A Simplified Graphical Comparison of Power-Reconfigurable TWTA-based Architectures
Aloisio, M.¹; Angeletti, P²; Colzi, E²; D'Addio, S^2
1European Space Agency ESA-ESTEC; ²ESA-ESTEC

In communications satellite systems, reconfigurability of the RF power among different beams and channels is of primary interest for its potentiality in maximizing payload utilization over the satellite life time. Nevertheless, power flexibility is contrasting with the well established trend in tessellating the satellite coverage area with multiple beams providing, on one side higher gains and frequency reuse capability, and, on the other side, an increase of the physical antenna ports to which RF-amplifiers (primarily consisting of Traveling-wave Tube amplifiers - TWTAs) must be interconnected.
Multi-amplifiers architectures capable of moving the RF power from one output port to another are the subject of the present paper that reports analyses and trade-offs of different available solutions.
An ideal reconfigurable payload should allow redistributing the available RF power based on the real-time traffic needs. The achievement of this objective is strictly dependent on the architecture of the high-power section. The leading idea is to have a common power pool from which a channel can adaptively draw the power it requires.
Considering self standing power sections (not integrated with the antenna), two candidate solutions are nowadays debated: Flex-TWTAs and Multiport Amplifiers.

Flex-TWTAs are bias-adjustable (flexible) amplifiers whose saturated RF power can be changed by modifying the amplifier’s bias conditions. Recent developments have proven the possibility of controlling the TWTA saturated power with negligible degradation in efficiency. With Flex-TWTAs, the power pool is realized at the level of the satellite platform DC-power capacity. In a DC-power-pooled multi-amplifier configuration based on Flex-TWTAs each individual TWTA can arbitrarily change (within a range that in current developments is about 3-4 dB) its saturated RF output power to match the traffic demand while maintaining a constant efficiency. The tubes must be operated such that their aggregated power consumption does not exceed the overall available DC power. The non-linear performances of the system depend on the nature of the signals individually presented to each power amplifier (i.e. single- or multi-carrier) and on the linearity performances of the TWTAs themselves.

With Multiport Amplifiers, a plurality of input signals is transformed by the input microwave network (INET) and presented to the stack of amplifiers and recombined by the output microwave network (ONET). The cascade of the INET-ONET scattering matrices should correspond to an ideal permutation matrix. In this case, the MPA-based architecture guarantees that amplified replicas of the input signals appear at the permuted output ports. Any unbalance between matrices' ports result in some power lost at the desired output port and appearing elsewhere. Intermodulation products are generated due to the non-linear nature of the amplifier devices and recombined by the ONET. Nevertheless, thanks to the non-coherency of the intermodulation products, they are distributed among the output ports. This intermodulation product spreading can be exploited to increase the multi-carrier efficiency. In a MPA-based architecture, the power pool is realized at RF-power level. The power sharing flexibility is achieved through the parallel amplification of all signals by the stack of Power Amplifiers.

To better understand the close relationship between the architectural solution and the achievable efficiency and degree of flexibility in arbitrarily moving the RF power, we have developed a simplified behavioural model that provides an easy-to-use tool to quickly compare the two above-mentioned architectures in terms of overall efficiency and power flexibility capability. The results achieved shall be thoroughly presented and discussed.