【正文】 erence to other receivers while achieving privacy. Spread spectrum generally makes use of a sequential noiselike signal structure to spread the normally narrowband information signal over a relatively wideband (radio) band of frequencies. The receiver correlates the received signals to retrieve the original information signal. Originally there were two motivations: either to resist enemy efforts to jam the munications (antijam, or AJ), or to hide the fact that munication was even taking place, sometimes called low probability of intercept (LPI). Frequencyhopping spread spectrum (FHSS), directsequence spread spectrum (DSSS), timehopping spread spectrum (THSS), chirp spread spectrum (CSS), and binations of these techniques are forms of spread spectrum. Each of these techniques employs pseudorandom number sequences — created using pseudorandom number generators — to determine and control the spreading pattern of the signal across the alloted bandwidth. Ultrawideband (UWB) is another modulation technique that acplishes the same purpose, based on transmitting short duration pulses. Wireless Ether standard IEEE uses either FHSS or DSSS in its radio interface. Notes ? Techniques known since 1940s and used in military munication system since 1950s ? Spread radio signal over a wide frequency range several magnitudes higher than 2 minimum requirement. The core principle of spread spectrum is the use of noiselike carrier waves, and, as the name implies, bandwidths much wider than that required for simple pointtopoint munication at the same data rate. ? Two main techniques: sequence (DS) hopping (FH) ? Resistance to jamming (interference). DS is better at resisting continuoustime narrowband jamming, while FH is better at resisting pulse jamming. In DS systems, narrowband jamming affects detection performance about as much as if the amount of jamming power is spread over the whole signal bandwidth, when it will often not be much stronger than background noise. By contrast, in narrowband systems where the signal bandwidth is low, the received signal quality will be severely lowered if the jamming power happens to be concentrated on the signal bandwidth. ? Resistance to eavesdropping. The spreading code (in DS systems) or the frequencyhopping pattern (in FH systems) is often unknown by anyone for whom the signal is unintended, in which case it encrypts the signal and reduces the chance of an adversary39。s more, for a given noise power spectral density (PSD), spreadspectrum systems require the same amount of energy per bit before spreading as narrowband systems and therefore the same amount of power if the bitrate before spreading is the same, but since the signal power is spread over a large bandwidth, the signal PSD is much lower, often significantly lower than the noise PSD, therefore the adversary may be unable to determine if the signal exists at all. However, for missioncritical applications, particularly those employing mercially available radios, spreadspectrum radios do not intrinsically provide adequate security。spread spectrum39。s transmitting frequency. Features phasemodulates a sine wave pseudorandomly with a continuous string of pseudonoise (PN) code symbols called chips, each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate. 2. It uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the 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 consti