Rohit Iyer Seshadri

Analysis and compensation for nonlinear interference of two high-order modulation carriers over satellite link

We introduce analytical characterization of the nonlinear interference that results when passing more than one high-order modulation carrier through the same nonlinear transponder high-power amplifier (HPA). A Volterra filter is proposed which is novel in its implementation of this analytical characterization and modeling of intersymbol interference (ISI) and adjacent channel interference (ACI). Our focus is on adaptive algorithms with pilot-based training so that the solutions are completely blind to unknown transponder HPA characteristics, and can rapidly respond to varying operating back-off level. Furthermore, two families of adaptive solutions are provided to compensate for nonlinear ISI and ACI. The first set performs adaptive channel inversion and then applies equalization. The second set of solutions performs adaptive channel identification and then applies cancellation. The effectiveness of the proposed analysis and techniques is demonstrated via extensive simulations for high-order QAM and APSK modulations. Also included is the coded performance with selected LDPC codes designed for the DVB-S2 standard. Finally, computational complexity is assessed and performance impact is quantified when complexity is reduced by decreasing the number of Volterra coefficients.

Multicarrier Successive Predistortion for Nonlinear Satellite Systems

To satisfy the aggressive demand for higher satellite throughput, industry trend is moving toward sharing the transponder amplifier by multiple carriers, each employing high-order modulations that are spectrally compact. This trend, in conjunction with the inherently nonlinear nature of the amplifier when driven efficiently closer to saturation, creates significant intermodulation distortion that needs to be appropriately compensated. This paper presents a powerful compensation technique that is capable of mitigating the nonlinear intermodulation distortion and is placed at the transmitter or gateway. It is a novel multicarrier data predistortion technique that successively modifies the transmitted symbols to drive multicarrier distortion vector toward zero. This distortion vector results from passing the transmitted symbols from the multiple carriers, intrinsically accessible at the gateway, through the nonlinear satellite channel model. The novel successive predistortion technique and methods of estimating the distortion are described in detail. It is demonstrated using extensive computer simulations that the proposed multicarrier predistortion technique is capable of achieving near-optimum performance, even when only a simple linear receiver is employed and no exchange of data is assumed between receivers.

Faster-than-Nyquist Signaling and Optimized Signal Constellation for High Spectral Efficiency Communications in Nonlinear Satellite Systems

We here consider utilizing faster-than-Nyquist (FTN) signaling to increase spectral efficiency in combination with using tight frequency roll-off, optimized signal constellations that have better energy efficiency, and allowing the satellite transponder to operate near its saturation. FTN provides a degree of freedom that allows for increasing the spectral efficiency without the need for introducing additional rings in the signal constellation, which is helpful in the presence of nonlinear transponders. Also, FTN allows for increasing the symbol rate without being adversely affected by the input multiplexing and output multiplexing (IMUX/OMUX) filters. This is because FTN does not alter the signal spectral shape. However, these advantages are gained at the expense of introducing distortion that needs to be compensated successfully. We then utilize an advanced Turbo Volterra receiver that compensates for FTN-induced distortion and nonlinear impairments. Through extensive simulations, we demonstrate substantial performance gains over a wide range of spectral efficiencies in nonlinear satellite systems with adjacent carriers.

DVB-S2X: An Update to DVB-S2

After ten years of successful deployment, the second generation satellite standard for digital video broadcasting, DVB-S2, has been updated with several new features without changing its fundamental structure. This paper provides a high level discussion on several of the most important additions to the new standard.

OFDM-Like Signaling for Broadband Satellite Applications: Analysis and Advanced Compensation

Orthogonal frequency-division multiplexing (OFDM), special form of multicarrier modulation, is the cornerstone of broad range of current standards. In addition, fifth-generation (5G) terrestrial wireless networks, advancing at accelerated speed, continue to use OFDM air interface. The first part of this paper endeavors to apply OFDM-like signaling for broadband satellite transmission in the forward direction, from gateway to terminals. Two layers of multicarrier operation are invoked. One allows multiple signals to share single on-board high-power amplifier. The other layer permits transmitted symbols from individual signals to modulate multiple narrowband OFDM subcarriers, followed by interpolating filters to suppress interference into adjacent signals and limit out-of-band emission to be compatible with satellite uplink. The second part of this paper proposes novel nonlinear compensation techniques, applied at the transmitter or receiver, and combination of both. Two performance assessments are provided: one exploiting information-theoretic framework using ideal channel coding; another based on total degradation using practical forward-error correction codes. We show that performance loss from nonlinear distortion can be significantly reduced when adopting the proposed correction methods. Furthermore, it is possible to use OFDM-like signaling over satellite, while being competitive, and in some cases surpassing, traditional systems with single-carrier modulation employing state-of-the-art enhanced receiver architecture.

A Novel Receiver Based Technique for Monitoring Spectral Re-growth and Mitigating Adjacent-Channel Interference

We introduce a novel receiver based technique for estimating the spectral re-growth experienced by a modulated carrier transmitted through a power efficient nonlinear amplifier. Our approach builds upon the accurate characterization of the resulting nonlinear channel as a Volterra filter. The information contained in the filter coefficients is used to estimate the level of the principal side-lobes in the signal power-spectral density. Also, we demonstrate the ability of our proposed technique to provide accurate estimates using relatively short symbol sequences and its robustness to mismatch in amplifier characteristics. This makes our approach well suited for use in bandwidth and energy efficient satellite return links where adjacent carrier interference caused by spectral re-growth can severely impair performance.

Adaptive Volterra compensation for interference of two high-order modulation carriers over satellite

We introduce complete analytical characterization of the nonlinear interference that results when passing more than one high-order modulation carrier through the same nonlinear transponder HPA. Based on this analysis, a novel Volterra filter is proposed that models nonlinear intersymbol interference (ISI) and adjacent channel interference (ACI). Our focus is on adaptive algorithms so that the solutions are completely blind to unknown transponder HPA characteristics, and can rapidly respond to varying operating back-off level. Furthermore, two families of adaptive solutions are proposed to compensate for the nonlinear ISI and ACI. The effectiveness of the proposed techniques is demonstrated via extensive simulations for high-order QAM and APSK modulation schemes.