NB-IOT – Physical Broadcast Channel (NPBCH)

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NPBCH stands for narrow band physical broadcast channel similar to PBCH in LTE Technology. The NPBCH is used to deliver the NB-IoT Master Information Block (MIB), which provides essential information for the NB-IOT UE to operate in the NB-IoT network. NPBCH uses a 640 ms TTI, but within the TTI only subframe 0 is used in every radio frame. NPBCH is transmitted in entire subframe zero of a radio frame (except initial 3 OFDM symbols).

  • Subframe Location: 0
  • Subframe periodicity: 10 ms
  • Basic TTI 640 ms

NPBCH Channel Characteristics.

  • NPBCH carries MIB (Master Information Block) MIB contain 34 bits  information after adding CR, TBCC  it result in 1600 bits at physical layer.
  • MIB is QPSK modulated, QPSK can carry 2 bits in each symbol it required 1600/2 = 800 symbols or 800REs to transmitted one complete MIB.
  • One MIB transmission period is about in 640 ms i.e. 64 Radio Frames
  • One NB-IOT subframe can utilize about 100 RE out of 168 others are used for reference signals, LTE PDCCH etc, so to transmit entire MIB 800 REs, 8 sub-frames are required. each sub-frame’s data related to NPBCH is called a code sub block (CSB).
  • Each subblock is repeated 8 times and is transmitted in every subframe zero th of each radio frame and each sub block consider entire sub-frame’s bandwidth except initial 3 ODFM symbols.

Picture Below is depicts the location of NPBCH in frequency and in time domain a Narrow Band Resource Block.

If  we have a close look the Narrow band resource block, there are 100 REs available for NPBCH in a subframe. NPBCH uses QPSK modulation, and thus 200 encoded bits can be carried in an NPBCH subframe. As stated above, NPBCH will only be transmitted in subframe 0 of the radio frame. The Transport Block Size of NPBCH is 34 bits and a 16-bit CRC is attached to the Transport Block makes it 50 bits which are encoded using the LTE Tail Bit Convolution Coding  and rate-matched to generate 1600 encoded bits. The encoded bits are segmented into eight code sub blocks (CSBs), each of which is 200-bit long and mapped to 100 QPSK symbols.

The transmission of the eight NPBCH code sub blocks in an NPBCH TTI is depicted in figure below. Each NPBCH subframe is self-decodable, but all the NPBCH subframes can also be jointly decoded. For certain NB-IOT UEs in good coverage, a single transmission of one CSB may be sufficient to decode the NPBCH information correctly and  UEs in poor coverage or cell edge can make you of repetitive NPBCH

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