[1] Pseudorandom sequences are often used in radio systems; however, the nonzero out-of-phase autocorrelation of many binary sequences induces range sidelobes which significantly reduce the echo signal-to-noise ratio (SNR). In this paper, the use of almost perfect sequences, exhibiting zero out-of-phase autocorrelation except one value in the middle is examined with reference to common m sequences and perfect sequence. The ambiguity functions demonstrate that it is possible to use the almost perfect sequences for ranging without sidelobes and that their Doppler measurement performance is similar to m sequence of the same length. This is an important result for ionospheric oblique backscattering detection where the echoes are superposed and where range sidelobes can submerge the main lobes of weak signals. The 124-bit almost perfect sequence and the 127-bit m sequence are applied to the Wuhan Ionospheric Oblique Backscattering Sounding System for sequence testing. The test results have proven that the almost perfect sequence exhibits a higher echo SNR for the same detection conditions.

Bound states in the continuum (BICs) correspond to a particular leaky mode with an infinitely large quality-factor (Q-factor) located within the continuum spectrum. To date, most of the research work reported focuses on the BIC-enhanced light matter interaction due to its extreme near-field confinement. Little attention has been paid to the scattering properties of the BIC mode. In this work, we numerically study the far-field radiation manipulation of BICs by exploring multipole interference. By simply breaking the symmetry of the silicon metasurface, an ideal BIC is converted to a quasi-BIC with a finite Q-factor, which is manifested by the Fano resonance in the transmission spectrum. We found that both the intensity and directionality of the far-field radiation pattern can not only be tuned by the asymmetric parameters but can also experience huge changes around the resonance. Even for the same structure, two quasi-BICs show a different radiation pattern evolution when the asymmetric structure parameter d increases. It can be found that far-field radiation from one BIC evolves from electric-quadrupole-dominant radiation to toroidal-dipole-dominant radiation, whereas the other one shows electric-dipole-like radiation due to the interference of the magnetic dipole and electric quadrupole with the increasing asymmetric parameters. The result may find applications in high-directionality nonlinear optical devices and semiconductor lasers by using a quasi-BIC-based metasurface.

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