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Massive Machine-type Communications in 5G: Physical and MAC-layer solutions
Carsten Bockelmann, Nuno Pratas, Hosein Nikopour, Kelvin Au, Tommy Svensson, Cedomir Stefanovic, Petar Popovski, Armin Dekorsy
TL;DR
mMTC requires scalable, energy-efficient connectivity for massive numbers of low-rate devices sending short packets, beyond assumptions of human-centric cellular systems. The paper presents METIS PHY and MAC solutions for this challenge, with simulations showing that SCMA can support up to three times more devices than contention-based OFDMA for delay-sensitive small packets, while some approaches increase base-station processing and buffering demands.
Problem
mMTC uplinks involve massive numbers of low-rate devices and short packets, requiring technologies different from human-centric cellular systems such as LTE.
Method
The paper presents PHY and MAC technologies from METIS, including random access, SCMA contention-based grant-free transmission, CPM, and coded random access with CS-MUD.
Results
Up to 3 times more devices than contention-based OFDMA can be supported for delay-sensitive small packets, at the cost of higher base-station processing.
Takeaways & Limitations
METIS candidate solutions are a first step toward energy-efficient and low-cost massive access, with flexible waveforms identified as a promising way to adapt mMTC channels to varying cell loads and coexist with xMBB.
Takeaways & Limitations
Coded random access with CS-MUD significantly increases algorithmic complexity and base-station data buffering because active users, delays, and channels are generally unknown.
Abstract
from arXiv · showhide
Machine-type communications (MTC) are expected to play an essential role within future 5G systems. In the FP7 project METIS, MTC has been further classified into "massive Machine-Type Communication" (mMTC) and "ultra-reliable Machine-Type Communication" (uMTC). While mMTC is about wireless connectivity to tens of billions of machine-type terminals, uMTC is about availability, low latency, and high reliability. The main challenge in mMTC is scalable and efficient connectivity for a massive number of devices sending very short packets, which is not done adequately in cellular systems designed for human-type communications. Furthermore, mMTC solutions need to enable wide area coverage and deep indoor penetration while having low cost and being energy efficient. In this article, we introduce the physical (PHY) and medium access control (MAC) layer solutions developed within METIS to address this challenge.
2 Introduction
5G must extend beyond human-centric broadband to support diverse machine-type use cases. METIS distinguishes massive connectivity from ultra-reliable services and focuses this article on PHY/MAC solutions for massive uplink access.
- 5G introduces machine-type communications alongside human-type traffic, creating challenges for a unified radio solution.
- MTC spans scenarios from low-energy environmental monitoring and smart cities to wireless factories with stringent latency and reliability requirements.
- METIS classifies MTC into mMTC for connectivity to tens of billions of low-power devices and uMTC for stringent availability, latency, and reliability.
- mMTC prioritizes scalable connectivity, wide-area coverage, and deep indoor penetration rather than peak data rates.
- The article presents METIS PHY and MAC technologies for massive, energy-efficient access, emphasizing the uplink’s uncoordinated connections.
3 Requirements and Design Challenges
mMTC requires a different design point from LTE-era human-centric systems because many low-power devices send small, sporadic uplink packets. The section frames access, energy, resource granularity, and coding as central design challenges.
- Requirements and Design Challenges: PHY/MAC design trades performance against complexity and control overhead, unlike traditional systems optimized for high-rate downlink communication with large packets.
- Requirements and Design Challenges: LTE-style access can impose substantial overhead even when an MTC device transmits a single byte, including pilots, link adaptation, and control procedures.
- Access design: Non-orthogonal access can overload resources for many uplink users, accepting greater receiver algorithmic complexity at the base station.
- Access design: Grant-free access reduces control overhead and appears favorable for sporadic mMTC traffic, but collision resolution increases base-station complexity.
- Energy efficiency: Low-overhead MAC protocols combined with efficient PHY approaches can preserve device energy and support long battery lives.
- Short-packet design: Very short packets require finer resource granularity and novel short-block channel codes, while detailed coding treatment is outside the article’s scope.
- Candidate technologies: The article examines SCMA, CS-MUD, and CPM PHY approaches, plus SCMA- and CS-MUD-based MAC approaches for mMTC.
4 Physical Layer Solutions
The physical-layer solutions address massive uplink access through collision-resolving detection, overloaded sparse code-domain access, and energy-efficient constrained-envelope waveforms. These approaches trade receiver or baseband complexity against low overhead, massive connectivity, coverage, and terminal energy efficiency.
- Compressed Sensing based Multi‐User Detection (CS‐MUD): CS-MUD jointly estimates sporadically active users and their data from sparse uplink activity, enabling collision resolution in random access.Its low-signaling operation approaches grant-based performance, but shifts complexity to the base station.
- Sparse Code Multiple Access (SCMA): In the illustrated SCMA scenario, 6 layers occupy 4 OFDMA tones with 150% overloading, while sparsity limits each tone to 3 colliding layers.The codebook structure and low-projection techniques reduce message-passing detection complexity compared with nonsparse combinations.
- Sparse Code Multiple Access (SCMA): SCMA maps bits to layer-specific multidimensional codewords, allowing non-orthogonal overloading across orthogonal OFDMA tones.Sparse codewords limit the number of colliding layers on each tone and support practical iterative message-passing detection.
- Continuous Phase Modulation (CPM): Constant-envelope coded modulation enables cost-effective, power-efficient amplifiers to operate near saturation without adding distortion.This targets mMTC terminal energy efficiency and coverage, which are prioritized over spectral efficiency.
5 Access Layer Solutions
METIS develops uplink access solutions for sporadic mMTC based on SCMA random access and Coded Random Access combined with CS-MUD. These approaches improve collision handling and scalability, while increasing base-station processing and buffering requirements.
- Uplink SCMA Contention-Based Grant-free Transmission: SCMA random access combines contention regions, SCMA codebooks, and pilot sequences into contention transmission units for uplink access.Codebook reuse creates a larger CTU pool, while pilot collisions determine when users interfere in channel estimation.
- Uplink SCMA Contention-Based Grant-free Transmission: SCMA scales through configurable contention regions, codebooks, codeword lengths, and sparsity patterns adapted to users, traffic, detection complexity, coverage, and reliability.The number of contention regions and CTUs can also follow device counts and collision statistics.
- Uplink SCMA Contention-Based Grant-free Transmission: Blind SCMA detection jointly identifies active layers and their data, extending regular MPA reception for random-access mMTC.The receiver must recognize active layers because their number and codebooks are not known in advance.
- Uplink SCMA Contention-Based Grant-free Transmission: Up to 3 times more devices than contention-based OFDMA were supported for delay-sensitive small packets, at the cost of higher base-station processing.The result comes from system-level simulations.
- Coded Random Access and CS-MUD: Coded Random Access uses beacon-delimited contention periods, replicated packets, iterative belief propagation, and successive interference cancellation across slots.CS-MUD and FEC decoding recover packets within slots, after which decoded packets are subtracted and replicas are cancelled across slots.
- Coded Random Access and CS-MUD: CS-MUD combined with Coded Random Access supports asynchronous sporadic uplink communication when active users, delays, and channels are unknown.The processing requires significantly greater base-station algorithmic complexity and data buffering.
6 Conclusions
METIS presents candidate PHY and MAC technologies for energy-efficient, low-cost mMTC access, while identifying air-interface adaptation and higher-layer support as necessary next steps.
- 6 Conclusions: METIS presents candidate technologies for massive machine-type communication motivated by energy-efficient and low-cost solutions to massive access.The conclusion characterizes these technologies as a first step toward a complete solution.
- 6 Conclusions: Dynamic adaptation to different mMTC cell loads and integration with xMBB remain requirements for the overall air-interface design.Flexible waveform design enabling in-band mMTC channels is identified as promising in this respect.
- 6 Conclusions: Lean signaling and longer sleep cycles can be supported through connectionless one-shot transmission modes, while traffic prediction can support quasi-periodic traffic.These are presented as higher-layer considerations for completing mMTC support.
9 Biographies
The biographies describe researchers whose backgrounds span wireless communications, coding, channel coding, compressive sensing, PHY/MAC design, standardization, and wireless networking.
- Biographies: Carsten Bockelmann works on compressive sensing and its applications to communications, channel coding, and transceiver design.He coordinates related research activities at the University of Bremen.
- Biographies: Nuno K. Pratas researches wireless communications, networks, and analysis tools for Machine-to-Machine and Device-to-Device applications.He is an Assistant Professor of Wireless Communications at Aalborg University.
- Biographies: Hosein Nikopour has worked on MIMO, WiMAX, LTE, 5G cellular standards, and wireless solutions at Nortel, Huawei, and Intel Labs.His experience includes PHY design and standardization.
- Biographies: Kelvin Au works on air-interface research and standardization, with experience in PHY/MAC, radio-resource management, MIMO-OFDM, and radio software.His industry experience includes Nortel Networks and BlackBerry.
- Biographies: Tommy Svensson leads research on air interfaces and wireless backhaul networking technologies for future wireless systems.He is Associate Professor in Communication Systems at Chalmers University of Technology.
- Biographies: Čedomir Stefanović researches coding theory, communication theory, and wireless communications, while Petar Popovski works on wireless communications and networks.Both are associated with Aalborg University.