Path Loss Models for Wireless Access Network Systems Using High Frequency Bands

1. Introduction

In developing wireless communication systems, it is necessary to design their coverage areas and evaluate their performance such as the system throughput, the transmission capacity, and the system capacity. The system performance and the size of the area depend on the degree of propagation attenuation (path loss), as seen in Fig. 1. The propagation characteristics, including path loss, vary greatly depending on the frequency band. Therefore, it is necessary to develop a path loss model that can be used for the frequency band of the system.

Fig. 1. Path loss model for designing wireless communication systems.

Although it is assumed that the fifth-generation mobile communication system (5G) will use high frequency bands above 6 GHz, it is known that the propagation characteristics in high frequency bands differ from those for frequency bands below 6 GHz, which have long been used for conventional mobile communication systems. For example, the path loss increases as the frequency increases, and radio waves in high frequency bands do not reach shielded areas such as the areas behind buildings.

The frequency allocation of the high frequency bands for 5G will be discussed at the World Radiocommunication Conference in 2019. The propagation characteristics of the high frequency bands will be the basis of the discussion and should thus be studied in order to discuss appropriate allocation. Therefore, modeling the propagation characteristics of the high frequency bands has become a pressing issue.

At NTT Access Network Service Systems Laboratories, we are carrying out propagation modeling of a wide range of frequency bands in order to develop a variety of wireless communication systems. In this article, we introduce path loss models for the 20–40 GHz bands that can be useful for wireless communication systems in high frequency bands.

2. Path loss models

We clarified a propagation mechanism by measuring and simulating propagation, focusing on urban and residential environments. These environments are important use case scenarios of future wireless communication systems. They include mobile communication systems and wireless local area network systems. We obtained dominant paths from invisible propagation phenomena. The results enabled us to develop path loss models for high frequency bands that had been unclear except for the bands below 6 GHz. These models are depicted in Fig. 2.

Fig. 2. Path loss models developed for urban and residential environments.

For urban environments, we developed a model by deriving dominant path loss characteristics for three areas in which waves sent from a transmitter (Tx) to a receiver (Rx) are either visible or invisible: a line-of-sight (LoS) area, a non-line-of-sight (NLoS) area, and a corner (i.e., an intersection) area that is a transition area between LoS and NLoS areas [1]. The respective path losses in these areas are indicated as L_LoS, L_att, and L_C, as seen in Fig. 2(a). The radio waves transmitted from the Tx propagate to the Rx through these areas, with the three path losses added. We modeled the total path loss by using the frequency, the distance along a road between the Tx and the Rx, and the distance along a road between the Tx and the corner as parameters.

For residential environments, we developed a model by deriving three dominant paths: a path along a road, a path between buildings, and a path above a rooftop. The respective losses for these paths are indicated as L_r, L_b, and L_v, as seen in Fig. 2(b) [2]. We modeled these path losses using parameters including the distance along a road between the Tx and the Rx, building density, and building height. In our study, the equation for the residential environment model differs from that for the urban environment model because the received power at the Rx in a residential environment depends on which of the three paths has the least path loss.

In this way, we were able to develop models from the dominant paths that can easily predict actual path loss with high accuracy using simple parameters. These models have the potential to play a major role on the world stage in studies on the propagation characteristics of high frequency bands.

3. Prediction results obtained with developed path loss models

The results of predicting path loss using the models are plotted in Fig. 3. We verified the models’ validity by comparing the prediction results and the measurement results obtained using 26 GHz and 37 GHz frequencies in an urban and residential environments.

Fig. 3. Results of predicting and measuring path loss in urban and residential areas.

In the urban environment, as shown in Fig. 3(a), the Rx moves along the road about 180 m and then turns the corner. The path loss increases sharply at the corner because the Rx moves from a LoS area to a NLoS area.

In the residential environment on the other hand, as shown in Fig. 3(b), the path loss shows significant variation when the Rx moves from area A to area E. This is caused by situational variations such as the number of corners between the Tx and the Rx and the degree of building shielding.

In both environments, the developed models accurately predicted the actual path losses. The root mean square error (RMSE) values are less than 5 dB. These results indicate that the models provide sufficient prediction accuracy for designing future wireless communication systems.

4. Future overview

This article presented path loss models that can accurately predict actual path losses using simple parameters. The derived models are suitable for designing coverage areas and evaluating the performance of various future wireless communication systems using high frequency bands. The models are expected to be utilized for the 5G system, which has been the focus of much research and development in the wireless communication field worldwide.

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. He joined NTT Access Network Service Systems Laboratories in 2009, where he has been conducting research on 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, NTT Access Network Service Systems Laboratories. He received his B.E. and M.E. in electrical engineering from Tokyo University of Science in 2009 and 2011. He joined NTT Wireless Systems Innovation Laboratories in 2011, where he has been researching radio propagation for the 5G system 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, the Communications Society Best Paper Award, and the Best Paper Award from IEICE in 2006, 2011, and 2014, respectively. He is a member of IEEE.
	Takeshi Onizawa Senior Research Engineer, Supervisor, Group Leader of 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 research and development of personal communication systems and high data rate wireless LAN systems. He is now in charge of strategies and propagation of wireless communication systems. He received the Best Paper Award, the Young Researcher’s Award, and the Achievement Award from IEICE in 2000, 2002, and 2006, respectively. He also received the Maejima Hisoka Award from the Tsushinbunka Association in 2008. He is a senior member of IEICE and a member of IEEE.