Spread spectrum communication is a typical scheme for covert communication because of its low detectability and antijam characteristic. However, the associated design concerns multiple factors, such as cochannel multiple access interference (MAI) and spread spectrum gain. In this paper, the lattice reduction theory is applied to MAI cancellation of spread spectrum communication and a novel lattice reduction aided multiple user detection method is proposed. The near maximum likelihood (ML) performance of MAI resistance is verified by simulation and theoretical analysis. The superiority of detection performance in strong MAI scenarios is especially addressed. Based on the algorithm, a collaborative covert communication system design is proposed. Low-power covert signals can be transmitted at a higher bit rate with the same coverage as more high-power cochannel signals. The covert transmission performance can be improved significantly compared to traditional designs.

A novel network clock-offset resolution method for space network is proposed in the paper. In the process of clock-offset resolution based on the network inter-node measurement data, the prediction of satellite clock result is used as the priori information. A double weighted control is made with the consideration of the point clock performance and the measurement performance, and better measurement error suppression effect is achieved. The results of simulation experiment indicated that network clock-offset resolution result with better stability and accuracy is achieved compared with the traditional Least Squares method in different scenarios.

Abstract. At present, reliable ambiguity resolution in GPS precise point positioning (PPP) can be achieved through the traditional model called "EWL-WL-NL". In this paper, we proposed a new model of ambiguity resolution,"WL-WL-WL", where making use of linear independence of coefficient vector of wide-lane combination. Firstly, using the MelbourneWubbena combination observable on L2 and L5, we could resolve extra-wide-lane ambiguity instantaneously. Then, the resolved unambiguous extra-wide-lane carrier-phase assists wide-lane ambiguity resolution (AR). Three wide-lane combinations whose coefficient vectors are linearly independent are chosen to compose one full-rank matrix so that the three narrow-lane ambiguity resolution can be achieved. As a result, with the triple-frequency signals, the correctness rate of narrow-lane ambiguity resolution achieves 90% within 60s, in contrast to only 63% within 180s in dual-frequency PPP. Therefore, we demonstrate that triple-frequency PPP has the potential to achieve ambiguity-fixed solutions within a few minutes and the efficiency of ambiguity resolution in triple-frequency PPP is higher.