# Power Efficient Random Access for Massive NB-IoT Connectivity
## 1 Introduction  
The Internet of things (IoT) has been gaining popularity for its smart decision making capabilities using data acquired through several sensors. The narrowband Internet-of things (NB-IoT) is the standardized form of the IoT that is compatible with the legacy Long Term Evolution(LTE) mobile networks. It is especially attractive for low energy sensor based applications that require wireless data transmission using the backbone network. NB-IoT is imbued with features like low complexity, low cost, ubiquitous coverage, low data rate and low power computing. One of the important NB-IoT attributes is its offer of significant coverage extension beyond existing cellular technologies [1; 2]. The coverage extension feature of NB-IoT technology is especially useful when sensors are located in remote or hard to reach areas. Reliable coverage enhancement is achieved by the repeated transmission of data and control signaling. Each transmission can be configured to repeat for a designated number of times in order to achieve higher success opportunities at the desired coverage level [3]. When path loss at the desired coverage is high, a greater number of repetitions can be configured. The networkcan configure up to three coverage enhancement (CE) levels (CE0, CE1, CE2) to serve User Equipments (UEs)with different path losses [4] as shown in Figure 1. The main impact of the different CE levels is that the message is to be repeated a different number of times. If the device resides in the CE level with lower path loss (level CE0 in Figure 1) then the number of times that the signal needs to be repeated is less. However, the number of configured repetitions are higher for the device in the CE level that manifests high path loss (level CE2 in Figure 1). This ensures that a good signal quality is received in all the configured CE levels. Repeated transmission occupies more resource elements [5] leading to loss in spectral resources. Thus, several of the recent works on NB-IoT are focused on evaluation of the optimal number of repetitions. Not only the spectral resources, but the power consumption of the NB-IoT devices would also be affected by the increase in the number of repetitions. In light of distinctive NB-IoT requirements like the battery life of ten years, small form-factor battery, support for low-power and low complexity operation [1], it is not hard to visualize the importance of conservative methods for achieving the power efficiency objectives. In this paper, we specifically focus on manifesting power saving in the random access procedure (RAP). In RAP, the device that wishes to establish the connection with the base station randomly chooses and transmits a preamble. The legacy RAP was originally designed for a limited number of connections. However, the number of devices that attempt RAP in massive connectivity would be high. The vision of a connected IoT is based on the usage of a high number of low complexity wireless devices. As the technology advances, people would be using an increasingly broader range of services and applications [6]. The requirements for enormous connectivity would only become more acute in the future. In such a congested environment, the RAP would often fail due to frequent collisions of preambles [7]. The resulting multiple RAP reattempts would adversely affect the power saving of the NB-IoT devices. It is notable that each RAP reattempt in turn comprises of several repetitions of preamble transmission. Thus, we believe that it is important to address the power saving challenge when a congested RAP is considered together with the transmission repetition characteristics of NB-IoT devices. If an RAP attempt by an NB-IoT device fails, then the device can first reattempt the RAP in the same CE level as the one with the preceding attempt; 3 GPP defines the parameter'maxNumPreambleAttemptCE' such that when the preamble transmission counter of the CE level becomes higher than this parameter, the CE level is increased [8].