This paper investigates several maximum selection and serial search based code acquisition techniques for direct sequence spread spectrum receivers, which utilize either a single or multiple antenna elements. The techniques are studied with different combinations of threshold comparison and verification. Paper includes large number of analytical results calculated in static, fast and slow fading channels to obtain an insight of the performances of these methods. Numerical results will give an overview of the methods relative performance in different channels.

This paper investigates three code acquisition methods for direct sequence spread spectrum systems (DS/SS) utilizing smart antennas. The methods are suitable especially for receivers that consist of a smart antenna followed by a single correlator or a matched filter (IMF). The first method is the known fixed beam strategy where the whole angular uncertainty region is divided into small cells using conventional beamforming techniques. Therein, the receiver searches through all angular and delay cells via a serial search procedure. In the second method, the fixed division is made using advanced beamforming techniques, which provide improved tolerance against interference. In the third strategy, the direction-of-arrival (DOA) of incoming signals are estimated and utilized in the acquisition process. An advantage of this DOA estimation-based strategy, when compared to fixed beamforming methods, is a decrease in the region of uncertainty. Disadvantages are increased computational complexity and signal-to-noise ratio (SNR) limitations. The acquisition strategies are compared when the serial search acquisition of the code phase is made using either the correlator or the matched filter. The results indicate that a single antenna receiver gives the best acquisition performance when SNR is high. However, single antenna methods are sensitive to interference. On the other hand, DOA estimation-based methods offer shorter mean acquisition times than fixed beam methods, especially when the number of arriving signals is small.

Matched filter (MF) time and frequency synchronization method for orthogonal frequency division multiplexing (OFDM) systems is presented. The proposed method uses the same pseudo-noise (PN)-preamble generation as was used in [1], but signal processing in the receiver is different. The performance of the proposed method is investigated using theoretical and simulated synchronization probabilities. The obtained results indicate that the proposed synchronizer attains the optimum theoretical performance.

In this paper, code synchronization probabilities of the direct sequence spread spectrum (DS/SS) system are investigated when the receiver utilizes an adaptive antenna array. Performance studies of three beam forming algorithms in the presence of direction-of-arrival (DOA) errors are presented. The investigated algorithms are the conventional, the minimum variance distortionless response (MVDR), and the MVDR+ADL where the MVDR algorithm is enhanced against DOA error via the adaptive diagonal loading (ADL). The paper includes a large number of analytical and simulation results where the effects of DOA errors are investigated. It can be concluded that the MVDR beam former is more sensitive to DOA errors than the conventional beam former, especially, at large DOA errors and high signal-to-noise-ratio (SNR) values. However, this sensitivity can be reduced notably by using the ADL.

This paper introduces a method used for code synchronization in direct sequence spread spectrum (DS/SS) systems when the receiver is equipped with a smart antenna followed by the matched filter (MF). The presented method first estimates the direction-of-arrival (DOA) of the DS-signal, then performs beamforming, and finally makes synchronization decision based on the maximum output of the MF. Because the amount of data needed in DOA estimation depends on the prevailing signal-to-noise ratio (SNR), the proposed method operates iteratively. The method starts by collecting a piece of data for DOA estimation. Due to the inaccurate DOA estimation, the main beam of the radiation pattern may be steered to erroneous direction and code acquisition may be failed. After a fail more data is collected and the accuracy of DOA estimate improves. This process is repeated until DOA and delay estimation succeeds. The results show that it is advantageous to start by collecting only a small amount of data for DOA estimation because the shortest mean acquisition times are achieved in that case