# On the Performance of a UAV-Aided Wireless Network Based on NB-IoT
## 3 The Narrowband-IoT Technology
### Uplink Channels, Parameters, and Implications  
Only two channels are defined in the UL, the narrowband physical random access channel (NPRACH) and the narrowband physical uplink shared channel (NPUSCH). The NPRACH is used to trigger the RA procedure. It is composed of a contiguous set of either 12, 24, 36, or 48 subcarriers with 3.75 kHz spacing, which are repeated with a predefined periodicity, that may take several discrete values between 40 ms and 2560 ms. The RA procedure starts with the transmission of a preamble, with a duration of either 5.6 ms or 6.4 ms (Format 0 and 1, respectively) depending on the size of the cell, and can be repeated up to 128 times to improve coverage. A preamble is composed of four symbol groups, each transmitted on a different subcarrier. The initial subcarrier is chosen randomly by the UE, while the following ones are determined according to a specific sequence depending on the first one. Two UEs selecting the same initial subcarrier will thus collide for the entire sequence, as described in Section 3. A special mechanism can help resolving the collisions and thus the access probability of a node \(u\) can be approximated as Equation (1):  
\[P_{\mathrm{acc,\mu}}=e^{-\frac{\mathrm{n_{RA}}}{\mathrm{N_{\mathrm{RU}}}}} \tag{1}\]  
where \(n_{\mathrm{RA}}\) is the number of nodes entering RA and \(N_{\mathrm{RU}}\) are the total available subcarriers.  
In case of standalone deployment, the NPUSCH occupies all the UL resources left available after the allocation of the NPRACH. NPUSCH is used for UL data and UL control information. The eNB decides how many resources to allocate to the UEs depending on the amount of data to be sent, the modulation-coding scheme (MCS) used and the numberof repetitions needed to correctly receive the data. The minimum resource block which can be allocated, referred to as a resource unit (RU), depends on the UE capabilities and the configured numerology. Specifically, in the case of 3.75 kHz subcarrier spacing and single-tone operation, which is the configuration assumed in this paper, the RU is 32 ms long. The number of the RUs (ranging from one to ten) to be allocated depends on the size of the transport block size (up to 2536 bits in Rel. 15) and the MCS chosen to meet the required success probability. In addition, the eNB specifies the desired number of repetitions. Thus, the total number of resources allocated to a UE is equal to:  
\[N_{\text{RU}_{\text{TOT}}}=N_{\text{RU}}\cdot N_{\text{REP}} \tag{2}\]  
where \(N_{\text{RU}}\) is the number of resources needed to send a packet of size \(B\) depending on the chosen MCS and \(N_{\text{REP}}\) is the number of transmission repetitions the UE is configured to send.  
Without the loss of generality, in what follows we imply 3.75 kHz subcarrier spacing with 48 carriers allocated for RACH. Furthermore, we need to set a number of other uplink parameters to define a resource unit and resource availability on the NPUSCH. For example, we have to define the setting for the MCS used to determine the Transport Block Size (TBS), that is the number of bits which can be transmitted given a certain number of RUs assigned in the NPUSCH. In our model, it holds \(I_{TBS}=I_{\text{MCS}}=6\), and, for clarity, we report in Table 1 only the possible TBSs for the selected scheme.  
Moreover, we need to set the uplink parameters for the three different coverage classes. For simplicity, we refer to them as coverage extension (CE), so that the Normal becomes CE0, the Robust CE1 and the Extreme CE2. These are characterized for a different number of repetitions, subcarrier NPRACH assignment, periodicity and receiver sensitivity. Please note that the optimal choice of these parameters' values is out of the scope of this paper, and most of them are taken from [5]. Table 2 summarizes our settings, that should be handled easy by a UAB.