# Power Efficient Random Access for Massive NB-IoT Connectivity
## 3 Collision--CE Level Based RAP (PE-RAP)  
The network configures CE levels and broadcasts the list of power thresholds for the received reference signals of the CE levels in the cell [3]. The device measures RSRP for the selection of its CE level. Subsequently, it randomly selects the preamble from amongst the ones mapped to the chosen CE level; 3 GPP defines that each PRACH resource contains a set of subcarriers and every subcarrier corresponds to a random access preamble [8]. The number of preambles allocated to each CE level are limited and are configured by the network. In a massively connected IoT where enormous number of devices may perform simultaneous random access, more than one device may choose the same preamble resulting in an RAP failure. In the existing NB-IoT system shown in Figure 2(a), the device on an RAP failure first reattempts in its initially chosen CE level. The device keep trying RAP reattempts in its selected CE level until the maximum number of configured attempts in this CE level are executed [9]. Subsequently, if the failure persists, the RAP restarts in the next higher CE \begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline
**Technology in Focus/Approach** & **Refs.** & **Work Summary** & **Objective(s)** \\ \hline NB-IoT Overview & [1, 21; 22] & Review of NB-IoT evolution, technologies, and issues & Provide overview of design for NB-IoT \\ \hline RAP in NB-IoT & [7] & NB-IoT & RAP modelled probabilistically using Markov chain & To calculate system throughput \\ \hline RAP in NB-IoT & [14] & Trade off between repetition and RAP reattempts & Increasing the detection probability \\ \hline RAP in NB-IoT & [9] & Analytical model for considering three CE levels & RAP & Success probability and access delay estimations \\ \hline RAP in NB-IoT & [23] & Joint optimization technique under a target delay constraint & Optimal configuration of NPRACH parameters & Maximization of the access success probability \\ \hline RAP in NB-IoT & [24] & Access reservation protocol with partial preamble transmission & Reducing collision probability \\ \hline RAP in NB-IoT & [4] & Superimposed NPRACH preambles with multiple RAPs & Derive detection threshold \\ \hline RAP in NB-IoT & [26] & Classification of back-off in massive NB-IoT connectivity & Capacity gain is estimated \\ \hline \end{tabular}
\end{table} 
Table 1: Related work on random access in NB-IoT. level [8]. Several RAP reattempts are allowed until the device preempts all possible trials up to the highest CE level. We propose that in PE-RAP, the device measures RSRP after every failure and is able to re-select the CE level. Thus, on an RAP failure, the device do not necessarily move to the higher CE level in a sequential order. In PE-RAP, the device that fails an RAP attempt has the flexibility to transit to any of the CE levels as shown in Figure 3b. Moreover, in PE-RAP the NB-IoT device is not required to perform power ramping when reattempting an RAP in the same CE level. Higher CE levels are configured for transmission at higher power. When the device selects higher CE level for its RAP reattempt then the power ramping becomes implicit. By enabling the re-selection of the CE level on an RAP failure, PE-RAP enables the correct selection of the number of repetitions and the power level such that they are neither unnecessarily high nor inadequately low. This is because different CE levels are configured with different power level and different number of repetitions. The proposed PE-RAP is presented in Algorithm 1.