Receiver Design for Downlink MIMO MC-CDMA in Cognitive Radio Systems

Abstract

Multiuser interference (MUI) and co-space<br><br>interference (CSI) are two challenging issues in design of<br><br>multicarrier code-division multiple-access (MC-CDMA) systems<br><br>when multiple antennas are used at both the transmitter and<br><br>receiver sides (MIMO transmission). On the other hand, in<br><br>cognitive radio (CR) system, the secondary users that use MCMCDA<br><br>technique must deactivate the subcarriers used by the<br><br>primary users. This leads to loss of orthogonality of the<br><br>conventional spreading codes such as Hadamard Walsh codes. In<br><br>this paper, we have modified two recently proposed linear<br><br>receivers to be deployed in MIMO MC-CDMA based CR<br><br>downlink transmission where some of the subcarriers are turned<br><br>off. To solve the resultant non-orthogonality problem, the<br><br>complex carrier interferometry (CI) codes are utilized that exist<br><br>for any length. The first method, called total interference<br><br>cancellation (CI-TIC), is able to completely eliminate the CSI and<br><br>the MUI. The other method is called CI symbol-chip level<br><br>minimum mean square error (CI-SC-MMSE) that shows<br><br>superior bit error performance with respect to CI-TIC. For<br><br>different scenarios, simulation results show that the proposed CI<br><br>based receivers are able to effectively mitigate the MUI and the<br><br>CSI in comparison with the case where Hadamard Walsh codes<br><br>are utilized.