Traveling-Wave Tube Amplifiers Performance Optimisation by Means of a Digital End-to-end Figure-of-Merit
Aloisio, M; Casini, E; Colzi, E; D'Addio, S
ESA-ESTEC

The ever increasing demand for higher quality TV transmissions (e.g. HDTV) as well as broadband connectivity for interactive services (e.g. Internet like) via satellite has generated the need to define new communication standards for broadcast and broadband satellite systems, aiming at making the satellite link more efficient. This is achieved by introducing a number of new physical layer features such as advanced coding schemes and power and spectral efficient modulations. The use of complex high order digital modulation schemes (up to 32-ary APSK) brings new requirements for the whole on-board RF chain, in particular in terms of linearity and power efficiency, as compared to quasi-constant envelope modulations (such as, for example, the standard QPSK). Of particular criticality is the Traveling-wave Tube (TWT) amplifier that provides efficient and high power amplification from C to Ka-band, but whose nonlinearity may strongly affect the performance communication link when these new modulations schemes are used, especially in multicarrier operation. As a result, a reassessment of the linearity versus efficiency trade-offs is required first to re-optimize the operating point (in terms of OBO) of TWTs under operation, and second to provide guidelines for the design of future TWTs.

The impact of high order modulation schemes on the TWT requirements usually translates into a revision of the classical TWT non-linearity requirements (such as C/IM3, AM/PM transfer coefficient or NPR). Actual linearity versus efficiency trade-offs are based on the analysis of figures-of-merit, incorporating various level of complexity. For example, the carrier-to-noise-plus-intermodulation ratio C/(N+I) is widely used; however it does not take into account the digital nature of the modulation.

During the last years, digital system-level figures-of-merit have been proposed based on end-to-end simulations, implementing actual modulation and coding schemes. In particular, the Total Degradation (TD) proposed by ESA is a digital figure-of-merit that can be extended to systems employing advanced coding schemes. We will discuss in detail how the TD comprises the contribution of the TWT output power losses (OBO), the TWT DC-RF conversion efficiency, and the signal degradation expressed as demodulation losses (i.e. the additional transmitted power necessary for achieving, on a non-linear channel, the same performance as that on a linear channel). The latter term brings into the figure-of-merit the characteristic of any coded digital system and the performance of the end-to-end communication link (including co-channel and adjacent channel interference). For a more realistic representation of modern satellite broadband systems, our analysis considers a TWT operated with one, two or more digitally modulated carriers. Both numerical simulations and experimental measurements have been performed, for fully assessing the system-oriented TWT optimization methodology based on TD. We will show how such methodology can be applied to re-optimizing the operating point (in terms of IBO) of currently used TWTs as well as to comparing their end-to-end performance in the satellite link. In addition, we will discuss how the standard TWT linearity requirements are expected to evolve towards linearity-versus-efficiency requirements, in light of the constraint put by the new digital, high order modulation schemes on the overall system performances.