ABSTRACT
NB-IoT can be implemented in three different operation modes:
- Standalone mode,
- Guard-band mode,
- In-band mode.

And while NB-IoT signals may operate under any of the three different implementation modes described above, the most common at this early stage of the technology is the so-called In-band mode that has already been implemented by large service providers in Europe like Vodafone, Deutsche Telekom, Telecom Italia Mobile, etc. with many other wireless operators worldwide following in their footsteps. These operators need to ensure that existing LTE networks and new NB-IoT services coexist in harmony, while maintaining the required quality of experience and service level agreements expected for each.

The main advantage of NB-IoT In-Band mode is that is the most cost-effective and seamless for mobile operators to implement since it does not require any hardware changes to the radio access network, and efficiently uses spectrum resources for LTE or NB-IoT services based on demand from mobile users or devices. Notwithstanding the above, NB-IoT In-Band also presents an inherent technical challenge: its narrowband signal occupies 180kHz or basically one physical resource block (PRB) within the LTE broadband carrier, which can produce a type of internal interference referred to as Inter-PRB or the interference between two different PRB elements that may collide with each other.

The NB-IoT PRB in In-Band mode differentiates from the other PRBs of the broadband LTE signal using a power dynamic feature (power boosting) that is defined by 3GPP in terms of LTE physical resource block (PRB) to be larger than or equal to +6dB for In-band or for Guard-band operation modes. The 3GPP standard also specifies other NB-IoT signal structure features like supported modulation types, frequency error tolerances or EVM values allowed for the synchronization and control channels.

VIAVI CellAdvisor™ is a cost-effective, handheld solution that can support acceptance testing in compliance with 3GPP specifications, focusing on the following compliance aspects for NB-IoT signals:
- Max output power of PRB (Physical Resource Block) carrying NB-IoT signal and channels,
- Total Power dynamic range of the NB-IoT active PRB, is it equal or greater than 6dB,
- EVM measurements for NPDSCH and other control and synch channels,
- Downlink NRS (Narrowband Reference Signal) channel power and EVM measurements,
- Frequency Error measurement.