Time-of-Arrival Estimation in Block-IFDMA Systems for LTE in Unlicensed Spectrum

Time-of-arrival estimation is a critical aspect of wireless systems, both for initial access and ongoing communications. The timing estimates are needed in uplink and downlink, where the initial timing estimation in LTE uplink is based on a physical random access channel (PRACH) preamble transmitted by the user. Block Interleaved Frequency Division Multiple Access (B-IFDMA) is a transmission scheme that recently emerged as an attractive choice for LTE operation in unlicensed spectrum due to its benefits in allowing increased power levels under unlicensed band regulations. In this paper, we introduce a new PRACH preamble format suitable for unlicensed operation based on B-IFDMA. In addition, we present a receiver structure for the PRACH preamble together with simulation results on its timing estimation performance. We show that the PRACH preamble design is robust against possible aliasing effects caused by frequency-domain properties of B-IFDMA and provides accurate timing estimates. We also propose non-uniform frequency interleaving for the PRACH preamble that further improves timing estimation performance.