# Performance Analysis of NB-IoT Uplink in Low Earth Orbit Non-Terrestrial Networks +
## 2 System Model
### NB-IoT Uplink Transmission Scheme  
In 3 GPP Release 13, a cellular-based NB-IoT standard to support low-cost, low-power UE is confirmed. NB-IoT is a long-distance IoT communication technology that uses the long-term evolution (LTE) licensed band. LTE architecture is heavily reused in NB-IoT, including the numerologies, downlink orthogonal frequency-division multiple access (OFDMA), uplink single-carrier frequency-division multiple access (SC-FDMA), and other features. NB-IoT uplink channels include a narrowband physical uplink shared channel (NPUSCH) and a narrowband physical random access channel (NPRACH) [19]. This system model considers only the NPUSCH. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Satellite altitude & 552 [km] \\ Satellite effective isotropic & \\ radiated power (EIRP) & 34 [dBW/MHz] \\ Satellite antenna gain & 30 [dBi] \\ Equivalent satellite antenna aperture & 2 [m] \\ Antenna gain-to-noise-temperature (G/T) & 1.1 [dB/K] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: LEO satellite parameters. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Frequency band (\(f_{c}\)) & S-band (2 GHz) \\ Antenna type and & (1,1,2) with omnidirectional \\ configuration & antenna element \\ Polarization & Linear: \(+/-45^{\circ}X-pol\) \\ Antenna temperature & 290 [K] \\ Noise figure & 7 [dB] \\ Transmit power & 200 [mW] \\ Antenna gain & 0 [dBi] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: UE parameters. Figure 2: Architecture of the NB-IoT NTN. The NB-IoT SCS supports frequencies of 15 kHz and 3.75 kHz, setting bandwidths up to 180 kHz. The NB-IoT adds a resource unit (RU) concept to manage resources efficiently. One RU is the transport block size (\(TBS\)), which is defined by setting the number of tones and slots. Figure 3 shows a configurable transport block (TB) in an NPUSCH. In this study, the \(TBS\) is set by defining 12 tones and two slots as one RU. Figure 4 depicts the NB-IoT resource grid: an enlarged view of a TB configured with one \(DMRS\) symbol per slot. The NPUSCH format determines the number of \(DMRS\) symbols. Two NPUSCH formats are defined in [19; 31]. Format 1 is used for general NB-IoT data transmission and has one \(DMRS\) symbol per slot. Format 2 uses a single tone and specifies three \(DMRS\) symbols per slot. In this study, NPUSCH Format 1 is selected. value each time for retransmission. RV can be viewed as basic information, and additional bits are transmitted to estimate the data. In LTE, the RV is divided into \(\{0,2,3,1\}\), allowing four repetitions. The RV is divided and sent because the receiver stores the received data in a buffer before sending the NACK and compares it with the retransmitted data to correct the error. Unlike LTE, NB-IoT is divided into \(\{0,2\}\). Instead, it corrects data errors by allowing an increased \(N_{Rep}\). In the NB-IoT uplink, data transmission is possible by increasing \(N_{Rep}\) up to 128 [34]. However, because NTN has a long \(RTT\), the data rate decreases as \(N_{Rep}\) increases. Therefore, in this study, the maximum analyzed value of \(N_{Rep}\) is 2. \(N_{Rep}\) is the number of retransmissions, including the first transmission. For example, if \(N_{Rep}\) is 2, it is a value of one-time retransmission. The HARQ process is a technique used in a system using HARQ, and increasing the number of HARQ processes improves the throughput of a system with a long \(RTT\). The \(RTT\) of the satellite set in this study is about 25.77 ms. Currently, in the 3 GPP, the maximum number of HARQ processes of the NB-IoT NTN is proposed to be two [7; 34; 35]. Figure 5 illustrates the effect of varying the number of HARQ processes. Figure 5a is a communication method with 1 HARQ process. When the number of HARQ processes is one, 1 TB (1 ms) is transmitted during \(RTT\) and not sent for 24.77 ms. Therefore, there is a delay until the subsequent TB is transmitted. Figure 5b is a communication method with 2 HARQ processes.