NR: PointA,offsetToPonitA,kSSB三者关系。

1. PointA,offsetToPonitA,kSSB三者关系示意图

NR: PointA,offsetToPonitA,kSSB三者关系。_第1张图片
在频域上,NR中的SSB可以在传输载波的任何位置,SSB的位置服从synchronization Raster, 而PDCCH/PDSCH的载波中心频率服从Channel Raster, SSB的子载波0的位置甚至不与物理资源块RB对齐。如上图所示, SSB的子载波0与CRB0的偏移等于offsetToPointA(单位:RB)+kSSB(单位:子载波).

2. 38.211协议定义

4.4.4.2 Point A 和 offsetToPointA
Point A serves as a common reference point for resource block grids and is obtained from:

  • offsetToPointA for a PCell downlink represents the frequency offset between point A and the lowest subcarrier of the lowest resource block overlapping 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 for all other cases where absoluteFrequencyPointA represents the frequency-location of point A expressed as in ARFCN.

上面提到的overlapping 就是指SSB和CRB(最小)的重叠区域,也就是的是 N C R B S S B N_{CRB}^{SSB} NCRBSSB RB 区域. 后面的in units of 讲述了offsetToPointA 的单位,单位是RB, 1RB = 12sc, 那子载波间隔SCS为多少呢? 如果FR1,则SCS为15,也就是1个单位等于 15 * 12 = 180 kHz. 如果FR2,则SCS为60,也就是1个单位等于 60 * 12 = 720 kHz.

4.4.4.3 中有一句话是关于PointA的:
Common resource blocks 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’.
也就说PointA 指向CRB0的子载波0的中心位置

7.4.3.1 kSSB定义
NR: PointA,offsetToPonitA,kSSB三者关系。_第2张图片
kSSB : 子载波偏移( N C R B S S B N_{CRB}^{SSB} NCRBSSB 的子载波0 -> SSB子载波0 ). 单位是子载波个数,那子载波间隔是多少呢?
NR: PointA,offsetToPonitA,kSSB三者关系。_第3张图片
FR1: SCS=15 kHz,
FR2: SCS由上层提供subCarrierSpacingCommon.
举例: FR1, offsetToPointA = 253, kSSB = 12, 则SSB子载波0与PointA的长度:
253 * 12 * 15 + 12 * 15 = 45720 kHz = 45.72 MHz.

3. How to get offsetToPonitA/kSSB

offsetToPonitA:
在38.331中可以找到SIB1中有offsetToPonitA。详细包含如下:
SIB1 -> ServingCellConfigCommonSIB->DownlinkConfigCommonSIB->FrequenceInfoDL-SIB -> offsetToPointA

kSSB: 38.211 - 7.4.3.1
FR1, kSSB的范围{0,1,2…,23} , 需要5个bit来表示
FR2, kSSB的范围{0,1,2…,11}, 4bit 足以表示这个范围NR: PointA,offsetToPonitA,kSSB三者关系。_第4张图片
kSSB, 低4个bits来源于 ssb-SubcarrierOffset, 而 ssb-SubcarrierOffset 来自于MIB.
最高位(5bit) 来自于PBCH编码中的 a A ˉ + 5 a_{\bar{A}+5} aAˉ+5
NR: PointA,offsetToPonitA,kSSB三者关系。_第5张图片
写在最后:
UE在PSS->SSS->MIB->SIB1,得到了offsetToPointA 和 kSSB, 就可以根据上图来反推出PointA的位置。PointA的位置在NR中是如此的重要:以Point-A为基准,为每一种SCS建立独立的CRB/BWP网格

BWP建立的细节可以参考:https://blog.csdn.net/wowricky/article/details/102990219

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