Code-division multiple access

Code-division multiple access (CDMA) is a way for many radios to talk at the same time on the same channel. It lets lots of people share the same group of frequencies.

CDMA works by spreading the signal over a wide range of frequencies. Each transmitter gets a special code so their signals can be separated.

CDMA is used in many mobile phone systems, like IS-95 and CDMA2000. Even though some companies have stopped using older versions, CDMA is still important.

CDMA is also used in space communications because it can handle changes in signal strength from moving satellites. It is often used with other methods like binary phase-shift keying and quadrature amplitude modulation to make it work better.

History

Code-division multiple access (CDMA) has been studied and developed for many years. In the United States, early work on CDMA began at the Massachusetts Institute of Technology in 1950. More studies on anti-jamming followed in 1952 at Lincoln Lab.

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

Uses

A CDMA2000 mobile phone

Code-division multiple access (CDMA) has been used in many important systems. An early version called Synchronous CDM was used in the Global Positioning System before it was used in mobile phones. Later, standards like IS-95, called cdmaOne, and IS-2000, called CDMA2000, were developed by Qualcomm for use by mobile phone companies such as the Globalstar network. The UMTS 3G mobile phone standard uses a version called W-CDMA. CDMA has also been used 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, 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)

An example of 4 mutually orthogonal digital signals

Code-division multiplexing, also known as synchronous CDMA, is a way for many people to use the same radio channel at the same time. It works by giving each person a special code. These codes are made so they don’t mix up each other’s messages, even when lots of people are talking or sending information together.

Each person’s signal is mixed with their unique code. On the other end, the system uses those same codes to find and understand each person’s message, even when many messages are sent together. This lets lots of people use the same frequencies without bothering each other.

Step	Encode sender0	Encode sender1
0	code0 = (1, −1), data0 = (1, 0, 1, 1)	code1 = (1, 1), data1 = (0, 0, 1, 1)
1	encode0 = 2(1, 0, 1, 1) − (1, 1, 1, 1) = (1, −1, 1, 1)	encode1 = 2(0, 0, 1, 1) − (1, 1, 1, 1) = (−1, −1, 1, 1)
2	signal0 = encode0 ⊗ code0 = (1, −1, 1, 1) ⊗ (1, −1) = (1, −1, −1, 1, 1, −1, 1, −1)	signal1 = encode1 ⊗ code1 = (−1, −1, 1, 1) ⊗ (1, 1) = (−1, −1, −1, −1, 1, 1, 1, 1)

Step	Decode sender0	Decode sender1
0	code0 = (1, −1), signal = (0, −2, −2, 0, 2, 0, 2, 0)	code1 = (1, 1), signal = (0, −2, −2, 0, 2, 0, 2, 0)
1	decode0 = pattern.vector0	decode1 = pattern.vector1
2	decode0 = ((0, −2), (−2, 0), (2, 0), (2, 0)) · (1, −1)	decode1 = ((0, −2), (−2, 0), (2, 0), (2, 0)) · (1, 1)
3	decode0 = ((0 + 2), (−2 + 0), (2 + 0), (2 + 0))	decode1 = ((0 − 2), (−2 + 0), (2 + 0), (2 + 0))
4	data0=(2, −2, 2, 2), meaning (1, 0, 1, 1)	data1=(−2, −2, 2, 2), meaning (0, 0, 1, 1)

Step	Decode sender0	Decode sender1
0	code0 = (1, −1), signal = (1, −1, −1, 1, 1, −1, 1, −1)	code1 = (1, 1), signal = (1, −1, −1, 1, 1, −1, 1, −1)
1	decode0 = pattern.vector0	decode1 = pattern.vector1
2	decode0 = ((1, −1), (−1, 1), (1, −1), (1, −1)) · (1, −1)	decode1 = ((1, −1), (−1, 1), (1, −1), (1, −1)) · (1, 1)
3	decode0 = ((1 + 1), (−1 − 1), (1 + 1), (1 + 1))	decode1 = ((1 − 1), (−1 + 1), (1 − 1), (1 − 1))
4	data0 = (2, −2, 2, 2), meaning (1, 0, 1, 1)	data1 = (0, 0, 0, 0), meaning no data

Asynchronous CDMA

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

CDMA uses these spreading sequences to let many users share the same frequency band. Controlling signal strength is important. Stronger signals can block weaker ones. In mobile phone networks, base stations adjust the power of each device. This helps keep communication clear. New advances use smart ways to make better spreading sequences. This improves performance.

Collaborative CDMA

Collaborative CDMA is a special way to help many users talk on the same radio channel at the same time. It groups a few users together to share one code. This lets 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.