**6. Conclusion**

It was shown earlier that because of the numerous impairments and distortions that take place in Double Selective channels, it is rather "hard" to obtain high values for characteristics, such as noise immunity and spectral efficiency in Doubly Selective Channels which usually appears in 5G and beyond networks for information transmission from HSV terminals.

The material presented above is devoted to two new options for users'separation in hybrid NOMA /OMA schemes related to the power CR-NOMA information transmission over-application of NOMA to improve the spectrum efficiency paradigm in SISO channels. It has been demonstrated that non-linear filtering is effective to separate OMA/NOMA users in hybrid schemes.

Moreover, two possible approaches for SISO channels were proposed for mitigation of the multiuser's interferences, namely a Chaos-based filtering approach and an approach based on the m sequential hypothesis testing. Both, show several attractive invariant features for application over Doubly Selective SISO channels such as high fidelity for multiuser decoding, robustness, and reduced decision time, which allows recommend them not only for Doubly Selective channels but also for the "conventional" channels as well, besides a large number of already reported algorithms for them (see [25–29] and references therein).

Previously published papers, where chaotic filtering is used for other applications [19–21], show that proposed algorithms here can be effectively implemented in hardware.

Proposals for future research

It is worth mentioning the existence of a solid number of unsolved problems for OMA/NOMA separating users, particularly for Doubly Selective channels. They are as follows:

