CDMA

CDMA
Name	Code-division multiple access
Caption	A cell site antenna using the technology.
Invented	1949
Inventor	Hedy Lamarr, George Antheil
Industry	Telecommunications
Connector	Base station

CDMA. Code-division multiple access is a channel access method used by various radio communication technologies. It allows numerous transmitters to send information simultaneously over a single communication channel, enabling multiple users to share a band of frequencies. This is achieved by assigning each transmitter a unique code to spread its signal across the wide bandwidth. The method forms a foundational technology for several major mobile phone standards, particularly those under the 3G umbrella.

Overview

The core concept enables multiple signals to occupy the same frequency band simultaneously by using unique spreading codes. This approach is fundamentally different from traditional methods like frequency-division multiple access or time-division multiple access. It is a form of spread-spectrum communication, a technique with origins in military applications for secure and robust signaling. Major commercial implementations were pioneered by companies like Qualcomm, and it became a key air interface for networks operated by carriers such as Verizon Wireless and Sprint Corporation.

Technical principles

The system operates by using pseudo-noise sequences to modulate the data signal, spreading it across a bandwidth much wider than the original information requires. Each user's signal is recovered at the receiver by correlating the received wideband signal with a synchronized copy of the assigned unique code. This process, known as despreading, isolates the desired user's transmission from all others. Key mathematical operations involve digital signal processing and correlation, managed by specialized integrated circuits within handsets and base station equipment.

History and development

The foundational idea of spread spectrum was first conceived by actress Hedy Lamarr and composer George Antheil in 1941 for guiding torpedoes. Further development occurred within the United States Armed Forces, leading to its use in secure military communications. In the late 1980s, Irwin M. Jacobs and his company Qualcomm pioneered its commercial application for cellular networks. A major milestone was its adoption by the Telecommunications Industry Association as the IS-95 standard, also known as cdmaOne, which launched the first large-scale digital networks in the 1990s.

Standards and variants

The primary second-generation standard was IS-95, branded as cdmaOne. Its evolutionary path led to CDMA2000, a family of 3G standards that included 1xRTT and 1xEV-DO for high-speed data. Another significant but incompatible variant is Wideband CDMA, which was the chosen technology for the Universal Mobile Telecommunications System in Europe and Japan. These standards were developed and maintained by bodies like the 3rd Generation Partnership Project 2 and the International Telecommunication Union.

Applications and deployment

Beyond its dominant role in mobile telephony, the principles are used in the Global Positioning System for satellite navigation and in some Wi-Fi standards like IEEE 802.11b. Commercially, it was extensively deployed across North America and parts of Asia, notably in South Korea. Major infrastructure vendors like Lucent Technologies, Nortel, and Ericsson produced network equipment, while handset manufacturers like Samsung Electronics and LG Electronics were key suppliers.

Advantages and disadvantages

Key advantages include efficient use of spectrum, inherent security from the spreading codes, soft handoff capabilities that improve call quality, and resistance to multipath interference. However, it faced disadvantages such as complexity in power control to solve the near-far problem, and the challenge of global fragmentation, as the dominant GSM ecosystem adopted W-CDMA for 3G. This incompatibility ultimately led to its decline in the 4G era with the universal adoption of Orthogonal frequency-division multiple access standards like Long-Term Evolution.

Category:Channel access methods Category:Telecommunications Category:Mobile technology