Marinella Aloisio

Analysis of helical slow-wave structures for space TWTs using 3-D electromagnetic simulators

This paper presents a detailed analysis of traveling wave tube (TWT) helical slow-wave structures (SWSs) using three-dimensional (3-D) electromagnetic simulators. The main advantages of this approach are its capability to take into account the real 3-D geometry of this complex structure, and consequently to eliminate the approximations used in the analytical approach. In addition, it provides higher accuracy than experimental results. Two different methods have been applied (finite elements and finite integration) to predict the cold parameters (phase velocity, coupling impedance, and attenuation) at a level of accuracy higher than the usual practice. An extended analysis has been performed to guarantee the stability and accuracy of the results, as well as to study the sensitivity of the results to the discretization. This paper has shown that both methods, as implemented in two commercially available simulators, are perfectly suitable for the accurate modeling of such complex geometries.

Evolution of Space Traveling-Wave Tube Amplifier Requirements and Specifications for Modern Communication Satellites

Complex nonconstant envelope high-order digital modulation schemes used in modern communication satellites bring new requirements for the linearity and power efficiency of the on-board traveling-wave tubes (TWTs). As a result, a reassessment of typical TWT specifications is required. A digital system-level figure-of-merit, the total degradation, is proposed. It accounts for the characteristics of any coded signaling scheme, as well as for the performance of the overall communications link, including cochannel and adjacent channel interference. This figure-of-merit allows for reoptimizing the operating point of currently used TWTs, comparing the end-to-end performance of different tubes, and revising the standard TWT linearity-versus-efficiency requirements based on carrier-to-third-order-intermodulation (IM) ratio (C/IM3), noise power ratio, and carrier-to-noise-plus-IM ratio [C/(N + I)].

Accurate Characterization of TWTA Distortion in Multicarrier Operation by Means of a Correlation-Based Method

Complex payload architectures, together with the use of high-order digital modulations and advanced coding schemes, call for an accurate evaluation of traveling-wave tube amplifier (TWTA) nonlinearity impairments. To assess in a simple yet accurate manner the level of signal distortion generated by the TWTA nonlinearity, we adopt a correlation-based methodology. The proposed method provides an accurate evaluation of the TWTA intermodulation, particularly in the presence of multicarrier operation with nonconstant envelope modulations.

One-third-of-pitch reduction technique for the analysis of ternary azimuthally periodic helical slow-wave structures

This paper presents a technique to reduce the standard single-pitch helical model when performing a full-wave analysis of helical slow-wave structures (SWSs). The technique makes use of azimuthally periodic boundary conditions applied to the conventional helical SWS; the ternary angular symmetry of its dielectric rods allows reducing the standard computational domain by two thirds. It has been demonstrated that this method, as implemented with a commercially available simulator (high-frequency structure simulator), has no impact on the stability and accuracy of the cold parameters but allows a significant reduction of simulation time, which increases with the accuracy of the results. The proposed technique is therefore perfectly suitable for the accurate modeling of complex geometries and is expected to be particularly attractive for optimization analyses and applications in which accuracy and short simulation time are main concerns.

Performance Assessment of Output Sections of Satellite Flexible Payloads

In communication satellite systems, the payload utilization can be improved by means of the reconfigurability of the RF power among different beams and channels as an alternative to the well-known multiple antenna beams. The performance of a reconfigurable payload strictly depends on the architecture of its high-power section. In particular, the use of Multiport Amplifiers (MPAs) is generally considered the most effective means of achieving a high level of on-board RF power flexibility. This paper trades-off two multi-amplifier architectures based on both standard traveling-wave Tubes (TWTAs) and MPAs, with the objective of assessing the provided on-board power flexibility and overall DC-RF power efficiency. In addition, the two solutions will be evaluated according to their capability of achieving the required quality in end-to-end satellite link with a minimum DC power consumption, still guaranteeing some realistic margins on the link budget. The comparisons are performed by means of an in-house ESA developed software simulator able to analyze the impact of any payload non-ideality on the end-to-end link performance; in particular, the reference signal impairments are estimated by the carrier-to-interference ratio, evaluated with the correlation method.

Flexibility performance in advanced Ka-band multibeam satellites

Multibeam satellite systems are typically designed assuming a uniform traffic demand, while, in reality, the uncertainty of the actual traffic creates load imbalances among different beams. In this case, the global capacity balance yields to a paradox: the system is not able to meet the demand while some of the available capacity is left unused. Multibeam systems can mitigate this effect by embarking flexible payloads that allow to allocate the available resources on demand, depending on the actual traffic conditions.

Exploitation of Q/V-band for future broadband telecommunication satellites

In order to support the deployment of satellite broadband services able to satisfy the growing need for broadcast interactive applications, future broadband telecommunication satellites will have to exploit frequencies beyond Ka-band. This entails new key challenges and the use of critical techniques and technologies, which will be discussed.

Accurate characterization of TWTA distortion in multicarrier operation by means of a correlation-based method

Complex payload architectures, together with the use of high-order digital modulations and advanced coding schemes, call for a new interpretation of the traveling-wave tube (TWT) linearity parameters. We propose a correlation-based methodology to assess in a simple, yet effective, manner the level of intermodulation (IM) generated by the TWT nonlinearity; this method is particularly suited to accurate IM evaluation in the presence of multicarrier operation with non-constant envelope modulations.

Multi-Amplifiers Architectures for Power Reconfigurability

In communication satellite systems, the reconfigurability of the RF power among different beams and channels can be employed in alternative to the well-known multiple antenna beams, in order to improve the payload utilization. The performance of a reconfigurable payload strictly depends on the architecture of the high-power section. Two multi- amplifiers architectures, one based on bias-adjustable Traveling-wave Tubes (flex-TWTAs), the other based on Multiport Amplifiers, will be traded-off against the achievable efficiency and degree of flexibility at payload level.

End-to-end performance evaluation methodology for TWTA-based satellite flexible payloads

Reconfigurability of the RF power among different beams and channels is of primary interest in communications satellite systems for its potentiality in maximizing the payload bandwidth and power utilization over the satellite lifetime. The performance of reconfigurable payloads strictly depends on the architecture of the high-power section. We will compare two multi-amplifier architectures, one based on bias-adjustable Traveling-wave Tubes, the other based on Multiport Amplifiers. These architectures will be traded-off against their performance in terms of power flexibility and overall DC power efficiency by means of a suitable end-to end figure-of-merit.