MIMO Feedback in LTE Rel 8
CSI feedback allows downlink transmission to be adaptively optimized based on the instantaneous DL channel, so that closed loop beamforming and adaptive link adaptation can be enabled to optimize the system performance.
The DL reference signal used for CSI measurement is different in Rel-8 and Rel-10. In Rel-8, CSI measurement is based on CRS, which is also used for data demodulation. In contrast, CSI measurement in Rel-10 is based on a set of newly introduced CSI-RS signals, which is low-duty cycle and low-density, and allows a higher reuse factor than Rel-8 CRS. The feedback mechanisms of LTE Rel-8 and LTE-Advanced Rel-10 are both based on the implicit feedback framework that has been well established and tested since early 3GPP releases.
In brief, UE measures the DL channel through measurement reference signals and feeds back the channel state information (CSI) in the form of recommended transmission formats.
- Rank indicator (RI): number of layers recommended for SU-MIMO transmission
- Precoding matrix indicator (PMI): index of the recommended SU-MIMO precoding matrix in the feedback/precoding codebook, corresponding to the RI
- Channel quality indicator (CQI): indication of the channel quality corresponding to the reported RI/PMI in LTE, CQI is defined as a set of transport block sizes, each of which translates to a maximum code rate and quadrature amplitude modulation (QAM) order that can be received by the UE at a certain block error rate (BLER).
As a criterion for testing the CQI report accuracy, when the reported code rate and QAM order is used for actual data transmission, the UE must be able to decode the data with a BLER below 10 percent.
Note that PMI and RI jointly represent the spatial directions of the MIMO channel, while CQI indicates the strength of the corresponding spatial directions. It is easy to see that the same feedback mechanism (RI/PMI/CQI) can be applicable and followed in LTE/LTE-Advanced.
This is independent of the exact measurement reference signals (CRS in Rel-8 and CSI-RS in Rel-10) that are used for CSI feedback, as long as they reflect the un-precoded antenna signals. More important, this commonality ensures backward compatibility so that eNB and UE of different releases can operate together seamlessly.
Benefits of this implicit PMI/CQI/RI framework include those listed here.
- Overhead: It is well known in the literature that codebook-based feedback is an effective means to achieve reasonably accurate CSI quantization with manageable overhead.
- UE receiver transparency: UE receiver implementation (usually proprietary) is implicitly reflected in the CSI report and therefore can stay transparent. For example, UE with an advanced interference rejection combining (IRC) receiver may report a higher CQI value than other UE with a simple minimum mean square error (MMSE) receiver. UE vendors are therefore encouraged to differentiate their products through advanced receiver implementation, offering better user experiences.
- Testability: Interoperability is an essential part of any multivendor ecosystem, including LTE/LTE-Advanced. The implicit feedback framework (e.g., reported RI/PMI/CQI must pass 10 percent BLER test when used for data transmission) has a proven track of testability to ensure that the CSI report is reliable.