1. Preamble Preamble

Since the distance between the mobility of the terminal, the terminal and the network is uncertain, so if the terminal needs to send a message to the network, it must be maintained in real time uplink synchronization management. The purpose is to achieve the PRACH uplink synchronization , uplink synchronization establishment, and network relationships and network requests dedicated resources allocated to the terminal, the normal traffic transmission.

Preamble the actual content of the preamble sent by the UE in PRACH channel, Tcp is the cyclic prefix length from the length of the CP and sequence of Sequence Tseq composition:

PRACH: Physical Random Access Channel, for the transmission of random access preamble.

LTE-TDD five preamble formats, respectively Preamble Format 0/1/2/3/4, wherein format0-3 for frametype 1 (FDD), format 0-4 for frametype 2 (TDD), as follows Figure:

See the preamble of the relevant parameters from the above table. Followed by the duration Ts of T_CP, long T_CP the MS, the duration Ts of T_SEQ, long T_SEQ the MS, the preamble duration, the number of sub-frames occupied by a preamble, and the length of the cell radius protection.

Duration of the preamble

And a preamble T_cp T_SEQ composition, length = T_CP + T_SEQ when a preamble;

Preamble format such as 0, its preamble duration = 3168Ts + 24576Ts = 27744Ts = 0.9031ms

The number of subframes occupied by the preamble format

Each TDD-LTE sub-time frame is long 30720Ts, preamble formats Preamble 0 time = 27744Ts <30720Ts, so only occupies an uplink subframe, can be calculated similarly to other sub-frame formats occupancy.

Time protection

Before transmitting OFDM symbols, a guard interval is inserted between symbols, when the protective space is large enough, the influence caused by multipath delay does not extend into the next symbol period, thereby eliminating the inter-symbol interference and multi-carrier interference .

The length of time and distance on the protection. Can be seen in the above table, GT and Cell Radius are strongly correlated. GT longer the time, the greater the coverage area of the cell.

The length of each subframe is 30720Ts, the time it takes to remove the preamble is a guard time, such as the guard time of the preamble format 0:

GT=（30720-3168-24576）Ts = 2976Ts=2976*[1/(15000*2048)]s = 96.875us。

Cell radius

Consider round-trip transfer between the eNB and UE, the formula is divided by 2:

The maximum cell radius = (the number of subframes occupied * 10 ^ 8) * GT / 2;

Such as maximum cell radius of the preamble format 0 = (3 * 10 ^ 8) m / s * 96.875 us / 2 = 14.53 km.

Similarly, the other can be calculated the maximum cell coverage radius preamble format.

CP

OFDM, a guard interval using the CP ( Cyclic the Prefix, the cyclic prefix ). The so-called cyclic prefix is a copy of the tail section of each OFDM symbol before a symbol.

UE decoding SSS (Secondary Synchronization Signal, secondary synchronization signal) when the normal CP value may be determined downlink. ul-CyclicPrefixLength the SIB parameters for configuring an uplink CP type. Typically, the same uplink and downlink CP type.

CP types are divided into normal and extended.

Normal CP configuration slot structure shown in FIG. In the first slot, the 0th OFDM symbol CP length 160TS, about 5.2us; 6 OFDM symbols and the other of 144 Ts CP length, about 4.7us; length of the useful symbol period of each OFDM 2048Ts, about 66.7us. 7 OFDM symbol periods, and the length of the useful symbol CP length and exactly 15360Ts, is about 0.5ms.

扩展CP配置下的时隙结构如图，每个时隙的OFDM符号数不再是7个，而是6个。而且一个时隙内每个OFDM符号周期长度都是512Ts。

SEQ

seq部分是前导码的内容部分，每个小区有64个前导码seq。基站负责前导码的分配。分给竞争的随机接入和非竞争的随机接入。用于竞争的随机接入的前导码又分为groupA和groupB，groupA是必须存在的。如下图示例，

基站通过SIB2消息中的RACH-ConfigCommon信元中告诉终端groupA和groupB包含哪些前导码，剩下的就是留给非竞争随机接入的。终端如果在竞争的随机接入流程，则先选择group，然后在对应的group中随机选取一个前导码seq使用。

前导码格式4的使用

上面的表中可以看到，格式4的前导码时长=（448+4096）Ts=4544Ts。协议明确规定，格式4只能在4384Ts或5120Ts的UpPTS上传输。36211-Table4.2-1给出了各种特殊子帧配置下的Ts长度。从表格中可以看到，当下行CP=上行CP=normal CP的时候，特殊子帧配置5、6、7、8配置的UpPTS时长满足条件；当下行CP=上行CP=extended CP的时候，特殊子帧配置4、5、6配置的UpPTS时长满足条件。

2、SIB1、SIB2及前导码的选择

SIB1

特殊子帧配置Special subframe configuration参数在SIB1的TDD-Config信元中，如下图。

SIB2

PRACH configuration Index参数来自于RRC层的SIB2消息（36331协议），UE和eNB侧都会有同样的一套参数。PRACH configuration Index的具体参数路径是：SystemInformationBlockType2->radioResourceConfigCommon->RadioResourceConfigCommonSIB->prach-Config->PRACH-ConfigSIB->prach-ConfigInfo->prach-ConfigIndex。

PRACH configuration Index参数决定了前导码的格式，

PRACH configuration Index值为0~19时，使用Preamble Format 0；
PRACH configuration Index值为20~29时，使用Preamble Format 1；
PRACH configuration Index值为30~39时，使用Preamble Format 2；
PRACH configuration Index值为40~47时，使用Preamble Format 3；
PRACH configuration Index值为48~57时，使用Preamble Format 4。

3.Preamble时域频域资源

UE发送前导码的时刻和位置是由SIB-2的prach-ConfigIndex和prach-FrequencyOffset字段决定的：

时域资源

Spec: TS36.211 - Table 5.7.1-2
SIB2中的prach配置参数：prach-ConfigIndex决定了preamble format（也就是子帧的个数）和SFN/子帧的位置，下图中prach-ConfigIndex = 9，指示了prach时域的位置：任何SFN里面的1,4,7子帧可以作为起始子帧来发送prach preamble，持续子帧的个数由preamble 格式确定，这里为1个子帧。

频域位置

PRACH 在频域上占用6 个连续的RB.

PRACH子载波间隔为1.25kHz, 这与其它上行子帧的子载波间隔15kHz不同。

下图prach-FreqOffset为8，指示PRACH在频域上位于从PRB8开始持续的6个PRB上。

总结：
对于FDD 而言，通过SIB2中的prach-ConfigIndex查36.211 的Table 5.7.1-2 得到preamble format 以及可以用于传输preamble 的系统帧和子帧号，从而确定可选的时域资源。通过SIB2中的prach-FrequencyOffset得到在频域上的起始RB，从而确定频域资源。

PRACH资源格示意图, :