Research For The Key Technologies Of LTE Downlink Systems In High-mobility Environment

High Doppler frequency shift and fast time-varying channel will severely degradethe communication quality in the multicarrier systems in the high-mobility scenario.Doppler frequency shift results in Carrier Frequency Offset (CFO) and inducesInter-Carrier Interference (ICI) in the Orthogonal Frequency Division Multiplexing(OFDM) systems. Channel changing in one OFDM symbol also destroys theorthogonality of subcarriers and induces ICI, which will make the channel estimationand equalization more complex and difficult.At the same time,3GPP Long Term Evolution (LTE) technology, as the futuremobile communication standard, adopts the key technology of OFDM and MIMO(Multiple Input Multiple Output). It has shorter transmission delay, higher data rate andlarger capacity and coverage area than the existed systems. The LTE technology willimprove the communication quality effectively in the high-mobility scenario. So LongTerm Evolution for Railway (LTE-R) systems is proposed at the right time. It will beapplied to the construction of railway private network and improve the communicationquality of high speed railway. This article will mainly research the problems about theestimation of Doppler CFO and time-varying channel in LTE downlink in the highmobility scenario. The main work is as follows:First, we model the time-varying channel in the high mobility scenario. Here wededuce the general expression of time-varying channel, analyze the time-variation ofthe Doppler shift in Light of Sight (LOS) component, and then we modify thestatistical channel model and the WINNER II channel model, so as to fit thecharacteristics of the high-mobility channel better, and we give the simulation curve ofthe modified channel in two dimensions of time and frequency.Second, this article studies the estimation of Doppler CFO and time-varyingchannel in the high mobility scenario. This part presents the characteristics of ICIinduced by Doppler CFO and time-varying channel respectively and provides themethods of CFO estimation which are suitable to the LTE downlink and are used to thehigh mobility scenario. Specific to the feature of LTE downlink systems, we modifythe conventional channel estimators and the novel time-varying channel estimators andapply them to the LTE downlink. Channel equalization is usually involved in matrixinversion which has a high computational complex. Here we give the simplifiedalgorithms of equalization in frequency domain, which have obvious advantages inbroadband LTE systems. At last, the verification results of simulation are given out.Third, this article studies the ICI suppression technique in LTE-R downlink systems which adopt RRU chain in the high speed railway scenario. Based on theparticular scenario of the overlap region of two RRUs, we propose a scheme forestimation and compensation of multi-CFO. At the same time, in order to exploit theadvantage of multi-antenna of the distributed RRUs, we adopt two schemes of transmitdiversity. At last, combined the above-mentioned channel estimators, the simulationsof Bit Error Rate (BER) performance is given, which verify the validity of theproposed scheme.At last, the whole article is summarized, and the future research directions of thekey communication technologies in high mobility scenario are discussed here.