A Study On Blind Despreading Of Direct-Sequence Spreed-Specturum Signals

Direct sequence spread spectrum (DSSS) signals have been widely used for military and civil communications due to their anti-multipath, anti-jamming, low probability of intercept and multiple access properties. In non-cooperative applications such as communication reconnaissance, wireless spectrum surveillance and localization tracking of illegal communication transmitters, the transmitted information should be recovered without prior knowledge of the spreading sequence at the receiver. Therefore, blind despreading of DSSS signals plays an important role in non-cooperative scenarios.Although in the past years the study on blind despreading of DSSS signals has witnessed many developments, not so much in problem solving means and with new measures due to lack of profound theoretic investigation. However, some important problems have not been solved satisfactorily. For an example, the theoretical lower bound of spreading waveform estimation has not been studied. Another example is that the Maximum Likelihood Estimate (MLE) of long-code DSSS signals has seldom been evaluated comprehensively under the non-cooperative context.These problems are comprehensively studied from theoretical viewpoint. The main contributions of this dissertation include some aspects as follows:1. For the first time, under the stochastical and the deterministic models for the DSSS signals, we derive the Cramér–Rao bound (CRB) for the spreading waveform estimation problem. The derived CRB holds for both long-code and short-code DSSS signals, and gives a performance lower bound for all spreading waveform estimators.2. For the problem of blind synchronization for single-user short-code DSSS signals, an improved algorithm based on m1 -norm of the covariance matrix is proposed. The proposed algorithm has low computational complexity and can be used for the multi-user signals with unknown carrier offset. For the spreading sequence estimation problem of short-code QPSK-DSSS signals, an improved algorithm based on constant modulus property is proposed. Through theoretical analysis, we prove that the existing eigenanalysis-based spreading waveform estimator, which can approach the CRB, is essentially the Gaussian MLE. Using de-noising and reduced-dimension techniques, an improved blind despreading algorithm for asynchronous CDMA signals is presented, which has low computational complexity and shows good performance.3. For single-user long-code DSSS signals, we derive the MLE of spreading waveform. Note that the MLE problem belongs to the field of combinatorial optimization. We propose a blind despreading algorithm based on semidefinite programming and extend it to the case when unknown carrier offset exists. The proposed algorithm provides significant performance improvements compared to existing methods in the case of low numbers of data samples and low SNR, and can be applied to the short-code DSSS signals.4. For multi-user long-code DSSS signals, under the deterministic model and the assumption that the user number is known apriori, we propose a blind despreading algorithm based on low-rank approximation of missing-data model, and the proposed algorithm can approach the CRB in the single-user case. Under the stochastical model, based on the MLE for incomplete multivariate Gaussian data, we propose a blind despreading algorithm which can estimate the user number. For the single-user DSSS signals, to overcome the drawbacks concerning convergence towards local minima and the difficulty to choose the initial value and iterative step precisely in optimization methods, a non-iterative blind despreading algorithm based on monotone missing-data model is proposed.5. Using multi-channel receiver, we propose a blind despreading algorithm based on missing-tensor model for multi-user long-code DSSS signals through the three-dimensional extension of the two-dimensional missing-data model. Due to fully exploiting the structure of spatial-temporal-spreading diversity, the proposed algorithm provides better performance compared to the ones using single-channel receiver.