On the Road to Third Generation Wireless
BByy PPaauull W
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AAuugguusstt 55,, 22000022
Abstract
The wireless revolution, or more correctly evolution, is in full swing worldwide. In
this paper, the author presents a short history of mobile wireless telephony with
an emphasis on the relevant air interface technologies. FDMA, TDMA, and
CDMA are put in perspective as the wireless networks evolved from the first
generation to the second. The paper concludes with an explanation of the
evolutionary paths to third generation for GSM and CDMA systems.
Introduction
Cellular telephony arrived on the North American scene in 1983 with the rollout of
the Advanced Mobile Phone System (AMPS). After almost forty years in the
making, projections of only one million subscribers by 1990 led many to believe
that cellular phones were for a small segment of the population only. By 1990,
the U.S. had over five million cellular subscribers and today there are almost 140
million subscribers in the U.S. From the world perspective, there are now over
one billion users of wireless telephony. In fact, early this year wireless telephones
surpassed wired telephones in the world.
Early systems, now referred to as first generation (1G), used analog technology
called frequency division multiple access (FDMA) to deliver a radio-based voice
channel to a mobile telephone user. Problems included poor quality, limited
coverage, and less than adequate system capacity—but mobility ruled the day. In
the late 1980s, second generation (2G) systems were deployed using digital
technologies. The first U.S. system used time division multiple access, and was
known as North American Digital Cellular (NADC). We no longer use the term
NADC and simply call the system TDMA. In the early 1990s, TDMA technology
was used to introduce the Global System for Mobile Communication (GSM) to
Europe. In the mid 1990s, code division multiple access (CDMA) became the
second type of digital 2G system, with the U.S. introduction of Interim Standard95 (IS-95), now referred to as cdmaOne.
All of the 2G systems provided enhanced quality and better capacity. Roaming
became part of the service offerings and coverage continued to improve. Today
we have a combination of 1G and 2G systems and still face problems of limited
capacity in many markets. The industry is now moving to a third generation (3G)
system that promises better voice capacity, higher speed mobile data
connectivity, and multimedia applications.
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On the Road to Third Generation Wireless
FDMA, TDMA, and CDMA Explained
Before pursuing the 3G future, it is worthwhile to examine the operation of each
of the three air interfaces. First, one must remember that a mobile telephone is
nothing more than an FM radio with about 400 pairs of radio channels. Second,
these channels are paired so that one channel is from mobile to base and the
other channel is from base to mobile; this allows for duplex communication. In
Figure 1 we refer to the air interface as the uplink and downlink. Third, there is a
set of two-way control channels that control the voice channels. Last, the air
interface needs a process by which voice channels are allocated to multiple
users simultaneously. Enter FDMA, TDMA, and CDMA as the air interface
channel allocation schemes.
Figure 1: Wireless System Overview
FDMA was the first allocation method and it is the easiest to understand. A user
wishing to make a phone call signals their intention to do so by means of the
control channel. The operation is to enter the called party’s phone and depress
the send button. If there is voice capacity available in the cell, a channel pair is
assigned to the mobile station for the duration of the call—one channel for one
voice call. Assuming a typical layout of cells, the maximum number of voice calls
in any given cell would be about 60. Clearly, one cannot support millions of users
with such limited capacity.
TDMA systems alleviated the channel capacity issue by dividing a single radio
channel into timeslots and then allocating a timeslot to a user. For example, the
U.S. TDMA system had three timeslots per channel while the GSM system had
eight timeslots per channel (there are other significant differences that are
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On the Road to Third Generation Wireless
beyond the scope of this paper). To use these timeslots, the analog voice had to
be converted to digital. A voice coder, known as a vocoder, performs this
process. The initial capacity gains were small but with the advent of low bit rate
vocoders, the number of voice channels per radio channel could be increased
significantly.
CDMA systems took a very different approach to the capacity issue. It also used
the vocoder to digitize the voice but instead of allocating time slots, each voice
call was assigned a unique code before being added into the radio channel. The
process is often called noise modulation because the resulting signal looks like
background noise. The mathematical details behind the process are significant
but a real world observation can be used to somewhat explain the concepts.
Imagine that you have just landed at a major international airport and you are
entering the transit lounge in preparation for boarding your next flight. As you
enter the crowded room, you first notice the noise. Because you speak English,
you catch snippets of English conversations. Similarly, French ears hear French
voices; German ears hear German voices, and so on through the languages of
the world. You can pick out each conversation as long as the overall noise level
is below some maximum. This means that the maximum number of voice calls in
a CDMA system is a function of the background noise plus the noise created by
each voice call. Compared with TDMA, CDMA offers better capacity at
essentially the same or better quality. Figure 2 shows a simple graphical
comparison of the three air interfaces.
Figure 2: Comparison of FDMA, TDMA, and CDMA
Of the one billion plus mobile telephony subscribers in the world, about 690
million use GSM, 120 million use CDMA, and the remaining 290 million use
FDMA or TDMA. Across all the digital systems, one finds a remarkable similarity
between voice and data services. As we move to 3G, the GSM and CDMA
systems will evolve whileTDMA and FDMA will be sent to the dustbin of history.
The GSM path ends with Wideband CDMA (WCDMA) whereas the CDMA path
ends with cdma2000.
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On the Road to Third Generation Wireless
The 3G Vision
In the 1990s, the International Telecommunication Union – Telecommunication
Standardization Section began work on a vision of the future for public land
mobile telecommunications systems. The resulting product was called
International Mobile Telecommunications-2000 (IMT-2000). As an aside, the
“2000” was added to imply that these services would be available around the
year 2000. It now appears that these services will become available during 2002.
IMT-2000 is much more than a set of services, it fulfills the dream of anywhere
and anytime communications. To do this, it provides a framework for the
integration of terrestrial and/or satellite-based networks. Moreover, IMT-2000
discusses the networks’ aspects of wireless Internet, convergence of fixed and
mobile networks, mobility management (roaming), mobile multimedia functions,
internetworking, and interoperability.
As specified, the 3G systems should work in a universally acceptable spectrum
range and provide voice, data, and multimedia services. For the technically
stationary user operating in a picocell, the data rate would be up to 2.048 Mbps.
For a pedestrian user operating in the microcell, the data rates would be up to
384 kbps. For a user with vehicular mobility operating in the macrocell, the data
rates would be up to 144 kbps. Figure 3 shows the relationship of the various
IMT-2000 service areas. A critical part of this system is providing packetswitched data services. The evolution from 2G to 3G begins with the creation of
robust, packet-based data services.
Figure 3: IMT-2000 Service Areas
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On the Road to Third Generation Wireless
From GSM to 3G
The only true version of 3G wireless in the GSM evolution is Wideband CDMA. In
the European market, one hears WCDMA being referred to as the Universal
Mobile Telecommunications System (UMTS). WCDMA and UMTS are one and
the same; the names have been changed to confuse the populace. The major
question in this evolution is: How many steps will it take to get there?
In the structure of 3G services, there is a need for a tremendous amount of
bandwidth and thus a need for more spectrum. The European carriers spent over
$100 billion to purchase spectrum for 3G services; other carriers in the world
have also allocated 3G spectrum. In the U.S., the FCC has not allocated any
spectrum for 3G services and an allocation is not expected soon. As an aside,
the U.S. has about 190 MHz allocated for mobile wireless services whereas the
rest of the world has about 400 MHz allocated. One thing is certain; the 3G
evolution in the U.S. will be different from the rest of the world.
Starting with a basic GSM system, the first step in any evolution is to introduce a
packet-switched data service that is more sophisticated than the Short Message
Service. The General Packet Radio Service (GPRS) meets this need and today
there are over 50 GPRS-capable networks worldwide including three in the U.S.
market.
The major problem in implementing GPRS is choosing the number of channels to
allocate for GPRS data. GSM uses eight timeslots per 200 kHz radio channel.
Without GPRS, these timeslots can accommodate at least eight voice users. If
the spectrum is already over utilized with just voice, then where does the data
go? The solution is to make trade-offs between data capacity and voice capacity.
For each timeslot allocated to data, we have a data rate of 14.4 kbps. If all
channels were allocated to data, the rate would be 115.2 kbps. In reality, most
providers begin with an uplink (mobile to base) of one data channel and a
downlink (base to mobile) of three data channels. With overhead, the effective
rates are somewhere in the 20–40 kbps range. Since true 3G services start at
144 kbps, some U.S. providers are calling their GPRS implementations 2.5G to
differentiate the service from the older 2G offering.
The second step to 3G actually delivers a true 3G data rate. The Enhanced Data
Rates for GSM (or Global) Evolution (EDGE) can provide data rates up to 384
kbps. EDGE uses the same 200 kHz channel with eight timeslots and gets its
improved speed by using a more efficient modulation scheme. Instead of 14.4
kbps per timeslot, EDGE achieves 48 kbps per timeslot. Allocating the eight
timeslots for data yields the 384 kbps speed. Most analysts believe the actual
rates will be in the 64–128 kbps range.
The strength of EDGE is that it uses the traditional channel size, thus requires no
additional spectrum. As of this writing, it appears that only the U.S. market will
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On the Road to Third Generation Wireless
move to EDGE. In other markets the move will be directly to WCDMA using the
new 3G spectrum.
WCDMA is truly a broadband radio service. It will use at least a 5 MHz channel to
deliver data at rates of up to 2 Mbps. Currently, there are WCDMA trials in both
Europe and Japan so the technology is well on its way to commercial availability.
From IS-95 to cdma2000
The CDMA world will not instantly morph into a 3G scenario because of the lack
of spectrum in the U.S. market. Interestingly, the Korean market is already
experimenting with cdma2000 in its 3G spectrum. As we saw with the GSM
evolution, the U.S. and the rest of the world will take different roads to 3G
systems.
Cdma2000 is structured in a way that allows some 3G service levels in the
traditional 1.25 MHz IS-95 channel. These services are referred to as cdma2000
1xRTT(one times the IS-95 channel size radio transmission technology). At full
3G capability, cdma2000 uses a 3.75 MHz channel, three times the traditional
channel, and is called 3xRTT.
The 1xRTT system uses a more efficient modulation scheme to double the
number of voice users and create data channels of up to 144 kbps. This upper
speed has allowed some carriers to claim that they are offering 3G today. In
reality, the user speeds will be in the range of 50–60 kbps. Data in the 1xRTT
scheme would be packet-switched to ensure efficient channel use.
Speeds of up to 2.4 Mbps can be achieved by implementing 1xEvolution-Data
Only (1xEV-DO) but this is a data only service—no voice allowed in the channel.
When 1xEV-Data/Voice (1xEV-DV) is eventually offered, then the true
multimedia channel will be available.
Beyond 1xEV-DV, one gets into the realm of multichannel cdma2000. The
3xRTT would be a 3.75 MHz channel implemented in 5 MHz of spectrum—the
remaining 1.25 MHz is used for upper and lower guard bands. There are
operational scenarios for 10 MHz, 15 MHz, and 20 MHz of spectrum. Figure 4
compares the channel sizes and chip rates for UMTS and the CDMA 1x and 3x
scenarios.
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On the Road to Third Generation Wireless
Figure 4: Defining the Chip Rate
Summary
It is clear that there will be several roads to 3G mobile wireless systems. It is also
clear that the vision of IMT-2000 has won widespread acceptance. However, the
incompatibility of 3G technologies, the shortage of spectrum in many markets,
and the lack of 3G applications and handsets pose some significant near term
problems.
From the technology perspective, WCDMA and cdma2000 both use spread
spectrum techniques. However, they have different channel configurations,
chipping codes, chipping rates, and synchronization procedures. It will be some
time before harmonization of these technologies occurs.
As for spectrum, some countries have it and others don’t. Moreover, the
spectrum varies from country to country and most, if not all of it, is in use today
for other applications. It will be an expensive and time-consuming task to sort out
all the spectrum issues worldwide.
Finally, there needs to be a set of compelling applications. Wireless packet data
services will allow for the advent of always-on services. We are already seeing
the popularity of e-mail and instant messaging to PDAs and handsets. Now we
need to get the array of multimedia applications that will require the data speeds
provided by 3G systems.
These issues aside, there are competing wireless technologies that may obviate
the need for the 3G wireless systems described herein. Already the 802.11
wireless LANs with speeds in the order of 10–50 Mbps are becoming the de facto
connection method for laptops. Can the use of 802.11 in PDAs and handsets be
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On the Road to Third Generation Wireless
far behind? Factor in things like Bluetooth and ultrawideband communications
and the field of broadband mobile wireless communications gets filled with a
number of viable players. As a final thought, Figure 5 puts the alternatives to
some aspects of 3G in perspective. The question is whether these services will
complement 3G or compete with it.
Figure 5: Ultrawideband, 802.11 and Bluetooth
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On the Road to Third Generation Wireless
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