Direct-sequence spread spectrum
In telecommunications, direct-sequence spread spectrum (DSSS) is a spread spectrum modulation technique used to reduce overall signal interference. The spreading of this signal makes the resulting wideband channel more noisy, allowing for greater resistance to unintentional and intentional interference.
A method of achieving the spreading of a given signal is provided by the modulation scheme. With DSSS, the message signal is used to modulate a bit sequence known as a Pseudo Noise (PN) code; this PN code consists of a radio pulse that is much shorter in duration (larger bandwidth) than the original message signal. This modulation of the message signal scrambles and spreads the pieces of data, and thereby resulting in a bandwidth size nearly identical to that of the PN sequence. In this context, the duration of the radio pulse for the PN code is referred to as the chip duration. The smaller this duration, the larger the bandwidth of the resulting DSSS signal; more bandwidth multiplexed to the message signal results in better resistance against interference.
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- DSSS phase-shifts 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 information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate.
- 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.
Direct-sequence spread-spectrum transmissions multiply the data being transmitted by a "noise" signal. This noise signal is a pseudorandom sequence of
−1 values; at a frequency much higher than that of the original signal.
The resulting signal resembles white noise, like an audio recording of "static". However, this noise-like signal is used to exactly reconstruct the original data at the receiving end, by multiplying it by the same pseudorandom sequence (because 1 × 1 = 1, and −1 × −1 = 1). This process, known as "de-spreading", mathematically constitutes a correlation of the transmitted PN sequence with the PN sequence that the receiver already knows the transmitter is using.
The resulting effect of enhancing signal to noise ratio on the channel is called process gain. This effect 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.
While for useful process gain the transmitted DSSS signal must occupy much wider bandwidth than simple a.m. of the original signal alone would require, its frequency spectrum can be somewhat restricted for spectrum economy by a conventional analog bandpass filter to give a roughly bell-shaped envelope centered on the carrier frequency. In contrast, frequency-hopping spread spectrum which pseudo-randomly re-tunes the carrier, instead of adding pseudo-random noise to the data, requires a uniform frequency response since any bandwidth shaping would cause amplitude modulation of the signal by the hopping code.
If an undesired transmitter transmits on the same channel but with a different PN sequence (or no sequence at all), the de-spreading process has reduced 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.
- Resistance to unintended or intended jamming
- Sharing of a single channel among multiple users
- Reduced signal/background-noise level hampers interception
- Determination of relative timing between transmitter and receiver
- The United States GPS, European Galileo and Russian GLONASS satellite navigation systems; earlier GLONASS used DSSS with a single PN code in conjunction with FDMA, while latter GLONASS used DSSS to achieve CDMA with multiple PN codes
- 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 both OFDM and DSSS)
- 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 Automotive vehicles
- Frequency-hopping spread spectrum
- Linear feedback shift register
- Barker code
- Orthogonal frequency-division multiplexing
- Complementary code keying
- Haykin, Simon (2008). Communication systems (4 ed.). John Wiley & Sons. pp. 488–99. Retrieved 11 April 2015.
- "DSSS - Direct Sequence Spread Spectrum - Telecom ABC". www.telecomabc.com. Retrieved 2016-11-11.
- Rappaport, Theodore (January 2010). Wireless Communications Principles and Practice (2 ed.). Prentice-Hall, Inc. p. 458. Retrieved 11 April 2015.
- Capacity, Coverage and Deployment Considerations for IEEE 802.11G (PDF), Cisco Systems, Inc, 2005, p. 1
- The Origins of Spread-Spectrum Communications
- This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".
- NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management