Path Loss Model to Evaluate Interference for Small Cells between Different Floors

1. Introduction

NTT Access Network Service Systems Laboratories and NTT DOCOMO have been conducting a joint study toward development of the fifth-generation mobile communications system (5G). The results of measurements carried out jointly made it possible to clarify the path loss characteristics of the indoor environment in the high frequency band. This article presents the developed path loss model.

2. Path loss model

Frequency allocation of 5G, particularly the high frequency bands above 6 GHz, will be discussed at the World Radiocommunication Conference to be held in 2019. To discuss the appropriate frequency allocation, it is important to evaluate the interference between 5G systems and other existing systems. In the interference evaluation, it is necessary to understand the path loss (attenuation of the radio wave) in order to understand the interference power.

Path loss varies depending on the frequency, so it is essential to construct a path loss model in the frequency band that will be used. It is assumed that the high frequency band above 6 GHz will be used for 5G, and therefore, the standardization group 3rd Generation Partnership Project (3GPP) is standardizing the path loss models of the high frequency band [1].

In 3GPP, office environments are assumed as the indoor small cell scenarios, but the path loss model that can be used in that environment is only for a single floor (the same floor). In order to accurately evaluate interference, an evaluation of inter-floor interference [2] is also required (Fig. 1). NTT Access Network Service Systems Laboratories has developed a path loss model for evaluating inter-floor interference for a wide range of frequencies as part of efforts to realize various wireless communication systems.

Fig. 1. Interference between different floors in indoor small cells.

3. Prediction results produced with developed path loss model

To understand the inter-floor interference of indoor small cells in an office environment, we measured path loss using five frequency bands (0.8, 2.2, 4.7, 26.4, and 37.1 GHz), including the bands above 6 GHz in a multistory building of NTT DOCOMO (from the third to sixth floors). As a result, we clarified that the path loss characteristics between different floors were represented by two dominant propagation paths inside and outside the building in a wide frequency band (Fig. 2). Here, the propagation path inside the building is the path transmitted through the floors (and ceilings) between the transmitter (Tx) and the receiver (Rx). The propagation path outside the building is the path that is diffracted at the window frame of the Tx floor, propagated outside the building, which then re-enters from the window of the Rx floor. The propagation path inside the building attenuates due to transmission loss caused by ceilings and floors, and the propagation path outside the building attenuates because of the diffraction loss at the window.

Fig. 2. Dominant paths between different floors.

The results of calculating the path loss using the proposed model for these two dominant paths are shown in Fig. 3(a), and the results of measuring the path loss between different floors are shown in Fig. 3(b). The measurement results show the median values of path loss at each floor. The value is normalized by the path loss when the Tx and Rx are set on the same floor, and it therefore represents the floor penetration loss when the Tx and Rx are on different floors.

Fig. 3. Results of calculating and measuring path loss between different floors.

The calculation results in Fig. 3(a) show the path loss of the path inside the building transmitted through floors when varying the floor thickness w, and that of the path outside the building that is diffracted at the window frame. The path losses are calculated from 0.1 GHz to 100 GHz. In the measurement environment, the thickness of the floor material was 1.5 m per floor. It can be seen that the transmission loss increases rapidly as the floor thickness increases (that is, as the distance between floors increases) or as the frequency increases.

In contrast, the diffraction loss of the path outside the building increases gently with respect to the frequency increase. Therefore, when the frequency is low or the distance between floors is small, the path inside the building (transmission path) becomes dominant, and when the frequency is high or the distance between floors is large, the path outside the building (diffraction path) becomes dominant.

The frequency dependence of the measurement results in Fig. 3(b) can be explained by using the two dominant paths. The path loss for a distance consisting of one floor level increases sharply as the frequency increases up to 4.7 GHz, but over 4.7 GHz the path loss increases gently. Also, in the case of two or three floors, the path loss increases gently with respect to the frequency increase. It is thought that the path inside the building (transmission path) is dominant up to 4.7 GHz at a distance of one floor level (e.g., the distance between the fifth floor and the sixth floor), and the path outside the building (diffraction path) is dominant under the other conditions.

As discussed above, when the two dominant paths are used, it is possible to express the path loss characteristics in a wide frequency band. From this result, for example, when base stations for indoor small cells are installed near a window, it is necessary to pay attention to the fact that interference between floors becomes especially large.

4. Future overview

This article presented a developed path loss model that can represent the path loss characteristics between different floors. The proposed model is suitable for evaluating inter-floor interference of indoor small cells. We expect that the model will be utilized for 5G, which is a key topic in the research, development, and standardization in the wireless communication field worldwide. A paper describing the proposed model won the Best Paper Award at the 2016 International Symposium on Antennas and Propagation (ISAP 2016). Please refer to the paper [3] for more detailed results.

In order to contribute to the construction of various wireless communication systems, we intend to actively engage in the modeling of propagation phenomena for a wider range of frequencies and environments.

References

	Motoharu Sasaki Research Engineer, NTT Access Network Service Systems Laboratories. He received a B.E. in engineering and an M.E. and Ph.D. in information science and electrical engineering from Kyushu University, Fukuoka, in 2007, 2009, and 2015. In 2009, he joined NTT Access Network Service Systems Laboratories, where he has been engaged in researching propagation modeling of interference between mobile terminals for spectrum sharing wireless access systems. He received the Young Researcher’s Award and the Best Paper Award from the Institute of Electronics, Information and Communication Engineers (IEICE) in 2013 and 2014, respectively. He is a member of the Institute of Electrical and Electronics Engineers (IEEE).
	Minoru Inomata Research Engineer, Wireless Access Systems Project, NTT Access Network Service Systems Laboratories. He received a B.E. and M.E. in electrical engineering from Tokyo University of Science in 2009 and 2011. In 2011, he joined NTT Wireless Systems Innovation Laboratories. Since then, he has been involved in R&D of radio propagation for 5G systems and system design for unlicensed wireless communication systems. He received the Young Researcher’s Award from IEICE in 2016. He is a member of IEEE.
	Wataru Yamada Senior Research Engineer, Planning Section, NTT Access Network Service Systems Laboratories. He received a B.E., M.E., and Ph.D. from Hokkaido University in 2000, 2002, and 2010. Since joining NTT in 2002, he has been researching propagation characteristics for wide band access systems. From 2013 to 2014, he was a visiting research associate at the Centre for Telecommunications Research at King’s College, London, UK. He received the Young Researcher’s Award, a Communications Society Best Paper Award, and a Best Paper Award from IEICE in 2006, 2011, and 2014, respectively. He is a member of IEEE.
	Naoki Kita Senior Research Engineer, Supervisor, Group Leader, NTT Access Network Service Systems Laboratories. He received a B.E. from Tokyo Metropolitan Institute of Technology in 1994, and an M.E. and Ph.D. from Tokyo Institute of Technology in 1996 and 2007. He joined NTT in 1996, where he researched radio propagation characteristics and radio propagation modeling for wireless access systems. From 2009 to 2010, he was a visiting scholar at Stanford University, CA, USA, and from 2012 to 2013, he was a visiting research scholar at Waseda University, Tokyo. From 2013 to 2015, he was a senior manager in the planning department of NTT Information Network Laboratory Group. He is currently responsible for the R&D of future satellite communication systems and radio propagation modeling for future wireless communication systems. He received the Young Researcher’s Award, a Communications Society Best Paper Award, and a Best Paper Award from IEICE in 2002, 2011, and 2014, respectively. He is a senior member of IEICE and a member of IEEE.
	Takeshi Onizawa Senior Research Engineer, Supervisor, Group Leader - Wireless Access Systems Project, NTT Access Network Service Systems Laboratories. He received a B.E., M.E., and Ph.D. from Saitama University in 1993, 1995, and 2003. Since joining NTT in 1995, he has been engaged in the R&D of personal communication systems and high data rate wireless local area network systems. He is now in charge of strategies and propagation of wireless communication systems. He received a Best Paper Award, Young Investigators award, and Achievement Award from IEICE in 2000, 2002, and 2006, respectively. He also received the Maejima Award in 2008. He is a senior member of IEICE and a member of IEEE.
	Masashi Nakatsugawa Executive Manager, NTT Information Network Laboratory Group. He received a B.E. and M.E. from Waseda University, Tokyo, in 1987 and 1989, and an M.S. from the California Institute of Technology, USA, in 1999. He joined NTT Radio Communication Systems Laboratories in 1989. He has researched MMIC circuit design, packaging technologies, software-defined radio, and wide-area wireless access systems. From 2010 to 2012, he was a senior manager in the Radio Division, Technical Planning Department, NTT, where he was involved in regulatory and standardization activities for wireless systems. He is currently on secondment at NTT Advanced Technology Corporation. He received the Young Researcher’s Award in 1996 and the Distinguished Contributions Award (Electronics Society) in 2017 from IEICE, and the YRP Award from the YRP (Yokosuka Research Park) R&D Promotion Committee in 2002. He is a member of IEEE, IEICE, and the Japan Society of Applied Physics.
	Koshiro Kitao Research Engineer, Research Laboratories, NTT DOCOMO, INC. He received a B.S., M.S., and Ph.D. from Tottori University in 1994, 1996, and 2009. He joined NTT Wireless Systems Laboratories in 1996. Since then, he has been researching radio propagation for mobile communications.
	Tetsuro Imai Senior Research Engineer, 5G Laboratory, NTT DOCOMO, INC. He received a B.S. and Ph.D. from Tohoku University, Miyagi, in 1991 and 2002. He joined NTT Radio Communications Systems Laboratories in 1991. Since then, he has been engaged in the R&D of radio propagation, antenna systems, and system design for mobile communications. He received the Young Researcher’s Award in 1998, Best Paper Awards in 2006 and 2014, and the Communications Society Best Paper Award in 2015 from IEICE. Dr. Imai is a member of IEICE and IEEE.