No.6, 405–410

1 Introduction To meet increasing demand for wireless transmission rates, some of the hot areas identified in the last decade include applying multiple-input and multiple-output (MIMO). The use of MIMO, however, inevitably results in the need for high computing cost for signal processing. Our solution for this issue was to propose a new spatial division (SD) transmission method that utilizes the fading characteristics of two-ray ground reflection propagation [1]. This method utilizes two-ray fading, taking advantage of the characteristic that high attenuation locations change along with the antenna heights. Multi-stream transmission performance of the SD method relies on the high attenuation in the unwanted path. To obtain these attenuations, reception power level through the direct (or LOS, line of sight) ray and one through the ground reflection (GR) ray should be close to each other, ideally equal. When antennas have directivity, the difference in antenna gains in the LOS ray and GR ray directions makes reception power levels in both rays different. Hence, multi-stream transmission performance is reduced by that directivity. In general, to obtain sufficient signal-to-noise ratio (SNR), high-frequency transceivers have high-gain antennas with high directivity (e.g., 14 dBi in [2]). In fact, some commercial 60 GHz transceivers for WLANs employ narrow-beam antenna arrays with 16 or even 32 elements [3]. To make antenna gains in LOS and GR rays closer to each other and consequently improve transmission performance, we here propose, to adopt depression of the beam in each antenna element.

2 Description of the proposed method In the system analyses that have been presented so far [1], the antenna beam direction has been ﬁxed to be horizontal. Beam tilting methods themselves have been already used in various systems to enhance channel gain. For example, the cellular base stations use beam tilting to increase antenna gain in the direction of the mobile stations that are mostly located lower than the base station’s antenna. However the concept of the beam tilting method introduced this paper is in totally diﬀerent from classical ones. This method tilts beams purposely towards out of the direction of the communicating peer. Fig. 1 illustrates the proposed method. Transmitting and receiving arrays with M elements (here M is three) are placed on the ground facing each other. The distance between the arrays is D. The i-th transmitting element Tx#i and the i-th

receiving element Rx#i have the same height from ground hi and directly face each other, where i ¼ 1; 2; 3. LOS rays and GR rays cancel each other in paths except

the one between Tx#i and Rx#i. The elevation angles of beams are i for Tx#i and Rx#i.

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Fig. 1. An illustration of the proposed method

3 Performance simulation procedures Fig. 1 shows the simulation model. Antenna heights, key parameters for the SD method, are optimized for each transmission distance using the equations below. These are derived with the same procedure shown in [1], setting the reflection coefficient on the ground surface to negative real number, because when the polarization is parallel to the ground and the grazing angle is sufficiently low, the ground reflection coefficient is negative whether the ground is conductive or not. When M ¼ 2, the optimum antenna heights are ppffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffi rffiffiffiffiffi h 5 1 D h 1 D 1 ¼ ; 2 ¼ ppffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ; ð1Þ 0 2 0 0 5 1 0

where 0 is the wavelength. When M ¼ 3, the optimum antenna heights are

expressed as Eq. (2), where N12, N23, and N31 are unequal odd numbers excluding

1 [1]. In this paper these parameters are set to be the smallest combination, N12 ¼ 3,

N23 ¼ 5, and N31 ¼ 7. rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi h N N D h N N D h N N D 1 ¼ 12 31 ; 2 ¼ 12 23 ; 3 ¼ 23 31 : ð2Þ 0 2N23 0 0 2N31 0 0 2N12 0 In this model it is assumed there is antenna directivity in the plane on which all

antenna elements are located. The directivity of Tx#i or Rx#i is denoted by GiðÞ, where θ is the elevation angle. Here θ of depression (below horizon) is set to be

negative. The antenna’s radiation pattern without tilt G0ðÞ is set to have no side lobes and is deﬁned by using the simpliﬁed cosine model shown in eq. (15) and

(18) in [4]. Using the half-power beamwidth, 3dB, we express the model’s radiation

pattern without tilting G0ðÞ as a function of the elevation angle θ. pffiffiffi ð Þ¼ q ¼ log2 2 ð Þ G0 cos ; where q ð Þ 3 log2 cos 3dB Here, using eq. (12) in [4], we assumed the beam was rectangular and was equivalent to a beam of half-wave dipole array. The antenna gain g is defined as

g ¼ 41253=ð78° 3dBÞ; ð4Þ where η is the radiation eﬃciency of the antenna. Each antenna element in the

© IEICE 2017 arrays on both sides has equal directivity. Each antenna is assumed to have the DOI: 10.1587/comex.2016XBL0213 same gain, g. The radiation pattern of Tx#i or Rx#i with beam tilting, G ðÞ,is Received December 11, 2016 i Accepted February 27, 2017 Publicized March 28, 2017 Copyedited June 1, 2017

407 IEICE Communications Express, Vol.6, No.6, 405–410

GiðÞ¼G0ð iÞ; ð5Þ

where i is the elevation angle of the beam of Tx#i or Rx#i, as described in the previous section.

From the calculation shown in [5], E0 ij, the complex valued electric ﬁeld at

Rx#j, which is produced by the LOS ray r0 ij transmitted from element Tx#i, and

Er ij, which is produced by the GR ray rr ij transmitted from the same element, are expressed as pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 30P gG ð Þ 2 30P gG ð Þ 2 t j 0 ij j r0 ij t j r ij j rr ij E0 ij ¼ e 0 ; Er ij ¼ Rij e 0 : ð6Þ r1 ij rr ij

The elevation angle toward the LOS ray is denoted by 0 ij and that toward the GR

ray by r ij. Rij is the reflection coefficient for the reflection from the ground surface

in the GR ray rr ij and Pt is the transmission power. The path length of the direct

wave is r0 ij and that of the reflected wave is rr ij; these lengths are calculated from the numbers given in Fig. 1. The actual reception power at an antenna Rx#j is calculated, taking the phase rotations due to the path lengths into account, by using the formulated calculations shown in [6]. pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 G ð ÞG ð Þ 2 G ð ÞG ð Þ 2 0g j 0 ij i 0 ij r0 ij j r ij i r ij rr ij ¼ 0 þ 0 ð Þ Pij Pt e Rij e : 7 4 r0 ij rr ij Since signal-to-interference-plus-noise ratio (SINR) at Rx#i is P SINRðiÞ¼ ii ;i¼ 1; 2; ;M; ð8Þ XM Pij þ PN i j¼1;j≠i

where the noise level at each receiving antenna is PN i. To calculate the channel capacity, the equation below is used.

XM ¼ ½ þ SINRð Þ ð Þ C log2 1 i 9 i¼1

4Effectiveness of proposed method and optimum beam tilt angles In this section, using the ray-tracing whose procedure is described above, we show the effectiveness of the proposed method. It is assumed that the ground is a conductor and that the polarization is parallel to the ground. Each antenna height is set using eq. (1) and (2). Taking the transmission of modulated signal into account, we can clarify this type of SD transmission system’s performance by evaluating the SIR (signal-to- interference ratio) in each transmission path. Here, SIR(i) denotes an SIR at Rx#i. Fig. 2(a) shows the SIR enhancement obtained for M ¼ 2 in the 10°–90° half- power beamwidth range by using the proposed beam tilting at the transmission distance of 2000 wavelengths. Fig. 2(b) shows the same for M ¼ 3. It can be seen that beam tilting significantly improves the SIR values when half-power beamwidth

© IEICE 2017 is less than 60°. This indicates that the proposed method improves the performance. DOI: 10.1587/comex.2016XBL0213 Received December 11, 2016 On the other hand, it cannot be expected to enhance the performance of SD Accepted February 27, 2017 Publicized March 28, 2017 Copyedited June 1, 2017

408 IEICE Communications Express, Vol.6, No.6, 405–410

transmission systems with large beamwidth because relatively little degradation occurs in them even without beam tilting.

(a) (b)

Fig. 2. SIR enhancements. (a) M ¼ 2, (b) M ¼ 3. D ¼ 2000 0.

We can also see that SIR of M ¼ 3 is lower than that of M ¼ 2 in a whole range. When M ¼ 2, the optimum antenna heights and beam tilt bring complete canceling of LOS ray and GR ray simultaneously in both diagonal paths (the path between Tx#1 and Rx#2 and the path between Tx#2 and Rx#1). On the other hand,

when M ¼ 3 there is no combination of i (i ¼ 1; 2; 3) and antenna heights that brings complete canceling in all diagonal paths, because, for example, the tilt angles are not fully optimum for both path between Tx#1 and Rx#3 and path between Tx#1 and Rx#2. There is a trade-oﬀ for these two paths when we optimize the beam tilts. Fig. 3 shows the optimum beam tilt angles as the function of half-power

beamwidth on the same conditions as Fig. 2. Here 1 is ﬁxed to zero for M ¼ 2

because either 1 or 2 has to be tuned when M ¼ 2, as explained above. When the beam is narrow, the path gains of the LOS ray and GR ray can be equalized with small beam tilt. So the optimum tilt is small for narrow beamwidth.

On the other hand, when the beam is wide (e.g., 3dB > 60° for M ¼ 2), SIR degradation is very small even without beam tilting, hence there is no need for tilting and the optimum tilt gets smaller along with the increase of the beamwidth. Accordingly the curves shown in this ﬁgure have maximum values. When M ¼ 3,

the optimum tilt angles increase along with 3dB in the range of up to 80°, which is

© IEICE 2017 DOI: 10.1587/comex.2016XBL0213 Received December 11, 2016 Accepted February 27, 2017 Fig. 3. Optimum tilt angles. D ¼ 2000 0.ForM ¼ 2, 1 ¼ 0°. Publicized March 28, 2017 Copyedited June 1, 2017

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larger value than M ¼ 2 system. This is because, in M ¼ 3 system, antenna heights are higher than those in M ¼ 2 system with the same transmission distance, so larger tilt angles are still required even with wider beam in order to minimize the diﬀerence between the received powers of LOS and GR rays. Accordingly the maximum value of the curve is at larger beamwidth.

5 Conclusions This paper proposed a method to enhance the transmission performance in a spatial division transmission system without any MIMO signal processing by exploiting two-ray fading. Using ray-tracing simulation, we clariﬁed the method’s eﬀectiveness.

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