Seung-Hyun Kong

A Deterministic Compressed GNSS Acquisition Technique

In the cold start of a Global Navigation Satellite Systems (GNSS) receiver, fast acquisition of the GNSS signal requires either an extensive usage of hardware resources for massive parallel correlators or a high computational complexity for fast Fourier transform (FFT) and inverse FFT operations. Because GNSS uses direct-sequence spread spectrum (DSSS) signaling with binary phase-shift keying (BPSK) or with BPSK and binary offset carrier, any GNSS signal can have a sparse representation so that the concept of compressed sensing can be applied to detect GNSS signals. To achieve a fast acquisition of the GNSS signal with a reduced number of correlators and low computational complexity, we propose a two-stage deterministic compressed GNSS acquisition technique using the Walsh-Hadamard matrix. The proposed technique makes fast acquisition possible for a receiver using a much smaller number of correlators than the conventional parallel-correlator-based technique, which requires much less computational complexity than the FFT-based technique. We provide complexity analysis of the proposed technique and compare the statistical performance of the proposed technique with other techniques applicable to the fast GNSS acquisition. The proposed technique is easy to implement and is the first compressed-sensing-based GNSS acquisition technique.

Statistical Analysis of Urban GPS Multipaths and Pseudo-Range Measurement Errors

In urban environments one of the causes of pseudo-range measurement error in Global Positioning System (GPS) is short-delay multipaths due to the scatterers around the receiver. Therefore knowledge of the temporal distribution of GPS multipaths based on a statistical scatterer distribution in an urban environment is essential to estimating the positioning performance and to developing an efficient multipath mitigation technique for urban GPS applications. The work presented here introduces a scatterer distribution model for the urban environment, derives analytical expressions of the consequent time-of-arrival (TOA) probability density function (pdf) with respect to satellite elevation angles, and analyzes the effect of short-delay multipaths on the pseudo-range measurement errors. The expressions derived provide insights into the statistical properties of GPS multipaths and pseudo-range measurement errors in GPS code phase measurement functions due to the short-delay multipaths.

Low Computational Enhancement of STFT-Based Parameter Estimation

Short-time Fourier transform (STFT) is one of the most widely used tools to analyze frequency and phase of local sections of time-varying signals using a time window function. Since the fixed window size of STFT causes limitation in time-frequency representation (TFR), adaptive STFT (ASTFT) techniques have been studied to adjust the window size depending on the local signal characteristics. However, ASTFT techniques suffer from heavy computational complexity in general. In this paper, we propose low computational enhanced temporal and spectral parameter estimation techniques for STFT output of radar signals; forward and backward STFT enhancement techniques. The proposed forward STFT enhancement technique is to enhance the spectral resolution of a received radar signal by multiple times using a number of successive STFT outputs without applying additional STFT with wider time window. On the other hand, the proposed backward STFT enhancement technique is to improve the temporal resolution of a received radar signal using an STFT output with a wide time window, without applying multiple STFTs with smaller time window. The performance of the proposed techniques are theoretically analyzed, and the advantage of the proposed techniques to the conventional techniques in terms of computational complexity is demonstrated with numerous simulations.

Two-Dimensional Compressed Correlator for Fast PN Code Acquisition

Acquisition of an incoming long pseudo-noise (PN) code signal requires a fast hypothesis testing function for a large number of code phase hypotheses. In addition, when a transmitter is moving at a high speed, the hypothesis testing function needs to search for the signal in a 2-Dimensional (2D) search space that includes all possible combinations of code phase hypothesis and Doppler frequency hypothesis. Since a receiver has limited hardware resources (in terms of number of correlators and computational capacity) in practice, fast PN code acquisition is not an easy goal to achieve. In this paper, we propose a double dwell search scheme, where a 2D compressed correlator (TDCC) tests a number of coherently combined neighboring code phase hypotheses and Doppler frequency hypotheses at a time in the 1st dwell search, and all individual neighboring hypotheses found in the 1st dwell search are tested in the 2nd dwell search using a conventional correlator. We present theoretical performance analysis of the proposed technique and Monte Carlo simulation results to demonstrate the performance of the proposed technique and to compare it to the conventional double dwell search technique.

Two-Dimensional Compressed Correlator for Fast Acquisition of Signals

The long spreading code and the binary offset carrier (BOC) modulation technique are used in the next-generation Global Navigation Satellite System (GNSS) to improve positioning performance and to reduce inter-GNSS interference; however, the signal acquisition process of a GNSS receiver can take more time and requires additional hardware resources than legacy Global Positioning System (GPS) receivers. This paper presents the BOC 2-D compressed correlator (TDCC) technique for the fast acquisition of the next-generation GNSS signals. In the proposed BOC-TDCC, signal power in neighboring code-phase hypotheses and Doppler frequency hypotheses can be coherently combined and tested by a single compressed correlator in the first stage, and the conventional correlator-based serial search technique is employed in the second stage to test each hypothesis combined in the first stage. The performance of the BOC-TDCC is demonstrated with numerous Monte Carlo simulations and tested with real data. The BOC-TDCC has much lower mean acquisition time (MAT) than other conventional search schemes, which demonstrates that the BOC-TDCC is an effective search scheme for next-generation GNSS signals.

Determination of Detection Parameters on TDCC Performance

Due to the longer PRN code employed in next-generation GNSS (Global Navigation Satellite System), receivers in the signal acquisition process should test a larger number of code phase hypotheses. Recently, to reduce acquisition time and computational complexity, TDCC (Two-dimensional Compressed Correlator) is introduced, and this technique is different from the conventional double dwell search technique in terms of parameters used to reduce acquisition time and computational complexity. Studies for optimizing the performance of TDCC have not been introduced yet, therefore, in this paper, detection thresholds of TDCC are investigated to optimize its acquisition performance. The optimal detection thresholds minimizing the MAT (Mean Acquisition Time) and MAC (Mean Acquisition Computation) are numerically evaluated and analyzed for widely used detection strategies in its serial and parallel search schemes. It is demonstrated that the minimized MAT and MAC are much smaller than the MAT and MAC using a detection threshold based on CFAR (Constant False Alarm Rate).

Least-Squares-Based Iterative Multipath Super-Resolution Technique

In this paper, we investigate the multipath resolution problem for direct sequence spread spectrum signals. To resolve multipath components arriving within a very short interval, we propose a new multipath super-resolution algorithm based on the iterative least-squares method. The proposed least-squares-based iterative multipath super-resolution (LIMS) algorithm exploits a triangular shaped auto-correlation function (ACF) of the pseudo-noise (PN) sequence and simplifies the least-squares parameter estimation procedure using iterative and algebraic operations. This results in an algorithm demanding low computational load with a high multipath resolution capability. It is also discussed that the LIMS algorithm can be applied for recursive multipath tracking of source localization systems, such as the global navigation satellite systems (GNSS). Simulation results show that the LIMS algorithm maintains its good performance even in a low [(C)/(N0)] or severe multipath interference conditions.

Fast Multi-Satellite ML Acquisition for A-GPS

Successful position fix in harsh environments such as indoors and dense urban canyons is a strongly required capability for an assisted global positioning system (A-GPS) receiver. In recently developed cellular networks, receiving fine time assistance and maintaining high-frequency accuracy using downlink measurements are not possible for A-GPS receivers, since node-Bs are asynchronous and are not equipped with a source for precise time and frequency. In this paper, we propose a correlator-based fast multi-satellite maximum likelihood (MSML) algorithm, for A-GPS receivers in asynchronous networks, that achieves fast acquisition utilizing fast computation techniques. From numerous Monte Carlo simulations, it is demonstrated that the proposed fast MSML algorithm, when compared with conventional correlator-based acquisition techniques used in standalone GPS and A-GPS receivers, provides higher detection sensitivity for weak signals in the presence of other strong signals by removing strong inter-satellite interference (ISI).

A Novel Indoor Positioning Technique Using Magnetic Fingerprint Difference

Utilizing the pattern of magnetic field intensity (MFI) variation over a distance as a fingerprint for indoor positioning of mobile devices has gained increasingly wide attention. However, pattern matching with a large fingerprint is computationally expensive, and magnetometers are vulnerable to ferromagnetic perturbations and prone to have unknown magnetic offset (MO) in MFI measurements, which may result in large positioning error. In addition, the time-varying attitude of mobile devices and unreliable attitude determination with power consuming gyroscopes in mobile devices can render the 3-D MFI measurement useless for indoor positioning. In this paper, we propose a low power consuming and computationally efficient indoor positioning technique using the difference in 2-D (horizontal and vertical) MFI measurements that are collected over a distance, where 3-D accelerometers are used to find the vertical direction, and the MFI measurement difference is used to enhance robustness to unknown MO fixed in the body frame but time varying due to user dynamics in the navigation frame. The analysis of real data verifies that the proposed technique affords superior positioning performance relative to the conventional magnetic fingerprint techniques and achieves a similar performance to the conventional magnetic fingerprint techniques with MO-free measurements.

Design of FFT-Based TDCC for GNSS Acquisition

Due to the longer spreading code used for the next generation GNSS (Global Navigation Satellite System) signals, receivers have to spend longer time or require larger amount of hardware resources for signal acquisition. Since many recent GNSS receivers use DSP (Digital Signal Processor) to realize parallel signal acquisition scheme in the frequency domain, this paper proposes FFT-based TDCC (Two-Dimensional Compressed Correlator) with which coherently compressed hypotheses are tested using a reduced number of IFFT points in the 1st stage, and the individual hypotheses that construct the compressed hypothesis found in the 1st stage are tested using the conventional parallel search scheme in the 2nd stage. The performance of the proposed technique is demonstrated with numerous Monte Carlo simulations and a comparison to the conventional FFT-based search technique is provided. The results show that the proposed technique requires lower computation and has lower mean acquisition time than the conventional FFT-based search technique for moderate and high C/N0 (Carrier-to-Noise Density Ratio) GNSS signals.