Enabling Grant-Free URLLC: An Overview of Principle and Enhancements by Massive MIMO

Abstract

<div>Enabling ultra-reliable low-latency communication (URLLC) with stringent requirements for transmitting data packets (e.g., 99.999% reliability and 1 millisecond latency) presents considerable challenges in uplink transmissions. For each packet transmission over dynamically allocated network radio resources, the conventional random access protocols are based on a request- rant scheme. This induces excessive latency and necessitates reliable control signalling, resulting overhead. To address these problems, grant-free (GF) solutions are proposed in the fifth-generation (5G) new radio (NR). In this paper, an overview and vision of the state-of-the-art in enabling GF URLLC are presented. In particular, we first provide a comprehensive review of NR specifications and techniques for URLLC, discuss underlying principles, and highlight impeding issues of enabling GF URLLC. Furthermore, we explain two key phenomena of massive multiple-input multiple-output (mMIMO) (i.e., channel hardening and favorable propagation) and build several deep insights into how celebrated mMIMO features can be exploited to enhance the performance of GF URLLC. Moving further ahead, we examine the potential of cell-free (CF) mMIMO and analyze its distinctive features and benefits over mMIMO to resolve GF URLLC issues. Finally, we identify future research directions and challenges in enabling GF URLLC with CF mMIMO.</div>

Non-Terrestrial Networks with UAVs:A Projection on Flying Ad-Hoc Networks

“…Overall, UAVs may outperform all other NTN components in some NTN use cases determined by 3rd generation partnership project (3GPP) such as i) service continuity to provide NTN access when terrestrial networks are unavailable; ii) service ubiquity to improve NTN availability in the event of disasters that cause a temporary outage or destruction of a terrestrial network, and iii) service scalability to provide traffic from terrestrial networks even during peak hours [11], [63]. Furthermore, the UAV networks can support services proposed by the international telecommunication union (ITU), which are enhanced mobile broadband (eMBB), massive machine type communications (mMTC), and ultra-reliable and low latency communications (URLLC) [11], [63]- [65]. For these use cases, the actual value of UAVs is in the payloads they can carry and their ability to effectively establish networks of flying nodes, i.e., FANETs.…”

SIC Aided $K$-Repetition for Mission-Critical MTC in Cell-Free Massive MIMO

“…Since the SIC procedure in (8) was tailored by exploiting the strong favorable propagation and channel hardening effects in co-located massive MIMO under the condition of perfect power control, its viability relies on the two following features: 1) the multi-user interference is negligible, thanks to the favorable propagation; 2) |Q l k | = 1 ⇐⇒ z k,l is decodable, thanks to the channel hardening with perfect power control.…”

“…Thus, existing SIC scheme cannot be effective when applied in the context of CF massive MIMO. In particular, as shown in (8), by subtracting ĥu 2 s u from z k,l , the BS CPU could only cancel the coherent interference caused by the replicas from device u due to the preamble collisions. Although this might lead to new cases that new data can be decoded, the signal from device u is not fully cancelled in z k,l and r k , which still causes multi-user interference to other devices and error propagation that affects the cancellation and decoding performance in consecutive SIC iterations.…”

“…Nevertheless, these works are simply based on the assumption that each active device has a unique preamble, which overlooks the key impact of preamble collisions 1 on the data detection performance when a number of devices contend for the shared channel resources in GF random access. In practice, preamble collisions do happen in contention-based random access, which result in contaminated channel estimation and thus lead to a potential transmission failure for the collided devices [8]. Therefore, if the preamble collision issue can be addressed, the reliability in K-repetition can be further improved.…”

Statistical Analysis of Received Signal Strength in Industrial IoT Distributed Massive MIMO Systems

“…mMIMO relies in the use of Base Stations (BSs) with a very high number of receive/transmit antenna elements, thus providing great spatial multiplexing capabilities. Recent works have also studied the performance gains that mMIMO can provide to mMTC [4] and cMTC use cases [5], [6]. However, as networks become denser, the inter-cell interference, inherent to the traditional cellular paradigm, represents a major bottleneck for the system performance.…”