Spread Spectrum
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22-12-2009, 03:22 PM

.ppt   Spread Spectrum.ppt (Size: 200.5 KB / Downloads: 226)
Spread spectrum is a form of wireless communications in which the frequency of the transmitted signal is deliberately varied. This result in a much greater bandwidth than the signal would have if its frequency were not varied.A conventional wireless signal has a frequency, and are methods by which electromagnetic energy generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth usually specified in megahertz (MHz) or gigahertz gigahertz), that does not change with time (except for small, rapid fluctuations that occur as a result of modulation). When you listen to a signal at 103.1 MHz on an FM stereo receiver, for example, the signal stays at 103.1 MHz. It does not go up to 105.1 MHz or down to 99.1 MHz. The digits on the radio's frequency dial stay the same at all times. The frequency of a conventional wireless signal is kept as constant as the state of the art will permit, so the bandwidth can be kept within certain limits, and so the signal can be easily located by someone who wants to retrieve the information. There are at least two problems with conventional wireless communications that can occur under certain circumstances. First, a signal whose frequency is constant is subject to catastrophic interference. This occurs when another signal is transmitted on, or very near, the frequency of the desired signal. Catastrophic interference can be accidental (as in amateur-radio communications) or it can be deliberate (as in wartime). Second, a constant-frequency signal is easy to intercept, and is therefore not well suited to applications in which information must be kept confidential between the source (transmitting party) and destination (receiving party). To minimize troubles that can arise from the above mentioned vulnerabilities of conventional communications circuits, these techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference and jamming, to prevent detection, and to limit the power flux density on satellite downlinks. The frequency of the transmitted signal can be deliberately varied over a comparatively large segment of the electromagnetic radiation spectrum. This variation is done according to a specific, but complicated mathematical function. In order to intercept the signal, a receiver must be tuned to frequencies that vary precisely according to this function. The receiver must "know" the frequency-versus-time function employed by the transmitter, and must also "know" the starting-time point at which the function begins. If someone wants to jam a spread-spectrum signal, that person must have a transmitter that "knows" the function and its starting-time point. The spread-spectrum function must be kept out of the hands of unauthorized people or entities. Most spread-spectrum signals use a digital scheme called frequency hopping. The transmitter frequency changes abruptly, many times each second. Between "hops," the transmitter frequency is stable. The length of time that the transmitter remains on a given frequency between "hops" is known as the dwell time. A few spread-spectrum circuits employ continuous frequency variation, which is an analog scheme. An example of commercial spread spectrum systems are systems that are designed to be used in so-called unlicensensed bands, such as the Industry, Scientific, Medical (ISM) band around 2.4 GHz. Typical applications are here cordless telephones, wireless LANs, and cable replacement systems as Bluetooth. Since the band is unlicensed, there is no central control over the radio resources; interference is from other communication systems and other electrical and electronic equipment (e.g., microwave ovens, radars, etc.). Here the jamming is not intentional, but the interference
Applications Code-division multiple access systems
(CDMA systems) use spread spectrum techniques to provide communication to several concurrent users. CDMA is used in one second generation (IS-95) and several third generation wireless cellular systems (e.g., cdma2000 and WCDMA). One advantage of using jamming-resistant signals in these applications is that the radio resource management is significantly reduced.
This spread spectrum multiple access technique allow multiple signals (user) occupying the same RF bandwidth to be transmitted simultaneously without
Interfering with one another.

Each user is assigned a code of its own, which performs either direct sequence or Frequency hop spread spectrum modulation.

Each code is orthogonal to all others.
Operation is asynchronous. “all users need not transmit simultaneously.
Advantage over TDMA
CDMA does not require external synchronization network
Performance degradation is gradual in case of increase in number of user that add to system.
Jamming Resistance
CDMA offers an external interference rejection capability like multipath rejection or resistance to jamming.
All users can share full spectrum simultaneously because each user has different code .
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seminar flower
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30-05-2012, 02:44 PM

Spread Spectrum

.ppt   Spread Spectrum.ppt (Size: 470 KB / Downloads: 14)

Spread Spectrum

Analog or digital data
Analog signal
Spread data over wide bandwidth
Makes jamming and interception harder
Frequency hoping
Signal broadcast over seemingly random series of frequencies
Direct Sequence
Each bit is represented by multiple bits in transmitted signal
Chipping code.

Spread Spectrum Concept

Input fed into channel encoder
Produces narrow bandwidth analog signal around central frequency
Signal modulated using sequence of digits
Spreading code/sequence
Typically generated by pseudonoise/pseudorandom number generator
Increases bandwidth significantly
Spreads spectrum
Receiver uses same sequence to demodulate signal
Demodulated signal fed into channel decoder.

Direct Sequence Spread Spectrum

Each bit represented by multiple bits using spreading code
Spreading code spreads signal across wider frequency band
In proportion to number of bits used
10 bit spreading code spreads signal across 10 times bandwidth of 1 bit code
One method:
Combine input with spreading code using XOR
Input bit 1 inverts spreading code bit
Input zero bit doesn’t alter spreading code bit
Data rate equal to original spreading code
Performance similar to FHSS.

Code Division Multiple Access (CDMA)

Multiplexing Technique used with spread spectrum
Start with data signal rate D
Called bit data rate
Break each bit into k chips according to fixed pattern specific to each user
User’s code
New channel has chip data rate kD chips per second
E.g. k=6, three users (A,B,C) communicating with base receiver R.

seminar flower
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Posts: 10,120
Joined: Apr 2012
08-08-2012, 01:50 PM

Spread Spectrum

.ppt   SpreadSpectrum.ppt (Size: 389 KB / Downloads: 112)

Input is fed into a channel encoder
Produces analog signal with narrow bandwidth
Signal is further modulated using sequence of digits
Spreading code or spreading sequence
Generated by pseudonoise, or pseudo-random number generator
Effect of modulation is to increase bandwidth of signal to be transmitted

Frequency Hoping Spread Spectrum (FHSS)

Signal is broadcast over seemingly random series of radio frequencies
A number of channels allocated for the FH signal
Width of each channel corresponds to bandwidth of input signal
Signal hops from frequency to frequency at fixed intervals
Transmitter operates in one channel at a time
Bits are transmitted using some encoding scheme
At each successive interval, a new carrier frequency is selected

Channel sequence dictated by spreading code
Receiver, hopping between frequencies in synchronization with transmitter, picks up message
Eavesdroppers hear only unintelligible blips
Attempts to jam signal on one frequency succeed only at knocking out a few bits

FHSS Performance Considerations

Large number of frequencies used
Results in a system that is quite resistant to jamming
Jammer must jam all frequencies
With fixed power, this reduces the jamming power in any one frequency band

Direct Sequence Spread Spectrum (DSSS)

Each bit in original signal is represented by multiple bits in the transmitted signal
Spreading code spreads signal across a wider frequency band
Spread is in direct proportion to number of bits used
One technique combines digital information stream with the spreading code bit stream using exclusive-OR

PN Sequences

PN generator produces periodic sequence that appears to be random
PN Sequences
Generated by an algorithm using initial seed
Sequence isn’t statistically random but will pass many test of randomness
Sequences referred to as pseudorandom numbers or pseudonoise sequences
Unless algorithm and seed are known, the sequence is impractical to predict


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