# LoRaWAN Communication Protocols: A Comprehensive Survey under an Energy Efficiency Perspective
## 2 Background Knowledge
### Collisions, Scalability, and Robustness Issues  
In LoRaWAN, two frames are collided in case two or more packets overlap in time and use the same transmission LoRa parameters, i.e., the SF, BW, and carrier frequency (CF). Adoption of the same transmission parameters for a high number of end-devices increases the possibility of collisions and consequently the packet error rate (PER). However, a packet received with a higher power level (at least 6 dB) can still be decoded during a collision [33]. Collisions lead to a degradation of the overall network performance and decrease network reliability. The PER and packet delivery ratio (PDR) are used to demonstrate the network reliability. Due to collisions, LoRaWAN cannot support more than a few hundred end-devices connected to the same GW [37]. Therefore, the choice of both SF and transmission power (TP) parameters will affect the final number of packets collisions and determine the coverage area of GWs and end-devices within it. Coverage range depends on transmission parameters such as SF, TP, BW, and CR, and conditions where the GWs and end-devices are deployed. Therefore, protocols proposing a proper SF assignment to end-devices and adjusting the transmission parameters are expected to achieve improved scalability, since the optimized allocation of SF to end-devices allows the simultaneous transmission of multiple packets [3]. Another factor that can increase collisions is that all GWs in the coverage area of end-devices, regardless of the network they belong to, receive packets transmitted by any other end-device. Collisions are also affected by transmission frequency and the type of payload. Larger payloads in size and more frequent transmissions result in longer ToA and medium occupation time.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Mtype** & **Description** \\ \hline
000 &
\begin{tabular}{l} Join-Request \\ Join-accept \\ \end{tabular} \\
010 &
\begin{tabular}{l} Unconfirmed Data Up \\ Unconfirmed Data Down \\ \end{tabular} \\
100 &
\begin{tabular}{l} Confirmed Data Up \\ Confirmed Data Down \\ \end{tabular} \\
101 &
\begin{tabular}{l} Rejoin-request \\ Proprietary \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 3: MAC message types.  
As collisions contribute significantly to packet loss, several techniques have been proposed for collisions management through proper coordination and synchronization of end-devices and scheduling of their transmissions [38], as well as through detection of the common medium [39] and monitoring of the medium access following the listen-before-talk (LBT) concept [40], while others focused on slotted-ALOHA [3]. Additionally, deploying more than one GWs or using directional antennas at the device side can decrease the possibility of collisions. However, the increase in the number of GWs cannot eliminate all the issues related to scalability [3]. As aforementioned, DC restriction is another factor that decrease LoRaWAN scalability, as it limits the number of downlink messages, considering ACK messages, that a GW can send to the end-devices. The problem becomes more severe as the network size increases [11] since a large number of end-device are requesting ACKs and the GW must comply with DC restriction. In addition, end-devices may have to retransmit their data packets when a collision occurs. Retransmissions result in higher energy consumption and latency, as well as waste bandwidth. Therefore, the increase in retransmissions leads to eventually network collapse.