Timing Advance is a MAC CE that is used to control Uplink signal transmission timing. Network (eNodeB in this case) keep measuring the time difference between PUSCH/PUCCH/SRS reception and the subframe time and can send a 'Timing Advance' command to UE to change the PUSCH/PUCCH transmission to make it better aligned with the subframe timing at the network side. If PUSCH/PUCCH/SRS arrives at the network too early, network send a Timing Advance command to UE saying "Transmit your signal a little bit late", If PUSCH/PUCCH/SRS arrives at the network too late, network send a Timing Advance command to UE saying "Transmit your signal a little bit early".

 Timing Advance是MAC層的控制單元，用于控制上行信號發(fā)送時機(jī)；

網(wǎng)絡(luò)側(cè)（eNodeB）一直監(jiān)測PUSCH/PUCCH/SRS上接收（的信息）和下行子幀（發(fā)送的信息）時間差，用“Timing Advance”命令通知UE在PUSCH/PUCCH上調(diào)整信號發(fā)送時機(jī)，以便與網(wǎng)絡(luò)側(cè)（子幀）時間一致；

如果PUSCH/PUCCH/SRS信號到達(dá)的時間過早，網(wǎng)絡(luò)下發(fā)“延遲發(fā)送信號“命令；

如果PUSCH/PUCCH/SRS信號到達(dá)的時間過晚，網(wǎng)絡(luò)下發(fā)“提早發(fā)送信號“命令；

MAC PDU for Timing Advance is as follows. It is one byte data and the first two bits are reserved and set to be always 0. The remaining 6 bits carries Timing Advance command value ranging from 0 to 63.

Timing Advance 在MAC層PDU為1byte數(shù)據(jù)，其中前2bits保留，全設(shè)為0，           剩下的6bits承載Timing Advance命令，取值范圍：0~63；

As you see in the following figures, for Rel 8,9,10 there is no special tag for each component carrier, meaning that even in Carrier Aggregation single Timing Advanced value apply to all the component carriers. But in Rel 11, the first 2 bits are allocated to indicate whether the value is for PCC or SCC. If TAG id is 0, it means it is for PCC.

LTE在R8，9，10版本中對載波沒有區(qū)別標(biāo)識，即便在載波聚合（CA）中每個載波都相同；

從R11版本，前2bits被用來區(qū)分PCC或SCC載波；如果標(biāo)識為0，指示載波為PCC;

< 36.321 Rel 8,9,10 - Figure 6.1.3.5-1: Timing Advance Command MAC control element >

< 36.321 Rel 11 - Figure 6.1.3.5-1: Timing Advance Command MAC control element >

Then how to translate each value of TA(Timing Advance) value to physical 'time' delay or advance value. It is described in detail in 36.213 4.2.3 Transmission timing adjustments. Simply put, the UL transmit timing is controlled by following equation.

 在36.213 4.2.3中對TA(Timing Advance)與物理上的時間（提前or延后）換算有專門描述；上行發(fā)送時間計算公式如下：

UL Transmission Time = (UL Transmittion Time for Previous subframe) + (TA value - 31) x 16 samples.

上行發(fā)送時間=上行前一幀發(fā)送時間+（TA值-31）*16（采樣點(diǎn)）

where 1 sample is about 0.033 us and 16 samples is about 0.52 us.

1個采樣點(diǎn)約為0.033us,16個采樣點(diǎn)約為0.52us; 

By this calcuation, you can see that the maximum timing change by single TA value (0 or 63) is about 16.7 us (I hope my calculation is right. please let me know if this calculation is wrong).

根據(jù)公式：TA（0 or 63）最大值為16.7us 

Example 1 > Timing Advance in a Live Network (in the field) 

Following plot is from the data captured by a drive test tool Azenqos Drive Test tool (AZQ Android). I got the log captured by the tool and exported the data as csv file and then plot it on Microsoft Excel. The map displaying the path of the measurement shown at the bottom is the one automatically created by AZQ reporting tool. It is hard to interpret the exact meaning of each points unless you have the exact location of eNB displayed on the map and the distance between UE and the eNB is recorded, but at least you would see some interesting pattern of TA value changes as the UE moving along the path getting closer to and farther away from the serving eNB. 

                 原文來自 http://www.sharetechnote.com/

LTE中的TA.docx