Opportunistic semantic and bit communications in uplink NOMA

A novel opportunistic semantic and bit communication strategy is proposed for uplink non-orthogonal multiple access (NOMA). Specifically, a secondary far user (F-user) employs either semantic communication (SemCom) or bit-based communication (BitCom) to participate in NOMA with a primary near user (N-user) employing the BitCom. For each fading channel state, the secondary F-user has to select the most suitable communication method, thus striking a good tradeoff between its own achieved performance and the interference imposed on the primary N-user. The optimal communication policy at the F-user over fading channels is derived for maximizing the ergodic (equivalent) semantic rate achieved at the F-user, subject to the minimum ergodic bit rate constraint of the N-user. Numerical results show that the proposed opportunistic scheme can achieve higher communication performance for NOMA than the baseline schemes merely employing SemCom or BitCom. In addition, SemCom can better guarantee the performance of the F-user admitted in NOMA than BitCom when the communication requirement of the primary N-user is high.