## 4 A Hybrid-ISL System with Ranging and LoS Support
### Proposed Network Protocol Stack for Hybrid-ISL System  
In Figure 4, the H-ISL's network protocol stack is depicted, highlighting the layers on the left side and the corresponding hardware partitioning on the right side, implemented on a software defined radio (SDR) platform consisting of a ZCU102 FPGA development board [24] and the AD-FMCOMM-S5 daughter board [25] featuring the AD9361 highly integrated RF transceiver chip. The protocol stack consists of the PHY and DLL layers, where the PHY is segmented into the High-PHY and Low-PHY sublayers.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & & Proximity-1 & Hybrid-ISL \\ \hline \multirow{4}{*}{\begin{tabular}{c} \\ \end{tabular} } & Frequency band & UHF & S-band \\ \cline{2-4}  & Bandwidth & 1.5 MHz & 10 MHz \\ \cline{2-4}  & Data rate & up to 1 MBps & \textgreater{}1 MBps \\ \cline{2-4}  & Modulation scheme & OFDM & PSK \\ \cline{2-4}  & Ranging support & yes & yes \\ \cline{2-4}  & Line-of-sight support & no & yes \\ \hline \multirow{4}{*}{\
\begin{tabular}{c} \\ \end{tabular} } & Synchronization & Standardized & Custom algorithm \\ \cline{2-4}  & Channel access & Combined time/frequency division & Frequency division \\ \cline{2-4}  & Channel coding & Convolutional &
\begin{tabular}{c} Read–Solomon \\ (configurable) \\ \end{tabular} \\ \cline{2-4}  & Quality of service & ARQ/expedited & expedited \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of the Proximity-1 protocol and the proposed Hybrid-ISL.  
Figure 4: Network protocol stack and its hardware partitioning.  
The use of a DLL layer is indicated to enable data communication, metrology, and timing (for accurate time stamping) channels. Each channel can be accessed by its service access points (SAPs) (i.e., Metrology SAP and Timing SAP). The DLL is also responsible for hailing, joining, and time synchronization functionalities in the network.  
The data flow in the PHY layer is as follows. On the transmit side of the PHY (downlink (DL) from the viewpoint of a base station/access point):  
* The stream from the DLL will be first randomized and then scrambled;
* After that channel encoding is performed, possible candidates for channel encoding are the Reed-Solomon (RS), low-density parity check (LDPC), or convolutional coding, in compliance with [5];
* The channel encoding is followed by a Binary Phase Shift Keying (BPSK) baseband modulation;
* The 1024 subcarrier data is then applied to a 1024-point Inverse Fast Fourier Transform (IFFT) block, implementing the orthogonal frequency division modulation (OFDM). Note: the sub-carrier spacing is 15 kHz and a 15.36 MHz sampling frequency and 9 MHz channel width is applied. The first 212 and, respectively, the last 211 subcarriers are used as a guard interval, and the remaining 600 subcarriers are used for data transmission. The direct current (DC) component is not modulated. Each OFDM symbol is preceded by a cyclic prefix (CP), with a length of 72 samples (4.68 \(\upmu\)s); thus the total length of a symbol is 1096 samples (with a total duration of 71.35 \(\upmu\)s);
* The resulting symbol is then modulated into RF frequencies by the AD9631 transceiver chip. On the receiver side of the PHY (up-link (UL) from the viewpoint of a base station/access point):
* Carrier frequency synchronization is is carried out as in [26]
* The beginning of a symbol in the I/Q stream is detected by an autocorrelation function of the prescribed and the received preamble;
* The cyclic prefix is removed;
* The OFDM demodulation takes place in a 1024-point FFT block;
* Channel equalization is carried out based on the received preamble;
* Baseband demodulation;
* Channel decoding;
* Descrambling and normalization is executed.