Frame structure and physical resourcesV15.8.0

Basic timing units

• ns, where
• ns, where

Frame structure

Multiple OFDM numerologies are supported, where 𝜇 and the cyclic prefix for a bandwidth part are obtained from the higher-layer parameter subcarrierSpacing and cyclicPrefix, respectively.

Downlink and uplink transmissions are organized into frames with ms duration, each consisting of ten subframes of ms duration. The number of consecutive OFDM symbols per subframe is . Each frame is divided into two equally-sized half-frames of five subframes each with half-frame 0 consisting of subframes 0 – 4 and half-frame 1 consisting of subframes 5 – 9.

What it means:

• one frame = 10ms = two half-frames = 10 subframes
• one subframe = 1ms = 1/2/4/8 slots for μ=0/1/2/3, i.e. SCS of 15/30/60/120kHz
• one slot = 1/0.5/0.25/0.125ms for μ=0/1/2/3

There is one set of frames in the uplink and one set of frames in the downlink on a carrier.

What it means: uplink and downlink can have different numerologies, and hence different frame structure

For subcarrier spacing configuration μ, slots are numbered in increasing order within a subframe and in increasing order within a frame. There are consecutive OFDM symbols in a slot where depends on the cyclic prefix. The start of slot in a subframe is aligned in time with the start of OFDM symbol in the same subframe.

What it means:

• one slot = 14 or 12 symbols, for normal or extended CP, respectively.
• For extended CP, every symbol have the same time duration, which is .
• For normal CP, every 0.5ms there is one symbol with longer duration than the others. This is true for all numerologies. The symbol index with longer duration is symbol 0 and symbol . The time difference between longer and normal symbols is 0.521 us.

OFDM symbols in a slot can be classified as 'downlink', 'flexible', or 'uplink'. Signaling of slot formats is described in subclause 11.1 of TS 38.213.

• In a slot in a downlink frame, the UE shall assume that downlink transmissions only occur in 'downlink' or 'flexible' symbols.
• In a slot in an uplink frame, the UE shall only transmit in 'uplink' or 'flexible' symbols.

Note: SCS of 240 kHz (μ = 4) is only applicable to SSBs, not for data transmission.

Physical resources

• Antenna ports:
  • An antenna port is defined such that the channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed. For DM-RS associated with a PDSCH, the channel over which a PDSCH symbol on one antenna port is conveyed can be inferred from the channel over which a DM-RS symbol on the same antenna port is conveyed only if the two symbols are within the same resource as the scheduled PDSCH, in the same slot, and in the same PRG as described in clause 5.1.2.3 of [6, TS 38.214].
  • For DM-RS associated with a PDCCH, the channel over which a PDCCH symbol on one antenna port is conveyed can be inferred from the channel over which a DM-RS symbol on the same antenna port is conveyed only if the two symbols are within resources for which the UE may assume the same precoding being used as described in clause 7.3.2.2.
  • For DM-RS associated with a PBCH, the channel over which a PBCH symbol on one antenna port is conveyed can be inferred from the channel over which a DM-RS symbol on the same antenna port is conveyed only if the two symbols are within a SS/PBCH block transmitted within the same slot, and with the same block index according to clause 7.4.3.1.
  • Two antenna ports are said to be quasi co-located if the large-scale properties of the channel over which a symbol on one antenna port is conveyed can be inferred from the channel over which a symbol on the other antenna port is conveyed. The large-scale properties include one or more of delay spread, Doppler spread, Doppler shift, average gain, average delay, and spatial Rx parameters.
• Resource grid:
  • For each numerology and carrier, a resource grid of subcarriers and OFDM symbols is defined. starting at common resource block indicated by higher-layer signalling. There is one set of resource grids per transmission direction (uplink or downlink) with the subscript x set to DL and UL for downlink and uplink, respectively.
  • The carrier bandwidth for subcarrier spacing configuration μ is given by the higher-layer parameter carrierBandwidth in the SCS-SpecificCarrier IE. The starting position for subcarrier spacing configuration μ is given by the higher-layer parameter offsetToCarrier in the SCS-SpecificCarrier IE. The frequency location of a subcarrier refers to the center frequency of that subcarrier.
  • What it means: resource grid is defined for all subcarriers in frequency domain, times 1subframe in time domain. Uplink and downlink can have different numerologies (SCS), and hence different resource grids.
• Resource elements:
  • Each element in the resource grid for antenna port p and subcarrier spacing configuration μ is called a resource element and is uniquely identified by where k is the index in the frequency domain and l refers to the symbol position in the time domain relative to some reference point. Resource element corresponds to a physical resource and the complex value .
  • What it means: a resource element is the smallest resource unit for NR, which transmit one complex value.
• Resource block: A resource block is defined as 12 consecutive subcarriers in the frequency domain.
  • Point A:

    serves as a common reference point for resource block grids and is obtained from:

    • offsetToPointA for a PCell downlink where offsetToPointA represents the frequency offset between point A and the lowest subcarrier of the lowest resource block, which has the subcarrier spacing provided by the higher-layer parameter subCarrierSpacingCommon and overlaps with the SS/PBCH block used by the UE for initial cell selection, expressed in units of resource blocks assuming 15 kHz subcarrier spacing for FR1 and 60 kHz subcarrier spacing for FR2;
    • absoluteFrequencyPointA (in FrequencyInfoUL-SIB, FrequencyInfoUL, or FrequencyInfoDL) for all other cases where absoluteFrequencyPointA represents the frequency-location of point A expressed as in ARFCN.
  • Common resource blocks (CRB):
    • CRBs are numbered from 0 and upwards in the frequency domain for subcarrier spacing configuration μ. The center of subcarrier 0 of common resource block 0 for subcarrier spacing configuration μ coincides with 'point A'.
    • The relation between the common resource block number in the frequency domain and resource elements (k, l) for subcarrier spacing configuration μ is given by , where k is defined relative to point A such that k = 0 corresponds to the subcarrier centered around point A.
  • Physical resource blocks (PRB):
    • PRBs for subcarrier spacing configuration μ are defined within a bandwidth part and numbered from 0 to where i is the number of the bandwidth part. The relation between the physical resource block in bandwidth part i and the common resource block is given by , where is the common resource block where bandwidth part starts relative to common resource block 0.
  • Virtual resource blocks (VRB):
    • VRBs are defined within a bandwidth part and numbered from 0 to where i is the number of the bandwidth part.
  • Bandwidth part (BWP):
    • A bandwidth part is a subset of contiguous common resource blocks for a given numerology in bandwidth part i on a given carrier. The starting position and the number of resource blocks in a bandwidth part shall fulfil and , respectively. Configuration of a bandwidth part is described in clause 12 of TS 38.213.
    • A UE can be configured with up to four bandwidth parts in the downlink with a single downlink bandwidth part being active at a given time. The UE is not expected to receive PDSCH, PDCCH, or CSI-RS (except for RRM) outside an active bandwidth part.
    • A UE can be configured with up to four bandwidth parts in the uplink with a single uplink bandwidth part being active at a given time. If a UE is configured with a supplementary uplink, the UE can in addition be configured with up to four bandwidth parts in the supplementary uplink with a single supplementary uplink bandwidth part being active at a given time. The UE shall not transmit PUSCH or PUCCH outside an active bandwidth part. For an active cell, the UE shall not transmit SRS outside an active bandwidth part.

Diagram below shows the time and frequency resources with 2880 kHz X 1ms. In this diagram, each block corresponds to:

• 1 RB in frequency domain, which is 12 sub-carriers, spans kHz, i.e., 720 kHz for μ=2
• 1 slot in time domain, which is 14 or 12 symbols, spans μs, i.e., 250 μs for μ=2

Note with the same bandwidth, the number of resource elements within a certain time period is roughly the same. This means changing SCS alone won't affect the throughput. However, be aware that higher SCS can support wider bandwidth, hence higher peak throughput per carrier.