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We propose to apply chip interleaving and iterative detection to self-encoded multiple access (SEMA) communications. In SEMA, the spreading code is obtained from user bit information itself without using a pseudo noise code generator. The proposed scheme exploits the inherent diversity in self encoded spread spectrum signals. Chip interleaving not only increases the diversity gain, but also enhances the performance of iterative detection. We employ user-mask and interference cancellation to decouple self-encoded multiuser signals. This paper describes the proposed scheme and analyzes its performance. The analytical and simulation results show that the proposed system can achieve a 3 dB power gain and possess a diversity gain that can yield a significant performance improvement in both Rayleigh and multipath fading channels.Index Terms: Chip-interleaving, interference cancellation, iterative detection, self-encoded multiple access (SEMA), spread spectrum.

Spread spectrum and code-division m u l t i p l e access s y s t e m s t r a d i t i o n a l l y h a v e e m p l o y e d d e t e r m i n i s t i c pseudo-random s e q u e n c e s for s i g n a l s p r e a d i n g and de-spreading. Although random sequences have often been used for the purpose o f analysis analysis, t h e y cannot be p r a c t i c a l l y implemented because data r e c o v e r y at the receiver requires a knowledge of the sequences that have been generated i n the t r a n s m i t t e r .In t h i s paper, w e present a novel spread spectrum c o m m u n i c a t i o n s y s t e m f o r secure t r a n s m i s s i o n s of digi t a l i n f o r m a t i o n . The approach is based on the unconventional self e n c o d i n g p r i n c i p l e s that w e have developed for the m o d u l a t i o n and detection of spread s p e c t r u m signals. The proposed s y s t e m does not usepseudo-random codes, and i s u n i q u e i n that tradit i o n a l t r a n s m i t and receive code generators based on m a x i m a l -l e n g t h sequences or c h a o t i c signals are not needed. In f a c t , the enhanced t r a n s m i s s i o n s e c u r i t y arises not o n l y due to the spread-spectrum nature of the signal, but also f r o m the stochastic nature of the u n i q u e spectrum spreading and de-spreading processes. T h i s paper describes the self-encoded system and presents the t h e o r e t i c a l and s i m u l a t i o n results of the s y s t e m p e r f o r m a n c e i n a d d i t i v e -

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