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LTE NB-IoT NPDSCH Function | LTE NB NPDSCH Physical Layer Processing


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

Function:
The NPDSCH channel is used to carry unicast data from eNB (i.e. base station) to NB-IoT device. The data packet from upper layers is segmented into one or more TBs. NPDSCH transmits one TB unit at one time. NPDSCH is used to transmit broadcast information such as SIBs. Following table-1 mentions useful NPDSCH parameters with their values.

NPDSCH information Value
Subframe Any
Basic TTI 1, 2, 3, 4, 5, 6, 8, 10
Repetitions 1, 2, 4, 8, 16, 32, 64, 128, 192, 256, 384, 512, 768, 1024, 1536, 2048
Subcarrier Spacing 15 KHz
Bandwidth 180 KHz
Carrier Any

Location: NPDSCH follows similar subframe level resource mapping as described for NPDCCH. Following points summarize difference between NPDSCH and NPDCCH.
➤NPDCCH multiplex resources to transmit two DCI messages in a single subframe, where as the single NPDSCH subframe carry one TB at most. Basic RU for NPDSCH is one PRB pair.
➤ The starting OFDM symbol in NPDSCH subframe may be different from NPDCCH subframe in in-band mode when subframe is used to carry SIB1-NB informations.

NPDSCH TB Size

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

LTE NB NPDSCH Physical Layer Processing

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

Reference: 3GPP 36 series



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