| Summary: | Grant-free multiple access is a critical mechanism introduced in 5G new radio (NR) to support ultra-reliable low-latency communication (URLLC) services. Pilot authentication (PA) is a key security mechanism to guarantee reliable performance of grant-free URLLC. However, PA can be easily paralyzed by pilot-aware attack since pilot signals are usually publicly known, and unprotected. To solve this, we develop the concatenated graph coding (CGC) theory by which time-frequency resources on bandwidth part (BWP) can be encoded flexibly to protect PA securely. Particularly, we use bipartite graph, and multigraph theory to model PA on BWP as transmission, and retrieval of pilot (TRP). Each transmitter in the uplink needs transmit a unique random pilot sequence as subcarrier activation pattern (SAP) on BWP. After observing SAPs from multiple transmitters, the receiver decodes a pilot sequence of interest, and tests its authenticity. The retrievability of authentic pilots is defined, and formulated analytically. We also derive the analytical closed-form expression of system failure probability, and accessibility in the regime of large-scale antenna arrays, and short data packets. Interestingly, we find that four trade-offs exist: retrievability-latency, retrievability-accessibility, reliability-latency, and reliability-accessibility. Simulation results show the security advantage of our proposed theory in grant-free URLLC system.
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