1, CQI definition
CQI-Channel Quality Indication, a channel quality indication, CQI measurements obtained by the UE, thus, CQI generally refers to downlink channel quality. eNodeB scheduling algorithm to select the appropriate data block size and the downlink CQI information to ensure that the UE to obtain the best performance under different downlink wireless environment.
2. The reason for introducing the CQI
PDSCH supports three encoding: QPSK, 16QAM and 64QAM, corresponding to three of its constellation, such that the channel conditions do not require the same, namely: the higher the coding scheme (QPSK <16QAM <64QAM), channel conditions, the higher the demand .
Since the downlink scheduling is determined by the eNodeB, the eNodeB as a transmitting terminal does not know the channel quality is good or bad, to such a person is speaking, listeners hear their perception is clear, and therefore the channel quality is good or bad measured by the UE. eNodeB determines what encoding to be used to feedback the channel quality is good or bad need UE, LTE protocol quantized channel quality is good or bad sequence would be 0 to 15 ( 4 bit to carry ), and is defined as the CQI.
3, CQI selection criteria
And report the CQI value measured by the UE. LTE specifications are not explicitly defined measurement of CQI, CQI only defined selection rules, i.e. to ensure a PDSCH decoding error rate (i.e., the BLER) CQI value is less than 10% is used. That is, the UE needs (such as SINR) characteristic evaluation of a downlink according to the measurement results, and can use an internal algorithm determines SINR acquired under this BLER value condition, and in accordance with BLER 10% limit <, corresponding to the reported CQI value.
4, CQI values and its corresponding encoding
Wherein the modulation scheme determining a modulation order, which represents the number of bits per one symbol transmitted. QPSK modulation order corresponding to 2,16QAM is 4,64QAM 6. Code rate is the ratio between the total number of bits of information bits of the physical channel in the transport block, namely:
Rate = number of information bits in the transport block / physical channel bit number = total number of information bits / (total number of symbols channel physical channel modulation order *) = Efficiency / modulation order
Thus, different CQI values determines the difference between a downlink modulation scheme and transport block size. The larger the CQI value, the higher modulation and coding scheme employed, the greater the efficiency of the transport blocks corresponding to about also large, so the higher peak throughput downlink is provided.
5, CQI factors
The UE SINR measured values to determine and report CQI available to eNodeB, so the CQI value is mainly related to the downlink reference signal SINR.
In addition, CQI is also related to sensitivity, MIMO transmission mode and the radio link characteristics of the UE receiver. Specific performance:
- Under the same conditions the channel quality, the higher the sensitivity of the UE receiver, the higher the measured SINR value, thus the reported CQI value becomes.
- MIMO mode, the number of retransmissions and the number of antennas BLE affect performance. Since the CQI corresponding to the desired 10% BLER SINR value, therefore, under the same conditions SINR, 3 retransmission CQI value higher than 0 retransmissions, the higher TM3 / 4 than that of TM2 CQI, antenna than antenna. 4 corresponding to a higher CQI.
6, CQI impact on performance
Seen from the above analysis, CQI plays a key role in the downlink scheduling. The UE estimates SINR values and the CQI by the periodic or aperiodic manner reported, an eNodeB according to the extracted different modes corresponding CQI wideband or sub-band CQI information informed UE interference on a particular frequency band, to achieve frequency selection or non-selective scheduling. It is important, and an eNodeB acquires MCS based on the CQI information and PRB TBS information, which directly affect the downlink throughput.
The relationship between the downlink CQI and the single-user throughput exemplified below.
CQI reported by the UE is assumed that the maximum value 15, it corresponds to the modulation scheme is 64QAM, code rate is 0.926. The 20MHz (corresponding to 100 PRB) under, TD-LTE system, the maximum peak PHY rate is calculated as follows:
①PRB the number of RE: (14 symbols / subframe) x (100 th PRBx 12 RE / symbol) = 16800RE / subframe
② assumed that each sub-frame PDCCH three symbols, the number of occupied RE CFI removed, to give: 16800 RE / subframe - (3 PDCCH symbols x (100 th PRBx 12RE / symbol)) = 13200RE / subframe
③ the number of bits associated with the physical layer modulation scheme, 64QAM modulation order corresponding to 6 , it is: 6 x 13200 = 79200 bits / subframe
④ The code rate calculated transport block size: the transport block of information bits = the number of physical channels x number of total bit rate = 79200 x 0.926 = 73340
This means that the CQI = 15, the maximum TBS can carry a bandwidth of 20MHz is 73,340. Assuming uplink-downlink ratio is 1: 3, TD-LTE i.e., a half-frame of 5ms, there are three downlink slots, and in accordance with regulatory requirements, special subframe DwPTS 5 can not be transmitted in the downlink data, the MIMO mode (2 codewords simultaneously transmitted), downlink peak rate: 73340 (TBS) x 2 (the number of streams) x 3 (number of downlink slots) x 200 (half the number of frames 1s) = 88008000 bit = 88Mbps.
In the above example, if other CQI value corresponding to the code rate and modulation scheme are different, the size of transport blocks transmittable each TTI will be different, causing a difference in downlink throughput. Therefore, CQI plays a key role in the downlink scheduling.
ENodeB and UE scheduling algorithm and the measured CQI evaluation mechanism has a direct impact on system performance. For example in terms of lower if the CQI reported by the UE, but the system mistakenly transmitted TBS large, it may cause a decoding failure and the UE transmits ACK information to generate retransmission affect the resource utilization of the system. Conversely, if the actual radio environment is poor, but a higher CQI value reported by the UE, the network selection according to the CQI TBS large, and this may cause the UE fails to decode the same, resulting in reduced utilization of system resources.
Speed greater impact on the accuracy of CQI report. The higher the speed, the greater the CQI deviation, the transmission cycle should be reduced, increasing the transmission frequency to ensure the accuracy of CQI information.
