IS-95

IS-95 employs direct-sequence spread spectrum, where each user's signal is multiplied by a unique pseudo-random code, allowing all users to share the same frequency band simultaneously. The standard defines a 1.25 MHz channel, which is derived from the 1.2288 Mcps chip rate, and uses soft handoff to maintain calls as users move between base stations. It supports variable-rate voice coding, with the Qualcomm code-excited linear prediction (QCELP) codec operating at rates from 8 kbit/s down to 1.2 kbit/s during silence, conserving battery and capacity.

Commercial IS-95 networks launched in 1995 in Hong Kong and the United States, with operators such as Sprint and Verizon adopting the technology. The standard evolved through revisions A and B, adding features like enhanced 911 and higher data rates. IS-95B increased peak data rates to 64 kbit/s by aggregating up to eight supplemental channels, but it was soon superseded by IS-2000 (CDMA2000 1x), which offered backward compatibility and improved data services. IS-95 networks were widely deployed in the Americas, Asia, and Oceania, with some remaining in service into the 2020s.

IS-95 laid the groundwork for third-generation (3G) CDMA2000 and influenced the development of WCDMA, the air interface used in UMTS. Its use of soft handoff and power control became foundational to modern CDMA systems. The standard also introduced the concept of a pilot channel for synchronization, which was later adopted in LTE and 5G. Although IS-95 itself is obsolete, its principles persist in contemporary wireless systems, and its commercial success demonstrated the viability of CDMA for cellular telephony.

IS-95 was originally designated as a U.S. standard, but it was adopted internationally by the International Telecommunication Union (ITU) as part of IMT-2000. A notable quirk is that the standard's name derives from the TIA's numbering system, not from any technical attribute. The first commercial CDMA call was made in 1995 by Qualcomm, and the technology was initially met with skepticism due to the 'near-far' problem, which was solved by precise power control. IS-95 also supported a 'gated' transmission mode to reduce interference, and its forward link used a 64-ary Walsh code structure that was later simplified in CDMA2000.

CDMA

Code-division multiple access, a channel access method where multiple users share the same frequency by using unique codes.

Soft handoff

A handoff technique where a mobile device communicates with multiple base stations simultaneously before switching.

QCELP

Qualcomm code-excited linear prediction, a speech codec used in IS-95.

Chip rate

The rate at which the spreading code is applied, measured in chips per second.