Power-Efficient Transmissions in LoRa Uplink Systems

LoRa has become a promising low-power wide-area network (LPWAN) technology for long-range communications, enabling various Internet of Things (IoT) applications. However, LoRa requires appropriate settings of transmission parameters on end devices (EDs) to achieve good performance for different IoT applications. We investigate in this paper power-efficient transmissions in LoRa uplink systems. To adapt to the diverse situations of IoT applications, we define two typical scenarios considering per-ED and sum-ED quality-of-service (QoS) requirements, respectively, to optimize the system power consumption. We formulate the problem to minimize the overall transmit power of LoRa EDs, in consideration of factors such as ED and frequency resource block (RB) pairing, spreading factor (SF) allocation, and ED's minimum data rate requirement. To solve the joint optimization problem efficiently, we first develop an ED-RB pairing scheme, which computes the average signal-tonoise ratio (SNR) for each ED, then sorts the EDs in an ascending order according to their average SNRs, and finally selects one RB with the highest SNR for the first ordered ED and then for other EDs sequentially. We also propose to select the appropriate SF to meet the transmission data rate requirements and adjust the transmit power subsequently to facilitate power-efficient uplink transmissions. Furthermore, the complexity of the proposed schemes are analyzed as well. Both numerical simulation and experimental results demonstrate that the proposed schemes save more than 34.6% power in comparison with the baseline.