[Paper Review] Multiple Transmit Power Levels based NOMA for Massive Machine-type Communications

Wenqiang Yi, Wenjuan Yu+4 more|arXiv (Cornell University)|Nov 24, 2020

Advanced Wireless Communication Technologies13 references4 citations

TL;DR

This paper proposes a multiple transmit power levels-based non-orthogonal multiple access (MTNOMA) scheme for massive machine-type communications (mMTC) to enhance uplink connectivity and spectral efficiency. By enabling open-loop power control via a power map that assigns region-specific transmit power levels, the framework supports semi-grant-free (semi-GF) transmission with user barring, achieving up to 360% higher average arrival rate (AAR) than grant-based and 32 times higher than grant-free schemes under high load, with performance stabilizing even as device counts surge.

ABSTRACT

This paper proposes a tractable solution for integrating non-orthogonal multiple access (NOMA) into massive machine-type communications (mMTC) to increase the uplink connectivity. Multiple transmit power levels are provided at the user end to enable open-loop power control, which is absent from the traditional uplink NOMA with the fixed transmit power. The basics of this solution are firstly presented to analytically show the inherent performance gain in terms of the average arrival rate (AAR). Then, a practical framework based on a novel power map is proposed to associate a set of well-designed transmit power levels with each geographical region for handling the no instantaneous channel state information problem. Based on this framework, the semi-grant-free (semi-GF) transmission with two practical protocols is introduced to enhance the connectivity, which has higher AAR than both the conventional grand-based and GF transmissions. When the number of active GF devices in mMTC far exceeds the available resource blocks, the corresponding AAR tends to zero. To solve this problem, user barring techniques are employed into the semi-GF transmission to stable the traffic flow and thus increase the AAR. Lastly, promising research directions are discussed for improving the proposed networks.

Motivation & Objective

• To address the scalability and connectivity limitations in massive machine-type communications (mMTC) with high device density and intermittent traffic.
• To overcome the lack of open-loop power control in conventional uplink NOMA, which relies on closed-loop feedback unsuitable for mMTC.
• To design a practical framework using a power map that maps geographical regions to transmit power levels, reducing collisions without requiring instantaneous channel state information (ICSI).
• To enhance connectivity in mMTC by integrating semi-grant-free (semi-GF) transmission with user barring techniques to stabilize performance under heavy load.
• To explore future research directions for MTNOMA-mMTC, including QoS-aware SIC, dynamic user barring, and MIMO integration.

Proposed method

• Proposes a multiple transmit power levels (MTPL) scheme enabling open-loop power control in uplink NOMA, eliminating the need for feedback in mMTC.
• Introduces a power map that maps geographical regions to a set of predefined transmit power levels (TPLs), reducing collisions and energy consumption.
• Employs a semi-grant-free (semi-GF) transmission protocol combining the reliability of grant-based and the flexibility of grant-free access.
• Designs a user barring mechanism to limit the number of simultaneously transmitting devices, stabilizing system load and preventing AAR collapse under high device density.
• Uses stochastic geometry and statistical channel models to derive the mapping between transmit power levels and received power levels (RPLs), enabling performance analysis.
• Applies successive interference cancellation (SIC) at the base station based on RPL strength order, with decoding success dependent on SINR thresholds.

Experimental results

Research questions

• RQ1 How can open-loop power control be effectively implemented in mMTC to replace feedback-dependent closed-loop power control?
• RQ2 What is the performance gain of MTNOMA over conventional OMA and NOMA in terms of average arrival rate (AAR) under varying device loads?
• RQ3 How can a practical power map be designed to assign transmit power levels to geographical regions without requiring instantaneous channel state information?
• RQ4 How does semi-GF transmission with user barring improve system stability and AAR when the number of active devices exceeds resource block capacity?
• RQ5 What are the key challenges and future research directions for integrating QoS-aware SIC, dynamic user barring, and MIMO into MTNOMA-mMTC networks?

Key findings

• Semi-GF transmission with MTNOMA achieves up to 360% higher average arrival rate (AAR) than conventional grant-free (GF) transmission under 10 resource blocks and 2 RPLs.
• The proposed semi-GF scheme with user barring increases AAR by 32 times compared to GF-only transmission when 200 devices are active with 10 RBs and 4 RPLs.
• Without user barring, AAR drops sharply under heavy load, but with user barring, AAR remains stable and close to the theoretical maximum even as device count increases.
• The power map framework enables effective collision reduction and energy efficiency by assigning region-specific TPLs based on geographical and statistical channel information.
• Theoretical analysis confirms that the system operates at optimal performance in the long run when the power map is properly designed.
• Future research directions include developing non-Gaussian channel models for short-packet communications, improving ML-based power map design, and enabling QoS-aware SIC and dynamic user barring.