As in legacy LTE, LTE-NB also transmit cell specific reference(CRS) signal called NRS. The sequence (data) is generated in a very similar way to legacy LTE, but RE mapping (the position in resource map) is a little bit modified in such way that NRS does not overwriting legacy LTE CRS).

Followings are the topics to be covered in this page.

• Signal Generation of NRS
• Resource Element Mapping
• In which subframes can the UE rely on NRS ?
• Reference

Signal Generation of NRS

The signal generation method (formula) for NRS (LTE-NB) is same as RS(LTE) except the Cell ID parts as indicated below. If you are really interested and motivated, you may try writing a small program (in C, Matlab or whatever you are familiar with) to generate these sequence. Once you really tried this, you would understand the meaning of each of these parameters even though the resulting data itself is not 100% accurate. Just trying would give you much deeper understanding. The most tricky part of generating this sequence would be to implement the pseudo random sequence(c()). LTE Psuedo Random Sequence would help you on this.

NRS reuses the LTE CRS sequence, with the NB-IoT cell ID N_ID^Ncell in place of N_ID^cell. The label on the upper formula reads 36.211-10.2.7.2, but the NRS sequence is in 36.211 clause 10.2.6.1, and clause 10.2.7.2 is the NSSS.

The NRS sequence is the LTE CRS sequence of 36.211 clause 6.10.1.1, with N_ID^cell replaced by N_ID^Ncell. 36.211 v19.3.0 clause 10.2.6.1 defines it that way. So every term in the lower formula carries over, including the ones that NB-IoT never uses.

Two of those terms are worth a second look. N_CP is always 1 for NB-IoT, because the NB-IoT downlink supports normal cyclic prefix only. The alternative n'_s for frame structure type 3 belongs to LTE LAA, so an NB-IoT UE always uses n_s itself. In practice the NRS sequence is the LTE CRS sequence for normal CP, seeded with the NB-IoT cell ID.

• Same generator as the LTE CRS : clause 10.2.6.1 points straight back to clause 6.10.1.1.
• Only the cell ID changes : N_ID^Ncell replaces N_ID^cell in c_init.
• N_CP is always 1 : NB-IoT downlink supports normal cyclic prefix only.
• m runs over two resource blocks : the formula keeps 2N_RB^max,DL elements, and the mapping picks two of them per symbol.

Resource Element Mapping

The location of NRS (LTE-NB Reference Signal) is as shown below. It has two different pattern, one for Single Antenna mode and the other one for two antenna mode. You may refer to 36.211-10.2.6.2 for formal description.

NRS sits in the last two OFDM symbols of every slot. Each symbol carries two NRS resource elements per port, six subcarriers apart.

• One antenna port : R0 is at subcarriers 0 and 6 in symbol 5, and at 3 and 9 in symbol 6, in both slots.
• Two antenna ports : R1 takes the opposite pattern: 3 and 9 in symbol 5, and 0 and 6 in symbol 6.
• The grey squares are reserved : each port leaves the other port's positions empty, so the two never overlap.
• The picture assumes v_shift = 0 : a different cell ID moves every NRS up the subcarrier axis.

The Resource Map shown above is just an example. The exact location would vary slighly depending on LTE NB Physical Cell ID and following equation. If you look into this very carfully, you would notice that the time domain location for NRS does not change, but frequency domain location varies with parameters m, v and v_shift. m varies between 0 and 1. v varies with antenna port number and v_shift varies with Physical Cell ID. It means that the reference signal  position in frequency domain would vary depending on Physical Cell ID(PCI). To be more accurate, the frequency location of NRS shifts by 1 as PCI increments by 1, but after the shift cycle through 6 RE(Resource Element) in frequency domain. This is same logic as in legacy LTE.

The time position is fixed at symbols 5 and 6 of each slot. The frequency position depends on the port through v and on the cell ID through v_shift.

• m takes only 0 and 1 : two NRS resource elements per symbol and per port.
• m' picks the middle of the LTE sequence : m' = m + N_RB^max,DL - 1, so NRS uses the two elements at the centre.
• v separates the ports : 0 or 3, depending on port 2000 or 2001 and on the symbol.
• v_shift separates the cells : N_ID^Ncell mod 6 gives six frequency shifts.

36.211 v19.3.0 clause 10.2.6.2 adds one more rule for the in-band case. Higher layers can indicate that the NB-IoT cell ID equals the LTE cell ID. In that case, the UE may assume that CRS antenna ports 0 and 1 are equivalent to NRS antenna ports 2000 and 2001. It may also assume the same number of CRS and NRS ports, and that the CRS is available wherever the NRS is. So in the inband-SamePCI case, the UE can use the LTE CRS inside the NB-IoT PRB to improve its channel estimate. Otherwise, the UE takes the number of CRS ports from eutra-NumCRS-Ports.

In which subframes can the UE rely on NRS ?

The frame structure maps on this site draw NRS in almost every subframe. The specification is more careful, because a UE may only rely on NRS where 36.211 says it may assume it. The answer changes as the UE learns more about the cell, so it depends on the stage of cell acquisition and on the operation mode.

The table below summarises 36.211 v19.3.0 clause 10.2.6 for an FDD anchor carrier. Each row adds to the one above it.

From 36.211 v19.3.0 clause 10.2.6, for frame structure type 1 on an anchor carrier. The UE may assume NRS in these subframes, and in no others.

A non-anchor carrier works differently. When DL-CarrierConfigCommon-NB configures it, NRS is only guaranteed around the transmissions the UE is waiting for. For paging, random access and the common search spaces, the UE may assume NRS in 10 NB-IoT downlink subframes before and 4 after each search space, and around each scheduled NPDSCH. So a UE on a non-anchor carrier cannot expect a continuous reference signal to track.

• NRS is guaranteed in subframes 0, 4 and 9 from the start : those are the subframes a UE may use before it knows anything else about the cell.
• Guardband and standalone add subframes 1 and 3 : in-band adds subframe 3 only when it carries an additional SIB1-NB transmission.
• SIB1-NB unlocks the rest : after it, NRS is present in every NB-IoT downlink subframe.
• Non-anchor NRS follows the traffic : it is guaranteed only before, during and after the search spaces and NPDSCH the UE monitors.

Reference

[1] 3GPP TS 36.211 v19.3.0 - clause 6.10.1.1 for the LTE CRS sequence, clause 10.2.6 for NRS sequence generation, mapping and availability