# The SF12 Well in LoRaWAN: Problem and End-Device-Based Solutions
## 3 Related Work  
Previous work has pointed out that confirmed traffic in LoRaWAN may be impaired by DC restrictions that limit downlink capacity, rendering gateways as bottlenecks that cause packet delivery ratio decrease [7; 8; 9; 21; 22; 23]. However, the consequent problem of congestion collapse remains unexplored as of this writing. The gateway bottleneck issue aggravates as the SF used by the devices increases, where using SF12 corresponds to the worst scenario. SF12 actually offers the best link range in LoRaWAN [6; 24]. However, such an advantage is achieved at the expense of minimizing frame payload size and bit rate, which increases frame Time-on-Air (ToA) [14], the ED power consumption [14], and the uplink collision probability [10]. In the downlink, as aforementioned, a gateway uses the same SF as the one used in the uplink in RX1, and SF12 in RX2 [17]. Hence, when the use of SF12 by the EDs increases, the use of SF12 by the gateways for downlink transmission (e.g., to send ACKs) increases as well. In turn, this behaviour minimizes the downlink capacity of the gateways [11], which may result in congestion losses [9]. Different proposals intended to mitigate the bottleneck problem involve the use of multiple gateways [23], ACK aggregation (i.e., grouping ACK information in fewer, longer \begin{table}
\begin{tabular}{c c c c c} \hline
**Data Rate (DR)** & **Modulation** & **Spreading Factor** & **Bandwidth (kHz)** & **Bit Rate (bit/s)** \\ \hline
0 & LoRa & SF12 & 125 & 250 \\
1 & LoRa & SF11 & 125 & 440 \\
2 & LoRa & SF10 & 125 & 980 \\
3 & LoRa & SF9 & 125 & 1760 \\
4 & LoRa & SF8 & 125 & 3125 \\
5 & LoRa & SF7 & 125 & 5470 \\
6 & LoRa & SF7 & 250 & 11,000 \\
7 & GFSK & & & 50,000 \\ \hline \end{tabular}
\end{table} 
Table 1: DRs, SFs, and physical layer bit rates for the EU 868 MHz band channels. Figure 2: Uplink transmission followed by the two receive windows, RX1 and RX2, defined in LoRaWAN class A.  
sized ACKs) [25], and sending a single ACK to a group of EDs [26]. The first type of solution obviously increases network cost, whereas the other mentioned solutions require major MAC layer modifications, affecting both communication endpoints: the EDs and the NS. In order to minimize the negative impact of SF12, one possible approach could be the leveraging of the ADR mechanism [15; 19; 27]. Only recommended when propagation conditions are stable, the ADR aims to optimize the uplink data rate, air time, and energy consumption by using the ED link budget as the input. Many research works propose ADR optimizations [28; 29; 30; 31; 32; 33]. Such optimized ADR schemes focus on different main objectives, such as throughput improvement [29; 30], energy efficiency [31; 32], or scalability [33]. In [29], a new ADR algorithm called EXPLoRa-C is proposed and evaluated in terms of throughput by means of theorical analysis and simulations and taking into account the capture effect. On the other hand, in [30] a scheme for power transmission and SF allocation is presented to achieve a fairer network operation by optimizing the packet error rate for nodes far from the gateway. Energy efficiency is an objective in [32], where an analytical model is developed to select the SF, channel width, and code rate to optimally satisfy a given Quality of Service (QoS). In [31], the authors implement an improved ADR scheme that yields a better data extraction ratio and a lower number of collisions, while keeping energy consumption. On the other hand, the authors in [33] propose an improved ADR mechanism that increases reliability and energy efficiency fairly for any network size and increases the delivery ratio in dense networks. However, the ADR ignores possible congestion problems. In addition, the ADR relies on the exchange of MAC commands between the EDs and the NS. Thus, its usage adds more pressure to the downlink bottleneck and is inefficient in scenarios with a high number of packet losses [8]. Another important drawback of the ADR is its data rate backoff sequence [21].