# On the Benefits of Listen before Talk Scheme for NB-Fi Networks
## 3 A Short Description of NB-Fi  
In this section, we describe the most relevant details of NB-Fi, while the detailed description of NB-Fi can be found in [4; 15].  
An NB-Fi network has a \"star\" topology and consists of a server, base stations (BSs), and end devices (hereinafter called sensors). Typically, sensors transmit their data frames via the wireless channel to a BS, which forwards the frames to the server through a wired link. In the reverse direction, the server sends service frames, e.g., acknowledgments or data.  
In many countries, NB-Fi networks usually use the 868.7-869.2 MHz ISM band and have separate uplink and downlink channels. The uplink channel is at least 51.2 kHz wide, while the downlink channel is at least 102.4 kHz wide.  
NB-Fi devices usually use differential binary phase-shift keying (DBPSK) with data rates of 50, 400, 3200, and 25,600 bps. All frames in NB-Fi have the same size of 288 bits, so their duration and the width of the subchannel required for transmission are determined by the used data rate (see Table 2). Note that in NB-Fi, all data rates have the same spectral efficiency (bits/Hz/s): the lower ones have higher reliability because of narrower bands and, consequently, higher signal power spectral density.  
By default, sensors access the channel according to an Aloha-like scheme: a sensor does not sense the channel before transmission and sends the frame immediately if it is the first transmission attempt, or after a random delay before a retry. Sensors can also use the LBT mode: before transmitting a packet in some frequency band, the sensor evaluates the signal strength in this band to ensure that no other devices are transmitting. If the sensor does not detect any transmission, it transmits its packet, otherwise, the sensor postpones its transmission until the signal strength falls below a threshold.  
To transmit a frame, each sensor selects a subchannel within the uplink channel. Effectively, the central frequency of the subchannel is randomized within the allocated frequency band for each frame transmission attempt, including the retries. This rule has an exception if the required subchannel width is comparable to the channel width (e.g., the data rate is 25,600 bps, while the channel width is 51.2 kHz). In this case, the central frequency of the subchannel equals the central frequency of the channel.  
The subchannel for downlink transmissions is determined by the sensor's identifier and does not change at different transmission attempts.  
In this paper, we consider that sensors operate in discontinuous reception (DRX) mode, and their frames require acknowledgment. In this mode, after a frame transmission, the sensor waits for \(T_{delay}\) and listens to the downlink channel until it either receives an acknowledgment or a \(T_{listen}\) interval passes (see Table 2). If no acknowledgment is received, the sensor retries after a random time distributed uniformly over the interval \((0,T_{rnd})\)  
\begin{table}
\begin{tabular}{c c c c} \hline
**Date Rate \(r\), bps** & \(T_{Frame}\)**ms** & \(T_{delay}\)**ms** & \(T_{listen}\)**, ms** & \(T_{rnd}\)**ms** \\ \hline
50 & 5760 & 140 & 60,000 & 5000 \\
400 & 720 & 20 & 30,000 & 1000 \\
3200 & 90 & 5 & 6000 & 100 \\
25,600 & 11.25 & 3.75 & 6000 & 100 \\ \hline \end{tabular}
\end{table}
Table 2: Time intervals.  
\begin{table}
\begin{tabular}{c c c} \hline
**Paper(s)** & **Main Contribution** & **Difference with This** **Research** \\ \hline
[22] & Comparison of NB-Fi, LoRaWAN, and Sigfox in a wide range of scenarios. & Does not study LBT in NB-Fi. \\ \hline \end{tabular}
\end{table}
Table 1: _Cont._Retransmissions are performed until the reception of an acknowledgment or upon reaching the configurable retry limit.