904 research outputs found
TS-MUWSN: Time synchronization for mobile underwater sensor networks
Time synchronization is an important, yet challenging, problem in underwater sensor networks (UWSNs). This challenge can be attributed to: 1) messaging timestamping; 2) node mobility; and 3) Doppler scale effect. To mitigate these problems, we present an acoustic-based time-synchronization algorithm for UWSN, where we compare several message time-stamping algorithms in addition to different Doppler scale estimators. A synchronization system is based on a bidirectional message exchange between a reference node and a slave one, which has to be synchronized. Therefore, we take as reference the DA-Sync-like protocol (Liu et al., 2014), which takes into account node's movement by using first-order kinematic equations, which refine Doppler scale factor estimation accuracy, and result in better synchronization performance. In our study, we propose to modify both time-stamping and Doppler scale estimation procedures. Besides simulation, we also perform real tests in controlled underwater communication in a water test tank and a shallow-water test in the Mediterranean Sea.Peer ReviewedPostprint (author's final draft
Degrees of Freedom and Achievable Rate of Wide-Band Multi-cell Multiple Access Channels With No CSIT
This paper considers a -cell multiple access channel with inter-symbol
interference. The primary finding of this paper is that, without instantaneous
channel state information at the transmitters (CSIT), the sum
degrees-of-freedom (DoF) of the considered channel is
with when the number of users per cell is sufficiently large,
where is the ratio of the maximum channel-impulse-response (CIR) length
of desired links to that of interfering links in each cell. Our finding implies
that even without instantaneous CSIT, \textit{interference-free DoF per cell}
is achievable as approaches infinity with a sufficiently large number
of users per cell. This achievability is shown by a blind interference
management method that exploits the relativity in delay spreads between desired
and interfering links. In this method, all inter-cell-interference signals are
aligned to the same direction by using a discrete-Fourier-transform-based
precoding with cyclic prefix that only depends on the number of CIR taps. Using
this method, we also characterize the achievable sum rate of the considered
channel, in a closed-form expression.Comment: Submitted to IEEE Transactions on Communication
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