This paper examines how to minimize the energy consumption of a user equipment (UE) when transmitting short data payloads. The receiving base station (BS) controls a reconfigurable intelligent surface (RIS), which requires additional pilot signals to be configured, to improve the channel conditions. The challenge is that the pilot signals increase the energy consumption and must be balanced against energy savings during data transmission. We derive a formula for the energy consumption, including both pilot and data transmission powers and the effects of imperfect channel state information and discrete phase-shifts. To shorten the pilot length, we propose dividing the RIS into subarrays of multiple elements using the same reflection coefficient. The pilot power and subarray size are tuned to the payload length to minimize the energy consumption. Analytical results show that there exists a unique energy-minimizing solution. For small payloads and when the direct path loss between the BS and UE is weak compared to the path loss via the RIS, the solution is using subarrays with many elements and low pilot power and vice versa. The optimal percentage of energy spent on pilot signaling is in the order of 10-40%.

翻译：本文研究了用户设备（UE）在传输短数据载荷时如何最小化能耗。接收基站（BS）控制一个可重构智能表面（RIS），该表面需通过额外导频信号进行配置以改善信道条件。挑战在于导频信号会增加能耗，必须与数据传输过程中的节能效果相权衡。我们推导了能耗公式，涵盖导频与数据传输功率，以及非完美信道状态信息和离散相位偏移的影响。为缩短导频长度，我们提出将RIS划分为使用相同反射系数的多个子阵列，每个子阵包含多个单元。导频功率和子阵列尺寸根据载荷长度进行调节，以最小化能耗。分析结果表明存在唯一的能量最小化解。对于短载荷，且当BS与UE之间的直接路径损耗相对于通过RIS的路径损耗较弱时，该解采用包含大量单元的子阵列和低导频功率，反之亦然。导频信号所消耗的最优能量百分比约为10-40%。