求助一个问题，PDSCH资源分配的type2如何理解

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由RIV指示所分配的RB起始位置和序号连续的VRB长度。
能否有比较通俗的说明一下，协议看了一下还是一头雾水。

把协议相关的一段贴在后面了，请高手帮忙说明一下。

In resource allocations of type 2, the resource block assignment information indicates to a scheduled UE a set of contiguously allocated localized virtual resource blocks or distributed virtual resource blocks. In case of resource allocation signalled with PDCCH DCI format 1A, 1B or 1D, one bit flag indicates whether localized virtual resource blocks or distributed virtual resource blocks are assigned (value 0 indicates Localized and value 1 indicates Distributed VRB assignment) while distributed virtual resource blocks are always assigned in case of resource allocation signalled with PDCCH DCI format 1C. Localized VRB allocations for a UE vary from a single VRB up to a maximum number of VRBs spanning the system bandwidth. For DCI format 1A the distributed VRB allocations for a UE vary from a single VRB up to   VRBs, where   is defined in [3], if the DCI CRC is scrambled by P-RNTI, RA-RNTI, or SI-RNTI. With PDCCH DCI format 1B, 1D, or 1A with a CRC scrambled with C-RNTI, distributed VRB allocations for a UE vary from a single VRB up to   VRBs if   is 6-49 and vary from a single VRB up to 16 if   is 50-110. With PDCCH DCI format 1C, distributed VRB allocations for a UE vary from   VRB(s) up to   VRBs with an increment step of  , where   value is determined depending on the downlink system bandwidth as shown in Table 7.1.6.3-1.

Table 7.1.6.3-1:   values vs. Downlink System Bandwidth
System BW ( )


        DCI format 1C
6-49        2
50-110        4

For PDCCH DCI format 1A, 1B or 1D, a type 2 resource allocation field consists of a resource indication value (RIV) corresponding to a starting resource block ( ) and a length in terms of virtually contiguously allocated resource blocks . The resource indication value is defined by
if   then

else

where  1 and shall not exceed  .
For PDCCH DCI format 1C, a type 2 resource block assignment field consists of a resource indication value (RIV) corresponding to a starting resource block ( = ,  ,  ,…,  ) and a length in terms of virtually contiguously allocated resource blocks ( = ,  ,…,  ).  The resource indication value is defined by
if   then

else

where  ,   and  . Here,
 1 and shall not exceed  .