Español | Português
After an initial deployment of TDMA digital technology, Canada's
Bell Mobility discovered that its customers' needs were not
being met. Following a detailed technical evaluation and exploration
of the options, Bell Mobility replaced the TDMA system with
CDMA digital technology to capitalize on CDMA's technical
and financial advantages today and into the future.
Initial Digital Decision
Over the last ten years, Bell Mobility expended a significant
amount of effort evaluating the alternatives at its disposal
to evolve from the highly successful first generation AMPS
analog system to a second generation digital technology. This
effort was influenced by two factors:
• In 1989 it became very evident that the existing AMPS
network was running out of capacity, and given the rapid growth
expected in customers and traffic, Bell Mobility needed an
• Technology had evolved significantly since the first
AMPS systems were introduced in the 1970's, which meant new
service functionality could be introduced with new digital
Carriers around the world therefore started looking at various
alternatives to analog in the late 1980's, including NAMPS,
TDMA and FDMA systems. The consensus of the industry in 1989
was that TDMA was the only viable alternative and development
began on that technology. CDMA technology was still in the
research and development phase at the time and therefore not
a commercially available option.
In 1992, after numerous technology trials, Bell Mobility
decided to deploy TDMA in its network. The TDMA service was
officially launched in November 1993. The technology introduced
at that time offered capacity enhancements (three calls in
the same spectrum required to support one AMPS call) and two
new features: Calling Number Display and Message Waiting Indicator.
The other promised service enhancements were to be developed
in the future.
Soon after the launch, Bell Mobility discovered that while
it was possible to get threefold capacity improvement, it
was done at the expense of call quality. Customers were initially
happy to move to the new TDMA digital network in order to
get a lower priced rate plan that Bell Mobility was offering
to attract customers to the digital service, but once customers
started using the service, they were very dissatisfied with
the voice quality. This result was a serious concern to Bell
Mobility, causing its engineers to redouble their efforts
in determining how to improve the quality. After significant
analysis and network optimization effort, Bell Mobility came
to the conclusion in 1995 that the only way to improve the
TDMA service was to go back to the drawing board and redesign
the core TDMA technology.
CDMA Alternative Emerges
While the deployment and optimization of TDMA was going
on, a new technology known as CDMA emerged as an alternative
as it matured in the standards process and research labs of
North America. So while Bell Mobility was faced with the delay
required for the industry to redesign the TDMA technology,
it performed theoretical and field trial evaluations of the
CDMA technology. In the end, Bell Mobility concluded that
a major change to introduce CDMA technology was preferable
to continuing to incrementally improve the old TDMA technology.
As a result of its extensive evaluation, Bell Mobility believed
that TDMA could be made to work eventually, but it would never
be able to match the quality, capacity or functionality advantages
Since launching CDMA digital PCS technology in 1997, Bell
Mobility has been very happy with the response from its customer
base to CDMA call quality and the ability to add new enhanced
services to its portfolio.
Key Factors Favor CDMA
Five key factors helped Bell Mobility conclude that CDMA
was the appropriate technology to deliver on the evolving
needs of its customers while allowing Bell Mobility to meet
its own goals.
As evidenced by Bell Mobility's experience with TDMA, in
the eyes of the customer the most significant criteria in
selecting a new digital technology is the resulting voice
quality. In this area, CDMA truly excels by employing numerous
technical advancements to provide a voice quality that is
as good as or better than that produced by AMPS, and is significantly
better than that of TDMA. Two technical features that allow
for the superior voice quality are discussed below.
High quality 13 Kb voice coder: The TDMA channel structure
allows no more than an 8 Kb voice coder and is planned to
evolve to a 4 Kb coder. The CDMA structure uses 13 Kb in the
near term providing the best quality at launch and an improved
8 Kb coder is now available for implementation.
Soft handoff: TDMA employs a hard handoff when moving between
cell sites, resulting in a noticeable mute on the call. This
mute can be significant in cities such as Toronto or Montreal,
where handoffs will occur 2-4 times per minute. CDMA, on the
other hand, uses a technique known as soft handoff to eliminate
this problem. Soft handoff allows the phone to receive signals
from 3-5 adjacent cell sites at the same time, and combines
the signals to eliminate the handoff muting as well as improving
overall signal quality (by always selecting the best signal
of the 3-5 that are received).
The capacity considerations for the digital technology were
very important in order to minimize the need to evolve the
network in a significant way in the near future. At issue
were several items: customer annoyance at yet another change,
the company's desire not to have to invest additional capital
in another network as well as the need to support the fast
growing traffic load on the network.
CDMA capacity has been a much-debated item in the industry
and was the subject of significant scrutiny by Bell Mobility.
At the end of the day, Bell Mobility believed that even the
most conservative views of CDMA capacity were that it offered
at least double that of TDMA. Further, even assuming that
conservative view, Bell Mobility could meet its capacity requirements
for the foreseeable future, as can be seen in the following
CDMA achieves this capacity advantage by employing the following
- Variable rate voice coders reduce the rate being transmitted
when the speaker is not talking. This technique allows the
channel to be packed more efficiently, resulting in additional
- Power control techniques are used to keep the transmitted
power at the absolute minimum required to support a high
quality call. The relationship to capacity is as follows:
Less Power = Less Energy = Less Interference = Greater
CDMA has been selected as the second generation digital technology
of choice by more than half of the North American industry
with the remainder of the industry split across two other
digital technologies: TDMA and GSM. Bell Mobility therefore
believed that the best way to meet the needs of its customers,
who use their service in all of Canada and in the United States,
was to select a technology that was more broadly deployed
than the other alternative digital technologies.
In evaluating the cost structure of TDMA versus CDMA, Bell
Mobility concluded that CDMA was a more cost effective solution
in the long run. This cost benefit is realized largely due
to two factors:
- Capacity gains: Higher capacity per site means that fewer
sites need to be deployed to meet a given traffic demand.
- Coverage efficiencies: The coverage generated by CDMA
technology is better than that of AMPS or TDMA (by 3-6 dB).
Better coverage means better quality and fewer sites
required to provide service outside the major urban centers.
Third Generation Technology of Choice
AMPS analog systems are referred to as first generation wireless
technology and digital systems such as CDMA, TDMA and GSM
are referred to as second generation. The world is currently
working on third generation systems to deliver enhanced data
service support early in the next decade. Regardless of whether
these third generation systems are deployed in the near term
or as part of a long range plan, it is important to consider
this next technology evolution when selecting the current
one. This type of technology planning ensures a graceful and
cost effective migration when the market does require the
introduction of advanced services and features.
As Bell Mobility was evaluating its second generation options,
TDMA and CDMA, it believed that CDMA would likely be the basis
of third generation systems given its inherent technical capabilities
to support enhanced capacity and data services, two key elements
of third generation wireless. Today most of the world has
agreed that third generation systems will be based on CDMA,
and therefore Bell Mobility's belief that its second generation
CDMA solution offered the most 'future proof' alternative
has been proven true.
CDMA's Promise Fulfilled
Bell Mobility's CDMA system has been in operation for
nearly two years and the advantages expected for the company
and its customers have been realized. As the technology continues
to evolve to meet emerging market demands it is expected to
continue fulfilling its promise long into the next generation
About the Author
Brian O'Shaughnessy is Vice President, Technology Development for Bell Mobility and the President of the CDMA Development Group. The Bell Mobility family of companies provides a complete range of wireless communications solutions - cellular, 1- and 2-way paging, data, satellite, PCS and airline passenger communications services -to over 2 million Canadian customers. The CDMA Development Group is a non-profit trade association formed to foster the worldwide development, implementation and use of CDMA technology.