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The Move to CDMA: Bell Mobility's Technology Decision

Brian O'Shaughnessy
Vice President, Technology Development
Bell Mobility

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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 alternative.
• 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 systems.

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 of CDMA.

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.

Call Quality

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).

Network Capacity

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 chart:

CDMA achieves this capacity advantage by employing the following techniques:

  • 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 capacity.
  • 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 Capacity

Expansive Footprint

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.

Cost Effective

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.



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