Aperiodic Codes Research Articles

Hybrid aperiodic coding for SNR improvement in a BOTDA fiber sensor.

The measurement accuracy of a Brillouin optical time domain analysis (BOTDA) fiber sensor is determined by the signal-to-noise ratio (SNR) of the received sensing signal. Here, a new hybrid aperiodic coding method is proposed to improve the SNR. In the proposed method, two pre-discovered short seed aperiodic codes (SA-codes) are used to construct a new hybrid aperiodic code (HA-code) in a nested way. The HA-code inherits the good denoising capabilities of the two SA-codes and features a high coding gain. In the proof-of-concept experiment, a SNR improvement up to 8 dB is obtained, which improves the measurement certainty to 1.67 MHz over a 117.46 km sensing range under a spatial resolution of 2.6 m.

Genetic-optimised aperiodic code for distributed optical fibre sensors

Distributed optical fibre sensors deliver a map of a physical quantity along an optical fibre, providing a unique solution for health monitoring of targeted structures. Considerable developments over recent years have pushed conventional distributed sensors towards their ultimate performance, while any significant improvement demands a substantial hardware overhead. Here, a technique is proposed, encoding the interrogating light signal by a single-sequence aperiodic code and spatially resolving the fibre information through a fast post-processing. The code sequence is once forever computed by a specifically developed genetic algorithm, enabling a performance enhancement using an unmodified conventional configuration for the sensor. The proposed approach is experimentally demonstrated in Brillouin and Raman based sensors, both outperforming the state-of-the-art. This methodological breakthrough can be readily implemented in existing instruments by only modifying the software, offering a simple and cost-effective upgrade towards higher performance for distributed fibre sensing.

Blind channel estimation in aperiodic time-hopping ultra-wideband communications

Impulse radio ultra-wideband (UWB) communication systems employ a time-hopping technique to enable multiple access. Each user is assigned a distinct time-hopping sequence, either periodic or aperiodic, to suppress multiuser interferences. Aperiodic codes have many promising features compared with periodic codes, such as smoothed spectrum and enhanced security. But they pose additional challenges to channel estimation. This paper studies channel estimation issues in aperiodic time-hopping systems. Both a least squares technique and a correlation matching technique are developed. An instantaneous estimate of the mean or correlation of received signals is exploited for simultaneous estimation of all users' channels based on a pulse-rate discrete-time model. If only the desired user's time-hopping codes are available, single-user channel estimators are developed by utilizing the time-varying property of aperiodic time-hopping codes. Performance of each proposed channel estimator is analyzed and numerically evaluated by computer simulations. This paper shows that the correlation matching approach has better performance while the least squares approach has lower complexity. Matched filter receivers based on proposed channel estimators yield detection performance close to corresponding ones with known channel state information. Significant improvement of detection performance is observed by using modified matched filter receivers or multiuser receivers that explicitly take into account interferences to the desired signal.

Coded pulse excitation for ultrasonic strain imaging

Decorrelation strain noise can be significantly reduced in low echo-signal-to-noise (eSNR) conditions using coded excitation. Large time-bandwidth-product (>30) pulses are transmitted into tissue mimicking phantoms with 2.5-mm diameter inclusions that mimic the elastic properties of breast lesions. We observed a 5-10 dB improvement in eSNR that led to a doubling of the depth of focus for strain images with no reduction of spatial resolution. In high eSNR conditions, coded excitation permits the use of higher carrier frequencies and shorter correlation windows to improve the attainable spatial resolution for strain relative to that obtained with conventional short pulses. This paper summarizes comparative studies of strain imaging in noise-limited conditions obtained by short pulses and four common aperiodic codes (chirp, Barker, suboptimal, and Golay) as a function of attenuation, eSNR and applied strain. Imaging performance is quantified using SNR for displacement (SNRd), local modulation transfer function (LMTF), and contrast-to-noise ratio for strain (CNRepsilon). We found that chirp and Golay codes are the most robust for imaging soft tissue deformation using matched filter decoding. Their superior performance is obtained by balancing the need for low-range lobes, large eSNR improvement, and short-code duration.