# A Systematic Analysis of Narrowband IoT Quality of Service
## 4 Results
### PHY Measurements: Signal Quality Analysis  
In this section, the signal quality parameters SNR, SINR, and RSRQ were used to analyze the Interference Margin (IM), the noise figure of the modem, as well as the cell load during the measurements. Various signal levels between excellent coverage and total signal loss were simulated in 5 dB steps using the attenuator. For each attenuation step, 30 packets were sent in the uplink and downlink, and the RSRP, SNR, and RSRQ were recorded. Additionally, a long-term measurement was conducted over 24 h. Every minute, one uplink and one downlink message were sent for a total of 1440 measurements per direction. For each packet, the SNR, RSRP, RSSI, and latency measurements were recorded.  
Figure 8(a) compares the theoretical SNR to the measured SINR as a function of RSRP. The SNR could be derived for an effective noise floor \(P_{N,eff,15 kHz}=-125.2\) dBm and a noise figure NF = 7 dB:  
\[SNR[dB]=RSRP-P_{N,eff,15 kHz}=RSRP-P_{N,15 kHz}-NF=RSRP-125.2\ dBm \tag{23}\]  
The SINR additionally included interference, so the SNR was an upper bound for the measured SINR:  
\[SINR[dB]=SNR[dB]-10 log\bigg{(}1+\frac{P_{L,15 kHz}[W]}{P_{N,eff,15 kHz}[W]} \bigg{)} \tag{24}\]  
The IM, which was considered in the link budget in Section 2.4, could then be derived as:  
\[IM=10 log\bigg{(}1+\frac{P_{L,15 kHz}[W]}{P_{N,eff,15 kHz}[W]} \bigg{)} \tag{25}\]  
Figure 8(a) shows an offset between the theoretical SNR and the measured SINR samples, which was an indication of the presence of network interference during the measurement. The resulting IM of 2-6 dB as defined in Equation (25) increased the effective noise floor and reduced the MCL in a real NB-IoT network. Furthermore, Figure 8(a) provides an upper bound of 7 dB for the NF, since a higher NF would shift the slope of the theoretical SNR to the right and violate the condition that SNR was an upper bound for SINR.  
In Figure 8(b), the theoretical SNR for 0% cell load (\(x=2/12\)) and 100% cell load (\(x=1\)) is compared to the measured SINR values for varying RSRP signal levels. The measurement matched the theoretical curve for 100% load (see Equation (3)), which confirmed that all 12 subcarriers were allocated in every modem QoS measurement. Since all modem measurements were performed in the downlink, this verified the 3 GPP NB-IoT specification [15] that required downlink resources to be allocated in subframes over the full system bandwidth.  
On the other hand, Figure 9(a) illustrates the signal conditions over a period of 24 h. The measurement points were filtered to only include one cell tower. There were significant fluctuations in the RSRP level, which reflected changes in the signal power received by the modem. These changes could be the result of human activity and created an effect similar to small-scale fading. At night, the signal conditions stabilized dramatically. As such, UE in extreme coverage might benefit from collecting the sensor values during the day and transmitting at night.  
Figure 9(b) compares uplink and downlink latencies over time. Once again, the measurement points were filtered to only include one cell tower. Despite the significant changes in the RSRP and SNR levels shown in Figure 9(a), the latency remained relatively constant. While there were latency spikes in the downlink, these values were well within the 10 s latency requirement of 3 GPP [4].  
Figure 10: Long-term measurement of NB-IoT QoS parameters over a full day. **(a)** Signal quality parameters over one day. **(b)** Uplink and downlink latency over 24 h.  
Figure 9: Analysis of the DL-SINR due to the presence of network interference for varying attenuation. **(a)** Measured DL-SINR over reported RSRP. **(b)** Measured DL-SINR over RSRQ. The solid lines show the theoretical curves for x = 2/12 (0% load) and x = 1 (100% load) according to Equation (3).