# On the Benefits of Listen before Talk Scheme for NB-Fi Networks
## 4 LBT Variants  
The NB-Fi specification mentions that devices can operate in LBT mode but does not provide much detail on how it should work. The straightforward interpretation of the description of LBT in the standards means that the device listens to the channel before transmission and, if the power of the signal in the channel is greater than some level, it should wait until the channel becomes idle, and only then should it start the transmission. Such operation of LBT corresponds to the persistent CSMA algorithm. Apart from this algorithm, we consider other modes of sensors' operation with LBT that implement different CSMA algorithms, see Figures 1-3. Namely, we consider:  
* Non-persistent CSMA;
* Non-persistent CSMA with frequency hopping;
* \(p\)-persistent CSMA.  
They all have in common that, before transmission, the sensor listens to the channel and compares the power of the signal with a threshold. If the signal is greater than the threshold, the channel is considered busy, and the sensor performs different actions depending on the used algorithm. Let us describe the considered types of CSMA in more detail.  
In the non-persistent CSMA mode, the sensor selects a random subchannel when a packet arrives, see Figure 1, and senses the subchannel for the \(\sigma\) time interval. The specification does not define the value of \(\sigma\), but it is reasonable to make it equal to the symbol duration at the used data rate (see Table 3) because of two reasons. First, this value is sufficient to estimate the signal power. Second, this value is enough for the signal to propagate between any sensors within their transmission range. If the signal strength is below the sensitivity threshold \(S\) (see Table 3), the channel is considered idle, and the sensor transmits the packet. Otherwise, the channel is considered busy, and the sensor repeatedly senses the channel with a random delay between the sensing attempts until the channel becomes idle. The distribution of this delay is not specified in the standard, so we choose it uniformly from the interval \(\left[0,T_{Frame}\right]\), where \(T_{Frame}\) is the frame duration at the used data rate. Although the interval for the random delay seems quite narrow, we choose such an interval because preliminary results show that the further increase in its upper bound does not provide any significant improvement in the P  
Figure 1: Schemes of the non-persistent CSMA.  
A variation of the non-persistent CSMA mode is implemented in the non-persistent CSMA mode with frequency hopping, see Figure 2. In this mode, when the sensor finds the selected subchannel busy, it senses another random subchannel, chosen equiprobably from the available frequency band.  
In the \(p\)-persistent CSMA mode, when the sensor generates a data frame, it senses the subchannel for a \(\sigma\) time interval, see Figure 3. In the case of an idle subchannel, the sensor transmits the frame. Otherwise, the sensor waits until the subchannel becomes idle. Then, it transmits the packet with the probability \(p\) and repeats listening to the subchannel within the time interval \(\sigma\) with the probability \(1-p\). We further denote a special case of \(p\)-persistent CSMA with parameter \(p=1\) as persistent CSMA.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Data Rate, bps** & \(\sigma\), \(\mu\)**s** & **S, dBm** & \(R_{sensor}\)**, m** & \(R_{base}\)**m** \\ \hline
50 & 20,000 & \(-150\) & 4270 & 12,150 \\
400 & 2500 & \(-141\) & 2460 & 7650 \\
3200 & 312.5 & \(-132\) & 1420 & 4810 \\
25,600 & 39.06 & \(-123\) & 820 & 3030 \\ \hline \end{tabular}
\end{table}
Table 3: Network and device characteristics.  
Figure 2: Schemes of the non-persistent CSMA with frequency hopping.