Grant-free multiple access (GFMA) protocol has been regarded as a key element to support sporadic traffic generated from massive internet-of-things (IoT) networks. In GFMA protocol, each IoT device transmits data packets without grant from a base station (BS) via pre-reserved uplink resources. Packet collisions inherently occur when multiple IoT devices transmit packets by using the same radio resource, but the collision effect can be alleviated with multi-packet reception (MPR) capability of the BS. Since a number of studies have focused on improving the physical layer performance such as bit error rate, they may be hard to provide intuitions from the MAC layer perspective when a number of IoT devices sporadically generate uplink packets and attempt the GFMA. In this paper, we thoroughly investigate the GFMA from the MAC layer perspective. We provide an analytical framework based on a Markov chain to capture the performance of the GFMA in terms of packet transmission success probability, ergodic throughput, and access delay. Through simulations, we validate our analytical framework and verify the necessity of adopting MPR technique for supporting a massive number of IoT devices generating sporadic traffic.

Introduction

Internet-of-things (IoT), which connects a massive number of IoT devices with a wide range of applications through IP-based networks, has been considered as a key enabler for Industry 4.0 [1]. Due to the advantage of cellular networks such as coverage and security, the cellular networks have attracted great attention as one of candidates for implementing IoT. Accordingly, there have been a number of studies for implementing IoT in practical cellular networks such as LTE/5G new radio (NR) [2], [3]. A number of IoT devices are expected to sporadically transmit small-sized packets in uplink direction for reporting purpose [4]. In this case, each IoT device may transit to a sleep mode after completion of packet transmissions for saving energy consumption and release its connection with the base station (BS) [5]. This implies that each IoT device should perform a random access (RA) procedure to (re-)establish the connection with the BS when it has a new packet to be sent to the IoT server. The RA procedure consists of 4-steps of handshaking [6], which takes several tens of millisecond (ms) [7]. From the perspective of data packet transmission, the RA procedure can be regarded as additional signaling procedure required in advance. In particular, it has been considered as a critical signaling overhead for supporting sporadic traffic generated from IoT devices. As the size of packet becomes smaller, this signaling procedure becomes more inefficient. Without considering significant modifications of the legacy protocol, minimizing the RA delay spent before the actual data transmission as small as possible can be the straightforward approach to improve the latency performance.