Similar
to LTE, a resource block (RB) in NR is defined as 12 consecutive subcarriers.
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 OFDM 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
irrespective of the numerology.
An
LTE resource block is defined in both time and frequency domain i.e., an RB occupies
12 subcarriers in frequency domain and one slot in time domain. In contrast, an
RB in NR is defined only in frequency domain. This approach provides NR system
a lot of scheduling flexibility in time domain.
As
shown in the following table, the bandwidth occupied by an RB depends upon the numerology
being used.
µ
|
SCS
|
Bandwidth per RB = 12 x SCS
|
0
|
15 kHz
|
180 kHz
|
1
|
30 kHz
|
360 kHz
|
2
|
60 kHz
|
720 kHz
|
3
|
120 kHz
|
1440 kHz
|
4
|
240 kHz
|
2880 kHz
|
Point A
Bandwidth
Adaptation (BA) in NR is achieved by configuring
the UE with Bandwidth Part(s) (BWPs). This allows the NR carrier to
support multiple numerologies across the carrier bandwidth. Different UE’s may
be configured with different BWPs based on the need and the UE capability. Therefore,
the UEs need a common reference point from which the relative position of a BWP
could be determined.
Point
A serves as a common reference point for resource block
grids. For all subcarrier spacings, the lowest subcarrier (subcarrier #0) of
Common RB #0 (discussed later) is referred to as Point A.
After
decoding SSB, the UE doesn’t automatically know starting PRB of the bandwidth
part. The UE needs to first determine the position of Point A using one
of the following parameters;
1. offsetToPointA represents
frequency offset between Point A and the lowest subcarrier of the common resource block which overlaps with the start of SSB. This field is provided by the network via
FrequencyInfoDL-SIB as part of SIB1.
2. absoluteFrequencyPointA
represents the frequency-location of point A expressed
as in ARFCN. It provides absolute frequency position of the reference resource
block (Common RB 0) whose lowest subcarrier is Point A.
-
For
downlink, this field is mandatorily configured within FrequencyInfoDL-SIB or
FrequencyInfoDL.
-
For
uplink, this field is not configured for TDD and the UE uses same value provided
for the downlink. In case of FDD or SUL, the network separately configures this
field in FrequencyInfoUL-SIB or FrequencyInfoUL.
Common Resource Blocks
(CRBs)
Common
resource blocks are numbered from 0 and upwards in the frequency domain for
each subcarrier spacing configuration ‘µ’.
The
CRBs associated with each and every SCS are aligned at Point A i.e.,
the center of subcarrier#0 of CRB#0 for each subcarrier spacing coincides with Point
A. This is illustrated in the figure below;
As can be seen from the above figure, the CRB edges for different SCS are not aligned. The CRB edge with SCS = 2∆f is shifted by ∆f kHz as compared to CRB edge with SCS = ∆f. For example, the CRB edge with SCS = 30 kHz is shifted by 7.5 kHz as compared to CRB edge with SCS = 15 kHz.
The relation between the CRB number nCRB in the frequency domain and resource element (k, l) is given by nCRB = floor (k/12). The value k is defined relative to Point A such that k = 0 corresponds to the subcarrier centered around Point A.
The relation between the CRB number nCRB in the frequency domain and resource element (k, l) is given by nCRB = floor (k/12). The value k is defined relative to Point A such that k = 0 corresponds to the subcarrier centered around Point A.
Physical Resource
Blocks (PRBs)
PRBs
are the resource blocks which are used for actual transmission/reception. A set
of PRBs belongs to and form a BWP. PRBs for a specific subcarrier
configuration defined within a BWP are numbered from 0 to (size of BWP
- 1). Each BWP has its own set of PRBs.
For
a specific subcarrier configuration, the relation between the physical resource
block nPRB in BWP ‘i’ and the common resource block nCRB
is given by;
nCRB =
nPRB + NBWP,i
start
where NBWP,i start is the CRB where BWP
starts relative to CRB0.
The
BWP’s starting PRB location relative to CRB0 is derived as
follows;
NBWP,i start
= Ocarrier + RBstart
-
Ocarrier provided by offsetToCarrier
is the offset in frequency domain between Point A and
the lowest usable subcarrier on the carrier in number of PRBs.
- RBstart
is derived from locationAndBandwidth (part of BWP
configuration).
The
example in figure below illustrates relation between PRBs and CRB0
Virtual Resource
Blocks (VRBs)
Similar
to PRBs, Virtual Resource Blocks are defined within a specific BWP and
are numbered from 0 to (size of BWP - 1).
The
resource allocation for PUSCH/PDSCH using PDCCH is given in terms of VRBs, which
are then mapped onto PRBs. Resource allocation will be covered in a future
post.
Bandwidth Part (BWP)
A
bandwidth part (BWP) is subset of contiguous common resource blocks for
a given numerology.
A
UE can be configured with up to four DL BWPs and up to four
UL BWPs for each serving cell. In case of SUL, there can be up to four
additional UL BWPs on the SUL carrier.
Per
serving cell, only one BWP in DL and one in the UL are active at a given time.
For
downlink, the UE is not expected to receive PDSCH, PDCCH, or CSI-RS (except for
RRM) outside an active BWP. For uplink, the UE shall not transmit PUSCH
or PUCCH outside an active BWP and for an active cell, the UE shall not
transmit SRS outside an active BWP.
The
BWP’s starting PRB location relative to CRB0 is derived as
follows;
NBWP,i start
= Ocarrier + RBstart
-
Ocarrier provided by offsetToCarrier
is the offset in frequency domain between Point A and
the lowest usable subcarrier on the carrier in number of PRBs. This is configured
per subcarrier spacing within SIB1 or via dedicated RRC signalling.
- RBstart
is derived from locationAndBandwidth (part of BWP
configuration).
BWP
will be discussed in detail in a future post.
Unused Resource
Blocks
Not
all resource blocks in channel bandwidth are used for transmission. This is due
to the fact that the guardband at both edges of the RF carrier is needed
to meet the out-of-band emission requirements.
The
guardband is between the edge of the carrier and the lowest usable
subcarrier. As can be seen from the below table from 38.101, guardband
size increases with the increase of subcarrier spacing.
Minimum guardband (in kHz) for each channel bandwidth and SCS for FR1
|
||||||||||||
SCS
|
5 MHz
|
10 MHz
|
15 MHz
|
20 MHz
|
25 MHz
|
30 MHz
|
40 MHz
|
50 MHz
|
60 MHz
|
80 MHz
|
90 MHz
|
100 MHz
|
15 kHz
|
242.5
|
312.5
|
382.5
|
452.5
|
522.5
|
592.5
|
552.5
|
692.5
|
NA
|
NA
|
NA
|
NA
|
30 kHz
|
505
|
665
|
645
|
805
|
785
|
945
|
905
|
1045
|
825
|
925
|
885
|
845
|
60 kHz
|
NA
|
1010
|
990
|
1330
|
1310
|
1290
|
1610
|
1570
|
1530
|
1450
|
1410
|
1370
|
Minimum guardband (in kHz) for each channel bandwidth and SCS for FR2
|
||||
SCS
|
50 MHz
|
100 MHz
|
200 MHz
|
400 MHz
|
60 kHz
|
1210
|
2450
|
4390
|
NA
|
120 kHz
|
1900
|
2420
|
4900
|
9860
|
This
increased guardband with increased SCS is extremely useful while implementing
the necessary filters to meet out-of-band emission requirements which avoids
excessively steep filtering requirements (for higher SCS).
Based
on the above discussion, due to varying sizes of guardband, the first usable
(for transmission) resource block location is different for different subcarrier spacings. For this reason,
the network always provides the location of first usable subcarrier using offsetToCarrier.
- offsetToCarrier
is the offset in frequency domain between Point A and
the lowest usable subcarrier on the carrier in number of PRBs. This is configured
per subcarrier spacing within SIB1 or via dedicated RRC signalling.
Let
us consider an example of 40 MHz channel bandwidth. From 38.101, SCS of
15 kHz gives 216 RBs which should yield 108 RBs when SCS of 30 kHz is
used. But according 38.101, SCS of 30 kHz defines only 106 RBs. The
first and last RBs are left out unused. Similarly, for 60 kHz, RBs 0 and 52 are
unused and only 51 RBs are used for transmission.
The
following example illustrates the unused RBs for different subcarrier spacings
for 40 MHz channel bandwidth. The maximum number of RBs per SCS are
defined in 38.101 and will be discussed in the subsequent section.
Note: As discussed already, it is expected that the CRB
edges for different SCS are not aligned.
Maximum Bandwidth
in NR (in RBs)
The
largest possible carrier bandwidth in NR is limited to 275 RBs for any SCS.
Per-carrier
maximum bandwidths (in MHz) for different SCSs are given in the table below;
SCS
|
Max. per-carrier Bandwidth
|
FR1
|
|
15 kHz
|
50 MHz
|
30 kHz
|
100 MHz
|
60 kHz
|
100 MHz
|
FR2
|
|
60 kHz
|
200 MHz
|
120 kHz
|
400 MHz
|
The
following table summarises the maximum number of RBs for each channel bandwidth
and subcarrier spacing (refer to 38.101).
|
Maximum Transmission Bandwidth configuration (in RBs) for FR1
|
||||||||||||
|
SCS
|
5 MHz
|
10 MHz
|
15 MHz
|
20 MHz
|
25 MHz
|
30 MHz
|
40 MHz
|
50 MHz
|
60 MHz
|
80 MHz
|
90 MHz
|
100 MHz
|
|
15 kHz
|
25
|
52
|
79
|
106
|
133
|
160
|
216
|
270
|
NA
|
NA
|
NA
|
NA
|
|
30 kHz
|
11
|
24
|
38
|
51
|
65
|
78
|
106
|
133
|
162
|
217
|
245
|
273
|
|
60 kHz
|
NA
|
11
|
18
|
24
|
31
|
38
|
51
|
65
|
79
|
107
|
121
|
135
|
|
Maximum Transmission Bandwidth configuration (in RBs) for FR2
|
||||
|
SCS
|
50 MHz
|
100 MHz
|
200 MHz
|
400 MHz
|
|
60 kHz
|
66
|
132
|
264
|
NA
|
|
120 kHz
|
32
|
66
|
132
|
264
|
Note for example (from the
above table) that maximum RBs for 5 MHz with SCS = 30 kHz is defined to
be 11 RBs. However, in order for the UE to detect and decode SSB, 20 RBs are needed.
So, 5 MHz bandwidth with SCS = 30 kHz is not used for transmission.
Reference: 3GPP TS 38.211, 38.213, 38.331, and 38.101
