How exactly do you simulate the RF performance of Narrowband IoT (NB-IoT), a standards-based, low-power wide area technology developed to enable a wide range of new IoT devices and services? That’s the question ASTRI— Hong Kong’s Applied Science and Technology Research Institute—faced when it discovered that the RF performance specified in the receiver wideband intermodulation definition for the 3GPP standard might not be accurate.
The issue, as ASTRI saw it, was fairly straightforward. The wideband intermodulation definition for NB-IoT, as defined in 3GPP TS 36.101 (version 13.6.1), originally established the signal power parameter as REFSENS+6dB. But this parameter did not take into account the sensitivity of NB-IoT. In later standardization, the narrowband benefit of NB-IoT was taken into account in the sensitivity definition, with the NB-IoT sensitivity being set to around 7.7 dB lower than that of the enhancements for Machine-Type Communications (eMTC) standard. And that made ASTRI wonder: Is it logical to take signal power into account again in the NB-IoT wideband intermodulation definition?
Figure 1. Shown here is the wideband intermodulation definition for NB-IoT, category NB1, from 3GPP TS 36.101 (version 13.6.1).
As an active participant in the 3GPP NB-IoT standardization process, ASTRI knew it had to get to the bottom of this question, but how? The answer lay in the use of the SystemVue electronic design automation (EDA) environment (FREE trial available) to simulate NB-IoT RF performance.
Using the software and its embedded NB-IoT library, ASTRI set up a simulation to study the effect of the NB-IoT receiver wideband intermodulation definition. First, it built a system-level simulation platform for an NB-IoT receiver (Figure 2). The Low Intermediate Frequency (Low-IF) NB-IoT receiver design used for the simulation, with the local oscillator phase noise, nonlinearity and noise figure of each module modeled, is shown in Figure 3. Then, the Bit Error Rate (BER) for the design was measured to evaluate the effect of the receiver’s performance on signal demodulation.
Figure 2. The NB-IoT receiver simulation platform, modeled in Keysight SystemVue.
Figure 3. The Low-IF NB-IoT receiver, modeled in Keysight SystemVue.
Figure 4 shows the signal and interferers at the receiver input in simulation. For simplicity, a QPSK signal with 1.4-MHz bandwidth was used as the modulated interferer.
Figure 5 shows an example of the LO phase noise requirement with different signal power definitions in the NB-IoT receiver design. As can be seen by the simulation result, if the input signal is set to REFSENS+6dB, the receiver’s LO phase noise would need to be no worse than the red curve in Figure 5 to achieve a BER of zero. Wideband intermodulation, therefore, becomes the bottleneck for the receiver LO phase noise requirement. Moreover, much more current would be needed to achieve the phase noise requirement, especially for high-frequency bands. And this means that the NB-IoT wideband intermodulation definition would essentially lead to higher power consumption for NB-IoT user equipment; something that runs counter to the low power requirement of most IoT applications.
Figure 4. Signal and interferers at the receiver input.
Figure 5. An example for the LO phase noise requirement with different signal power definitions.
On the other hand, if the input signal power in the wideband intermodulation definition for NB-IoT is set to REFSENS+12dB, rather than REFSENS+6dB—as it is for LTE and eMTC in 3GPP TS 36.101 (version 13.6.1)—then the LO phase noise for 1 MHz to approximately 7.5 MHz can be relaxed by 10 dB (shown by the blue curve in Figure 5) to achieve a BER of zero from the simulation. By doing so, wideband intermodulation no longer acts as the bottleneck of the LO phase noise requirement. Instead, it is mainly related to the linearity of the RF front-end, which is in line with the definitions for LTE and eMTC.
Based on these SystemVue simulation results, ASTRI proposed that the Category NB1 signal power specification shown in Figure 1 be revised from REFSENS+6dB to REFSENS+12dB in 3GPP RAN4 #82 meeting held in Athens, Greece this February. The 3GPP standards body accepted this proposal after in-depth discussion and the parameter will be updated accordingly in the coming version of 3GPP TS 36.101. The revision on the parameter further optimized NB-IoT User Equipment (UE) standard, and will facilitate the implementation of low power NB-IoT terminal chip.
For more information on the value of SystemVue software for baseband and RF simulation go to: www.keysight.com/eesof-systemvue. For more information on the NB-IoT, go to: http://www.3gpp.org/news-events/3gpp-news/1785-nb_iot_complete.