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1、毕业（设计）论文外文翻译外文题目 AN INTRODUCTION TO SPEREAD-SPECTRUM COMMUNICATION 译文题目 扩频通信系统的介绍 系 别 机电工程系 专 业 测控技术与仪器 班 级 162902 学生姓名 王 延 学 号 092151 指导教师 吴鹏飞 报告日期 2012.3.18 AN INTRODUCTION TO SPEREAD-SPECTRUM COMMUNICATION Introduction As spread-spectrum techniques become increasingly popular, electrical engineer
2、s outside the field are eager for understandable explanations of the technology. There are books and websites on the subject, but many are hard to understand or describe some aspects while ignoring others(e.g., the DSSS technique with extensive focus on PRN-code generation).The following discussion
3、covers the full spectrum(pun intended).A Short HistorySpread-spectrum communications technology was first described on paper by an actress and a musician! In 1941 Hollywood actress Hedy Lamarr and pianist George Antheil described a secure radio link to control torpedoes. They received U.S. Patent #2
4、.292.387.The technology was not taken seriously at that time by the U.S. Army and was forgotten until the 1980s, when it became active. Since then the technology has become increasingly popular for application that involve radio links in hostile environments.Typical applications for the resulting sh
5、ort-range data transceivers include satellite-positioning systems GPS，3G mobile telecommunications, W-LAN(IEEE802.11a,IEEE 802.11b,IEEE 802.11g),and Bluetooth. Spread-spectrum techniques also aid in the endless race between communication needs and radio-frequency availability-situations where the ra
6、dio spectrum is limited and is, therefore, an expensive resource.Theoretical Justification for Spread Spectrum Spread-spectrum is apparent in the Shannon and Hartley channel-capacity theorem: C=Blog2(1+S/N) (Eq.1)In this equation, C is the channel capacity in bits per second (bps), which is the maxi
7、mum data rate for a theoretical bit-error rate (BER). B is the required channel bandwidth in Hz, and S/N is the signal-to-noise power ratio. To be more explicit, one assumes that C, which represents the amount of information allowed by the communication channel, also represents the desired performan
8、ce. Bandwidth (B) is the price to be paid, because frequency is a limited resource. The S/N ratio expresses the environmental conditions or the physical characteristics (i.e., obstacles ,presence of jammers ,interferences, etc.).There is an elegant interpretation of this equation, applicable for dif
9、ficult environments, for example, when a low S/N ratio is caused by noise and interference. This approach says that one can maintain or even increase communication performance (high C) by allowing or injecting more bandwidth (high B), even when signal power is below the noise floor. (The equation do
10、es not forbid that condition!)Modify Equation 1 by changing the log base from 2 to e (the Napier Ian number) and by noting that in=loge.Therefore:C/B= (1/ln2) ln(1+S/N)=1.443ln(1+S/N) (Eq.2)Applying the MacLaurin series development forln(1+x)=x-x2/2+x3/3-x4/4+(-1)k+1xk/k+:C/B=1.443(S/N-1/2(S/N)2+1/3
11、(S/N)3-) (Eq.3)S/N is usually low for spread-spectrum applications. (As just mentioned, the signal power density can even be below the noise level.) Assuming a noise level such that S/N 1, Shannons expression becomes simply:C/B1.443S/N (Eq.4)Very roughly:C/NS/N (Eq.5)Or:N/SB/C (Eq.6)To send error-fr
12、ee information for a given noise-to-signal ratio in the channel, therefore, one need only performs the fundamental spread-spectrum signal-spreading operation: increase the transmitted bandwidth. That principle seems simple and evident. Nonetheless, implementation is complex, mainly because spreading
13、 the baseband (by a factor that can be several orders of magnitude) forces the electronics to act and react accordingly, which, in turn, makes the spreading and dispreading operations necessary.DefinitionsDifferent spread-spectrum techniques are available, but all have one idea in common: the key (a
14、lso called the code or sequence) attached to the communication channel. The manner of inserting this code defines precisely the spread-spectrum technique. The term spread spectrum refers to the expansion of signal bandwidth, by several orders of magnitude in some cases, which occurs when a key is at
15、tached to the communication channel.The formal definition of spread spectrum is more precise: an RF communications system in which the baseband signal bandwidth is intentionally spread over a larger bandwidth by injecting a higher frequency signal (Figure 1).As a direct consequence, energy used in transmitting the signal is spread
