5G NR 频率计算
5G中引入了频率栅格的概念，也就是小区中心频点和SSB的频域位置不能随意配置，必须满足一定规律，主要目的是为了UE能快速的搜索小区；其中三个最重要的概念是Channel raster 、synchronization raster和pointA。
1、Channel raster
可以理解为载波的中心频点的可选位置；
一般频点值都以NR-ARFCN（NR绝对射频频率信道编号）数值间接表示，即下面表格中的Nref，一般在RRC消息中传递的都是这个信道编号，如果需要知道具体代表的频率值， 参考下面公式中的频率Fref 的计算：
ΔFGlobal全局频率栅格间隔(granularity of the global frequency raster)，不同频率范围取值见下表。
Fref RF reference frequencies，也即具体频率值
NR-ARFCN（NR Absolute Radio Frequency Channel Number）绝对信道号取值范围FR1为0…2016666，FR2为2016667 – 3279165，与RF reference frequency Fref的关系见下式。
FREF = FREF-Offs + ΔFGlobal (NREF – NREF-Offs)
channel raster 是RF reference frequencies的子集，对每个band来说中心频点不能随意选，需要按照一定起点和步长选取，具体可用的见下表。
ΔFRaster为间隔粒度，大于等于ΔFGlobal。
比如对n41，如果步长是3，换算出对应的频率的步长是3Fglobal=3×5=15Khz；如果步长是6，换算出对应的频率的步长是6Fglobal=6×5=30Khz，这里有两种ΔFRaster，根据 I 确定。未找到 I 如何确定。
Table 5.4.2.3-1: Applicable NR-ARFCN per operating band
| NR operating band |
ΔFRaster (kHz) |
Uplink Range of NREF (First – |
Downlink Range of NREF (First – |
| n1 |
100 |
384000 – <20> – 396000 |
422000 – <20> – 434000 |
| n28 |
100 |
140600 – <20> – 149600 |
151600 – <20> – 160600 |
| n41 |
15 |
499200 – <3> – 537999 |
499200 – <3> – 537999 |
| 30 |
499200 – <6> – 537996 |
499200 – <6> – 537996 |
|
| n77 |
15 |
620000 – <1> – 680000 |
620000 – <1> – 680000 |
| 30 |
620000 – <2> – 680000 |
620000 – <2> – 680000 |
|
| n78 |
15 |
620000 – <1> – 653333 |
620000 – <1> – 653333 |
| 30 |
620000 – <2> – 653332 |
620000 – <2> – 653332 |
|
| n79 |
15 |
693334 – <1> – 733333 |
693334 – <1> – 733333 |
| 30 |
693334 – <2> – 733332 |
693334 – <2> – 733332 |
整个载波的中心频率channel raster 位置和RB总数有关系，在RB数量为偶数时，表示Nprb的子载波0，当RB数量为基数时，表示Nprb的子载波6。也即比小区频率的绝对中心向上偏移了半个子载波。
2、synchronization raster
synchronization raster可以理解为SSB块的中心频点可选位置；也是为了让UE更快速的找到SSB；5G里面SSB的中心和载波的中心不需要重合；
参见下面表格，SSB的中心频率即下表中的SSref；也是按照一定规律步进的；
SSB的中心频率一般也是通过GSCN的编号值间接表示的，方便消息传递；
GSCN Global Synchronization Channel Number，即全球同步信道号，是用于标记SSB的信道号。
每一个GSCN对应一个SSB的频域位置SSREF（SSB的RB10的第0个子载波的起始频率），GSCN按照频域增序进行编号。
Table 5.4.3.1-1: GSCN parameters for the global frequency raster
| Frequency range |
SS Block frequency position SSREF |
GSCN |
Range of GSCN |
| 0 – 3000 MHz |
N * 1200kHz + M * 50 kHz, N=1:2499, M ϵ {1,3,5} (Note 1) |
3N + (M-3)/2 |
2 – 7498 |
| 3000 – 24250 MHz |
3000 MHz + N * 1.44 MHz N = 0:14756 |
7499 + N |
7499 – 22255 |
| NOTE 1: The default value for operating bands with which only support SCS spaced channel raster(s) is M=3. |
|||
Table 5.4.3.1-1: GSCN parameters for the global frequency raster
| Frequency range |
SS block frequency position SSREF |
GSCN |
Range of GSCN |
| 24250 – 100000 MHz |
24250.08 MHz + N * 17.28 MHz, N = 0:4383 |
22256 + N |
22256 – 26639 |
Synchronization raster也不是SSB块的绝对的中心（1/2处），SSB块是20个RB，共计20*12=240个子载波；absoluteFrequencySSB对应于第10个RB（从0编号）的第0号子载波的中心，也就是和绝对的中心向上偏了半个子载波；
Table 5.4.3.2-1: Synchronization raster to SS block resource element mapping
| Resource element index k |
0 |
| Physical resource block number nPRB of the SS block |
nPRB = 10 |
3、PointA
根据公共参考点absoluteFrequencyPointA的定义，这个参考点是第0个RB（RB0）的第0个子载波的中心点；注意不是边沿（edge），很多网上文章都理解为edge；
Absolute frequency position of the reference resource block (Common RB 0). Its lowest subcarrier is also known as Point A. Note that the lower edge of the actual carrier is not defined by this field but rather in the scs-SpecificCarrierList. Corresponds to L1 parameter 'offset-ref-low-scs-ref-PRB' (see 38.211, section FFS_Section)
参见38.211 定义
absoluteFrequencyPointA for all other cases where absoluteFrequencyPointA represents the frequency-location of point A expressed as in ARFCN
The center（中心） of subcarrier 0 of common resource block 0 for subcarrier spacing configuration μ coincides with ‘point A’.
4、各参数的关系
offsetToPointA ：表示SSB最低RB的最低子载波与point A 之间的频域偏移，单位为RB，注意这里频域偏移计算时不是以真实的子载波间隔来计算的，而是对于FR1假设子载波间隔为15kHz，对于FR2假设子载波间隔为60kHz。
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;
Kssb：poin A 和 SSB的0号RB0号子载波相差的RB数量不一定正好差整数个RB，可能还会差出几个子载波；Kssb就表示还差出几个子载波；这里也是假设子载波间隔为固定值FR1为15kHz，FR2为60kHz。Kssb的低4比特由高层参数ssb-SubcarrierOffset给出。对于SS/PBCH block type B（μ∈{3,4} ）来说，kSSB∈{0,1,2,...,11} ，4比特就足够了；而SS/PBCH block type A（μ∈{0,1} 的kSSB∈{0,1,2,...,23} ，需要5比特表示。
所以poin A 和 SSB的RB0的0号子载波相差的频率等于offsetToPointA*15*12+Kssb*15；
而absoluteFrequencySSB和SSB的RB0的0号子载波相差的频率等于10×12×subCarrierSpacingCommon（SSB的RB数量为20）
offsetToCarrier：Point A（CRB0最低子载波）与最低可用子载波之间的频域偏移，单位为PRB。
Offset in frequency domain between Point A (lowest subcarrier of common RB 0) and the lowest usable subcarrier on this carrier in number of PRBs (using the subcarrierSpacing defined for this carrier). The maximum value corresponds to 275*8-1. Corresponds to L1 parameter 'offset-pointA-low-scs' (see 38.211, section 4.4.2)
对于3GHz以下
ΔFGlobal = 5
absoluteFrequencyPointA × ΔFGlobal + offsetToPointA×15×12+Kssb×15 = SSREF - 10×12×subCarrierSpacingCommon
SSREF = absoluteFrequencySSB * ΔFGlobal = absoluteFrequencySSB × 5
也即：
absoluteFrequencyPointA + offsetToPointA×15×12/5 + Kssb ×15/5 = absoluteFrequencySSB - 10×12×subCarrierSpacingCommon/5
载波中心NR-ARFCN：
NREF = 载波中心NR-ARFCN= absoluteFrequencyPointA + N_CRB×12/2× subcarrierSpacing/5 + offsetToCarrier × 12 × subcarrierSpacing/5
5、NSA配置
NSA里，基站会通过RRC重配置消息通知UE关于频点的信息，帮助UE快速搜索到目标小区；
NR=band 41，是小于3Ghz的band，可以套用上面表格5.4.2.1-1对应的第一列的参数；
carrierBandwidth=273，代表载波里面是273个RB，273RB * 12 * 30Khz = 98.280Mhz，并没有完全占满100Mhz带宽，因为两边需要留出保护带宽（guard band）。
subcarrierSpacing；代表子载波间隔是30Khz；每个RB有12个子载波
absoluteFrequencyPointA=503172：代表公共参考点A；503172 *5Khz = 2515860Khz
absoluteFrequencySSB= 504990 代表SSB块的中心频点；504990*5Khz = 2524950Khz，也就是SSB中心位于2524950Khz；
offsetToCarrier = 0
NREF = 载波中心ARFCN = absoluteFrequencyPointA + N_CRB×12/2× subcarrierSpacing/5 + offsetToCarrier × 12 × subcarrierSpacing/5
载波中心NR-ARFCN = 503172 + 273*12/2* subcarrierSpacing / 5 + 0 = 503172 + 9828 = 513000
频率为513000 ×5 = 2565000kHz
absoluteFrequencyPointA + offsetToPointA×15×12/5 + Kssb ×15/5 = absoluteFrequencySSB - 10×12×subCarrierSpacingCommon/5
503172 + offsetToPointA×15×12/5 + Kssb ×15/5 = 504990 - 10 ×12 × 30 / 5
offsetToPointA × 36 + Kssb ×3 = 1098
得出offsetToPointA = 30，Kssb = 6
SSB GSCN = 6312，N = 2104，M = 3
各变量的关系如下图所示：
6、SA配置
可以看到MIB中配置的sb-SubcarrierOffset = 6,offsetToPointA = 30，offsetToCarrier =0。
7、限制条件
1、最小保护间隔
最小保护间隔在38.101中定义，
Table 5.3.3-1: Minimum guardband for each UE channel bandwidth and SCS (kHz)
| SCS (kHz) |
5 MHz |
10 MHz |
15 MHz |
20 MHz |
25 MHz |
30 MHz |
40 MHz |
50 MHz |
60 MHz |
70 MHz |
80 MHz |
90 MHz |
100 MHz |
| 15 |
242.5 |
312.5 |
382.5 |
452.5 |
522.5 |
592.5 |
552.5 |
692.5 |
N/A |
N/A |
N/A |
N/A |
N/A |
| 30 |
505 |
665 |
645 |
805 |
785 |
945 |
905 |
1045 |
825 |
965 |
925 |
885 |
845 |
| 60 |
N/A |
1010 |
990 |
1330 |
1310 |
1290 |
1610 |
1570 |
1530 |
1490 |
1450 |
1410 |
1370 |
Table 5.3.3-1: Minimum guardband for each UE channel bandwidth and SCS (kHz)
| SCS (kHz) |
50 MHz |
100 MHz |
200 MHz |
400 MHz |
| 60 |
1210 |
2450 |
4930 |
N. A |
| 120 |
1900 |
2420 |
4900 |
9860 |
NOTE: The minimum guardbands have been calculated using the following equation: (BWChannel x 1000 (kHz) - NRB x SCS x 12) / 2 - SCS/2, where NRB are from Table 5.3.2-1.
The number of RBs configured in any channel bandwidth shall ensure that the minimum guardband specified in this clause is met.
Figure 5.3.3-2: UE PRB utilization
2、UE在不同频段支持的带宽
Table 5.3.5-1 Channel bandwidths for each NR band
| NR band / SCS / UE Channel bandwidth |
|||||||||||||||
| NR Band |
SCS kHz |
5 MHz |
101,2 MHz |
152 MHz |
202 MHz |
252 MHz |
30 MHz |
40 MHz |
50 MHz |
60 MHz |
70 MHz |
80 MHz |
90 MHz |
100 MHz |
|
| n1 |
15 |
Yes |
Yes |
Yes |
Yes |
||||||||||
| 30 |
Yes |
Yes |
Yes |
||||||||||||
| 60 |
Yes |
Yes |
Yes |
||||||||||||
| n28 |
15 |
Yes |
Yes |
Yes |
Yes7 |
||||||||||
| 30 |
Yes |
Yes |
Yes7 |
||||||||||||
| 60 |
|||||||||||||||
| n41 |
15 |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
||||||||
| 30 |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
|||||
| 60 |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
|||||
| n77 |
15 |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
|||||||
| 30 |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes4 |
Yes |
Yes4 |
Yes |
|||
| 60 |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes4 |
Yes |
Yes4 |
Yes |
|||
| n78 |
15 |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
|||||||
| 30 |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes4 |
Yes |
Yes |
Yes |
|||
| 60 |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes4 |
Yes |
Yes |
Yes |
|||
| n79 |
15 |
Yes |
Yes |
||||||||||||
| 30 |
Yes |
Yes |
Yes |
Yes |
Yes |
||||||||||
| 60 |
Yes |
Yes |
Yes |
Yes |
Yes |
||||||||||