# Extending the Coverage of IEEE 802.15.4z HRP UWB Ranging
## 3 Link Analysis of HRP UWB Communication  
### Packet Structure of HRP UWB  
In the previous section, we examined how HRP UWB communication is a system whose performance is limited only by the average power. In this section, we will examine how this principle is implemented in the actual IEEE 802.15.4z HRP UWB standard. Figure 3 shows the packet structure presented in the IEEE 802.15.4z HRP UWB standard [5,7]. As shown in Figure 3, the packet structure of IEEE 802.15.4z HRP UWB is divided into a synchronization header (SHR) used to set the reference time for synchronization and distance measurement and a data portion (DP) for transmitting data, and both use separate modulation methods. The IEEE 802.15.4z HRP UWB standard also includes a scrambled time sequence (STS) for encrypted distance measurement, but since STS uses the same modulation as the SHR, Figure 3 shows a packet without an STS.  
Figure 3: Typical packet structure of IEEE 802.15.4z HRP UWB.  
Figure 2: Pulse characteristics of UWB radar and HRP UWB communication (**a**) in the case of UWB radar and (**b**) in the case of HRP UWB communication.  
First, the SHR is divided into a preamble and start-of-frame delimiter (SFD). The preamble is transmitted by repeating the same symbol, and its length is determined by the number of preamble symbol repetitions (PSRs). One preamble symbol is defined by 31 or 127 sub-symbols, and each sub-symbol has a value of (\(-\)1, 0, 1). Here, '1' means a UWB pulse with positive (+) polarity, '\(-\)1' means a UWB pulse with negative (\(-\)) polarity, and '0' means no pulse. These pulses have the time interval of the inverse of the PRF. The basic PRF (BPRF) mode of the IEEE 802.15.4z standard uses a PRF of 64 MHz. Next, the SFD consists of 8 or 64 symbols and indicates the end of the preamble and the start of the DP. The preamble can obtain processing gain by repeatedly sending a known pulse without any special modulation. In HRP UWB, this is defined as the number of packet acquisition chunks (PACs). In addition to the processing gain through the PACs, the preamble uses the leading edge detection (LED) algorithm to increase the ranging accuracy; that is, if the number of PSRs is 1024, and the PAC count is 32, then one channel impulse response (CIR) is generated by collecting the preambles of 32 PACs, and the LED algorithm is used to increase the ranging accuracy by using 32 (=1024/32) of these CIRs [1].  
Next, the DP consists of a physical header (PHR) and a payload. The PHR includes information about the length and speed of payload data and uses different channel coding and modulation schemes from the SHR. Reed-Solomon (RS) coding and convolutional coding are used for channel coding, and burst position modulation-binary phase shift keying (BPM-BPSK) modulation is used as a modulation method. The DP supports multiple data rates, including 850 kbps and 6.8 Mbps. The difference between the 850 kbps and 6.8 Mbps modes in the IEEE 802.15.4z standard is achieved by adjusting the symbol duration and the internal burst structure while maintaining the same transmission bandwidth. For example, the 850 kbps mode adopts a much longer symbol duration with more burst positions and chips per symbol. In contrast, the 6.8 Mbps mode uses a shorter symbol duration with fewer burst positions and chips, allowing faster data transmission at the expense of the link margin. The symbol structures for the 850 kbps and 6.8 Mbps data rates are shown in Figure 4.  
This is because unlike the SHR, pulses are not sent repeatedly in the data section, and thus there is no processing gain; rather, coding gain is obtained through channel coding  
Figure 4: HRP UWB PHY BPM-BPSK modulation structure and symbol duration: (**a**) data rate of 6.8 Mbps and (**b**) data rate of 850 kbps.  
to balance the link between the preamble and payload. In HRP UWB communication, the ranging limit is determined by the smaller link margin between the SHR and DP links, making it necessary to adjust the parameters to appropriately balance the two.