Research On The Frequency Offset And The Time Offset Estimation For LTE-based Air-ground Wireless Broadband Communication System

With the rapid development of wireless communication technology, the 4th generation mobile communication system (4G) -LTE technology has made a comprehensive achievement in the operator field, and many operators around the world have been fully carried out the operation of LTE. Due to the outstanding performance of LTE technology in operations, more and more people are concerned about the development of LTE technology in other areas (e.g. civil aviation communications). At present, there are mainly two solutions to realize mobile communication in the civil aircraft:one is using satellite to communicate with terrestrial network; the other is employing ground specific radio base station to achieve coverage of civil aircraft airborne. Although the satellite communication is relatively mature, the disadvantages such as the conversion of satellite communication systems, the narrow bandwidth and high costs make it difficult in spreading utilization of civil aviation. It is usually applied on international flights which cannot be covered by ground station. Therefore, the solution that employing ground specific radio base station to achieve coverage has attracted more and more attention. Compared to other techniques, it can provide a higher download speed by using the 4thgeneration LTE mobile communication system to realize coverage of civil. Meanwhile, owing to the low demand on switching period adapts to switch in high speed mobile scenes for LTE, it has a broader commercial prospect of air-ground communication system based on LTE.Since the LTE system is initially designed to provide more than 100kbps access service for the high-speed mobile users whose maximum speed is 350Km/h, while the civil aircraft flight rate is usually in 800～1000Km/h, therefore, the technical difficulty in air-ground communication system based on LTE is how to support high-speed data transmission at 800～1000 Km/h high-speed mobile scenario. OFDM is the key technology of LTE system’s physical layer. The aircraft flying at high speed will bring huge Doppler frequency shift, then frequency offset will destroy the orthogonality between OFDM subcarriers, which will result in inter carrier interference, multi-user interference, and further affecting the demodulation performance. Thus, this thesis focuses on the study of frequency and time offset estimation and compensation of air-ground communication system based on LTE.Firstly, this thesis introduces several traditional OFDM frequency offset estimation algorithm, and analyses the maximum terminal velocity of traditional OFDM frequency offset estimation algorithm in LTE system that can support. Theoretical analysis and simulation results show that conventional OFDM frequency offset estimation algorithm can’t meet the requirements of air-ground wireless broadband communication system. Therefore, this thesis firstly puts forward a frequency offset estimation algorithm which is based on the peak power of PRACH channel. The frequency estimation algorithm can extend the range of frequency offset estimation to [-1875Hz,1875Hz] and meet the requirement of air-ground broadband wireless communication system. But the algorithm is largely influenced by noise and with inaccurate estimation in certain frequency offset range, and UE will be launched only when PRACH channel is in the initial access. Therefore, this algorithm only can be used for initial frequency offset estimation. In order to ensure the performance of frequency offset estimation to meet the requirement of PUSCH channel demodulation, a joint frequency offset estimation method is proposed in this thesis. This method combines the traditional frequency offset estimation algorithm based PUSCH channel with the algorithm proposed herein based on PRACH channel. The joint frequency estimation algorithm not only expands the range of frequency offset estimation to [-1875Hz,1875Hz], but also guaranties the accuracy of the frequency offset estimation. Finally, this thesis describes the process of frequency offset estimation and frequency offset compensation of air-ground wireless broadband communication system based LTE, and the simulation results show that this algorithm can actually be applied to air-ground wireless broadband system. About the time offset estimation, we proposed a new joint time offset and frequency offset estimation algorithm for LTE uplink system. We proposed a new cross correlation method to estimate time offset and frequency offset simultaneously, when compared with conventional correlation time and frequency offset estimation, our proposed algorithm can decrease complexities about 33%. The simulation results demonstrate that compared with the conventional algorithms, our proposed algorithm can achieve a better performance on frequency offset estimation in low SNR and the same performance on time offset estimation.