An NB-IoT UE reports one field, ue-Category-NB, instead of the separate downlink and uplink categories of LTE. 36.306 clause 4.1C says that the field defines a combined uplink and downlink capability. The value fixes the largest transport blocks the UE can handle, its soft buffer, its layer 2 buffer and its half-duplex type.

Followings are the topics to be covered in this page.

• What does each NB-IoT UE category support ?
• What does Category NB2 add ?
• How does the UE report its category ?
• Reference

What does each NB-IoT UE category support ?

The four tables below come from 36.306 clause 4.1C, and they show the Release 13 version with Category NB1 only. Each one sets one kind of limit that the eNB must respect when it schedules the UE.

< 36.306-Table 4.1C-1: Downlink physical layer parameter values set by the field ue-Category-NB >

Category NB1 receives at most 680 bits of DL-SCH in one TTI, in one transport block, with 2112 soft channel bits for HARQ.

< 36.306 - Table 4.1C-2: Uplink physical layer parameter values set by the field ue-Category-NB >

Category NB1 sends at most 1000 bits of UL-SCH in one TTI, in one transport block.

< 36.306 - Table 4.1C-3: Total layer 2 buffer sizes set by the field ue-Category-NB >

Category NB1 has a total layer 2 buffer of 4000 bytes.

< 36.306-Table 4.1C-5:Half-duplex FDD operation type set by the field ue-Category-NB for a half-duplex FDD capable UE >

Category NB1 uses half-duplex FDD operation type B.

The uplink limit is larger than the downlink one, which is unusual. The 1000 bits match the largest value in the Release 13 NPUSCH TBS table, 36.213 Table 16.5.1.2-2. The table numbers jump from 4.1C-3 to 4.1C-5, and 36.306 v19.3.0 has no Table 4.1C-4 for NB-IoT.

Type B is the stricter half-duplex type. 36.211 v19.3.0 clause 6.2.5 defines it: the UE does not receive the downlink subframe just before an uplink subframe, or the one just after it. Each of those becomes a half-duplex guard subframe. Type A only drops the last part of one downlink subframe. 36.211 clause 10.2.2.3 allows only type B for NB-IoT.

• One field for both directions : ue-Category-NB covers downlink and uplink together.
• NB1 downlink : 680 bits per TTI and 2112 soft channel bits.
• NB1 uplink : 1000 bits per TTI.
• Type B half-duplex : a whole guard subframe before and after each uplink subframe.

What does Category NB2 add ?

The tables above stop at Release 13. 36.306 v19.3.0 adds Category NB2 to every one of them, and it gives NB2 two options that depend on 16QAM support. The table below collects the current values.

From 36.306 v19.3.0 Tables 4.1C-1, 4.1C-2, 4.1C-3 and 4.1C-5. The larger NB2 values apply when the UE indicates support of npdsch-16QAM-r17.

NB2 roughly quadruples the NB1 transport block in the downlink, and 16QAM almost doubles it again. The uplink rises to 2536 bits, which is the largest value in the 36.213 v19.4.0 NPUSCH TBS table. The half-duplex type does not change, so an NB2 UE still cannot receive and transmit at the same time.

36.306 also sets one rule between the two categories. A UE that indicates Category NB2 shall also indicate Category NB1. So an eNB that knows only Release 13 still sees NB1 from an NB2 UE, and it schedules that UE within the NB1 limits.

• NB2 downlink : 2536 bits per TTI, or 4968 bits with 16QAM.
• NB2 uplink : 2536 bits per TTI.
• Every NB2 UE is also NB1 : 36.306 clause 4.1C requires both indications.
• Still half-duplex type B : NB2 changes the buffers, not the duplex mode.

How does the UE report its category ?

The eNB needs the category before it schedules anything large, and it learns it from the UE capability. 36.331 v19.3.0 carries the category in more than one field, because each release that added a value added a new field for it.

From 36.331 v19.3.0. Value nb1 corresponds to Category NB1, and nb2 to Category NB2.

The field description of ue-Category-NB states that a UE shall always include ue-Category-NB-r13 in this version of the specification. An NB2 UE therefore sends nb1 in ue-Category-NB-r13 and nb2 in ue-Category-NB-r14, which is the 36.306 rule above written in ASN.1. A TDD UE reports NB2 through the separate TDD capability field. UE-RadioPagingInfo-NB carries the category to the MME, so that paging can take it into account.

The capability itself travels in a pair of RRC messages. The eNB sends UECapabilityEnquiry-NB, and the UE answers with UECapabilityInformation-NB. Both use SRB1 or SRB1bis, depending on whether AS security is active, as the SRB mapping page shows. So a Control Plane CIoT UE, which never activates AS security, still reports its category, and it does so over SRB1bis.

• ue-Category-NB-r13 is always present : it carries nb1 for every UE.
• nb2 comes in a later field : ue-Category-NB-r14 for FDD, ue-Category-NB-r15 for TDD.
• Paging carries the category too : UE-RadioPagingInfo-NB holds both the r13 and the r16 field.

Reference

[1] 3GPP TS 36.306 v19.3.0 - clause 4.1C for ue-Category-NB

[2] 3GPP TS 36.211 v19.3.0 - clause 6.2.5 and clause 10.2.2.3 for half-duplex FDD guard periods

[3] 3GPP TS 36.331 v19.3.0 - UE-Capability-NB and UE-RadioPagingInfo-NB