Acquisition Of Pseudo-Noise Sequence In Direct-Sequence Spread Spectrum Systems

Acquisition of Pseudo-noise signal is an important issue in direct sequence spectrum system (DSSS) and some studies is carried out on this in this dissertation. The research work can be divided into two parts. The first part is focused on the maximum likelihood code timing estimator which is near far resistant in direct-sequence code division multiple access (DS-CDMA) systems with short PN code of symbol length. The second part is focused on the acquisition of PN code and adaptive beamforming weight using antenna array in DSSS systems with long PN code. The main contributions included in this dissertation can be summarized as below:1. The PN code acquisition problem is addressed. Based on the discussion of the sptial-temporal channel model, a modified spatial-temporal channel model is proposed for channel simulation and algorithm evaluation. Some approaches are also introduced to reduce the computation complexity of the simulation model.2. For DS-CDMA wireless communication systems with near-far effect, the large sample maximum likelihood (LSML) approach is extended to the case of multipath fading channel and further to the multi-rate systems. With the idea of CLEAN and RELAX algorithm, the multidimisional optimization problem is simplified as multiple one-dimensional optimization problem to reduce the complexity. The Crame-Rao Bound (CRB) of the extended estimator is derived.3. For DSSS system with antenna array, the spatial-temporal correlator (STC) is proposed for the acquisition of the PN code and adaptive beamforming weight. In the proposed approach the Wiener filter based adaptive beamforming is incorporated with the conventional correlator to acquire the PN code and the adaptive beamforming weight with the help of the pilot signal. Since adaptive beamforming is used to suppress the interferences coming from directions different with the signal, the proposed approach have satisfactory detection performance for low signal-to-noise ratio (SNR). The suboptimum combining and optimum combining is used for spatial white noise and spatial color noise/interference environment, respectively. A modified implementation approach of the STC is proposed to reduce the computation complexity of the optimum combining approach by dividing the received data into multiple blocks, and the computation complexity is significantly reduced at the cost of negligible performance loss. The STC is compared with the code acquisition approach of code-filtering based oneigenvalue decomposition, it is shown that the two approaches are equivalent under special conditions. The STC is extended to the case with data modulation and blind code acquisition is achieved.4. Detection performance of the STC with suboptimum combining is theoretically analyzed and also simulated for DS-CDMA systems in additive white Gaussian noise environment. For DSSS with spatially colored interference, detection performance of the optimum combining approach with Gaussian jamming is theoretically analyzed and also simulated. When optimum combining approach is used, the decision threshold is independent of the SNR and constant false alarm rate (CFAR) is achieved. The effect of the length of the observation interval on the performance analysis is considered. The effect of the statistical distribution of the interference on the detection performance is investigated and the results of the analysis and simulation show that the performance of the STC is not sensitive to the probability distribution of the interference.5. The effect of the carrier frequency error and the angle spread of the received signal on the detection performance of the STC is studied. The impact of the carrier frequency error on the code acquisition with optimum combining and suboptimum combining is analyzed, and the results show that the carrier frequency error only result in negligible detection performance loss and the frequency error has no effect on the false alarm rate of the optimum combining approach. The detection performance of STC with angle spread is derived, and th...