关于TAC，个人感觉像是用于手机作上行同步监控的。
The UE has a configurable timer timeAlignmentTimer which is used to control how long the UE is considered uplink time aligned
并且当timer超时的时候，手机会通知RRC释放PUCCH/SRS
when timeAlignmentTimer expires:
- flush all HARQ buffers;
- notify RRC to release PUCCH/SRS;
- clear any configured downlink assignments and uplink grants.
只是个人看法，欢迎指教
时间: 2012-7-5 09:46
作者: kin5000 标题: 回复 3# 的帖子
这个在协议里有规定啊，36.133 7.6节。
The UE shall monitor the downlink link quality based on the cell-specific reference signal in order to detect the downlink radio link quality of the serving cell.
The UE shall estimate the downlink radio link quality and compare it to the thresholds Qout and Qin for the purpose of monitoring downlink radio link quality of the serving cell.
The threshold Qout is defined as the level at which the downlink radio link cannot be reliably received and shall correspond to 10% block error rate of a hypothetical PDCCH transmission taking into account the PCFICH errors with transmission parameters specified in Table 7.6.1-1.
The threshold Qin is defined as the level at which the downlink radio link quality can be significantly more reliably received than at Qout and shall correspond to 2% block error rate of a hypothetical PDCCH transmission taking into account the PCFICH errors with transmission parameters specified in Table 7.6.1-2.