As you may easily guess, LTE-NB has simpler than legacy LTE as illustrated below (Simple but not extremely simpler than legacy LTE. It is almost same structure as Release 8 legacy channel structure).

Followings are illustrated summary of 36.212 - 6.1 Mapping to physical channels and 36.211-10.1.1 Overview.

The two diagrams below show the mapping in each direction, and the table after them lines up every legacy LTE channel with its LTE-NB counterpart. Most of the simplification comes from channels that LTE-NB drops, not from new ones.

Followings are the topics to be covered in this page.

• Downlink
• Uplink
• Which legacy LTE channels are missing in LTE-NB ?
• Reference

Downlink

The downlink has to carry the same three kinds of traffic as LTE: broadcast system information, paging and user data. It also needs physical signals for synchronization and channel estimation. The diagram below shows how LTE-NB maps these onto three physical channels and three physical signals.

BCH maps to NPBCH, PCH and DL-SCH both map to NPDSCH, and DCI maps to NPDCCH. NRS, NPSS and NSSS are physical signals, so no transport channel maps to them.

• Three transport channels : BCH, PCH and DL-SCH, as in 36.212 Table 6.1-2.
• PCH shares NPDSCH with DL-SCH : paging and user data travel on the same physical channel, scheduled by different DCIs.
• DCI is control information : 36.212 Table 6.1-3 maps it to NPDCCH.
• The yellow blocks are physical signals : NRS for channel estimation, NPSS and NSSS for synchronization and the cell ID.

36.211 v19.3.0 clause 10.2.1 lists two more downlink physical signals than the diagram. NPRS is the narrowband positioning reference signal, and NWUS is the narrowband wake up signal. NWUS tells a UE in idle mode whether it needs to decode NPDCCH for the paging occasion that follows. When no NWUS is sent, the UE can skip that paging occasion without decoding NPDCCH at all.

All three downlink transport channels use the same channel coding. 36.212 clause 6.4 uses tail biting convolutional coding for BCH, DL-SCH and PCH, and Table 6.2-2 uses it for DCI too. LTE uses turbo coding for DL-SCH, so this is one more place where the NB-IoT receiver is simpler.

Uplink

The uplink is simpler still. It needs a way to access the cell, a way to send data, and a way to return HARQ feedback. LTE uses three physical channels for these, PRACH, PUSCH and PUCCH, and the diagram below shows how LTE-NB does it with only two.

RACH maps to NPRACH. UL-SCH maps to NPUSCH format 1 and UCI maps to NPUSCH format 2. DMRS is the only uplink physical signal.

NOTE : No PUCCH in NB-LTE. UCI information in NB LTE is also carried by NPUSCH

• Two transport channels : RACH and UL-SCH, as in 36.212 Table 6.1-1.
• NPUSCH has two formats : format 1 carries UL-SCH, and format 2 carries UCI.
• UCI is mainly HARQ-ACK : a 1-bit ACK or NACK for NPDSCH, coded with a rate 1/16 block code.
• DMRS is the only uplink signal : 36.211 clause 10.1.1.2 lists no SRS for NB-IoT.

NPUSCH format 2 carries HARQ-ACK for NPDSCH. 36.212 clause 6.3.3 turns the single bit into 16 identical bits, which is the rate 1/16 block code of Table 6.2-2. In 36.213 v19.4.0, a UE configured with sr-with-HARQ-ACK-Config can also send a scheduling request in the same NPUSCH format 2. A UE with sr-WithoutHARQ-ACK-Config sends its scheduling request on NPRACH instead.

UL-SCH uses turbo coding with rate 1/3, like LTE. 36.211 clause 10.1.2.1 fixes a single antenna port for all uplink transmissions.

Which legacy LTE channels are missing in LTE-NB ?

The introduction says that LTE-NB is almost the same structure as legacy LTE. That is true for the channels it keeps, but several LTE channels have no LTE-NB counterpart at all. The table below lines the two up, and the channels that disappear show where the simplification really comes from.

From 36.211 v19.3.0 clauses 10.1.1 and 10.2.1 and 36.213 v19.4.0 clauses 16.5.2 and 16.6.1. NWUS has no legacy LTE counterpart, because it was defined for NB-IoT.

Each missing channel removes a job from the receiver. Without PCFICH, the UE does not decode a control format indicator in every subframe. Without PHICH, the UE does not look for a separate HARQ indicator after each NPUSCH. Without PUCCH, the uplink has no separate control region, and the eNB schedules HARQ-ACK like any other NPUSCH transmission.

• Six LTE downlink channels shrink to three : PCFICH, PHICH and PMCH disappear, and only NPBCH, NPDCCH and NPDSCH remain.
• Three LTE uplink channels shrink to two : PUCCH disappears, and NPUSCH format 2 takes over its job.
• SRS disappears : one PRB and a single antenna port leave nothing to sound.
• NWUS is new : it saves UE power in idle mode, which LTE never needed a signal for.

Reference

[1] 3GPP TS 36.211 v19.3.0 - clause 10.1.1 and clause 10.2.1, NB-IoT uplink and downlink physical channels and signals

[2] 3GPP TS 36.212 v19.3.0 - clause 6.1 for mapping to physical channels, clause 6.2 for channel coding

[3] 3GPP TS 36.213 v19.4.0 - clause 16.4.2 for ACK/NACK, clause 16.5.2 for NPUSCH retransmission, clause 16.6.1 for the NPDCCH start symbol