Wireless Network Experiment ZHANG Qianyun 073011Lab2 Direct Sequence Spread Frequency Techniques直序扩频通信仿真ContentAbstract-3Experiment Background-3Experiment Procedure-5Analysis and Conclusion-10Reference -10Appendix-121. Abstract The objective of this lab experiment is to learn the fundamentals of the direct sequence spread spectrum and code division multiple address techniques. To get familiar with the direct sequence spread spectrum modulator and demodulator. And the direct sequence spread spectrum system can be shown as:Figure 1. Direct sequence spread spectrum system2. Experiment Background2.1 Introduction of Direct Sequence Spread Spectrum 1In telecommunications, 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 devices transmitting frequency.Figure 2.1 Procedure to generate a DSSS signal2.2 Generation of Direct Sequence Spread SpectrumTo generate a spread spectrum signal one requires: 1. A modulated signal somewhere in the RF spectrum 2. A PN sequence to spread it2.3 Features of Direct Sequence Spread SpectrumDSSS has some features as following:1. DSSS phase-modulates 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.2. 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.2.4 Transmission of Direct Sequence Spread SpectrumDirect-sequence spread-spectrum transmissions multiply the data being transmitted by a noise signal. This noise signal is a pseudorandom sequence of 1 and 1 values, at a frequency much higher than that of the original signal, thereby spreading the energy of the original signal into a much wider band. 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.For de-spreading to work correctly, the transmit and receive sequences must be synchronized. This requires the receiver to synchronize its sequence with the transmitters sequence via some sort of timing search process. However, this apparent drawback can be a significant benefit: if the sequences of multiple transmitters are synchronized with each other, the relative synchronizations the receiver must make between them can be used to determine relative timing, which, in turn, can be used to calculate the receivers position if the transmitters positions are known. This is the basis for many satellite navigation systems.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