|dc.description.abstract||The growing demand for capacity in wireless communications is the driving force behind improving
established networks and the deployment of a new worldwide mobile standard. Capacity
calculations show that the direct sequence code division multiple access (DS-CDMA)
technique has more capacity than the time division multiple access technique. Therefore, most
3rd generation mobile systems will incorporate some sort of DS-CDMA.
In this thesis DS-CDMA receiver structures are investigated from the view point of pattern
recognition which leads to new DS-CDMA receiver structures. It is known that the optimum
DS-CDMA receiver has a nonlinear structure with prohibitive complexity for practical implementation.
It is also known that the currently implemented receiver in 2nd generation DSCDMA
mobile handsets has poor performance, because it suffers from multiuser interference.
Consequently, this work focuses on sub-optimum nonlinear receivers for DS-CDMA in the
First, the thesis reviews DS-CDMA, established equalisers, DS-CDMA receivers and pattern
recognition techniques. Then the new receivers are proposed. It is shown that DS-CDMA can
be considered as a pattern recognition problem and hence, pattern recognition techniques can be
exploited in order to develop DS-CDMA receivers. Another approach is to apply known equaliser
structures for DS-CDMA. One proposed receiver is based on the Volterra series expansion
and processes the received signal at the chip rate. Another receiver is a symbol rate radial
basis function network (RBFN) receiver with reduced complexity. Subsequently, a receiver is
proposed based on linear programming (LP) which is especially tailored for nonlinearly separable
scenarios. The LP based receiver performance is equivalent to the known decorrelating
detector in linearly separable scenarios. Finally, a hybrid receiver is proposed which combines
LP and RBFN and which exploits knowledge gained from pattern recognition. This structure
has lower complexity than the full RBF and good performance, and has a large potential for
Monte-Carlo simulations compare the proposed DS-CDMA receivers against established linear
and nonlinear receivers. It is shown that all proposed receivers outperform the known linear receivers.
The Volterra receiver’s complexity is relatively high for the performance gain achieved
and might not suit practical implementation. The other receiver’s complexity was greatly reduced
but it performs nearly as well as an optimum symbol by symbol detector.
This thesis shows that DS-CDMA is a pattern recognition problem and that pattern recognition
techniques can simplify DS-CDMA receiver structures. Knowledge is gained from the DSCDMA
signal patterns which help to understand the problem of a DS-CDMA receiver. It
should be noted that from the large number of known techniques, only a few pattern recognition
techniques are considered in this work, and any further work should look at other techniques.
Pattern recognition techniques can reduce the complexity of existing DS-CDMA receivers
while maintaining performance, leading to novel receiver structures.||en