Random Access for Machine‐Type Communications

Abstract

The success of 5G wireless networks (5G) substantially relies on efficiently embracing machine‐type communications (MTC), intended to support autonomous exchange of data between devices. Typical MTC applications include smart metering, e‐health care, environmental and structural monitoring, and asset tracking. Therefore, MTC is an essential component of smart cities and the Internet of Things (IoT). However, cellular networks have traditionally been designed to efficiently handle human‐to‐human (H2H) traffic, whose characteristics greatly differ from those of MTC traffic. Only recently the cellular networks started to be optimized for MTC and, in that sense, a special focus has been put on the random access (RA), which occurs whenever a device requests initial access to a base station (BS). Specifically, the RA mechanism of cellular networks is based on the slotted ALOHA protocol, whose efficiency degrades rapidly as the number of users increases, also presenting a large signaling overhead when short packets are transmitted. The RA channels are further challenged by massive machine‐type communications (mMTC) applications, where a great number of MTC devices attempt to access the BS, while the class of MTC applications with strict latency requirements is challenged by the signaling overhead. These problems have been inherited from the H2H‐oriented cellular systems and, while numerous solutions have been proposed, there is still no consensus on the MTC enhancements for the second phase of 5G standardization.

Priority Enabled Grant-Free Access With Dynamic Slot Allocation for Heterogeneous mMTC Traffic in 5G NR Networks

“…The DSA-GF scheme focuses on the NR frame structure and features the flavors of both 4G and 5G access mechanisms such as access class barring and unified access control [1] [23], imposing different permission probabilities to heterogeneous types of traffic. It is operated on a subframe-by-subframe basis.…”

“…In LTE-A, the time required for initial four-step handshaking, which occurs prior to a data transmission, is in the order of 15 ms [1]. This is not a major concern since many 4G applications do not have stringent low latency requirements.…”

“…In other words, no dedicated preamble transmission for granting access and allocating radio resources is required for GF communications before starting a data packet transmission [3]. The benefits brought by this principle in terms of shortened delay and reduced protocol overhead make GF mechanisms attractive for various applications with URLLC/mMTC requirements and small data packets [1].…”

“…On the other hand, channels in multi-channel slotted ALOHA regard to different kinds of orthogonal resources such as codes or preambles which are used in the same, for instance time slot, during the initial access procedure. Using different orthogonal resources, multiple devices can access to a common channel simultaneously [1]. However, the amount of resources is still limited.…”

Random-Access Accelerator (RAA): A Framework to Speed Up the Random-Access Procedure in 5G New Radio for IoT mMTC by Enabling Device-To-Device Communications

“…The ZC generates orthogonal preambles with zero correlation, which avoids inter-signal interference. However, the generation process is difficult to perform in real-time and requires a large amount of memory to store the sequences [ 22 , 23 ]. Therefore, the shortage of preambles and the great number of devices derivate in very long periods of blackouts (no connection with the network) and many missed transmission opportunities.…”