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NB-IoT Frame Structure | LTE-NB Frame Structure


This page covers NB-IoT Frame Structure as per LTE-NB standard. It describes NB-IoT Frame Structure with 15 KHz Subcarrier spacing and 3.75 KHz Subcarrier Spacing.

Introduction:
• The figure-1 depicts overall time frame structure used in NB-IoT AS (Access Stratum).
• In downlink (DL) and uplink (UL), NB-IoT supports 15 KHz subcarrier spacing (SCS) frame structure. This frame consists of 20 slots as shown in figure-1.
• In UL (Uplink), one more additional frame structure with 3.75 KHz SCS is supported. In this frame structure, each frame is divided into 5 slots with each having 2ms duration as shown below in figure-3.

NB-IoT Frame Structure for 15 KHz SCS

NB-IoT Frame Structure for 15 KHz SCS
Figure-1

➤One hyperframe Cycle = 1024 hyperframes
➤One hyperframe = 1024 frames
➤One frame = 10 subframes
➤One subframe = 2 slots
➤One slot duration = 0.5 ms

Hyperframe/Frame are designated as H-SFN and SFN respectively. H-SFN stands for Hyper System Frame Number, SFN stands for System Frame Number and SN stands for Subframe Number. H-SFN, SFN and SN range from 0-1023, 0-1023 and 0-9 respectively.

PRB Pair in NB-IoT Downlink
Figure-2

In the downlink, concept of PRBs is used to specify mapping of physical channels/signals onto REs. As shown, RE (Resource Element) is used as smallest physical channel unit. RE identified by its index i.e. subcarrier index (k) and symbol index (l) within PRB. A PRB spans 12 subcarriers over 7 OFDM symbols with total 12 x 7 (i.e. 84 ) REs. One PRB pair is used as smallest schedulable unit in most DL cases which occupies 2 consecutive slots.

NB-IoT Frame Structure for 3.75 KHz SCS

NB-IoT Frame Structure for 3.75 KHz SCS
Figure-3

In the uplink, RU is used to specify mpping of UL channels onto REs. The definition of RU depends on configured SCS and number of allocated subcarriers to UL transmission. For 15 KHz SCS and 12 subcarriers, RU corresponds to PRB pair as shown.

In sub-PRB scheduling case with assignments of 6/3/1 subcarrier, RU is expanded in time to compensate for diminishing frequency allocation.
In single subcarrier allocation case which is known as single tone allocation, RU supports additional subcarrier spacing of 3.75 KHz. Following RU formats are supported in various UL transmit configurations.

NB-IoT NPUSCH RU

The table summarizes definitions of RU for various NPUSCH configurations.



LTE and NB-IoT RELATED LINKS

This tutorial section on LTE basics covers following sub topics:
LTE tutorial  NB-IoT tutorial  LTE features  LTE terminologies  Frame  TDD FDD  Channel types  PHY  stack  throughput  CA   cell search  network entry  Timers  PSS vs SSS  Security   LTE Bands  EARFCN  Hotspot  router  What is VoLTE  VoLTE originating call flow  VoLTE UE attach and registration procedure 

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