Soft Turbo Despreading and Decoding for Self Spread-Spectrum Communications

In self-spread spreading (SSP), data are spread with a sequence which is generated from past transmitted data. While proving an improved security with respect to conventional spread-spectrum system, an error propagation phenomena may be a limiting factor in its use. We propose a new turbo soft receiver for coded SSP which iterates between despreading and decoding. Despreading uses the extrinsic likelihood provided by decoding to linearly combine the received signal with the aim of minimizing the mean square error at its output. Moreover, the redundancy of the SSP signal is exploited by a soft chip decoder included in the despreader. Decoding is implemented with a soft-input soft-output decoder. By performance evaluation, we show that the turbo receiver allows a significant reduction of the error propagation phenomenon typical of SSP systems.