36.321中
5.4.3  Multiplexing and assembly
5.4.3.1  Logical channel prioritization
The UE shall maintain a variable Bj for each logical channel j. Bj shall be initialized to zero when the related logical channel is established, and incremented by the product PBR × TTI duration for each TTI, where PBR is Prioritized Bit Rate of logical channel j. However, the value of Bj can never exceed the bucket size and if the value of Bj is larger than the bucket size of logical channel j, it shall be set to the bucket size. The bucket size of a logical channel is equal to PBR × BSD, where PBR and BSD are configured by upper layers.
The UE shall perform the following Logical Channel Prioritization procedure when a new transmission is performed:
-  The UE shall allocate resources to the logical channels in the following steps:
-  Step 1: All the logical channels with Bj > 0 are allocated resources in a decreasing priority order. If the PBR of a radio bearer is set to “infinity”, the UE shall allocate resources for all the data that is available for transmission on the radio bearer before meeting the PBR of the lower priority radio bearer(s);
-  Step 2: the UE shall decrement Bj by the total size of MAC SDUs served to logical channel j in Step 1
NOTE:  The value of Bj can be negative.

首先step1中BJ不可能超过PBR，因为有 before meeting the PBR of the lower priority radio bearer，除非PBR是无穷的，不然allocated resources 不可能比PBR大。那么在step2种BJ减去served SDUs怎么可能为负数呢？