Spectral and Energy Efficient Wireless Powered IoT Networks: NOMA or TDMA? (1801.09109v1)

Abstract: Wireless powered communication networks (WPCNs), where multiple energy-limited devices first harvest energy in the downlink and then transmit information in the uplink, have been envisioned as a promising solution for the future Internet-of-Things (IoT). Meanwhile, non-orthogonal multiple access (NOMA) has been proposed to improve the system spectral efficiency (SE) of the fifth-generation (5G) networks by allowing concurrent transmissions of multiple users in the same spectrum. As such, NOMA has been recently considered for the uplink of WPCNs based IoT networks with a massive number of devices. However, simultaneous transmissions in NOMA may also incur more transmit energy consumption as well as circuit energy consumption in practice which is critical for energy constrained IoT devices. As a result, compared to orthogonal multiple access schemes such as time-division multiple access (TDMA), whether the SE can be improved and/or the total energy consumption can be reduced with NOMA in such a scenario still remains unknown. To answer this question, we first derive the optimal time allocations for maximizing the SE of a TDMA-based WPCN (T-WPCN) and a NOMA-based WPCN (N-WPCN), respectively. Subsequently, we analyze the total energy consumption as well as the maximum SE achieved by these two networks. Surprisingly, it is found that N-WPCN not only consumes more energy, but also is less spectral efficient than T-WPCN. Simulation results verify our theoretical findings and unveil the fundamental performance bottleneck, i.e., "worst user bottleneck problem", in multiuser NOMA systems.

• The paper analyzes spectral and energy efficiency of NOMA versus TDMA in wireless powered IoT networks under a "harvest and transmit" protocol.
• Contrary to some assumptions, NOMA demonstrates higher energy consumption and lower spectral efficiency compared to TDMA in this specific WPCN setup.
• Higher energy demands from simultaneous access and circuit power make NOMA less suitable than TDMA for energy-sensitive wireless powered IoT environments.

Spectral and Energy Efficiency in Wireless Powered IoT Networks: A Comparison of NOMA and TDMA

The paper under review explores the intricacies of achieving spectral and energy efficiency in wireless powered communication networks (WPCNs), which are envisaged as pivotal components of future Internet-of-Things (IoT) infrastructures. The central focus of this research is to evaluate and compare the efficacy of non-orthogonal multiple access (NOMA) and time-division multiple access (TDMA) as communication schemes in the uplink of WPCNs, particularly in scenarios involving a vast number of IoT devices.

The paper outlines the conventional challenges posed by battery-based solutions in sustaining large numbers of IoT devices, citing the prohibitive costs and environmental concerns associated with battery replacement. To address these issues, the adoption of wireless power transfer (WPT) is proposed, where devices harvest energy from ambient radio frequency signals. The efficacy of WPT, however, remains limited by signal attenuation characteristics, necessitating effective energy utilization protocols.

Two primary schemes—TDMA and NOMA—are scrutinized within the context of a "harvest and then transmit" protocol. For both, the early research promises improved spectral efficiency (SE); however, this paper specifically questions the overall energy efficiency when NOMA is implemented, considering device-level energy constraints.

The authors formalize a mathematical model to determine the optimal time allocations aimed at maximizing SE for both TDMA-based (T-WPCN) and NOMA-based WPCNs (N-WPCN). The analysis reveals that contrary to traditional assumptions, N-WPCN exhibits higher energy consumption and lower spectral efficiency compared to T-WPCN. This outcome arises from the inherent energy demands of NOMA's simultaneous multi-user access method, which results in higher circuit and transmit energy consumption. Particularly, the "worst user bottleneck problem" in NOMA systems is identified, where the user with the lowest channel performance dictates the system efficiency, leading to suboptimal usage of resources.

Theoretical results are supported through simulations, affirming that NOMA configurations demand longer downlink energy transfer phases, leading to greater energy wastage. Additionally, the SE in NOMA configurations remains inferior, primarily due to unavoidable increased circuit energy consumption, essential for the operation of simultaneous multi-user access. Therefore, NOMA, contrary to certain expectations, could impede the realization of energy-efficient and sustainable IoT infrastructures.

The implications of these findings are multi-fold. Practically, the research suggests a reevaluation of NOMA's suitability in energy-sensitive IoT environments. From a theoretical standpoint, it identifies critical performance bottlenecks and provides insights into the non-trivial trade-offs posed by NOMA in energy-constrained scenarios. Potential future developments may explore energy-efficient adaptations of NOMA, ensuring its feasibility and optimization as part of sustainable IoT network strategies.

In conclusion, while NOMA has been considered for improving SE in WPCNs, this research indicates T-WPCN's superiority in both SE and energy efficiency under realistic operating conditions for IoT networks. The community stands to benefit from further exploration into hybrid models or novel protocols that mitigate NOMA's identified drawbacks, promoting more balanced and efficient resource utilization.