Reducing energy consumption is fundamental to address the requirements of Massive Machine-Type Communication (mMTC) applications. In this sense, D2D relaying could play an important role in some scenarios. D2D discovery process is a key phase of D2D relaying since in this phase the MTC device (MTD) has to select an optimal relay. This process should ensure low energy consumption and low complexity while taking into account the specific constraints of the MTDs.In this paper, we analyze the total (i.e., during the discovery phase and the data transmission phase) energy consumed by the MTD when it uses a D2D relaying mechanism. We propose a simple distributed discovery protocol, which allows reducing the MTD energy consumption. Our numerical and simulation results confirm that when using the proposed discovery mechanism, the total MTD energy consumption can be significantly reduced depending on the data packet size and the location of the MTD with respect to the base station.

Massive machine-type communications (mMTC) is one of the main services delivered by the fifth Generation (5G) mobile network. The traditional cellular architecture where all devices connect to the base station is not energy efficient. For this reason, the use of device-to-device (D2D) communications is considered to reduce the energy consumption of mMTC devices. The main idea is to use nearby user equipment (UE) as a relay and establish with it D2D communication. However, the relay selection process also consumes energy, and this consumption can be significant compared to the energy consumed during the data transmission phase.In this paper, we propose a distributed energy-efficient D2D relaying mechanism for mMTC applications. This mechanism favors the selection of the UEs with low path loss with the mMTC device. Through mathematical analysis and simulations, we show that our mechanism allows a reduction of the total energy consumption of mMTC devices (up to 75% compared to direct transmission) when they have an unfavorable link budget. Moreover, our mechanism achieves almost constant energy consumption for a large range of UE densities and distances between the mMTC device and the base station.

Reducing energy consumption is a key requirement for Massive Machine-Type Communication (mMTC) devices operating on battery power. Device-to-Device (D2D) communication is a promising technology that can be used in 5G networks for this purpose. In this paper, we identify the interest of using a D2D relay mechanism compared with a direct link with an unfavorable link budget. We first look for an optimal Relay-MTC device configuration that minimizes the energy consumption. We then compare the energy consumption in direct and relaying mode expressing it in terms of the distance between the base station and the MTC device and between the base station and the relay. We show that when the MTC device is close to the base station, D2D relay mechanism can reduce the energy consumed by the MTC device at the expense of an increase in the global energy consumption (i.e. the energy consumed by the MTC device plus the energy consumed by the relay). On the other hand, when the MTC device is far from the base station, D2D relay mechanism allows reducing both the energy consumption by the MTC device and the global energy consumption.

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