# Collision Avoidance Resource Allocation for LoRaWAN
## 2 Related Works  
[PERSON] et al. [5] proposes an approach to increase the single channel capacity using the Slotted ALOHA protocol for the communication between end-nodes and gateways. They tested this proposal in a real environment with 24 nodes accessing the same channel and demonstrated that, in a high traffic set-up, the measured improvement was up to 5.8 in comparison with the Pure-ALOHA measured throughput. This solution requires the implementation of a different medium access protocol and, therefore, it is not compatible with current LoRaWAN networks.  
In order to meet the requirements of the real-time traffic management in Industrial IoT Applications, [PERSON] et. al. [6] worked on a solution based on a centralized MAC scheme. In this scheme, a central node synchronized all end-nodes broadcasting a periodic beacon. After a beacon, the time to send information is divided in two periods--the first is intended for non-periodic unconfirmed data in which end-nodes use the ALOHA mechanism for channel access, and the second is intended for periodic real-time confirmed data flows in which end-nodes access the channel with a TDMA mechanism. This solution seems to be devised for periodic transmissions in the contention-free period, guaranteeing no collisions, high reliability and bounded end-to-end delays. However, in the content-access period (with the Pure ALOHA medium access strategy), reliability is not ensured for the non-periodic transmissions.  
In [7] a synchronization and scheduling solution is proposed, in which a central entity assigns the communication slots to the end-nodes based on several policies and infrequent signaling messages. In this assignment, it is necessary to know the traffic periodicity and the clock accuracy of end-nodes, so the central entity can assign the required time slots. The limitation of this work is the requirement of periodic traffic, otherwise the central entity will not know the number of time slots to assign.  
Another time-slotted based solution is presented in [8], where it is computed a hash algorithm with the nodes' addresses to assign them a specific time slot to transmit. The network server leverages the last slot sent in the broadcast frame in order to handle the acknowledgements and time synchronization at the expense of extra energy cost.  
To achieve energy-efficiency and increase the network performance, the work in [9] designs a new LoRaWAN network architecture and a new MAC protocol based on unicast and broadcast TDMA communications. These communications are started by a sink and an intermediate entity, the cluster head, is responsible for the rely of wake-up beacons to synchronize the end-devices and avoid collisions. This solution needs the implementation of a non-standard LoRaWAN transceiver.  
The different solutions proposed in EXPLoRa [10] are useful when taking into account different traffic types from different applications or in situations in which there are areas with different traffic demands requiring a balanced resource allocation. However, this solution requires a centralized entity that has to monitor and manage periodically the network behaviour.  
FCA-LoRa [11] achieves a fairness resource allocation with collision avoidance. It is based on the broadcasting of periodic beacon frames sent by the gateways. This novel MAC protocol uses an alternative version of the CSMA/CA algorithm for listening the medium before the transmission of a packet (instead of using the ALOHA protocol used in original LoRaWAN).  
The main differences between these works and our proposal are the following: (1) our proposal is compatible with current LoRaWAN networks since it works on top of the existing MAC protocol, and (2) after the initial LoRaWAN join procedure, no extra signaling or communication with the end-device is required.