Non-Linear Distortion Cancellation and Symbol-Based Equalization in Satellite Forward Links

Abstract

In this paper, a low-complexity symbol-based equalizer that performs non-linear distortion cancellation is proposed for application at the user terminal in the DVB-S2X satellite forward link. The channel is comprehensively modelled, including the non-linear travelling wave tube amplifier (TWTA) characteristics, the input-multiplexing (IMUX) and output-multiplexing (OMUX) filter responses at the satellite transponder, and the phase noise at the user terminal, according to the very-small aperture terminal (VSAT) reference scenario. Two detectors in the cancellation loop are considered, comparing the packeterror rate (PER) performance of simple maximum likelihood (ML) demodulation with hard decision to soft information exchange with low-density parity-check (LDPC) decoder, and showing only marginal improvement with the latter solution. The PER performance is compared against a number of predistortion techniques at the transmitter, such as dynamic data pre-distortion, successive data pre-distortion, and static data predistortion. The novel receiver demonstrates superior performance even with one iteration of distortion cancellation, while the joint application of successive data pre-distortion and iterative symbol-based equalization shows up to 4.95-dB energy efficiency gain for 32-level amplitude and phase-shift keying (32-APSK). The computational complexity is also evaluated. The improved receiver is particularly suitable for application with higher order modulation, a wide-band carrier, and low roll-off factors.

Estimation and cancellation of transponder distortions in satellite forward links using memory polynomials

“…To obtain the second term, the detected symbols are used to estimate the distorted received symbols. This is performed in the state‐of‐the‐art non‐linear equalizer by passing the detected symbols through the signal processing blocks along the transmission chain, under the assumption of ideal channel knowledge, including the magnitude and group delay responses of the IMUX/OMUX filters and the non‐linear transfer characteristic of the TWTA onboard the satellite. In this paper, instead, the non‐linear channel with memory is estimated practically and applied to the detected symbols according to the memory polynomial model: where x ( n ) is the input sequence with length of N symbols, n =0, …, N −1, y ( n ) is the output sequence, K is the non‐linear order, Q is the memory depth, and a k q is the complex‐valued coefficient associated with the term with non‐linear order k , k = 1,…, K , at time q , q = 0, …, Q .…”

“…A symbol‐based equalizer with non‐linear distortion cancellation, introduced by Dimitrov, reduces the complexity of the receiver by using maximum likelihood (ML) demodulation with hard decision in the cancellation loop, eliminating the need of iterations with the decoder. This equalizer shows only a marginal degradation of the packet‐error rate (PER) performance in the single‐carrier satellite forward link as compared with soft information exchange with the decoder . In addition, the iterative non‐linear equalizer has been also compared with a variety of predistortion techniques by Dimitrov .…”

“…This equalizer shows only a marginal degradation of the packet‐error rate (PER) performance in the single‐carrier satellite forward link as compared with soft information exchange with the decoder . In addition, the iterative non‐linear equalizer has been also compared with a variety of predistortion techniques by Dimitrov . While predistortion at the transmitter has been shown to be an effective channel compensation technique in a multicarrier transponder in the work of Piazza et al, the symbol‐based equalizer with non‐linear distortion cancellation at the receiver presents significantly better results in a single‐carrier transponder.…”

“…The state‐of‐the‐art iterative channel compensation techniques at the receiver assume ideal channel knowledge for estimation and cancellation of the non‐linear distortion. While knowledge of the Volterra model is assumed by Ampeliotis et al, ideal knowledge of the transponder characteristics is assumed at the receiver by Dimitrov, ie, measured magnitude and group delay responses of the IMUX/OMUX filters, as well as the input amplitude/output amplitude (AM/AM) and input amplitude/output phase (AM/PM) characteristics of the TWTA. Such information can be obtained before the commission of the satellite.…”

“…The PER performance of the fractionally spaced adaptive linear equalizer from the DVB‐S2X standard is simulated to provide reference for the improvements. In addition, the best‐case performance of the symbol‐based equalizer with non‐linear distortion cancellation is included, assuming ideal knowledge of the satellite transponder characteristics at the receiver . With the increase of the memory depth and the non‐linear order, the proposed practical equalizer implementation is shown to approach the PER performance of the implementation with ideal channel knowledge.…”