Code-division multiple access

Code-division multiple access
The Anome · CC BY-SA 3.0 · source
Name	Code-division multiple access
Caption	Example of spectrum usage in a spread-spectrum system
Type	Multiple access method
Invented	1940s–1990s
Inventor	Claude Shannon, Harry Nyquist, Irwin Jacobs, Andrew Viterbi
Initial release	1940s

Code-division multiple access is a channel access method used in various wireless communication systems that allows multiple transmitters to send information simultaneously over a single communication channel. It employs spread-spectrum techniques and unique code sequences to separate signals from different users, enabling multiple access without fixed time or frequency allocation. CDMA influenced standards, companies, and research institutions across industry and academia, shaping cellular networks and satellite communications.

Overview

CDMA exploits spread-spectrum concepts pioneered by Claude Shannon, Harry Nyquist, Irwin Jacobs, Andrew Viterbi, and researchers at Bell Labs, Qualcomm, Massachusetts Institute of Technology, Stanford University, and University of California, Los Angeles. Implementations appear in systems developed by Motorola, Nokia, Ericsson, Samsung Electronics, AT&T, Verizon Communications, and China Mobile. CDMA contrasts with Frequency-division multiple access and Time-division multiple access systems standardized by bodies such as the 3GPP, 3GPP2, International Telecommunication Union, European Telecommunications Standards Institute, and Institute of Electrical and Electronics Engineers. Regulatory agencies like the Federal Communications Commission and European Commission influenced spectrum allocation policies that affected CDMA deployment.

Technical principles

The core principle uses pseudo-random spreading sequences (e.g., Walsh codes, Gold codes, Kasami sequences) developed in mathematical work at Bell Labs, MIT, Caltech, and Princeton University. Signals are multiplied by orthogonal or pseudo-orthogonal codes so that receivers using matched filters or rake receivers can despread the desired signal amid interference from other users, a technique informed by theories from Claude Shannon and Harry Nyquist. Implementation relies on channel estimation and multipath handling techniques studied by researchers at University of California, Berkeley, Georgia Institute of Technology, University of Texas at Austin, and Cornell University. Error-correcting codes such as convolutional codes, turbo codes, and LDPC codes from Nokia, Ericsson, Motorola, and researchers like Claude Shannon and Gérard Gallager often complement CDMA systems. Power control algorithms from groups at Qualcomm, Bell Labs, and NTT DoCoMo mitigate the near–far problem described in papers from IEEE conferences.

Implementations and variants

Major CDMA-based standards include cdmaOne and cdma2000 from Qualcomm and 3GPP2, and Wideband CDMA (W-CDMA) from 3GPP used in Universal Mobile Telecommunications System deployments by Vodafone, T-Mobile, Orange S.A., and Telefonica. Variants and related technologies include IS-95, Enhanced Data rates for GSM Evolution driven by GSM Association, Universal Terrestrial Radio Access Network work at 3GPP, and hybrid approaches combining CDMA with OFDM as seen in projects at IEEE 802.11, Intel Corporation, Nokia Siemens Networks, and Ericsson. Satellite systems from Iridium Communications and Globalstar used spread-spectrum concepts inspired by CDMA research at NASA and European Space Agency. Proprietary and academic implementations emerged from Qualcomm, Motorola, Nokia, Ericsson, Alcatel-Lucent, and university labs including Stanford University, MIT Lincoln Laboratory, and UC Berkeley.

Performance and limitations

Performance depends on code orthogonality, receiver complexity, power control, and spectrum availability discussed in papers at IEEE Communications Society, ACM SIGCOMM, and IET. CDMA systems can offer robustness to multipath and intersymbol interference relative to some Frequency-division multiple access or Time-division multiple access deployments studied by researchers at Bell Labs and Lucent Technologies, but suffer from capacity limits due to multiple-access interference and the near–far problem analyzed by Qualcomm and Bell Labs. Complexity of rake receivers, scaling challenges in dense urban deployments encountered by Vodafone and China Mobile, and spectral efficiency trade-offs considered by 3GPP led to migration toward OFDMA in later generations promoted by Intel, Samsung Electronics, and Qualcomm.

History and development

Foundations trace to spread-spectrum research in the 1940s and 1950s at Bell Labs, Harvard University, MIT, and Stanford University; early patents and papers exchanged among inventors like Hedy Lamarr and engineers at Bell Labs influenced later work. The 1980s and 1990s saw commercialization and standardization by Qualcomm, Motorola, Nokia, Ericsson, 3GPP2, and 3GPP, culminating in deployments such as cdmaOne, cdma2000, and W-CDMA in networks run by Sprint Corporation, SK Telecom, China Telecom, and NTT DoCoMo. Regulatory and industry milestones involving the Federal Communications Commission, European Commission, International Telecommunication Union, and the GSM Association shaped spectrum policy and standard adoption. Academic contributions from Caltech, Princeton University, Cornell University, Georgia Institute of Technology, and University of Southern California further advanced coding, modulation, and multiple-access theory.

Applications and standards

CDMA technology underpinned 2G/3G cellular standards including cdmaOne, cdma2000, and W-CDMA adopted by carriers such as Verizon Communications, Sprint Corporation, Vodafone, China Mobile, and NTT DoCoMo. It informed satellite communications projects at Iridium Communications and Globalstar, military systems developed by Raytheon, Lockheed Martin, and BAE Systems, and research testbeds at MIT Lincoln Laboratory and NASA. Standards bodies including 3GPP, 3GPP2, IEEE, ETSI, and ITU published specifications that referenced CDMA techniques while the industry migration to LTE and 5G involved organizations such as 3GPP, Qualcomm, Ericsson, Nokia, and Samsung Electronics.

Category:Mobile telephone standards