With the expansive deployment of ground base stations, low Earth orbit (LEO) satellites, and aerial platforms such as unmanned aerial vehicles (UAVs) and high altitude platforms (HAPs), the concept of space-air-ground integrated network (SAGIN) has emerged as a promising architecture for future 6G wireless systems. In general, SAGIN aims to amalgamate terrestrial nodes, aerial platforms, and satellites to enhance global coverage and ensure seamless connectivity. Moreover, beyond mere communication functionality, computing capability is increasingly recognized as a critical attribute of sixth generation (6G) networks. To address this, integrated communication and computing have recently been advocated as a viable approach. Additionally, to overcome the technical challenges of complicated systems such as high mobility, unbalanced traffics, limited resources, and various demands in communication and computing among different network segments, various solutions have been introduced recently. Consequently, this paper offers a comprehensive survey of the technological advances in communication and computing within SAGIN for 6G, including system architecture, network characteristics, general communication, and computing technologies. Subsequently, we summarize the pivotal technologies of SAGIN-enabled 6G, including the physical layer, medium access control (MAC) layer, and network layer. Finally, we explore the technical challenges and future trends in this field.

Satellite Communication systems represent a solid solution to extend and complement terrestrial networks in un-/under-served areas. The recent standardisation endeavours within 3GPP led to the Non-Terrestrial Networks (NTN) study item for New Radio, aimed at deploying satellite systems either stand-alone or as integration to terrestrial networks in mobile broadband and machine-type communication scenarios. In this context, the main focus is currently related to: i) evaluating the system level performance; and ii) understanding the impact at link level of typical satellite channel impairments on the PHY/MAC procedures. In this paper, we focus on the former; in particular, we provide: i) a detailed discussion on the current system architecture and assumptions; ii) we thoroughly describe the methodology to perform link budget computations; and iii) we show the numerical results for GEO and LEO systems.

To face the challenges of the new era of heterogeneous networks, mobile satellite systems need to provide an increased level of flexibility. As a consequence, a more effective use of the available resources and the exploitation of interference mitigation techniques are considered necessary to tackle the issue. The limits of a traditional configuration are even more noticeable in specific areas of high traffic requests i.e., in hot spots. In this paper, focusing on the forward link of a typical mobile interactive satellite system operating in L-band, where a large imbalance in traffic demand between the multiple beams is common, new configurations that aim at improving the performance of the system are studied. The proposed approach, based on a higher frequency reuse factor and the application of precoding techniques, boosts the system in terms of total capacity and provides higher flexibility, without making any change whatsoever to the space segment. A general analytic formulation of the problem and a pragmatic solution are provided for hot spot scenarios. Simulations are carried out to validate the proposed solution.

We investigate and compare different options for updating the error correcting code currently used in space mission telecommand links. Taking as a reference the solutions recently emerged as the most promising ones, based on Low-Density Parity-Check codes, we explore the behavior of alternative schemes, based on parallel concatenated turbo codes and soft-decision decoded BCH codes. Our analysis shows that these further options can offer similar or even better performance.

The mobile satellite broadcasting arena is witnessing a constant increase in demand for improved transmission quality and higher number of services in order to remain on par with the corresponding terrestrial counterparts. A promising way forward in this regard is the migration from the conventional single polarization per beam to an advanced dual polarization per beam architecture for effective spectrum exploitation; this also enables the application of advanced multiple antenna (MIMO) techniques. The present paper moves from DVB-SH the current state-of-the-art for mobile satellite services towards a more advanced system architecture doubling the overall system capacity with a minimum power increase. It turns out that a multiplexing gain equal to two comes at a cost of about 6 dB for the conventional single polarization system, whereas for the advanced system the corresponding cost is about 3.5 dB.

In this paper we propose the usage of Single Carrier Frequency Division Multiple Access (SC-FDMA) in the Broadband Satellite Return Channel. Notably, SC-FDMA is able to improve the physical layer spectral efficiency with respect to single carrier systems, which has been used so far in the Return Channel Satellite (RCS) standards. However, it requires a strict synchronization and it is also sensitive to phase noise of local radio frequency oscillators. For this reason, we propose an effective pilot tone arrangement within the SC-FDMA frame, and a novel Joint Multi-User (JMU) estimation method for the SC-FDMA. As shown by numerical results, the proposed scheme manages to satisfy strict synchronization requirements and to guarantee a proper demodulation of the received signal.

The paper investigates coding and encapsulation techniques for the possible extension of the digital video broadcasting-return channel satellite (DVB-RCS) to ensure a global coverage for mobile terminals. specifically, link layer- forward error correction (LL-FEC) appears to be a reliable solution, allowing to improve the C/N performance and to counteract the Doppler effect typical of mobile channels, avoiding scalability and latency issues of Automatic Repeat reQuest (ARQ) approaches. In the paper we evaluate LL-FEC performance when applied to the railway environment. It is shown how this scenario deeply affects the LL-FEC code performance both in the multi protocol encapsulation (MPE), and the generic stream encapsulation (GSE) cases. In order to counteract this performance reduction two different approaches are considered. In the first case, we increase the number of rows of the LL-FEC Reed Solomon (RS) matrix. In the second case we exploit the sliding encoding mechanism. We also introduce a modified version of GSE that shows reduced overhead and improved performance. Results show that DVB-RCS can greatly benefit from LL-FEC RS coding in reducing residual errors that cannot be counteracted at the physical layer for efficiency reasons.

This paper tackles the problem of unified frame and symbol timing synchronization for the return link of a DVB satellite system with Continuous Phase Modulation. Robust schemes are introduced to cope with the carrier frequency uncertainty affecting the received signal. The novel approach employs the same detector to recover the frame alignment and the symbol synchronism, based on Post Detection Integration strategy. It is shown that this unified approach is able to provide quasi-optimal performance limiting complexity at the same time.

In any conditions where DC and RF power are at a prime, such as in satellite systems, transmit amplifiers are operated close to the saturation region to maximize both output power and DC-to-RF efficiency, at the price of introducing non-negligible spectral regrowth and in-band distortion. In order to mitigate these effects, predistortion techniques can be applied at the transmitter; however, mass-produced terminal equipment have operating characteristics that exhibit large dispersion, and for cost reasons calibration procedures are ruled out. Therefore, feedback from the power amplifier output is required to apply predistortion, but the presence of phase noise over the feedback signal impairs the correct recovery of the amplifier phase characteristics, and attention must be focused on amplitude only. This paper proposes two adaptive predistortion schemes which are able to track and compensate the amplitude distortion in the power amplifier, based on neural networks and on look-up-tables, respectively. The two techniques are compared both in terms of performance and complexity, giving insights on the available design trade-offs. Thanks to the reduced spectral regrowth, the techniques are shown to enable the adoption of high order modulations also in those cases where it would have been prevented by out-of-band spectral masks.

In this paper the symbol timing estimation problem is addressed for bursty transmission in mobile satellite communications. In particular, a classical approach and iterative techniques are compared in terms of estimation performance. The iterative techniques are based on the expectation-maximization approach and they exploit the soft a-posteriori data information delivered by the iterative decoder in order to aid the synchronization process. A new iterative approach is here proposed with the aim to overcome the biased results observed on exiting algorithms for large timing offset values.

High data rate (HDR) CDMA systems are designed for cellular data distribution networks with maximized overall throughput and quality of service (QoS). Both figures of merit rely critically on the adoption of highly performant medium access control (MAC) algorithms, which are the focus of this paper. In particular, the closed loop resource allocation (CLRA) MAC algorithm for the reverse link of an HDR system is described. CLRA entails different properties and advantages that make it attractive for application in HDR or similar mobile Internet networks. First of all, CLRA is a macro control loop with distributed intelligence stimulated by busy tones issued by the base stations to inform terminals about their current load. Rate allocation is performed at terminals, whereby fairness is ensured by a stochastic rate assignment algorithm based on data queue states, rate history, and minimum rate guarantee. Network stability is guaranteed by controlled ramping driven by base stations busy tones. Further, a specific feature is introduced to account also for possible effects on base stations which are not in the rake active set; this feature is identified as candidate set protection. Finally, power headroom margins are used to minimize the occurrence of outage events. The analytical characterization of this MAC algorithm is derived using Markov chain modeling, and the results confirm the CLRA robustness, fairness and efficiency. The proposed algorithm is applicable to the 1xEV-DO system standardized by 3GPP2 within the cdma2000 family.

In all short-medium term business/market forecasts, the efficient provision of digital multimedia broadcasting (DMB) services in mobile systems appears to be of key importance for mobile operators and broadcasters. Several DMB systems are being developed around the world, with several proposed competing standards. In this framework, the DVB (digital video broadcasting) forum is working to define a new air interface, identified as DVB-SSP, to deliver satellite services to portable devices, and several possibilities are being considered as viable options, including the use of DVB-H (handheld). A critical factor to be considered is the resilience to non linear distortion effects. In fact, DVB-H signals are obtained through an OFDM modulation, which traditionally was considered unfit for satellite communications. Recently (S. Cioni et al., 2005), it has been proved that the adoption of strong channel coding techniques in conjunction with the use of non-linear distortion compensation techniques can lead to good OFDM performance even when operating close to amplifier saturation. Following this avenue, this paper investigates the adaptation of the DVB-H physical layer to the satellite environment, and validates the improved robustness of the proposed scheme to non linear distortion

In a recent paper, the modified Gardner detector has been proposed for advanced symbol-timing recovery of M-ary phase-shift keying signals. Compared with the original algorithm, the self-noise jitter can be considerably reduced for highly bandlimited systems and smaller values of M. In this letter, a completely different approach is investigated, which improves the self-noise performance for larger values of M, as well

The paper considers the comparison of orthogonal frequency division multiplexing (OFDM) and high speed downlink packet access (HSDPA) techniques in the physical layer of a satellite system devoted to the provision of digital multimedia broadcasting (DMB) services. In particular, the paper demonstrates that the inherently high spectrum efficiency of OFDM can be retained even in unfavorable propagation channels affected by heavy non-linear and linear distortion, respectively introduced by the on-board high power amplifier and by frequency selective propagation. This encouraging performance, achieved through the proper design and application of coding and pre-distortion techniques, is compared with the standardized HSDPA radio interface for 3G terrestrial mobile networks, and is shown to be in line on non-selective channels. Advantages for OFDM are demonstrated in frequency selective channels, using low complexity equalization in the frequency domain

The effective implementation of adaptive coding and modulation for the reverse (user-to-satellite) link of broadband satellite systems, operating at Ka-band and above, calls for the resolution of a number of problems. The many-to-one reverse link nature and the related distributed interference characteristics make efficient and robust link adaptation challenging, particularly when in the presence of bursty traffic and time varying propagation conditions. In this paper, a channel estimation and adaptation algorithm, suitable for the system under analysis, are devised and their performance investigated in a Ka-band multibeam system study case for different multiple access schemes.

The exploitation of adaptive coding and modulation (ACM) techniques for the forward link (satellite-to-user) of broadband communication satellites operating at Ka-band and above has been shown to theoretically provide large system capacity gains. In this article, the problem of how to accurately estimate the time variant channel and to adapt the physical layer mitigating the effects of estimator errors and propagation delays is analyzed. A comprehensive system simulator is then developed to assess the performance of the proposed solution and the impact on outage probability and system capacity due to estimation errors.

In recent years large scientific interest has been devoted to joint data decoding and parameter estimation techniques. In this paper, iterative turbo decoding joint to channel frequency and phase estimation is proposed. The phase and frequency estimator is embedded into the structure of the turbo decoder itself, taking into consideration both turbo interleaving and puncturing. Results show that the proposed technique outperforms conventional approaches both in terms of detection capabilities and implementation complexity.

In unknown channels, nonadaptive maximum likelihood sequence detection (NA-MLSD) is far from optimum in the presence of channel-induced uncertainty. To counteract this uncertainty, conventional solutions augment MLSD with external channel estimators. However, for very low SNR values this estimation procedure becomes marginal and the conventional adaptive MLSD (CA-MLSD) approaches perform poorly. We consider joint data decoding and parameter estimation techniques, and in particular per survivor processing (PSP) techniques, as a potentially effective solutions to this problem, at the expense of increased complexity. The performance/complexity tradeoff is addressed for a convolutionally encoded Q-PSK return link of an SHF/EHF satellite system for multimedia services impaired by phase noise and frequency offsets. By contrasting NA-MLSD, CA-MLSD, and PSP, ranges are provided for channel conditions where the tradeoff favors a particular technique. Further, CA-MLSD and PSP are also contrasted in terms of scalar/ vector estimation performance.

The application of per-suvivor processing (PSP) to digital satellite communications at Ka-band and beyond is considered. The PSP approach is used in a convolutionally encoded M-PSK transmission scheme to effectively counteract the degradations caused by phase noise and frequency offsets. Second order tracking loops are considered for per-survivor parameter estimation. A comparison is carried out between conventional adaptive maximum likelihood sequence detection (CA-MLSD) and PSP schemes both in terms of detection and estimation performance. Emphasis is placed on the analysis of the estimation quality one of the most unexplored research areas in the field of PSP-based systems. To this aim, novel definitions for a decision-related tracker and virtual tracker are introduced and their performance analyzed. Monte-Carlo simulations are provided to show the improvement in both estimation and detection quality brought about by PSP with respect to CA-MLSD.