2 research outputs found
Fast antijamming timing acquisition using multilayer synchronization sequence
Pseudonoise (PN) sequences are widely used as preamble sequences to establish timing synchronization in military wireless communication systems. At the receiver, searching and detection techniques, such as the full parallel search (FPS) and the serial search (SS), are usually adopted to acquire correct timing position. However, the synchronization sequence has to be very long to combat jamming that reduces the signal-to-noise ratio (SNR) to an extremely low level. In this adverse scenario, the FPS scheme becomes too complex to implement, whereas the SS method suffers from the drawback of long mean acquisition time (MAT). In this paper, a fast timing acquisition method is proposed, using the multilayer synchronization sequence based on cyclical codes. Specifically, the transmitted preamble is the Kronecker product of Bose–Chaudhuri-Hocquenghem (BCH) codewords and PN sequences. At the receiver, the cyclical nature of BCH codes is exploited to test only a part of the entire sequence, resulting in shorter acquisition time. The algorithm is evaluated using the metrics of MAT and detection probability (DP). Theoretical expressions of MAT and DP are derived from the constant false-alarm rate (CFAR) criterion. Theoretical analysis and simulation results show that our proposed scheme dramatically reduces the acquisition time while achieving similar DP performance and maintaining a reasonably low real-time hardware implementation complexity, in comparison with the SS schem
Scintillation on global navigation satellite signals and its mitigation
PhD ThesisThe scintillation effects on the Global Positioning system (GPS) or other GNSS (global
navigation satellite system) receivers have been investigated by many researchers and
several mitigation strategies have been proposed in this regard but the problem is not yet
fully solved. This thesis covers the investigation of scintillation effects on GPS receivers
and developing a mitigation approach which can play an important role in mitigating the
effects of scintillation on these and other GNSS receivers.
Firstly, a new GPS signal acquisition method known as the repetitive block acquisition
(RBA) is presented which can be used to speed up the GPS signal acquisition in case fast
acquisition is required. This acquisition method is implemented using coarse-acquisition
(C/A) codes and tested by collecting real GPS data. The RBA method can also be used for
other codes as well. It is rather difficult to show that how scintillation affects the acquisition
process in a GPS receiver because mostly it results in tracking loop loss of lock due to
cycle slip. However, during strong amplitude scintillation which is usually most important
at low or near-equatorial latitudes, deep power fades resulting from amplitude scintillation
result in the selection of long data records which leads to slow acquisition due to long
acquisition times. It is shown in this thesis that, by using the RBA method, the acquisition
time can be reduced to a fairly low level by reducing the number of computations involved
in acquisition compared to other well-known methods such as the parallel FFT-based
method and zero padding method (ZP).
Secondly, the scintillation effects on the GPS tracking loop have also been investigated
in this thesis and, based on this investigation, a new improved analogous phase scintillation
index, σw
φa, has been designed to more accurately represent the phase scintillation intensity
at European high latitudes. This is then also validated using the real GPS data from
Trondheim, Norway (63.41o N, 10.4o E). The σw
φa uses dual frequency (L1 & L2) based
vi
time and spatial variations of total electron contents (TEC) at 1 Hz for estimating the
phase scintillation values. For deriving the σw
φa, the low frequency TEC fluctuations
due to Doppler shift of the satellite/receiver motion and also due to the slowly varying
background ionosphere need to be removed in order to consider only the high frequency
TEC fluctuations which are responsible for scintillation due to the fast moving electron
density irregularities which is done by using the wavelet transform. The σw
φa is really
an improved version of σφa where, rather than using time-invariant digital high pass
filters (HPF), which according to several researchers are in-appropriate for filtering the
non-stationary raw GPS signals affected by the ionospheric scintillation, a wavelet-based
filtering technique is used. Although, the wavelet transform has been used previously in
detrending raw amplitude and phase observations at 50 Hz for estimating the scintillation
indices (amplitude and phase), due to the high sample data rate it may not be desirable to
use this transform due to its very high computational cost. Since, σw
φa uses TEC data at 1
Hz so this problem has been overcome. The performance of the new improved index (σw
φa)
is investigated and is also compared with the previously proposed σφa and σφ indices using
one whole year of data from a GPS receiver at Trondheim, Norway (63.41o N, 10.4o E).
The raw TEC observations and the σw
φa index are then used in estimating the tracking
phase jitter using two different methods. The phase jitter helps in defining the tracking
thresholds for the tracking loops in a receiver which is useful in updating the tracking loop
parameters during scintillation conditions as required in robust GPS/GNSS receiver designs
because the phase jitter decides how wide the tracking (and thus the noise) bandwidth
should be allowed in the tracking loop for the tracking to remain efficient. It is shown
that if the phase jitter is estimated using the new proposed methods, generally a better
estimate can be obtained compared to the previously proposed phase jitter estimation
methods which employs σφa and σφ indices. These new phase jitter estimation methods
can further be used in GPS/GNSS receivers for updating the tracking loop parameters
during scintillation conditions and hence can serve as a good alternative for mitigating the
effects of scintillation on GPS/GNSS receivers.Higher Education
Commission (HEC) of Pakistan and the Sukkur Institute of Business Administration,
Pakistan