The LTE standard defines three quality indicators that serve as a benchmark for the transmission quality in the downlink: CQI, PMI and RI (channel state information – CSI). The user equipment (UE) can measure all three and transmit the information in the uplink to the base station (BS), which then adapts the signal transmission in the downlink accordingly, although this is not mandatory. To actually improve transmission quality through a modification in the downlink, the statistical properties of the channel must remain constant between the time a quality indicator is reported to the eNB and the time the transmission is modified (coherence time).
Channel quality indicator (CQI)
The CQI indicates the highest modulation and the code rate at which the block error rate (BLER) of the channel being analyzed does not exceed 10 %. The CQI accepts discrete values between 0 and 15. Index 0 indicates that the UE has not received any usable LTE signals and that the channel is inoperable. The CQI report for the UE has a wide variety of settings. As an example, the UE can use one of two methods to send the CQI value to the eNB via the uplink:
- periodically via the PUCCH or PUSCH channels,
- aperiodically via the PUSCH channel.
In this case, the eNB explicitly requests the UE to send a CQI report. In addition, the frequency domain resolution in the CQI report can be varied. Apart from the wideband CQI for the entire channel bandwidth, there are different subband CQIs, each of which indicates the transmission quality for a specific frequency subrange.
The CQI index reported to the eNB by the UE is derived from the quality of the downlink signal. In contrast to other mobile radio systems such as HSDPA, the LTE CQI index is not directly associated with the measured signal-to-noise ratio. Instead, it is also influenced by the signal processing in the UE. With the same channel, a UE featuring a powerful signal processing algorithm is able to forward a higher CQI index to the BS than a UE that has a weak algorithm.
Precoding matrix indicator (PMI)
The precoding matrix determines how the individual data streams (called layers in LTE) are mapped to the antennas. Skillfully selecting this matrix yields a maximum number of data bits, which the UE can receive together across all layers. However, this requires knowledge of the channel quality for each antenna in the downlink, which the UE can determine through measurements. If the UE knows what the allowed precoding matrices are, it can send a PMI report to the eNB and suggest a suitable matrix.
Rank indicator (RI)
The channel rank indicates the number of layers and the number of different signal streams transmitted in the downlink. When using a single input multiple output (SIMO) or a transmit diversity configuration, only one layer is utilized. In contrast, 2×2 MIMO (multiple input multiple output) with spatial multiplexing uses two layers. The goal of an optimized RI is to maximize the channel capacity across the entire available downlink bandwidth by taking advantage of each full channel rank. RI is not the sole benchmark for the state of the channel when using LTE. CQI and PMI are taken into account as well, since the value of the RI also influences the allowed precoding matrices and CQI values. In contrast, the eNB can only utilize the CQI reporting to adapt the downlink channel (assuming the RI does not change such as in pure SIMO mode). The eNB is not forced to react to the feedback from the UE and modify the signal in the downlink accordingly. In most cases, it nevertheless makes sense to do this in order to reduce the error rate and increase the data throughput. However, inaccurate feedback from the UE regarding the state of the channel can lead to exactly the opposite situation. For this reason, it is vital to ensure that the UE accurately indicates the state of the channel by means of the CQI, PMI and RI parameters.