# Performance Analysis of NB-IoT Uplink in Low Earth Orbit Non-Terrestrial Networks +
## 6 Numerical Results  
### Link-Level Simulation  
This section provides simulation results of uplink NB-IoT performance in the LEO satellite communication system. We implement a link-level simulator for an NB-IoT NPUSCH. The 3 GPP TDL-D channel model [36] is adopted and the maximum Doppler shift is set to 950 Hz. The simulator outputs the _BLER_ and throughput results with the SNR. \(N_{Rep}\) is set to 2. The two slots comprise one TB with a transmission time of 1 ms. The _TBS_ is set to 120 bits [34]. The NB-IoT receiver checks for errors when it receives one TB. If an error occurs, retransmission is requested; otherwise, new data is requested. The simulation reports the _BLER_ and throughput results when 5000 _TBS_ have been transmitted. The link-level simulation parameters are summarized in Table 5.  
Figure 10(a) shows _BLER_ curves for zero residual Doppler shift. One _DMRS_ symbol has the lowest required SNR for satisfying \(BLER\leq 0.1\)[30]. However, in Figure 10(b), the link performance deteriorates due to the residual Doppler shift of 950 Hz. The distance between _DMRS_ symbols determines the NB-IoT receiver Doppler shift compensation range; the more significant the Doppler shift, the greater the influence of the _DMRS_ symbols. Therefore, in the residual Doppler shift calculated in this study, the performance is good at two _DMRS_ symbols. Terrestrial network NB-IoT has improved link performance through many iterations. However, in NB-IoT NTN, many reiterations are impossible due to time delays. Therefore, _DMRS_ symbol addition can be a practical solution to improve _BLER_ performance.  
Figure 12 shows \(BELR\) performance results according to \(TBS\), illustrating the advantages and disadvantages of adding \(DMRS\) symbols instead of data symbols. In Figure 12, when \(N_{dmrs}=1\), a possible SNR satisfying \(BLER\leq 0.1\) cannot be obtained owing to an excessively high Doppler shift [30]. On the contrary, \(DMRS\) symbol addition can obtain a feasible SNR thanks to its improved Doppler compensation range. Despite increased overhead, it achieves a much higher data rate than \(N_{dmrs}=1\). Figure 12 plots \(BLER\) curves when more bits (\(TBS=328\) bits) are sent than the maximum number of bits that can be sent with \(N_{dmrs}=3\). Here, \(N_{dmrs}=3\) yields the worst \(BLER\) because the overhead is too large to convey the transport block; rather, \(N_{dmrs}=2\) shows the best BLER. In conclusion, the appropriate number of \(DMRS\) symbols should be selected according to the Doppler shift and \(TBS\).  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline FFT size & 128 \\ SCS & 15 [kHz] \\ Multiple access & SC-FDMA \\ Modulation & OPSK \\ Channel coding & 1/3 Turbo code \\ \(TBS\) & 120 bits \\ Fading channel model & 3 GPP TDL-D \\ Residual Doppler shift & 0, 950 [Hz] \\ \(N_{Rep}\) & 2 \\ Bandwidth & 180 [kHz] \\ \hline \hline \end{tabular}
\end{table}
Table 5: Link-level simulation parameters.  
Figure 11: Uplink transmission \(BLER\) result for varying numbers of \(DMRS\) symbols. (**a**) Doppler shift = 0 Hz, (**b**) Doppler shift = 950 Hz.  
Figure 13 plots throughput curves of NB-IoT NTN with \(N_{Rep}=2\) for one and two HARQ processes. Compared to the terrestrial NB-IoT link, the NTN uplink transmission involves a much longer \(RTT\) of 25.77 ms. Therefore, in Figure 13a, the throughput of one HARQ process is limited to 2.3 kbps. Figure 13b shows that the maximum throughput doubles up to 4.6 kbps with the help of the two HARQ processes. Among current NTN target performance scenarios, IoT connectivity requires a data rate of 10 kbps or more [20]. Although increasing the number of HARQ processes can satisfy the requirement, it is not preferred due to the increase in the data buffer and the risk of shortening battery life. Therefore, it is necessary to analyze this issue in future studies.