扩频通信相关文章 [2]
论文作者:www.51lunwen.org论文属性:作业 Assignment登出时间:2016-05-16编辑:lily点击率:15472
论文字数:1017论文编号:org201605091710114850语种:英语 English地区:英国价格:免费论文
关键词:扩频通信调制宽频信号
摘要:本文对扩展频谱通信相关信息进行了介绍,说明其概念、方式和在现实生活中的具体应用,对其原理和应用方式进行了重点说明,并对其应用前景进行了展望。
nal, spread spectrum is used in satellite communication and is being considered for use in local area networks.
The object of our project is to Design & Implement a “Digital Speech Security System” (DSSS) which can provide protection against externally generated interfering (jamming) signals with finite power. The jamming signal may consist of fairly powerful broadband noise or multitude waveform that is detected at the receiver for the purpose of disrupting communications. Protection against jamming waveforms is provided by purposely making the information - bearing signal occupy a bandwidth necessary to transmit it. This has the effect of making the transmitted signal assume a noise like appearance so as to blend into the background. The transmitted signal is thus enabled to propagate through the channel undetected by any one who may be listening.
The additional security to speech can be provided by inverting two bits at transmitter as well as at receiver station. Conventionally, speech can be protected from unauthorized interception or eavesdropping by analog techniques using scramblers. There are time, frequency and bandwidth scramblers in diplomatic use.
In a Digital Speech Security System (DSSS), the analog signal is converted into Pulse Code Modulated (PCM) digital signal from using an analog-to-digital converter. This digital signal is combined directly with the output from a pseudo-random noise (PN sequence) generator to obtain an encrypted speech before transmission. This technique is known as direct sequence spread spectrum technique. Such a system has property that for the interceptor, the received message appears like noise & thus prevents him from eavesdropping. However, the desired party can decipher the message with local replica of the pseudo-random noise (PN sequence) available with him.
脉码调制——Pulse code modulation
Pulse code modulation (PCM) was developed by AT & T in 1937 at their Paris laboratories Alex H. PCM is the preferred method of communication within the public switched telephone network. Pulse code modulation is the only one of the digitally encoded modulation techniques shown in Figure 2.1 that is used for digital transmission.
With PCM, the pulses are of fixed length and fixed amplitude. PCM is a binary system where a pulse or lack of a pulse within a prescribed time slot represents either a logic 1 or logic 0 condition. PWM, PPM, and PAM are digital but seldom binary, as pulse does not represent a single binary digit (bit).
Figure 2.2 shows a simplified block diagram of a single- channel, simplex (one way only) PCM system. The bandpass filter limits the frequency of the input analog signal to the standard voice - band frequency range of 300 Hz to 3000Hz. The sample and hold circuit periodically samples the analog input signal and convert those samples to a multilevel PAM signal. The analog - to digital converter (ADC) converts the PAM samples to parallel PCM codes, which are converted to serial data in the parallel- to-serial converter then outputted onto the transmission line. The transmission line repeaters periodically regenerate the PCM codes.
In the receiver, the serial-to-parallel converter converts serial data from the transmission line to parallel PCM codes. The digital-to-analog converter (DAC) converts the parallel PCM code to multilevel PAM
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