الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
An important aspect of all spread spread spectrum receivers is synchronization. Military communications, packet radio networks, wireless communication channel, and code division multiple access have necessitated the development of fast and robust techniques. Current techniques for direct sequences spread spectrum (DSSS) synchronization, although rapid, are not robust to multiple access interfaces, this is because they require the spreading codes of the multiple access network to have very good cross correlation properties. There is another approach to synchronization called soft synchronization; by which spread code obtained from the received signal. As more data is collected, the code estimate improves and so does the quality of the despread data. The key advantage of the soft synchronization technique is that excellent despread data can be obtained directly.
The soft technique exploits the cyclostationarity property of the DSSS signal. It can theoretically provide two schemes of despreader in time and frequency domain for estimating data received in arbitrary levels of white background of only the code repetition rate. The technique is robust to multiple access interface provided the users have different code repetirion r ates.
The previous study of soft technique in time domain provides that the technique has large reception code periods and computational complexity ender finite averaging assumptions.
In this thesis , the study examines and simulates the soft dispreading technique in frequency domain. Results are compared with the previous technique in time domain in case of single signal containing arbitrary additive Gaussian noise (AWGN), and multiple access interface(MAI).
The study examines the performance of soft technique in frequency domain for different code length and signal constellations.
It gives a complete picture for the performance charts in different cases.
These cases are :
1-PSK signal with different code lengths (short-medium-long)and AWGN.
2-QPSK signal with different code lengths(short-medium-long)and AWGN.
3- DPSK signal with different code lengths (short-medium-long)and AWGN.
4- DQPSK signal with different code lengths(short-medium-long)and AWGN.
5- QPSK signal with different code lengths (short-medium-long)and AWGN for various RMS.
6- Multiple access interface with different code lengths(short-medium-long)and AWGN for CDMA.
7-Fading signal with different modulation schemes (PSK&QPSK) and AWGN for medium code length.
The study shows that the software technique in frequency domain simulation is better than in time under finite time averaging assumptions, the com[lexity of time domain simulation is relaxed, and short receptions is admitted.
CONCLUSIONS:
1- The performance of the dispreading technique frequency domain is better than in time domain.
2- 2- The performance of PSK DS/SS scheme requires short number of code periods to establish synchronization.
3- 3- The soft dispreading technique gives better performance for QPSK DS/SS scheme with suitable code periods required at low signal to noise ratios.
4- 4- The designer of DS/SS system can choose the suitable scheme with lower short reception code repetition.
5- 5- The immunity of frequency domain dispreading technique in multiple access interface for CDMA.
6- 6- The immunity of frequency domain dispreading in Rayleigh fading environment for mobile.
7- Chapter 1 includes an introduction I spread spectrum techniques, importance of synchronization, and soft technique for synchronization. Chapter 2 deines the stages of synchronization and provides an overview of conventional acquisition techniques. Chapter 3 briefly reviews the cyclostationarity property of a DSSS signal, which is exploited in the development of the algorithm for the soft estimation. The results of computer simulation are presented in chapter 4. Finally, in chapter 5, conclusions are presented and possible future work is proposed.