If your cellphone battery makes it through the day and you can’t remember the last time you had a dropped call, you likely have code-division multiple access (CDMA) to thank.
Introduced by Qualcomm in 1989, CDMA relies on relatively low-power signals spread over a frequency, and assigns a unique code to each signal. As a result, many signals can travel over the same frequency simultaneously. CDMA technology results in fewer dropped calls, higher capacity and bandwidth, better voice clarity, and longer battery life. Third-generation (3G) cellular technical standards were based on the technology.
On the 28th anniversary of CDMA’s first demonstration by Qualcomm, the technology was named an IEEE Milestone. Administered by the IEEE History Center and supported by donors, the Milestone program recognizes outstanding technical developments around the world.
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The first generation of analog cell networks (referred to as 1G) managed traffic by assigning a specific frequency to each mobile phone user. It was one channel per user for each phone call, which limited the capacity of a cell to the number of channels at the site.
When 2G digital cell networks debuted in the 1980s, network providers served multiple subscribers with a time division multiple access (TDMA) system. Channels were divided into time slots and each slot held a single user. The signal switching across time slots was so fast that users didn’t notice they were sharing a channel with others. With the North American 2G standard, D-AMPS, three users occupied a 30-kilohertz channel. In Europe, relying on the GSM standard, a 200-kHz channel supported up to eight users.
Because neighboring cells were on different frequencies to reduce interference, a hard switch occurred when callers moved from one cell to the next. This switch sometimes caused calls to disconnect. As cellphones became more popular, it became clear that the TDMA approach did not have enough capacity.
Qualcomm entered the cellular industry by the late 1980s. The company was cofounded in 1985 by IEEE Life Fellows and Medal of Honor recipients Irwin Jacobs and Andrew Viterbi to offer satellite tracking services. At the time, a method similar to CDMA was being used in satellite communications for the military. Despite resistance from the mobile industry, which was already largely invested in TDMA, Qualcomm proposed a more radical approach to the capacity problem: It would apply a spread-spectrum technology to commercial networks.
The company’s perseverance in developing CDMA and demonstrating it to be a vast improvement over TDMA led it to becoming the standard of the Telecommunications Industry Association in 1995—the first step to widespread adoption.
Qualcomm’s CDMA method became crucial to the development of 3G networks. Its low-power signals, each with its own unique code, could travel over the same frequency. At the receiving end, the desired signal was separated from the rest so that the speaker’s voice could be heard with minimal background noise.
Silence, or dead time, makes up more than half of a typical telephone conversation. CDMA systems did not transmit the silence—which greatly increased network capacity as well as cellphones’ energy efficiency. And 3G networks introduced variable rate coding, in which dead time would be transmitted at 1,200 bits per second, while sound was transmitted at up to 9,600 bps. That greatly improved quality.
Third-generation networks ushered in the era of power-efficient, low-cost smartphones with Internet access speeds that rivaled what could be had at home.
CDMA was honored on 7 November at Qualcomm’s headquarters, in San Diego. Jacobs and Viterbi attended along with Kathleen Kramer, IEEE Region 6 director; IEEE Fellow Roberto Padovani, and former Qualcomm CTO; and James H. Thompson, the company’s current CTO. A drone lifted the curtain to reveal the plaque, which reads:
On 7 November 1989, Qualcomm publicly demonstrated a digital cellular radio system based on Code Division Multiple Access (CDMA) spread spectrum technology, which increased capacity, improved service quality, and extended battery life. This formed the basis for IS-95 second-generation standards and third-generation broadband standards that were applied to cellular mobile devices worldwide.
This article was written with assistance from the IEEE History Center, which is partially funded by donations to the IEEE Foundation.