A combined coherent carrier recovery and decision-directed delay-lock-loop scheme for DS/SSMA communication systems employing complex spreading sequences

The majority of papers on code synchronization loop design for DSSS communication systems concentrate on noncoherent schemes employing binary spreading sequences with loop structures comprising early and late arm structures with squaring devices which are very prone to arm imbalance effects. In this paper a novel decision-directed code-lock-loop (DD-CLL) synchronization scheme is proposed for DS/SSMA communication systems employing either binary or complex spreading sequences. The novelty of the proposed CLL structure lies in the use of difference signals in both the spreading, demodulation/despreading and CLL processes, which results in the elimination of unwanted cross product interference terms when employing complex spreading sequences. Additional advantages of the new structure include features such as less sensitivity to arm imbalance problems due to the elimination of squaring devices, improved tracking capability and simplified hardware by requiring only two correlators in stead of four in the CLL structure. The loop response is analysed and its performance verified through computer simulation in the presence of AWGN and typical non-stationary channel effects, employing both binary as well as a relatively new family of complex linearly and non-linearly root-of-unity (Land NL-RU) filtered spreading sequences, including a novel class of previously introduced analytical bandlimited complex (ABC) sequences.