Direct-sequence spread spectrum (DSSS)

Presented by:Chinmay S. Kotwaliwale (47)Chetan Kautakar (46)Ashutosh Sharma (44)Bharat Jadhav (45)

GUIDED BY:-Ms. Seema Chandak (Asst. Prof.)

In telecommunications, direct-sequence spread spectrum (DSSS) is a modulation technique.

As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that modulates the carrier or broadcast frequency. The name 'spread spectrum' comes from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency.

Note:- (Certain IEEE 802.11 standards use DSSS signaling.)

Introduction

Features:-

The DSSS is having several features which seperetes this from the other modulation techniques.

1) DSSS phase-modulates a sine wave pseudorandomly with a continuous string of pseudonoise (PN) code symbols called "chips", each of which has a much shorter duration than an information bit. That is, each informationinformation bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate.

2) DSSS uses a signal structure in which the sequence of chips produced by the transmitter is already known by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.

Transmission Method

Multiplies the data being transmitted by a "noise" signal.

The noise signal is a pseudorandom sequence of 1 and −1 values, at a frequency much higher than that of the original signal.

Despreading:-

Process of extract the original signal at reciever by using some basic logic operations on the recieved noise singnal. This is done using following operation on recieved pattern.

Transmitter and reciever >> same chip signal .

The resulting effect of enhancing signal to noise ratio on the channel is called process gain.

Can be made larger by employing a longer PN sequence and more chips per bit, but physical devices used to generate the PN sequence impose practical limits on attainable processing gain.

If an undesired transmitter transmits on the same channel but with a different PN sequence (or no sequence at all), the de-spreading process results in no processing gain for that signal.

This effect is the basis for the code division multiple access

(CDMA) property of DSSS, which allows multiple transmitters to share the same channel within the limits of the cross-correlation properties of their PN sequences.

Code Division Multiple Access (CDMA)

Multiplexing Technique used with spread spectrumStart with data signal rate DCalled bit data rate

Break each bit into k chips according to fixed pattern specific to each userUser’s code

New channel has chip data rate kD chips per secondE.g. k=6, three users (A,B,C) communicating with base receiver RCode for A = <1,-1,-1,1,-1,1>Code for B = <1,1,-1,-1,1,1>Code for C = <1,1,-1,1,1,-1>

CDMA Example

Benefits

Resistance to intended or unintended jamming Sharing of a single channel among multiple users Reduced signal/background-noise level hampers

interception Determination of relative timing between

transmitter and receiver

Uses

The United States GPS, European Galileo and Russian GLONASS satellite navigation systems

DS-CDMA (Direct-Sequence Code Division Multiple Access) is a multiple access scheme based on DSSS, by spreading the signals from/to different users with different codes. It is the most widely used type of CDMA.

Cordless phones operating in the 900 MHz, 2.4 GHz and 5.8 GHz bands

IEEE 802.11b 2.4 GHz Wi-Fi, and its predecessor 802.11-1999. (Their successor 802.11g uses OFDM instead)

Automatic meter reading

IEEE 802.15.4 (used, e.g., as PHY and MAC layer for ZigBee, or, as the physical layer for WirelessHART)

Radio-controlled model vehicles