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Spread Spectrum Communications
Published in Jerry D. Gibson, The Communications Handbook, 2018
Laurence B. Milstein, Marvin K. Simon
Spread spectrum is a communication technique wherein the transmitted modulation is spread (increased) in bandwidth prior to transmission over the channel and then despread (decreased) in bandwidth by the same amount at the receiver. If it were not for the fact that the communication channel introduces some form of narrowband (relative to the spread bandwidth) interference, the receiver performance would be transparent to the spreading and despreading operations (assuming that they are identical inverses of each other). That is, after despreading the received signal would be identical to the transmitted signal prior to spreading. In the presence of narrowband interference, however, there is a significant advantage to employing the spreading/despreading procedure described. The reason for this is as follows.
Digital Modulation
Published in Jerry C. Whitaker, The RF Transmission Systems Handbook, 2017
A class of modulation that is important for military and cellular radio communications is spread spectrum [Peterson, Ziemer, and Borth, 1995]. Spread spectrum modulation is any modulation format that utilizes a transmission bandwidth much wider than that required to transmit the message signal itself, independent of the message signal bandwidth. It can be categorized several ways. The two most common types of spread spectrum modulation are direct-sequence spread spectrum (DSSS) and frequency-hop spread spectrum (FHSS). In the former type, the modulated signal spectrum is spread by multiplication with a pseudonoise binary code, which changes state several times during a symbol interval. In the latter type, the spectrum is spread by hopping the modulated signal spectrum about in a pseudorandom manner. The receiver knows the code that is used for spreading at the transmitter. Thus, it is possible to despread the received signal at the receiver once the receiver’s code is synchronized with the spreading coding on the received signal. Spread spectrum modulation is used for several reasons. Among these are to hide the modulated signal from an enemy interceptor, to lower the susceptibility of jamming by an unfriendly or unintentional source, to combat multipath, to provide multiple access capability for the modulation scheme, and to provide a means for range measurement. An example of the latter application is the global positioning system (GPS). An example of the fourth application is the second North American digital cellular-radio standard mentioned earlier.
Security in Wireless Sensor Networks
Published in Shafiullah Khan, Al-Sakib Khan Pathan, Nabil Ali Alrajeh, Wireless Sensor Networks, 2016
Jamming attack may be defended by employing variations of spread-spectrum communication such as frequency hopping and code spreading [29]. “Frequency-hopping spread spectrum” (FHSS) is a method of transmitting signals by rapidly switching a carrier among many frequency channels using a pseudorandom sequence that is known to both the transmitter and the receiver. As a potential attacker would not be able to predict the frequency selection sequence, it will be impossible for him or her to jam the frequency being used at a given point of time. Code spreading is another technique for defending a network against jamming. However, it requires greater design complexity and more energy consumption than FHSS and is thus not suitable for WSNs. In general, sensor devices are limited to single-frequency use and are highly susceptible to jamming attacks. One approach to tolerate jamming attacks in a WSN is to identify the jammed part of the network and effectively avoid it by routing around it. Wood et al. [29] have proposed an approach in which nodes along the perimeter of a jammed region report their status to their neighbors and the affected region is identified collectively and packets are routed around it.
Comparative analysis of quaternion modulation system with OFDM systems
Published in International Journal of Electronics Letters, 2021
Anam Zahra, Qasim Umar Khan, Shahzad Amin Sheikh
Every communication system tries to achieve a lower bit error rate (BER) ensuring minimum data loss. With the increasing use of wireless communication technology, the demand for improved BER services over both wired and wireless links is also increased. New techniques are required to transfer data with better BER which existing techniques cannot support (Saxena & Prasad, 2003; Zou & Wu, 1995). In communication system, BER can be achieved at a certain limit at the expense of bandwidth or PAPR (Gangwar & Bhardwaj, 2012). There are various applications to achieve lower BER is more essential as compared to bandwidth such as military communication, point-to-point communication and point-to-multipoint communication (Gallagher, 1979). In the past, various techniques have been implemented in military communication systems to provide protection against noise interference. Spread spectrum is a modulation technique, extensively used in military (Zou & Wu, 1995). It resists interference and makes it hard for enemies to intercept, whereas in spread spectrum modulation technique, the bandwidth of transmitted signal is much greater than the bandwidth of original message. One technique to improve BER is CDMA; it has been used in military applications (Gangwar & Bhardwaj, 2012; Saxena & Prasad, 2003). CDMA is a multiple access technique used with spread spectrum, which allows numerous signals to occupy a single transmission channel in 2G and 3G wireless communications.
Supraharmonics reduction in LED drivers via random pulse-position modulation
Published in International Journal of Electronics, 2018
Joaquin Garrido-Zafra, Antonio Moreno-Munoz, Aurora Gil-De-Castro, Manuel A. Ortiz-López, Tomás Morales
Spread Spectrum Techniques simply mean distributing the energy of a given signal across a wide frequency band, thereby reducing peak energy. These usually involve the introduction of a random (or at least pseudo-random) quantity. If the switching algorithm is non-deterministic, then its frequency spectrum will be continuous due to the non-periodicity of the signal. As a result, the average spectral power density of broadband emission can be drastically limited. This method could be used to overcome the interference caused by supraharmonics in a conventional PWM self-commutated PEC stage. In addition, low-frequency emission is minimal.