Design Of Optimal LTE-A Pilot Patterns And Polar-Cap Differential Precoding Codebook Under High Mobility Scenarios

Under the high mobility scenario, some factors have strong influence on LTE-A system performance, such as the velocity of mobile user, pilot patterns, multiple-input-multiple-output (MIMO) transmission strategies, wireless channel characteristics, etc. The Doppler shift or spread due to the user velocity will not only cause the inter-carrier-interference (ICI) for orthogonal frequency division multiplexing (OFDM) transmission but also increase the channel estimation error in time-varying channels, which further deteriorates the efficiency of precoding and symbol detection. In this thesis, the impact of pilot spacing and the number of MIMO transmission streams is evaluated under high mobility scenario. Moreover, an improved polar-cap differential codebook is introduced.Generally, smaller pilot spacing in time domain provides better channel estimation accuracy, but also results in larger system overhead and lower transmission efficiency. Similarly, larger number of MIMO transmission streams definitely enables more data symbols to be transmitted simultaneously, but requires more pilot symbols for channel estimation, thus leading to larger system overhead. In this thesis, the effects of user velocity, pilot patterns and the number of MIMO transmission streams on system throughput are investigated. The analysis and simulation results show that under certain user velocity and channel condition, there exists an optimal pilot spacing and an optimal number of MIMO transmission streams in terms of the system throughput.A polar-cap differential codebook is then investigated. Simulation results show that the conventional polar-cap differential codebook suffers serious performance degradation under high mobility scenarios. Thus the parameters such as the polar-cap size need to be improved. By comparing the time-varying polar-cap size in the conventional scheme, the fixed polar-cap size value is also considered, and the optimal value of the polar-cap size is found for different speeds by comparing the time-varying polar-cap size with different fixed polar-cap size value. It is shown that the improved polar-cap differential codebook outperforms the one-shot RVQ codebook at low speed and outperforms the conventional polar-cap differential codebook at high speed.