As mentioned before, traditional ARQ based schemes are not suitable for UASNs, since sender relies on acknowledgments to decide whether to retransmit the packet or not, leading to an increase in end-to-end delay [4].
We show that although content adaptive approach may enhance the stego-security, Luo et al.'s PVD-based scheme is not a good choice for realizing adaptive embedding since it contains a serious design flaw in data embedding procedure and this flaw can lead to possible attacks.
We also believe that the OPPF-based scheme is not the only demodulation scheme for PDMA-PM, and there must be other better schemes which it is still an open issue.
Adaptation of recalibration to a fixed time-based scheme is not substantiated by our data, since "time to last calibration" was neither associated to accuracy nor precision of CIpc.
From the tables, it is clear that even though the difference in work between using DIRECT for the local optimization compared to using a gradient-based scheme is not dramatic, significantly less time is spent in the local optimization phases when the gradientbased methods are used.
However, an extension to an SC-FDE based multiaccess scheme is not obvious and it has been developed only recently, called single-carrier FDMA (SC-FDMA) [4].
This scheme is not based on the so-called "Landau Log" formulation of the operator and ensures the physically relevant long-time behavior of the solution.
This alternative scheme is not based on internal coefficients, but on the transfer functions, cf. Eq. (10), and thus independent of the source.
In both works[9, 11], the decision whether a user is served by an adaptive or non-adaptive access scheme is not done based on analytical calculations.
