Code-division multiple access

Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA allows several transmitters to send information simultaneously over a single communication channel, enabling many users to share a band of frequencies (bandwidth). It works by using spread spectrum technology and a special coding scheme where each transmitter is assigned a unique code.

CDMA optimizes the use of available bandwidth by transmitting over the entire frequency range, rather than limiting users to specific frequencies. It is used in many mobile phone standards, including IS-95 (also called "cdmaOne") and its 3G evolution CDMA2000. Although many carriers like AT&T, UScellular, and Verizon have shut down their 3G CDMA networks, CDMA remains an important technology in communication systems.

CDMA can also serve as a channel access technology, similar to ALOHA, and is often used in space-based communications due to its ability to handle large path loss and Doppler shift from satellite motion. It is frequently combined with modulation schemes like binary phase-shift keying (BPSK), quadrature amplitude modulation (QAM), or orthogonal frequency-division multiplexing (OFDM) to enhance robustness and efficiency.

History

Code-division multiple access (CDMA) has been studied and developed for many years. In the United States, early work on CDMA was part of a research project at the Massachusetts Institute of Technology in 1950, with more studies on anti-jamming techniques following in 1952 at Lincoln Lab.

In the Soviet Union (USSR), research on signal separation methods began in 1935. By 1957, a young engineer named Leonid Kupriyanovich created an early mobile phone called the LK-1, which could work over distances of up to 30 kilometers. Later developments led to the "Altai" mobile phone service for cars, which began in Moscow in 1963 and later expanded to other cities.

Uses

Code-division multiple access (CDMA) has been used in many important systems. For example, an early version of CDMA called Synchronous CDM was used in the Global Positioning System (GPS) before it was used in mobile phones. Later, standards like IS-95, marketed as cdmaOne, and IS-2000, known as CDMA2000, were developed by Qualcomm and used by mobile phone companies such as the Globalstar network. Additionally, the UMTS 3G mobile phone standard uses a version called W-CDMA. CDMA has also been applied in the OmniTRACS satellite system for transportation logistics.

Steps in CDMA modulation

CDMA is a way for many people to talk on the same radio channel at the same time without getting mixed up. It works by spreading out the signal over a wider space, kind of like stretching out a piece of music so it takes longer to play but still sounds the same. Each person or device uses a special code to send their message. When the message arrives, the receiver uses the same code to pick out the right message and ignore the others, which helps keep things clear.

Think of it like people speaking different languages in the same room. If you understand one language, you can hear that person clearly, but other languages sound like background noise. In CDMA, each user has their own unique code, allowing many users to share the same radio frequencies without interfering with each other. This makes CDMA a powerful tool for managing many conversations or data transmissions at once.

Code-division multiplexing (synchronous CDMA)

Code-division multiplexing, also known as synchronous CDMA, is a way for many users to share the same radio channel at the same time. It works by giving each user a special code. These codes are designed so that they don’t interfere with each other, even when many users are talking or sending data at once.

Each user’s signal is mixed with their unique code. At the receiving end, the system uses the same codes to pick out and understand each user’s message, even when many are sent together. This lets many people use the same frequencies without getting in each other’s way.

Asynchronous CDMA

See also: Direct-sequence spread spectrum and near–far problem

When mobile devices cannot be perfectly coordinated, a different method is needed. In asynchronous CDMA, special "spreading sequences" are used. These sequences are like secret codes that help each user's signal stand out from others. They make the signals of other users look like noise to the intended receiver.

CDMA uses these spreading sequences to allow many users to share the same frequency band. Controlling signal strength is important because stronger signals can drown out weaker ones. In mobile phone networks, base stations carefully adjust the power of each device to keep communication clear. Recent advances use smart techniques to create better spreading sequences, improving performance.

Collaborative CDMA

Collaborative CDMA is a special way to help many users talk on the same radio channel at the same time. Instead of giving each user their own unique code, this method groups a few users together to share one code. This helps more users connect without making the system too complicated. Another version called interleave-division multiple access (IDMA) separates users by mixing up their data in a specific pattern, instead of using unique codes.