【正文】 cturer. The AntheilLamarr version of frequency hopping used a pianoroll to change among 88 frequencies, and was intended to make radioguided torpedoes harder for enemies to detect or to jam. The patent came to light during patent searches in the 1950s when ITT Corporation and other private firms began to develop Code Division Multiple Access (CDMA), a civilian form of spread spectrum, though the Lamarr patent had no direct impact on subsequent technology. It was in fact ongoing military research at MIT Lincoln Laboratory, Magnavox Government amp。s book was the first prehensive unclassified review of the technology and set the stage for increasing research into mercial applications.Initial mercial use of spread spectrum began in the 1980s in the US with three systems: Equatorial Communications System39。s radio navigation system for navigation of aircraft for crop dusting and similar applications, and Qualm39。 Techniques known since 1940s and used in military munication system since 1950s Two main techniques: sequence (DS) hopping (FH) 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。 Resistance to fading. The high bandwidth occupied by spreadspectrum signals offer some frequency diversity, . it is unlikely that the signal would encounter severe multipath fading over its whole bandwidth, and in other cases the signal can be detected using . a Rake receiver.s electromagnetic emissions to ply with the electromagnetic patibility (EMC) regulations. It is a popular technique because it can be used to gain regulatory approval with only a simple modification to the equipment.Spreadspectrum clocking has bee more popular in portable electronics devices because of faster clock speeds and the increasing integration of highresolution LCD displays in smaller and smaller devices. Because these devices are designed to be lightweight and inexpensive, passive EMI reduction measures such as capacitors or metal shielding are not a viable option. Active EMI reduction techniques such as spreadspectrum clocking are necessary in these cases, but can also create challenges for designers. Principal among these is the risk that modifying the system clock runs the risk of the clock/data misalignment.It is important to note that this method does not reduce the total energy radiated by the system, and therefore does not necessarily make the system less likely to cause interference. Spreading the energy over a large frequency band effectively reduces the electrical and magnetic field strengths that are measured within a narrow window of frequencies. Spreadspectrum clocking works because the measuring receivers used by EMC testing laboratories divide the electromagnetic spectrum into frequency bands approximately 120 kHz the system under test were to radiate all of its energy at one frequency, it would register a large peak at the monitored frequency band. Spreadspectrum clocking distributes the energy so that it falls into a large number of the receiver39。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 methodDirectsequence 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 position if the transmitters39。 Resistance to intended or unintended jamming Reduced signal/backgroundnoise level hampers interception (stealth) The United States GPS and European Galileo satellite navigation systems Cordless phones operating in the 900 MHz, GHz and GHz bandsGHz WiFi, and its predecessor . (Their successor uses OFDM instead) IEEE (used . as PHY and MAC layer for ZigBee)Frequencyhopping spread spectrumFrequencyhopping spread spectrum (FHSS) is a method of tra