FPGA Design, Implementation, and Breadboard Development of an Innovative SCCC Telemetry + Pseudo-Noise Ranging Satellite System
Round 1
Reviewer 1 Report
Comments and Suggestions for Authors1. Some minor errors need to be corrected, such as two sections 2.2 and no sections 4.4.
2. Theoretically, Ranging signal is similar to phase noise in the combined signals. Parameter modulation index plays a key role in the performance of ranging. When the modulation index increases, the performance of ranging becomes better, while the performance of premetry decreases. When parameter modulation index decreases, vice versa. Therefore, parameter modulation index should be changed to an optimal value to take into account the best performance tradeoff of the premetry and ranging. Based on Figures 1.6, 1.7 and 1.8, it is suggested to supplement the performance of ranging when the modulation index is 0.1 rad-pk.
Author Response
Dear Reviewer,
We sincerely thank you for your valuable comments and observations on our work. Your feedback has been instrumental in enhancing the clarity and readability of the manuscript, as well as in allowing us to address and correct any sections that were previously unclear. Please find below our detailed responses to your comments.
Comment 1: Some minor errors need to be corrected, such as two sections 2.2 and no sections 4.4.
Response 1: Fixed sections enumeration in new manuscript version.
Comment 2: Theoretically, Ranging signal is similar to phase noise in the combined signals. Parameter modulation index plays a key role in the performance of ranging. When the modulation index increases, the performance of ranging becomes better, while the performance of telemetry decreases. When parameter modulation index decreases, vice versa. Therefore, parameter modulation index should be changed to an optimal value to take into account the best performance tradeoff of the telemetry and ranging. Based on Figures 1.6, 1.7 and 1.8, it is suggested to supplement the performance of ranging when the modulation index is 0.1 rad-pk.
Response 2: Once the two nested loops of the receiver have converged, the modulation index should no longer have a direct impact on the degradation of the telemetry system's performance (if not for the ranging jitter which influences the cancellation quality). It is absolutely true that the performance of the ranging are influenced by the modulation index, but the use of high order modulations generate a “floor like” effect at higher SNRs that is not influenced by such parameter (see eq. 7, ref. 12, and ref. 13). For such reason, only for very high ModCods, it becomes legit considering the idea of reducing the modulation index in order to improve telemetry cancellation, as the impact of such change on ranging jitter performances is minimal due to the presence of the floor (please consider that the floor for 64-APSK is higher compared to that in Figure 18, detailed in ref.13)
Reviewer 2 Report
Comments and Suggestions for AuthorsIn order to solve the problem that traditional satellite ranging technology occupies dedicated spectrum and limits the telemetry bandwidth,this paper proposes a hardware system combining CCSDS 131.2-B SCCC telemetry and CCSDS 414.1-B pseudo-noise ranging to transmit high-speed telemetry and ranging signals at the same time,and maximizes spectrum utilization through phase modulation superposition instead of frequency division multiplexing. The cross-cancellation algorithm of the receiver design iteration forms negative feedback,which gradually separates the signals and suppresses mutual interference. The real-time switching of coding modulation under fixed bandwidth and the uninterrupted ranging function provide a new idea for adaptive link optimization. However,the following problems remain:
1 Only the AWGN channel is simulated, and the complex environment such as Doppler frequency shift and sudden interference in the real satellite communication system is not considered.
2 Only the fixed symbol rate and ranging bitrate were tested,and the generality at different rates was not verified, which may mask the potential problems in the actual scene.
3 The relationship between higher-order modulation and ranging performance is not thoroughly analyzed,and the influence of higher-order modulation on ranging jitter is not considered,lack of innovation.
4 The specific reasons for the 2.1dB implementation loss of 64-APSK have not been analyzed in depth,and there is room for optimization. It is recommended to add targeted analysis.
5. The synchronization problem of coding modulation adaptive adjustment and switching is not considered,and the loop reconvergence time is not qualitatively analyzed.
6. The convergence analysis and error propagation model of the double-loop crossover elimination algorithm are not analyzed theoretically,and only the bit error rate simulation comparison results are given. A mathematical model of mutual interference of TM/RG channels can be established,and the residual error can be eliminated by quantifying the crossover.
Author Response
Dear Reviewer,
We sincerely thank you for your valuable comments and observations on our work. Your feedback has been instrumental in enhancing the clarity and readability of the manuscript, as well as in allowing us to address and correct any sections that were previously unclear. Please find below our detailed responses to your comments.
Comment 1: Only the AWGN channel is simulated, and the complex environment such as Doppler frequency shift and sudden interference in the real satellite communication system is not considered.
Response 1: Correct, only AWGN effects were considered within the scope of this ESA activity, as the primary objective was to provide a preliminary demonstration of the feasibility of the proposed technology and to establish an initial foundation towards a potential CCSDS green-book (as for GMSK+PN-Ranging technique). Consequently, the project operated under a very limited budget, but future work will certainly address more complex scenarios, including Doppler shift, Doppler rate, and phase noise profiles. These phenomena, tailored to specific mission scenarios, are certainly crucial for the full characterization and performance evaluation required for actual mission deployment. Nevertheless, this work addresses key implementation challenges, such as the complexity of cross-cancellation at medium data rates in structured transmission schemes, the lock-in performance at varying SNR levels according CCSDS 131.2-B standard, and the ability to perform dynamic modulation changes without link interruption. These aspects, directly related to real hardware implementation, have not been thoroughly investigated in the existing scientific literature, to the best of our knowledge. A corresponding sentence has been added to the Conclusions section (page 19) of the revised manuscript to explicitly highlight that the implementation of a comprehensive channel with all impairments is surely a future work to arrive an actual EM of the system.
Comment 2: Only the fixed symbol rate and ranging bitrate were tested,and the generality at different rates was not verified, which may mask the potential problems in the actual scene.
Response 2: Correct, the activity was however deliberately proposed by ESA with a simplified configuration based on a fixed set of symbol and chip rates, in order to contain complexity and budget, as it is intended as an initial step toward system validation. Implementing a fully dynamic system capable of adapting to variable symbol and chip rates would had significantly increased complexity, by a factor of two or three, due to the need to reconfigure interpolators, delay line structures, filtering chains, equalization stages, and other DSP blocks. Such an effort was beyond the scope of this preliminary assessment, but will surely be part of future activities on the subject or during the development of an EM for actual missions. Nonetheless, as discussed in the paper, particular care was taken from the beginning to avoid choosing rates that exhibit simple integer relationships. This was done intentionally to prevent artificial alignment effects or metric correlations that could mask true system behavior. The selected rates (4.25 Msym/s for telemetry and 2.987 Mchip/s for ranging) were chosen to mitigate such risks and ensure that the performance evaluation remains representative of real mission scenarios, where similar correlation effects must also be carefully avoided.
Comment 3: The relationship between higher-order modulation and ranging performance is not thoroughly analyzed,and the influence of higher-order modulation on ranging jitter is not considered,lack of innovation.
Response 3: The impact of higher-order modulation on ranging jitter had already been explored in earlier theoretical and simulation-based studies by B. Ripani (Politecnico di Torino) and A. Modenini (ESA, and Technical Officer for the activity underlying this paper), as reported in references [12] and [13] of the submitted manuscript. The analysis presented in this work, based on real hardware, serves primarily as a cross-validation of those theoretical foundations. It also aims to identify and resolve hardware-related issues (as detailed in Section 4) and to highlight any limitations associated with directly implementing the algorithms. Based on these practical insights, the paper proposes a concrete approach for implementing cooperative cancellation with precise matching. This is something previously demonstrated in hardware only by BAE Systems on the ESA TTCP receiver, and only in the context of very low data rates and GMSK modulation where the loops could be closed in a few clock cycles. Instead, the timestamp based approach stated in the paper allows the system to be closed with precise cancellation on long delay lines, as required by a standard implementing framing structure and high-order modulations.
Comment 4: The specific reasons for the 2.1dB implementation loss of 64-APSK have not been analyzed in depth,and there is room for optimization. It is recommended to add targeted analysis.
Response 4: Indeed, this work does not include an in-depth analysis of the implementation losses affecting the telemetry BER performance for 64-APSK modulation. As stated in the manuscript, only basic checks were carried out using the available hardware to verify that the system’s tracking loops were functioning with results reasonably close to those predicted by theoretical studies, although some implementation loss is naturally present. A more comprehensive analysis will surely be addressed in future work, where performance optimization and functionality characterization will be a primary focus. However, due to the high complexity of such an investigation, it was beyond the scope of this initial ESA activity. ESA considered that, for this preliminary validation, a reduction in modulation index would be sufficient to verify the core functionalities of the system. This approach, and the plan to revisit the detailed performance analysis in future studies, is explicitly mentioned in the Conclusions section (page 19) of the revised manuscript.
Comment 5: The synchronization problem of coding modulation adaptive adjustment and switching is not considered,and the loop reconvergence time is not qualitatively analyzed.
Response 5: Modulation and coding changes have been implemented during the activity to verify that ACM/VCM (Adaptive and Variable Coding and Modulation) adaptation modes can be effectively supported. This fact has been stated in the last part before the conclusion. These modulation format changes do not affect telemetry synchronization when already locked, as the system relies on a framed structure with automatic modulation recognition through the frame header. The only component potentially affected by modulation changes is the ranging loop, which may experience increased jitter. However, this only happens when operating in the saturation region of the jitter curve, which is driven by the residual amplitude error of the telemetry signal. In all other scenarios, the ranging jitter remains unaffected by modulation changes, as the modulation-related effects that behave like noise remain well below the actual noise level at the input of the ranging loop. The system is therefore exactly proposed such that no-unlock and resynchronization/reconvergence to a stady state is needed during operative scenarios.
Comment 6: The convergence analysis and error propagation model of the double-loop crossover elimination algorithm are not analyzed theoretically,and only the bit error rate simulation comparison results are given. A mathematical model of mutual interference of TM/RG channels can be established,and the residual error can be eliminated by quantifying the crossover.
Response 6: The goal of the proposed paper is to provide an insight of the functionalities and the way of implementing such complex cancellation in a system supporting multiple modulation formats with framing from an hardware prospective. Drawing mathematical model for the nested loops in order to evaluate if there’s a way to further eliminate the residual effect is therefore out of the scope for the paper. Please consider that investigation of the cross cancellation has from a mathematical prospective has already been deeply performed in Section 4.1 of reference 13 (specifically on Section 4.1.2 regarding the ranging performance and the term defining the floor-like effect).
Sincerely, Authors
Reviewer 3 Report
Comments and Suggestions for AuthorsThe paper is well written and the proposed system is interesting.
I have only minor comments:
page 4. Section 2. What is the carrier frequency ? Before it was indicated Ka/Ku frequency bands, but it is not clear.
page 7. It seems that the channel emulator only add AWGN, but a channel emulator usually also simulates or generates other effects like fading, disturbances, rain or doppler effects. Please, can the authors elaborate on this aspect ? Are additional features future developments ?
page 15. Table I. please, could you indicate the unit of measure ?
Figure 14,15,18. The differences between the curves are scarcely distinguishable. I think that the authors should report the differences in some other form: a table or another figure with the differences.
page 19. The abbreviations should be sorted in the alphabetical order.
page 19 For the funding, the numeric identifier of the project should be provided.
After reading the paper, I did not get the information if the components of the proposed prototpe are space enabled. For example, are the components certified for radiation ?
Author Response
Dear Reviewer,
We sincerely thank you for your valuable comments and observations on our work. Your feedback has been instrumental in enhancing the clarity and readability of the manuscript, as well as in allowing us to address and correct any sections that were previously unclear. Please find below our detailed responses to your comments.
Comment 1: page 4. Section 2. What is the carrier frequency ? Before it was indicated Ka/Ku frequency bands, but it is not clear.
Response 1: The carrier frequency of the breadboard has been set to 620 MHz, as indicated in Section 2 (pag. 4). This frequency was selected because it is commonly used in ground stations, including ESA's TTCP. The references to Ka-band and Ku-band in Section 1 pertain to LEO missions, which were the original target for the CCSDS 131.2-B standard. However, this specific frequency range is not relevant to the scientific missions that the proposed SCCC + PN-Ranging scheme aims to support, despite the fact that CCSDS 131.2-B is still used as the foundation for telemetry communication in such scheme. Added a phrase in the paper stating the 620MHz relationship to common ground station intermediate frequencies. Updated figures to indicate the carrier frequency.
Comment 2: page 7. It seems that the channel emulator only add AWGN, but a channel emulator usually also simulates or generates other effects like fading, disturbances, rain or doppler effects. Please, can the authors elaborate on this aspect ? Are additional features future developments ?
Response 2: Correct, only AWGN effects were considered within the scope of the ESA activity, as the primary objective was to provide a preliminary demonstration of the feasibility of the proposed technology. Consequently, the project operated under a very limited budget. Future work will certainly address more complex scenarios, including Doppler shift, Doppler rate, and phase noise profiles, which are essential for the proper characterization of an Engineering Model (EM) suitable for actual mission implementation. Nevertheless, this work already addresses the complexity of cross-cancellation at medium data rates, where precise alignment of the cross-cancellation signal is required, as well as the lock-in capability of a Higher-order Modulation (HoM) system, aspects that, to our knowledge, have not been explored in previous scientific literature. A corresponding sentence has been added to the Conclusions section (page 19) of the revised manuscript to explicitly highlight that the implementation of a comprehensive channel with all impairments is surely a future work to arrive an actual EM of the system.
Comment 3: page 15. Table I. please, could you indicate the unit of measure ?
Response 3: The numbers presented in the table refer to quantities of various FPGA components (e.g., CLBs, DSPs, …) and are therefore dimensionless. A clarification has been added in the text just above, including small phrase related to DSP blocks utilization.
Comment 4: Figure 14,15,18. The differences between the curves are scarcely distinguishable. I think that the authors should report the differences in some other form: a table or another figure with the differences.
Response 4: The limited distinguishability of the BER curves in Figure 15 is intended to show that values are absolutely overlapped. Implementation losses are usually evaluated for a defined work point, which in our experience on multiple ESA activities has always be BER = 10-5 or BER = 10-6, and meaningful values to be considered are usually multiples of 0.1 dBs. In the latter case, the loss at BER = 10-6 is below 0.1dB, resulting in a totally negligible values. A phrase stating the values of figure 15 are below 0.1dB of implementation loss has been added. Concerning values in figure 18, tabled values (Table 2) have been provided and added to the paper.
Comment 5: page 19. The abbreviations should be sorted in the alphabetical order.
Response 5: The abbreviations have been reordered alphabetically in the new version of the manuscript.
Comment 6: page 19 For the funding, the numeric identifier of the project should be provided.
Response 6: Modified fundings section content in the new manuscript version
Comment 7: After reading the paper, I did not get the information if the components of the proposed prototpe are space enabled. For example, are the components certified for radiation ?
Response 7: Given that the system described in this work serves as a breadboard prototype (proof of concept) implemented in a commercial board, the hardware setup is not intended to be space-qualified. Added a clarification in the Conclusions section (page 19).
Sincerely, Authors
Round 2
Reviewer 2 Report
Comments and Suggestions for Authors1. Add hypothetical analysis, qualitative analysis of possible sources of loss, and cite relevant literature.
2. Identify the possible technical challenges of dynamic symbol rates in the "Conclusions " section.
Author Response
Dear Reviwer,
Thank you for the comments, whose responses are detailed below.
Comment 1: Add hypothetical analysis, qualitative analysis of possible sources of loss, and cite relevant literature.
Response 1: An explicit clarification regarding the observed implementation loss has been added in the "Results" section, emphasizing the unprecedented nature of the problem encountered and therefore the resulting difficulty of the authors to formulate relevant hypotheses concerning the root causes of the performance degradation in this preliminary work. This is particularly attributed to the extremely complex and time-varying interactive behaviour of the dual-loop structures. It is further underlined that, despite the unknown origin of this limitation, it is not attributable to any malfunction of the ranging cancellation mechanism itself, as dedicated tests have confirmed its correct operation. Moreover, a potential mitigation strategy has been proposed, ensuring full compliance of the system under all operational conditions defined by the standards.
Comment 2: Identify the possible technical challenges of dynamic symbol rates in the "Conclusions " section.
Response 2: An additional statement has been included in the “Conclusion” section to explicate the challenges associated with employing configurable, rather than fixed, rate values.
Sincerely, Authors
