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

Svilen Dimitrov
2018 International Journal of Satellite Communications And Networking  
In this paper, a practical realization of a non-linear equalizer with distortion cancellation at a satellite receiver is studied, using a memory polynomial model for channel estimation and distortion cancellation. Varying the memory depth and the non-linear order of the memory polynomial model, the packet-error rate (PER) performance of this non-linear equalizer is compared to the standard fractionally-spaced linear adaptive equalizer, as well as to a setup of the non-linear equalizer with
more » ... channel knowledge at the receiver. In a single-carrier transponder setup similar to the direct-to-home (DTH) broadcast DVB-S2X reference scenario, the improved receiver with linear and non-linear equalization and practical channel estimation is shown to consistently approach the PER performance of the setup with ideal channel knowledge when increasing the memory depth and the non-linear order. The improved receiver demonstrates superior performance as compared to the standard fractionally-spaced linear adaptive equalizer with up to 5.48-dB energy efficiency gain for 64-APSK for a practical memory polynomial setup. Furthermore, it enables the use of high-order modulation up to 256-APSK in this scenario, improving significantly the spectral efficiency of the air interface. Prepared using satauth.cls [Version: 2010/05/13 v2.00] 2 S. DIMITROV over satellite is known to suffer from linear and non-linear signal distortions due to limitations of the onboard satellite transponder. In the forward-link, for example, in order to maximize the spectral efficiency, state-of-the-art satellite communication systems resort to using a very low carrier roll-off and high symbol baud rates close to the maximum transponder bandwidth [2] . Due to imperfect magnitude and group-delay responses of the input-multiplexing (IMUX) and outputmultiplexing (OMUX) onboard filters, however, linear distortion is introduced in the form of intersymbol interference (ISI). In addition, due to the power limitation of the satellite and operation of the travelling wave tube amplifier (TWTA) close to saturation, waveforms with higher-order modulation and multiple amplitude rings, e.g., 256-APSK, suffer from non-linear distortion in the form of constellation warping and clustering at the receiver demodulator. Without adequate channel compensation techniques, these adverse effects can reduce the spectral efficiency gains in the air interface expected with the application of tighter roll-off factors, higher symbol rates, and higher modulation orders. Non-linear compensation schemes include a number of pre-distortion techniques at the transmitter, such as signal pre-distortion [3], [4], [5] and data pre-distortion [3], [6], [7], [8], [9], [10]. Signal pre-distortion [3] is commonly used for amplifier linearization. A solution against spectral regrowth is band-limited signal pre-distortion [4], [5] which applies digital filters to the out-ofband components and compensates the effect of the non-linear device on the in-band portion of the signal. Data pre-distortion [3], [6], [7], [8], [9], [10] is applied to the constellation symbols prior to pulse shaping, and it preserves the signal spectrum. Static data pre-distortion [2], [3], [6], [9] is the state-of-the-art solution in DVB-S2X. It accounts for static nonlinearities in the channel, whereby the transmitted constellation is modified such that the received centroids are close to the original transmitted constellation. Dynamic data pre-distortion [6], [7], [8], [9], [10] compensates in addition the memory effects in the channel. It takes into account the current symbol, as well as symbols preceding and succeeding each symbol. A common approach to handle the ISI is equalization at the receiver [3], where knowledge of the channel, obtained for instance by channel estimation, is used to minimize the ISI. A fractionally-spaced adaptive linear equalizer has been introduced in the DVB-S2X receiver [2]. Pre-distortion at the transmitter has been shown to be an effective channel compensation technique for a multi-carrier transponder in [10] . However, in a single-carrier transponder, symbolbased equalization with non-linear distortion cancellation at the receiver has shown significantly better results [11] . Decision-directed equalization [12] applied at the receiver is known to provide very good performance in channels with memory. A turbo equalizer using soft information exchange with the low-density parity check (LDPC) decoder has been proposed for application over non-linear satellite channels in [13] . A novel symbol-based equalizer with non-linear distortion cancellation, introduced in [14], reduces the complexity of the receiver by using maximum likelihood (ML) demodulation with hard decision in the cancellation loop with only marginal degradation of packeterror rate (PER) performance [11] in the single-carrier satellite forward link. Both approaches assume perfect channel knowledge at the receiver for estimation and cancellation of the non-linear distortion. In [11] , knowledge of the transponder characteristics is assumed at the receiver, i.e., 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. While such information can be obtained before the commission of the satellite, diurnal
doi:10.1002/sat.1239 fatcat:xt2zb3ceovf5vn2zeorktpxy7m