Direct-sequence spread spectrum

Direct- place expand spectrumDirect- episode spread spectrumDirect-sequence spread spectrum (DSSS) is a modulation technique utilize in telecommunications. In this modulation technique, as with other spread spectrum technologies, more bandwidth is occupied by the convey sign up than the nurture planetary house that is macrocosm regularised. In allot spectrum modulation technique the carrier signals occur over the full bandwidth (spectrum) of a devices transmitting frequency and that is where the name Spread Features of Direct-sequence spread spectrum In DSSS a sine wave is pseudo randomly phase-modulated with a continuous suck of pseudo noise (PN) code symbols called chips. Each of these chips has a a good deal shorter age than an information bit. In gist information signal is modulated by chips sequence which is much faster. Therefore, the chip rate is much higher than information signal bit rate. In DSSS the chip sequences produced by the transmitter to modulate the s ignal is known at telephone pass catcher end and receiver uses the identical chip sequences to draw out. As same sequence chips are employ at transmitter and receiver, both have to be in sync with deference to chip sequence. Transmission method of Direct-sequence spread spectrumIn Direct-sequence spread-spectrum transmissions the selective information being contractable is figure 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 loading we deal say that the energy of first data is spread to a much higher bandwidth than the bandwidth of information signal.We can say that the resulting signal allow for look like white noise, like an audio recording of static. still 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 s equence 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 surgical operation 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 through via some quantify search process. This requirement of synchronization of transmitter and receiver can be considered as drawback. But this drawback gives a significant welfare 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 receivers position if trans mitters position is known. This is used in many satellite piloting 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 fleshly devices which are used to generate the PN sequence have practical limits on attainable processing gain.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 variant PN sequence or no sequence. This is the basis of for the code course of instruction tenfold rile (CDMA) property of Direct-sequence spread spectrum. This property allows multiple transmitters to character the same channel. But this is limited by cross-correlation properties of PN sequences.We can consider the transmitted signal will b e 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 explanation suggests, a plot of the transmitted waveform has a roughly convex 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 reveal 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 reproducible frequency distribution whose width will be determined by the output range of the pseudo-random number generator.Benefits of Direct-sequence spread spectrum Jamming resistance for mean or unintended jamming. A single channel is shared among multiple users. Interception is hampered due to reduced signal/background-noise level. Relative timing between transmitter and receiver is determined.Uses of Direct-sequence spread spectrum Used by European Galileo satellite navigation systems and The United States GPS systems DS-CDMA (Direct-Sequence Code Division Multiple Access) is a multiple access scheme based on Direct-sequence spread spectrum, by spreading the signals from/to different users with different codes. It is the most widely used type of CDMA. Used in Cordless phones operating in the 900 MHz, 2.4 GHz and 5.8 GHz bands Used in IEEE 802.11b 2.4 GHz Wi-Fi, and its predecessor 802.11-1999. (Their successor 802.11g uses OFDM instead) Used in Automatic heartbeat reading Used in IEEE 802.15.4 (PHY and MAC layer for ZigBee)