CDMA射频系统工程：Samuel C. Yang的权威指南

CDMA.RF.

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"Artech.House.CDMA.RF.SystemEngineering" 是一本由Samuel C. Yang编著的专业书籍，属于Artech House移动通信图书馆系列。该书深入探讨了CDMA（码分多址）射频系统工程，是无线通信领域的重要参考资料。 CDMA是一种广泛应用于无线通信的技术，其核心原理是通过独特的编码方式来区分不同的用户信号，允许在同一频率上同时传输多个数据流，从而提高频谱效率。在CDMA系统中，每个用户的数据被一个唯一的码序列调制，这些码序列相互正交，即使在相同频率上，也能在接收端通过解码分离出各个用户的信号，实现了多址接入。书中详细讲解了CDMA系统的各个方面，包括基本原理、系统设计、射频工程、信号处理、功率控制、多径衰落以及干扰管理等关键知识点。这些内容对于理解CDMA系统如何在实际环境中高效运行至关重要。例如，射频系统工程部分可能涵盖了射频前端设计、天线选择、无线传播模型以及RF信号的发射和接收过程。此外，书中还涉及个人通信服务系统（PCS），这是CDMA技术在移动通信中的一个重要应用领域。PCS通常指的是提供语音、数据和多媒体服务的无线通信网络，CDMA技术因其在频率复用和容量扩展上的优势，在这一领域得到了广泛应用。书中的参考文献和索引为读者提供了进一步研究的途径，可以深入到特定主题或技术细节中。ISBN号0-89006-991-3标识了这本书，并根据美国国会图书馆和英国图书馆的分类标准进行了编目，方便读者在图书馆或在线查找。《Artech.House.CDMA.RF.SystemEngineering》是无线通信工程师、研究人员和学生深入了解CDMA射频系统设计与实施的宝贵资源。它不仅阐述了CDMA的基本理论，还详细讨论了实际工程中的挑战和解决方案，对于提升对CDMA技术的理解和应用能力具有极大的帮助。

Introduction

1.1 Motivation

The market of wireless communication is expected to increase dramatically in

the late 1990s and beyond, and this expected high demand has led many service

providers to investigate digital technology to satisfy the increasing demand.

Spread spectrum has been used for a long time in military communications to

resist intentional jamming and to achieve low-probability of detection. How-

ever, in recent years, spread spectrum has moved from military to commercial

communications, culminating in the introduction of Interface Standard IS-95

code division multiple access (CDMA) technology as an alternative standard for

commercial digital cellular and personal communication system (PCS) networks.

Service providers, both cellular and PCS carriers, have deployed commercial

CDMA systems in major metropolitan areas. The IS-95 CDMA is now being

used in numerous cellular and PCS markets around the world. Service

providers are deploying these systems in their markets, where there are mount-

ing demands for higher capacity.

Multiple access systems share a fixed resource (i.e., frequency spectrum)

to provide voice channels on demand to users. At first it seems counter-

intuitive that deliberately increasing the bandwidth required for transmission

increases capacity. After all, in a traditional frequency division multiple access

(FDMA) scheme, increasing the required bandwidth per user decreases the

total number of users a fixed spectrum can support. We start with a definition

of spread spectrum [1]:

interested in sending two separate messages: m

, which is (+1, −1, +1), and m

which is (+1, +1, −1).

Figure 1.2 shows the waveforms and spectrums for the two messages

(t) and

(t), the two orthogonal codes

(t) and

(t), and the two spread

messages m

(t)c

(t) and m

(t)c

(t). While we do not go into the details of calcu-

lating the spectrums of these time waveforms, it suffices for our purposes to

state that the bandwidth of a random digital waveform is limited to 1/T, where

T is the bit interval of the random digital waveform. We further make the dis-

tinction between

and

, where

is the bit interval (in seconds) of the mes-

sage and

is the chip interval of the running orthogonal code. In this example,

the chip rate (

1/T

) of the orthogonal code is running at four times the bit rate

(

1/T

). Therefore, we have an effective bandwidth expansion factor of

four. The bandwidth expansion factor is sometimes called the processing gain or

(

WR/

), where W is the final bandwidth of the spread message and R is the band-

width of the baseband message. Note that in this example, W is equivalent to

(1/T

), R is equivalent to (

1/T

), and the processing gain (

WR/

)is4,or6dB.

For an excellent treatment of power spectra of different digital waveforms,

consult [4].

Note that after spreading by the orthogonal codes, the spread messages

(t)c

(t) and m

(t)c

(t) now occupy a larger bandwidth than the original messages.

Figure 1.3 shows the waveforms at different points of the receiver. The

signal at point A is the result of the summation of the two spread messages.

The spectrum at A now contains two separate signals. In order to recover the

two separate messages from the composite spectrum, the signal at A is multi-

plied by the two respective orthogonal codes to obtain

and

. Figure 1.4

shows the signals at

and

, the outputs

and

of the decision thresholds,

and the recovered messages

(t) and

(t). The integrator adds up the signal

power over one bit interval

of the baseband message, and the decision

threshold decides, based on the output of the integrator, whether or not the

particular bit is a +1or−1. If the output of the integrator is greater than 0, then

the decision is a +1; if the integrator output is less than 0, then the decision is a

−1. The digital-to-analog (D/A) converter transforms the decision into the

recovered waveforms

(t) and

(t). As one can see in this idealized example,

the recovered messages

(t) and

(t) match perfectly the original baseband

messages

(t) and

(t).

This example only serves to illustrate the principle of DS-SS multiple

access. We have just demonstrated that, using DS-SS techniques, separate mes-

sages can be sent through the same channel in the same frequency band at the

same time, and the messages can be successfully recovered at the receiver. How-

ever, there are many real-world phenomena, especially in a mobile communica-

tions environment, that degrade the performance of such a DS-SS multiple

Introduction 5

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