Spread spectrum is a form of wireless communications in which the frequency of the transmitted signal is deliberately varied. This result in a much greater bandwidth than the signal would have if its frequency were not varied.A conventional wireless signal has a frequency, and are methods by which electromagnetic energy generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth usually specified in megahertz (MHz) or gigahertz gigahertz), that does not change with time (except for small, rapid fluctuations that occur as a result of modulation). When you listen to a signal at 103.1 MHz on an FM stereo receiver, for example, the signal stays at 103.1 MHz. It does not go up to 105.1 MHz or down to 99.1 MHz. The digits on the radio's frequency dial stay the same at all times. The frequency of a conventional wireless signal is kept as constant as the state of the art will permit, so the bandwidth can be kept within certain limits, and so the signal can be easily located by someone who wants to retrieve the information. There are at least two problems with conventional wireless communications that can occur under certain circumstances. First, a signal whose frequency is constant is subject to catastrophic interference. This occurs when another signal is transmitted on, or very near, the frequency of the desired signal. Catastrophic interference can be accidental (as in amateur-radio communications) or it can be deliberate (as in wartime). Second, a constant-frequency signal is easy to intercept, and is therefore not well suited to applications in which information must be kept confidential between the source (transmitting party) and destination (receiving party). To minimize troubles that can arise from the above mentioned vulnerabilities of conventional communications circuits, these techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference and jamming, to prevent detection, and to limit the power flux density on satellite downlinks. The frequency of the transmitted signal can be deliberately varied over a comparatively large segment of the electromagnetic radiation spectrum. This variation is done according to a specific, but complicated mathematical function. In order to intercept the signal, a receiver must be tuned to frequencies that vary precisely according to this function. The receiver must "know" the frequency-versus-time function employed by the transmitter, and must also "know" the starting-time point at which the function begins. If someone wants to jam a spread-spectrum signal, that person must have a transmitter that "knows" the function and its starting-time point. The spread-spectrum function must be kept out of the hands of unauthorized people or entities. Most spread-spectrum signals use a digital scheme called frequency hopping. The transmitter frequency changes abruptly, many times each second. Between "hops," the transmitter frequency is stable. The length of time that the transmitter remains on a given frequency between "hops" is known as the dwell time. A few spread-spectrum circuits employ continuous frequency variation, which is an analog scheme. An example of commercial spread spectrum systems are systems that are designed to be used in so-called unlicensensed bands, such as the Industry, Scientific, Medical (ISM) band around 2.4 GHz. Typical applications are here cordless telephones, wireless LANs, and cable replacement systems as Bluetooth. Since the band is unlicensed, there is no central control over the radio resources; interference is from other communication systems and other electrical and electronic equipment (e.g., microwave ovens, radars, etc.). Here the jamming is not intentional, but the interference
Applications Code-division multiple access systems
(CDMA systems) use spread spectrum techniques to provide communication to several concurrent users. CDMA is used in one second generation (IS-95) and several third generation wireless cellular systems (e.g., cdma2000 and WCDMA). One advantage of using jamming-resistant signals in these applications is that the radio resource management is significantly reduced.
This spread spectrum multiple access technique allow multiple signals (user) occupying the same RF bandwidth to be transmitted simultaneously without
Interfering with one another.
Each user is assigned a code of its own, which performs either direct sequence or Frequency hop spread spectrum modulation.
Each code is orthogonal to all others.
Operation is asynchronous. â€œall users need not transmit simultaneously.
Advantage over TDMA
CDMA does not require external synchronization network
Performance degradation is gradual in case of increase in number of user that add to system.
CDMA offers an external interference rejection capability like multipath rejection or resistance to jamming.
All users can share full spectrum simultaneously because each user has different code .