A more accurate gain metric to assess predistortion techniques Dieter Duyck * , Alberto Mengali and M.R. Bhavani Shankar † , Joel Grotz ‡ S. Cioni § Communication systems have multiple performance metrics, such as flexibility, scala- bility and efficiency. The latter is however one of the oldest concerns of communication technology and still very relevant, for example in satellite communications. An efficiency improvement directly translates to the significant improvement in operational expenditures while sometimes also allowing capital expenditure gains. While efficiency or throughput gain within a given frequency bandwidth is typically the ultimate purpose of new signal processing techniques such as predistortion techniques, publications mostly focus on the signal-to-noise ratio (SNR) gain, as this is easier to measure and does not depend on the granularity of the applicable modulation and coding technology. Furthermore, it is com- monly assumed that this SNR gain must be significantly large, e.g. larger than 1 dB, to yield a measurable guaranteed throughput gain. This motivates a revisit of using SNR gains as the benchmark for predistortion techniques. We explain that considering SNR gains assumes that the same signaling is used in the baseline and the improved system. However, we show by means of an example that systems with predistortion may achieve higher throughputs using non-orthogonal signaling which can exhibit higher symbol rates in the same occupied bandwidth. We therefore conclude that achieved average throughput over a range of SNRs in a given occupied bandwidth is the most comprehensive metric to assess predistortion techniques and we explain how to compute it, enabling the relevant players in the satellite ecosystem to take informed decisions, as the presented technologies are no long theoretic but ready for sale. I. Introduction Predistortion improves the error rate performance over non-linear channels, such as forward links (from hub to terminal) in satellite communications, given high SNR in the uplink (from hub to satellite). An overview of recent predistortion technologies is given in [11]. Typically, the predistortion gain is expressed as total degradation gain achieved over a given non-linear channel [15, 4, 7]. Total degradation accounts for the non-linear distortion as well as the power penalty induced by back-off given at the input of the on-board traveling wave tube amplifier (TWTA). For any Frame Error rate (FER) greater than zero, total degradation (TD) is defined in dB as, TD| FER = E s N 0 | NL - E s N 0 | AWGN + GOBO, (1) * Technical Labs, Newtec † Interdisciplinary centre for Security, Reliability and Trust (SnT), University of Luxembourg ‡ SES S.A. § European Space Agency, TEC-EST Section 1 of 9