| Summary: | In this paper, we consider a device-to-device communication system, where a multi-antenna mobile user directly communicates with nearby multiple devices, and propose a low-latency random access protocol utilizing random beams at the user. In our system model, each device tries to access the user with randomly selected one among multiple orthogonal (near-orthogonal) preambles, so the devices with different preambles can be discerned at the user. Meanwhile, the mobile user has multiple antennas and tries to fully utilize multiplexing gains. In our proposed protocol, the user adopts the orthogonal random beams both for reception and transmission regardless of channel conditions. Thus, in some cases, the multiple devices with the same preamble can be decoded at the user. Moreover, the user of random beamforming can reduce the computational complexity and time delay for beamforming. We analyze the access probability with our proposed random access protocol and find each device's one-shot access probability with the approximated one. Our simulation results show that our proposed protocol increases the random access probability compared to the conventional protocol that does not utilize the multiplexing gains, and our analysis well matches in various scenarios.
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