# A Survey on Adaptive Data Rate Optimization in LoRaWAN: Recent Solutions and Major Challenges
## 3 Review of Adaptive Data Rate Algorithms
### Throughput  
ADR can be used to improve network throughput by improving DER. [37] propose two algorithms of incremental complexity EXPLoRa-SF and EXPLoRa-AT. EXPLoRa-SF is a heuristic which attempts to equally distribute SFs to the end nodes in the gateway's radio range, only restricted by their Received Signal Strength Indicator (RSSI) values and relevant thresholds. EXPLoRa-AT on the other hand, propagates impartial allotment of the ToA between the end nodes in the network. The proposedscheme does not configure the SF on the basis of distance and received power measurement, but instead considers the quantity of linked end nodes, enabling the equalisation of the ToA of the packets in each SF. EXPLoRa-AT guarantees equalisation of the RF channel utilization by the end nodes by leveraging the use of multiple SFs in order to have orthogonal sub-channels. The two algorithms combine SF orthogonality and radio range visibility to expand the density of end nodes with concurrent transmission in the grid, reducing collisions, improving DER and ultimately improving throughput. The ordered waterfalling technique is used for even distribution of channel load between the end nodes in the system. For dense networks, the algorithms substantially improve data rates and robustness. Simulation of EXPLoRa-SF and EXPLoRa-AT in \"LoRaSim\" has a superior performance compared to the standard ADR. These two algorithms are implemented using one gateway. The drawback of this approach is that it is designed under the assumption that end nodes transmit with the uniform data rate and payload. The work in [38] attempts to address this drawback. There are circumstances where the assigned SF in EXPLoRa-SF does not adhere the restrictions stipulated in LoRaWAN that degrade performance. Additionally, the proposed approach in [37] does not support implementation with multiple gateways which is an open gap for future research. A contention-aware ADR scheme, developed by [39] traces the distribution of end nodes for every SF and seeks to expand the number of gadgets utilizing low SFs. The authors attempt optimization of throughput in networks where the same SF can be used by a huge number of end nodes. The scheme uses constrained optimization and defines the total throughput as an objective function with respect to the quantity of end nodes utilizing specified SFs. The authors develop the theoretic optimum throughput which could be accomplished by changing only the data rate of the end nodes. They adopt the gradient projection method to solve the constrained optimization problem, which enables the gateway to effectively obtain the optimum configuration subject to the constraints, irrespective of the number of end nodes implemented in the system. When numerous end nodes have analogous link quality, specifically in relation to the partial use of the SF, the propositional approach achieves significantly better throughput than the existing scheme due to the load balancing effect. The strength of the approach is factoring in the contention issue in optimising ADR. Although this approach improves throughput, the transmission success ratio of the end nodes declines, so if reliability is more important than throughput in an application, this approach would not be sufficient. For future research, a more comprehensive evaluation and efficient solution for the SF update via DL is an area that is open. An optimization technique can be developed that considers the transmission success ratio at the same time, thus extending the solution to the multi-objective optimization problem. A Fair Adaptive Data Rate algorithm (FADR) that calculates data rate and TP assignment with the purpose of achieving fairness in data rate and reducing collisions between the end nodes is propositioned in [40]. The algorithm uses RSSI values in its computations while deciding SF and TP allocations to maximize DER among all end nodes.