# The SF12 Well in LoRaWAN: Problem and End-Device-Based Solutions
## 1 Introduction  
Low Power Wide Area Network (LPWAN) technologies have gained significant momentum as wireless connectivity solutions for enabling the Internet of Things (IoT) applications. LPWAN technologies are typically based on the star topology paradigm, whereby the IoT devices communicate directly with neighboring radio gateways. This feature, along with the support of long range (in the order of kilometers), offers reduced infrastructure cost and complexity compared with short-range multihop topologies. In addition, LPWAN technologies provide low energy consumption, enabling multiyear lifetimes for battery-operated devices. However, the advantages of LPWAN technologies come at the expense of challenging constraints in terms of message sizes, bit rates, and message rates. These unique features of LPWAN technologies have attracted the attention of industry, academia, and standards development organizations [1; 2; 3; 4].  
One of the most prominent LPWAN technologies is LoRaWAN [5]. Two of the main reasons for the popularity of this technology are the following: (i) It is based on the sub-GHz unlicensed spectrum, which facilitates adoption, even if it also involves duty cycle constraints due to spectrum access regulations in some world regions; and (ii) it is possible to deploy a private LoRaWAN network without the need to rely on third-party entities (e.g., network operators).  
Since the initial specification of LoRaWAN was published, many researchers have devoted their efforts to investigating the performance of this technology [1; 6; 7; 8; 9; 10; 11]. However, LoRaWAN deployments are often still in the relatively initial stages, and network performance may become compromised as the number of connected end-devices increases, even for a relatively low number of such devices.  
In this paper, we identify and illustrate a problem which we call the _Spreading Factor 12 (SF12) Well_ and evaluate a number of solutions to counter it. LoRaWAN offers optional link-layer reliability, based on link-layer acknowledgments and retransmissions. Whenacknowledgments are not received by a sender after transmitting a frame in reliable mode, the sender typically tends to reduce its physical layer bit rate. However, this reaction increases uplink and, even worse, downlink channel utilization. As a result, network performance may degrade steadily into congestion collapse. In this paper, we characterize the SF12 Well problem in a range of scenarios, and we show that it is possible to mitigate it, maintaining good performance as the offered load increases by using different SF management techniques.  
In order to carry out the study, we developed and used a simulator called Advanced Framework for LoRa (AFLoRa), a LoRaWAN simulation environment that uses FLoRa [12] as a basis, albeit with significant enhancements and additions. As a side-contribution of this paper, we offer the simulator publicly [13].  
The remainder of the paper is organized as follows. Section 2 presents the main features of LoRaWAN, focusing on the parameters and mechanisms most relevant to the present work. Section 3 reviews related work. Section 4 illustrates the SF12 Well problem in different scenarios in terms of the offered load and the ratio of acknowledged traffic. Section 5 evaluates a number of techniques intended to counter the SF12 Well problem, showing their performances and trade-offs. Finally, Section 6 provides the main remarks from this work.