Direct sequence code division multiple access (DS-CDMA) provides flexible data transmissions in wide range of data rates by the use of orthogonal multicode multiplexing. In a frequency selective fading channel, when code multiplexing order becomes higher, the bit error rate (BER) performance is severely degraded since the destruction of othogonality among spreading codes becomes severer due to increasing inter-path interference. In this paper, we propose a technique to restore the othogonality while achieving frequency diversity effect by the application of frequency-domain equalization technique used in MC-CDMA receivers to multicode DS-CDMA systems. It is found by computer simulations that the BER performance of orthogonal multicode DS-CDMA in frequency selective fading channel can be significantly improved by the combined use of minimum mean square error combining (MMSEC) and antenna diversity reception.
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