CDMA and IS-95
CDMA (Code Division Multiple Access) was Qualcomm's digital cellular technology that competed with GSM. Using spread spectrum and unique codes for each user, CDMA offered superior voice quality and capacity. It dominated the US market but eventually lost globally to GSM's ecosystem.
The Birth of CDMA
CDMA emerged from military spread-spectrum technology. In 1949, Claude Shannon at Bell Labs published the theoretical foundations showing that digital spread-spectrum coding could enable multiple users to share the same frequency simultaneously. This remained unused for decades until Qualcomm's founders—Irwin Jacobs, Andrew Viterbi, and others—developed it for commercial cellular in the late 1980s.
The Soviet Union actually built an experimental CDMA mobile phone system in 1957, when Leonid Kupriyanovich created a wearable phone with a 3kg base station. However, the technology was not pursued at scale until Qualcomm commercialized it in the 1990s.
How CDMA Works
Unlike GSM's TDMA (time slots) or FDMA (frequency slots), CDMA gives each user a unique pseudo-random code sequence. All users transmit on the same frequency at the same time. The receiver correlates incoming signals with the user's code to extract their data; other users' signals appear as low-level noise.
- Processing Gain: Spreading factor of 128 in IS-95 (1.2288 Mcps / 9.6 kbps)
- Walsh Codes: 64 orthogonal codes for channel separation
- Soft Handoff: Phone connects to multiple towers simultaneously
- RAKE Receiver: Exploits multipath to improve signal quality
IS-95 and cdmaOne
The IS-95 standard, marketed as cdmaOne, was the first commercial CDMA system:
- Channel Bandwidth: 1.25 MHz (same as IS-95A, cdmaOne)
- Voice Codec: 13 kbps QCELP (Qualcomm CELP)
- Data Rates: Up to 115.2 kbps (IS-95B)
- Power Control: Open and closed loop, 800 Hz rate
CDMA2000 Evolution
CDMA2000 was the 3G evolution of IS-95:
- 1xRTT: Single carrier, 153.6 kbps data
- 1xEV-DO: Data-optimized, up to 3.1 Mbps (Rev A: 4.9 Mbps)
- 1xEV-DV: Voice and data on same carrier
Advantages Over GSM
- Higher Capacity: Up to 10x GSM per MHz in theory
- Soft Handoff: Seamless tower switching reduces dropped calls
- Variable Rate: Lower data rate when speaking quietly
- No SIM Card Needed: Phone identity burned into hardware
Decline and Sunset
Despite technical advantages, CDMA lost the standards war to GSM. GSM's open development, European regulatory support, and the ability to roam globally made it the dominant standard. By 2007, GSM had 80% of the world market. US carriers eventually migrated to LTE.
AT&T and Verizon shut down their 3G CDMA networks in 2022, marking the end of the technology in the United States. Sprint's CDMA network was retired when it merged with T-Mobile.