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.

The interleaving span of coded frequency-hopped (FH) systems is often constrained to be smaller than the decoder memory length, i.e. nonideal interleaving is performed. An upper bound on the performance of a Viterbi decoder of a convolutional code with nonideal interleaving is presented. A soft decision diversity combining technique is introduced, and the performance of combined convolutional and diversity coding subject to worst-case partial band noise jamming is investigated. Optimization of the FH system performance subject to constraints of allowed delay and synthesizer settling time provides the best combination of interleaving span and hopping rate. The FH system considered employs M-ary frequency-shift key (MFSK) modulation and noncoherent demodulation with 2-b soft decision based on Viterbi's ratio-threshold technique.>

The fading characteristics of the HF channel make communication over this channel difficult, even at moderate data rates. In this correspondence, a practical communication system is presented which has a near-optimal receiver for data detection and adaptive optimization of the receiver to channel parameters. The channel tracking is performed during the training sequence as well as during the detected data sequence. The receiver is fast converging, therefore suitable for frequency hopping. The correspondence presents the performance obtained on fading HF channels and the computational complexity via comparisons to previously published results. The performance is shown to be superior at comparable computational complexity.

Practical communication systems must cope with many uncertainties in addition to determining the transmitted data, e.g., the direction, timing, and distortion of the desired signal, and the spectral and spatial distribution of the interference, all of which may change with time. This paper describes exact least-squares (LS) recursive lattice algorithms which resolve these uncertainties in a direct-sequence spread-spectrum digital communication system. The adaptive LS algorithm is recursive beth in order and time, and converges rapidly to the uncertain parameters. Time-discrete algorithms may be mechanized by a receiver containing integrate-and-dump circuits operating at the chip rate of the pseudonoise (PN) sequence, one in each in-phase and each quadrature channel of each sensor array element's output. Different configurations of optimal time-discrete receivers are presented and transformed into adaptive receivers by taking advantage of the spectral properties of the different kinds of LS filters. Simulation results are presented and some guide lines are given for the architecture of an adaptive direct-sequence spread-spectrum system.