Bassel F. Beidas

Higher-order correlation-based approach to modulation classification of digitally frequency-modulated signals

A general framework that theoretically links the higher-order correlation (HOC) domain with statistical decision theory is explored. It is then applied to the problem of classification of M-ary frequency shift keying (MFSK) signals when contaminated by additive white Gaussian noise (AWGN). In particular, we propose a novel class of classifiers that utilizes time-domain HOC operations while completely avoiding the explicit determination of the spectrum of the observed signal. It is shown that this method delivers a performance that tightly lower-bounds that of the optimal likelihood-ratio test. In addition, this intrinsically wideband HOC-based method possesses an immunity to imperfect knowledge of exact frequency locations. Substantial performance improvement is also reported over the energy-based rule whenever it is applicable. >

Asynchronous classification of MFSK signals using the higher order correlation domain

The problem of asynchronous classification of M-ary frequency-shift keying (MFSK) signals when contaminated by additive white Gaussian noise (AWGN) is addressed. Two approaches are adopted. The first is based on the classical likelihood-ratio theory, which provides performance that is optimal, but sensitive to unknown frequency offsets. The second completely eliminates the fixed-frequency structure and instead utilizes measurements made strictly in the higher order correlation (HOC) domain. Assessed are the sensitivity gaps in performance incurred by the synchronous rules when the unknown signal time of arrival or epoch offsets are introduced. This sensitivity is ameliorated by averaging over a reduced-uncertainty epoch model. Fairly satisfactory results are reported with a small number of the discretized epoch uncertainty levels.

Intermodulation Distortion in Multicarrier Satellite Systems: Analysis and Turbo Volterra Equalization

The urgent objective of transmitting high data rates over satellite, coupled with the challenge to maximize satellite mass efficiency, has necessitated that multiple carriers share the same transponder high-power amplifier (HPA). This paper presents analytical framework that characterizes the resulting intermodulation distortion (IMD) by utilizing Volterra series representation to account for the memory within the carrier itself and those associated with other carriers. Also provided is analytical evaluation of nonlinear IMD which involves computing statistical averages of higher-order products of Volterra series containing complex-valued symbols from multiple carriers. Using this theoretical characterization, novel algorithms are developed to overcome IMD in highly distortion-limited environments by employing the powerful Turbo equalization method with linear minimum mean-squared error criterion. Further, the solution is adaptive so compensation does not require prior knowledge of the HPA characteristics and can be rapidly responsive to variations in the environment. Through extensive simulations, it is shown that the proposed multicarrier analysis and Turbo Volterra techniques can be used to substantially remove IMD resulting from operating the nonlinear transponder HPA, shared by multiple carriers, near saturation. By applying more iterations of joint equalization and decoding, the solution can approach the ideal performance when feeding back correct decisions.

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.

Modulation classification of MFSK signals using the higher-order correlation domain

The problem of classification of M-ary frequency shift keying (MFSK) signals when contaminated by additive white Gaussian noise (AWGN) is addressed. In particular, we propose a novel class of classifiers that utilizes time-domain higher-order correlation (HOC) operations while completely avoiding the explicit determination of the spectrum of the observed signal. Families of classification rules are identified for three different cases that exhaust all possible relations between symbol duration and bandwidth. The performances are numerically established in terms of the probability of correct classification. It is shown that the HOC-based method delivers a performance which tightly lower-bounds that of the optimal likelihood-ratio test. In addition, this intrinsically wideband method possesses an immunity to imperfect knowledge of exact frequency locations. Substantial performance improvement is also reported over the energy-based rule whenever it is applicable.

Adaptive Digital Signal Predistortion for Nonlinear Communication Systems Using Successive Methods

Highly-efficient operation of communication systems requires effective compensation of nonlinear distortion with memory. The main contributor to nonlinearity is the high-power amplifier (HPA) when operated close to saturation. This results in two major detrimental effects: spectral regrowth causing interference in adjacent frequency bands and in-band distortion in the form of constellation warping and clustering. This paper introduces a novel family of adaptive digital signal predistortion schemes that successively modifies the HPA input to drive nonlinear distortion with memory toward zero. This family of schemes is capable of suppressing the spectral regrowth and in-band distortion simultaneously, while keeping the HPA operating efficiently close to saturation. In addition, the proposed solution offers the system designer a beneficial tunability feature to select the levels of suppression. Reduced-complexity Volterra model is adopted and is implemented on-the-fly to cope with systems with high degree of nonlinearity and large memory span. Furthermore, the proposed schemes are made adaptive by applying stochastic gradient method offline during training phase to effectively deal with nonlinear systems whose characteristics are unknown a priori. Extensive computer simulations demonstrate that the proposed adaptive family of predistortion schemes approaches the performance of the perfectly predistorted solution, and can considerably outperform techniques based on adaptive inverse, commonly adopted in the literature.

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.

General framework for the higher-order correlation domain

A general framework that theoretically links the higher-order correlation (HOC) domain with statistical decision theory is explored. This is achieved by showing that the HOC domain provides an equivalent implementation of an average likelihood-ratio function (ALF) under certain conditions. Within this framework, statistics such as the energy of the first- and second-order correlation functions, among others, arise as terms in a power series expansion of an ALF. This ALF is associated with an AWGN-mutilated sinusoidal signal whose frequency is continuous and uniform. General analytical results on detection are produced. Special emphasis is placed on comparing the performance associated with the HOC with those achieved by the standard channelized and radiometric approaches. Receiver operating characteristic expression is also given to determine the effect on the overall performance of truncating the optimal non-linearity at different orders.

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.