【正文】 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 more chips per bit, but physical devices used to generate the PN sequence impose practical limits on attainable processing gain.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 spectrumFrequencyhopping 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: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 can share a frequency band with many types of conventional transmissions with minimal interference. The spreadspectrum signals add minimal noise to the narrowfrequency munications, and vice versa. As a result, bandwidth can be utilized more efficiently.Basic algorithmTypically, the initiation of an FHSS munication is as follows1. The initiating party sends a request via a predefined frequency or control channel. 2. The receiving party sends a number, known as a seed. 3. The initiating party uses the number as a variable in a predefined algorithm, which calculates the sequence of frequencies that must be used. Most often the period of the frequency change is predefined, as to allow a single base station to serve multiple connections.4. The initiating party sends a synchronization signal via the first frequency in the calculated sequence, thus acknowledging to the receiving party it has correctly calculated the sequence.5. The munication begins, and both the receiving and the sending party change their frequencies along the calculated order, starting at the same point in time.Military useSpreadspectrum signals are highly resistant to deliberate jamming, unless the adversary has knowledge of the spreading characteristics. Military radios use cryptographic techniques to generate the channel sequence under the control of a secret Transmission Security Key (TRANSEC) that the sender and receiver share.By itself, frequency hopping provides only limited protection against eavesdropping and jamming. To get around this weakness most modern military frequency hopping radios often employ separate encryption devices such as the KY57. . military radios that use frequency hopping include HAVE QUICK and SINCGARS.Technical considerationsThe overall bandwidth required for frequency hopping is much wider than that required to transmit the same information using only one carrier frequency. However, because transmission occurs only on a small portion of this bandwidth at any given time, the effective interference bandwidth is really the same. Whilst providin