# Performance of Narrow Band Wide Area Networks with Gateway Diversity
## 3 System Model  
In this section, the channel access and interference model along with the path loss model are presented which are used in our NS-3 simulations.  
1. Channel Access  
In this section, the channel access scheme that is used throughout the analysis and simulations are presented. The LPWAN module follows an ALOHA-based sporadic channel access model in the UL with spatial diversity among the GWs, where all the GWs listen to the UL traffic at all times [13; 14]. The network architecture follows a star topology. When a given GW can successfully decode an ENs UL packet, it passes it up to the cloud server to be sent to the application layer. The cloud server processes multiple copies of the same packet on a first come, first served basis and discards duplicates sent from multiple GWs. Coherent combining of the packets from the GWs as presented in [15] is beyond the scope of this paper.  
The underlying PHY modulation is LoRa but the upper layer channel access is different than the LoRaWAN protocol. The ENs use a single SF and a single channel. The GW diversity concept is similar to LoRaWAN, but our system model assumes single radio gateways unlike LoRaWAN, which can support multiple channels and data rates simultaneously. The best GW is determined by the cloud server based on the strongest UL RSSI among multiple GWs receiving a UL packet from a given EN, which best GW serves for the DL direction for that EN.  
By allowing the end-nodes to transmit when they have traffic, their transceivers can go to sleep to save energy, rather than periodically waking up to search for a transmit  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**C/I [dBc]** & \multicolumn{8}{c}{**Interference (LoRa)**} \\ \hline  & SF/BW (kHz) & 5/500 & 6/500 & 7/500 & 8/500 & 9/500 & 10/500 & 11/500 \\ \hline \multirow{8}{*}{5/500} & \multirow{8}{*}{6/500} & 1.26 & \(-0.83\) & \(-2.1\) & \(-3\) & \(-3.6\) & \(-3.8\) & \(-4.3\) \\ \cline{3-10}  & & \multicolumn{8}{c}{with IQ inversion} \\ \cline{3-10}  & & \(-3.1\) & \(-3.5\) & \(-3.6\) & \(-3.7\) & \(-3.9\) & \(-4.1\) & \(-4.6\) \\ \cline{3-10}  & & \(-4\) & \(1\) & \(-3.33\) & \(-4.8\) & \(-5.65\) & \(-6.2\) & \(-6.5\) \\ \cline{3-10}  & & \multicolumn{8}{c}{with IQ inversion} \\ \cline{3-10}  & & \(-5.3\) & \(-5.8\) & \(-6.1\) & \(-6.4\) & \(-6.5\) & \(-6.7\) & \(-7\) \\ \cline{3-10}  & & \(-7.8\) & \(-6.6\) & \(0.8\) & \(-6\) & \(-7.7\) & \(-8.6\) & \(-9\) \\ \cline{3-10}  & & \multicolumn{8}{c}{with IQ inversion} \\ \cline{3-10}  & & \(-7.8\) & \(-8.1\) & \(-8.7\) & \(-8.7\) & \(-9.1\) & \(-9.2\) & \(-9.6\) \\ \cline{3-10}  & & \(-10.6\) & \(-10.6\) & \(-9.4\) & \(0.54\) & \(-9\) & \(-10.7\) & \(-11.6\) \\ \cline{3-10}  & & \multicolumn{8}{c}{with IQ inversion} \\ \cline{3-10}  & & \(-10.5\) & \(-10.6\) & \(-10.9\) & \(-11.6\) & \(-11.4\) & \(-12.1\) & \(-12\) \\ \cline{3-10}  & & \(-13\) & \(-13.5\) & \(-13.5\) & \(-12.2\) & \(0.36\) & \(-11.9\) & \(-13.7\) \\ \cline{3-10}  & & \multicolumn{8}{c}{with IQ inversion} \\ \cline{3-10}  & & \(-13.1\) & \(-13.3\) & \(-13.5\) & \(-13.8\) & \(-14.6\) & \(-14.2\) & \(-15.1\) \\ \cline{3-10}  & & \(-15.7\) & \(-16\) & \(-16.2\) & \(-16.4\) & \(-15\) & \(0.14\) & \(-15\) \\ \cline{3-10}  &opportunity or a beacon from the GW. In the DL, the GW aligns its transmission to the periodic receive windows of the ENs after timing a handshake between the GW and the EN. The expected network load is sporadic and light enough to make the energy consumption on end-nodes the main consideration rather than the bandwidth efficiency. Moreover, the lack of overhead and decentralized channel access decisions are well-suited for applications with battery operation. The drawback of this scheme, however, is the interference in the UL towards the GW, as there is no coordinated channel access mechanism.