LTE NB-IoT Preamble and reference signals | NB-IoT NPSS, NSSS, NRS, DMRS

This page describes LTE NB-IoT Preamble (NPSS, NSSS) and reference signals (NRS, DMRS) used in downlink/uplink. It covers LTE NB-IoT NPSS, NSSS, NRS and DMRS mapping with location in the NB-IoT frame structure. The page describes difference between NB-IoT NPSS and LTE PSS as well as difference between NPSS and NSSS.

Introduction:
The NB-IoT frame consists of downlink and uplink channels and signals for various functions. NPSS, NSSS are used in preamble part where as NRS is used as reference signal in the downlink direction. DMRS is used as demodulation reference signal in the uplink direction. NPSS and NSSS are used to help device synchronize to NB-IoT cell.

The figure-1 depicts NB-IoT architecture. The signals and channels from eNB to UE are known as downlink channels/signals and frame is known as downlink frame. The signals and channels from UE to eNB are known as uplink channels/signals and frame is known as uplink frame.

Both NPSS and NSSS are transmitted in certain subframes based on 80ms repetition interval as shown in the figure-2 above. NPSS and NSSS synchronization help device to detect cell identity number and identify framing information within 80ms repetition interval.

NB-IoT NPSS

• Full Form: NPSS stands for Narrowband Primary Synchronization Signal.
• Function: It is used by device to achieve synchronization in both time and frequency. All the cells in NB-IoT network uses same NPSS. Hence device needs to search for one NPSS. In contrast LTE network uses three PSSs. This is the difference between NB-IoT NPSS and LTE PSS.
• Location: Each frame (of 10ms) consists of 10 subframes (numbered 0, 1, 2, ....9). The NPSS uses subframe number #5.
• NPSS is sequence generated using base sequence 'p' and binary cover code 'c'. Base sequence 'p' is length-11 frequency domain ZC sequence of root index '5' whose n^th frequency domain element is given by following equation.
➤p(n) = e^{-j*5*π*n (n+1)/11}, n = 0, 1, ....,10
Binary cover code, c = (1,1,1,1,-1,-1,1,1,1,-1,1)

Resource mapping within NPSS subframe is shown in the following figure.

➤Difference between NB-IoT NPSS and LTE PSS:

NB-IoT NSSS

• Full Form: NSSS stands for Narrowband Secondary Synchronization Signal.
• Function: After device completes coarse synchronization using NPSS, it uses NSSS to detect cell identity and to acquire more information about frame structure.
• Location: The NPSS uses subframe number #9 as shown. It has 80ms repetition interval which is used by four NSSS sequences as shown. Same set of four sequences are repeated in every 80 ms interval. The four occurences of NSSS are differentiated by phase shift θ_l, where θ_l equals 33*l/132. Only last 11 OFDM symbols are used to carry NSSS in the subframe.
• NB-IoT system uses 504 unique PCIDs (Physical Cell Identities) which is indicated by NSSS.

➤Difference between NPSS and NSSS: Compare to NPSS, NSSS is mapped to all 12 subcarriers of the PRB resulting in 132 REs in the subframe used for NSSS.

NB-IoT NRS

• Full form: NRS stands for Narrowband Reference Signal.
• Function: It is used to allow device to perform downlink channel estimation. It is also used to perform signal strength measurements and quality measurements both in Idle and connected modes.
• Location: It is mapped to certain subcarriers in last two OFDM symbols in every slot within subframe which carries NPBCH, NPDCCH or NPDSCH. Following rules apply to mapping of NRS.
- In all modes, NRS is present in subframes 0 and 4 as well as in subframes 9 not carrying NSSS.
- In stand-alone and guard band modes, NRS is also present in subframes 1 and 3.
- In all modes, NRS is present in all valid NB-IoT downlink subframes.
• The NRS symbol sequence is generated based on cell identity/port number. The pseudorandom QPSK sequence is used for randomizing interference between cells. The NRS sequence repeats itself in every 10 ms radio frame.

NB-IoT DMRS

• Full form: DMRS stands for Demodulation Reference Signal.
• Location: It is always associated with NPUSCH, either format-1 or 2. It is transmitted in every NPUSCH slot. The bandwidth of DMRS is identical to associated NPUSCH.
• NB-IoT DMRS of 180 KHz BW reuses LTE DMRS sequences defined for one PRB. New DMRS sequences are added to support NB-IoT DMRS with BW smaller than 180 KHz.
• For all the multiple formats, DMRS sequences are QPSK. For single tone formats, either 15 or 3.75 KHz BW , BPSK sequences are used.

Reference: 3GPP 36 series

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