LTE NB-IoT Preamble and Reference Signals: 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 the difference between NB-IoT NPSS and LTE PSS as well as the difference between NPSS and NSSS.

Introduction

The NB-IoT frame consists of downlink and uplink channels and signals for various functions. NPSS and NSSS are used in the preamble part, whereas NRS is used as a reference signal in the downlink direction. DMRS is used as a demodulation reference signal in the uplink direction.

NPSS and NSSS are used to help devices synchronize to an NB-IoT cell.

Figure 1: LTE NB Access Network Architecture

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

Figure 2: LTE NB IoT NPSS and NSSS

Both NPSS and NSSS are transmitted in certain subframes based on an 80ms repetition interval, as shown in Figure 2 above. NPSS and NSSS synchronization helps the device detect the cell identity number and identify framing information within the 80ms repetition interval.

NB-IoT NPSS

• Full Form: NPSS stands for Narrowband Primary Synchronization Signal.

• Function: It is used by the device to achieve synchronization in both time and frequency. All the cells in the NB-IoT network use the same NPSS. Hence, the device needs to search for one NPSS. In contrast, the 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 a sequence generated using base sequence ‘p’ and binary cover code ‘c’. Base sequence ‘p’ is a length-11 frequency domain ZC sequence of root index ‘5’ whose $n^{th}$ frequency domain element is given by the following equation:

  $p(n) = e^{-j*5*\pi*n*(n+1)/11}$, $n = 0, 1, ..., 10$

  Binary cover code, $c = (1, 1, 1, 1, -1, -1, 1, 1, 1, -1, 1)$

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

Figure: LTE Resource mapping in NPSS and NSSS

Difference between NB-IoT NPSS and LTE PSS: (This was already explained above under “Function”.)

NB-IoT NSSS

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

Difference between NPSS and NSSS: Compared 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

Figure: LTE NB IoT NRS

• Full form: NRS stands for Narrowband Reference Signal.
• Function: It is used to allow the 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 the last two OFDM symbols in every slot within a subframe which carries NPBCH, NPDCCH, or NPDSCH. The following rules apply to the 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 the 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 LTE