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Special Feature: R&D of Wide-area Ubiquitous Network to Create New Services

Technical Development Activities for Wide-area Ubiquitous Network

Daisei Uchida^†, Hajime Matsumura, Shigeru Kuwano,
Fusao Nuno, Nobuaki Mochizuki, Satoshi Kotabe, Kenji Suzuki,
Takafumi Fujita, and Koki Mitani

Abstract

This article describes the main technologies that underlie the wide-area ubiquitous network, which we expect will create a new industry.

†	NTT Network Innovation Laboratories Yokosuka-shi, 239-0847 Japan

1. Introduction

The wide-area ubiquitous network (WAUN) is characterized by the use of compact, long-life battery-driven wireless terminals, services that are less expensive than existing public wireless services, high security for customer information, and both indoor and outdoor service availability. It is a communication infrastructure that can provide bidirectional network services for collecting data from objects and delivering data to them [1].

The system configuration is shown in Fig. 1. This system is made up of a wireless system and a core network system. The wireless system consists of wireless terminals and base stations (remote base stations and master base station). The core network system consists of network management servers and internet protocol gateways (IP-GWs). A network management server authenticates wireless terminals and manages terminal identities (IDs) and locations. An IP-GW handles admission control for data transfer between the IP-GW and application servers. The wireless terminals have limited functionality to reduce their cost and to save energy; their functionality is supplemented by the functionalities residing in the rest of the network.

Fig. 1. WAUN system configuration.

The main technologies we have developed for the WAUN are described below.

2. Wireless system technologies

We have developed several technologies to enable the wireless system to offer a large cell size and cover wide service areas using compact low-power wireless terminals: site diversity allows each terminal to be served by multiple base stations, digitized radio on fiber (DROF) synchronizes the time of base stations and collects/delivers radio signals to support site diversity, and energy-saving technology enables compact low-power-consumption wireless terminals. These technologies have allowed the transmission power of wireless terminals to be reduced to 10 mW, which enables the system to satisfy two conflicting demands: allowing the cell radius to be as large as 3.5 to 5 km (wide area coverage) with wireless terminals powered by coin batteries that can operate for more than five years (energy saving).

2.1 Site diversity

In the traditional wireless system, some locations within a cell may not receive sufficiently strong radio signals from the base station. Therefore, to achieve a large cell radius in inter-object communication, it is necessary to overcome two major problems.

(i) Unlike humans, objects cannot move autonomously to a location where they can receive a strong radio signal.

(ii) The strength of the signal transmitted by a simple wireless terminal is so limited that the area that can be covered by the signal depends strongly on the surrounding circumstances.

In particular, in urban areas, it is necessary to suppress shadowing loss and other forms of degradation caused by the shielding effects of high-rise buildings. To solve these problems and achieve a large cell radius, we have chosen to use site diversity. The way this is implemented in our network is shown in Fig. 2 and an overview of the developed site diversity technology is shown in Fig. 3.

Fig. 2. Site diversity configuration.

Fig. 3. Site diversity technology.

Unlike the cell configuration of a mobile phone network, in which a base station is normally installed at the center of a cell (cell center configuration), in the WAUN, a cell is made up of multiple base stations, which are installed at cell edges (site diversity configuration). The directivity of the base station antennas is directed towards the center of each cell. This configuration’s technologies and advantages are described below.

2.1.1 Downstream link

The conventional application of site diversity to downstream signals from a base station has some problems. First, the effect of site diversity is not as great as would be expected from the number of base stations. Second, the demodulation performed by wireless terminals becomes more complex as the number of base stations increases. We have solved these problems in the following manner. The frequency of transmission signals from each base station is intentionally shifted from those of other base stations according to a certain rule, and terminals demodulate signals according to the same rule. This makes it possible to increase the effect of site diversity as the number of base stations is increased while the complexity of the demodulation performed by each terminal remains unchanged [2]. This allows simple terminals to benefit from site diversity without any constraints on the number of base stations, so the signals from base stations can reach a large number of terminals.

2.1.2 Upstream link

Conventionally, each base station demodulates upstream signals from terminals independently. Therefore, in the conventional application of site diversity to these upstream signals, individual base stations cannot cover dead spots. To solve this problem, signals from different base stations are gathered at one location and combined by applying maximal ratio combining (common-mode synthesis of signals after they have been weighted according to the signal reception level). They are then demodulated. This concerted operation of base stations makes it possible to cover the dead spots of individual base stations.

This technology maximizes the effectiveness of the base stations in a site diversity configuration and enables base stations to cover more terminals even when their transmission power is low.

2.2 Digitized radio on fiber

In site diversity, one effective approach is to install base stations away from each other so that multiple propagation routes can be established. Since these base stations need to transmit or receive the same radio signal simultaneously, their timing must be synchronized precisely.

For the WAUN, we developed DROF to achieve the required time synchronization. The base station functions are divided into multiple wireless transmission/reception parts (performed by remote base stations) and a wireless modulation/demodulation part (performed by the master base station), and these are connected to each other by DROF [3]. Since DROF allows remote base stations to be installed anywhere, it is possible to install remote base stations at the cell edges to achieve an effective site diversity configuration.

The distribution of outdoor reception levels within a cell is shown in Fig. 4 for two cases: three base stations (site diversity configuration) and only one base station (cell center configuration). It shows the distribution of average reception levels within a 400-m² area (20 m × 20 m). The reception level of the site diversity configuration is 6 dB higher than that of the cell center configuration at the point where the cumulative distribution is 1%. This effect is equivalent to increasing the cell radius by about 1.5 times.

Fig. 4. Experimental result: reception level within a cell.

2.3 Energy-saving technology

In the inter-object communication services supported by the WAUN, applications are expected to communicate only infrequently. In a remote meter reading application, for example, a meter will be read only a few times a month. In an environmental sensing application, data will be sent roughly once an hour. In other words, the communication interval will be extremely long. In addition, terminals remain inactive most of the time. For such a terminal, in terms of its overall energy consumption, the standby energy consumed while it is inactive is more significant than the energy consumed during communication. We have chosen a multithreshold CMOS (complementary metal oxide semiconductor) circuit technology to greatly minimize the leakage power in standby mode. It suppresses the terminal’s overall energy consumption to a few microwatts in standby mode. This suppression technology enables an intermittently operating terminal powered by a coin battery to have a battery lifetime of more than five years.

3. Core network system technologies

The core network system of the WAUN supplements the functions of the low-performance terminals. We have developed two technologies that make the core network system secure and scalable: terminal management technology achieves high security by managing a variety of terminal information within a limited wireless bandwidth and network configuration technology offers high scalability by using distributed processing of network functions for a large number of wireless terminals.

(1) Terminal management technology

When mobile terminals are connected to a wireless network and provided with communication services, they should be authenticated and their locations should be managed. We have developed an authentication and mobility management protocol for the WAUN. This protocol minimizes bandwidth consumption and the amount of computation so that it is applicable to narrowband wireless links and low-performance terminals.

To ensure high security, we use authentication to prevent spoofing, encryption to prevent sniffing, and message authentication to prevent data alteration [3]. In the message exchange between wireless terminals and the core network system, we have reduced bandwidth consumption and the amount of computation by utilizing hash functions and wireless parameters for these security processes. To ensure efficient utilization of narrowband wireless links, a short identifier that is unique in each cell area is used to identify terminals in a wireless link instead of a long global identifier. These technologies provide authentication and mobility management of low-performance terminals. The security level is equivalent to those of other wireless systems.

(2) Network configuration technology

Since the WAUN requires authentication, location management, and data transfer to/from a huge number of terminals, it is necessary to use distributed processing on multiple servers to handle the enormous amount of processing. Although the WAUN supports a huge number of terminals, each one imposes a small load because the communication volume and frequency is very small, in contrast to conventional mobile networks. To reduce the overhead for the transfer of individual small items of data, the information obtained from mobility management is used as control information. This reduces the communication overhead for transferring control information.

The configuration of the core network system is shown in Fig. 5. We have divided the functionality of the core network system into the radio access network server (RANS), IP-GW, and management database (MDB). RANS performs terminal authentication and mobility management, IP-GW is a gateway to and from the Internet, and MDB manages terminal information. In addition to this functional distribution, RANSs are distributed geographically in accordance with the locations of paging areas, and IP-GWs are distributed in accordance with the volume of traffic from application servers. With this configuration, the network offers high scalability and flexibility against changes in the number of connected terminals [5].

Fig. 5. Core network system configuration.

4. Main specifications of WAUN

The main specifications of the WAUN are listed in Table 1.

Table 1. Main specifications.

4.1 Frequency band used

From the macroscopic viewpoint of ensuring that radio signals cover as many wireless terminals as possible, it is advantageous to use relatively low frequencies because this reduces the radio propagation loss. On the other hand, to cope with microscopic changes in the radio signal strength caused by reflected waves (fading), an effect that all wireless systems are subject to, it is necessary to use space diversity, in which each base station uses multiple antennas. If the spacing between antennas is fixed, space diversity is more effective when the frequency is higher. Considering this tradeoff regarding the frequency level, we expect that a frequency within the VHF/UHF band (200–500 MHz) will be used.

4.2 Modulation method

To achieve a large cell size, accommodate many terminals, and meet the low terminal power requirements, we have chosen π/4-shift quadrature phase-shift keying (QPSK) for both its modulation/demodulation characteristics and high efficiency in frequency and power resource utilization.

4.3 Transmission speed

Since machine-to-machine (M2M) communication is characterized by a low data transfer volume and infrequent communication, it does not require as high a transmission speed as wireless local area networks and mobile telephone systems. We expect that the transmission speed will be around 9.6 kbit/s, which is equivalent to that of low-speed wireless systems, such as radio-frequency identification (RFID) systems.

4.4 Cell radius and terminal transmission power

For the network to cover large geographical areas economically, it is necessary to use macro-cells, which have a cell radius of the order of several kilometers. Since we expect the transmission power of a battery-powered terminal to be around 10 mW, we have chosen a cell radius of 3.5–5 km.

5. Conclusion

As the WAUN becomes more widely used, it will become possible to collect and accumulate information from a wider variety of objects. We will continue with the development of technologies, such as data processing technology, that take full advantage of such information and contribute to enhancing the safety and security of communication and environmental friendliness.

References

[1]	O. Kagami, M. Matsuo, M. Harada, H. Hayashi, and S. Yoshino, “Wide-area Ubiquitous Network that Allows Communication with Objects,” NTT Technical Review, Vo. 8, No. 6, 2010. https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr201006sf1.html
[2]	Y. Fujino, D. Uchida, T. Fujita, and K. Watanabe, “Diversity technology for a wide-area ubiquitous network—Part II: frequency offset transmission diversity—,” 2008 IEICE General Conference (in Japanese).
[3]	S. Kuwano, Y. Suszuki, Y. Fujino, T. Fujita, D. Uchida, O. Kagami, and K. Watanabe, “Diversity technology for a wide-area ubiquitous network––Part I: diversity technology employing digitized radio on fiber technique—,” 2008 IEICE General Conference (in Japanese).
[4]	H. Toshinaga, K. Mitani, H. Shibata, S. Kotabe, S. Ishihara, J. Ichimura, and H. Sakuma, “Wide Area Ubiquitous Network Service System Architecture and Evaluation of Development,” IEICE Technical Report, IN2008-194, March 2009 (in Japanese).
[5]	H. Shibata, K. Mitani, K. Takasugi, H. Toshinaga, S. Kotabe, S. Ishihara, and H. Saito, “A Load Balancing Method for Radio Access Servers to Handle a Large Number of Wireless Terminals,” IEICE Technical Report, IN2008-42, July 2008 (in Japanese).

	Daisei Uchida Senior Research Engineer, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories. He received the B.E. and M.E. degrees in applied physics from Tokyo Institute of Technology in 1994 and 1997, respectively. Since joining NTT Wireless Systems Laboratories in 1997, he has worked on the access scheme for satellite systems, radio channel assignment and tree topology of PHS-based local positioning and information systems to form wireless multihop networks autonomously, radio channel assignment and transmission power control for mesh-type broadband fixed wireless access systems, and modulation-demodulation schemes for SDM-COFDM systems. He is currently engaged in R&D of modulation-demodulation schemes in wireless access system for the WAUN. He is a member of the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan. He received the Young Engineers Award from IEICE in 2001.
	Hajime Matsumura Senior Research Engineer, Supervisor, Ubiquitous Network Systems Laboratory, NTT Network Innovation Laboratories. He received the B.E., M.E., and D.E. degrees in engineering from the University of Tokushima in 1987, 1989, and 2000, respectively. Since joining NTT in 1989, he has been engaged in R&D of STM switching systems, a common software platform for switching systems, a distributed network architecture and distributed network performance simulation, a network-linked broadcasting system based on home servers, and business operation support systems for FLET’S and NGN services. He is currently developing WAUN systems. He is a member of IEEE and IEICE.
	Shigeru Kuwano Senior Research Engineer, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories. He received the B.E., M.E., and Ph.D. degrees in communication engineering from Osaka University in 1987, 1989, and 1992, respectively. He joined NTT in 1992. His current field of work is transmission techniques for wired and wireless technologies. He received the Best Paper Award from IEICE in 1996. He is a member of IEEE and IEICE.
	Fusao Nuno Senior Research Engineer, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories. He received the B.E. degree from Ehime University and the M.E. degree from Kumamoto University in 1991 and 1993, respectively. He joined NTT in 1993. He is currently engaged in developing WAUN wireless access systems.
	Nobuaki Mochizuki Senior Research Engineer, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories. He received the B.E. and M.E. degrees from Tohoku University, Miyagi, in 1992 and 1994, respectively. He joined NTT in 1994 and engaged in R&D of a modulation/demodulation scheme for a broadband 5-GHz wireless access system and synchronization scheme for coded OFDM. He is currently engaged in developing WAUN wireless access systems. He is a member of IEICE.
	Satoshi Kotabe Senior Research Engineer, Ubiquitous Network Systems Laboratory, NTT Network Innovation Laboratories. He received the B.E. and M.E. degrees in electronic engineering from Ibaraki University in 1993 and 1995, respectively. He joined NTT in 1995. He has mainly been working on high-speed data communication network architectures. His current research is WAUN protocols and architecture. He is a member of IEEE and IEICE.
	Kenji Suzuki Research Engineer, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories. He received the B.E. and M.E. degrees in electrical and electronic engineering from Tokyo Institute of Technology in 1999 and 2001, respectively. In 2001 he joined NTT Telecommunications Energy Laboratories, where he worked on a wireless transceiver LSI architecture for low power dissipation. His interests include analog and RF IC design for wireless communications. In 2009, he moved to NTT Wireless Systems Innovation Laboratory. He is currently working on WAUN wireless terminals. He is a member of IEEE and IEICE.
	Takafumi Fujita Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories. He received the B.S. and M.S. degrees from the University of Tokyo in 1999 and 2001, respectively. He joined NTT in 2001 and engaged in R&D of high-data-rate wireless LAN systems. He is currently engaged in R&D of wireless access systems suitable for ubiquitous network infrastructures. He is a member of IEICE.
	Koki Mitani Ubiquitous Network Systems Laboratory, NTT Network Innovation Laboratories. He received the B.S. degree in information and computer science and M.S. degree in computer science from Keio University, Kanagawa, in 2003 and 2005, respectively. He joined NTT Network Innovation Laboratories in 2005. He is currently working on WAUN protocols and architecture. He is a member of IEICE.

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