Loading...
| Friend's email | |
| Your name | |
| Your email | |
| enter code | |
This page was sent successfuly
- Type of Document: Ph.D. Dissertation
- Language: Farsi
- Document No: 46552 (05)
- University: Sharif University of Technology
- Department: Electrical Engineering
- Advisor(s): Marvasti, Farokh
- Abstract:
- In this thesis, detection problem in CDMA systems in overloaded regim is investigated. To thid end, both wireless and optical CDMA systems are considered. In overloaded CDMA systems, if the spreading matrix is invertible and channel is ideal and noiseless, then in the receiver the information of users can be detected without any error. Previusely, it was proven that for limited information symbols space, there are some matrices which are invertible or detecting. However, optimal receiver in these systems is comlex and infeasible. The only case that the optimal receiver can be implemented in a fesible way, is when the spreading matrix is constructed using kronecker technique. Therefore, in this thesis, a near optimal Bielef Propagation (BP) algorithm is considered. Based on BP algorithm, three multiuser receivers are proposed. The proposed receivers have different compelexity and performances. Then, code design problem in optical CDMA in overloaded regime is investigated and it is shown that random sparse codes can be detecting if ratio of the number of users to the number of chips is kept constant. It is also proven that there is no detecting matrix with tree factor graph representation. As results, the proposed receivers exhibit acceptable performances in overloaded regime. Moreover, it is shown that the performance of the receivers in poisson optical channel is very close to the optimal receiver.
In another part of this thesis, a result in CDMA systems with random spreading codes is extended. In fact previously it was shown that an optimum receiver in CDMA systems with random spreading has a close performance to single user receiver in a single user channel if the loading factor is kept constant in large system limit. It this thesis, it is shown that this result is valid for random binary and Gaussian codes even if the loading factor grows logarithmically with the number of users.
In the last part of this thesis, performance of asynchronous Spectral-Amplitude-Coding (SAC) OCDMA is evaluated. It is shown that PIIN effect is worse in asynchronous regime compared to synchronous regime. This result shows that unlike linear CDMA systems, in SAC-OCDMA the performance of practical system which is asynchronous is not better than synchronous regime - Keywords:
- Phase Noise ; Code Devision Multiple Access (CDMA) ; Over Loads ; Asynchronous System
Digital Object List
-
محتواي کتاب
- view
Bookmark