5G(NR): Frame structure( Slots and symbols Formats)

5G(NR): Numerologies and Frame structure( Slots and symbols Formats)

               In this post, we will discuss about NR numerologies and frame structure. Numerology (3GPP term) is defined by Sub Carrier Spacing (SCS) and Cyclic Prefix (CP).

   In LTE, there is no need for any specific term to indicate the subcarrier spacing because there is only one subcarrier spacing, which is 15KHz, but there are several different types of subcarrier spacing in NR.

Slot Structure:

       The transmission of Downlink and Uplink are organized into frames. Each frame is of 10-millisecond duration. Each frame is divided into 10 subframes of 1 millisecond, and the subframe is further divided into slots according to numerology.

In LTE, only 2 slots are available. But in NR, the number of slots varies according to the numerology. in 1 slot, the number of symbols are fixed that is 14-with normal cyclic prefix(CP) and 12-with extended CP. 

The following table summaries number of slots in a sub-frame/frame for each numerology with normal prefix.

Normal CP

Numerology = 0

Numerology 0 means 15 kHz subcarrier spacing. in this a sub-frame has only one slot available in it, it means that a radio frame contains 10 slots in it. The number of OFDM symbols are 14 within each slot.

 Numerology = 1

Numerology 1 means 30 kHz subcarrier spacing. in this configuration, a subframe is divided into 2 slots, it means a radio frame contains total of 20 slots in it. The number of OFDM symbols within a slot are 14 symbols.

Numerology = 2

Numerology 2 means 60 kHz subcarrier spacing. In this configuration, a subframe is divided into 4 slots, it means a radio frame contains total 40 slots in it. The number of OFDM symbols within a slot is 14 symbols.

Numerology = 3:

Numerology 3 means 120 kHz subcarrier spacing. In this configuration, a subframe is divided into  8 slots, it means a radio frame contains total 80 slots in it. The number of OFDM symbols within a slot is 14 symbols.

Numerology = 4:

Numerology 4 means 240 kHz subcarrier spacing. In this configuration, a subframe is divided into 16 slots in it, it means a radio frame contains total 160 slots in it. The number of OFDM symbols within a slot is 14-symbols.

Extended CP

Numerology = 2

In this configuration, a subframe is divided into 8 slits, it means a radio frame contains total 80 slots in it. The number of OFDM symbols within a slot are 12-symbols.

Slot Formats:

        As we have seen above a slot has fixed 14-symbols with normal CP and how these 14 symbols are getting configured during transmission, is indicated by Slot Format. A slot can be categories as downlink (all symbols are dedicated for downlink) or uplink (all symbols are dedicated for uplink) or mixed uplink and downlink transmissions.

In the case of FDD(for UL and DL there are two different carriers), all symbols within a slot for a downlink carrier are used for downlink transmissions and all symbols within a slot for an uplink carrier are used for uplink transmissions because there are two separate carriers for uplink and downlink transmitions.

TDD Slot configuration:

      5G provides a feature using which each symbol within a slot can either be used to schedule Uplink packet (U) or Downlink packet(D) or Flexible (F). A symbol marked as Flexible means it can be used for either Uplink or Downlink as per requirement.

In NR, slot format configuration can be done in a static, semi-static or fully dynamic fashion. The configuration for Slot format would be broadcast from SIB1 or/and configured with the RRC Connection Reconfiguration message. The configuration of Static and semi-static for a slot is done using RRC while dynamic slot configuration is done using PDCCH DCI.
Note that if a slot configuration is not provided by the network through RRC messages, all the slots/symbols are considered as flexible by default.

Slot configuration via RRC consists of two parts. 
 1-  Providing UE with Cell-Specific Slot format Configuration (tdd-UL-DL-ConfigurationCommon)
 2-  Providing UE with dedicated Slot format configuration (tdd-UL-DL-ConfigurationDedicated)