Special Feature: Fourth-generation Wireless Access

Overview of the Fourth-generation
Mobile Communication System
Narumi Umeda†, Toru Otsu, and Tatsuro Masamura
Abstract
NTT DoCoMo is conducting research on the fourth-generation (4G) mobile communication system.
This article outlines the technical issues involved in establishing such a system taking into consideration
the capability and performance expected from future mobile communication systems. It also overviews
the system configuration and discusses activities related to the standardization of 4G mobile communication systems.

1. Introduction
The third-generation (3G) of mobile communication services, standardized as IMT-2000 (international mobile telecommunications 2000) [1], whichbegan in October 2001 in Japan, have generated muchanticipation regarding the development of a variety
of multimedia services such as video communications. We believe that this will lead to mobile communications taking a more central role in our daily
lives and to the expansion of this role as our lifestyles
use mobile communications as a stepping stone toimprove the quality of life over the next ten years.
Such an era will require a fourth-generation (4G)
mobile communication system that far surpasses thecapability of IMT-2000. Providing mobile communication services based on new technology involves
more than simply proposing and proving technology—it also requires field-testing of functions and performance, standardization of technical specifications,development of mobile terminals, and construction of
network facilities. New mobile communication services will require more time and effort to establish
than other types of communication services do.
Research and development for the timely introduction of a 4G system that has the performance required
to serve as a part of the future foundation for our society and lifestyle is in progress in NTT DoCoMo.
† NTT DoCoMo, Inc.
Yokosuka-shi, 239-8536 Japan
E-mail: umedan@nttdocomo.co.jp

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In this article, we describe a basic approach to thetechnical issues and system configuration involved inachieving the capabilities and performance requiredof the 4G system based on the research at NTT
DoCoMo. We also describe the trends in standardization concerning future mobile communication systems.
2. System objectives
2.1 Requirements(1) Broadband communications
Up until now, the traffic carried by mobile communication systems has mainly been voice communications. The second-generation (2G) system, the personal digital cellular (PDC) system, introduced imode services [2] that have brought about the currently popular form of Internet access, electroniccommerce, and e-mail, which are mainly text-based
data communications via a cellular phone. The IMT2000 system offers high bit-rate transmission services from 64 to 384 kbit/s, and the proportion of data
to voice traffic is expected to increase. Moreover, therising popularity of broadband services such as
ADSL (asymmetric digital subscriber line) and optical fiber access systems and office and home LANs islikely to lead to a demand for comparable services in
the mobile communications environment.
(2) Low cost
To make broadband services available so that users
can exchange various kinds of information, it is necessary to lower charges dramatically to keep the cost
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Technologies

at or below that of existing services. The IMT-2000
system aimed at lowering the bit cost and establishingeconomical rates, but the 4G system requires a broadband channel and an even lower bit cost.
(3) Wide service area
One feature of mobile communications is that it isavailable for use anytime and anywhere. These capabilities are also important for future mobile communications. When a new system is first introduced, it isgenerally difficult to provide such an extensive service area as the existing system, but customers will
not buy the new terminals if they have restricted service areas. Moreover, to support terminals that haverelatively large display screens, such as personal digital assistants (PDAs) and personal computers withwireless capability, especially ones used withadvanced services, which will often be used indoors,
we need to provide better coverage of indoor service
areas.
(4) Diversified services and ease of use
The target subscriber base for mobile communications comprises various types of users. In the future,
we expect to enhance the system performance andfunctionality to introduce a variety of services that
include not only ordinary telephone services, but also
services that transfer information utilizing all fivesenses. These services must be made easier for anyone to use.
2.2 Design objectives
The design objectives for meeting the above
requirements are shown in Fig. 1. Considering thatvideo and data communications will be the main features, the 4G system must provide even higher transmission rates and larger capacity (i.e., both number ofusers and traffic volume) than IMT-2000. Also, considering that the video transmission quality in current
broadcasting is achieved by a transmission rate ofseveral megabits per second, that LAN transmission
rates are from 10 to 100 Mbit/s, and that the rate of
ADSL is several megabits per second, the design
objective is a transfer rate of approximately 100
Mbit/s in an outdoor mobile environment and gigabitclass rates indoors. It will not be possible to accommodate future mobile communication traffic unless atransmission capacity of at least ten times that of the
IMT-2000 is achieved. To ensure throughput for communications between terminals and achieve highlevel realtime communications, it is necessary toachieve a low transfer delay time of 50 ms. Also,
assuming that future services will be based on Internet protocol (IP) networks, efficient transmission of
IP packets over wireless connections is also a necessity. While increased capacity is also effective in lowering the bit cost, the cost per bit must be reduced to
between 1/10 and 1/100 of the current levels by
reducing the infrastructure equipment, operation, andconstruction costs. The design objectives describedabove focus on services that have higher performance
than existing services, yet are easy to use. It is necessary to pioneer new markets by making use of thecapabilities and performance of the 4G system, suchas integration with indoor wireless LAN and wired
systems, and by implementing a mechanism for introducing new services in a short time.
3. Basic approach to 4G system configuration
3.1 Technical issues
The technical issues concerning wireless technolo-

Capability and performance

New service platform-Rapid deployment of new services-Easy development of new services

Start new services
based on new capabilities

Seamless connection and handoff
between heterogeneous access systems

Enhance services throughperformance enhancements

100 Mbit/s (peak rate in mobile environment)
1 Gbit/s (peak rate in indoor environment)

Information bit rate: 384 kbit/s
System capacity

10 times that of 3G
1/10 to 1/100 per bit

Cost

All-IP

Base station networktransport system: ATM
Transmission delay time

50 ms or lessIMT-2000

4G system (2010)

Fig. 1. Design objectives.

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Special Featuregy that need to be addressed to achieve the systemobjectives described above are shown in Fig. 2.
(1) High-capacity and high-rate transmission
IMT-2000, which employs wideband code divisionmultiple access (W-CDMA), achieves a transmission
rate of 2 Mbit/s with a 5-MHz frequency bandwidth.
Furthermore, technology for transmission at approximately 10 Mbit/s with the same frequency bandwidth using multi-level adaptive modulation anddemodulation is under development [3]. To achieverates of 100 Mbit/s to 1 Gbit/s, we must use a larger
frequency bandwidth and new transmission systems
that are suited to high-rate transmission. For data
communications, we will need a radio access systemthat can transmit packets efficiently. Considering theimportance of indoor area coverage in the future,technologies that allow use both indoors and outdoors must also be developed. To obtain the broadband frequencies for achieving high-rate transmission and meet the expected large increase in data traffic demand, we must consider new frequency bands
and we must develop the circuit technology needed tomake amplifiers and filters and understand radio
wave propagation in these bands. At the same time,
technology for making efficient use of limited spectrum resources is also important.
(2) Lower costs
With conventional system configuration technology, using a higher frequency band to achieve a higher
Signal transmission and antenna technology• High-rate transmission/high-performance
modulation and demodulation• High-capacity, broadband packet access• Adaptive array antenna• High-performance FEC, ARQ, TPC, diversity
reception
Radio wave propagation andradio link budget design• Microwave mobile propagation• Broadband propagation• Indoor propagationtransmission rate generally reduces the area of thecell that one base station can cover. Retaining theoriginal coverage area requires more base stations
and increases the network cost. To avoid that problem, it is necessary to expand the cell radius by using
higher performance radio transmission and circuit
technology such as improved modulation/demodulation techniques that can cope with a low signal-tonoise ratio, adaptive array antennas, and low-noise
receivers. To further reduce the system construction
and operating costs, we must study diversified
entrance links that connect base stations to the backbone network, autonomous base station control technology, and multi-hop radio connection technology
employing simple relay stations.
(3) Interconnection based on IP networking
One way to ensure that users of the new system do
not have restricted service areas is to ensure that new
terminals can handle the existing system as well as
the new one. Moreover, considering the demand forinternational roaming, a terminal that can be configured to work with multiple systems based on software
defined radio (SDR) technology [4] is an effective
way to cope with introductory periods and differences in operating frequency bands among different
countries and regions. Furthermore, future mobile
communication networks will be integrated with heterogeneous access methods and various kinds of cellswith interconnection capabilities based on IP netRAN architecture and control• IP packet control/distribution control RAN• QoS packet transmission control• Microcell configuration• Seamless control for connection to othernetworks

Entrance link

Optical link
RF circuit/EMC• Highly efficient amplifier• Super-low-noise receiver• Software defined radio• Microwave EMC

Entrance links• Broadband wireless links• Integrated optical/wireless network

Microcell
FEC: forward error correction
ARQ: automatic retransmission request
TPC: transmission power control
RAN: radio access network
QoS: quality of service
RF: radio frequency
EMC: electromagnetic compatibility

Fig. 2. Technical issues concerning wireless technology.

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Special Featureworking. Accordingly, interconnection and handover
between such various access systems are required inaddition to handover and roaming within one mobile
communication system.

(2) Cell classification and configuration according
to communication environment
The 4G system has cells for outdoors, indoors, andinside moving vehicles, as shown in Fig. 3. Outdoor
cells cover a wide area, unlike the hotspot areas ofwireless LANs, and allow high-rate packet transfer
for fast-moving terminals. Indoor areas are coveredby indoor APs, because the radio waves to/from outdoor base stations suffer large attenuation. Indoor
APs are designed not only to provide a high ratetransfer and simple operation, but also to competewith expected future wireless LANs. Furthermore,cells within moving vehicles such as buses and trains(moving cells/networks) are served by a mobilerouter (MR) that has wireless functions and relays
signals between a base station and each terminal in
the vehicle, rather than the terminals individuallycommunicating directly with the base station in theconventional method. This configuration is designed
to achieve efficiency in terms of terminal transmission power, transmission rate, control signal volume,
etc. A multi-hop connection, which is effective in
expanding the cell size, is being investigated as a way
to overcome dead spots caused by shadowing. Datatransmission via relay stations is expected to allow
communications even when the effects of limited terminal transmitting power and radio wave propagation
attenuation are large, as shown in Fig. 4 [5].
(3) Multimedia communications
Conventional IP networks have provided mainly
best-effort services, but with realtime applications
expected to increase as multimedia communication

3.2 System configuration(1) IP-based connection configuration
The 4G system will be configured for connection to
IP networks, considering efficient transmission of IP
packets, co-existence with other access systems, easeof system introduction, expandability, and other suchfactors. IP networks can also connect with or accommodate wireless access systems other than 4G systems. The 4G wireless access point (hereinafter 4GAP) will be connected to an access router (AR) as
shown in Fig. 3 and will have wireless control functions for wireless transmission, handover, etc., allowing communication with mobile nodes operating on
IP. The 4G-APs will form their respective cells. Whena mobile node moves between cells, handover will beaccomplished by simply switching access points andwireless areas if the two 4G-APs are connected to thesame AR. If the 4G-APs belong to different ARs,then the packet transmission route on the IP network
must be changed rapidly. The cooperative operationof 4G-AP switching and IP routing is important forsmooth handover. For handover between a 4G-AP
and an AP of another system, the mobile node musthave functions for accessing both systems. Handover
will be performed by monitoring and comparing different systems to select the one that is more suitable.

To other networks(Internet, etc.)

IP network

AR

4G-AP

Other
accessIndoor cell
Other
accessOutdoor cell
4G-AP

4G-AP

4G-AP

4G-AP

MR
4G-AP

AR: access router
4G-AP: 4G wireless access point
MR: mobile router

Moving cellFig. 3. 4G system configuration.

Vol. 2 No. 9 Sep. 200415

Special Feature

Direct communication isblocked by the building.

Direct communication is not possible because
the distance exceeds the single-hop range.
Building

Base station

Multi-hop
Multi-hopSingle-hop

Fig. 4. Communication problems overcome by multi-hop connection.

diversifies, the importance of services that take into
account quality of service (QoS) is also expected toincrease. The 4G system configuration allows for amechanism that guarantees the transmission rate tosome extent and that prioritizes packet transfer by the
packet type in cooperation with the IP network for
QoS-aware packet transmission on a mobile radio
link, which is the bottleneck.
4. Trends in standardization
4.1 ITU-R activities
In 2000, the year in which the prospect of introducing the IMT-2000 system came into view, the international telecommunication union (ITU) began
research on future development of IMT-2000 andother systems. In the ITU radiocommunication sector(ITU-R), investigation of Q.229/8 on future development of IMT-2000 and systems beyond IMT-2000
was assigned to study group 8 (SG8) working party
8F (WP8F), which was established in November
1999, and work on this topic began in March 2000. At
the world radiocommunication conference held in
June 2000 (WRC-2000), ITU-R resolved to conduct
research on future systems, including spectrum
requirements, to investigate the research situation at
WRC-2003, and to review spectrum requirements at
subsequent WRCs. ITU-R WP8F formulated a recommendation regarding a future vision to give direction to future technological development. The recommendation was approved at the February 2003 meeting of SG8 and forwarded to a higher-level organization, the radiocommunication assembly (RA). In RA,
it was approved as the framework recommendation inJune 2003. In WRC-2003 held in July, approval was
given for the agenda items of WRC-2007 to include
the frequency assignment for systems beyond IMT2000. In that recommendation, “systems beyond
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IMT-2000” is considered to cover all future mobile
communication systems, including the current IMT2000 and its enhanced versions. The various wirelessaccess systems will need to cooperate via the network
so that users can use the full range of capabilities ofthe systems beyond IMT-2000 without being awareof individual wireless access systems. Furthermore,
there is now recognition of the need for a new wireless access system and a frequency band for it to operate in to cover the performance region that cannot be
achieved by advanced IMT-2000 systems (transmission rates of approximately 100 Mbit/s during highspeed movement and approximately 1 Gbit/s when
not moving, although these bit rates assume sharing
by users and the specific values are research targets).
Furthermore, the target time for implementation of
the new wireless access system is 2010 [6]. In the
future, the study of spectrum requirements andresearch on specific technological issues are expected to make progress.
4.2 Activities in Japan
In Japan, the New Generation Mobile Communication Committee was established in the Information
and Telecom Council of the Ministry of Public Management, Home Affairs, Posts and Telecommunications between October 2000 and June 2001, the same
period during which ITU began research, and it formulated a vision for future mobile communications[7]. The results were reflected in ITU-R’s vision recommendation described above. Furthermore, basedon the committee’s findings, the mobile IT Forum(mITF) was established in June 2001 to investigate
and research the early implementation of the 4G system, mobile commerce, and other topics. [8].
4.3 Activities in other organizations
The Wireless World Research Forum (WWRF), an
NTT Technical Review

Special Featureorganization of mainly European vendors, is also
conducting research concerning a future vision for
wireless communications [9], and new research projects are being organized on the basis of the results. In
Europe, the 6th Framework Programme is being promoted. It established “Ambient Network” [10] and“WINNER” [11] this January and initiated researchon future systems.
4.4 Future plans
The vision for systems beyond IMT-2000 proposedso far by ITU-R coincides with the research objectives of NTT DoCoMo. We will continue to contribute to the standardization work being accomplished by ITU-R in coordination with future
progress in research. It is also important to makeglobal standards through cooperation with otherresearch organizations. IEEE, which has establishedwireless LAN standards, has begun to study nextgeneration wireless LANs. Considering the expected
increase in indoor use of the 4G system and the growing affinity for the Internet, further cooperation withIEEE standardization is necessary.
5. Conclusion
We outlined research projects toward the 4G system. We described system requirements, topics forstudy, and a basic approach to the system configuration. We also presented trends related to standardization in this field.
References
[1]
[2]
[3]

http://www.itu.int/home/imt.html
http://www.nttdocomo.com/corebiz/imode/index.html
F. Adachi and M. Uesugi, “Latest Trends in CDMA Technology,”
IEICE Journal, Vol. 86, No. 2, pp. 96-102, 2003.
[4] K. Uehara, K. Araki, and M. Umehira, “Trends in Research andDevelopment of Software Defined Radio,” NTT Technical Review,Vol. 1, No. 4. pp. 10-14, 2003.
[5] A. Fujiwara, S. Takeda, H. Yoshino, T. Otsu, and Y. Yamao, “System
Capacity Expansion Effect of Multi-hop Access in a Broadband
CDMA Cellular System,” IEICE Trans. B, Vol. J85-B, No. 12, pp.
2073-2079, 2002.
[6] T. Otsu, “The Challenge of Systems beyond IMT-2000—approachfrom wireless,” ITU Journal, Vol. 33, No. 3, pp. 26-30, 2003.
[7] http://www.soumu.go.jp/joho_tsusin/policyreports/joho_tsusin/
bunkakai/abstract.pdf[8] http://www.mitf.org/index_e.html[9] http://www.wireless-world-research.org/[10] http://www.ambient-networks.org/[11] https://www.ist-winner.org/

Vol. 2 No. 9 Sep. 2004

Narumi Umeda
Director, Communication Systems Laboratory,
Wireless Laboratories, NTT DoCoMo, Inc.
He received the B.E. and M.E. degrees in electronic engineering from Hokkaido University,
Sapporo, Hokkaido in 1985 and 1987, respectively. He joined NTT Laboratories in 1987. In
1992, he transferred to NTT Mobile Communications Network, Inc. (now NTT DoCoMo, Inc.).
He has been engaged in research on the radio linkcontrol for the personal digital cellular (PDC)
system and IMT-2000. His current researchinterests are in the radio control for 4G mobile
communications systems (beyond 3G). He was aco-recipient of the Japan Institute of Invention
and Innovation (JIII) Imperial Invention Prize in1998 and the best paper award of the International Conference on Telecommunications at
ICT2002. He is a member of the Institute of Electronics, Information and Communication Engineers (IEICE) and IEEE.

Toru Otsu
Director, Global Business Department, NTT
DoCoMo, Inc. (Formerly with the Wireless Laboratories)
He received the B.E., M.E., and Ph.D. degrees
in global information and telecommunications
from Waseda University, Tokyo in 1983, 1985,
and 2002, respectively. In 1985, he joined NTT
and in 2000 he transferred to NTT DoCoMo, Inc.
From 1992 to 1993, he was a visiting researcher
at ENST (Ecole Nationale Superieure desTelecommunications), France. Since joining
NTT DoCoMo, he has been involved in research
projects on 4G mobile communications systems
and related standardization activities. His
research interests are network architectures, network control protocols, and resource management in mobile and satellite networks. He is amember of IEICE and IEEE.

Tatsuro Masamura
Associate Senior Vice President, Managing
Director of Wireless Laboratories, NTT DoCoMo, Inc.
He received the B.E. and M.E. degrees in electrical engineering from the University of Tokyo,
Tokyo in 1974 and 1976, respectively. He joined
the Electrical Communication Laboratories, Nippon Telegraph and Telephone Public Corporation (now NTT) in 1976. Since then he has been
engaged in R&D of satellite communication systems based on CDMA and TDMA. He worked at
the Communication Research Center, Department of Communication of Canada as an
exchange scientist from 1983 to 1984. From1992 to 1994, he was a manager of satellite communications systems at NTT Network Development Center. From 1994 to 1999, he wasengaged in the development of multimedia services using satellite communications. From 1999
to 2002, he was the executive manager of NTT
Network Innovation Laboratories. Since April
2002, he has been with NTT DoCoMo as theManaging Director of Wireless Laboratories
responsible for R&D activities for next-generation mobile communications.

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