Terminology » LTE » LTE NB-IoT NPBCH: Function and Physical Layer Processing

LTE NB-IoT NPBCH: Function and Physical Layer Processing

lte nb-iot npbch physical layer mib

This page describes the function and basics of the LTE NB-IoT NPBCH (Narrowband Physical Broadcast Channel) as per the LTE-NB IoT Standard. It covers LTE NB NPBCH Physical Layer Processing and NPBCH applications within the system.

Function of NPBCH

The NPBCH stands for NB-IoT Physical Broadcast Channel. It’s used to carry MIB (Master Information Block) information to the device in the downlink by the eNB (evolved NodeB, i.e., Base station). This helps the device to operate within the NB-IoT network.

NPBCH mapping in NB-IoT Frame

Location: As shown in the figure above, NPBCH uses a 640 ms TTI (Transmission Time Interval), but within that TTI, only subframe #0 is used in every radio frame.

The following table outlines key information about the NPBCH.

NPBCH informationValue
Subframe0
Subframe periodicity10 ms
Basic TTI640 ms
Subcarrier spacing15 KHz
Bandwidth180 KHz
CarrierAnchor

LTE NB NPBCH Physical Layer Processing

NPBCH resource mapping

  • As shown in the figure above, 100 REs (Resource Elements) are available for NPBCH in a subframe. NPBCH utilizes QPSK (Quadrature Phase Shift Keying) modulation. Therefore, 200 encoded bits can be carried in an NPBCH subframe.
  • A TBS (Transport Block Size) of 34 bits, along with 16 bits of CRC (Cyclic Redundancy Check), results in 50 bits during NPBCH processing.
  • These 50 bits are encoded by the LTE TBCC (Tail Biting Convolutional Code) and subsequently rate-matched to produce an output of 1600 encoded bits.
  • These 1600 encoded bits are segmented into 8 CSBs (Code Subblocks), each with a size of 200 bits. Each of these CSBs is mapped to 100 QPSK symbols.

NPBCH CSBs Transmission

The figure above depicts the transmission of NPBCH CSBs. Symbol-level scrambling is applied to each CSB to provide robust protection against inter-cell interference.

Reference: 3GPP 36 series Rel.13, 14, 15