Optimum linear single user detection in direct-sequence spread-spectrum multiple access systems

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Virginia Tech


After Qualcomm's proposal of the IS-95 standard, code-division multiple access (CDMA) gained popularity as an alternative multiple-access scheme in cellular and personal communication systems (PCS). Besides the advantage of allowing asynchronous operation of the users, CDMA direct-sequence spread spectrum (DS-SS) offers resistance to frequency selective fading and graceful degradation of the performance as the number of users increases.

Orthogonality of the signals in time-division multiple access and frequency-division multiple access is inherent from the nature of the multiple access scheme. In a CDMA system, orthogonality of the signals is not guaranteed in general. Consequently, the performance of conventional correlation receivers suffers.

Sub-optimum receivers which use knowledge of the interfering signals have been investigated by other researchers. These receivers attempt to cancel the multi-user interference by despreading the interfering users. Hence, these receivers require knowledge about all the spreading codes, amplitude levels, and signal timing, and are, in general, computationally intensive.

In this thesis, a technique is presented for which a high degree of interference rejection can be obtained without the necessity of despreading each user. It is shown that exploiting spectral correlation can help mitigate the effects of the multiple-access interference. If code-on-pulse DS-SS modulation is used, a cyclic form of the Wiener filter provides substantial improvements in performance in terms of bit error rate and user capacity. Furthermore, it is shown, that a special error-criterion should be used to adapt the weights of the filter.

The computational complexity of the receiver is equivalent to that of conventional equalizers.



code-division multiple access, code-on-pulse DS-SS modulation, CDMA