Jean-Michel Friedt
email: jmfriedt@femto-st.fr

Michel Lours

Gwenhaël Goavec-Merou

M Dupont

B Chupin

O Chiu

E Meyer

François Meyer

J Achkar

J.-M Friedt

É Meyer

Development of an opensource, openhardware, software-defined radio platform for two-way satellite time and frequency transfer

Keywords:

I. INTRODUCTION

Two-Way Satellite Time and Frequency Transfer (TWSTFT) is currently used for the computation of the Coordinated Universal Time (UTC) by direct comparison between atomic clocks disseminated worldwide. The classical means of spectrum spreading by introducing a pseudo-random noise (PRN) binary sequence modulating, using binary phase shift keying (BPSK), of a carrier is used to transfer time information, with the PRN generator being clocked by the reference frequency and triggered by a one pulse per second (1-PPS) signal.

While the historical development of the TWSTFT modem, initially the academic project of MITREX then the SATRE instrument sold by TimeTech GmbH, goes back to the 1980s, hardly any published reference [START_REF] Hartl | Present state of long distance time transfer via satellites with application of the mitrex-modem[END_REF]- [START_REF] Jong | Results of the calibration of the delays of earth stations for twstft using the vsl satellite simulator method[END_REF] justifies the selected PRN pattern and the technological limitations at the time that might have driven some of the scientific decision whose relevance to current computational power might be questionable.

With the advent of Software-Defined Radio (SDR) solution, as demonstrated for the computation of UTC in [START_REF] Jiang | Use of software-defined radio receivers in two-way satellite time and frequency transfers for utc computation[END_REF], prototyping various codes, code lengths, or modulation schemes has become a matter of reprogramming a Field Programmable Gate Array (FPGA) matrix of logic gates, while recording the raw IQ stream of received signal allows for prototyping multiple processing algorithms on a given dataset. This technology has become available for the development of a TWSTFT prototyping framework thanks to 1) large storage capacity despite the datarates involved with SDR: at a sampling rate of 5 MHz and two channels This work is supported by the ANR OscillatoIMP (Oscimp) and FIRST-TF grants. N. Gallone contributed the gateware configuration software during his internship and C. Calosso (INRIM, Torino, Italy) provided the algorithm for computing the SNR in addition to fruitful discussions on SDR and digital signal processing. recorded as 16-bit integer complex values, a datarate of 2.4 Gbytes/minute is produced 2) large FPGA capacity on opensource and openhardware SDR platforms allowing to add PRN generation next to the data handling logic 3) efficient FPGA programming frameworks for efficient prototyping of novel modulation schemes and PRN sequences, in our case the Python-derived Amaranth framework.

In this abstract, we address the technical implementation of an SDR emitter complying with satellite service provider requirements, emitting the standard 70 MHz Intermediate Frequency (IF) signal feeding the upconverter of the Very Small Aperture Terminal (VSAT) transmitter (14 GHz) solely relying on an FPGA and output bandpass filter. We follow with the implementation of an SDR receiver relying on a dual channel radiofrequency frontend, either as Ettus Research B210 (at LNE-LTFB) with the AD9361 providing the local oscillator and variable gain amplifiers, or the Ettus Research X310 (at LNE-SYRTE). Finally, the signal processing steps for computing the relevant link indicators such as frequency translation offset of the downlink frequency introduced by the satellite transponder, Signal to Noise Ratio (SNR) and most relevant time delay in ranging and two-way transfer are discussed. We conclude with a link demonstration between LNE-SYRTE in Paris and LNE-LTFB in Besanc ¸on [START_REF] Achkar | Two-way satellite time and frequency transfer using an opensource, openhardware software defined radio platform[END_REF].

II. EMITTER: CODING SCHEME

The emitted signal is synthesized by an FPGA clocked by the metrological 10 MHz and 1-PPS references, the only means with discrete logic to meet the stringent requirements of aligning the PRN code with the reference frequency and time signals. The design is portable to any FPGA fitted with a PLL fast enough to define the 70 MHz IF carrier BPSK modulated: in the case of the Zynq processor fitted on the Zedboard or the Spartan6 and Kintex7 FPGA fitted on B210 and X310 respectively, a core clock of 280 MHz is selected as an integer multiplication of the reference 10 MHz readily divided to generate the 70 MHz IF and the four states of the QPSK modulation.

While SDR provides a generic and flexible radiofrequency interface, emission is plagued by spurious spectral components produced by the digital synthesis: here. the 70 MHz is produced as a square wave as a General Purpose Input/Output (GPIO) interface of the FPGA associated with numerous overtones and unwanted spectral components. Dedicated 70 MHz Surface Acoustic Wave (SAW) filters are selected as C-TECH 321823 or SAWTEK 851547 for keeping the signal within the 4-MHz wide currently allocated satellite transponder band and reject by over 50 dB out-of-band spectral components. The high losses of these filters does not impact the link budget since the digital GPIO voltages must be attenuated anyway to match the SATRE modem output power (typically -20 dBm range) feeding the VSAT upconverter.

III. RECEIVER: SIGNAL PROCESSING On the receiver side, a single board or laptop computer fetches samples from a dual channel SDR receiver, with one channel acting as reference through a loopback link from the emitter and the second channel connected to the VSAT downconverter.

Unlike the closed Costas loop classically used for real time frequency tracking of the BPSK signal, in the context of SDR post-processing the identification of the frequency offset between nominal carrier frequency and received signal frequency is achieved by squaring to cancel BPSK spectrum spreading and identifying the spectral component at twice the frequency offset. A numerically controlled oscillator compensates for the frequency offset before correlating with the local copy of the code. A classical improvement on the time resolution of the correlation peak is brought by parabolic fitting, improving the delay resolution by a factor equal to the SNR on the correlation peak.

Under such considerations, the tradeoff lies between selecting a short PRN code leading to lower Pulse Compression Ratio (PCR) SNR improvement during the correlation but allowing for averaging multiple successive delay estimates, or a long code with fewer estimates for averaging but improved PCR. In order to avoid the ranging and two-way transfer uncertainty related to a short Pulse Repetition Interval (PRI) induced by a short code, a PRN length of at least the twoway trip of 270 ms is selected, and 1-s long code appears as best suited to avoid introducing additional coding schemes for determining the correlation peak associated with the 1-PPS of the beginning of the second.

IV. RESULTS Correlating the 2.5 Mchips-long PRN code sampled at 5 MS/s introduces artefacts as the satellite is moving at a rate of a few nanoseconds/s depending on the orbital parameters. For example at a rate of 3.2 ns/s and a sampling period of 200 ns, correlation peak position fluctuations of a few nanoseconds are observed every 62.5 s (Fig. 1), inducing unwanted excessive standard deviations on the measurements sessions lasting 180 s. This session duration is derived from the 8 GB broadband Random Access Memory (RAM) used for storing the datastream on the single board Raspberry Pi4 computer used in the current implementation.

This issue is solved by oversampling the received signal: since computing the correlation is most efficiently achieved by computing the Fourier transform of the code (pre-processing) and the received signal whose complex conjugate is multiplied with the former prior to inverse Fourier transform, efficient oversampling is implemented by zero-padding the Fourier transform product prior to the inverse Fourier transform. Such a procedure cancels the unwanted delay fluctuations and provides typical sub-ns standard deviation on the measured ranging or two-way delays over 3-min. sessions as determined by the available broadband storage resources (Fig. 2).

V. CONCLUSION

We have demonstrated a functional SDR-based TWSTFT framework for analyzing the various parameters of the digital communication. All software for reproducing the proposed setup is available at https://github.com/oscimp/amaranth twstft

Fig. 1 .

 1 Fig.1. Top: ranging delay over a 3-min. session, and bottom after removal of the parabolic trend. Top without oversampling (5 MS/s), bottom at 15 MS/s by interpolating using zero-padding prior to inverse Fourier transform when computing the correlation in the Fourier domain.

Fig. 2 .

 2 Fig.2. Ranging measurement over 36 h using the SDR setup (circles) during odd UTC hours and the SATRE modem (dots) during the even UTC hours.


Oscar Werner
email: oscarwerner88@gmail.com

Victor Pommier

Sophie Guillaumont

Marie Vincenti

Multimodal imaging

Keywords: Giant aneurysms, Kawasaki disease, Coronary artery bypass grafting, Case report

come

Multimodal imaging for cardiac follow-up and management of giant coronary aneurysm related to Kawasaki disease in childhood: a case report

Background

Kawasaki disease (KD) is a rare paediatric condition that can lead to giant coronary aneurysms. Follow-up of such complex coronary lesions remains a challenge, and their management is difficult to standardize.

Case summary

Our present case concerns a 17-year-old boy who suffered a giant aneurysm of the left coronary artery, complicated by an asymptomatic stenosis. During regular follow-up, his annual cardiopulmonary exercise test revealed signs of ischaemia (ST depression and premature ventricular complexes). After several further stress tests with inconsistent results, he underwent invasive coronary angiography that revealed significant stenosis with a positive fractional flow reserve (FFR)

Discussion

We discuss the challenges of diagnosing and managing coronary artery stenosis in paediatric patients with KD, particularly in cases with calcified and thrombosed lesions. A multimodal approach is crucial, including non-invasive imaging, and coronary angiography with optical coherence tomography and FFR. The evaluation of the lesion and its follow-up is an important factor in anticipating the best therapeutic choice for each patient.

Introduction

Kawasaki disease (KD) mainly affects children under the age of 5. This inflammatory disease results in necrotizing vasculitis of small-and medium-sized arteries. Its severity is related to the onset of cardiac complications, especially coronary artery aneurysms.1These aneurysms usually appear during the first 6 weeks after the initial symptom and can lead to permanent giant aneurysms (GAs), defined as having a diameter >8 mm or +10 Z-score.2The mortality rate of KD fell from 2% in the 70s to 0.01% nowadays, due to high-dose immunoglobulin therapy and early anti-thrombotic prophylaxis. There is a greater risk of thrombosis or stenosis in GA, leading to acute or delayed myocardial ischaemia. ESC 2019 guidelines highlighted the value of multimodal imaging in the assessment of the severity of chronic coronary syndrome in the adult population.3However, there are no specific guidelines for the diagnosis and management of chronic coronary artery lesions in the paediatric population with KD.

We present here the case of a 17-year-old boy presenting with a GA of the left coronary artery complicated by severe stenosis. We discuss the management of this case, and the benefits to decision-making offered by a more accurate imaging modality.

Case presentation

Kawasaki disease was diagnosed when the boy was 7 months old. In the first weeks following the diagnosis, a GA of the proximal part of the left anterior descending artery (LAD) occurred. A combined antiplatelet (aspirin) and anticoagulation (Vitamin K Antagonist) therapy was conducted. However, thrombosis and stenosis of the LAD aneurysm developed over time, as observed by systematic coronary computed tomography (CT) angiography.

The patient remained asymptomatic, without clinical symptoms or signs of ischaemia in regular stress tests performed throughout childhood. At the age of 17, his annual cardiopulmonary exercise test revealed a decreased aerobic capacity, an ST segment depression in lateral leads, and premature ventricular complexes at peak effort, without any simultaneous symptom.

Therefore, complementary exams were conducted to confirm myocardial ischaemia: the coronary CT angiography underscored a worsening of the proximal LAD stenosis at the level of a voluminous calcified plaque downstream of the 7 mm aneurysm (Figure 1). The myocardial stress scintigraphy was negative; the coronary angiography showed a decreased fractional flow reserve (FFR) at 0.69 in front of the LAD stenosis. The optical coherence tomography (OCT) illustrated the persistence of the GA of the proximal LAD up to the first diagonal artery division, and a proximal stenosis (stenosis fraction 72%) associated with a stenosis of the diagonal (hence a bifurcation lesion) (Figures 2and 3). Finally, stress cardiac magnetic resonance imaging (CMR) confirmed reversible ischaemia in the LAD territory (Figure 4).

The surgical team decided to perform a double coronary artery bypass grafting using a 'Yassembly' with the right internal mammary artery artery connected to the post-aneurysm LAD segment and the left intern mammary artery connected to the first diagonal. The patient did not present any post-surgical complications. One month later, he engaged in a post-operative rehabilitation programme with benefit as he regained a normal physical capacity with a normalization of the ECG.

A repeated coronary angiography at 6-month post-operatively showed an evolution of the stenosis of the LAD (90-99%), a visual decrease in the GA size, and no competitive flow between the native coronary and the bypass graft.

The 9-month post-operative cardiopulmonary exercise test showed a normalized aerobic capacity without any significant cardiological anomaly.

Discussion

Giant aneurysm related to KD is rare (0.13%), but is associated with a greater risk of thrombosis or stenosis leading to myocardial ischaemia.2These heterogeneous vascular lesions evolve due to calcified and thrombosed mechanisms, and challenge both the diagnosis of myocardial ischaemia and its management. While ∼50% of patients will experience a coronary stenosis and/or an ischaemic event during the 15 years following a KD, the prediction of a significant myocardial ischaemia remains difficult. Therefore, lifelong personalized and regular follow-up is highly recommended.2

In this case, a positive cardiopulmonary exercise test led to the performance of a coronary CT angiography. This showed a worsening of the LAD stenosis. However, the anatomic evaluation of a GA with extended calcified lesions can be limited by the use of non-invasive imaging, given that calcified lesions can lead to blooming artefacts. Regular coronary angiography appears to be one of the best ways to assess the level of stenosis, extension, and stability of coronary lesions.4Intravascular coronary imaging, which includes intravascular ultrasound (IVUS) and OCT, is a cornerstone tool in the description of coronary parietal lesions (atherosclerosis, intimal hyperplasia, calcification). This is recommended for the diagnostic and therapeutic processes offered to KD patients where intravascular intervention is required. The technical means most adapted to paediatric patients would appear to be OCT, given that its spatial resolution is 10 times higher than IVUS.5,6In this case, it showed specific signs of KD lesions (intimal hyperplasia and calcified lesions.6) That said, OCT also highlighted the underestimated focal stenosis of the initial part of the first diagonal, which was included in the GA. This information was crucial in evaluating which type of therapeutic plan could be considered for this patient.

Evaluating inducible ischaemia to warrant risk stratification is recommended in the follow-up of KD. AHA guidelines recommend the reasonable use, prior to any interventional procedure, of non-invasive stress tests, such as the stress scintigraphy that was performed in our case.7Since 2012, ESC guidelines regarding adults with chronic coronary diseases recommend coronary angiography to evaluate the haemodynamic significance of a lesion with the FFR.3This invasive examination is still considered a second-line test. However, the safety of coronary angiography should expand its use in paediatrics, as proposed by the 2020 JCS guidelines.2The advocated cut-off value is 0.8 in the adult population. The cut-off value in the paediatric population would appear to be closer to 0.75.2Our case underscores the value of a positive FFR result, suggesting a need for a second ischaemic test (cardiac stress MRI) to validate the recommendation for coronary revascularization, even after a previously normal myocardial scintigraphy.

The type of coronary revascularization in this case merits discussion, as it was a single vessel lesion.2Different percutaneous intervention techniques are available to treat KD coronary stenosis as follows: percutaneous transluminal coronary rotational ablation, stenting, and angioplasty. However, the long-term result depends on the patient's age and body size, and the number and locations of the coronary stenoses.8Percutaneous intervention techniques also present a higher risk of complication in the paediatric population, because of a lack of dedicated catheters.9In this case, the pre-operative assessment advocated for a surgical coronary artery bypass graft, given the complexity of LAD stenosis and its high level of obstruction, which constitute a positive criterion for the long-term patency of bypass grafts with no risk of competitive flow.10Lastly, the anatomic coronary angiography overview supported this therapeutic choice. Indeed, the significant focal stenosis of the diagonal, in addition to the LAD stenosis, supported the case for an upgrade to a double coronary artery bypass graft.11

Conclusion

We have presented the case of an asymptomatic 17-year-old child with a stenosis of a giant coronary aneurysm related to KD. Several examinations were performed to confirm the indication of revascularization and its modality, highlighting the usefulness of anatomical (OCT) and functional (FFR) coronary angiography in decision-making regarding complex and heterogeneous coronary lesions.

  Paediatric and Congenital Cardiology Department, M3C Regional Reference Centre, Clinical Investigation Centre, University Hospital, 371 Avenue du Doyen Giraud, 34295 Montpellier, France; 2 Paediatric Cardiology and Rehabilitation Unit, St-Pierre Institute, Palavas-Les-Flots, France; and 3 PhyMedExp, University of Montpellier, CNRS, INSERM, CHU Montpellier, Montpellier, France

	Oscar Werner1*, Victor Pommier1,2, Sophie Guillaumont 1,2, and Marie Vincenti 1,2,3
	1

Consent:

This case report was written with patient consent and is in accordance with COPE guidelines.

Funding:

No funding.

Data availability

The data underlying this article will be shared on reasonable request to the corresponding author

Conflict of interest:

None declared.

Lead author biography

Paediatric cardiologist in the University Hospital of Montpellier with a specialization in cardiac imaging.

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