Abstract:
Wireless communications rely on Multiple Access Techniques to share limited
radio spectrum resources. There are three main types of Multiple Access Techniques
i.e. FDMA, TDMA and CDMA. These days CDMA is gaining popularity due to its
high capacity and performance. In CDMA different users occupy the same bandwidth
(unlike FDMA) at the same time (unlike TDMA). The user data is separated via the
use of signature waveforms, sequences or codes assigned to each user. In order to
eliminate interference among different user’s data these signature waveforms should be
mutually orthogonal (Cross-correlation = 0).
DS-CDMA has been the most popular and widely used technique among all
other techniques for CDMA. In DS-CDMA each user is assigned a PN Code which is
multiplied by the user data before transmission. At the receiver detection is done on the
basis of a filter matched to the PN Code of the user. This detector is known as Matched
filter detector. Initially this conventional matched filter detector used only orthogonal
sequences so there was no issue regarding Multiple Access Interference (MAI), but in
order to increase capacity the condition of orthogonality was removed so MAI comes
in to the scene. So in order to remove this MAI different techniques were proposed and
Multi-user detection (MUD) is one of them. MUD offers high capacity and better
performance as compared to Matched Filter Detector.
We implemented two MUD Algorithms in Matlab and Simulink, in order to
remove and mitigate the effects of MAI. The first one is the Decorrelating Detector,
which consists of a bank of matched filters followed by a linear transformation that
multiplies the output of the matched filter bank with the inverse of the correlation
matrix. The second one is the Successive Interference Cancellation, which involves
successive cancellation of MAI from the outputs of the matched filters. First the users
are ranked according to increasing received signal strength. Then, the strongest signal
is cancelled out first followed by the second strongest signal and so on.
