Direct-sequence spread spectrum
Encyclopedia
In telecommunication
s, 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 is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency. Certain IEEE 802.11 standards use DSSS signaling.
The resulting signal resembles white noise
, like an audio recording of "static". However, this noise-like signal can be 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 believes 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.
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.
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.
In contrast, frequency-hopping spread spectrum
pseudo-randomly re-tunes the carrier, instead of adding pseudo-random noise to the data, the latter process resulting in a uniform frequency distribution whose width is determined by the output range of the pseudo-random number generator.
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
s, direct-sequence spread spectrum (DSSS) is a modulation
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
technique. As with other spread spectrum
Spread spectrum
Spread-spectrum techniques are methods by which a signal generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth...
technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency. Certain IEEE 802.11 standards use DSSS signaling.
Features
- DSSS phase-modulatesPhase modulationPhase modulation is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave.Unlike its more popular counterpart, frequency modulation , PM is not very widely used for radio transmissions...
a sine waveSine waveThe sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...
pseudorandomPseudorandomnessA pseudorandom process is a process that appears to be random but is not. Pseudorandom sequences typically exhibit statistical randomness while being generated by an entirely deterministic causal process...
ly with a continuous stringString (computer science)In formal languages, which are used in mathematical logic and theoretical computer science, a string is a finite sequence of symbols that are chosen from a set or alphabet....
of pseudonoisePseudorandom noiseIn cryptography, pseudorandom noise is a signal similar to noise which satisfies one or more of the standard tests for statistical randomness....
(PN) codeCodeA code is a rule for converting a piece of information into another form or representation , not necessarily of the same type....
symbols called "chipsChip (CDMA)In digital communications, a chip is a pulse of a direct-sequence spread spectrum code, such as a pseudo-noise code sequence used in direct-sequence code division multiple access channel access techniques....
", each of which has a much shorter duration than an information bitBitA bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rateBaudIn telecommunications and electronics, baud is synonymous to symbols per second or pulses per second. It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes made to the transmission medium per second in a digitally modulated signal or a...
is much higher than the informationInformationInformation in its most restricted technical sense is a message or collection of messages that consists of an ordered sequence of symbols, or it is the meaning that can be interpreted from such a message or collection of messages. Information can be recorded or transmitted. It can be recorded as...
signal bit rateBit rateIn telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....
. - DSSS uses a signal structure in which the sequence of chips produced by the transmitter is known a priori 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
Direct-sequence spread-spectrum transmissions multiply the data being transmitted by a "noise" signal. This noise signal is a pseudorandom sequence of1 and −1 values, at a frequency much higher than that of the original signal.The resulting signal resembles white noise
White noise
White noise is a random signal with a flat power spectral density. In other words, the signal contains equal power within a fixed bandwidth at any center frequency...
, like an audio recording of "static". However, this noise-like signal can be 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
Correlation
In statistics, dependence refers to any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence....
of the transmitted PN sequence with the PN sequence that the receiver believes the transmitter is using.
The resulting effect of enhancing signal to noise ratio on the channel is called process gain
Process gain
In a spread spectrum system, the process gain is the ratio of the spread bandwidth to the unspread bandwidth. It is usually expressed in decibels ....
. 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.
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
Code division multiple access
Code division multiple access is a channel access method used by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne, CDMA2000 and WCDMA , which are often referred to as simply CDMA, and use CDMA as an underlying channel access...
(CDMA) property of DSSS, which allows multiple transmitters to share the same channel within the limits of the cross-correlation
Cross-correlation
In signal processing, cross-correlation is a measure of similarity of two waveforms as a function of a time-lag applied to one of them. This is also known as a sliding dot product or sliding inner-product. It is commonly used for searching a long-duration signal for a shorter, known feature...
properties of their PN sequences.
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
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
transmission, except that the added noise causes the distribution to be much wider than that of an AM transmission.
In contrast, frequency-hopping spread spectrum
Frequency-hopping spread spectrum
Frequency-hopping spread spectrum is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver...
pseudo-randomly re-tunes the carrier, instead of adding pseudo-random noise to the data, the latter process resulting in a uniform frequency distribution whose width is determined by the output range of the pseudo-random number generator.
Benefits
- Resistance to intended or unintended jammingRadio jammingRadio jamming is the transmission of radio signals that disrupt communications by decreasing the signal to noise ratio. Unintentional jamming occurs when an operator transmits on a busy frequency without first checking whether it is in use, or without being able to hear stations using the frequency...
- 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 GalileoGalileo positioning systemGalileo is a global navigation satellite system currently being built by the European Union and European Space Agency . The €20 billion project is named after the famous Italian astronomer Galileo Galilei...
and Russian GLONASSGLONASSGLONASS , acronym for Globalnaya navigatsionnaya sputnikovaya sistema or Global Navigation Satellite System, is a radio-based satellite navigation system operated for the Russian government by the Russian Space Forces...
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 phonesCordless telephoneA cordless telephone or portable telephone is a telephone with a wireless handset that communicates via radio waves with a base station connected to a fixed telephone line, usually within a limited range of its base station...
operating in the 900 MHz, 2.4 GHz and 5.8 GHz bands - IEEE 802.11b 2.4 GHz Wi-FiWi-FiWi-Fi or Wifi, is a mechanism for wirelessly connecting electronic devices. A device enabled with Wi-Fi, such as a personal computer, video game console, smartphone, or digital audio player, can connect to the Internet via a wireless network access point. An access point has a range of about 20...
, and its predecessor 802.11-1999. (Their successor 802.11g uses OFDM instead) - Automatic meter readingAutomatic meter readingAutomatic meter reading, or AMR, is the technology of automatically collecting consumption, diagnostic, and status data from water meter or energy metering devices and transferring that data to a central database for billing, troubleshooting, and analyzing.This technology mainly saves utility...
- IEEE 802.15.4IEEE 802.15.4IEEE 802.15.4 is a standard which specifies the physical layer and media access control for low-rate wireless personal area networks . It is maintained by the IEEE 802.15 working group....
(used, e.g., as PHY and MAC layer for ZigBeeZigBeeZigBee is a specification for a suite of high level communication protocols using small, low-power digital radios based on an IEEE 802 standard for personal area networks. Applications include wireless light switches, electrical meters with in-home-displays, and other consumer and industrial...
, or, as the physical layer for WirelessHARTWirelessHARTWirelessHART is a wireless sensor networking technology based on the Highway Addressable Remote Transducer Protocol .-Description:The protocol utilizes a time synchronized, self-organizing, and self-healing mesh architecture...
) - Radio-controlled modelRadio-controlled modelA radio-controlled model is a model that is steerable with the use of radio control. All types of vehicles imaginable have had RC systems installed in them, including cars, boats, planes, and even helicopters and scale railway locomotives....
vehicles