Modern communication systems are required to provide services based on high data rates burst-mode packet-data transmission, capable of operating at very low SNR conditions. Turbo codes enable the operation at low SNR, close to the Shannon limit. However, carrier frequency and phase synchronization, needed for optimal coherent performance of the receiver, still remains a problem in low SNR and short bursts conditions. This paper proposes a new carrier synchronization method, the Maximum-Likelihood Iterative-Soft-Decision-Directed (ML-ISDD), which uses the turbo-decoder soft decisions to improve the carrier synchronization performance at low SNR values. The ML-ISDD method operates iteratively and jointly with the turbo decoder, enhancing both the turbo-decoder and the synchronization performance. The ML-ISDD method has been shown by simulation to significantly increase the allowed initial frequency and phase uncertainty region, thus allowing the use of very short training sequences for initial carrier synchronization.

Turbo codes, which provide superior performance, close to the Shannon limit, have found their way into WLAN and mobile system standards. Synchronization of turbo-coded signals operating at low SNR conditions may be difficult, especially for short packet transmission. This paper proposes two techniques for using the turbo decoder information to improve the carrier phase and frequency offset synchronization for short packets communication systems. One of those techniques, which are based on soft-decisions produced by the Turbo decoder, can be used for tracking. The other technique is based on maximizing a newly invented cost function, the MSSO (/spl Delta/f,/spl Phi/) function, to acquire and track the carrier frequency and phase errors, with the cost of complexity. A combination of the two techniques is introduced as well to provide a wholesome solution.