| Summary: | Grant-free access is an attractive approach to enable spectrum-efficient low-latency access for systems with massive number of users. Pilot design plays a crucial role for grant-free access as it needs to provide a large number of access codes with fast collision detection capability and good channel estimation performance. Recently, pilot designs with fast collision detection capability have been proposed for compressed sensing (CS) based channel estimation. But the existing designs are not optimized for the estimation of highly sparse and block-sparse channels. In this paper, we present several propositions related to the performances of the CS based sparse and block-sparse channel estimation. Utilizing these propositions, we develop a novel non-orthogonal pilot design with fast collision detection capability for grant-free access in block-sparse channels. We also propose two methods to optimize the Peak-to-Average Power Ratio (PAPR) of the proposed non-orthogonal pilot codes. The simulation results illustrate that the proposed design provides similar or better channel estimation and collision detection performances with much better pilot resource efficiency when compared to the existing designs. Finally, we investigate the trade-offs among different design parameters and the channel estimation performances to facilitate better design choices.
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