Research On Cqi Feedback And Performance Analysis In Lte Systems

In order to meet the increasing demand of communications, higher data rate, larger system capacity, wider range, multiple services and lower expenditure are necessary for the future wireless communication system. Long-term evolution system (LTE) called3.9G is the product of this trend. LTE system adopts a series of key technologies to meet the demands of higher system performance. Link adaptive technology as one important technology, is used to adjust data rate for each user to achieve the best channel capacity according to the state of wireless channel.In a practical system, because of the fading characteristics of wireless channel, the channel estimation error and the time delay error of feedback information exist in channel state information. However, channel state information is the key factor in link adaptation technology. Its accuracy affects the system performance directly. Thus, research on the error and designing a robust scheme to deal with this error is the vital subject. Under this background, this thesis focuses on the study of channel quality indicator (CQI). The main contributions are described as follows:Firstly, the CQI report modes specified in LTE standard are studied. Through the system-level simulations based on the LTE R8configuration, the performance of each CQI report model is evaluated and the mapping application scene of each mode is summarised. A feasibility analysis about the adaptation model is given.Secondly, this thesis analyzes the reasons of CQI unrealiablity. There are three main reasons:1) channel estimation error (CEE);2) CQI quatilization error;3) time delay of feedback. The CEE problem is formulated. Considering the randomness of CEE, three types of link adaptation algorithms are proposed:scheduling algorithm, weighted average (WA) algorithm based on three different parameters and an enhanced adaptive time window (ATW) algorithm. Both the theoretical derivation and system-level simulation prove that the WA algorithm based on channel estimation value can effectively mitigate CEE and achieve significant gain. Subsequently, better system performance can be obtained with ATW algorithm by adjusting the window size dynamically to offset the loss of channel estimation error.Thirdly, the evaluation of CQI delay compensation schemes is considered. The classic time-vary ing channel model-Jakes’model and five typical channel senarios proposed by international Telecommunication Union (ITU) are introduced. This thesis analyzes the channel characteristics and the variation of CQIs of five typical scenarios in detail. Based on the prediction theory, two types of CQI delay compensation schemes, namely weighted mean scheme (WMS) and prediction scheme based on normalized least mean square (NLMS-PS), are proposed. Evaluation results show that these proposed schemes can boost the system performance effectively based on Jakes’model when the CQI delay is less than5ms. However, increasing the accuracy of CQI purely can not work well on ITU channel models.Finally, a brief summary of this thesis is made and possible future research contents are pointed out.