The Wide World of Wireless
From AMPS to IMT-2000, the international
wireless scene has a huge cast of systems
and standards. Here?s your program guide to
the big show.
by Gilbert Held
Any discussion of wireless communications inevitably brings up an alphabet
soup of acronyms. Along with Advanced Mobile Phone Service (AMPS),
Frequency Division Multiple Access (FDMA), Code Division Multiple
Access (CDMA), and Global System for Mobile Communications (GSM),
you have CdmaOne, IS-95, and of course International Mobile
Telecommunications Year 2000 Initiative (IMT-2000). It?s enough to make
the Sprint dime lady scream.
The easiest way to understand what
all these acronyms mean is to go
back to the beginning of the wireless
communications industry and see
how the technology evolved. You
can easily learn the meanings of
these terms while exploring the
history of the industry.
IN THE BEGINNING
In the 1970s, Bell Telephone
Laboratories developed the first
wireless transmission system to
provide service to mobile
subscribers. The resulting commercial system consisted of three major
components? cellular phones, base stations, and a Mobile Telephone
Switching Office (MTSO).
Although significant technological developments have occurred since the
1970s, the basic components of a cellular system haven?t changed. In
addition, the basic concept behind cellular communications has also remained
the same. This concept divides a serving area into geographic zones referred
to as ?cells.?
AMPS
The analog cellular telephone system invented by Bell Laboratories uses
FDMA. This system assigns the 800MHz to 900MHz frequency spectrum
(also called the 800MHz band) to cellular operators and subdivides the
spectrum into 25KHz channels. This analog cellular system is also known as
AMPS.
Under AMPS, a cell is subdivided by frequency into distinct channels, which
allows multiple access to cells. The service can assign only one subscriber at a
time to each channel within a cell, and the channel remains occupied until a
mobile subscriber reaches a location where signal strength decreases to a
predefined level. At that point the base station serving the subscriber informs
the MTSO that the subscriber is reaching the limits of the cell. The MTSO
then hands off the call to the antenna of the base station cell where the
subscriber is heading.
That handoff to an adjacent cell occurs on a channel on a different frequency.
This alleviates the possibility of interference between adjacent cells. However,
the amount of bandwidth allocated to AMPS, its use of 25KHz per channel,
and the restriction on using the same frequencies on adjacent cells limits the
number of simultaneous calls a cell can support. Figure 1a illustrates the
FDMA concept, where conversations are restricted to a predefined
frequency associated with a channel in a cell.
AMPS currently has the most widespread wireless coverage in North
America, due to the way the FCC licensed both wireline and nonwireline
carriers to serve major metropolitan areas. It can also, in emergencies, be
used to transmit data (see ?AMPS to the Rescue?). However, success began
to kill the golden goose. As more and more users purchased cell phones with
one-year service contracts, they soon began to encounter blocking, in which
calls couldn?t be placed. Also, when a cell lacked capacity to assign a channel
to the arriving mobile user, some users experienced dropped calls when
leaving one cell and moving into another.
Seeing that fixed-cell analog cellular systems
couldn?t serve the growing base of
subscribers, cellular developers investigated
different access methods and came up with
two new systems: Time Division Multiple
Access (TDMA) and CDMA.
TDMA
TDMA divides radio channels into time
slots, each slot consisting of a small fraction
of a second. These time slots are then
assigned among eight subscribers, which
substantially increases the capacity of a cell.
Figure 1b illustrates how two conversations
that would appear on separate channels
under FDMA are allocated to one channel
under TDMA.
TDMA-based cellular systems operate at
800MHz or 1900MHz in North America
and are referred to as digital cellular or
Personal Communications Services (PCS).
Digital cellular systems operate at 800MHz
and North American PCS systems operate
at 1900MHz. Because PCS systems
operate at higher frequencies and the
wavelength is proportional to the reciprocal
of frequency, the wavelength is shorter. The
shorter wavelength results in a smaller cell
diameter, which means a 1900MHz system
requires more cells per geographic area than
an 800MHz system.
TDMA normally coexists with analog
channels on the same network, which
enables subscribers to enjoy widespread
analog network coverage while TDMA
coverage continues to grow. Of course, you
need a dual-mode phone to take advantage
of this.
TDMA is also referred to as Digital-AMPS
(D-AMPS) and North America TDMA
(NA-TDMA). To add to the confusion, you
may periodically see the terms IS-54 and
IS-136 associated with TDMA. The TIA
standardized the first implementation of
TDMA as IS-54.
A proposed ?next generation? version of
cellular TDMA is referred to as IS-136.
This technology will provide data
transmissions of up to 43.2Kbits/sec. It also
has the ability to evolve into a third generation (3G) system that supports
transmissions up to 384Kbits/sec from mobile users and up to 2Mbits/sec
from fixed users. I?ll talk more about 3G systems later in the article.
CDMA
CDMA is another multiple-access technique employed by cellular operators.
In North America, CDMA is based on the IS-95 standard developed by
Qualcomm (www.qualcom.com). Instead of creating time slots or dividing the
radio frequency spectrum into separate channels by frequency, CDMA uses
spread spectrum transmission to obtain radio channels approximately six times
wider than TDMA and AMPS. The CDMA system assigns digital codes to
active subscribers. This enables pieces of a conversation to be spread out
within a channel by frequency and time, as shown in Figure 1c.
To ensure that a sufficient amount of information reaches the destination to
accurately reproduce the conversation, each small voice sample is repeated
several times. Although some voice samples may be lost to interference,
usually a sufficient amount reaches the receiver to ensure the receipt of an
acceptable signal.
The TIA adopted the IS-95 CDMA standard as a digital cellular standard in
1992. Since its introduction, the CDMA subscriber base has rapidly
expanded, and now has over 30 million subscribers. CDMA deployments are
under way in more than 35 countries, adding to the 27 countries where the
technology is already in commercial service. North America currently has
over 10 million subscribers, while the Asia-Pacific region has approximately
20 million. Other areas of the globe using CDMA include the Caribbean,
Latin America, Europe, the Middle East, and Africa.
One advantage of CDMA is its ability to extend system capacity. CDMA can
provide approximately 10 to 20 times the capacity of analog AMPS and four
to six times the capacity of TDMA. Unlike analog and digital systems, a
CDMA system permits a more graceful handoff, as it enables a subscriber to
monitor and communicate with multiple cells.
Similar to TDMA, CDMA operates in the 800MHz and 1900MHz frequency
bands. Digital cellular systems use 800MHz, while PCS systems use the
1900MHz band. North American carriers using CDMA include AirTouch,
Bell Atlantic, GTE, and Sprint. England?s Vodafone Group recently acquired
AirTouch, resulting in a giant company with operations in the United States
and most of Europe.
In September 1999, British Telecommunications (BT) and AT&T announced
an alliance to market a worldwide mobile phone service, perhaps to compete
with the expanded Vodafone Group. Although the BT-AT&T alliance is
oriented toward global customers who want to use their phones everywhere,
for the next year BT and AT&T will skirt compatibility issues by offering a
two-phone package to subscribers. One phone will operate in the
subscribers? home market, while the second will be carried across the
Atlantic. According to BT, a universal telephone should be available in 12 to
18 months.
CDPD
Cellular Digital Packet Data (CDPD), commonly referred to as wireless IP, is
implemented as an overlay to a communication carrier?s existing digital cellular
network. Based on TCP/IP, CDPD provides Internet access to mobile users
at a transmission rate of up to 19.2Kbits/sec.
The CDPD specification uses the Reed Solomon forward error-correction. In
addition, CDPD protocols include automatic error detection and correction
so that bad packets are either corrected by Reed Solomon coding or
retransmitted. According to AT&T, this results in the highest level of
throughput accuracy of any wireless data-transmission technology currently
available. In addition, CDPD supports encryption and authentication.
CDPD currently covers approximately 75 percent of the U.S. business
population with transmission support in over 135 major metropolitan areas
and 4,000 towers throughout the country.
Although CDPD represents an interesting convenience for the mobile
executive or sales person, it?s expensive. In addition to various pricing plans
based upon the number of kilobytes transmitted per month, you must
purchase a CDPD modem. With a price ranging from between $450 and
$1,600 for a CDPD modem, it takes a lot of ?convenience? to justify this
purchase, especially compared to a $79.95 V.90 modem that provides at
least a 28.8Kbit/sec connection through 10-cent per minute, circuit-switched
long distance service.
GSM
If AMPS, FDMA, TDMA, and CDMA aren?t enough for you, there?s
always GSM (see Table 1 for a comparison of all these technologies). GSM
is the uniform European version of TDMA. More than 90 countries have
adopted GSM, with the highest coverage in Europe, where it allows
Europeans to roam without having to carry dual-technology phones.
Worldwide, approximately half of all wireless users employ GSM technology.
One of the more interesting capabilities of GSM is its support of a Short
Message Service (SMS). This service enables GSM phones to receive short
text messages. In addition, you can connect a GSM phone to a PC to
transmit data and fax messages at data rates up to 9.6Kbits/sec.
Originally restricted to operating at 900MHz, a newer European version of
GSM operates at 1800MHz, while the North America version operates at
1900MHz. Today you can acquire dual-band (900/1800 and 900/1900)
phones, and a few ?universal? GSM tri-band 900/1800/1900 phones that
enable roaming were recently advertised. Without a universal phone, most
GSM phones used on North American systems can?t operate when traveling
in Europe (and vice versa).
However, you can roam between North American and Europe by removing
the user-specific Subscriber Identity Module (SIM) card from inside a phone
used in North America and placing it in a European phone, or vice versa. The
SIM stores your assigned phone number, secret user key, the portfolio of
services you subscribe to (such as call forwarding, voice mail, and so on), and
authentication algorithms used to validate subscribers.
Currently, out of a worldwide base of over 170 million subscribers,
approximately 5 million North Americans use GSM. GSM is supported in 46
states, and North America is now the third largest nonEuropean market in the
world.
GSM1900 operators in North America include Powertel, SBC
Communications, Aerial Communications, and Omnipoint. Another GSM
operator, VoiceStream Wireless, recently announced plans to acquire both
Omnipoint Communications and Aerial Communications. If successful, this
could create a nationwide wireless operator on the scale of AT&T and Sprint.
3G NETWORKS
Some evolving wireless technologies may enhance our ability to communicate.
Collectively, these technologies are referred to as third-generation (3G)
wireless. AMPS represents the first generation of wireless, while both TDMA
and CDMA represent second-generation wireless technology. 3G research
dates back to 1986, when the ITU launched IMT-2000. IMT-2000 is a
family of systems that provides wireless access through satellite and terrestrial
systems for both fixed and mobile users.
Under the ITU?s IMT-2000 roadmap, network services will include support
for all existing PSTN services, voice message services (including Teletex,
paging, Telefax, and SMS), point-to-multipoint dispatch services, as well as
multimedia (including high-speed data). Both packet- and circuit-switched
bearer services will be supported, with proposed minimum data rates
indicated in Table 2.
Perhaps following the Not Invented Here syndrome, cellular operators and
product manufacturers in Europe and Japan selected a technology called
Wideband Code Division Multiple Access (WCDMA). Unfortunately,
WCDMA is substantially different from North American CDMA systems
based upon technology pioneered by Qualcomm, which included efforts by
Lucent Technologies, Motorola, and Nortel Networks. (For readers
enamored with acronyms, CDMA standard developers were originally
dubbed the LMNQ group in honor of each vendor, which lasted until
Samsung joined the group.)
The previously mentioned vendors, along with other manufacturers, formed
the CDMA Development Group (CDG) and branded the trademark
CdmaOne, which describes a complete wireless system that includes the
IS-95 CDMA air interface, the ANSI-41 network standard for switch
interconnection, and other standards that complete the wireless system.
The next phase proposed for CdmaOne is a standard referred to as 1XRTT.
1XRTT, approved for publication by the TR45.5 subcommittee of the TIA in
July 1999, enables 144Kbit/sec packet data transmission in a mobile
environment. Other plans under the 1XRTT standard include a two-fold
increase in standby time and voice capacity within the existing 1.25MHz
CDMA channel. According to the CDG, CdmaOne will eventually support
different channel sizes, as well as provide circuit and packet data rates up to
2Mbits/sec.
To move toward 3G the TIA assigned the name Cdma2000 to identify the
planned evolution of CDMA from a second generation CdmaOne system to
3G services and features defined by the 1XRTT standard.
Until recently it appeared that once again Europe and the United States would
go their separate ways, but the June 1999 meeting of the ITU group of radio
experts on IMT-2000 in Beijing endorsed harmonization for the CDMA
component of the IMT-2000 standard. The proposed harmonized parameters
for CDMA are structured to develop inexpensive multimode phones, which
will enable WCDMA and CDMA2000 phones to interoperate.
Regardless of the acronym, IMT-2000 will result in phones that support
5MHz of bandwidth by 2004. Because transmission is proportional to
bandwidth, this supports 384Kbit/sec data transmission in a mobile
environment and 2Mbits/sec from a fixed location.
For the homeowner or apartment dweller, the ability to surf the Internet at
data rates up to 2Mbits/sec will be appealing, especially when compared to
other evolving technologies. While fixed wireless may go head to head with
Digital Subscriber Line (DSL) and cable modems, the new kid on the block
might be a winner, since DSL for the residence in the form of G.lite still
requires microfilters, while cable modems represent a shared media solution.
Concerning the latter, the 6MHz TV channel used for cable modems could
have its bandwidth shared among thousands of subscribers. If wireless
operators construct distributed cell sites that enable capacity to match
demand, the battle for the consumer dollar between the Baby Bells and the
cable operators might have an unexpected winner.
With this in mind, you can look toward a future where the need for speed for
data applications may be satisfied from the evolving mobile phone, allowing
business and residential users to snip the bonds of the local loop.
Gilbert Held is an award-winning author and lecturer. He has written
over 40 books and 300 technical articles, including Voice and Data
Internetworking from McGraw-Hill. He can be reached at
gil_held@yahoo.com.
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