Wireless Personal Communications 9: 255–270, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands.

High-Quality and High-Speed Wireless Multimedia Transmission Technology for Personal Handy Phone System HITOSHI TAKANASHI, TOSHIAKI TANAKA, TOMOYOSHI OONO, HIDEO MATSUKI and TAKESHI HATTORI NTT Wireless Systems Laboratories, 1-1 Hikari-no-oka, Yokosuka-shi, Kanagawa-ken, 239 Japan E-mail: [email protected]

Abstract. The Personal Handy-phone System (PHS), developed as a PCS, has been very widely accepted. This system offers many benefits as the infrastructure of the mobile data communication services that are expected to be available soon with the popularity of nomadic computing. Data communication systems based on PHS and some applications are proposed in this paper. One of the key technologies, an error control scheme, is also introduced. Three kinds of possible system configurations are discussed as a data communication platform for PHS. One of them has an ARQ function which corrects all errors that occur in the propagation path. Conventional application software on nomadic computers can be used in the mobile environment because this ARQ function is installed as a lower layer protocol. The introduced system connects all mobile computers to all servers that are on the networks which include ISDN, the conventional analog PSTN, cellular networks and PHS networks. Keywords: PCS, data communication, wireless, modem, and Rayleigh fading.

1. Introduction The expanding availability of increasing more convenient and advanced services has been realized by advances in telecommunication technology, and there is growing expectation for further advances in the future. PHS [1, 2] service is very promising and more than six subscribers have joined since the service started on July 1995 in Japan. PHS offers several advantages to both users and operators due to its enhancements and variations, all of which are achieved by making the best use of the digital network (ISDN). PHS is not considered just as an extension of the cellular phone, but as an enhancement of the radio access function, and so contributes to the personalization of telecommunication services. As PHS was developed as a PCS system that should offer better service quality than the conventional cellular systems, PHS transmits high quality voice on a 32 kbit/s-channel with ADPCM as the voice coder. The demand for data transmission in the mobile environment is very likely to rapidly increase in the near future, because of the continued penetration of nomadic computing. Many kinds of computers such as personal digital assistants (PDA), laptop personal computers and so on have been put on the market. Wireless data transmission up to 4800 bit/s is possible with voice band data modems through the ADPCM coder of PHS and wired analog networks. Since 4800 bit/s is not sufficient, a high speed data transmission scheme should be established for PCS. We propose a 19.2 kbit/s data transmission scheme based on the V.110 protocol; a high-speed, 28.8 kbit/s data transmission system that has a sophisticated error control scheme to overcome severe Rayleigh fading is also introduced. A mobile DTE (Data Terminal Equipment) with an adapter (ADP) is connected to the protocol transformer equipment (PTE) proposed here. The adapter and the PTE have a modified ARQ

256 Takanashi et al. function that compensates for errors occurring in poor PHS radio channels. The PTE has voice band data modems for connection to other DTEs with voice band modems through the analog network. Voice band modems are effective when using the 32 kbit/s radio channel. High-speed data from a mobile DTE can be transmitted to another DTE across the ISDN with a terminal adapter or with the ADP for a mobile DTE. The ADP supports ARQ and has an RS-232C (V.24) interface to a DTE. The terminal adapter (T/A) has the same ARQ function as mentioned above. Consequently, a mobile DTE can access any DTE on any network. Facsimile data can also be sent by using voice band data modems. Since DTEs are now supported in the mobile environment, they can be used far more effectively. It is now time to discuss various application services of PHS for these nomadic computers. Mobile multimedia service is one of the most promising services; it can be achieved by transmitting high quality and high speed data across the PHS environment. We propose two data transmission methods for PCS and key technologies for high quality data transmission. Furthermore, placement of the network equipment for data transmission is discussed in this paper. Finally, applications for the mobile multimedia environment are shown. 2. PHS Specifications PHS has been growing rapidly as a new mobile communication service. Its frequency band is allocated in the FPLMTS (Future Public Land Mobile Telecommunication Systems) band as shown in Figure 1, this band is similar to the DECT band. There are 77 carriers in the assigned 23 MHz band. Forty carriers (12 MHz) are assigned for public use, where each carrier has four duplexed TDMA/TDD slots. Base stations of this system are sited rather low to the ground on public telephone boxes, walls of buildings and so on. This placement and low transmitting power yield street micro cells which offer increased frequency utilization. This micro cell structure makes it possible to accommodate the really heavy traffic that is expected in the near future. 2.1. S YSTEM C ONFIGURATION The PHS service concept is to provide a convenient service like the cellular phone services in city areas cheaply. To this end, it is indispensable to utilize the existing digital network rather than installing a dedicated network as was done for the cellular phone service. This approach makes it possible for subscribers to use the wide range of services that the digital network supports. This PHS system configuration is shown in Figure 2. NTT added the PHS equipment to the existing public ISDN in order to realize the necessary functions such as location registration, hand off and authentication. The network interface between cell stations and the digital network has been standardized by the Telecommunication Technical Committee (TTC), a Japanese standard organization for network issues. This standardized network interface is based on the ISDN interface and modifications were carried out to add the functions listed above. 2.2. C OMMON A IR I NTERFACE The common air interface was standardized by the Association of Radio Industries and Businesses (ARIB: ex-Research and development Center for Radio systems) as RCR-STD-28 [3]. The main features of the air interface is listed in Table 1. This air interface is commonly used

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Figure 1. PHS frequency allocation.

Figure 2. PHS system configuration.

258 Takanashi et al. Table 1. Main features of PHS system. Frequency band Access method Traffic channels / RF carrier Modulation scheme Voice codec Transmitting power Radio transmitting rate Carrier spacing

1.9 GHz TDMA – TDD 4 π/4 – QPSK 32 kbit/s ADPCM CS: 20 mW / PS: 10 mW 384 kbit/s 300 kHz

for public service systems and private systems. Thus, the same terminal can be used outdoors and indoors. The terminal works as a cordless phone in offices and houses. 3. Data Transmission Infrastructure on PCS As shown in Table 1, PHS and DECT offer high speed 32 kbit/s digital radio channels. Each channel can be applied to data transmission for realizing a mobile multimedia infrastructure. This is an important advantage over the conventional cellular systems. High data transmission quality can be achieved because the cell radius is rather small. a few hundred meters, so the delay spread due to multipath fading does not degrade transmission quality. The same equipment for data communication on PHS can be used in small offices and houses as a local area network. One can access the LAN of his/her office, Internet access points via ISDN/PSTN from ones house and so on. 3.1. U NRESTRICTED D IGITAL B EARER As the network cell station interface is based on ISDN, the common air interface has some affinity with the ISDN protocol. It is easy to provide unrestricted digital bearer channels by adding the definition of bearer capability to the air interface and the ADPCM coding scheme. It is also possible to add a function that transforms the data speed of 32 kbit/s to 64 kbit/s to the cell stations as shown in Figure 3. The transformation scheme of the standardized I.460 method is suitable for PHS because of its 32 kbit/s air interface and the 64 kbit/s network interface. This digital bearer is not protected by an error control scheme, though. 3.2. DATA C OMMUNICATION

WITH

V.110 P ROTOCOL

The speed of 32 kbit/s is convenient because it is the intermediate speed adopted by the V.110 protocol. Considering the DTEs serial data interface, the start-stop synchronized interface is the most convenient. Placing a V.110 adapter between the DTE and the portable (mobile) station makes it possible to connect to conventional terminal adapters (T/A) as shown in Figure 4. Serial data streams with rates of 4.8 kbit/s, 9.6 kbit/s and 19.2 kbit/s are transformed into synchronized 32 kbit/s speed data in conformance with the V.110 procedure. This data is sent to the cell station and the speed is converted to 64 kbit/s using the I.460 scheme. The converted data passes to the terminal adapter via ISDN and is reconverted to start-stop synchronized serial data that can be accepted by DTE2: the reverse operations are performed

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Figure 3. PHS data transmission on 32 kbit/s bearer.

Figure 4. PHS data transmission using V.110 protocol.

on the counter propagating channel data. Asynchronous V.110 framing is possible due to burst error caused by Rayleigh fading. Some errors can be controlled by the upper layer applications. Conventional terminal adapters (T/A) on ISDN can be utilized in this method. This is the most important advantage of this configuration. 3.3. PHS DATA C OMMUNICATION

VIA

H IGH -S PEED DATA M ODEM

Most data communication users are using data modems on conventional analog networks. The users must be connected to PHS mobile users. Since PHS is connected to ISDN, digital/analog conversion should be performed in the network. We have developed a new data communication system on the conventional PHS as shown in Figure 5. To achieve high quality data communication, a new error control scheme was developed. The error control function is performed between an adapter (ADP in the figure) for data communication at the mobile site and protocol transform equipment (PTE in the figure). Digital data signals are converted into analog signals at PTE which has data modems and the error control scheme. The equipment is described in this section. 3.3.1. PHS High Speed Data Modem System Equipment The high-speed PHS data modem system has following equipment. (1) An adapter between DTE and PS (ADP) This adapter terminates the start-stop synchronization signals using a standardized in-

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Figure 5. PHS high-speed data modem system configuration.

terface such as V.24 (RS-232C) and terminates the 32 kbit/s bearer data from the PS. It has an ARQ function between PTE (Protocol Transformation Equipment) as is described later. (2) Portable Station (PS) and Cell Station (CS) The PS and CS transmitting and receiving the bearer data stream can be those designed for the conventional voice telephone service. (3) Protocol Transformation Equipment (PTE) The PTE consists of a terminal adapter that terminates the 64 kbit/s bearer signal. Stopstart signals on the bearer are converted into data modem signals. It has an ARQ function corresponding to that of the ADP. This PTE has a digital network (ISDN) interface corresponding to CS and an analog network interface as a data modem. The analog network interface can be replaced with the ISDN interface of 3.1 kHz audio bearer capability. Figure 5 shows one configuration that makes it possible to offer data communication services via a data modem. In this configuration, PTE is not part of the network equipment, but a protocol transform center which lies outside the network. Users should pay for two calls, one is the fee for a link between the mobile station and PTE, the other one is the fee for a link between PTE and DTE2. 3.3.2. Available Data Modem Speed Due to the high speed of the 32 kbit/s radio bearer channel, a 28.8 kbit/s data stream can be sent even if an error correction scheme is employed between the ADP and the PTE. The difference between the bearer speed and the modem speed corresponds to 10% of the bearer speed. V.34 is adopted as the data modem specification and gives the fastest data transmission rate. These three data transmission services are summarized in Table 2. The advantage of the V.110 protocol is that a conventional terminal adapter for the ISDN can be utilized. Notice that the high speed data modem offers faster data transmission and highest quality. This scheme makes it possible to connect terminals on the analog network to those on the digital network. Throughput and error control are discussed in the following section. The bit error rate depends

Wireless Multimedia Transmission Technology for Personal Handy Phone System 261 Table 2. Data transmission service comparison. Data transmission method

Transmission Quality rate

32 kbit/s digital bearer 32 kbit/s V.110 (start-stop synchronization) 19.2 kbit/s High-speed data modem 28.8 kbit/s

Terminal on wired network

; ;

BER 10−3 Dedicated terminals on ISDN BER 10−3 General purpose ISDN terminals (T/A) BER < 10−6 T/A + data modem (ISDN) Data modem (analog)

on the propagation environment and system design. The BER of 10−3 is an example. High speed data transmission systems of the future are being developed as the Advanced Wireless Access System [4]. Data compression (V.42 bis) is installed in ADP and PTE to enhance data transmission speed. 4. Error Control Techniques High layer data communication protocols have error control functions, however, they do consider the poor data communication channel characteristics due to Rayleigh fading. These channels not only suffer lower throughput, but the link is often aborted. In order to achieve high-quality data transmission, an error control scheme is indispensable. This section compares the techniques of automatic repeat request (ARQ) and forward error collection (FEC). 4.1. ARQ ARQ is categorized into the following three schemes: (1) “Stop and Wait (SW)” is the basic ARQ. A transmitter waits for a response indicating that the sent data was received correctly. After receiving the response, the transmitter sends the next data packet. (2) “Go back N (GBN)” can be more effective than the SW. A transmitter sends packets one after another and does not wait for any response. When a repeat packet is requested, the packet is sent again and packets following the requested packet are also re-sent. These re-sent packets may be redundant, however, ARQ frame numbering is greatly simplified. In a high quality channel, the degradation due to this redundancy is negligible. (3) “Selective Repeat (SR)” is the most effective ARQ scheme, especially for poor channels but it is the most complex method. In the SR scheme, the only requested packets are re-sent. To carry out the SR method, packet numbering management is the most important issue. Each packet has its own number, however, very large numbers can not be assigned due to the limited number bits available. Usually, a finite number is used cyclically. However, if a request signal is not received for longer than the cyclic duration due to consecutive errors, the request number can not be distinguished in the transmitter side. We have proposed a modified SR method that assigns numbering bits dynamically [5]. Bits in the data field are used to differentiate identically numbered frames.

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Figure 6. Average transmission speed various ARQ protocols on PHS (ARQ: 160 bits).

We call this ARQ scheme “MODS-ARQ” (Modulo Operation using Data field Selective repeat ARQ) [5]. It gives better throughput performance than conventional ARQs as shown in Figure 6. PHS offers much higher channel quality and speed than ordinary cellular phones. Considering these advantages, the throughput of 24 kbit/s is possible according to the computer simulation results shown in Figure 6 because the average bit error rate may be lower than 10−3 inside a cell. Burst errors in the radio channel can be offset by MODS ARQ. A comparison of GBN and WORM ARQ [6] developed for PDC (Personal Digital Cellular) as applied to PHS is shown in the same figure. This figure indicates that MODS ARQ has the best performance in the PHS environment. Since MODS ARQ was developed for mobile systems, it can be applied to other systems such as DECT, GSM and so on. Figure 7 shows the performance of MODS ARQ as applied to DECT. As the figure shows, it achieves higher throughput than MODS ARQ on PHS. Since the data clock rate of the air frame of DECT is faster than that of PHS, burst errors can occur more frequently. In other words, the frame error rate of DECT is lower than that of PHS. 4.2. FEC FEC codes can be categorized into block codes and convolutional codes. Generally speaking, convolutional codes offer the best performance in random error channels. Accordingly, we discuss block codes here. One of the most effective codes for burst errors is the Reed–Solomon (RS) code. The performance of this code, obtained by computer simulations, is shown in Figure 8. It is obvious that FEC is not effective in the Rayleigh fading channels that cause burst errors. Despite reducing the throughput by 27/31 (=0.871) due to coding, few gains are

Wireless Multimedia Transmission Technology for Personal Handy Phone System 263

Figure 7. Average transmission speed of various ARQ protocols on DECT (ARQ: 160 bits).

obtained. To get a significant coding gain, a huge degree of bit interleaving is indispensable. This, however, causes excessive transmission delay. All these things confirm that MODS ARQ is the most suitable error control technique for high quality data transmission. MODS ARQ does not offer a transparent channel (transmitting delay does exist), however, this is not much of a problem for data transmission. On the other hand, the residual errors permitted by FEC are a significant problem for data transmission. It should be concluded, that ARQ – especially, MODS-ARQ – should be employed as the error control scheme for the multimedia infrastructures such as DECT and PHS. 5. Mobile Multimedia Infrastructure and Applications It follows from what has been discussed that PHS can yield the infrastructure for mobile multimedia services if it adopts MODS ARQ and the V.110 protocol. We would like to discuss some applications for this high-performance infrastructure. 5.1. PHS H IGH -S PEED DATA T RANSMISSION A PPLICATION Figure 9 shows an example of a high speed PHS data modem application. As introduced above, the DTE is connected to another DTE via a digital bearer channel using MODS ARQ. In this figure, the connection data rate is 28.8 kbit/s. TCP/IP on PPP (Point to Point Protocol) is possible over this radio channel. The high quality (error free) channel makes it possible to provide high-throughput TCP/IP connections between DTEs. There are many TCP/IP appli-

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Figure 8. Effects of Reed–Solomon code in Rayleigh fading channels.

Figure 9. DTE to DTE file transfer system with high-speed PHS data modem.

cations such as “ftp” and many more multimedia applications on TCP/IP will be released in the near future. Because the PHS high speed data modem service offers error free channels, TCP/IP error control, which can degrade the throughput characteristics is not needed. This means that the throughput of TCP/IP can be maximized. 5.2. V.110 DATA T RANSMISSION A PPLICATIONS The V.110 adapter and the I.460 data speed transformation procedure make it possible to connect a mobile DTE to another DTE at the serial data speed of 19.2 kbit/s as shown in Figure 10. The same applications supported by the PHS high speed data modem are also supported in this environment. The difference from the PHS data modem service is the data transmission rate, transmission data quality, and lack of connection to the analog network. Error control is performed by the TCP/IP protocol in this application.

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Figure 10. DTE to DTE file transfer system with high-speed PHS V.110 protcol.

6. Function Arrangement of High-Speed Data Modem System The PTE can be placed at CS, local switch on a network or the external network as shown in Figure 11. PTE placement at the CS, shown in figure (a), makes high performance ARQ characteristics possible because the radio channel information like received signal strength can be used most effectively [7]. However, system cost is high due to the additional function needed in the CS. The PTE function should be aggregated because data traffic will be small compared to voice calls. If PTE is placed at a local switch as shown in figure (b), the PTE function can be aggregated to some extent because each local switch deals with many cell stations. Placing the PTE outside the network as shown in figure (c) realizes the most economical system from the viewpoint of system equipment cost. However, two calls are needed to communicate between DTEs. This means that the total charge would be more expensive even if both DTEs and the PTE were in the same local area. The best placement depends on the high speed data modem service traffic. In the first stage of service introduction, the PTE should be placed at the local switch or external to the network. 7. Current Development Since PHS is connected to ISDN as the figure of system configuration shows, this system has the potential of higher rate data transmission. Two slots can be assigned to the one user forming a 64 kbit/s channel to ISDN. Standardization is being worked on in ARIB and should be finished by the end of 1997. The radio channel control scheme (including dynamic channel allocation) is the main issue. An error control scheme based on MODS ARQ will be employed to establish a robust data communication channel against unstable mobile radio channels. Most ISDN applications that utilize a data channel of 64 kbit/s can be adopted. By utilizing four slots of PHS, up to 128 kbit/s data rate is available, however, there is no standardization schedule so far because the current regulation prohibits the use of three or more slots. A comfortable mobile multimedia environment is realized by utilizing various slots corresponding to the requested transmission speed up to 128 kbit/s. The 64 kbit/s channels, composed of two 32 kbit/s channels, can be bundled up by “Multilink PPP” to offer a higher speed channel for a higher layer. This system is under study in our laboratory.

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Figure 11. Possible functional arrangements.

8. Conclusions We introduced PHS and a promising PHS data transmission scheme. The PHS data transmission service makes it possible to offer a mobile multimedia infrastructure with effective error control. There are two suitable ways of transmitting data via PHS; 19.2 kbit/s start-stop synchronization with the V.110 protocol and the high speed of 28.8 kbit/s using a data modem and protocol transform equipment that performs error control. These functions will make multimedia ubiquitous in PHS. A really effective ARQ technique, the MODS-ARQ technique, was also introduced. By using these schemes, PHS services will become more attractive and mobile multimedia applications for the PHS will be released in the near future. Higher rate channels (64–128 kbit/s) will be available to enhance the mobile multimedia services based on PHS in the near future. Acknowledgement The authors wish to thank Prof. Kenji Kohiyama, as well as their colleagues of the Personal Communication Systems Department for several fruitful discussions.

Wireless Multimedia Transmission Technology for Personal Handy Phone System 267 References 1. K. Kohiyama, T. Hattori, H. Sekiguchi and R. Kawasaki, “Advanced Personal Communication System”, in IEEE Proc. of 40th VTS, Orlando, May 1990, pp. 161–166. 2. T. Hattori, H. Sekiguchi, K. Kohiyama and H. Yamamoto, “Personal Communication – Concept and Architecture –”, in IEEE Proc. of ICC, April 1990, pp. 1351–1357. 3. Research & Development Center for Radio Systems, “RCR Standard – Personal Handy Phone System (RCRSTD-28)”, December 1993. 4. K. Kohiyama and A. Hashimoto, “Advanced Wireless Access System”, in ITU Telecom ’95, Proc. of Technology, Summit 6-2, October 1995. 5. H. Matsuki, H. Takanashi and T. Tanaka, “A Study of ARQ with Modulo Operation using Data Field in Rayleigh Fading Environment”, IEICE B-495, in Japanese, March 1995. 6. Research and Development Center for Radio systems, “RCR-STD-27C”, November 1994. 7. K. Ishioka and H. Takanashi, “Improved Throughput Characteristics by ARQ with Weighted Majority Decision”, IEICE B-493, in Japanese, March 1995.

Hitoshi Takanashi is a member of the IEEE and the IEICE. He received the B.S. and M.S. degrees in electronic engineering from Keio University, Tokyo, in 1985 and 1987, respectively. He joined NTT Wireless Systems Laboratories in 1987, where he was initially engaged in work on trellis-coded modulation for 256 QAM. In 1991 he became involved in the development of mobile radio systems including Personal Handy-phone System (PHS) and the PHS Data Communication System using the PHS Internet Access Forum Standard, PIAFS. His current interests are in media access control (MAC) and physical layer issues for high speed ratio access. He received the Scholarship Encouragement Award in 1994. He is currently a Senior Research Engineer at NTT Wireless Systems Laboratories. During 1996–1997 he was a Visiting Scholar at the State University of New York (SUNY) at Stony Brook with Leading Prof. Stephen. S. Rappaport (advisor).

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Toshiaki Tanaka received his B.S., M.S., and Ph.D. degrees from Osaka University in 1975, 1977, and 1988 respectively. He joined Nippon Telegraph and Telephone Public Corporation in 1977. He is currently a Senior Research Engineer, and Supervisor of the Personal Communication Systems Laboratory, NTT Wireless Systems Laboratories. He is now engaged in the development of wireless multimedia communication systems including PHS and future PCS. He is a member of IEEE and IEICE of Japan.

Tomoyoshi Oono received his B.E. and M.E. degrees from the Science University of Tokyo, in 1987 and 1989, respectively. He joined the Radio Communication Systems Laboratories, Nippon Telegraph and Telephone Corporation in 1989. He was engaged in research and development of PHS (personal handy phone system). He is currently engaged in the development of wireless data communication systems in PHS. He is a member of the Institute of Electronics and Communications Engineers (IEICE) of Japan.

Wireless Multimedia Transmission Technology for Personal Handy Phone System 269

Hideo Matsuki received his B.S. degree from Tohoku University, Sendai, Japan, in 1991. He joined the Radio Communication Systems laboratories, Nippon Telegraph and Telephone Corporation in 1991, where he has been engaged in research and development of Personal Communication Systems. He is currently engaged in the development of wireless data communication systems especially the ARQ scheme on PCS. He is a member of the Institute of Electronics and Communications Engineers (IEICE) of Japan.

Takeshi Hattori received his B.S., M.S., and Ph.D. degrees from the University of Tokyo, Tokyo, Japan in 1969, 1971, and 1974, respectively. He joined the Electrical Communication Laboratory, NTT, Japan in 1974, and was engaged in research on the 800-MHz land mobile telephone system and high capacity mobile communication system. From 1986, he was head of the Mobile Communication Applications Section in the Radio Communications Networks Laboratory, and responsible for the development of advanced cordless telephone systems, maritime telephone systems, and high speed paging systems. From 1987, he was Research Group Leader of the Radio Communication Systems Laboratory, and was involved in the research of high speed digital mobile radio transmission technologies. From 1992, he was Executive Manager of Personal Communication Systems Laboratory in NTT Wireless Systems Laboratories and was engaged in the research and development of wireless personal communication systems including Personal Handyphone Systems and future personal wireless access systems. From 1997, he has been with Faculty of Science and Technology, Sophia University as a professor.

270 Takanashi et al. Dr. Hattori was awarded the IEEE Vehicular Society Paper of the Year in 1981. He has guest-edited two special issues on Wireless Personal Communications for the IEEE Journal on Selected Areas in Communications (JSAC). He is a member of IEEE Communication Society, IEEE Vehicular Technology Society, IEEE Computer Society, and Institute of Electronics and Communications Engineers (IEICE) of Japan.