# Power-Efficient Random Access Design for Machine Type Communication
## 1 Introduction  
Over the past few decades, wireless communication has been mainly used for human devices, such as cell phones, laptops. We referred to this kind of communication scenario as human type communication (HTC) in which high data rate and low latency are the primary goals. In recent years, machine type communication (MTC) has gained substantial attention due to the requirement of the smart city [1; 2; 3; 4]. Devices are required to access the network so that the messages can be easily exchanged between devices. So the MTC is the most important part for the Internet of Things (IoT) which aims to connect all of the physical objects in the word. There are three categories of communication technologies that can realize the MTC. The first of which is the unlicensed low power local area network (LAN) which is popular in smart homes, such as ZigBee, WiFi, and Z-Wave [5]. The advantages are the low cost and low power consumption. It however has a limitation on coverage, which is typically from ten meters to several hundred meters. The second of which is the unlicensed low power wide area network (LPWAN), such as Utra-Narrow Band (UNB) [6], LoRa [7], Weightless [8]. Most of these networks are private solutions and work on the ISM band such as 868 MHz or 2.4 GHz. The advantages are low cost, low power consumption and large coverage which can reach 100 km at most. But the operating band limits its quality of service (QoS) that the interference on the ISM band is uncontrollable. The last of which is the licensed LPWAN, such as LTE-eMTC, EC-GSM [9], NB-IoT [10]. Apparently, the advantage is the large coverage with controllable QoS. The development of licensed LPWAN is slower than the other two technologies due to its complex standardization procedure. Besides this, the dense and heterogeneous network can also be used to support the MTC, such as femtocell networks [11; 12; 13; 14; 15]. In ref. [11], an efficient uplink power control method based on the game theoretic is proposed in the two-tier femtocell networks supportingmultiple services. In ref. [12], they proposed an optical solution based on the game theoretic to the cell selection and power control in the uplink of multi-service open access two-tier femtocell networks. In ref. [13; 14; 15], they focus on the interference management with the help of power control and resource allocation in the femtocells. LTE is the latest commercial and mature system to serve for the HTC. And from 2009, 3 GPP began to study on cellular MTC, i.e., the LTE-MTC, aiming to let the LTE system to support MTC which is well-documented in [16; 17; 18]. LTE-eMTC is the enhanced LTE-MTC that more constraints are proposed, such as reduced peak data rate, reduced transmission power, simplified hardware and the narrowband communication [19]. The reduced capability significantly degrades the performance of MTC devices, resulting in coverage shrinkage. Yet the coverage is very important in the MTC scenario, as the MTC devices may be located in the basement and fixed. Depending on the actual environment, additional coverage up to 15 dB is thus required to ensure the connectivity in extreme cases, such as the basement. For ease of discussion, we divide the coverage extension levels into four different classes: Coverage class (CC) 1 includes the MTC devices that are in the coverage extension level of 15 dB, which leads to a maximum coupling loss of 140 + 15 = 155 dB, where the 140 dB is the maximum coupling loss for the traditional LTE coverage. Coverage classes 2 to 4 correspond to maximum coupling losses of 150, 145, and 140 dB, respectively. Clearly, LTE must continue its evolution in order to meet the challenges imposed by emerging applications and markets by taking advantage of the available small bands. The focus of this article is the design of random access in the MTC scenario, especially the physical layer signaling structure design. Our main contributions are as follows. We propose a common random access signal structure for different coverage classes.