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Direct-sequence spread spectrum
Direct-sequence spread spectrum (DSSS) is a modulation technique used in telecommunications. In this modulation technique, as with other spread spectrum technologies, more bandwidth is occupied by the transmitted signal than the information signal that is being modulated. In Spread spectrum modulation technique the carrier signals occur over the full bandwidth (spectrum) of a device’s transmitting frequency and that is where the name Spread
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Features of Direct-sequence spread spectrum
Ø In DSSS a sine wave is pseudo randomly phase-modulated with a continuous string of pseudo noise (PN) code symbols called “chips”. Each of these chips has a much shorter duration than an information bit. In effect information signal is modulated by chips sequence which is much faster. Therefore, the chip rate is much higher than information signal bit rate.
Ø In DSSS the chip sequences produced by the transmitter to modulate the signal is known at receiver end and receiver uses the same chip sequences to demodulate. As same sequence chips are used at transmitter and receiver, both have to be in sync with respect to chip sequence.
Transmission method of Direct-sequence spread spectrum
In Direct-sequence spread-spectrum transmissions the data being transmitted is multiplied by a noise signal. The noise signal used is a pseudorandom sequence of 1 and −1 values. Also the frequency of noise signal is much higher than that of the information signal. In effect we can say that the energy of original data is spread to a much higher bandwidth than the bandwidth of information signal.
We can say that the resulting signal will look like white noise, like an audio recording of “static”. But this noise signal will be used to reconstruct the original data at the receiver end where it will be multiplied with pseudorandom sequence of 1 and −1 values which is exactly the same sequence which was used to modulate the data signal. As 1 × 1 = 1, and −1 × −1 = 1 so multiplying two times the data signal with pseudo random sequence will restore the original signal. The process of multiplying the signal at receiving end with same chip sequence used at transmitter end is known as de-spreading. In De-spreading a mathematical correlation of the transmitted PN sequence with the PN sequence at receiver is constituted.
As it would have been clear by now that to reconstruct data at receiver end, transmit and receive sequences must be synchronized. It is done via some timing search process. This requirement of synchronization of transmitter and receiver can be considered as drawback. But this drawback gives a significant benefit also. If we synchronize sequences of various transmitters, the relative synchronization which we will do for receiver can be used to determine relative timing. This relative timing can be used to determine receiver’s position if transmitters’ position is known. This is used in many satellite navigation systems.
Process gain is effect of enhancing signal to noise ratio on the channel. The process gain can be increased by using a longer PN sequence and more chips per bit. But there is a constraint here that physical devices which are used to generate the PN sequence have practical limits on attainable processing gain.
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If a transmitter transmits a signal with a PN sequence the de-spreading process give a process gain if we demodulate it with same PN sequence. It does not provide any process gain for the signals transmitted by other transmitters on the same channel but with a different PN sequence or no sequence. This is the basis of for the code division multiple access (CDMA) property of Direct-sequence spread spectrum. This property allows multiple transmitters to share the same channel. But this is limited by cross-correlation properties of PN sequences.
We can consider the transmitted signal will be roughly a bell shaped enveloped centered on the carrier frequency (same as in AM transmission) but the noise which we add causes the distribution to be wider.
As this description suggests, a plot of the transmitted waveform has a roughly bell-shaped 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.
If we compare frequency-hopping spread spectrum with Direct-sequence spread spectrum then we will find that frequency-hopping spread spectrum pseudo-randomly re-tunes the carrier, instead of adding pseudo-random noise to the data. This retuning of carrier results in a uniform frequency distribution whose width will be determined by the output range of the pseudo-random number generator.
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