On design of physical random access channel in high-speed railway LTE systems

For the uplink in the long-term evolution (LTE) systems, the physical random access channel (PRACH) adopts Zadoff-Chu (ZC) sequences as preamble sequences, and each user can choose a unique cyclic shift for user identification. In high-mobility scenario, the Doppler frequency shift raises up the false alarm probability by inducing the correlation peak offsetting at the receiver side. Traditional detection method utilizes multiple search windows for searching the potential uplink users. When the Doppler frequency shift is larger than the subcarrier spacing, the energy of correlation peak leaks outside the search window and increases the false alarm probability. In this paper, we present a novel design for PRACH preamble sequence for high-speed railway scenario. Through numerical observation, we find out that the offset of the correlation peaks only depends on the initial root sequence number and the length of ZC sequences. Therefore, the root number of ZC sequences should be carefully designed to generate a preamble sequence to fight against the Doppler frequency shift. Consequently, those offsets of the correlation peaks are concentrated within a single search window, resulting in lower complexity of detection and lower probability of false alarm for random access in LTE uplink.