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In this paper, blind signal classification and detection in a non-orthogonal multiple access (NOMA) system are explored. Since a NOMA scheme superposes the multiple-user signals within nonorthogonal resources, classical modulation classification methods used in orthogonal multiple access (OMA) systems are not sufficient to process the superposed NOMA signal. NOMA receivers require information about the multiple access schemes such as modulation order and need interference cancellation of the co-scheduled user's signal; therefore, a NOMA system causes more high-layer signaling overheads than OMA during packet scheduling. Blind detection algorithms used for multiplexing information are considered to be possible solutions; however, they pose various challenges and could cause performance loss while performing blind modulation classification in the order of OMA/NOMA classification, co-scheduled user's modulation classification, and classification of the signal, due to the necessity for successive interference cancellation. To improve the performance of blind detection, we propose a NOMA transmission scheme that applies phase rotation to data or pilot symbols depending on the NOMA multiplexing format, as an aid to the blind detection. The proposed classification algorithm can implicitly provide essential information on NOMA multiplexing without the need for any extra high layer signaling or resources. The performance improvement is verified through simulation studies, and it is found that the proposed algorithm provides a gain of more than 1 dB compared to the existing blind signal classification methods and shows almost equivalent performance as the genie information scheme. [ABSTRACT FROM AUTHOR]

In this correspondence, an uplink nonorthogonal multiple access (NOMA) system where the base station (BS) trends to decode the independent information of users occupying the same radio resource simultaneously is considered. Different from the traditional successive interference cancellation (SIC) decoding scheme where the later decoding can only be processed after the successful decoding of the previous users, an advanced SIC decoding strategy is proposed to achieve outage balancing for the later decoded users. Under the proposed SIC decoding mechanism, the BS regards the unsuccessfully decoded users’ information as the interference on the following users, i.e., even the outage occurs for any of the previous decoded users, the BS will still try to decode the following users’ information. The outage performance with the new decoding strategy is theoretically analyzed. The closed-form expressions of the outage probability for each user are derived for an uplink 2-user NOMA system. The accuracy of the theoretical results is verified by the simulations. [ABSTRACT FROM AUTHOR]