码分多址通信系统、扩频通信外文翻译

中文2800字
毕业设计英文翻译
专业电子信息工程
班级2010级
学生姓名
学号
课题码分多址通信系统的建模、仿真和设计
——初始化模块、基站接收模块
指导教师
2014 年06 月10 日
译文原文
1.1 The basic concept of spread-spectrum communication
Spread spectrum communication’s basic characteristics, is used to transmit information to the signal bandwidth(W) is far greater than practical required minimum(effective) bandwidth (F
∆),as the radio of processing gain P G.
=/
G P∆
F
W
As we well know,the ordinary AM,FM,or pulse code modulation,GP value in the area more than 10 times,collectively,the “narrow-band communication”,and spread-spectrum communication GP values as hundred or even thousands of times, can be called “broadband communication”.
Due to the spread-spectrum signal,it is very low power transmitters,transmission space mostly drowned in the noise,it is difficult to intercepted by the other receiver ,only spreading codes with the same (or random PN code) receiver, Gain can be dealt with ,and despreading resume the original signal.
1.2 The technology superiority of spread-spectrum communication.
Strong anti-interference, bit error rate is low. As noted above, the spread spectrum communication system due to the expansion of the transmitter signal spectrum, the receiver despreading reduction signal produced spreading gain, thereby greatly enhancing its interference tolerance. Under the spreading gain, or even negative in the signal-to-noise ratio conditions, can also signal from the noise drowned out Extraction, in the current business communications systems, spread spectrum communications systems, spread spectrum communication is only able to work in a negative signal-to-noise ratio under the conditions of communication .
Anti-multi-path interference capability, increase the reliability of system. Spread-spectrum systems as used in the PN has a good correlation, correlation is very weak. Different paths to the transmission signal can easily be separated and may in
time and re-alignment phase, formation of several superimposed signal power, thereby improving the system’s performance to receive increased reliability of the system.
Easy to use the same frequency, improving the wireless spectrum utilization. Wireless spectrum is very valuable,although long-wave microwave have to be exploited, and still can not meet the needs of community. To this end, countries around the world are designed spectrum management, users can only use the frequency applications,rely on the channel to prevent the division between the channel interference.
Due to the use of spread-spectrum communication related receive this high-tech,low signal output power(“a W,as a general-100mW),and will work in the channel noise and thermal noise in the background,easy to duplicate in the same area using the same frequency,can now all share the same narrow-band frequency communication resources.
Spread-spectrum communication is digital communication,particularly for digital voice and data transmission with their own encryption, only in the same PN code communication between users, is good for hiding and confidential in nature, facilitating communication business. Easy to use spread-spectrum CDMA communications, voice compression and many other new technologies, more applicable to computer networks and digitization of voice,image information transmission.
Communication in the most digital circuits, equipment, highly integrated, easy installation, easy maintenance, but also very compact and reliable. The average failure rate no time was very long.
1.3 Spread spectrum communication system
Spread spectrum communication,namely, spread spectrum communications (Spread spectrum communication), with fiber-optic communications,satellite communications,with access to the information age as the three major high-tech communications transmission. Spread spectrum communication is to send the information to be pseudo-random data is coded(Spread spectrum sequence: spread sequence) modulation, spread spectrum and then the realization of transmission; the
receiving end is using the same modem code and related processing, the restoration of the original data. Spread spectrum communication system has three main characteristics.
(1) Carrier is an unpredictable, or so-called pseudo-random broadband signal.
(2) Carrier data bandwidth than the modulation bandwidth is much wilder.
(3) Receiving process is generated by local broadband carrier signal and receiving a copy of the signal to the broadband signal to achieve.
The main way of spread spectrum are as follows: Direct Sequence Spread Spectrum(DSSS) using high-speed pseudo-random code on to the low-speed data transmission spread spectrum modulation; Frequency-hopping system using pseudo-random code to control the carrier frequency in a wider band of the change; TH is the data transmission time slot is a pseudo-random; chirp frequency system is a linear extension of the process of change. Combination of a number of ways of hybrid systems are often applied.
The most important measure pf spread-spectrum system is an indicator of spreading gain, also known as processing gain. It is precisely because of the spread spectrum system itself with its performance characteristics with a series of advantages.
1.4 Code division multiple access
Code division multiple access (CDMA) is a channel access method used by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne, CDMA2000(the 3G evolution of cdmaOne) and WCDMA (the 3Gstandard used by GSM carrier), which are often referred to as simply CDMA, and use CDMA as an underlying channel access method.
One of the concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a signal communication channel. This allows several users to share a band of frequencies (see bandwidth). This concept is called multiple access. CDMA employs spread-spectrum technology and a special coding scheme( where each transmitter is assigned a code) to allow multiple user to be multiplexed over the same physical channel. By contrast, time division multiple
access (FDMA) divides it by frequency. CDMA is a form of spread-spectrum signaling, since the modulated coded signal has a much higher data bandwidth than the data being communicated.
1.5 Spread-spectrum characteristic of CDMA
Most modulation schemes try to minimize the bandwidth of this signal since bandwidth is a limited resource. However, spread spectrum use a transmission bandwidth that is several orders of magnitude greater than the minimum required signal bandwidth. One of the initial reasons for doing this was military applications including guidance and communication systems. These system were designed using spread spectrum because if its security and resistance to jamming. Asynchronous CDMA has some level of privacy built in because the signal is spread using a pseudo-random code; this code makes the spread spectrum signals appear random or have noise-like properties. A receiver cannot demodulate this transmission without knowledge of the pseudo-random sequence used to encode the data. CDMA also resistant to jamming. A jamming signal only has a finite amount of power available to jam the signal. The jammer can either spread its energy over the entire bandwidth of the signal or jam only part of the entire signal.
CDMA can also effectively reject narrow band interference. Since narrow band interference affects only a small portion of the spread spectrum signal, it can easily be removed through notch filtering without much loss of information. Convolution encoding and interleaving can be used to assist in recovering this lost data. CDMA signal are also resistant to multipath fading. Since the spread spectrum signal occupies a large bandwidth only a small portion of this will undergo fading due to multipath at any give time. Like the narrow band interference this will result in only a small loss of data and can be overcome.
Another reason CDMA is resistant to multipath interference is because the delayed versions of the transmitted pseudo-random code, and will thus appear as another user, which is ignored at the receiver. In other words, as long as the multipath channel induces at least one chip of delay, 天the multipath channel induces at least one chip of delay,the multipath signals will arrive at the receiver.in other words, as long as the multipath channel induces at least one chip of delay, the multipath signals
will arrive at the receiver such that they are shifted in time by at least one chip from the intended signal. The correlation properties of the pseudo-random codes are such that this slight delay causes the multipath to appear uncorrelated with the intended signal, and it is thus ignored.
Some CDMA devices use a rake receiver, which exploits multipath delay components to improve the performance of the system. A rake receiver combines the information from several correlators, each one tuned to a different path delay, producing a stronger version of the signal than a simple receiver with a signal correlation tuned to the path delay of the strongest signal.
Frequency reuse is the ability to reuse the same radio channel frequency at other cell sites within a cellular system. In the FDMA and TDMA systems frequency planning is and important consideration. The frequencies used in different cells must be planned carefully to ensure signals from different cells do not interfere with each other. In a CDMA system, the same frequency can be used in every cell, because channelization is done using the pseudo-random codes. Reusing the same frequency in every cell eliminates the need for frequency planning in a CDMA system; however, planning of the different pseudo-random sequences must be done to ensure that the received signal from one cell does not correlate with the signal from a nearby cell.
Since adjacent cell use the same frequencies, CDMA systems have the ability to perform soft handoffs. Soft handoffs allow the mobile telephone to communication simultaneously with two or more cells. The best signal quality in selected until the handoff is complete. This is different from hard handoffs utilized in other cellular systems. In a hard handoff situation, as the mobile telephone approaches a handoff, signal strength may vary abruptly. In contrast, CDMA systems use the soft handoff, which is undetectable and provides a more reliable and higher quality signal.
Concluding remarks
spread-spectrum technology in the initial stages of development, it has become a theory and a major technological breakthrough. Later in the development process is the improvement and hardware performance improved. Development to the
present,spread-spectrum technology and the theory has been almost perfect,mainly from the point of view of overall performance, and the other new technology applications. Therefore, the application has been driven by the development of spread-spectrum technology is a power driving force, the future wireless communication systems, such as mobile communication. Wireless LAN, global personal communications, spread-spectrum technology will certainly play an important role.
译文正文
1.扩频通信系统概述
扩频通信，即扩展频谱通信（Spread spectrum communication)，它与光纤通信、卫星通信，一同誉为进入信息时代的三大高技术通信传输方式，扩频通信是将待传送的信息数据被伪随机码调制，实现频谱扩展后再传输；接收端则采用相同的编码进行解调及相关处理，恢复原始信息数据。

扩频通信系统具有3个主要特征：
（1）载波是不可预测的，或者称为伪随机的宽带信号
（2）载波的带宽比调制数据的带宽要宽得多
（3）接收过程是通过本地将本地产生的宽带载波信号的复制信号与接收到的宽带信号相关来实现的。

扩展频谱的方式主要有一下几种：直接序列扩频（DSSS）使用高速伪随机码对要传输的低速率数据进行扩频调制；跳频系统则采用伪随机码控制载波频率在一个更宽的频带内变化；跳时则是数据传输时隙是伪随机的；线性调频系统中的频率扩展则是一个线性变化的过程。

几种方式组合的混合系统也经常得到应用。

衡量扩频系统最重要的一个指标是扩频增益，又称为处理增益。

正是因为扩频系统本身具有的特征使其性能具有一系列的优势。

1.2扩频通信技术的基本概念
扩频通信的基本特点：其传输信息所用信号的带宽（W）远大于信息本身实际所需的最小（有效）带宽（F
∆），其比值称为处理增益
=/
G P∆
W
F
G值一般都在众所周知，在普通的调幅、调频或者脉冲编码调制通信中，P
十多倍范围内，称为“窄带通信”，而扩频通信的P
G值高达几百甚至几千倍，可以称为“宽带通信”。

由于扩频后的信号，它的发射功率很低，传输时大多数淹没在空间的噪声中，因此很难被其他接收机截获，只有具有相同的扩频码（或称PN码）的接收机，
才能获得处理增益，解扩并恢复出原发的信号。

1.3扩频技术的优越性
抗干扰能力强，误码率低，如上所述，扩频通信系统由于在发送端扩展信号频谱，在接收端解扩还原信号，产生扩频增益，从而大大提高了抗干扰容限。

根据扩频增益的不同，甚至在负的信噪比条件下，也可以将信号从噪声的淹没中提取出来，在目前商用的通信系统中，扩频通信是唯一能够工作于负信噪比条件下的通信方式。

抗多径干扰能力强，增加了系统的可靠性。

由于扩频系统中采用的PN码具有很好的自相关性，互相关性很弱，不同路径传来的信号很容易被分开，并可在时间相位上重新对齐，形成几路信号功率的叠加，从而改善了接收系统的性能增加了系统的可靠性，易于同频使用，提高了无线频谱利用率。

无线频谱十分宝贵，虽然从长波、微波都已得到开发利用，仍然满足不了社会的需求。

为此，世界各地都设立了频谱管理机构，用户只能使用申请获得的频率，依靠频率划分来防止信道之间的干扰。

由于扩频通信又采取了相关接收这一技术，信号发送功率极低（＜1W，一般为1~100mW），且可工作在信道噪声和热噪声背景中，易于在同一地区重复使用同一频率，也可以在现今各种窄带通信共享同一频率资源。

扩频通信是数字通信，特别适合数字语音和数据同时传输，扩频通信自身具有加密功能，只能在PN码相同的用户之间进行通信，隐蔽性好，保密性强，便于开展各种通信业务。

扩频通信容易采用码分多址、语音压缩等多项新技术，更加适用于计算机网络以及数字化的语音、图像信息传输。

扩频通信绝大多数是数字电路，设备高度集成，安装便捷，易于维护，也十分小巧可靠，平均无故障时间也很长。

1.4码分多址
码分多址（CDMA）是一种用于各种无线通信技术的信道接入方法。

它不应该与被用在移动电话标准的要求的cdmaOne，CDMA2000（cdmaOne的的3G演进）和WCDMA相混淆（用于GSM的载波3G标准），它往往被简称为CDMA，并且作为一个潜在的信道接入方法。

在数据通信中有一种想法是允许多个发射机同时发送的信息通过信号通信信道。

这允许多个用户共享一个频带（见带宽）。

这个概念被称为多址接入。

CDMA 采用扩频技术和特殊编码方案（其中每个发射机分配一个码），以允许多个用户被复用在同一物理信道。

相比之下，频分多址（FDMA）由频率划分。

CDMA 是扩展频谱信号的一种形式，由于调制的编码信号具有比被传送的数据更高的数据带宽。

1.5扩频CDMA的特点
大多数调制方案都尽量减少信号的带宽，因为带宽是有限的资源。

然而，扩频技术用比所需的最小信号带宽大几个数量级的传输带宽。

这样做的最初原因之一是军事上的应用，包括指导和通信系统。

这些系统的设计使用，因为其安全性和抗干扰扩频。

异步CDMA有一些内置的隐私级别，因为信号传播使用伪随机码，这个代码使扩频信号出现随机或有噪声性能。

不能接收解调不用于编码数据的伪随机序列的信号传输。

CDMA还耐干扰。

一个干扰信号只有有限的能量去干扰信号。

干扰信号可以传播其能量在信号的整个带宽或者信号的整个带宽的一部分。

CDMA还可以有效抑制窄带干扰。

由于窄带干扰只影响扩频信号的一小部分，它可以很容易地通过陷波滤波，没有多少信息损失就可以将它除去。

卷积编码和交织可以用来协助回收这丢失的数据。

CDMA信号也耐多径衰落。

由于扩展频谱信号占有相当大的带宽，只一小部分信号将经受衰落由于多路径。

像窄带干扰，这将导致只有一小数据丢失，并且可以被克服。

CDMA抗多径干扰的另一个原因是由于所发射的伪随机码的延迟，并且将显示为另一个用户，这样在接收机就会被忽略。

换句话说，只要在多径信道产生的至少一个芯片的延迟，多径信号将到达接收机。

换句话说，只要在多径信道引起至少一个芯片的延迟，多径信号将到达接收机，这样，它们比预定信号中在时间上移位一个芯片延迟。

伪随机码的相关特性是这样的，这稍微的延迟会导致多路径出现与预期不相关的信号，因此，它会被忽略。

一些CDMA设备使用RAKE接收器，它利用多径时延组件来提高系统的性能。

RAKE接收器整合多个相关器的信息，每一个调谐到不同的路径延迟，产生
一个比相关的简单调整，以最强的信号路径延迟接收的信号更强的信号版本。

频率复用是在蜂窝系统内的其他小区站点重复使用相同的无线电频率的能力。

在FDMA和TDMA系统中频率规划是重要的考虑因素。

在不同小区中使用的频率必须仔细规划，以确保来自不同小区的信号不会相互干扰。

在CDMA系统中，在相同的频率可以在每个小区中使用的，因为所使用的伪随机码会进行信道化。

在每一个小区内可以重复使用相同的频率，所以在CDMA系统中不需要频率规划;然而，必须得对不同的伪随机序列进行规划，以确保从一个小区接收到的信号不与来自相邻小区的信号相关。

由于相邻小区使用相同的频率，CDMA系统必须有进行软切换的能力。

软切换允许移动电话同时与两个或更多个用户通信。

选择最佳的信号品质，直到软切换完成。

这与其他蜂窝系统的硬切换不同。

在硬切换的情况下，当移动电话进行切换，信号强度可能会突然发生变化。

相比之下，CDMA系统中使用的软切换，这是不可检测的，并提供了更可靠和更高质量的信号。

结束语
扩频通信在发展的初始阶段，就已经实现了理论与技术上的重大突破，在此之后的发展过程中主要是硬件的改善和性能的提高。

发展到现在，扩频技术和理论已经趋于完善，主要应从系统的角度考虑总体性能，且与其他新技术结合使用。

因此，应用的驱动一直都是扩频技术发展的强大动力，未来的无线通信系统，如移动通信、无线局域网、全球个人通信等，扩频技术将发挥重要作用。