Direct-to-device (D2D) satellite connectivity, an exemplar of non-terrestrial network (NTN) integration with 5G and forthcoming 6G mobile ecosystem, is enjoying rapidly growing prominence. Delivering space-enabled broadband connectivity to smartphones, with their low-power, omnidirectional antennas, requires multitude of technological advancements. One innovation exploits distributed satellite systems using swarms of small regenerative satellites in low-Earth orbit (LEO) that synthesize multiple steerable beams. Another advancement optimizes efficiency of the already scarce radio-frequency (RF) spectrum with full frequency reuse, unfortunately intensifying intrabeam interference. This paper develops novel methodology for unified interference mitigation, comprised of selective precoder and high-performance receiver. Precoder is based on maximizing signal-to-leakage-and-noise ratio (SLNR), made selective about which subset of interference to leave unmitigated. It has low complexity, implemented in two stages to reduce computational requirements and alleviate user-scheduling constraints. High-performance receiver jointly processes signal and dominant interference, building on divide-and-conquer (DAC) paradigm to synergize with selective precoding. It has low complexity with computational load that does not grow exponentially with the number of dominant interferers. Extensive simulations reveal that unified interference mitigation significantly surpasses state-of-the-art schemes in sum-rate throughput, outage probability, and block error rate, also under scenarios of synchronization errors and imperfect channel state information at the transmitter (CSIT).

Next-generation satellite systems targeting multi-Terabit-per-second throughput require surmounting layers of design challenges. For feeder links, extremely high frequency (EHF) band is desired to overcome spectrum limitations and reduce equipment size. Multiple gateways are utilized to aggregate their bandwidths and serve as site diversity, mitigating the increased susceptibility of the EHF band to atmospheric attenuation. Toward this, multiple-input multiple-output (MIMO) technology is well suited for feeder links that share the same time, frequency, and polarization resources. This paper provides novel extension of MIMO-enabled feeder links when clustering more than two gateways and mounting more than two antennas onboard geostationary multibeam satellite, exploiting line-of-sight MIMO on uplinks and downlinks. Arsenal of countermeasures against inter-antenna interference and weather impairments is designed to maximize gains in signal-to-noise ratio, spatial multiplexing, and spatial diversity. Comparisons between offered sum rates and theoretical MIMO capacity are provided when gateway clusters form linear versus circular patterns. Methodology for gateway diversity is developed and analyzed when deploying redundant MIMO feeder links that handle rerouted traffic. Algorithms for ground-based channel state information (CSI) acquisition are designed and implemented in receiver architecture that tackles strong practical impairments. Numerical studies reveal that MIMO-enabled feeder links offer greatly enhanced overall network availability compared with state-of-the-art single-input single-output (SISO) solutions.

Aggressive frequency reuse alleviates the frequency-spectrum shortage in multibeam satellite systems targeting multi-Terabits-per-second throughput but creates harsh co-channel interference (CCI) environment. For forward-link, state-of-the-art receivers at user terminals assume memoryless CCI to avoid exponential increase in complexity. However, memory effects of CCI are inevitable, arising when co-channel signals combine asynchronously. First, we provide analytical characterization of CCI accounting for memory, a formulation that captures synchronization impairments. Then, low-complexity receivers, capable of realizing full frequency reuse, are developed to compensate for spatial and temporal CCI profiles, with computational complexity increasing linearly in the memory span. This is achieved by our novel soft-in soft-out, iterative divide-and-conquer (IDAC) paradigm, decomposing interference into smaller sets depending on intensity. A set is created from weak interferers which are not decoded but considered as thermal noise. The interferers which are decoded are further split into a set that subtracts strong ones and another set addressing intense interferers in optimal-Bayesian fashion. Extensive numerical studies reveal that IDAC architecture offers lossless compensation of dominant CCI with memory for beam-edge terminals, when rate coordination is guided by network information theory. Further, its performance is superior compared with approaches using iterative subtractive cancellation, commonly employed for return-link, random-access satellite applications.

The quest for massive satellite throughput is relentless, motivating the transmission of broadband signals at multiple Giga-Baud rates. Building low-cost, low-complexity user terminals is essential. These competing drivers conspire to cause analog radio-frequency (RF) components to exceed their tolerance limits. Specifically, analog mixers, anti-aliasing filters and amplification in quadrature frequency-conversion architectures create direct-current (DC) offset and in-phase/quadrature (I/Q) imbalance that is strong and frequency-selective. This paper provides two characterization models of analog RF impairments when the frequency offset is present. Novel digital compensation algorithms with immunity to frequency offset are presented, categorized into equalization with image-rejection capability and image cancellation. Adaptive techniques are utilized that operate without a priori knowledge of RF impairments. Two optimization methods are proposed for the multicarrier scenarios that are useful when using known data samples for factory calibration or in decision-directed mode during field re-calibration. Special consideration is given to estimating the frequency offset during initial factory calibration. Extensive computer simulations reveal that the proposed compensators are robust to frequency offset, provide lossless attenuation of imbalance-induced image and remove DC offset, under numerous scenarios. Cases include when the desired carrier is plagued by image interference from more than one source and on-board high-power amplifier is operated near saturation.

This special issue is devoted to Satellite Communication advances so as to gather the most important contributions in this field from academia and industry. In particular, the importance of this issue is also due to the fact that the last issue appearing on IEEE JSAC about SatCom is dated back to 2004 and therefore many important technology advances have been testified in the meantime, actually revolutionizing the satellite industry. In particular, important innovations have been brought to the design of both ground and space segments, spanning physical layers up to upper layers of the protocol stack.

This special issue is devoted to the satellite communications (SatCom) advances so as to gather the most important contributions in this field from academia and industry. In particular, the importance of this issue is also due to the fact that the last issue appearing on IEEE JSAC about SatCom is dated back to 2004 and therefore many important technology advances have been testified in the meantime, actually revolutionizing the satellite industry. In particular, important innovations have been brought to the design of both ground and space segments, spanning physical layers up to upper layers of the protocol stack.

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.

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.

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.

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.

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.

The problem of efficient utilization of the frequency spectrum for satellite systems is investigated; one which results as a consequence of highly crowding adjacent channels. An analytical characterization of the resulting interference channel is introduced and then exploited for interference cancellation. Two classes of cancelers are investigated. The first approach does not benefit from the forward error control (FEC) coding information which limits the performance gain. This motivates the second approach where a joint implementation of interference cancellation and decoding is developed using soft-input-soft-output (SISO) modules along with the iterative structure. It is shown that iterative interference cancellation techniques can achieve significant gains compared with the single-user matched filter receiver.

The problem of efficient utilization of the frequency spectrum for coded satellite systems is investigated; one which results as a consequence of highly crowding adjacent channels. An analytical characterization of the resulting interference channel is introduced based on concepts in statistical decision theory and is then exploited for interference cancellation. A joint implementation of MMSE interference cancellation and forward error control (FEC) decoding is considered where soft-input soft-output (SISO) modules are used along with the iterative structure. It is shown that, for example, one can operate a satellite system with convolutionally encoded QPSK modulation that uses practical pulse shaping at the channel spacing valve of 0.75 of the symbol rate, or bandwidth efficiency level of 2.67 bits-per-second/Hz, with minimum additional energy requirement. (This corresponds to a spectral efficiency improvement of 55% compared with a conservative baseline system.).

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.

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.>