【Practical Exercise】Introduction to CDMA Communication and MATLAB Simulation

# 1. Fundamentals of CDMA Communication

CDMA (Code Division Multiple Access) is a digital cellular mobile communication technology that enables multiple users to communicate simultaneously on the same frequency band by spreading their signals. CDMA communication offers strong resistance to interference, high capacity, and enhanced security, making it widely used in the field of mobile communications.

The basic principle of CDMA communication involves spread spectrum processing of user digital signals, significantly expanding their bandwidth beyond the original signal bandwidth. The spread spectrum signal possesses excellent resistance to interference, ensuring correct signal reception even in the presence of other signal interferences.

# 2. CDMA Communication Theory

### 2.1 Principles of CDMA Spread Spectrum

#### 2.1.1 Overview of Spread Spectrum Technology

Spread spectrum technology is a technique that expands the bandwidth of an information signal to a much larger extent than its original bandwidth. In CDMA systems, spread spectrum technology is used to convert narrowband information signals into broadband signals, thereby enhancing their resistance to interference.

#### 2.1.2 CDMA Spread Spectrum Methods

There are two common spread spectrum methods in CDMA systems: Direct Sequence Spread Spectrum (DS-CDMA) and Frequency Hopping Spread Spectrum (FH-CDMA).

- **DS-CDMA:** Multiplying the information signal with a pseudo-random code (PN code) directly to spread the signal bandwidth.
- **FH-CDMA:** Allocating the information signal across multiple frequency carriers and hopping between them based on a PN code sequence to spread the signal bandwidth.

### 2.2 CDMA Channel Structure

#### 2.2.1 Chips and Code Groups

A chip is the basic unit of a spread spectrum signal in CDMA systems, usually consisting of 1 or 2 bits. A code group is a sequence composed of multiple chips, used to uniquely identify each user.

#### 2.2.2 Code and Power Control

- **Code Control:** Adjusting chip rate and code group length to control the signal's spread spectrum factor.
- **Power Control:** Regulating transmission power to control the signal's power level, thereby reducing interference between adjacent channels.

### 2.3 CDMA Reception Technology

#### 2.3.1 Correlative Reception

Correlative reception is a commonly used reception technique in CDMA systems. By performing correlation operations between the received signal and a locally generated PN code, the target user's signal can be extracted.

#### 2.3.2 Interference Suppression

Since CDMA systems involve multiple users transmitting signals simultaneously, ***mon interference suppression techniques include:

- **Near-far Effect:** Prioritizing the reception of signals from nearby users since their signal strength is significantly greater than that of distant users.
- **Soft Decision:** Improving the quality of the received signal through soft decision processing, thereby suppressing interference.
- **Multiple Antenna Technology:** Enhancing signal diversity and suppressing interference through the use of multiple antennas.

# 3. CDMA Communication Practice

### 3.1 CDMA System Architecture

The CDMA system architecture mainly consists of base station subsystems and mobile station subsystems.

#### 3.1.1 Base Station Subsystems

The base station subsystem is the core of the CDMA system, responsible for providing wireless access services to mobile stations. Its main components include:

- **Base Station Co