TTI Codeword, Layer, And Precoding In LTE

LTE TDD中的TTI 是可变的且最小1MS。

TTI是TRANSPORT CHANNLE的量纲,代表最小数据传送时间,可以根据不同业务有很大范围的变化.具体是指无线链路一个能够独立解调的传输块的长度，当多个subframe也能够解调，譬如接收完整个10ms无线帧再进行解调，那么TTI就是10ms

概念
TTI：Transmission Time Interval，传输时间间隔。TTI 是指在无线链路中的一个独立解码传输的长度。
在3GPP LTE与LTE-A的标准中，一般认为 1 TTI = 1ms 。即一个Subframe（子帧=2slot）的大小，它是无线资源管理所管辖时间的基本单位。

 
主要原理及应用
 
TTI Bundling（时隙绑定）技术是将一个数据包在连续多个TTI资源上重复进行传输，接收端将多个TTI资源上的数据合并达到提高传输质量的目的。
LTE中物理层调度的基本单位是1ms ，这样小的时间间隔可以使得LTE中应用的时间延迟较小。然而在某些小区边缘，覆盖受限的情况下，UE由于受到其本身发射功率的限制，在1ms的时间间隔内，可能无法满足数据发送的误块率（BLER）要求。因此，LTE中提出了TTI Bundling的概念，对于上行的连续TTI进行绑定，分配给同一UE，这样可以提高数据解码成功的概率，提高LTE的上行覆盖范围，代价是增加了一些时间延迟。eNodeB只有在收到所有绑定的上行帧以后，才反馈HARQ的ACK/NACK。
几个重要结论：

1. 3GPP R8版本中定义TTI Bundling用于VoIP业务，最大连续使用的TTI资源数为4，往返时间RTT为16ms，调制格式为QPSK，最大分配RB资源数为3。
   
2. TTI Bundling既可以应用到FDD，也可以应用到TDD模式。
   
3. 利用4TTIbundling进行LTE上行覆盖增强，能够大概提高上行用户1~2dB的SINR。

Terms codeword, layer, and precoding have been taken to refer specifically to LTE signals and processing. The figure shows the processing steps to which they relate. The terms used in the following ways:

Codeword: codeword represents the user data before it is formatted for transmission. One or two code words, CW0 and CW1, may be used depending on the conditions of the channel and the use case. In the most common case of a single user MIMO (SU-MIMO), two codewords are sent to a single handset UE, but less common for downlink multi-user MIMO (MU-MIMO), each codeword is sent only one UE.

Layer: The term is synonymous with the current. For MIMO, the use of at least two layers. Up to four layers are allowed .The Number is always less than or equal to the number of antennas.

Precoding:  Precoding layer modifies signals before transmission. This can be done diversity, spatial multiplexing, or beam direction. MIMO channel conditions may favor one layer (data stream) to another. If the base station (eNB) is given channel (e.g., the information sent back from the UE), the complex may be added to counter the imbalance cross-coupling in the channel. In arrangement 2 * 2, LTE uses a single stop 1 to 3 Precoding, which improves the performance if the channel has not changed too rapidly.

Eigen beamforming: (sometimes known simply as “beamforming”) amends the transmission signals to provide the best transportation to interference and noise ratio (CINR) out of the channel.

Tags:

• reception diversity