Ultra-reliable and low-latency communication (URLLC) is one of three pillar applications defined in the fifth generation new radio (5G NR), and its research is still in its infancy due to the difficulties in guaranteeing extremely high reliability (say 10−9 packet loss probability) and low latency (say 1 ms) simultaneously. In URLLC, short packet transmission is adopted to reduce latency, such that conventional Shannon’s capacity formula is no longer applicable, and the achievable data rate in finite blocklength becomes a complex expression with respect to the decoding error probability and the blocklength. To provide URLLC service in a factory automation scenario, we consider that the central controller transmits different packets to a robot and an actuator, where the actuator is located far from the controller, and the robot can move between the controller and the actuator. In this scenario, we consider four fundamental downlink transmission schemes, including orthogonal multiple access (OMA), non-orthogonal multiple access (NOMA), relay-assisted, and cooperative NOMA (C-NOMA) schemes. For all these transmission schemes, we aim for jointly optimizing the blocklength and power allocation to minimize the decoding error probability of the actuator subject to the reliability requirement of the robot, the total energy constraints, as well as the latency constraints. We further develop low-complexity algorithms to address the optimization problems for each transmission scheme. For the general case with more than two devices, we also develop a low-complexity efficient algorithm for the OMA scheme. Our results show that the relay-assisted transmission significantly outperforms the OMA scheme, while the NOMA scheme performs well when the blocklength is very limited. We further show that the relay-assisted transmission has superior performance over the C-NOMA scheme due to larger feasible region of the former scheme.

中文翻译：

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工厂自动化场景中 URLLC 的联合功率和块长度优化

超可靠低延迟通信（URLLC）是第五代新无线电（5G NR）定义的三大支柱应用之一，由于难以保证极高的可靠性（比如10- 9 数据包丢失概率）和低延迟（比如 1 毫秒）同时进行。在URLLC中，采用短包传输来减少延迟，使得传统的香农容量公式不再适用，并且在有限块长度下可达到的数据速率成为关于解码错误概率和块长度的复杂表达式。为了在工厂自动化场景中提供 URLLC 服务，我们考虑中央控制器向机器人和执行器发送不同的数据包，其中执行器位于远离控制器的位置，并且机器人可以在控制器和执行器之间移动。在这种情况下，我们考虑四种基本的下行链路传输方案，包括正交多址 (OMA)、非正交多址 (NOMA)、中继辅助和协作 NOMA (C-NOMA) 方案。对于所有这些传输方案，我们的目标是联合优化块长度和功率分配，以在机器人的可靠性要求、总能量约束以及延迟约束下最小化执行器的解码错误概率。我们进一步开发低复杂度算法来解决每个传输方案的优化问题。对于具有两个以上设备的一般情况，我们还为 OMA 方案开发了一种低复杂度的高效算法。我们的结果表明，中继辅助传输明显优于 OMA 方案，而 NOMA 方案在块长度非常有限时表现良好。我们进一步表明，由于前一种方案的可行区域更大，中继辅助传输比 C-NOMA 方案具有更好的性能。