Spread Spectrum

السلام عليكم ورحمه الله وبركاته

الموضوع دا مهم جدا ............والتقنيه دى شغال بيها نظام الـCDMA

Spread spectrum is defined as a communication technique in which the signals are spread over a bandwidth in excess of the minimum bandwidth required for their transmission. It has its origin in the military scenario where robustness in communications is vital. In particular, two principal requirements must be satisfied:
(1) detect ability of the signal by the enemy must be made difficult and (2) enemy-introduced interference must be made easy to combat (strange expressions for a peaceful technical material!). Systems presenting these characteristics are referred to, respectively, as low-probability of intercept (LPI) and antijam communications (AJM) systems. Spread spectrum is accomplished by the use of wideband encoding signals at the transmitter, which is required to operate in synchronism with the receiver where the encoding signals, at least for the intended information, are also known. Coding, therefore, is an important element in the design of spread-spectrum signals.
By allowing the signals to occupy a bandwidth in excess of the required bandwidth for their transmission, spread-spectrum signals can be given pseudorandom characteristics, which cause the signals to have a noise like appearance.

The LPI characteristic of spread-spectrum communications can be attained by several means, such as by transmitting at a very low power level (the lowest possible), by limiting the transmission time to very small time intervals (the smallest possible), and by giving the signal a noise like appearance. It is possible to reduce the signal power to a level below the ambient thermal noise power so that an unintended listener may find it difficult to determine that a transmission is taking place. Besides reducing the transmission time to very short time intervals, the position of these intervals within the total time can be made pseudorandom so that an unintended listener has difficulty learning the hopping pattern. Noiselike appearance is imprinted by the use of pseudorandom sequences in the modulation process. The AJM characteristic of spread-spectrum communications is attained as follows. Spreading the signal at the transmitter and despreading the received signal at the receiver would not lend any better performance if the communication channel had not been affected by some sort of narrowband interference. Assuming that spreading and despreading operations are identical inverses of each other, the detected signal after the despreading would be identical to the transmitted signal prior to spreading. However, considerable gain is obtained in the presence of a narrowband interference, considering
the interference being introduced after the spreading operation. In this case, the composite signal arriving at the receiver constitutes a wideband signal (wanted signal) and a narrowband interference (unwanted signal)
The despreading operation at the receiver restores the wanted signal to its original bandwidth, whereas the interference is spread by the same amount with which the wanted signal had been shrunk. Therefore, the power density of the interference is diminished by the same amount, thus reducing its effect.
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