Investigation On Channel Estimation Of The Downlink Physical Layer Of LTE System

When it comes to receiving data in actual wireless communication systems,coherent demodulation process is broadly used,and to permit that some reference signals or pilotsymbols are enclosed into the time-frequency grid of the OFDM(Orthogonal Frequency Division Multiplexing)in order to achieve Channel Estimation.In that case,a portion of available transmission resource is kept aside and utilized for channel estimation,referred to as pilot symbols.In current wireless communication standards,namely Long-Term-Evolution(LTE),there is a possibility to customize the power radiated on pilot symbols.The Channel State Information is used to evaluate how the channel affects the signal that propagates through a communication system.Wireless channel indeed is more sophisticated than just an Additive White Gaussian Noise(AWGN)channel as it consists of fading,due by multipath propagation of the transmitted signal.Wireless channel can be designed as a train of impulses with random amplitude.Wireless channel can be treated as Gaussian,its amplitudes treated as Rayleigh random variable(Rayleigh Fading).Two main cases exist: Flat fading and Frequency selective.Wireless channels are time varying channels.We first investigate a pilot-aided channel estimation under an OFDM system with Least Squares method,we reproduced the power spectrum density of the transmitted signal,and the power spectrum density for both received signal before and after channel estimation with equalization process.The constellation diagram before and after channel estimation with equalization process;the modulation scheme used is 16-QAM.Then we also give the Bit Error Rate(BER)according to different modulation scheme and SNR(Signal to Noise Ratio)values.The adjustment based on power distribution between pilots and data symbols brings space for a further optimization of the LTE system performance.We then investigate on the distribution of available power,shared between data symbols and pilot symbols beneath transmission across time-variant channels so that the SINR(Signal-to-Interference plus Noise Ratio)is optimized.The post equalization signal-to-interference and noise ratio as the reference function to solve the problem of the system optimization.The simulation results are obtained by using the Vienna LTE simulator,which is a realistic transmission system.