Direct-sequence spread spectrum

Direct-sequence spread spectrum ( DSSS ) is a transition technique used in telecommunications. In this transition technique, as with other spread spectrum engineerings, more bandwidth is occupied by the familial signal than the information signal that is being modulated. In Spread spectrum transition technique the bearer signals occur over the full bandwidth ( spectrum ) of a device ‘s transmitting frequence and that is where the name Spread

Features of Direct-sequence spread spectrum

O In DSSS a sine moving ridge is pseudo indiscriminately phase-modulated with a uninterrupted twine of pseudo noise ( PN ) codification symbols called “ french friess ” . Each of these french friess has a much shorter continuance than an information spot. In consequence information signal is modulated by french friess sequence which is much faster. Therefore, the bit rate is much higher than information signal spot rate.

O In DSSS the bit sequences produced by the sender to modulate the signal is known at receiver terminal and receiving system uses the same bit sequences to demodulate. As same sequence french friess are used at sender and receiving system, both have to be in sync with regard to bit sequence.

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Transmission method of Direct-sequence spread spectrum

In Direct-sequence spread-spectrum transmittals the informations being transmitted is multiplied by a noise signal. The noise signal used is a pseudorandom sequence of 1 and ?1 values. Besides the frequence of noise signal is much higher than that of the information signal. In consequence we can state that the energy of original informations is dispersed to a much higher bandwidth than the bandwidth of information signal.

We can state that the ensuing signal will look like white noise, like an audio recording of “ inactive ” . But this noise signal will be used to retrace the original informations at the receiving system terminal where it will be multiplied with pseudorandom sequence of 1 and ?1 values which is precisely the same sequence which was used to modulate the informations signal. As 1 ? 1 = 1, and ?1 ? ?1 = 1 so multiplying two times the informations signal with pseudo random sequence will reconstruct the original signal. The procedure of multiplying the signal at having terminal with same bit sequence used at transmitter terminal is known as de-spreading. In De-spreading a mathematical correlativity of the familial PN sequence with the PN sequence at receiving system is constituted.

As it would hold been clear by now that to retrace informations at receiver terminal, transmit and receive sequences must be synchronized. It is done via some timing hunt procedure. This demand of synchronism of sender and receiving system can be considered as drawback. But this drawback gives a important benefit besides. If we synchronize sequences of assorted senders, the comparative synchronism which we will make for receiving system can be used to find comparative timing. This comparative timing can be used to find receiving system ‘s place if senders ‘ place is known. This is used in many satellite pilotage systems.

Procedure addition is consequence of heightening signal to resound ratio on the channel. The procedure addition can be increased by utilizing a longer PN sequence and more french friess per spot. But there is a restraint here that physical devices which are used to bring forth the PN sequence have practical bounds on come-at-able processing addition.

If a sender transmits a signal with a PN sequence the de-spreading procedure give a procedure addition if we demodulate it with same PN sequence. It does non supply any procedure addition for the signals transmitted by other senders on the same channel but with a different PN sequence or no sequence. This is the footing of for the codification division multiple entree ( CDMA ) belongings of Direct-sequence spread spectrum. This belongings allows multiple senders to portion the same channel. But this is limited by cross-correlation belongingss of PN sequences.

We can see the familial signal will be approximately a bell shaped enveloped centered on the bearer frequence ( same as in AM transmittal ) but the noise which we add causes the distribution to be wider.

As this description suggests, a secret plan of the familial wave form has a approximately bell-shaped envelope centered on the bearer frequence, merely like a normal AM transmittal, except that the added noise causes the distribution to be much wider than that of an AM transmittal.

If we compare frequency-hopping spread spectrum with Direct-sequence spread spectrum so we will happen that frequency-hopping spread spectrum pseudo-randomly re-tunes the bearer, alternatively of adding pseudo-random noise to the information. This retuning of bearer consequences in a unvarying frequence distribution whose breadth will be determined by the end product scope of the pseudo-random figure generator.

Benefits of Direct-sequence spread spectrum

  • Thronging opposition for intended or unintended jamming.
  • A individual channel is shared among multiple users.
  • Interception is hampered due to cut down signal/background-noise degree.
  • Relative timing between sender and receiving system is determined.

Uses ofDirect-sequence spread spectrum

  • Used by European Galileo orbiter pilotage systems and The United States GPS systems
  • DS-CDMA ( Direct-Sequence Code Division Multiple Access ) is a multiple entree strategy based on Direct-sequence spread spectrum, by distributing the signals from/to different users with different codifications. It is the most widely used type of CDMA.
  • Used in Cordless phones runing in the 900 MHz, 2.4 GHz and 5.8 GHz sets
  • Used in IEEE 802.11b 2.4 GHz Wi-Fi, and its predecessor 802.11-1999. ( Their replacement 802.11g utilizations OFDM alternatively )
  • Used in Automatic metre reading
  • Used in IEEE 802.15.4 ( PHY and MAC bed for ZigBee )
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