In this paper, we propose a novel system architecture to realize OFDMA in ad hoc networks. A partial time synchronization strategy is presented based on the proposed system model. This proposed scheme can support concurrent transmission without global clock synchronization. We also propose a null subcarrier based frequency synchronization scheme to estimate and compensate frequency offsets in a multiple user environment. The simulation results show a good performance of our proposed synchronization scheme in terms of frequency offset estimation error and variance.Index Terms-Ad hoc networks, OFDMA, time and frequency synchronization.

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The ultra-short-period (USP) planets are exoplanets with very short orbital periods (P < 1 day), and TOI-1807b is one such planet recently discovered by the TESS mission where it orbits in the TOI-1807 system that is still little known nowadays. In this paper, we re-analyzed the transit light curves of TOI-1807 using the latest TESS data from Sector 49, combined with previous data from Sector 22 and 23. By running the MCMC simulation through all three sectors, we found that our transit model fits the data from Sector 49 the best, and we deduced that TOI-1807b is a Super-Earth with a mass of 2.27 +0.49 −0.58 M ⊕ , a radius of 1.37 +0.10 −0.09 R ⊕ , a density of 0.875 +0.264 −0.285 ρ ⊕ , and a surface temperature of 1499 +82 −129 K. We confirmed that TOI-1807b orbits at approximately 0.0135 +0.0013 −0.0022 AU with a period of 0.54929 +0.00012 −0.00005 days, which raises the possibility of the planet being tidally locked due to spin-orbit synchronization. In addition, we renewed an estimate for the conjunction time as 2651.98224 +0.00112 −0.00064 BTJD. We suggest that TOI-1807b might slowly undergo its orbital decay process, and we further identify that TOI-1807b is in a circular, synchronous orbit and permanently deformed due to tides, leading to ∼4% correction in density. Since TOI-1807 is such a young star with an age of only 300 ± 80 Myr, we also imply that the radiation emitted from active TOI-1807 could be so intense that it might have destroyed most of the atmosphere over the surface of TOI-1807b.

Accretion onto a blackhole can indue extreme radiation stimulating emission lines in nearby gases. In the deep gravitational well, these clouds move at speeds up to 10,000 km/s, and so broadens the emission lines via the relativistic Doppler Effect. These broad emission lines on spectra are the most prominent feature of AGNs. The region around central black hole, where such broad emission lines are produced, is known as the broad-line region (BLR). We developed a self-consistent model to describe both the geometry of the BLR and the dynamics of the gas clouds in the BLR. The motion of clouds is described by a series of Keplerian orbits. The spectral shift of emission line can be derived by the radial velocity at each point in Keplerian orbits. Assuming that the BLR is stable, that is, the structure and distribution of gas in BLR doesn’t change over a considerable period of time, the accumulated emission line profile can be constructed by simple addition of all Keplerian orbits in the BLR. By comparing the calculated profiles to spectra observed in SDSS (Sloan Digital Sky Survey), we found that our model can satisfactorily match most observed profiles. We attempted to review and summarize potential binary AGNs. We applied the model to the study of binary AGNs and their profiles of emission lines. We considered the velocity offset due to mutual Keplerian motion of black holes in binary and added their respective profiles to produce the final, resulting emission line profile from binary AGNs. Three kinds of emission line profiles are featured in our simulations: twin-peak structures where two strong broad-lines are present; sub-peak structures where a smaller but still visible emission line accompanies the dominant line; single broad emission peak but with significant velocity offset from the systematic redshift that is derived from the narrow emission-lines on the same spectrum. We then analyzed 1,348 AGN spectra with high S/N ratio in SDSS database, and selected 26 as candidate binary supermassive blackhole systems. These candidates are valuable for following-up observation, their binary nature could be confirmed by detecting the profile variation due to orbital motions.

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