# Novel Calibration of MIESM and Reduction of CQI Feedback for Improved Fast Link Adaptation
## 1 Introduction  
In mobile communications systems, the channel experienced by each user is apt to having frequency selectivity and time variation. In this situation, the user selects a channel quality indicator (CQI) by estimating an appropriate link quality for the fading channel through fast link adaptation (FLA) and sends the feedback. The base station (BS) selects a modulation and coding scheme (MCS) for transmission based on the CQI feedback from the user [1, 2]. If the channel condition is determined to be good enough, the BS increases the modulation order and/or coding rate. In the opposite case, the BS decreases them. This FLA process with resource allocation increases the throughput of the system while maintaining the quality of service (QoS) between the user and the BS [3, 4, 5, 6, 7]. Although the FLA can use various metrics to predict link quality [8, 9, 10], in this paper, we use mutual information effective signal-to-noise-ratio mapping (MIESM) [11, 12], which is a tool to obtain an accurate link-quality metric (LQM) to determine the CQI value for the instantaneous channel. The MIESM utilizes the instantaneous signal-to-noise ratios (SNRs) of subcarriers to compute theeffective SNR, which is a constant value representing the whole-channel quality. In this process, a calibration factor is used to calculate the effective SNR in the MIESM procedure. The optimal calibration factor enables the most accurate block error rate (BLER) prediction for a given channel, allowing reliable CQI selection. There were a few research works handling the calibration of MIESM. In Reference [11], the calibration factor of MIESM and its derivation were addressed first, but no detailed calibration procedure was given. In Reference [12], the standard method of calibration was proposed. In Reference [13], the average method was proposed to reduce the complexity during the calibration compared to the standard method. In Reference [14], an approximated version of MIESM was proposed for 256 quadrature amplitude modulation (QAM) in long-term evolution (LTE), but no calibration procedure was required. All the existing calibration methods of MIESM were performed using only limited channel environments without consideration of various channel conditions of mobile communications. This is not an optimal method considering FLA in actual situations. Accurate CQI estimation of the FLA enables reliable communication with high throughput between the user and the BS [15; 16; 17]. The CQI reporting method of the user includes periodic reporting of CQI feedback at regular intervals and aperiodic reporting, which feeds back the CQI according to the request of the BS. There are three types of CQI feedback: wideband, higher layer-configured subband feedback, and UE selected subband CQI feedback [18]. The subband is defined by dividing the entire downlink bandwidth (wideband) by the subband size according to the number of resource blocks. One subband consists of consecutive physical resource blocks (PRBs), and it is the smallest unit for CQI feedback reporting. An efficient CQI feedback reporting reduces the overhead of uplink communication between the user and the BS. The CQI is reported differently depending on the channel state information (CSI) [19]. Differential CQI was adopted to reduce the amount of feedback bits in multi-input multi-output (MIMO) situations of the third generation partnership project (3 GPP) evolved universal terrestrial radio access (E-UTRA) [18]. The differential CQI feedback first obtains the wideband CQI from subband CQIs, and then calculates the difference between each subband CQI and the wideband CQI. It is noted that the differential CQI feedback scheme does not utilize channel correlations over time. In this paper, we propose an improved calibration method for MIESM by deriving the optimal calibration factor to improve the accuracy of CQI feedback in FLA. The improvement of the proposed method comes from two technical advances.