Narrowband-IoT (NB-IoT) has been released by 3GPP to provide extended coverage and low energy consumption for low-cost machine-type devices. Requiring only a reasonably low-cost hardware update to the already deployed Long Term Evolution (LTE) base stations and being compatible with current core network and enhanced core solutions that aim to reduce the battery consumption and minimize the signalling, NB-IoT deployments are quickly increasing, making NB-IoT a dominating technology for low-power wide area (LPWA) networks. To this aim, in this paper we focus on group communications (i.e., multicast) in NB-IoT to efficiently support the transmission of firmware, software, task updates or commands towards a large set of devices. We discuss the architectural and procedural enhancements needed to support the unique features of group communications in machine-type environments, such as customer-driven group formation. We also extend the NB-IoT frame to include a channel for multicast transmissions. Finally, we propose two transmission strategies for multicast content delivery and evaluate their performance considering the impact on the downlink background traffic and the channel occupancy.

INTRODUCTION

Narrowband-Internet of Things (NB-IoT) is a new cellular network technology that was designed for improved power consumption, spectrum efficiency and deep coverage. It is primarily targeting low cost IoT and Machine Type Communication (MTC) devices with infrequent connections, that are expected to operate for 10 years or more [1], [2] (e.g. sensors, meters). Based on the expectation for low traffic by these devices NB-IoT operates on just 180kHz of system bandwidth within the LTE spectrum, and thus provides very limited communication resources. Therefore, it is even more imperative than in LTE, that these resources are used efficiently. However, the majority of works so far (e.g. [3]–[۶]) have focused only on the uplink direction, as this constitutes the majority of the expected traffic. The long lifetime necessitates that devices can receive software updates to remain secure and up-to-date [2]. Furthermore, they are likely to be receiving commands to execute tasks. As such, efficient use of the downlink direction is equally important. The large amount of involved devices running the same software and performing the same tasks, leads us towards group communications, i.e. multiple devices receiving the common content simultaneously to save network resources, instead of being served one-by-one (unicast). In current LTE networks, group services are provided with the Multimedia Broadcast Multicast Service (MBMS) [7]. MBMS follows a subscription-based approach, where the mobile operator periodically announces all available services (e.g. firmware update) in the Multicast Control Channel (MCCH) that devices can subscribe to. Each device has to monitor the periodic service announcements for relevant services, and when these occur, it subscribes to the evolved NodeB (eNB). Then, after setting up the appropriate communication bearers, it has to keep monitoring the MCCH for the exact time that the service will be broadcasted. The Single Cell – Point to Multipoint (SC-PTM) framework is a similar approach that extends MBMS to provide group communications within a single cell. More recently, it was also standardized in NBIoT [8].