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FDMA,
TDMA, CDMA & GSM - What is the future?
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
Standard-95 (IS-95), now referred to as cdmaOneÒ.
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.
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 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 cdma2000Ò.
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 packet-switched data
services. The evolution from 2G to 3G begins with the creation of
robust, packet-based data services.
Figure 3: IMT-2000 Service Areas
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 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.
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 email 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
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
Courtesy:
Hill Associates |
