## 3 HD-VLL for Hybrid DS/FH Spread Spectrum Signals
### Hd-Vll-Nco  
An HD-VLL for hybrid DS/FH spread spectrum signal tracking is proposed in Section 3.2, but the below problems are faced in practical engineering implementation. When the number of channels involved in a communication system is relatively small, or when hardware resources are sufficient, we typically equip each channel with an independent filter module to ensure independent and stable tracking for each channel. In this case, the HD-VLL mentioned above can be directly adopted. However, when receivers simultaneously need to process signals from multiple channels in order to improve hardware resource utilization, time-division multiplexing technology is widely employed in the design of filter modules. It allows multiple channels to share the same filter module, and through precise timing control, each channel utilizes the filter during different time slots, thus achieving the parallel processing of multi-channel signals without additional hardware costs. However, the output of the HD-VLL loop filter needs to be updated with the system clock, which means the filter must complete data processing within each system clock. This high-frequency update requirement poses a certain conflict with the multiplexing of a traditional filter. The carrier NCO plays a crucial role in generating a locally replicated carrier signal, which is updated at each system clock. The working principle of the carrier NCO involves calculating the corresponding frequency word based on the input Doppler frequency value. The accumulator then accumulates the frequency words, and the accumulation results are used to look up the table and obtain the corresponding sine and cosine amplitude values. The multi-carrier NCO, however, differs from the carrier NCO, as illustrated in Figure 6. The workflow is as follows: The input filtered velocity value and acquisition velocity are added together to form a velocity measurement value. Based on the predicted frequency hopping pattern and the current velocity measurement value, the Doppler frequency shift variable introduced by the next frequency hopping point is estimated. Subsequently, this estimated variable is converted into a frequency word, which is then accumulated and used for the lookup table. The hybrid DS/FH spread spectrum system employs multiple carrier frequencies. When utilizing the traditional carrier NCO structure, each carrier frequency necessitates a dedicated NCO, leading to increased resource utilization. The carrier frequency shift is calculated by the carrier frequency shift and the carrier frequency shift. tion. The multi-carrier NCO, on the other hand, improves efficiency and reduces the use of hardware resources by centrally processing multiple carrier signals. Based on the preceding discussions, the acceleration compensation module and jerk compensation module can be integrated into the multi-carrier NCO module, leading to the design of the HD-VLL-NCO. At each loop update period, the filter module outputs the filtered jerk \(\hat{\hat{a}}\), the filtered acceleration \(\hat{a}\), as well as the filtered velocity \(\hat{v}_{d}\) to the multi-carrier NCO module, which uses these values to update the frequency word at each system clock and then accumulates the frequency word and looks up the table to obtain the corresponding amplitude values. The structure of the HD-VLL-NCO is shown in Figure 7.  
As is shown in Figure 7, the structure of the second-order VLL loop filter is displayed in the dashed box at the bottom, and the structure of the high dynamic multi-carrier NCO is displayed in the dashed box at the top. However, this method will bring new problems to the multi-carrier NCO. The dimension difference between jerk, acceleration, and velocity is \(T_{s}\). Since the sampling frequency \(f_{s}\) can reach tens or even hundreds of megahertz, its reciprocal \(T_{s}\) will be very small.