By 2020 there will be 20-50 billion devices connected to cellular networks around the world, and less than a third of those will be the cell phones and tablets that dominate cellular networks today. This massive growth will be driven by the Internet of Things (IoT), the name given to the ubiquitous connectivity of everything everywhere. In domains from healthcare, industry, agriculture, manufacturing, energy and entertainment, embedded devices will be seamlessly monitoring, calculating and communicating wirelessly. To thrive in this new world of IoT, cellular networks must evolve to handle the new ways in which these devices will connect and communicate. Machine-to-Machine (M2M) communications aims to provide the most efficient communication infrastructure for enabling IoT. Providing this infrastructure will require a dramatic shift from the current protocols designed for a much smaller number of very different users, which are mostly for human-to-human (H2H) applications.
Despite huge efforts of 3GPP towards making M2M communications a reality, there are still open challenges where efficient solutions must be proposed. Among others, the random access channel of LTE has been identified as a key area where improvements for M2M traffic are needed. In current cellular standards, the users wishing to communicate with the base station first request an access through the random access (RA) procedure. That is each user randomly chooses a preamble and sends it via the random access channel. Once the base station has detected the preambles, orthogonal radio resources are allocated to the users, to then transmit in separate channels. This is however inefficient for M2M communications with a large number of devices, as many devices may select the same preamble in the random access procedure, thus their data transmission will collide making the base station unable to decode them. Even if the devices choose separate preambles, there will most likely not be sufficient radio resources to orthogonally allocate to all users. A major challenge in current cellular systems is then to ensure that a large number of devices involved in M2M applications can be supported.
In this regard, we overview the current random access procedure proposed for LTE-Advanced and discuss the challenges and issues related to it. We then represent the basic concept of an alternative uplink non-orthogonal multiple access (NOMA) technique. The issues and challenges of random NOMA are then explored. Channel coding techniques are also discussed as an essential part of NOMA.
Mahyar Shirvanimoghaddam received the B. Sc. degree with 1’st Class Honours from University of Tehran, Iran, in September 2008, the M. Sc. Degree with 1’st Class Honours from Sharif University of Technology, Iran, in October 2010, and the Ph.D. degree from The University of Sydney, Australia, in January 2015, all in Electrical Engineering. He then held a research assistant position at the Centre of Excellence in Telecommunications, School of Electrical and Information Engineering, The University of Sydney, before pursuing his academic career at the University of Newcastle, Australia, where he is now a Postdoctoral Research Associate at the School of Electrical Engineering and Computer Science. His general research interests include channel coding techniques, cooperative communications, compressed sensing, machine-to-machine communications, and wireless sensor networks.
Mahyar Shirvanimoghaddam was a recipient of University of Sydney International Scholarship (USydIS), University of Sydney Postgraduate Award (UPA), and University of Sydney Norman I prize. He is currently serving as the Reviewer of several international prestigious journals, e.g., IEEE Transactions on Communications, IEEE Transactions on Wireless Communications, IEEE Communication Letters, IEEE Transactions on Vehicular Technology, IEEE Wireless Communications Letters, IEEE Signal Processing Letters, and several international flagship conferences.