CQI = channel quality Indicator; signal quality indication;
RI = rank indication; Rank indicator;
PMI = precoding matrix Indicator; pre-coded matrices indicator;
The CQI is used to reflect the channel quality of downstream Pdsch. Use 0~15 to represent the channel quality of Pdsch. 0 indicates the worst channel quality and 15 indicates the best channel quality.
–>ue sends CQI to ENB on Pucch/pusch. ENB get this CQI value, the quality of the current Pdsch wireless channel condition is good or bad. This makes it possible to schedule the Pdsch.
–> in other words, what is the basis for adaptive coded modulation (AMC) downlink in LTE. One of the bases is CQI.
–> again popular point of view: The channel quality is good, that ENB more send point data, the channel quality is not good, then the insurance point, less send point data.
RI is used to indicate the number of valid data layers for Pdsch. Used to tell Enb,ue the number of CW that can now be supported. That means ri=1,1cw,ri>1,2 CW. Cw-code Word code Word
PMI is used to indicate the index of the Codebook collection. MIMO technology with multiple antennas applied to LTE. In the baseband processing of the Pdsch physical layer, there is a pre-coding technique.
–> the pre-coding here simply means multiplying the various precoding matrices. And this matrix, can use TM3 so no feedback way.
–> can also use TM4 to escalate the PMI through the UE to determine the pre-coded matrix. In principle, this makes the Pdsch signal optimal.
Downlink transmission Mode (TM) Many, in the R9 version of the downstream definition of TM1~TM8, in which tm4,6,8 the case, only need to have PMI feedback.
For more detailed information, see 36.211, 213.
The CQI (channel quality indication) report is an important factor in LTE and has a significant impact on system performance. There are two types of CQI reported in LTE: Periodic and non-cyclical. Periodic CQI reports are carried by the PUCCH. However, if the UE needs to send the UL data in the same subframe, the periodic CQI report will use the Pusch, along with the UL data transfer. This is because the UE can not be pucch and Pusch at the same time send. In this case, the periodic PUCCH resource will be idle. Reporting granularity is relatively coarse due to periodic CQI reports that bring "always on" signaling overhead. In order to obtain a more detailed CQI report, ENB can trigger occasional CQI reports when needed. Non-cyclical CQI reports are sent on Pusch, along with UL data or sent alone.
The granularity of the CQI report can be divided into three levels: broadband, the sub-band of UE selection, and the sub-band of higher-level configurations. The broadband report is a CQI value for the entire downlink system bandwidth. The UE Select sub-band CQI report divides the system bandwidth into multiple sub-bands, selects a set of preferred sub-bands (preferably M-bands), and then reports the wideband and a differential CQI value setting a CQI value (assuming that the transmission can only be done by selecting M-sub-bands). The Sub-band report of the high-level configuration provides the highest granularity. It divides the entire system bandwidth into multiple sub-bands and then reports a wideband CQI value and multiple differential CQI values, one for each sub-band.
In the case of closed-loop MIMO, the PMI (pre-coded matrix indicator) and RI (rank indication) are also reported. Based on the evaluation of the received reference signal, PMI indicates that the ENB should use multiple antennas for the codebook (pre-agreed parameters) in the data transfer. RI indicates the number of transport layers that the UE can differentiate. Space multiplexing can be supported only if ri> 1. For spatial multiplexing, CQI is based on a per-code word report. The maximum number of code words in LTE is 2.
There are seven modes of transmission in LTE, each corresponding to a multi-antenna technology. For each transfer mode, certain combinations of CQI reports are defined in the specification based on periodic/irregular, wideband/UE-selected sub-bands/higher-level configurations of the sub-bands, no PMI/single PMI/multi-PMI. Because RI changes are slower than CQI/PMI, it is reported with a longer time interval. It is together with the CQI/PMI report on non-cyclical reporting, because Pusch resources are limited.