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This article was published in the December 1998 issue of
IEEE Personal Communications.

ABSTRACT

The sudden increase in the number of pagers and cellular phones, reflects the rising demand for person-to-person communication. This article will introduce the latest personal multimedia communication services and advanced techniques based on wireless access. Subjects discussed in some detail are digital data communication services using PHS terminals, personal mobility using premises intelligent network, and the nomad office using communication satellites. Possible future personal communication services will be also proposed.

Evolution of Personal Multimedia Communications Services in Japan

Takehiro Murase, NTT Advanced Technology Corporation
Minoru Ohyama, NTT Information and Communications Systems Laboratories

Recent progress in communications technology is about to bring dramatic changes to business and home life. In 1990, NTT announced the Visual Intelligent and Personal (VI&P) communications services plan as its service vision for the beginning of the 21st century [1], as shown in Fig. 1. We are engaged in wide-ranging research and development aiming at creating service systems for diverse applications.
The phase 1 experiments based on this VI&P plan, which were conducted between 1991 and 1993, focused on narrow-band integrated services digital network (N-ISDN) services to optimize the use of the existing INS Net. The phase 2 experiments were initiated in April 1993, and focused on perfecting new services and creating new applications based on optical communications and asynchronous transfer mode (ATM) technology [2]. The sudden increase in the number of pagers and cellular phones reflects the rising demand for person-to-person communication, even for private use.
The P in VI&P symbolizes this latest trend. Among these VI&P services, personal communication services will become the most advanced and should receive the most attention. Taking this into consideration, the phase 3 experiments are concentrated on personal-use multimedia services based on wireless access by means of the Personal Handyphone System (PHS) and satellites [3].
This article describes the phase 3 experiments and the technology involved. To cope with future demand for wireless personal multimedia services, research and standardization activities are being strongly carried out in many countries, such as advanced Global System for Mobile Communications (GSM) systems in Europe, the IS-95-based wideband code-division multiple access (CDMA) system in the United States, and a wideband CDMA system in Japan.
International Mobile Telecommunications in the year 2000 (IMT-2000) is to offer capacities of 144 kb/s outdoors and 2 Mb/s indoors. Under the existing wireless environment, the usable bit rate is limited to 28.8 kb/s; however, several interesting multimedia services can be provided using this level of capacity.
In Japan, the introduction of Personal Handyphone Service (PHS) has led to an explosion in mobile digital services. In this article, the expectations of personal multimedia communications are introduced. The main subjects discussed in some detail are digital data communication services using PHS terminals, personal mobility using premises intelligent network, and the nomad office using communication satellites. Attractive future personal communication services are also proposed.

The Present State of Mobile Communications in Japan

Cellular Phone and PHS Services in Japan

Cellular phone service started in Japan in December 1979, and for the first 10 years the service grew at a slow space. In 1992 the mobile business was separated from NTT, and a harsh competition environment was generated by the entry of new common carriers in mobile phone service. With this competition providing momentum, the mobile business has been growing remarkable well. The number of cellular phone subscribers has increased over the last several years to more than 30 million (Fig. 2).
Furthermore, new PCS services, namely PHS, were started in 1995, and new competition has been generated between cellular phone carriers and PHS carriers. At the end of August 1997, PHS had gained seven million subscribers during the short period of two years (Fig. 2).
The characteristics of cellular phones and PHS are summarized in Table 1.
Although PHS terminals cannot be used in cars or trains, they offer other advantages such as compactness, low power consumption, high-quality voice, and high-speed digital communication [4].
In opposition to PHS digital communication services, a cellular phone carrier began a new packet service in February 1997. Using three slots on a carrier of the packet physical channel, this packet system can achieve a user data rate of up to 28.8 kb/s [5].

The Market Size of Mobile Communication

Mobile communication is the market with the highest expected growth potential. The multimedia mobile communications report issued by the Japanese Ministry of Post and Telecommunications predicted the market size as follows. In 2000 the mobile services will be worth US$53 billion. In 2010, the market will grow to US$157 billion. Recently, MPT announced that its estimate of next year's market was US$40 billion. In a word, the mobile market in Japan is growing dramatically and at a higher rate than the forecast.

PHS

The Basic System

PHS can be implemented using cordless phone technology and the existing network. Not only are call placing and call receiving possible, but handover is also supported. PHS offers high-quality voice, high capacity, and very secure data transmission services. In addition, PHS gives one-number access service anywhere, which is one of the final targets in personal communications.
Figure 3 shows the network configuration of PHS. While PHS is connected to existing ISDN, signals particular to mobile communications become necessary, such as signals for location registration, information required for call termination, and signals for simultaneous calling in which multiple cells call a terminal at the same time.
The main parameters of the PHS air interface are shown in Table 1. The small cell radius of PHS enables base stations to be made smaller, because the transmission power is low. Cell stations are generally installed at locations that are not very high off the ground, such as the tops of telephone booths or telephone poles. PHS uses dynamic channel assignment and autonomous decentralized radio channel control technologies which enable an operator to efficiently and flexibly use the frequencies and avoid troublesome frequency reuse planning.
The air interface has been completely standardized, and the network interface between base stations and the digital network has been standardized as JT-Q921-b and JT-Q931-b by the Telecommunications Technical Committee (TTC), which is a Japanese standardization organization for network issues. The standardized network interface is based on an ISDN interface and was modified to support PHS-specific functions such as location registration, authentication, and handover.
There are a total of 77 frequency carriers allocated in the 1.9 GHz band for the system. Four control carriers are arranged for each PHS operator for public use, and one is retained as a spare. The spectrum currently reserved for republic use is 12 MHz. The spectrum for private use (11 MHz) can be shared by public use services. PHS uses time-division multiple access/duplex (TDMA/TDD) as the radio access mechanism. This makes it possible to increase user channels and frequency utilization efficiency. The cell station can automatically pick up carriers at random, select an available carrier which has no interference problem, and assign an available traffic channel. The small size of cells means more efficient frequency reuse and larger number of subscribers. However, it is difficult with PHS to expand service areas, and PHS does not support high-speed movement. Transmission quality is degraded due to the frequent handovers needed when the terminal user is moving at high speed.
PHS was designed to use existing digital networks. This has the effect of reducing system cost as well as the time for system construction. The most important point is that the PHS bearer bit rate is 32 kb/s, and this means that PHS has great potential to provide a mobile multimedia infrastructure in the present circumstances.

Enhancement of PHS

Personal communications are not limited to communications by voice. PHS will grow in four phases.

In phase one, modems over voice band are utilized.
In phase two, 32 kb/s bearer applications becomes popular.
In phase three, 64 kb/s bearer services will be supported by PHS.
In phase four, radio packet services will be developed.

32 kb/s Digital Bearer -- Because PHS currently uses adaptive differential pulse code modulation (ADPCM) codecs , data transmission at up to 9.6 kb/s is possible by transmitting analog signals from a data modem through the codec. Since the radio section has a speed of 32 kb/s, it is possible to offer this high speed to users. Standardization was finished at the end of 1996, and a service using the 32 kb/s interface was started in April 1997.
64 kb/s Digital Bearer -- As shown in Fig. 4, when two 32 kb/s lines in the radio section are used, the resulting 64 kb/s substantially improves connectability to ISDN. Standardization of the 64 kb/s air interface was already completed at the end of 1997. It is expected that the range for PHS data transmission applications will further expand when high-speed 64 kb/s transmission becomes possible. As a result, fairly good-quality video transmission can also be expected. PHS 64 kb/s transmission will be also used for ISDN services in mobile environment.
PHS Internet Access Forum Standard (PIAFS) -- To realize protected digital bearer services, an error control function is indispensable. According to our experiments, automatic repeat request (ARQ) is quite effective to control bursty errors in Rayleigh fading channels. ARQ techniques can be divided into three main types.

Stop and wait (SW) ARQ
Go Back N (GBN) ARQ
Selective repeat (SR) ARQ

Among them, SR ARQ is the most effective scheme. Since the SR scheme retransmits only the frames in which transmission errors occur, the order of the data arriving at the receiver is not guaranteed. Previously received frames must be stored on the receiver side until all the frames are assembled in order. This means theoretically that infinite buffering is needed to achieve ideal throughput performance. However, the capacity of actual receiver buffers is finite, and the sequence numbers assignable to frames are also limited. Usually, a finite set of numbers is repeatedly used with a cycle of modulo M. In the SR scheme with finite buffer capacity, the maximum number of new frames that can be transmitted continuously without waiting for collect reception acknowledgment from the receiver is referred to as the maximum outstanding frame number.
The ARQ scheme of a wired system can be designed with a sufficiently large modulo number that the possibility of the transmitted frames reaching the maximum outstanding frame is reduced to almost zero. However, bit errors occur in a bursty manner in the wireless environment, so it is virtually impossible to ensure a large enough modulo number. Thus, with SR-ARQ with finite buffering, some means of distinguishing frames with the same numbers but forming different modulo cycles is needed to ensure data continuity. The modulo operation using data field SR ARQ (MODS-ARQ) is proposed to distinguish identically numbered frames by comparing user data based on the notion that user data are random. This technique makes it possible to eliminate the fixed control field (e.g., modulo identifier or protocol identifier) that has traditionally been required in the frame format, thereby increasing the maximum transmission speed.
Standardization adopting the MODS-ARQ scheme is well in hand in the PHS Internet Access Forum as regards the PIAF standard (PIAFS) [6]. Comparing the user data fields should make it possible to distinguish identically numbered frames. Figure 5 shows the data transmission performance based on this protocol over a circuit with 200 ms delay response. The excellent 29.2 kb/s throughput can be achieved by the PIAFS protocol under the bad condition of a 0.5 percent frame error rate.
Figure 6 shows an advanced modem transmission network. Conversion into modem signals is carried out at a modem pool for transmission. By using a V.34 data modem and a protocol converter, 28.8 kb/s data transmission can be realized between a PHS data terminal and an ordinary modem data terminal. This service was also started in April 1997.

The Personal Communications Concept

Progress in personal communications can be considered to occur in three steps. The first step is that people always carry a portable telephone, and use it anytime and anywhere. This is called terminal mobility. The second step is to guarantee the mobility of the telephone number. Generally, a telephone number and a terminal have a one-to-one correspondence. In this second step, called personal mobility, the idea is to facilitate communication through the use of a personal number which is assigned to each person. This personal number is usually stored on an IC memory card; therefore, one can register the same personal telephone number to one's wired telephone and wireless telephone. The third step is to introduce personalized services. One can receive personalized communication services by storing one's own communication mode in the network.
Terminal mobility is already possible with conventional cellular phones and PHS services. Therefore, the primary issues are how to realize personal mobility and personalized services. As for network-type PMC services, the PHS network can detect the location of people using PHS terminals or wireless IC cards, which contain personal user numbers. The range of a PHS terminal is several hundred meters. Using the location information, the network-type PMC services can provide some custom-made services such as call forwarding. It is possible to access a user who has a wireless IC card by calling his/her personal number. Consequently, while terminal mobility has already been achieved with the conventional cellular phone, we have somewhat achieved personal mobility with the network-type PMC services.

The Progress of PMC Services Experiments

Although the term personal communications tends to remind us of personal telephones, the meaning of the term in the context of VI&P is that VI&P includes the concept of "my favorite service," which refers to custom-made services. Therefore, wired networks are also included in networks that offer personal communications services. The PMC services experiments are divided into two parts. One concerns terminal-type PMC services, which include services incorporating PHS terminals and non-telephone terminals. The other involves network-type PMC services, which include services incorporating PHS terminals and wired intelligent networks. Terminal-type PMC services are multimedia communications services and have the following advantages:

PHS terminals can be used as cordless telephones at home and as portable telephones outside. Thus, they provide excellent terminal mobility.
PHS terminals are more suitable for multimedia communications services than conventional personal digital cellular (PDC) phones because the radio bandwidth between the PHS terminals and the cell stations is 32 kb/s digital communications.

PMC Services Experiments

The system configuration for the PMC services experiments at the Musashino R&D Center is shown in Fig. 7. We set up 40 cell stations to cover the entire area of the center. Intelligent network equipment on the premises accommodates these cell stations with the I´ interface. The I´ interface has additional functions not included in the I interface: location information registration of PHS terminals and terminal authentication. The IN equipment, which is similar to the IN of the public network, consists of a switch and several workstations (WSs). The WSs handle call and service control. A private automated branch exchange (PABX) at the R&D center accommodates the intelligent network equipment with the I/I´ interface. This equipment supports many kinds of new services by processing location information to determine which PHS terminal is in each cell station. We have also constructed a PMC service experiment environment outside of the experiment room using a communication satellite.

Terminal-Type PMC Services

Modem-Type Non-Phone Services -- The PHS terminal includes an ADPCM codec that converts voice signals to 32 kb/s digital signals. In using this terminal for data communication (non-phone service), the data signal has to be converted to an analog signal that is the same as the voice signal. A modem can be used for this purpose. We used a 4.8 kb/s modem for this experiment. Figure 8 is a photograph of an e-mail service unit. It is possible to send and receive e-mail in a wireless environment by connecting personal digital assistants (PDA)s or notebook PCs to mail servers through PHS terminals. Similarly, it is easy to send and receive facsimile messages between handy facsimile units and facsimile equipment in conventional networks through PHS terminals. The quality is the same as that of G3 facsimile. These services enable us to manage delivery schedules and check inventories while outside the office. Therefore, the services help to move up delivery dates and improve customer service. Several advanced users have already introduced these modem-type non-phone services.
Digital Bearer Communications Services -- The communication capability of PHS at 32 kb/s, is about three times faster than that of conventional cellular phones. To make the most of this capability, a high-speed data communications service that does not incorporate modems was tested by bypassing the ADPCM codec in the PHS terminal. This is the most promising of the terminal-type PMC services. Figure 9 shows a unit running a high-speed database access service. By connecting a notebook PC with a PHS terminal to an in-house network via PHS wireless access, customers can create the same communications environment as in their offices, even though they are outside. A notebook PC connected to a PHS terminal can access multimedia information on Web servers anytime and anywhere in the world. The difference between this system and conventional wireless LANs is that a customer can use a communication environment similar to that available in his/her office even while outside, at the price of a lower data rate. Similarly, a digital still-picture transmission service, in which pictures taken by electronic cameras can be fed into PCs or printed out on printers via wireless access, is very effective for applications like prompt reporting of news photographs or insurance policy claim assessments (Fig. 10). The JPEG algorithm is used to compress the still pictures taken by the camera. Frame transmission time is about 10 s. We are also testing a field video service for transmitting video signals (Fig. 11). Effective video compression algorithms matched to mobile systems are expected. MPEG4, under development, shall play a role in the future. Here the tentative compression algorithms for video and voice signals are H.261 and CELP, respectively, both of which were modified to include an error collection algorithm [7]. The terminal shown in Fig. 11 is used for a video-on-demand service in mobile communications and includes PHS functions. The terminal can access multimedia information at a speed of 32 kb/s via wireless access. This is very effective for offering local information to tourists because the field video terminal can receive different multimedia information from each cell station, which covers a range of several hundred meters. By connecting the terminal to cameras installed at various sites through a network, the terminal can receive real-time video from these cameras. This is called the telescope service, and will be used for traffic surveillance and construction site safety management. A strong demand for faster wireless access services is expected in the future. Anticipating this demand, we have started testing a high-speed digital video transmission service that operates at a speed of 1.5 Mb/s and uses a submillimeter band. MPEG 1 coded videos from remote locations are displayed on portable terminal equipment on demand using fiber radio.
MPEG1 coded signals are sensitive to bit errors. In these experiments, however, the terminal is assumed to be stationary; moreover, physical-layer protection using convolutional coding and Viterbi decoding helps to achieve the good performance seen. We have also started testing the basic technology for voice calls, an essential service of PHS. Figure 12 shows a bone-conductive ear microphone/receiver unit [8]. This unit enables people to communicate with each other in noisy environments such as background levels of over 70 phones. By mixing conventional air-conductive sound with bone-conductive sound, low-noise and high-quality voice calls are possible in the noisiest downtown areas and at construction sites. Since this is a hands-free unit, users can work safely and efficiently during calls. Also, it can easily be used by physically challenged people.

Network-Type PMC Services

We conducted a series of tests to detect the location of people using IC cards and to provide sophisticated, adaptive services utilizing this user location information. The detection range of the IC cards is several meters, and is smaller than the range of PHS terminals. Therefore, we prepared exclusive antennas, which are not cell stations. Figure 13 shows a unit running a call-forwarding service. A network detects personal information from the IC card by wireless signaling or when the IC card is inserted in the terminal, and manages the information and the user's location information. The IN equipment always manages the latest user's personal and location information. The IN equipment transmits the user's call to the nearest company telephone based on the user's location information. Thus, the user can receive calls anywhere without having to rely on other call-forwarding procedures. We are also testing an enhanced IC card that includes a pager function. This function enables a user to determine whether calls are for him/her or for someone else in the room. If the user cannot receive the call, simple messages such as "I will call you back" can be sent.
Figure 14 shows an IC-card PHS phone. Users can access sophisticated transmission services at convention centers or stadiums by inserting their individual IC cards into leased IC-card PHS phones. The personal number makes it easy to process individual charges. Eventually, we should be able to construct a personal multimedia environment (a telephone, e-mail unit and other OA devices) anywhere by inserting the IC card in a PC [9]. This IC card has another function: wireless connection to a wired telephone and thus the wired network. Using the card, the following network services can be offered.
Figure 15 shows a WS running a multimedia auto-tracking service, which is being tested for use as a virtual LAN. This service can be used at exhibitions. As visitors move from one booth to another, the workstation (WS) in each booth is connected automatically to a presenter's computer. Thus, visitors can receive precise explanations with multimedia information from each exhibitor through the WSs. The WSs also function as TV phones. This service is called multimedia auto-tracking because it works as if the exhibitors were automatically tracking visitors as they move around the exhibition. We are also testing a security service based on the wireless IC card. By installing an antenna at the entrances of important rooms, the IN can open and close the doors depending on whether the personal number in the user's IC card permits access. In addition, we are testing PMC services using a communication satellite. These tests are expected to show that the PMC service environment can easily be constructed anywhere by using a satellite and a portable satellite Earth station. The Earth station accommodates two primary-rate ISDN circuits. E-mail, handy facsimile, high-speed database access, and video gathering are all being tested (Fig. 16) with this system.

The Next Stage of Personal Communication Services

Packet Data Services

NTT has developed a new network, called the Open Computer Network, to cope with the strong demand for computer communication. This network is based on connectionless techniques such as the Internet, and will provide drastically cheaper services than existing connection-based network services. In the radio fields, new cost-effective access to the open computer network is also demanded, and packet transmission is one attractive solution to this demand. One method is to use a control channel for packet data transmission, because control slots are rarely used. We can use this time slot when it is not occupied. Thus, we can deliver a packet service that does not degrade regular PHS services. Estimates by computer simulation are made here of the data transmission speed over the packet channel and the number of users that can be accommodated with a single-slot channel. Figure 17 shows a comparison of the maximum user data transmission speed between PHS-PD and circuit switching as specified by PIAFS. The actual transmission speed of PIAFS is 29.2 kb/s, slightly less than 32 kb/s because of the overhead needed for retransmission. In circuit-switched data communications, the channel secured by a call connection is used exclusively by that user. This means that the maximum speed can always be maintained at a constant level because the data transmission speed is not dependent on traffic. In the case of a packet channel, because a channel is shared by multiple users, the data transmission speed is reduced in proportion to the number of control packets needed for protocol control. Moreover, when traffic varies, the data transmission speed also varies because of the insertion of other user's data. In this comparison, the maximum user data transmission speed was regarded as the bit rate obtained with no other user.
The number of users that can be accommodated by a circuit-switched service is determined by the call loss rate. In the case of a packet channel, it is determined by the delay time because the protocol control characteristics result in delay-type service. Figure 18 shows the random access characteristics of reservation and permission in the MAC layer. Throughput along the vertical axis and traffic along the horizontal axis have been normalized on the basis of channel capacity of 1. Collisions occur when multiple users send reservation packets simultaneously, resulting in delays due to retransmission. Setting the maximum allowable value of the average number of retransmissions at 1 (average of two transmissions) results in a package channel usage rate of approximately 30 percent. The volume of packet data would be equivalent to approximately 25 random transmissions/hr by each of 100 users in a cell, with one transmission passing half an A4-size page (640 Japanese characters = 1280 bytes).
In this PHS packet system [10, 11] only one slot is currently allotted to packet transmission, but more slots can be assigned. In the structure shown in Fig. 19, four slots are allotted to packet transmission. Therefore, capacity is quadrupled, and 128 kb/s capacity becomes possible. Circuit-switched calls have higher priority, and only unoccupied slots are used for packet data transmission; thus, compatibility between both systems is easily attained. Figure 20 shows an example of the packet transmission network configuration. In the public environment, PHS cell stations distribute packet data and switched data to the OCN network and existing ISDN network, respectively. In the private environment, cell stations will distribute packet data directly to office LANs, and wireless LANs will easily be constructed based on PHS packet technologies.

An Advanced Handset for the Future

The technologies of downsizing and batteries are advancing dramatically. Only two and a half years have passed since PHS began commercial operation, but we are now into the third generation of PHS handsets. Handset size has shrunk by two-thirds, and continuous operation time has doubled. Figure 21 shows the wristwatch-type PHS terminal developed in NTT Laboratories. This unit contains all the basic PHS functions, including the antenna. It uses voice recognition for dialing. This kind of handset will be in commercial use in the near feature. These handsets were used in the Winter Olympics held in Nagano and were praised for their high convenience. Our dreams for the telephone have almost come true.

The Future Trend in Personal Multimedia Communications

Figure 22 shows the future trend in personal multimedia communications. In 1996, modem-type wireless data services remained popular in Japan. At the beginning of 1997 bearer services began, and the real multimedia mobile age started.
Toward the end of the 20th century, PHS 64 kb/s digital service and packet service will appear, and the tariff for wireless multimedia communications will be drastically reduced. At the beginning of the 21st century, IMT-2000 will have reached worldwide acceptance, and personal mobility will be greatly advanced.
Following IMT-2000, ATM wireless access services will appear and various personal multimedia communication services will be born. These systems will be merged or used together, and will suffice to provide advanced personal multimedia services. The final stage of the personal service concept, the really personalized service era, will emerge.

Conclusion

This article describes personal multimedia communications services experiments and the technology involved. The number of subscribers to PHS services has increased rapidly since the service began in July 1995. A lot of high-speed services will appear based on PHS since the standardization of high-speed digital transmission has already been accomplished. PHS is expected to become a prime player in personal multimedia communications in Japan.
Cellular phones are also spreading their business fields to multimedia services. NTT DoCoMo, the primary Japanese cellular company, just started packet services in February 1997 and this system is also expected to be a strong tool in the personal multimedia era.
In the next step of PMC service experiments, we will test a new secretary function using agent-based communication and a high-security system. These systems will use finger-print or voice recognition technology. Furthermore, we are planning to enhance user services that can be customized. VI&P aims to achieve personal services in the true sense of the word, and these trials will offer guidelines for future R&D.
The authors would like to thank their colleagues involved in developing this experimental system.

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Biographies
Takehiro Murase [M] received a B.S. degree in electric engineering, an M.S. degree in control engineering, and a Ph.D. degree from Tokyo Institute of Technology in 1968, 1972, and 1989, respectively. He joined NTT Electrical Communications Laboratories in 1972, and has been working in the research and development of equalizers, interference cancellers, and predistorters. He was involved in the development of 16-QAM microwave radio systems and the 256-QAM 400 Mb/s microwave radio system during 1980–1991. Since 1994, he had been involved in developing personal multimedia communication services and ATM Wireless Access systems in NTT Wireless Communication Laboratories. Currently he is general manager of the Wireless Communication Division of NTT Advanced Technology Corporation.
Minoru Ohyama [AM] received B.S. and a M.S. degrees from Tokyo Denki University, Japan, in 1972 and 1975, respectively, and a Ph. D. degree from the University of Tsukuba, Ibaragi, in 1991. Since 1975 he has been working in NTT Electrical Communications Laboratories, where he is engaged in research on telephone directory assistance systems, audio response equipment, voice store and forward systems, artificial intelligence for telecommunications, ISDN application systems, intelligent e-mail systems, and Visual Intelligent and Personal(VI&P) communications services.