# Joint Power and Time Allocation in Hybrid NoMA/OMA IoT Networks for Two-Way Communications
## 1 Introduction  
Non-orthogonal multiple access (NoMA) allows simultaneous transmission for multiple users on the same frequency via power domain superimposed coding, thereby achieving improved spectrum efficiency as compared to conventional orthogonal multiple access (OMA), such as time division multiple access (TDMA) [1, 2]. Since its advent, NoMA has appeared in a variety of communication scenarios from Sub-6 GHz communications, ground networks, and cognitive radio networks, to mmWave and terahertz communications, space-air networks, and energy harvesting communication networks [3, 4, 5]. Although the investigation on 6G is still in its infancy, NoMA, in addition to multiple input multiple output (MIMO), has been recognized as a potential enabling technique for the future 6G networks, and therefore the two techniques of NOMA and MIMO will work jointly in the 6G networks [6, 7]. As a branch of NoMA-based communication networks, NoMA-based wireless access networks are the most widely used. Till now, most study on the NoMA based wireless access networks has been focused on one-way transmissions wherein the uplink (UL) and downlink (DL) are separated considered [8]. For separated design on the UL and DL, it is difficult to make full use of available transmission resources to fulfill the network-wide performance optimum. On this account, some works studied relay aided two-way transmissions for wireless powered relaying networks and NoMA-based wireless networks [9, 10, 11, 12]. From the performance analysis perspective, one investigation [9] studied relay aided two-way information exchange between two groups of users. Within the cooperative NoMAframework and from the optimization perspective, the authors of [10] investigated two-way communications between a pair of users, and the authors of [11] researched two-way communications between a base station and several cellular edge users. For two-way communications, the above-mentioned works considered equal-length UL and DL transmission time, and for multiuser access, both the UL and DL used NoMA; and for system design, transmit power is the only resource that can be optimized. In some low-cost Internet of Things (IoT) networks, traffic levels on the UL and DL are asymmetric, and the traffic on one link may be heavier than on the other [13]. For this case, a time division duplex (TDD) with equal-length transmission duration cannot adapt to the asymmetric traffic. On the other hand, NoMA receiver involves complex multiuser detection (MUD), and it is a challenge for low-cost IoT terminals/devices (IoDs) to cope with the complex MUD, and thus, there may be difficulties for the DL to adopt NoMA in the low-cost IoT applications [14]. The computational complexity for the receiver performing MUD increases exponentially with the number of NoMA users, and thus the cost of the NoMA performance improvement is the soaring receiver complexity. For performance and complexity compromise, some works resort to hybrid NoMA/OMA for reduced NoMA size, in which users are organized into multiple groups so that NoMA is used in each group, and OMA is applied across different groups [15; 16; 17; 18; 19]. We specifically highlight that these studies on hybrid NoMA/OMA only considered one-way transmissions. For two-way communications in low-cost IoT applications, this article proposes a scalable TDD protocol that combines NoMA and TDMA. In the UL, spectrum-efficient NoMA is used relying on the access point (AP)'s powerful processing capability, and in the DL, TDMA is adopted to facilitate signal detection of the IoDs with weak processing capability. For the system design, we jointly optimized the transmission time on the UL and DL, the transmission time for the DL TDMA slots, and the transmit power for the IoDs in both UL and DL directions.