Henri Puska

Serial search code acquisition using smart antennas with single correlator or matched filter

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 Time and Frequency Synchronization Method for OFDM Systems using PN-sequence Preambles

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.

Synchronization Probabilities using Conventional and MVDR Beam Forming with DOA Errors

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.

Serial search and maximum selection based code acquisition techniques for single and multi antenna systems

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.

Performance Comparison of DS/SS Code Acquisition using MMSE and MVDR Beamforming in Jamming

This paper investigates code acquisition performance of a direct sequence spread spectrum (DS/SS) system where acquisition is aided either by the minimum mean square error (MMSE) or by the minimum variance distortionless response (MVDR) beamformer. The use of the MMSE beamforming increases receivers complexity because it requires a matched filter (MF) in each antenna branch separately, whereas only a single MF is needed in the MVDR method. However, the MMSE beamformer has an advantage that it does not need a priori information of the direction-of-arrival (DOA) of the desired signal unlike the MVDR and many other advanced beamforming algorithms. The purpose of this paper is to compare acquisition performances of the MMSE and the MVDR beamformers. In addition, performance of a single antenna receiver is used as a reference. Obtained results indicate that the MMSE beamformer has the best mean acquisition time performance, although its performance measured via detection probability is worse than in the MVDR method.

Comparison of antenna array algorithms in DS/SS code acquisition with jamming

This paper compares three different interference suppression methods when the receiver is equipped with an antenna array. The first one is the spatial minimum variance distortionless response (MVDR) beamforming. The second structure is the combination of the spatial MVDR beamforming and the frequency domain forward consecutive mean excision (FCME) interference suppressor. The third strategy is the space-time MVDR beamformer. The purpose is to investigate the advantages of these interference suppressor structures already during the initial code synchronization process. The results show that all the investigated methods are capable of eliminating all investigated interference types if the angle separation between desired and interfering signal is few degrees, i.e., interfering signal arrives outside from the mainlobe. If the interfering signal arrives from the mainlobe then the best performance is achieved using the space-time MVDR beamforming. However, the space-time processing has the largest computational complexity and therefore the spatial beamforming is favorable. The results show also that the use of the FCME algorithm after the spatial beamforming gives additional performance gain if compared to the situation where it is not utilized.

Narrowband main beam interference mitigation algorithms for space-time adaptive processing

We investigate space-time adaptive processing (STAP) algorithms that can mitigate narrowband main beam interference. It will be shown, through simulations for direct sequence code phase estimation, that only a form of three possible Capon beamformer implementations can do the required mitigation. Alternatively, a novel time coherent combining beamformer can be used. Both algorithms have to function in a single sample per symbol or chip mode in order to avoid mutual correlation that decreases the performance.

Comparison of matched filter acquisition using beamforming and CME algorithm in impulsive interference

This paper introduces a receiver structure which can be used to aid code acquisition in direct sequence spread spectrum systems when there are undesired impulsive interferences in the communication channel. Impulses can be suppressed using the consecutive mean excision (CME) algorithm in the time domain. Additional gain against interference is obtained using beamforming prior the code acquisition. Performance comparisons to receivers without the CME algorithm are executed in environments where the direction-of-arrival of impulses is either random or fixed. It is shown that the CME algorithm improves the performance in most of the cases.

Interference mitigation techniques for single and multi antenna receivers

Unintentional or intentional interference may disturb the communication links. The interference may have several characteristics; it may be impulsive or continuous, narrowband or wideband. There exist several methods to mitigate interference including modulation design, channel coding and diversity reception. This paper offers an overview of additional signal processing methods against interference. In particular, these include single antenna methods against impulsive and narrowband interference and adaptive antenna methods, which can mitigate also wideband interference.

Performance comparison of robust array algorithms in delay estimation

Impulsive interference mitigation capabilities of robust array algorithms are considered in delay estimation of a direct sequence spread spectrum (DS/SS) system. The simulations are done in the environment where the direction of arrival (DOA) of individual impulses is either random or fixed. It is shown that the minimum variance distortionless response (MVDR) beamforming algorithm without any interference suppression algorithm is sufficient if impulses are arriving from a fixed DOA. In the random DOA case the performance of the MVDR beamformer algorithm alone is very poor and impulse suppression methods are needed. The paper compares four different impulse suppression methods. It is shown that the best algorithm allow over 60% of snapshots to be corrupted.