Laura Camoriano

Impact of GPS acquisition strategy on decision probabilities

The first stage of processing within a GPS receiver consists of the signal acquisition process, the output of which provides a rough estimation of code delay and Doppler frequency. The strong dependence of the acquisition performance on the decision strategy is shown, establishing the role of decision probabilities. Three acquisition strategies are analyzed and a new model describing the performance of a hybrid acquisition system is developed. The theoretical models are validated by simulations, and secondary phenomena, generally neglected in the literature, are also discussed.

Time-Frequency Excision for GNSS Applications

Interference detection and mitigation in global navigation satellite systems (GNSSs) are important issues for both military and civilian applications. In this paper, a novel time-frequency algorithm for GNSS application is proposed. The use of infinite impulse-response notch filters for the interference excision is introduced and analytical formulas for the detection of the disturbing signals are derived. The proposed method is tested by simulations and compared with time-frequency excision algorithms reported in literature, proving its effectiveness for interference removal.

Two-Pole and Multi-Pole Notch Filters: A Computationally Effective Solution for GNSS Interference Detection and Mitigation

In a global navigation satellite system (GNSS) receiver, the presence of detection and mitigation units, capable of reducing the impact of disturbing signals, can extremely enhance the position accuracy. However, the presence of such units is usually limited to professional receivers that dispose of additional computational power that can be used for interference detection and mitigation. In this paper, the two-pole notch filter, that is the natural extension of the one-pole notch filter, is proposed as computationally effective solution for interference detection and mitigation. The notch filter structure and the adaptive algorithm employed for tracking the disturbing signal are analyzed, and an interference detection unit, based on the adaptive algorithm convergence, is proposed. The two-pole notch filter coupled with the detection unit is used as elementary block for the design of a multi-pole notch filter that can efficiently mitigate more than one CW interference. Theoretical and simulative analyses show the feasibility and the good performance of the proposed method.

Impact of the Acquisition Searching Strategy on the Detection and False Alarm Probabilities in a CDMA Receiver

Fine delay and Doppler frequency estimation is a stringent problem of all block-based transmission, such as Orthogonal Frequency Division Multiplexing (OFDM) and Code Division Multiple Access (CDMA). In particular the first stage of each CDMA system, in presence of relevant Doppler shift, consists in a bi-dimensional search leading to a raw estimation of code delay and Doppler frequency. This stage is called acquisition and results extremely critical, both because of the catastrophic consequences of an erroneous estimation and because of the required high computational load. For this reason different acquisition strategies have been developed in order to increase the system performance and reduce the computational complexity. This paper studies the performance of three widely used acquisition strategies. The concept of system probabilities is introduced and used as comparison term. The case of Global Navigation Satellite System (GNSS) is considered as a critical application of CDMA modulation, nevertheless the results are general and apply to all CDMA systems

DTFT-based frequency lock loop for GNSS applications

A frequency lock loop (FLL) for Global Navigation Satellite System (GNSS) applications is described here. The core element is the discrete time Fourier transform (DTFT), that leads to a maximum likelihood (ML) frequency estimation and can be conveniently implemented in a software defined radio (SDR) HW platform. The algorithm is based on the iterative evaluation of DTFTs in a single frequency until the incoming frequency is locked.

Beamforming and synchronization algorithms integration for OFDM HAP-based communications

High Altitude Platforms (HAPs) are currently experiencing a great interest as potential solutions for the growing demand of wireless broadband applications, but their rapid motion and the characteristics of the stratospheric channel, which may cause strong attenuation, multipath fading and Doppler shift, can strongly impact their performances. The described scenario requires the use of beamforming techniques, able to provide medium to high gains in a mobile environment. Given these premises, this paper discusses the conjugation of OFDM techniques with smart antennas for HAP-based communications, stressing the need for integration of the different functional blocks of an OFDM receiver. In fact, in order to face a time varying scenario, adaptive techniques are needed, and only a careful joint design of time and frequency synchronization with beamforming algorithms can guarantee reliable communication. The detailed and integrated receiver design described in the paper is based on the OFDM version of the IEEE 802.16a transmission standard.

A Time-Frequency Excisor for GNSS Interference

In this paper the use of time-frequency representations for interference mitigation is discussed. In particular an excision algorithm is proposed and applied to the context of GNSS receivers. The algorithm is tested under different working conditions and its performances are assessed by means of the Receiver Operative Characteristics (ROC). The method is effective for interference removal and outperforms past algorithms already present in the literature.