通信工程—外文翻译4300字

通信工程—外文翻译4300字内容摘要：

information signal. Transmission method Directsequence spreadspectrum transmissions multiply the data being transmitted by a noise signal. This noise signal is a pseudorandom sequence of 1 and −1 values, at a frequency much higher than that of the original signal, thereby spreading the energy of the original signal into a much wider band. The resulting signal resembles white noise, like an audio recording of static. However, this noiselike signal can be used to exactly reconstruct the original data at the receiving end, by multiplying it by the same pseudorandom sequence (because 1 1 = 1, and −1 −1 = 1). This process, known as despreading, mathematically constitutes a correlation of the transmitted PN sequence with the PN sequence that the receiver believes the transmitter is using. For despreading to work correctly, the transmit and receive sequences must be synchronized. This requires the receiver to synchronize its sequence with the transmitter39。 s sequence via some sort of timing search process. However, this apparent drawback can be a significant benefit: if the sequences of multiple transmitters are synchronized with each other, the relative synchronizations the receiver must make between them can be used to determine relative timing, which, in turn, can be used to calculate the receiver39。 s position if the transmitters39。 positions are known. This is the basis for many satellite navigation systems. The resulting effect of enhancing signal to noise ratio on the channel is called process gain. This effect can be made larger by employing a longer PN sequence and mo re chips per bit, but physical devices used to generate the PN sequence impose practical limits on attainable processing gain. 4 If an undesired transmitter transmits on the same channel but with a different PN sequence (or no sequence at all), the despreading process results in no processing gain for that signal. This effect is the basis for the code division multiple access (CDMA) property of DSSS, which allows multiple transmitters to share the same channel within the limits of the crosscorrelation properties of their PN sequences. As this description suggests, a plot of the transmitted waveform has a roughly bellshaped envelope centered on the carrier frequency, just like a normal AM transmission, except that the added noise causes the distribution to be much wider than that of an AM transmission. In contrast, frequencyhopping spread spectrum pseudorandomly retunes the carrier, instead of adding pseudorandom noise to the data, which results in a uniform frequency distribution whose width is determined by the output range of the pseudorandom number generator. Benefits  Resistance to intended or unintended jamming  Sharing of a single channel among multiple users  Reduced signal/backgroundnoise level hampers interception (stealth)  Determination of relative timing between transmitter and receiver Uses  The United States GPS and European Galileo satellite navigation systems  DSCDMA (DirectSequence Code Division Multiple Access) is a multiple access scheme based on DSSS, by spreading the signals from/to different users with different codes. It is the most widely used type of CDMA.  Cordless phones operating in the 900 MHz, GHz and GHz bands  IEEE GHz WiFi, and its predecessor . (Their successor uses OFDM instead)  Automatic meter reading  IEEE (used . as PHY and MAC layer for ZigBee) Frequencyhopping spread spectrum Frequencyhopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. It is utilized as a multiple access method in the frequencyhopping code division multiple access (FHCDMA) scheme. A spreadspectrum transmission offers three main advantages over a fixedfrequency transmission: 5 1. Spreadspectrum signals are highly resistant to narrowband interference. The process of recollecting a spread signal spreads out the interfering signal, causing it to recede into the background. 2. Spreadspectrum signals are difficult to intercept. An FHSS signal simply appears as an increase in the background noise to a narrowband receiver. An eavesdropper would only be able to intercept the transmission if they knew the pseudorandom sequence. 3. Spreadspectrum transmissions。