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LTE NB-IoT NPDSCH Function and Physical Layer Processing

lte nb-iot npsch physical layer data transmission

This page describes the LTE NB-IoT NPDSCH function, its location, and its basics with respect to the LTE-NB IoT Standard. It covers LTE NB NPDSCH Physical Layer Processing and its applications within the system. It also mentions the difference between NPDSCH and NPDCCH.

Function

The NPDSCH channel is used to carry unicast data from the eNB (i.e., base station) to the NB-IoT device. The data packet from the upper layers is segmented into one or more Transport Blocks (TBs). NPDSCH transmits one TB unit at a time. It is also used to transmit broadcast information such as System Information Blocks (SIBs).

The following table-1 mentions useful NPDSCH parameters along with their values.

NPDSCH informationValue
SubframeAny
Basic TTI1, 2, 3, 4, 5, 6, 8, 10
Repetitions1, 2, 4, 8, 16, 32, 64, 128, 192, 256, 384, 512, 768, 1024, 1536, 2048
Subcarrier Spacing15 KHz
Bandwidth180 KHz
CarrierAny

Location

NPDSCH follows a similar subframe-level resource mapping as described for NPDCCH.

The following points summarize the difference between NPDSCH and NPDCCH:

  • NPDCCH multiplexes resources to transmit two Downlink Control Information (DCI) messages in a single subframe, whereas a single NPDSCH subframe carries one TB at most. The basic Resource Unit (RU) for NPDSCH is one Physical Resource Block (PRB) pair.
  • The starting OFDM symbol in an NPDSCH subframe may be different from an NPDCCH subframe in in-band mode when the subframe is used to carry SIB1-NB information.

NPDSCH TB Size

NPDSCH TB Size: The table mentions TBS size as per TB size index and number of subframes.

LTE NB NPDSCH Physical Layer Processing

  • First, a 24-bit CRC is calculated and attached to the TB. NPDSCH uses QPSK modulation and supports TB sizes up to 680 bits for device category Cat-N1.
  • The CRC-attached TB is encoded using a TBCC encoder and rate-matched as per the codeword length. The coding rate depends on two parameters: TB size and the number of NPDSCH subframes.
  • Before mapping the bits of the encoded TB to QPSK symbols, the bits are scrambled first. The scrambling is re-initialized every min(NREP, 4) repetition of the codeword, where NREP is the number of configured repetitions. A maximum of up to 2048 repetitions can be transmitted.
  • After mapping the NPDSCH codeword onto a subframe, the subframe is repeated min(NREP, 4) times before the mapping of the codeword continues.
  • The baseband signal waveform is generated with QPSK modulated symbols as input, as per the waveform equation defined in the standard.

Reference

3GPP 36 series LTE