Exploring 5G — Frame Structure

5 min readJun 19, 2018

Numerologies

In 5G multiple OFDM numerologies are supported, each numerology μ represents subcarrier spacing .following is the table used for determining different subcarrier spacing based on selected μ (higher-layer parameters DL-BWP-mu and UL-BWP-mu) and CP (DL-BWP-cp and ULBWP-cp).

Frames & Subframes

Downlink and Uplink transmissions are organized into frames . Each frame is of 10 ms duration .
Each frame is divided into 10 subframes , subframe duration is 1 ms.
Each subframe is further divided into multiple OFDM symbols .number of OFDM symbols depends on selected numerology called as mu (greek letter μ).
Number of OFDM symbols can be determined using following table , blue highlighted column is about number of OFDM symbols in one slot and yellow column is number of slots in one subframe

Number of OFDM symbols in one slots is always fixed (14) but number of slots in one subframe is variable and it depends on selected μ.

lets take few example to understand frame structure in more details .

when μ = 0

from below figure you can see for μ = 0 (highlighted row)
Number of OFDM symbols in one slot = 14
Number of Slots in one subframe = 1,hence in 1 ms there will be 14 OFDM symbols.

when μ = 2

Number of Slots in 1 subframe = 4

when μ = 3

Number of Slots in 1 subframe = 8

when μ = 4

Number of Slots in 1 subframe = 16

for extended cyclic prefix , following table shall be used to determine number of slots in one subframe and number of OFDM symbols in one slot

Each OFDM symbol in a slot can be downlink(D) ,uplink(U) and flexible(X) and below table is used to determine characteristics of OFDM symbols based on selected slot format indication (when SFI RNTI is used)in a slot.
This table is similar to legacy LTE TDD systems where the division of UL and DL was based on subframe but in 5G its based on OFDM sysmbols.

Resource Blocks

A 5G resource block is defined as 12 subcarriers in frequency domain .there are few new terms added in 5G ,lets explore them in detail

Point A :

This is a term used for common reference point for other resource blocks . All resource blocks will start from this point onwards.
This point tells the starting RB location in frequency domain and is obtained using following parameters .

PRB-index-DL-common : frequency offset between point A and lowest subcarrier of the lowest RB of SS/PBCH.
PRB-index-UL-common : for a PCell uplink in paired spectrum represents the frequency offset between point A and the frequency location based on ARFCN of the uplink indicated in SIB1.
PRB-index-UL-common for a PCell uplink in unpaired spectrum represents the frequency offset between point A and the lowest subcarrier of the lowest resrouce block of the SS/PBCH block used by the UE for initial cell selection.
PRB-index-DL-Dedicated for an SCell downlink represents the frequency offset between point A and the frequency location based on ARFCN in the higher-layer SCell configuration.
PRB-index-UL-Dedicated for an SCell uplink represents the frequency offset between point A and the frequency location based on ARFCN in the higher-layer SCell configuration.
PRB-index-SUL-common for a supplementary uplink represents the frequency offset between point A and the frequency location based on ARFCN in the higher-layer SUL configuration.

Common resource blocks :

Common resource blocks(CRB) are from 0 till max RB supported for a given subcarrier spacing μ.

The first subcarrier of CRB-0 will start from ‘point A’. figure explain about locations of CRB.

Physical Resource Block :

Physical resource blocks are defined in a bandwidth part and numbered from 0 to N_BWP,i^size (higher layer parameter).

Bandwidth Part:

Bandwidth Part (BWP) consists of a set of contiguous resource blocks(PRB from PRB 0 to N) .
Starting location of any BWP is given by higher layer in the following RRC message SIB-1(locationAndBandwidth) . here locationAndBandwidth is described as distance of BWP from point A.

BWP ::=      SEQUENCE {
locationAndBandwidth   INTEGER (0..37949),
subcarrierSpacing    SubcarrierSpacing,
cyclicPrefix     ENUMERATED { extended }              OPTIONAL -- Need R
}

RRC configures List of BWP for DL and UL both. and for each item in the list, starting location of BWP is given.
A UE can be configured upto 4 BWP in DL and UL and at a time only one BWP for DL and one for UL will be active. Network can change DL or UL BWP using DCI.
For unpaired spectrum, the UE may assume that two bandwidth parts with the same index i are paired.