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Signature Ensembles and Receiver Structures for Oversaturated Synchronous DS-CDMA Systems

Jarkko Paavola, Signature Ensembles and Receiver Structures for Oversaturated Synchronous DS-CDMA Systems. TUCS Dissertations 91. Turku Centre for Computer Science, 2007.

Abstract:

This thesis considers aspects in signature ensemble design for oversaturated synchronous DS-CDMA systems. CDMA is a multiple access method, where different users are distinguished with a unique code signal, or signature, assigned to each user because with DS-CDMA all users transmit simultaneously in the common frequency band. The set of all signatures is called signature ensemble. Synchronized signatures permit orthogonality between them but the number users is limited by the code signal space dimension, which is determined by available bandwidth and required data rate. If more users are to be served beyond the signal space dimension, the system becomes oversaturated making multiple access interference unavoidable. In such a situation a trade-off must be decided between the increase in the number of users, reduced performance due to inflicted interference and the complexity of a receiver, since the performance of a conventional matched filter receiver is poor in an oversaturated situation.

Two criteria of optimality are of essence when signature ensembles are assessed. Channel capacity is a measure of how well available resources are utilized. On the other hand, a crucial factor is the error probability in the receiver.
Oversaturated CDMA can reach the theoretical limit of channel capacity, but only with Gaussian distributed signals.
With a binary data transmission, which is also considered in this thesis, the optimization of signature ensemble for the maximization of channel capacity should be performed the same way with low signal-to-noise ratios as with Gaussian distributed signals. For high signal-to-noise ratios the Euclidean distance between transmitted signals should be maximized. This optimization direction also results in optimal error performance.

In this thesis oversaturated signature ensembles are designed for a group orthogonal system meaning that users are divided to non-interfering groups, which are then oversaturated with additional users. Group orthogonality permits using very simple receiver structure. The signature ensemble design criterion is to maximize the minimum Euclidean distance between all possible superpositions of user signatures modulated by antipodal information bits. That is, the minimum bit error probability is targeted. When one transmitter controls all transmissions, such as a base station in a cellular environment, data bits
can be collaboratively encoded to improve the performance of group orthogonal oversaturated system. This way the minimum Euclidean distance can even surpass the minimum Euclidean distance of orthogonal signaling. It is concluded in this thesis that oversaturated systems can serve more users than the conventional orthogonal scheme with tolerable performance loss and with a very simple receiver. Analytical results are confirmed with simulations.

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BibTeX entry:

@PHDTHESIS{phdPaavola07a,
  title = {Signature Ensembles and Receiver Structures for Oversaturated Synchronous DS-CDMA Systems},
  author = {Paavola, Jarkko},
  number = {91},
  series = {TUCS Dissertations},
  school = {Turku Centre for Computer Science},
  year = {2007},
  ISBN = {978-952-12-1959-7},
}

Belongs to TUCS Research Unit(s): Communication Systems (ComSys)

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