# Resource Allocation to Massive Internet of Things in LoRaWANs
## 2 Literature Review
### Interference-Based Approaches  
For the experiments on interference and the capture effect in a LoRa channel, an error model is considered. It can be applied to experiments monitoring the interference between communications using different SFs, as presented in [8]. This work shows three different approaches for allocating an effective SF: (i) A random SF assignment approach, which allocates the SFs based on a uniform distribution; (ii) a fixed SF assignment approach, which allocates static SFs during the initial deployment; and (iii) the packet error rate (PER) approach, which assigns the lowest possible SF using an error model based on additive white Gaussian noise (AWGN). The PER mechanism achieves a higher transmission success ratio by finding an appropriate SF than both random and fixed SF-based allocation approaches. However, the authors in [8] did not take into account the inter-SF interference, shadowing, and building penetration losses. Another work based on collision types to manage the SF is presented in [4]. There are two types of collisions observed during communications: (a) Collision of two packets with the same SFs, and (b) collision of two packets with different SFs. The primary aim of this collision-aware SF assignment method is to improve the PER by enhancing channel fairness. Firstly, this method categorizes the EDs to form groups based on the radio frequency (RF) coverage and path loss, wherein each group uses a distinct channel. Secondly, the lowest SF is selected and allocated to each group based on the observed cumulative interference ratio (CIR). The proposed scheme in [4] decreases the PER up to 42% overall. However, this improved performance comes at the cost of increased energy consumption compared to the conventional distance-based scheme [5], and the simulation analysis is only limited to the collision case (a). An SF assignment mechanism for the LoRaWAN network introduced in [9] aims to improve the success ratio by reducing the interference caused by the same SFs and channels. To categorize the interference of two packets, authors in [9] measures the signal-to-interference-plus-noise ratio (SINR) of packets transmitted with the same SFs over the same channels. In the simulation, if the measured SINR is larger than the threshold limit, the packets survive the interference. Otherwise, the packets are lost to the interference. However, authors in [9] does not take into account the interferences between packets with different SFs over the same channel, as these SFs are not entirely orthogonal [10; 11; 12; 13; 14]. A similar scenario related to the impact of non-orthogonality concerning the data extraction rate is presented in [11]. The study in [11] reveals that the impact of non-orthogonality is limited when the traffic load in the network is low. The authors of [11] also show that higher SFs (e.g., 10, 11, 12) are more venerable to interference due to their high ToA. It is also assumed in [15] that a collision occurs due to the simultaneous transmission of the packets over the same SF and channel. However, the analysis in [15] is limited to the case with an SF of 12 only. Furthermore, the effect of the non-orthogonality concerning SFs is further deliberated in [16], where it computes the probabilities of successful UL packet and coverage based on [11]. The study in [16] shows that UL packets transmitted with different SFs can collide as long as their received power at GW is different. It also reveals that the impact of inter-SF interference can be high enough in a significantly large network. However, in a short distance (i.e., less than 1 km), the effect of inter-SF interference is less. Additionally, the scalability of LoRaWAN under the impact of interference in the absence of DL traffic was studied in [17]. The testbed and simulation results concluded that one of the two packets can be received if the headers of both packets do not collide. In contrast to previous works, authors in [18] considers both inter- and intra-SF interferences.