LTE Narrowband Internet of Things (NB-IoT) has been standardized by 3GPP in Release 13 as a new air interface with the aim to make LTE suitable for low-cost massive machine-type communication (m-MTC). NB-IoT is heavily based on LTE but makes a number of simplifications and optimizations in order to reduce device costs, minimize power consumption and to maximize performance under unfavourable radio conditions. Several network operators around the world have chosen to adopt NB-IoT for MTC communication in their networks, including Deutsche Telekom, who claimed to have activated the first NB-IoT end-to-end system , and Vodafone, who have committed to launch live NB-IoT networks in four European countries in the first quarter of 2017, including Spain, Germany, Ireland and the Netherlands . Vodafone began to roll out their network in Spain in late January 2017 starting in Madrid and Valencia, already announcing the expansion to other Spanish cities, such as Barcelona, Bilbao, Málaga and Seville . Only days later, on February 1st, we discovered a live NB-IoT carrier in downtown Barcelona in Vodafone’s 800 MHz frequency band. In the remainder of this post, we will analyze this NB-IoT signal more closely and describe how it can be received and decoded with srsLTE, the SRS LTE software radio suite. Furthermore, we highlight a demonstration showcased at Mobile World Congress 2017 at the Software Radio Systems booth.
A First Closer Look
The figure below shows a power spectral density and waterfall plot of the aforementioned Vodafone LTE band centered at 806 MHz over the full cell bandwidth of 10 MHz. The NB-IoT carrier is clearly visible on the left-hand side of the spectrum. It is transmitted with slightly higher power than the rest of the LTE signal. Because the NB-IoT carrier is embedded in and uses resources of an existing LTE cell, it is said to operate in in-band mode.
The NB-IoT carrier occupies one physical resource block (PRB) of its base cell, i.e., has a bandwidth of 180 kHz. It is interesting to note that most of the time also its left and right neighbor PRBs are kept empty, as can also be seen from the waterfall plot. The in-band carrier placed in PRB 5 of the LTE cell is centered at 802.3025 MHz. The figure below again shows a power spectral density and waterfall plot, but this time only the NB-IoT PRB is visible. In the upper part of the picture one can clearly observe the four subcarriers on which the Narrowband Reference Signal (NRS) is transmitted. In the lower part, one can see the periodic transmissions of the Narrowband Physical Broadcast Channel (NPBCH) and the Narrowband Primary and Secondary Synchronization Signals (NPSS/NSSS).
Obtaining System Information Messages
Software Radio Systems has started to enhance srsLTE with support for NB-IoT shortly after the standard has been finalized in late June 2016 . In doing so, SRS could leverage the modularity and extensibility of the library and consequently followed the same approach for the NB-IoT physical layer components, including all downlink and uplink channels and signals defined in the standard.
One of the provided test applications is a Narrowband Master Information Block (MIB-NB) decoder. This application synchronizes to a cell, compensates for time and frequency offsets and attempts to decode the cell’s MIB, which contains the most basic information about the cell, including (parts of) the hyper and system frame number (HFN/SFN), scheduling information for System Information Block 1 (SIB1) as well as the operation mode of the cell. The listing below shows the console output of the application, displaying the narrowband cell ID (n_id_ncell), the number of antenna ports used at the base station, the number of repetitions and the transport block size (TBS) of SIB1 and the operation mode. The output says that the operation mode is
Inband (same PCI) which means that the base cell, i.e., the normal LTE cell at 806 MHz, uses the same Physical Cell Identifier (PCI), i.e., 268, as the NB-IoT carrier.
$ nbiot_ue_mib_sync_test_usrp -f 802302500
Set RX rate: 1.92 MHz
Set RX gain: 30.0 dB
Set RX freq: 802.30 MHz
- N_id_ncell: 268
- Nof ports: 2
- SFN: 832
- HFN (2 LSB): 1
- Sched. Info SIB1 2
- First frame 0
- No. repetitions 16
- TBS 208
- System Info Val 6
- AC barring No
- Operating mode Inband (Same PCI)
Decoding the Full Downlink Signal
During Mobile World Congress 2017, SRS showcased a fully working downlink receiver and decoder for NB-IoT. This demonstration not only decoded the live transmission from the Vodafone cell but also displayed various information about the signal, such as the Carrier Frequency Offset (CFO), Sample Frequency Offset (SFO) and the Block Error Rate (BER) of the Narrowband Physical Downlink Shared Channel (NPDSCH).
Furthermore, SRS also showed a graphical user interface (shown in the figure below) that displayed the constellation diagram of the NPDSCH (upper left image), the correlation peak of the NPSS (upper middle image) and the channel response (upper right image). The GUI also showed the content of MIB, SIB1 and SIB2 (bottom three text fields).
With the finalization of Release 13 of the LTE Advanced Pro specification in June 2016, 3GPP has provided a new radio access technology based on LTE to address the requirements and challenges of the Internet of Things. After less than 6 months after the standard has been completed, Software Radio Systems could successfully test a software-based implementation of the NB-IoT stack and could demonstrate and verify it’s standard-compliance with one of the world’s first commercial deployments prior to Mobile World Congress in February 2017. If you want to learn more about our NB-IoT extensions to srsLTE and srsUE, please get in touch with to receive more information.