NR
supports multiple numerologies as a consequence of which, there are a multiple resource
grids i.e., there is one resource grid for each numerology and carrier. Different
numerologies are discussed in the post: 5G NR: Numerologies and Frame Structure.
The
following are valid for a resource grid;
-
A
resource grid is characterized by one subframe in time domain and full carrier
bandwidth in the frequency domain.
-
There
are a set of resource grids per transmission direction (uplink or downlink).
-
There
is one resource grid for a given antenna port p, subcarrier spacing
configuration µ, and transmission direction (downlink or uplink).
Resource grid in general is given in the following
figure assuming Normal CP.
Configuration
required to derive Resource Grid
The
following parameters are needed to derive the corresponding resource grid.
carrierBandwidth defines
the width of the corresponding carrier in number of PRBs which can be from 1 to
275 (max No. of PRBs). This provides size of the grid which is given
(see the figure above) by Ngridsize, µ.
offsetToCarrier
defines the starting position Ngridstart, µ for
subcarrier spacing configuration µ. Basically, this field provides offset
in frequency domain between Point A (lowest subcarrier of common RB 0) and the
lowest usable subcarrier on this (where this field is received) carrier in
number of PRBs.
Both
carrierBandwidth and offsetToCarrier are
configured within SCS-SpecificCarrier IE which is carried by FrequencyInfoDL/
FrequencyInfoUL/ FrequencyInfoUL-SIB/ FrequencyInfoDL-SIB within
ServingCellConfigCommon/ ServingCellConfigCommonSIB. This information
is either transmitted by the gNB in SIB1 or configured via dedicated RRC signalling.
Resource Grid Overview
Number of slots per subframe:
As
can be seen from the resource grid figure above, there are Nsymbsubframe,
µ OFDM symbols in the time-domain which depends on numerology
being used.
-
Nsymbsubframe, µ is equal to the number of slots per subframe (Nslotsubframe,
µ) multiplied by the number of symbols in a slot (Nsymbslot).
The following table summarizes number of symbols in a subframe against numerology.
Nsymbsubframe,
µ = Nsymbslot x Nslotsubframe,
µ
µ
|
SCS (kHz)
|
Number of OFDM symbols per Slot
Nsymbslot
|
Number of slots per frame
Nslotframe, µ
|
Number of slots per subframe
Nslotsubframe, µ
|
Number of symbols per subframe
Nsymbsubframe, µ
|
0
|
15
|
14
|
10
|
1
|
14
|
1
|
30
|
14
|
20
|
2
|
28
|
2
|
60
|
14
|
40
|
4
|
56
|
3
|
120
|
14
|
80
|
8
|
112
|
4
|
240
|
14
|
160
|
16
|
224
|
Resource Element (RE):
It is the smallest physical resource in NR and it consists of one subcarrier
during one OFDM symbol. It is uniquely identified by (k, l) 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 Block (RB):
An RB is defined as 12 number of consecutive subcarriers in frequency domain.
Reference: 3GPP TS 38.211
