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Spread Spectrum ppt
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Spread Spectrum


.ppt   Spread Spectrum[.ppt (Size: 223.5 KB / Downloads: 10)

Introduction to Spread Spectrum

Problems such as capacity limits, propagation effects, synchronization occur with wireless systems
Spread spectrum modulation spreads out the modulated signal bandwidth so it is much greater than the message bandwidth
Independent code(PN code) spreads signal at transmitter and dispreads signal at receiver

Frequency multiplex

Separation of spectrum into smaller frequency bands
Channel gets band of the spectrum for the whole time
Advantages:
no dynamic coordination needed
works also for analog signals
Disadvantages:
waste of bandwidth
if traffic distributed unevenly
inflexible
guard spaces

Spread Spectrum Technology

Side effects:
coexistence of several signals without dynamic coordination
tap-proof
Alternatives: Direct Sequence (DS/SS), Frequency Hopping (FH/SS)
Spread spectrum increases BW of message signal by a factor N, Processing Gain

FH/SS (Frequency Hopping Spread Spectrum) I

Discrete changes of carrier frequency
sequence of frequency changes determined via PN sequence
Two versions
Fast Hopping: several frequencies per user bit (FFH)
Slow Hopping: several user bits per frequency (SFH)
Advantages
frequency selective fading and interference limited to short period
uses only small portion of spectrum at any time
Disadvantages
not as robust as DS/SS
simpler to detect

Performance of DS/SS Systems

Pseudonoise (PN) codes
Spread signal at the transmitter
Despread signal at the receiver
Ideal PN sequences should be
Orthogonal (no interference)
Random (security)
Autocorrelation similar to white noise (high at =0 and low for  not equal 0)
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#2
In telecommunications and radio communications, spread spectrum techniques are methods by which a signal (eg, an electrical, electromagnetic or acoustic signal) generated with a particular bandwidth is deliberately propagated in the frequency domain, giving result a signal with a wider bandwidth. These techniques are used for a variety of reasons, including establishing secure communications, increasing resistance to natural interference, noise and jamming, to prevent detection, and to limit the power flux-density (e.g. by satellite).

A conventional wireless signal has a frequency, usually specified in megahertz (MHz) or gigahertz (gigahertz), which does not change over time (except for small and rapid fluctuations that occur as a result of modulation). When you hear a signal at 103.1 MHz on an FM stereo receiver, for example, the signal remains at 103.1 MHz. It does not go up to 105.1 MHz or up to 99.1 MHz. The radio frequency dial digits remain the same at all times. The frequency of a conventional wireless signal is kept as constant as the state of the art allows, so the bandwidth can be kept within certain limits, and therefore 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 may occur under certain circumstances. First, a signal whose frequency is constant is subject to catastrophic interference. This occurs when another signal is transmitted at, or very close to, the desired signal frequency. Catastrophic interference may be accidental (as in amateur radio communications) or may be deliberate (as in times of war). Second, a constant frequency signal is easy to intercept and therefore not suitable for applications where information must be kept confidential between the source (transmitting party) and the destination (receiving party).
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