# A Survey of LoRaWAN for IoT: From Technology to Application
## 7 LoRaWAN Improvements
### Network Scalability Assessment and Improvement Methods  
Several studies reported about LoRaWAN network scalability concerns [32; 40; 50; 75; 97; 98], especially for the downlink traffic impact on scalability. By means of network simulations, it is shown that LoRaWAN can end in a deadlock stage if certain scalability issues are not addressed by time. Thus, several ways how SFs and transmit powers are distributed, as well as improvements to ADR mechanism are proposed. One of the first studies on scalability analysis is shown in [99]. The capacity of single gateway deployment under perfect synchronization and number of end devices for optimal Aloha case are derived. According to channel attenuation model shown in [32], it is shown that less than 2% of total end devices will use SF12. In addition, it is demonstrated that end devices far away from gateway have lower throughput compared to those nearby due to near-far problem. This means that packets from far nodes will be collided by the packets from nearby end nodes, due to better reception conditions. Due to this, authors of [100] presented an algorithm for assigning SFs and power levels to end nodes. First, the fraction of end nodes using certain SF inside one cell is optimized by minimizing the probability of having inter-SF collision. The second step is to find the optimal distribution of SFs and power levels. All the end nodes are sorted according to their path loss and are divided in a number of groups as there are channels available in the network. Then, in each group, nodes with lower path loss will get higher SF compared to those with higher path loss. This assignment will follow the optimized fraction of SFs found in the first step. It is proved that using such an algorithm PER is decreased by 50% for nodes at the cell edge in a scenario with 1 end node per 1000 m\({}^{2}\) transmitting every 10 min. The impact of SF distribution among end nodes in packet success probability (PSP) is appreciated in [101]. In [101], the regions of SF distribution are determined with the intention to increase the average PSP of the network. When assigning SFs to end nodes two conditions are considered: (i) the received power from that node in gateway should exceed the receiver sensitivity threshold for the assigned SF; and (ii) SIR for that node must also exceed the correctly decoded SIR threshold for that SF. An optimization problem for assigning SFs is formulated, where average network PSP is maximized with respect to region distances covered by each SF. [PERSON] and [PERSON] [101] demonstrated that, when numerous end devices are present in the network, it is reasonable to increase the lower SF region by increasing the probability for Condition (ii) and lower the probability for Condition (i). It is shown that the proposed solution outperforms the equal-interval and equal-area based SF distribution schemes in terms of average network PSP [101]. Two new algorithms for SF distribution in a cell are proposed in [102]. The first one, ExpLoRa-SF, assigns SFs to end nodes by equalizing the number of end nodes using different SFs. To this end, the first group of end nodes that are nearer to gateway based on their RSSI values will get assigned lowest SF and so on until the last group of end nodes. The second proposed scheme, ExpLoRa-AT, tends to equalize transmission times between different end nodes by distributing different SFs. The second approach will optimize the usage of different SFs over time, achieving better network performance. However, the first approach will achieve the same in case when all end nodes will have the same traffic patterns as well as same packet lengths. In addition to SFs and transmit power assignment algorithms, other improvements in ADR mechanism are proposed in [46; 103; 104]. In [103], the performance of ADR mechanism [11] in terms of the average converge time is shown. The convergence time depends on network size (200 and 3000 min for networks with 100 and 4000 nodes, respectively) and channel conditions [103].