CDMA and IS-95

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.

IS-95: The CDMA Standard

IS-95 (Interim Standard 95) was the first commercial CDMA system, also marketed ascdmaOne. It operated on a 1.25 MHz channel bandwidth with a chip rate of 1.2288 Mcps (mega-chips per second). Key parameters:

• Channel Bandwidth: 1.25 MHz (compared to GSM's 200 kHz)
• Chip Rate: 1.2288 Mcps — the rate at which the spreading code is applied
• Voice Codec: 13 kbps QCELP (Qualcomm Code-Excited Linear Prediction), later enhanced to 8 kbps EVRC
• Data Rates: Up to 14.4 kbps (IS-95A), 115.2 kbps (IS-95B)
• Power Control: Open and closed loop at 800 Hz — 16 power adjustments per frame to combat the near-far problem

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 = 128). This means each data bit is encoded with 128 chips, providing a processing gain of ~21 dB that allows the signal to be recovered even when buried below the noise floor.
• Walsh Codes: 64 orthogonal Walsh codes for channel separation. Code 0 is used for pilot, codes 1–32 for traffic channels, codes 33–63 for paging and access channels. The orthogonality ensures that perfectly synchronized signals can be separated at the receiver.
• PN Sequences: Short PN sequences (2¹⁵ - 1 = 32,767 chips) with a period of 26.67 ms are used for quadrature spreading and cell identification. Each cell uses a unique PN offset to distinguish itself from neighbors.
• Soft Handoff: Phone connects to multiple towers simultaneously — "make before break" — reducing dropped calls. The mobile's RAKE receiver combines signals from multiple base stations, improving reliability during handoff.
• RAKE Receiver: Exploits multipath to improve signal quality. Multiple "fingers" track different multipath components (reflections off buildings, terrain) and combine them constructively, turning multipath from an enemy into an ally.

The Near-Far Problem and Power Control

CDMA's biggest technical challenge is the near-far problem: a mobile device close to the base station can overwhelm signals from distant devices. Without precise power control, one strong signal would "capture" the receiver, drowning out others. IS-95 solved this with 800 Hz closed-loop power control— the base station measures each mobile's received power 800 times per second and sends power-up/power-down commands. This adjusts each mobile's transmit power to within ±0.5 dB, ensuring all signals arrive at the base station with approximately equal power.

CDMA2000 Evolution

CDMA2000 was the 3G evolution of IS-95, maintaining backward compatibility:

• 1xRTT (IS-2000): Single 1.25 MHz carrier, 153.6 kbps peak data. Uses the same bandwidth as IS-95 but with improved coding and modulation. Voice capacity doubled through improved vocoder and sectorization.
• 1xEV-DO Rev 0: Data-optimized — dedicates the full 1.25 MHz carrier to data. Uses time-division multiplexing on the downlink, scheduling users based on channel conditions. Peak: 2.4 Mbps downlink.
• 1xEV-DO Rev A: Added QoS and improved uplink. 3.1 Mbps downlink / 1.8 Mbps uplink. Supports VoIP with latency under 150 ms.
• 1xEV-DV: Combined voice and data on same carrier. Never widely deployed — operators jumped to 1xEV-DO instead.

Advantages Over GSM

• Higher Capacity: Up to 10x GSM per MHz in theory, due to universal frequency reuse (every cell uses the same frequencies)
• Soft Handoff: Seamless tower switching reduces dropped calls — connects to multiple towers simultaneously
• Variable Rate: Lower data rate when speaking quietly — VAD (Voice Activity Detection) gates the transmitter, reducing interference for others
• Multipath Resilience: RAKE receiver turns multipath reflections into a signal advantage
• Soft Capacity Limit: No hard channel limit — capacity gradually degrades as more users join, rather than blocking new calls

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. Key factors in CDMA's decline:

• Qualcomm's proprietary licensing model made CDMA equipment more expensive
• GSM's SIM card enabled device portability and international roaming
• The CDMA2000 upgrade path fragmented operator investment
• W-CDMA (3GPP) offered a clearer evolution 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.