NB-IoT can operate in multi carrier mode. In this case, a specific carrier is configured for initial connection setup + data transfer and other carriers are configured only for data communication. The carrier for the initial connection setup is called Anchor carrier and the other carriers are called Non-Anchor carrier.

The split raises two questions. The UE has to find the Anchor carrier by itself, so the Anchor carrier cannot sit just anywhere inside an LTE carrier. The Non-Anchor carriers carry less, so the UE has to be told what it can expect on them. The two sections below take these questions in turn.

Followings are the topics to be covered in this page.

• Which LTE PRB can carry the Anchor carrier ?
• How do Anchor and Non-Anchor carriers differ ?
• Reference

Which LTE PRB can carry the Anchor carrier ?

A UE that has just switched on knows nothing about the cell. It searches for NPSS on the 100 kHz channel raster, which 36.101 v20.0.0 gives for NB-IoT in-band, guard-band and standalone operation. An in-band Anchor carrier is one PRB of the LTE carrier, and LTE PRBs sit on a 180 kHz grid. So only some PRBs come close enough to a raster point to work as an Anchor carrier.

When NB-IoT is operating in 'inband with the same PCI' mode, only a specific PRB within the legacy LTE System Band can be configured as Anchor carrier. The PRB index within LTE System BW are as follows.

The above table is derived from following equation and table described in 36.213-16.8. The values in above table is nPRB in the following equation. is the max number of RB for a specific channel bandwidth. eutra-CRS-SequenceInfo is the parameter in MIB-NB, carried on NPBCH.

For example, if eutra-CRS-SequenceInfo = 7 and LTE channel Bandwidth is 5 Mhz. The nPRB can be calculated as follows.

n'PRB = 5 (for 5Mhz, = 25 which is odd number)

5 = nPRB - floor(25/2) = nPRB - 12

Therefore, nPRB = 17

< 36.213 - Table 16.8-1: Definition of eutra-CRS-SequenceInfo >

eutra-CRS-SequenceInfo 0 to 13 covers an odd N_RB^DL, and 14 to 31 covers an even N_RB^DL. The PRB index steps by 5, and the raster offset is ±7.5 kHz for an odd N_RB^DL and ±2.5 kHz for an even one.

The step of 5 in the table comes from the two grids. A PRB is 180 kHz wide and the raster step is 100 kHz, and the two line up again only every 900 kHz, which is every fifth PRB. The DC subcarrier and the half-PRB shift of an even N_RB^DL leave a small remainder. That remainder is the raster offset in the table: 7.5 kHz for an odd N_RB^DL and 2.5 kHz for an even one.

The table also leaves out the PRBs at the centre of the LTE carrier. The LTE PSS, SSS and PBCH occupy the six PRBs there, so an NB-IoT carrier cannot use them. In the 5 MHz example above, n'_PRB = ±5 is the closest position to the centre on either side.

• The Anchor carrier must be found by cell search : so its centre has to sit within 7.5 kHz of a 100 kHz raster point.
• Only every fifth PRB qualifies : 180 kHz and 100 kHz line up every 900 kHz.
• MIB-NB tells the UE where it is : eutra-CRS-SequenceInfo gives the PRB index and the raster offset for inband-SamePCI.
• The centre six PRBs are excluded : they carry the LTE PSS, SSS and PBCH.

36.213 v19.4.0 clause 16.8 applies eutra-CRS-SequenceInfo only when operationModeInfo indicates inband-SamePCI. The UE then derives both the LTE CRS sequence and the raster offset from it. For inband-DifferentPCI and guardband, 36.331 v19.3.0 gives MIB-NB a rasterOffset-r13 field instead.

How do Anchor and Non-Anchor carriers differ ?

The Anchor carrier has to support a UE that knows nothing yet, so it carries the synchronization signals and the broadcast channels. A Non-Anchor carrier only serves a UE that is already connected and has been told where to go. The figures below compare the two carriers, both simulated in the in-band same-PCI mode.

In terms of physical channel composition, there are some differences between Anchor and Non-Anchor carrier.

• Anchor Carrier : As shown in Figure 1, Anchor Carrier carries NRS, NPSS, NSSS, NPBCH, NPDSCH, NPDCCH
• NonAnchor Carrier : As shown in Figure 2, NonAnchor Carrier carries only NRS, NPDCCH, NPDSCH

NOTE : I created Figue 1 and 2 using Matlab LTE Toolbox based on Matlab 2018b

< Figure 1 :  Physical Channel Composition - Anchor Carrier >

The upper grid covers 80 subframes and shows NPBCH, NPSS, NSSS and NRS repeating. The zoomed part shows NPBCH filling subframe 0, with NRS resource elements inside it and in subframe 1.

< Figure 2 :  Physical Channel Composition - NonAnchor Carrier >

The Non-Anchor carrier shows only NRS in this simulation, because no NPDCCH or NPDSCH is scheduled. 36.211 v19.3.0 clause 10.2.6 only guarantees NRS on a Non-Anchor carrier around the NPDCCH and NPDSCH the UE monitors.

• Figure 1 has seven legend entries : NRS, NPSS, NSSS, NPBCH, SIB1-NB, NPDCCH and NPDSCH.
• Figure 2 has three : NRS, NPDCCH and NPDSCH.
• No NPSS, NSSS or NPBCH on a Non-Anchor carrier : those subframes are free for NPDCCH and NPDSCH.
• SIB1-NB stays on the Anchor carrier : a UE reads all its system information there.

Release 13 used Non-Anchor carriers for unicast data only, as Figure 2 shows. 36.331 v19.3.0 adds SystemInformationBlockType22-NB for this. Its dl-ConfigList-r14 and ul-ConfigList-r14 fields configure Non-Anchor carriers for paging and random access as well. Two more fields, pagingWeightAnchor-r14 and nprach-ProbabilityAnchorList-r14, set how much of that load stays on the Anchor carrier. Later extensions of SIB22-NB add mixed operation modes, coverage-based paging and a Release 19 CB-Msg3 probability for the Anchor carrier.

• Release 13 Non-Anchor carriers carry unicast data : the UE is moved there by RRC signalling after connection setup.
• SIB22-NB moves idle-mode load too : paging and random access can use Non-Anchor carriers.
• Weights keep the Anchor carrier in use : pagingWeightAnchor-r14 and nprach-ProbabilityAnchorList-r14 share the load.

Reference

[1] Narrowband Internet of Things (R&S Whitepaper)

[2] 3GPP TS 36.101 v20.0.0 - channel raster for category NB1 and NB2

[3] 3GPP TS 36.211 v19.3.0 - clause 10.2.6 for NRS on Anchor and Non-Anchor carriers

[4] 3GPP TS 36.213 v19.4.0 - clause 16.8 and Table 16.8-1 for eutra-CRS-SequenceInfo

[5] 3GPP TS 36.331 v19.3.0 - MasterInformationBlock-NB and SystemInformationBlockType22-NB