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DS-CDMA Acquisition Using Adaptive Filters and Sampling at the Rate of Innovation

Nazari Hashemi, Hamid | 2008

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 39159 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Aleam Marvasti, Farrokh
  7. Abstract:
  8. In modern wireless mobile telephony networks, DS-CDMA based physical layers has gained much attention due to their ease of implementation at relatively low overall cost, ability to provide high speed data services, increased user capacity, and many other outstanding features. One of the challenges facing these systems is the synchronization of the users’ signature codes (received via communication channel) to the receiver’s locally generated replica, as the only distinguishing feature between users in such systems is the pseudo-orthogonal properties of these signature codes. To fully utilize this orthogonality, all sequences need to have a common time reference. But when relative sequence timing is distracted due to inherent users’ asynchrony in transmission, multipath fading or …, the codes can no longer be considered as orthogonal. Ignoring these phase shifts in recovering users’ data can impose severe performance degradation on the whole communication system. Therefore, it is mandatory to have proper delay estimations before beginning the data retrieval phase. The process of obtaining and maintaining such estimates is called Synchronization and is mainly achieved through maximization of signature codes’ autocorrelation function. On the other hand it is well-known that one of the main features of adaptive filters is their ability to offer a linear model of an unknown system by observing its input and output. If we assume that the channel delay along with other users’ distortion can be modeled as a linear system with unknown parameters, an adaptive filter can be put into work in order to obtain a linear model which can then be used to estimate the desired delay. Additionally we might be able to make use of the tracking property of adaptive filter in implementing the Tracking phase of the synchronization process. Along these, the introduction of new sampling schemes and reconstruction techniques for signals with finite rates of innovation has led us to a whole new method of sampling and retrieval of non-bandlimited signals such as a stream of Diracs or more interestingly the DS-CDMA signals at much lower rates than the traditional Nyquist rate. Combining this new sampling setup with the proposed adaptive acquisition method greatly reduces the computational complexity while not impairing the system performance. Computer simulations verify this complexity reduction and performance.
  9. Keywords:
  10. Adaptive Filters ; Direct Sequence Code Division Multiple Access (DS-CDMA) ; E-Spline ; Acquisition ; Innovation Rate ; Polynomial Generating Kernel

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