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We propose a novel semi-centralized framework for Internet-of-Things (IoT) networks with non-orthogonal multiple access to maximize the energy efficiency (EE) of two types of clients, namely grant-based (GB) and grant-free (GF). We use a proximal policy optimization algorithm to maximize the EE of GB clients and a multi-agent deep Q-network to optimize resource allocation for GF clients aided by a gateway node. The proposed algorithm combines the advantages of fully centralized and fully distributed frameworks to compensate for their shortcomings (complexity and long learning time). The numerical results show that the proposed algorithm enhances the EE of GB clients by 6% and 11.5%, respectively, compared with the fixed power allocation and random power allocation strategies. Moreover, the results demonstrate a 47.4% increase in the EE of GF clients over the benchmark scheme. Additionally, we show …