CDMA versus TDMA
Christina Richards and Scott
Creque
Project 1
EE 4984: Telecommunication Networks
April 11, 1996
- Overview
- General Structure and Technical Differences
- Interference
- Why is the debate so intense?
- The
State of the Art
- Present Market Status
- Trends in Development
- Impact
of the Controversy on the Consumer
- Compatibility Issues
- Roaming Difficulties
- Unification Possibilities
- Contenders
and Supporters
- Developers of the Technologies
- Technologies used by Service Peoviders
- Goverment and International Support
- Future
Projections
- Projections - Where will we go from here?
- Resources
The principle problem facing wireless
communications systems is how to allow a large number of users access to a small
allotment of frequencies. In today's digital wireless communication systems,
there are two principle methods being used to multiplex users on the same
frequencies.
General Structure and Technical Differences
The first, and older of the two methods, uses a combination of frequency
division and time division to provide multiple access (FDMA and
TDMA). Although this method has been proven to be an effective solution to the
problem, its major shortcoming is its limited capacity. As an alternative
solution, code division multiple access (CDMA) was developed. A CDMA
system uses a combination of frequency division and code division to provide
multiple user access. Although the capacity of a CDMA system is not unlimited,
its limitations are considerably higher than those of an TDMA system.
TDMA has been around for quite a while and has proven itself to be reliable
technology capable of providing a large number of users with good quality
service. In a traditional system, the total allotted bandwidth is divided into
frequency channels. The channels are then divided into a number of time slots.
Each user (device) is allocated one time slot. There are several standards for
digital FDMA/TDMA systems world wide. Two of these standards include United
States Digital Cellular (USDC), also known as IS-54, and Global System for
Mobile Communications (GSM) also known as PCS-1900.
FDMA/TDMA Standards
| Standard |
Channel Width (kHz) |
Time Slots |
| USDC |
30 |
3 |
| GSM |
200 |
8 |
CDMA is a very different modulation scheme than frequency or time division.
First, the frequency "channel" is 1.244 MHz wide. Second, all of the users of
the system share the same channel. A signal to be broadcast on the channel is
first "spread-out" over the entire bandwidth (this is known as "spread
spectrum"). The signal is encoded using unique codes known only to the
transmitter and receiver. Since each signal on the channel is then unique, it
can be distinguished from the others and properly received.
The spreading
codes are the most important aspect of CDMA technology. The codes are Walsh
codes based on a 64X64 Walsh matrix. There are several key characteristics of
the Walsh matrix that allow the codes to be unique. In the matrix, every row is
orthogonal to every other row. Thus, when the derived codes are used the unique
device IDs, no two codes are the same. Moreover, each code differs from every
other code by specific and predetermined amount. Theoretically there is no limit
on the capacity of a CDMA system. In practice, however, the coding mechanisms
are not completely orthogonal to each other, and this places a limit on the
capacity at which the system can operate without interference, but even with
this restriction the capacity is still tremendous.
As a simple, example let
us assume a user is talking into a mobile phone on a CDMA network. The
transmitted portion of a voice signal has frequency components from
approximately 300~3400 Hz. This analog signal is digitally encoded, using QPSK
(Quadrature Phase Shift Keying), at 9600 bps. The signal is then spread to
approximately 1.23 Mbps using special codes that add redundancy. Some of these
codes include a device ID that is unique to the phone (like a serial number).
Next the signal is broadcast over the channel. When broadcast, the signal is
added to the signals of the other users in the channel. On the receiving end,
the same code is used to decode the incoming signal. The 9600 bps signal is
obtained and the original analog signal is reconstructed. When the same code is
used on another user's signal, the redundancy is not removed and the signal
remains at 1.23 Mbps.
An analogous comparison of a USDC system versus a CDMA system is a
conversation in a crowded room. If a listener were using frequency division, he
would differentiate conversations by the pitch of the speaker (i.e. lower pitch
for men's voices or a higher pitch for women's voices). Time division would
necessitate listening to first one person, then the second, then the third, etc.
Frequency and time division would be analogous to listening to man#1, women#1,
man#2, women#2, etc. CDMA, however, would be analogous to listening to different
languages.
Interference
In a standard system the desired signal must be at least
18dB above any noise or interference. The effect of this is that adjacent cells
(coverage areas) cannot share the same portion of the spectrum. The problem with
this is that not all of the channels in adjacent cells can be used. Due to this
"adjacent interference" and the 18dB attenuation requirement, typically only one
out of 21 of the available channels can be used in any one cell.
In a CDMA
system, however, signals can be received in very high levels of interference.
Under worst case conditions, a signal can be received in the prescience of
interference that is 18dB higher than the signal. Because of this,
channels can be reused in adjacent cells. Typically half of the interference in
a cell is due to adjacent cells.
The ability of a CDMA system to receive a
signal under such high noise interference conditions is a result of the digital
coding process used in spreading the signal. The coding gain of the signal is
the ratio of the transmitted bits to the data bits. The North American standard
coding gain is 128 or 21dB. Since only 3dB of signal power is required for
signal reception, this means that 18dB of noise can be tolerated. The signal can
be resolved because there is minimal cross-correlation between all of the
signals on the channel. This is a characteristic of the orthogonality and
uniqueness of the spreading codes.
Why is the debate so intense?
Why is their a debate over the technology?
If CDMA is superior, why are companies still backing an "inferior" technology?
Two of the most important factors to consider when choosing a system for a
wireless network are:
- Capacity of the system
- Ability to implement the system
Until quite recently, neither
CDMA systems, like IS-95, or TDMA systems, like USDC and GSM, had an optimal
combination of these features.
A major concern about CDMA is the fact that it
has very little field experience, where as TDMA systems have been operable for
quite some time. Fast "time to market" is essential to companies due to the
phenomenal pace of today's wireless communications market, and many providers
choose to invest in TDMA systems that have already been developed and proven.
The very fact that CDMA technology is so new and unproven makes it a big risk
for companies to invest in when there is the option of going with a time-proven
technology such as TDMA.
CDMA has many benefits over a traditional
system. Some benefits of a CDMA system include:
8~10 times more users than traditional FDMA/CDMA
better signal quality
privacy of coded digital communications
no sense of handoff when changing cells
easy addition of more users
"soft" capacity limit - Additional users add more noise to the
cell
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Present Status
For the past few years, TDMA and CDMA supporters have
waged an intense lobbying war, arguing the merits of their respective systems.
It has not been possible until just recently to test the systems against each
other as IS-95 CDMA systems have only recently made it out of the
laboratories.
A big advantage of the TDMA technology is that it was the
scheme used in the Interim Standard 54, also known as United States Digital
Cellular (USDC) or digital AMPS (D-AMPS), that served to make the transition to
a digital system from the first-generation cellular analog AMPS system.
Operating with the same channel allocations as the AMPS standard, TDMA was a
boon to the market in the late 1980's. It was not, however, designed for the
incredible demand now present in many metropolitan areas, even though TDMA
increased by three times the capacity of the AMPS system. The rampant growth of
the wireless industry is quickly outgrowing the capacities of the USDC system in
many markets. Nonetheless, Ericsson, a telecommunications company headquartered
in Dallas, TX that is a strong advocate of TDMA, anticipates that the two TDMA
based solutions, D-AMPS and PCS 1900 (an adaptation of the European GSM), will
capture a large portion of the market. They maintain that though some large
operators will adopt CDMA, the technology requires considerable improvement to
become a commercially viable product. Since the CDMA standard was announced,
they assert that continuous improvements to TDMA have caught up with the
performance promised by CDMA.
The CDMA technology, used in the Interim
Standard IS-95, maximizes spectrum efficiency and enables more calls to be
carried over a single 1.25 MHz channel. The technology has been widely adopted
by major cellular and PCS carriers in the United States and also
internationally. CDMA networks provide operators with reliable digital systems
that offer higher capacity, large coverage area and improved voice quality. In a
December 1995 release, Qualcomm, the San Diego company that developed CDMA,
advocated their product by saying, "CDMA provides significant benefits to
customers including superior voice quality, enhanced privacy, fraud protection,
messaging services and longer-lasting battery power in the phones." Catching the
interest of clients is key, and Qualcomm has gained some major players recently.
The difficulty with IS-95 is that because CDMA is both new and very complex,
development and production of the hardware and software necessary for an
operation-capable system was very slow coming at first. Original plans for
commercial implementation of CDMA were successively pushed back from 1991 to
1995, and many industry observers were skeptical that even the 1995 deadline
would be met. With the break-neck pace of wireless communications today,
time-to-market is key, and this was a major handicap at first for CDMA
technology. Another major concern about CDMA is the fact that there is actually
little field experience with the system, where as TDMA systems have been
operable for quite sometime.
Trends in Development
In the past ten to twelve months, however, the
shift has been strongly towards CDMA. This shift can be attributed to many
factors, not the least of which being that the hardware and software needed for
a working CDMA system have been developed and are being supported and improved.
Also, positive preliminary testing has instilled confidence in the operation of
the system outside the laboratory environment. A major triumph came at the end
of 1995 when the first commercial CDMA system was launched in Hong Kong.
Although Hong Kong has one of the world's most difficult terrains and congested
radio frequency environments, the network delivered outstanding performance.
Jack Scanlon, executive vice president and general manager of Motorola's
Cellular Infrastructure Group said, "The successful deployment of the CDMA
Networks in Hong Kong is proof that CDMA will work well anywhere in the world."
Promoters of TDMA technology, however, are also working to make TDMA systems
more attractive to service providers.
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Compatibility Issues
The impact of the "controversy" on the common
consumer is severe. Various companies are spending millions to push CDMA
technology and this will result in aggressive marketing schemes and fast
technological changes. One of the primary concerns to the consumer is the
incompatibility between the two systems. Since CDMA currently has limited
coverage, consumers must choose which system will meet their needs. As CDMA
coverage grows, this will be less of a problem. To help with this situation,
some companies (i.e. Qualcomm) are offering Dual Mode Phones. These phones are
capable of utilizing either service, but are more expensive then a standard
phone.
Roaming Difficulties
Without a dual mode phone, using A CDMA phone while
traveling (roaming) will not be possible unless you are in within the coverage
of another CDMA provider.
Another concern for consumers is the growing GSM
presence. GSM is another TDMA standard which offers better performance than
USDC. Since providers using TDMA are eager to compete against CDMA, some are
choosing to follow GSM standards.
Unification Possibilities
As CDMA grows and as the war of wireless is
fought, the consumer is faced with a situation much like having a different
remote control for stereo, TV, and VCR. As of yet, the "universal remotes" are
few and expensive.
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The
debate over the competing technologies has sparked devotion in supporters almost
to the point of religious fervor. This is true particularly among the companies
who did much of the initial development of the technologies. Qualcomm was the
prime developer of the CDMA technology, and has worked tirelessly to promote its
adoption in all wireless markets by developing and improving hardware and
software for CDMA systems. Much development was done for TDMA technology by
Ericsson. Ericsson was the first supplier to introduce digital technology in the
form of D-AMPS (another name for IS-54) which more than tripled the potential
capacity of the existing analog AMPS networks and allowed the introduction of
new services such as data communications, voice encryption, and calling line
identification. (15)
There are companies and organizations involved at
all levels of the controversy, from the development of the technology, to the
use of the schemes by the servers in their markets, to the endorsement of
technology by governments. Some are staunchly on one side or the other, like
Ericsson and Qualcomm; while others keep an interest in both technologies, like
Nortel. The following is an overview of the distribution of support among some
of the major players in the wireless market.
Developers of the Technologies
The success of a technology depends first
on the existence of quality hardware and software to support it. As expected,
Ericsson and Qualcomm dominate in the race to produce new and better
products.
Qualcomm introduced its QEDesign network planning tool at PCS
'95, held in September. QEDesign is a premier design and analysis tool for CDMA
system deployment. Shortly thereafter in October of 1995, Qualcomm announced the
creation of an ASIC Products business unit to ensure their ready supply. "As
service providers move quickly to meet customer demand for CDMA digital service,
manufacturers must react by having high-quality wireless products available when
and where they are needed. The formation of this business unit enhances
Qualcomm's ability to provide that high level of responsiveness for our many
CDMA licensees, as well as our own manufacturing requirements," said Rich
Sulpizio, chief operating officer of Qualcomm, Incorporated. In January of 1996,
the first in a family of products to be produced by Qualcomm to support CDMA was
announced. This was the QCT-1000, the world's first wireless local loop phone
based on CDMA.
In the early 1990's Ericsson gained a crucial lead over its
competitors by being the first supplier to put D-AMPS networks into commercial
service. Ericsson's CMS 8800 system for AMPS and D-AMPS cellular and PCS service
is used in networks that currently serve more that 15 million subscribers,
making CMS 8800 the world's most successful wireless network system in terms of
subscribers served. At CeBIT in Hanover, Ericsson displayed the latest addition
to the range of Ericsson radio base stations - a 'micro' base station for GSM
900, DSC 1800, and PCS 1900 wireless networks, as well as a new family of mobile
phones for use on the GSM networks.
Other companies are contributing to
the development of the software/hardware technologies needed for CDMA and TDMA.
Nortel, formerly Northern Telecomm and Bell Northern Research, has a strategic
relationship with Qualcomm to jointly design, develop and manufacture complete
CDMA networks for mobile and wireless local loop applications. Nortel was chosen
by Sprint Telecommunications Venture to supply them with PCS network equipment
and services based on CDMA technology. In November of 1995, DSP Communications,
Inc. signed a worldwide ASIC license agreement with Qualcomm to enable them to
design, manufacture, and sell Application Specific Integrated Circuits using
CDMA technology, for use in cellular, PCS, and other wireless application
equipment. "CDMA is rapidly becoming the technology of choice for
next-generation cellular and PCS systems in the United States," said Shaul
Berger, vice president of sales and marketing for DSPC.
Technologies used by Service Providers
Upon completion of the A/B block
PCS auctions, CDMA emerged as the leading digital wireless technology in the US
marketplace. Most significant was the choice by Sprint Spectrum, formerly known
as Sprint Telecommunications Venture, to use CDMA. Sprint Spectrum holds 163.305
million Points of Presence (POPs), making it the largest provider of PCS
markets, and they announced in July of last year that they would deploy CDMA
technology because it represents the best long-term solution for a seamless
national wireless network. "Sprint Telecommunications Venture has selected CDMA
as the best technology for its customers, and has selected Nortel/Qualcomm
because of our product architecture and CDMA network expertise," said Dr. Irwin
M. Jacobs, chairman and CEO of Qualcomm. Other important companies who have
chosen CDMA as their system's multiple access technique are PCS PrimeCo. L.P.,
who boasts the third largest PCS market holdings at 60.538 million POPs, and GTE
Mobilenet, who provides PCS service to 20.877 million POPs. The dominance of
CDMA is even more prevalent in the Cellular market, where eleven of the top
fourteen service providers use CDMA in their network either alone or in
conjunction with a previously existing TDMA system that is being phased
out.
In contrast, TDMA systems have remained as the sole multiple access
scheme in only 3 of the top fourteen cellular providers, although one of these
is AT&T Wireless, the largest cellular carrier at 78.757 million POPs. It
also remains in the markets of two other providers in the top fourteen, but is
coexists with CDMA, and is intended to be phased out. TDMA has a presence in
half of the top fourteen PCS service providers as the PCS 1900 standard, the
American version of GSM. The most significant of these perhaps is Atlantic
Personal Communication's Washington-Baltimore market that successfully began
commercial business in November of 1995.
Government and International Support
The International
Telecommunications Union (ITU) has recommended CDMA as an approved wireless
standard. Developing countries are showing interest in both CDMA and TDMA.
"Wireless local loop applications offer developing countries the opportunity to
jump-start their economies by enabling people everywhere to communicate. CDMA,
with its increased coverage and capacity, makes wireless local loop economical
and rapidly available." Dr. Paul Jacobs, vice president and general manager of
Subscriber Products for Qualcomm. "In India and other developing countries,
we're focusing our efforts on manufacturing and deploying CDMA wireless local
loop systems for urban and residential use in densely populated areas," said
Chris Simpson, senior vice president of Interantional Marketing at Qualcomm
Incorporated. TDMA is also represented in the international market. Ericsson has
signed contracts with clients in the Ukraine and other developing countries to
implement full D-AMPS systems.
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With the current rate of
development, it is "obvious" that CDMA will displace TDMA as the primary
wireless multiple access technology. Because CDMA is ideally suited for the high
traffic demands of metropolitan areas, CDMA systems will dominate these markets.
TDMA is a proven technology that has already been widely implemented and
supported. For this reason it will remain a viable alternative for markets with
lower capacity demands. For the savvy investor, now is a prime time to invest in
a growing technology that is sure to provide profitable returns. This technology
is, of course, CDMA. Two thumbs up!
Qualcomm Home Page
Ericsson Home Page
Motorola Home Page
AT&T Wireless Home
Page
Nortel Home Page
CDMA Developement Group Home Page
Comsearch, Inc. Wireless Engineering Networks Division seminars and
presentations
_Wireless Communications_, Rappaport 95
_Digital and Analog Communications Systems_, Couch 91
"Applied Microwave and Wireless," Winter 94
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Comments to: Christina Richards or Scott Creque
Last updated: April 11, 1996
http://fiddle.ee.vt.edu/courses/ee4984/creque_richards.html