LTE stands for fast and reliable data transmission. Accurately measuring the channel quality with the user equipment plays a major role.
We have listed out three LTE Quality Indicators that serves as a benchmark for transmission quality in the downlink.
- CQI (Channel Quality Indicator)
- PMI (Precoding Matrix Indicator)
- RI (Rank Indicator)
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.
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 analysed does not exceed 10 %. The CQI accepts discrete values between 0 and 15.
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 BS via the uplink:
- Periodically via the PUCCH or PUSCH channels
- Aperiodically via the PUSCH channel. In this case, the BS explicitly requests the UE to send a CQI report.
The CQI index reported to the BS 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.
They are applicable to closed loop transmission modes
• Transmission Mode-5:Multi-User MIMO.
• Transmission Mode-6:Closed Loop Spatial Multiplexing using a single layer.
The LTE UE use PMI information to signal preferred set of weights to be applied during the precoding process. UE does this in order to maximize the downlink S/N ratio.
Rank indicator (RI)
The channel rank indicates the number of layers and the number of different signal streams transmitted in the downlink.
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 BS can only utilise the CQI reporting to adapt the downlink channel.
RIs are applicable for open loop transmission and closed loop transmission modes. These modes use more than a single layer between layer mapping and precoding modules.
• Transmission mode-4 indicates closed loop spatial multiplexing
LTE RI can be transmitted using PUSCH or PUCCH.
Source : R&S Test System Analyser