# A Throughput and Energy Efficiency Scheme for Unlicensed Massive Machine Type Communications +
## 1 Introduction  
Internet of things has revolutionized the way communication works and is slowly becoming a part of our daily lives. Many applications are currently becoming IoT based, from remotely controlling your house to different processes in an industrial setting [1]. Billions of devices are expected to join the Internet by the year 2020, which while providing a big economic impact, will also create new challenges such as the availability of spectrum resources [2; 3]. This makes finding ways to efficiently use the spectrum more valuable than ever. Machine-type communications (MTC) is a non-human centric concept introduced under the umbrella of IoT for the future communications technology, 5G, which can support a high number of connectivity in the network and can provide different quality of services [4]. MTC can be divided into three categories based on the expected properties, (i) enhanced mobile broadband (eMBB); which should be able to provide connectivity with high peak rates in addition to moderate rates for the cell-edge users; (ii) ultra-reliable MTC (uMTC), which focuses on makingultra reliable and low latency connections in the networks possible and (iii) massive machine type communication (mMTC), whose main goal is to provide massive connectivity for a large number of nodes (in the order of 10 times higher than the current number of connected devices) with different quality of service (QoS) [5; 6]. A mMTC network usually consists of billions of low-complexity low-power machine-type devices as nodes. A good example of this type of networks are smart grids where the data from a very large number of nodes (smart meters) needs to be collected [7; 8; 9]. Industrial control is also another application of mMTC. In both of these examples, the reliability level of the network needs to be high, and it also should be able to handle critical situations [10]. A very good recent study on why 5G and MTC are needed in order to achieve the expected performance in smart grids, specifically with regards to grid protection and control is done in [11]. Motivated by this, in our work, we focus on throughput optimization and energy efficiency in a mMTC network in the presence of retransmissions. The spectrum resources are limited, and hence, the availability of spectrum is a never ending challenge for wireless communications. Considering that mMTC is going to connect billions of devices together, this notion is becoming even more challenging in the upcoming 5G networks. Thus, studying different ways to efficiently use the available spectrum is very important [8]. This is specifically important in the future 5G technology, specially mMTC, since suitable spectrum resources below 6 GHz are limited. Moreover, it is not really possible to use mmWave for mMTC applications since they are limited in terms of resources and are mostly implemented in locations where we could face propagation problems, such as indoor or underground locations [12]. Keeping all these in mind, spectrum sharing can provide useful tools which can help mMTC networks to use the spectrum more efficiently [13; 14; 15; 16]. One of these methods is the unlicensed spectrum access, which is a suitable option for low-power IoT-based networks and is also the spectrum access method used in this paper. It should be noted that while the studied model here is based on the concept of cognitive radios and spectrum sharing, the focus is not on design and issues related to these networks but rather on evaluating different performance metrics important to these kinds of networks. Our analysis is motivated by the aforementioned lack of spectrum resources and better ways to use these limited resources and increase the spectral efficiency of the network. A valuable study has been done in [17] where the authors study the energy grid as an important case of mMTC and propose and evaluate using the LTE network as a communication tool in these networks.