【正文】 s position if the transmitters39。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 constitutes a 16 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。spread spectrum39。s electromagic emissions to ply with the electromagic 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 magic 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 electromagic 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。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。s OmniTRACS system for munications to trucks. In the Qualm and Equatorial systems, spread spectrum enabled small antennas that viewed more than one satellite to be used since the processing gain of spread spectrum eliminated interference. The Del Norte system used the high bandwidth of spread spectrum to improve location accuracy. In 1981, the Federal Communications Commission started exploring ways to permit more general civil uses of spread spectrum in a Notice of Inquiry docket. This docket was proposed to FCC and then directed by Michael Marcus of the FCC staff. The proposals in the docket were generally opposed by spectrum users and radio equipment manufacturers, although they were supported by the then HewlettPackard Corp. The laboratory group supporting the proposal would later bee part of Agilent. The May 1985 decision in this docket permitted unlicensed use of spread spectrum in 3 bands at powers up to 1 Watt. FCC said at the time that it would wele additional requests for spread spectrum in other resulting rules, now codified as 47 CFR 12 permitted WiFi, Bluetooth, and many other products including cordless telephones. These rules were then copied in many other countries. Qualm was incorporated within 2 months after the decision to mercialize CDMA. Spreadspectrum telemunications This is a technique in which a (telemunication) signal is transmitted on a bandwidth considerably larger than the frequency content of the original information. Spreadspectrum telemunications is a signal structuring technique that employs direct sequence, frequency hopping, or a hybrid of these, which can be used for multiple access and/or multiple functions. This technique decreases the potential interference 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