Pilot Sequences With Low Coherence and PAPR for Grant-Free Massive Access

To accommodate massive devices and facilitate activity detection in a grant-free access system, it is necessary to design non-orthogonal pilot sequences with low coherence. In this letter, we study the optimization problem to minimize the worst-case coherence among sequences under the peak-to-average power ratio (PAPR) constraint of each sequence. An efficient method is proposed to iteratively construct sequences by the conjugate gradient descent and space projection. Simulation results demonstrate that the proposed sequences can decrease the coherence under strict PAPR constraints by up to 62% compared with the benchmarks.