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INVITED PAPER

Special Section on 10th Anniversary of Trans. Fundamentals: “Last Decade and 21st Century”

Wireless Past and Future—Evolving Mobile
Communications Systems—
Fumiyuki ADACHI† , Regular Member

SUMMARY Nowadays, when people colloquially use theword “wireless,” they almost always mean a portable telephone.
Over the last 10 years, there has been tremendous growth in
the mobile communications markets not only in Japan but alsoworldwide. For these 10 years, the most popular service has
been dominated by voice communication. However, modern mobile communications systems are shifting their focus from solely
voice communication to electronic mailing and Internet access.
From now, we will evolve into a wireless multimedia society,where a combination of mobile communications and the Internet will play an important role. Wireless technology is the coreof mobile communications systems. This article, which focuses
on wireless technology, looks at how mobile communications systems have evolved over the last 10 years and looks to the futureof advanced wireless technologies that will be necessary to realize
a true wireless multimedia society in the coming decade.
mobile communications systems, wireless techkey words:
niques, multimedia communications, wireless Internet

1.

Introduction

Our ultimate goal is to communicate any information
with anyone, at anytime, from anywhere. This is only
possible through the aid of wireless technology. For
the decade, mobile communications have enhanced ourcommunications networks by providing an importantcapability, i.e., mobility. Before the introduction ofmobile communications systems, communications were
only possible from/to ﬁxed places, i.e., houses, oﬃces,
and public phones. It was December 1979 when theﬁrst public mobile communications services started inJapan. For the ﬁrst 10 years, its growth rate was very
low. However, through the liberalization of mobile communications services in 1988 and terminal markets in
1994, the growth rate of mobile communications services accelerated over the last 10 years. In Japan, thenumber of subscribers to cellular and Personal Handyphone System (PHS) services has reached close to 57
million by March 2000; this number is equivalent to apenetration rate of around 45% in the population. Oneof the most important technical factors for this successis the increased utilization eﬃciency of portable phones(lighter weight and longer talk time [1], made possibleby advanced LSI technology) and easier-to-use portablephones. Similar rapid growth rates in mobile commuManuscript received July 12, 2000.
Manuscript revised August 29, 2000.
†
The author is with the Electrical and Communication
Engineering, Graduate School of Engineering, Tohoku University, Sendai-shi, 980-8579 Japan.

nications services are evident worldwide. On the other
hand, the number of ﬁxed analog telephone users has
been continuously declining from its peak of 61 million
at the end of March 1997 to 55.4 million at the end of
March 2000 and was overtaken by the number of mobile
communications users. This clearly shows that people want to communicate with people, not with places.
Mobile communications systems have now become an
important infrastructure of our society. Mobile communications systems have now just started to evolve from
simply providing traditional voice and fax communications services to providing Internet services that can
exchange a mixture of voice, text, and images. Wireless technology is the core of mobile communications
systems. This article, which focuses on wireless technology, looks at how mobile communications systems
have evolved over the last decade and takes a look at
the future of advanced wireless technology that will benecessary to realize a true wireless multimedia society
in the coming 10 years.
2.

Wireless Internet Society is Our Future

In ﬁxed networks, voice conversation was a long-time
dominant service, but the introduction of Internet communications services has been changing our society ata very rapid pace. Figure 1 shows just how fast mobile
communications, personal computers, and the Internet
have grown in Japan [2]. It is evident that Internet
services have spread throughout our society at a much
faster rate than other services. Through the Internet,
users can easily browse WWW sites to retrieve vari-Fig. 1point.

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Fig. 3Fig. 2“i-mode services” available with 2G systems.

ous types of information including images, enjoy on-line
shopping and trading services, and almost instantly exchange electronic mail messages instead of using traditional postal services. In line with the increasing popularity of Internet communications in ﬁxed networks,
mobile communications services have shifted their focus from solely voice conversation to electronic mailingand Internet access. One good example is the success ofthe mobile Internet access services called “i-mode services” provided through the 2nd generation (2G) cellular systems using the Japan standard PDC-Packet
technology [3]. A variety of services are available, including e-mail, Web access, and on-line services ranging
from bank transactions to entertainment, in addition toconventional voice conversation (Fig. 2).
Now, it seems that a mobile phone is not just forvoice conversation, but is a communication tool thatenables various types of electronic communications forprivate as well as business use. Present 2G systems allow access to only text information due to their slow
data transfer rates, e.g., 9.6 kbps, and small displayson the portable phones. However, the 3rd generation(3G) systems, called the International Mobile Telecommunication Systems (IMT)-2000 in the International
Telecommunication Union (ITU) [4], will be deployedstarting around 2001–2002 with much better representation compared to present 2G systems. Up to a 2Mbps data transfer rate will be available.
3.

Cellular concept.

Past 10 Years of Wireless

3.1 Basics of Wireless Techniques
Today’s 2G systems are designed based on the wellknown cellular concept. Multiple base stations are distributed uniformly as much as possible over a wide geographical area to communicate with users. All the wireless channels in the available frequency bandwidth aregrouped into F groups and each base station is assigned
a diﬀerent channel group (an example of F = 7 is illustrated in Fig. 3). The same channel groups are reusedin diﬀerent clusters (F cells that are assigned diﬀerent

Fig. 4Multiple access techniques.

Fig. 5

FDMA, TDMA, and CDMA.

channel groups form a cluster). In this way, a wide geographical area can be covered using a limited frequency
bandwidth. To enable as many mobile users as possible to communicate with the same base station, wireless multiple access techniques must be adopted. There
are a number of multiple access techniques (see Fig. 4);
the type that is used depends on the type of traﬃc. If
there is continuous traﬃc requiring a very short transmission delay, e.g., voice conversation, demand-assign
based multiple access is applied, in which the channels
are divided in a static fashion and each user is allocated one or more channels by a base station during its
communication, irrespective of whether or not transmitted data is generated. The demand-assign based
multiple access includes frequency division multiple access (FDMA), time division multiple access (TDMA),
and code division multiple access (CDMA). Channels
are conﬁgured using the available bandwidth either infrequency, time, or code space (see Fig. 5). In CDMA,
unlike in FDMA and TDMA, all the base stations use
the same frequency bandwidth, i.e. F = 1, and allusers share the same frequency bandwidth and time,
but use diﬀerent spreading code sequences to separate
each user.

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On the other hand, if data generation is random
and has a high peak-to-average rate ratio, random multiple access is used, in which many users share one communication channel and users transmit their packets ina random or partially coordinated way. The most popular examples are ALOHA, carrier sense multiple access (CSMA), and idle signal casting multiple access(ICMA) [5]. Any random multiple access technique
can be combined with any demand-assign based multiple access technique. When a portable phone entersan active state, it requests a channel from a base station. The base station assigns one of multiple channels
based either on FDMA, TDMA, or CDMA. The assigned channel is shared by a group of active portablephones. The portable phone stays in the group until it
leaves the active state. The channel assignment acts as
admission control so that the channel does not become
overloaded. During the active state, portable phones inthe same group access a base station based on a random multiple-access technique. This type of wirelessaccess will be important for the 4th generation (4G)
systems that provide advanced Internet access services
to mobile users.
3.2 Evolution Path
Figure 6 summarizes the evolution path of cellular
mobile communications systems. Interestingly, every
decade, new technology has emerged that enhances thecommunications capability. We will look back throughthe evolution of wireless multiple access techniques inmobile communications history.
3.2.1 From 1G to 2G
The 1st generation (1G) mobile communications systems, i.e., AMPS, TACS, and NTT, were deployedaround 1980. These systems employed analog FM wireless access using narrowband FDMA with the channel
spacing of around 25–30 kHz [6]. Then, the 2G systems, i.e., North American standards IS54/136, European standard Global System for Mobile (GSM), andJapan standard Personal Digital Cellular (PDC), were

Fig. 6

Evolution path.

deployed in the 1990’s, all of which adopted TDMA
with the channel spacing ranging from 25 to 200 kHz.
Later, a wireless access technique based on narrowband direct sequence CDMA (DS-CDMA) was developed and IS-95 started its deployment [7]. Its channel
spacing is 1250 kHz, which is much wider compared toother 2G systems.
As mentioned earlier, major services provided by
the present 2G systems are shifting from voice conversation to multimedia communications over the Internet,
as indicated by the “i-mode services.” However, allof the 1G and 2G systems were designed so that they
can be optimized for basic services, i.e., voice, facsimile, and low-speed voice-band data. It is expected thatthe coming 3G systems can provide advanced “i-mode”
like services with much higher transfer rates and muchbetter representation.
3.2.2

From 2G to 3G

There are three strong reasons for developing 3G systems: multimedia, higher capacity, and a global standard. In 2G systems, the data transfer rate is onlyaround 9.6 kbps, which is far too slow for retrieving
rich information comprising text and images. A significantly wide data-rate range will be in demand, e.g.,
from as low as 8 kbps to a couple of megabits per second. Next, in order to cope with the still-continuing
rapid growth of mobile communications, the issue oflink capacity must also be addressed. Finally, establishing a global standard is becoming increasingly important in the 21st century, when more and more people
will travel around the world for businesses and leisure(2G system standards are, more or less, regional standards).
Data transfer rates of up to 2 Mbps and the same
quality as ﬁxed networks are the targets. Minimum
requirements in terms of data transfer rates and quality
for diﬀerent environments are summarized below.
– Indoor: 2.048 Mbps and bit error rate BER = 10−6
– Pedestrian: 384 kbps and BER = 10−6
– Vehicular: 144 kbps and BER = 10−6
For the transfer of image information of 1 Mbyte,
14 min. is necessary at a 9.6 kbps user rate in 2G systems, but the transmission time will be signiﬁcantly
shortened (to 4 sec.) with a 2 Mbps transfer rate.
The 3G system standardization process is in theﬁnal stage in the ITU. Wideband DS-CDMA has
been adopted as the wireless access technique [8].
As shown in Fig. 7, there are three operation modes:
Frequency division duplex (FDD) single-carrier, FDD
multi-carrier, and time division duplex (TDD) [9].
They will use the 2-GHz band. There are two fully
globally established core networks presently used forthe 2G systems: GSM-MAP for GSM systems and
ANSI-41 for AMPS and IS-95 systems. Both core net-

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Fig. 7

3G system.

works will evolve into 3G core networks and the abovementioned air-interfaces will connect to both GSMMAP and ANSI-41 core networks.
3G systems will be deployed worldwide starting
around 2001–2002.

Fig. 8

Conceptual conﬁguration of TCP/IP-based 4G systems.

Table 1Comparison of 1G to 4G systems.

3.2.3 Coming 10 Years of Wireless Technology
Figure 6 shows that the 4G systems should emerge
around 2010. A major objective is to oﬀer mobile users
broadband multimedia services, which will soon be infull force in ﬁxed networks based on next generation
Internet technology. Information transferred over theInternet will become increasingly rich. Most of the information may contain high-quality still and moving
images.
3.3 What is 4G?
As mentioned earlier in this article, the 3G systems
have a much higher data transfer capability and canhandle Transmission Control Protocol and Internet
Protocol (TCP/IP) traﬃc more eﬃciently than any
other 2G system. However, the 3G core networks willslowly evolve from 2G by taking advantage of theirlegacy and therefore, the 3G core networks may not be
fully optimized to the TCP/IP packet traﬃc. Therefore, it is the 4G system core network that must be fully
optimized to TCP/IP traﬃc. Data transfer rates of 3G
systems may not be suﬃciently high to handle a variety
of information comprising rich images. Completely different wireless and network architectures from 2G and
3G systems are thus, necessary.
Figure 8 illustrates a conceptual conﬁguration of
4G systems and Table 1 shows how 1G to 4G systems
are diﬀerentiated. The wireless part will become closer
to a wireless LAN, but with wide area mobility management as in the 2G and 3G systems. Mobile communications systems require many call control functions
and a distributed database, and quick and stable connections between these are necessary. These will beembedded in the TCP/IP based core networks basedon a virtual leased line concept. Voice traﬃc can betransferred as TCP/IP packets, i.e., voice over IP, but
how to guarantee QoS and reduce latency is a major

technical problem (this may be much easier to realize if
TCP/IP over asynchronous transfer mode (ATM) networks are used).
With respect to the wireless part, wireless Internet
access will be the core:– TCP/IP packet over the air– Broadband random multiple access– Signiﬁcantly asymmetric traﬃc between forward andreverse links.
We need to develop an eﬃcient wireless random multiple access technique. Probably, there will be demandfor peak throughput of more than 2 Mbps in a vehicularenvironment and more than 10–20 Mbps in stationaryto-pedestrian environments on the forward links [10].
Due to asymmetric traﬃc between the forward and reverse links, wireless access networks must be completely
redesigned from the 3G systems.
There are other approaches to provide multimedia
services to mobile users with the aide of wireless technologies [11]; they are wireless LAN and wireless ATM.
It is highly probable that cellular core network architectures will migrate to packet/cell-based architectures
and will converge with wireless LAN/wireless ATM architectures in the 4G era (or inter-technology mobility
management will be introduced among cellular, wireless LAN and wireless ATM networks). As one may
expect, TCP/IP traﬃc will dominate over the circuit
switched type traﬃc in the near future, e.g., voice, andthus, TCP/IP-based core networks are highly desirable

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for 4G services. Note that there are also many development activities to accommodate TCP/IP traﬃc overATM networks [12]. One advantage of the ATM network is that it can transfer eﬃciently diﬀerent typesof traﬃc, e.g., the delay sensitive voice traﬃc and besteﬀort type TCP/IP traﬃc, while guaranteeing their respective QoS requirements.
3.4 Will the Cellular Concept Disappear?
The frequency bands for the 4G systems will most likely
lie above 5 GHz. We must remember that propagation loss is in proportion to 2.6th power to the carrier
frequency [13], resulting in increased propagation loss.
The wireless channel links are not only interferencelimited but also severely power-limited. This suggeststhat a nano-cell or even a pico-cell structure must be
adopted and there is no doubt that an adaptive antenna array will play a key role in abating this power
problem.
Due to the nano/pico-cell structure, propagation
statistics are strongly inﬂuenced by microscopic structure of nearby propagation environments and dynamically change from cell to cell. Also, it is almost impossible for 4G systems to provide nationwide coverage. Only high multimedia traﬃc areas may be covered. These suggest that 4G systems may need to be
designed apart from the cellular concept that relies on
the statistical properties of propagation channels. Aninteresting question is: based on which concept should
4G systems be designed? One solution may be an ad
hoc wireless network. Base stations are installed where
they are needed and are added or deleted; they areconnected to each other in a self-conﬁguring way totransfer TCP/IP traﬃc. This is similar to the present
Internet architecture in ﬁxed networks. Of course, adynamic TCP/IP routing algorithm over ad hoc wireless networks according to user movement is necessary.
With very short-range coverage, coverage areas of ad
hoc network base stations cannot overlap. So, close
cooperation with 3G systems is necessary. The 3G cellular network will be the primary network, which can
be overlaid with the above ad hoc networks in the areas
such as business centers, shopping areas, airports, etc.
Inter-technology mobility management between the 3G
and 4G ad hoc networks is one of the important issues.
This requires so-called software radio technology to enable a single mobile terminal to access both 3G and 4G
systems.
One idea to reduce the portable phone’s transmit
power is to distribute many receive stations; transmit
base stations with large transmit powers can be colocated with the base stations of 3G systems. This is
illustrated in Fig. 9. This concept is particularly suitable to non-real time TCP/IP packet transfer, which
obviously does not require transmit and receive functions at the same base station.

Fig. 9Distributed receive stations.

3.5 Which Wireless Access Technique?
A debate similar to that concerning 3G wireless multiple access will undoubtedly occur again. When deciding
on multiple access techniques, we must remember thatthe propagation channel becomes a severe frequency selective channel; it is a diﬃcult challenge to realize a highrate and high-quality transmission. Advanced wirelesstechniques must be developed. The signiﬁcant diﬀerence between traﬃc in the forward and reverse links
may lead to diﬀerent wireless access techniques to be
adopted in the forward and reverse links.
One promising candidate is a hybrid random multiple access technique using orthogonal frequency division multiplex (OFDM) and CDMA, because both provide a good degree of robustness against multipath fading while ensuring ﬂexibility in the multi-rate and highspeed services. It should be emphasized that multiple
access may not be necessary in the same way on theforward and reverse links, e.g., DS-CDMA or OFDMCDMA on the reverse link and OFDM-CDM on theforward link. Furthermore, the frequency bandwidth
can be diﬀerent between the forward and reverse links.
In this respect, for the duplex scheme, TDD may be
more suitable than its counter part, i.e., FDD. Anotherpromising technique is adaptive transmission according
to the varying channel conditions; higher throughput isoﬀered for better channel conditions. This can be easily
achieved by random access with retransmission; however, minimum throughput must be guaranteed. For
this, the modulation level can be adapted to the channel condition.
4.

Conclusion

Mobile communications systems have now become an
important infrastructure of our society and now, areabout to evolve into wireless multimedia communications systems that can ﬂexibly oﬀer various types ofInternet services to mobile users. This article reviewed
the past 10 years of wireless technology and took a lookback over the evolution path of mobile communications
systems. In almost every decade, a new generation

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system appeared. Wireless access techniques evolved
from FDMA (1G) to TDMA (2G) and to CDMA (3G)
so that digital technology can be fully utilized to improve quality and frequency eﬃciency. The 4G systems
will emerge around 2010. We discussed the technicalissues for the 4G systems. Its core network will be
TCP/IP-based. Since the expected frequency bands areabove several GHz and data throughput over the air will
be more than several Mbps, wireless links are severely
power-limited. A hybrid random multiple access technique using OFDM and CDMA may be a promisingtechnique. Adoption of the well-known and long-timeused cellular concept may not be a good idea. The useof wireless ad hoc networks that allow ﬂexible installation of base stations may be a better solution. Furthermore, the receive functions can be separated fromthe base station and can be geographically distributed
to make it possible to reduce the transmit power of
portable phones.
Finally, the 21st century will be a wireless multimedia society, in which a combination of mobile communications and the Internet will play an importantrole. Before the realization of this society, very diﬃcult
but interesting technical challenges are waiting for us.
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Fumiyuki Adachi
received his B.S.
and Dr.Eng. degrees in electrical engineering from Tohoku University, Sendai,
Japan, in 1973 and 1984, respectively.
In April 1973, he joined the ElectricalCommunications Laboratories of Nippon
Telegraph & Telephone Corporation (nowNTT) and conducted various types of research related to digital cellular mobile
communications. From July 1992 to December 1999, he was with NTT Mobile
Communications Network, Inc. (now NTT DoCoMo, Inc.), where
he led a research group on wideband/broadband CDMA wirelessaccess for IMT-2000 and beyond. Since January 2000, he has
been at Tohoku University, Sendai, Japan, where he is a Professor of Electrical and Communication Engineering at Graduate School of Engineering. His research interests are in CDMAand TDMA wireless access techniques, CDMA spreading code
design, Rake receiver, transmit/receive antenna diversity, adaptive antenna array, bandwidth-eﬃcient digital modulation, andchannel coding, with particular application to broadband wireless communications systems. From October 1984 to September
1985, he was a United Kingdom SERC Visiting Research Fellow
in the Department of Electrical Engineering and Electronics at
Liverpool University. From April 1997 to March 2000, he was avisiting Professor at Nara Institute of Science and Technology,
Japan. He has published over 160 papers in journals and over 70
papers in international conferences. He has written chapters of
three books: Y. Okumura and M. Shinji eds., “Fundamentals ofmobile communications” published in Japanese by IEICE, 1986;
M. Shinji, ed., “Mobile communications” published in Japaneseby Maruzen Publishing Co., 1989; and M. Kuwabara ed., “Digital mobile communications” published in Japanese by Kagaku
Shinbun-sha, 1992. He was a co-recipient of the IEICE Transactions best paper of the year award 1996 and again 1998. He is asenior member of the IEEE and was a co-recipient of the IEEE
Vehicular Technology Transactions best paper of the year award
1980 and again 1990 and also a recipient of Avant Garde award
2000.