Research On 3GPP - LTE Downlink Cell Search Algorithm

Downlink initial frequency scanning is the first fundamental procedure in the cell search. In this thesis we investigate the downlink initial frequency scanning for the3rd generation partnership project (3GPP) long term evolution (LTE) system. We review the progress of3GPP LTE standardization, describe the LTE physical layer procedure generally, and introduce some power spectral density (PSD) estimations and their performance in detail.At present, the general approach of user equipment (UE) downlink initial cell search in cellular system is, that we segment the received signal into epochs by frequency domain window, and perform power spectral density estimation using periodogram on each segment to yield the average power of each segment, then we decide whether each spectral segment has cell base-station signal by comparing the average power of this segment with a fixed threshold. After that, we make coarse decision on the bandwidth and center fluency of the cell signals, then we perform fine search on the segment with cell signal according to the coarse search and its adjacent segment, acquire the accurate cell signal bandwidth and center frequency.According to the3GPP-LTE standard, the available frequency range is100MHz, the grid of frequency center is100KHz, and5optional bandwidth is offered for downlink cell signal, all these characteristics bring more challenge to the downlink cell initial search. Based on the investigation of the popular power spectral density estimation algorithm and cell search algorithm, we propose a new scanning scheme with Welch algorithm and a dual-threshold applied in it. The Welch PSD algorithm is the evolution of periodogram, which performs estimation by segmenting the time-domain signal into equal epochs with overlapping, averaging the estimation of segments, and windowing segments with various window function. The first one helps increase the utilization of data, the second one reduces the variance of estimation which decreases the turbulence of the estimation around its mean value, and the last one contributes to declining the false decision caused by lobe leakage. Simulation results on3GPP-LTE system platform show that compared with the current algorithm, the Welch PSD algorithm can achieve better accuracy on bandwidth and center frequency determination, especially under the circumstances of narrow frequency gap between adjacent cell signals. Besides, a dual-threshold in scanning scheme has been proposed, instead of using power as the only threshold, the slope of power is added to the threshold, which helps in reducing the false alarms and wrong decisions, and improves the accuracy of cell search.