Optimum Solutions for Weighted Sum-Rate of NOMA and TDMA in Wireless-Powered IoT Networks

Wireless-powered Internet of Things (IoT) systems allow small IoT devices to operate without accompanying dedicated power sources. A well-known protocol for such networks utilizes the harvest and then transmit concept which involves wireless energy transfer (WET) followed by wireless information transfer (WIT). We formulate two optimization problems for wireless-powered IoT systems to maximize the weighted sum-rate for time-division multiple access (TDMA) and nonorthogonal multiple access (NOMA) by optimizing the harvesting time and transmission time variables. First, we derive semi-closed form solutions, which achieve the global optimum, for these two problems. The proposed approach is highly computationally efficient for large IoT networks. We prove that the scalar equations in both TDMA and NOMA maintain a unique solution which can be found via bisection. It is revealed that when the device's circuit power consumption is negligible, NOMA outperforms TDMA. However, when devices consume large circuit power, TDMA is more efficient than NOMA. Numerical results determine a critical point where NOMA surpasses TDMA in weighted sum-rate if plotted versus WET transmit power. The critical point depends on the WET power, device's circuit power consumption, conversion efficiency and saturation level of the (non)linear energy harvester, and finally the number of devices and their associated weights.