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Code shift keying (CSK) was proposed to increase the transmission efficiency of DSSS systems, and to overcome the spreading gain versus data rate limitation. However, CSK modulation provides no multi-access capability, and degrades the flexibility of a spread spectrum system. In this work, a generalized M-ary DSSS modulation scheme, spreading code phase shift keying (SCPSK), is proposed to improve the system flexibility, and to provide the multi-access capability. This scheme involves switching the spreading code phase in accordance with the incoming data. The union bound of the symbol error probability, in an AWGN channel exposed to multi-user interference, are derived for performance evaluation. Furthermore, the performance of M-SCPSK in a multipath fading channel is investigated via simulation and compared with the conventional DSSS-BPSK scheme.