# Narrowband Internet of Things (NB-IoT):
## 3 Narrowband-IoT Protocol Stack
### Physical Layer
#### 3.1.2 Random Access Procedure  
Like in LTE, NB-IoT random access (RA) is intended for initial UE uplink synchronization through which the UE acquires its unique UE ID used for communication with the base station. RA is also used to regain the lost UE access due to the long state of inactivity which has led to the loss in uplink synchronization. In NB-IoT, RA faces several challenges as seen on the research discussions; some solutions to improve the RA performance have been proposed as described in what follows.  
In [57], the authors presented the random access procedure (RAP) model and analyzed the system performance by taking into consideration the configurable signal propagation model, a number of supported users per cell, and the RAP configuration parameters. The paper used the contention-basedrandom access with Msg3 collisions instead of Msg1 collision (as multipath transmission) for the random access procedure. The proposed model results show the impact of the parameters (Msg3 transmission mode, Msg3 modulation and coding scheme (MCS), power control schemes and power ramping step) in the single-tone and multi-tone transmissions Bit Error Rate (BER) performance. The results are presented in terms of the total number of preamble transmission success, preamble retransmissions and lost preamble attempts. The work concludes that Msg3 must be considered in the random access procedure analysis, the transmission mode as well as MCS, and for better system performance and fairness distribution of UEs in the cell, it is better to configure power control correctly.  
In [58], the authors analyze NB-IoT transmission delay as well as mathematical evaluation of the probability of success for the random access procedure preamble transmission. The analysis is based on three scenarios; scenario one uses minimum values of parameters, scenario two uses the intermediate values, and scenario three uses maximum parameter values. The used parameters are NPRACH periodicity, start time, number of repetitions, number of preamble attempts, and random access response window size. The average delay analysis was performed such that \(k\) preamble sequences are mapped in \(n\) subcarriers. The preamble collision occurs when multiple UEs send preamble sequences in the same subcarrier. A successful preamble attempt occurs when only one UE sends the preamble to a given subcarrier.  
In [59], the authors investigated a random access optimization algorithm and summarized the NPRACH feature and hence designed random access with differentiated barring (RADB) for NB-IoT system. It is observed that the RADB could solve the preamble request conflict caused by massive NB-IoT UEs and hence provide reliable random access for latency-sensitive devices. However, the authors did not consider the problems of channel resource distribution and resource use rate.  
In [60], the authors designed a new frequency hopping pattern of NPRACH preamble which uses all feasible hopping distances for a given number of subcarriers. It is seen that their proposed pattern was compatible with standards that is keeping the same NPRACH structure with only very small changes (hopping in the standard is allowed only between the subcarriers of the same resource group). Their simulation where they adopted their first traffic model which deploys 3000 devices, 48 ms and 40 ms of NPRACH preamble and periodicity, respectively, show that the proposed hopping pattern could improve the ToA estimation without additional system overhead.