使用SIMULINK模拟CDMA(IS-95)通信系统

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"CDMA（关于IS-95的SIMULINK资料）" 该资源是一份详细介绍如何使用MATLAB中的SIMULINK工具包来建模、模拟和实现基于北美IS-95A CDMA标准的无线通信系统的用户指南。CDMA（码分多址）是一种通信技术，它允许多个用户在同一频率上同时进行通信，通过独特的编码序列区分不同的信号。IS-95是北美广泛采用的CDMA标准。 SIMULINK是MATLAB的一个扩展，主要用于动态系统建模和仿真。在这个CDMA Reference Blockset中，用户可以找到一系列预定义的Simulink模块，这些模块专门设计用于构建和分析CDMA通信系统。这些模块可能包括信道编码、扩频、多路访问冲突解决、功率控制以及接收机和发射机的基本组件。用户指南涵盖了以下关键主题： 1. **Modeling（建模）**：这部分内容指导用户如何使用SIMULINK块来构建CDMA系统的模型，包括信号的产生、编码、扩频和解扩等过程。用户可以学习如何设置不同的参数，如扩频码、码片速率和系统带宽。 2. **Simulation（仿真）**：在建模的基础上，用户将学习如何运行仿真以分析系统的性能。这可能涉及到误码率（BER）、信噪比（SNR）和容量等关键性能指标的计算。 3. **Implementation（实现）**：该部分可能会讨论如何将SIMULINK模型转化为实际的硬件或软件实现，可能涉及MATLAB的代码生成功能，以及如何将模型转换为嵌入式系统或其他平台。 4. **Version 1**：表明这是该参考指南的第一个版本，可能随着软件更新和技术进步会有后续版本发布。此外，文档还提供了与The MathWorks公司的联系方式，包括网站、新闻组、技术支持邮箱、产品建议邮箱、错误报告邮箱、销售咨询和客户服务等，以便用户获取更多信息、报告问题或提出改进建议。该文档版权由The MathWorks, Inc.和Algorex, Inc.共同所有，并强调软件的使用必须遵循许可协议，未经许可，不得复制或以任何形式分发手册内容。对于联邦采购，文档还可能包含特定的法规遵从性信息。这份用户指南对于理解IS-95 CDMA标准，以及在MATLAB环境中进行无线通信系统设计和仿真，是非常宝贵的资源。它可以帮助工程师和研究人员快速上手，进行系统级别的性能评估和优化。

1 Tutorial

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Wireless Communication and CDMA

This section provides some background information on wireless technology,

wireless personal communications, and the basics of CDMA technology. If you

are familiar with CDMA technology and the IS-95A standard, you can skip to

these later sections:

•“Main Library Structure” on page 1-12, which describes the components of

the CDMA Reference Blockset

•“Hints for Using the CDMA Reference Blockset” on page 1-19, which

describes some important conventions in the CDMA Reference Blockset that

you should keep in mind while studying or creating simulations

•“Demo Models” on page 1-22, which uses demonstration models to illustrate

how to use the blockset for simulating CDMA techniques.

Growth of Wireless Communications Services

The wireless communications industry has grown extremely rapidly over the

past decade, with markets doubling roughly every 2 years. Much of this growth

has been due to the public’s increasing demand for mobile telephones, and

more recently, wireless data systems. The increasing number of wireless users

has spurred communications engineers to improve the quality of service and to

use the available spectrum resources more efficiently.

Types of Wireless Communications Services

Consumers today have access to many different kinds of wireless products and

services, having widely varying range of coverage. Short-range products

include cordless telephones and wireless local area networks (WLANs).

Long-range products include satellite-based wireless units. At various

intermediate ranges are products such as cellular telephones, wide-area

wireless data and radio paging services, specialized satellite-based message

services in the freight industry, and internet access from hand-held units.

Clearly, wireless communications today encompasses many technologies,

systems and services, aimed at many different applications.

Wireless technologies and systems can be divided into six distinct groups:

•Cellular mobile radio systems

•Cordless telephones

1 Tutorial

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Overview of CDMA

Code division multiple access (CDMA) is a modulation and multiple-access

scheme based on spread-spectrum communication. In this scheme, multiple

users share the same frequency band at the same time, by spreading the

spectrum of their transmitted signals, so that each user’s signal is

pseudo-orthogonal to the signals of the other users.

CDMA Signals

In a CDMA system, each signal consists of a different pseudorandom binary

sequence (called the spreading code) that modulates a carrier, spreading the

spectrum of the waveform. A large number of CDMA signals share the same

frequency spectrum. If CDMA is viewed in either the frequency or time domain,

the multiple access signals overlap with each other. However, the use of

statistically orthogonal spreading codes separates the various signals in the

code space.

CDMA Receivers

A CDMA receiver separates the signals by means of a correlator that uses the

particular binary sequence to despread the signal and collect the energy of the

desired signal. Other users’ signals, whose spreading codes do not match this

sequence, are not despread in bandwidth and, as a result, contribute only to the

noise. These signals represent a self-interference generated by the system. The

output of the correlator is sent to a narrow-bandwidth filter. The filter allows

all of the desired signal’s energy to pass through, but reduces the interfering

signal’s energy by the ratio of the bandwidth before the correlator to the

bandwidth after the correlator. This reduction greatly improves the

signal-to-interference ratio of the desired signal. This ratio is also known as the

processing gain. The signal-to-noise ratio is determined by the ratio of the

desired signal power to the sum of all of the other signal powers. It is enhanced

by the processing gain or the ratio of spread bandwidth to baseband data rate.

CDMA Channel Assignments

A CDMA digital cellular waveform design uses a pseudorandom noise (PN)

sequence to spread the spectrum. The sample rate of the spreading sequence

(called the chip rate) is chosen so that the bandwidth of the filtered signal is

several times the bandwidth of the original signal.

A typical system might use multiple PN sequences. In addition, it might use

repeated spreading codes of known lengths to ensure orthogonality between

Wireless Communication and CDMA

1-5

signals intended for different users. The channel assignment is essentially

determined by the set of codes that are used for that particular link. Thus, the

signal transmitted at any time in a logical channel is determined by:

•The frequency of operation for the base station

•The current symbol

•The specific orthogonal spreading code assigned for the logical channel

•The PN spreading code

CDMA Signal Processing

In the demodulation of CDMA signals, the different paths may be

independently received, which greatly reduces the severity of the multipath

fading. However, multipath fading is not completely eliminated because

occasionally there may be multiple paths that cannot be independently

processed by the demodulator.

Different users in CDMA employ signals that have very small

cross-correlation. Thus, correlators can extract individual signals from a

mixture of signals even though they are transmitted simultaneously in the

same frequency band. CDMA systems employ wideband signals with good

cross-correlation properties, which means the output of a filter matched to one

user’s signal is small when it receives a different user’s signal as input.

In direct-sequence spread-spectrum systems, a high-rate antipodal

pseudorandom spreading sequence modulates the transmitted signal so that

the bandwidth of the resulting signal is roughly equal to the rate of the

spreading sequence. The cross-correlation of the signals is then largely

determined by the cross-correlation properties of the spreading signals.

Although CDMA signals overlap in both time and frequency domains, they can

be separated, based on their spreading waveforms.

Spreading rates can be chosen to exceed the coherence bandwidth so that the

channel becomes frequency selective. For instance, different spectral

components are affected unequally by the channel, and only parts of the signal

are affected by fades. Expressing the same observation in time domain terms,

multipath components are resolvable at a resolution equal to the chip period

and can be combined coherently, for example, by means of a rake receiver.

Coherent combination of multipath components requires an estimate of the

channel impulse response. Such an estimate can be calculated from a training

sequence or by means of a pilot signal.

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