Search

A novel coding strategy is proposed for a broadcast setting with two transmitter (TX) antennas and two single-antenna receivers (RX). The strategy consists of using space-time block coding to send a common message (to be decoded by both RXs) across the two TX antennas, while each TX antenna also sends a private message to one of the RXs. The relative weight of the private and common messages from each TX antenna is tuned to maximize the instantaneous achievable sum-rate of the channel. Closed-form expressions for the optimal weight factors are derived. In terms of the generalized degrees of freedom (GDoF) metric, the new scheme is able to achieve the sum-GDoF with finite precision channel state information at the transmitter (CSIT) of the two user broadcast channel. Moreover, as opposed to the existing rate-splitting schemes, the proposed scheme yields instantaneous achievable rates that are independent of the channel phases. This property is instrumental for link adaptation when only magnitude CSIT is available. Our numerical results indeed demonstrate the superiority of the scheme for the 2-user setting in case of magnitude CSIT. Extension to a more generalK-user scenario is briefly discussed.

This paper presents a short review on the main high spectral efficient systems for multibeam satellite communications. In this regard, we introduce the pros and cons of systems based on Multiuser Detection System (MUD) and Successive Interference Cancellation (SIC) applied to aggressive frequency reuse, frequency packing and Non Orthogonal Multiple Access (NOMA). Furthermore, we have also considered the presence of co-channel interference and spectrum limitations. The experimental validations have been conducted using DVB-S2 waveform. The results point out that the residual co-channel interference reduce the benefits of using frequency packing schemes. Furthermore, the best strategy to target a given FER when two streams are received with the same energy is to select a combination of low and high order modulation formats. Moreover, we investigate the effect of asynchronous reception of data streams in our previously proposed interference management scheme based on cooperative NOMA in multibeam satellite systems.

This paper proposes a hybrid network architecture combining a cell-free Massive MIMO terrestrial layout with a low Earth orbit satellite segment jointly targeting the maximization of the minimum per-user rate in the coverage area. Towards this end, an optimization framework is proposed, alongside a greedy solution, that diverts the terrestrial users experiencing poor propagation conditions to the satellite segment. Simulation results show the substantial benefits this integrated approach brings along, very specially when the terrestrial segment operates single-antenna access points or is sparsely deployed., Grant numbers : TERESA - Hybrid TERrEstrial/Satellite Air Interface for 5G and Beyond (TEC2017-90093-C3-1-R).

In this paper, we propose a multibeam multicast non-orthogonal multiple access-based (MB-MC-NOMA) scheme for satellite communication systems. Building upon the multibeam transmission, we exploit precoding and NOMA techniques to deal with inter-and intra-beam interference, respectively. Combining these two techniques in the multicast transmission is not trivial as both scheduling, and the precoding design shall be reconsidered. Indeed, it is shown there is a contradiction between criteria for efficient user scheduling in MC-NOMA and MClinear precoding: while the first one requires a strong channel gain imbalance between user terminals, the latter seeks user channel vector similarities. To tackle this problem, we propose a user scheduling in MB-MC-NOMA, which provides a good balance between these two criteria. Besides, the MC-linear precoding is designed with aid of a mapping function that deals with the lack of spatial degrees of freedom. The numerical simulation results show that the performance of the MB-MC-NOMA scheme is improved by using the proposed user scheduling and a novel precoding design based on the singular value decomposition (SVD). Moreover, the MB-MCNOMA scheme outperforms its orthogonal version scheme. In particular, we observe that the throughput of the MB-MC-NOMA is increased a 25% with respect to MB-MC-OMA scheme under certain fair conditions., Grant numbers : TERESA - Hybrid TERrEstrial/Satellite Air Interface for 5G and Beyond (TEC2017-90093-C3-1-R).

This paper addresses the application of nonorthogonal multiple-access techniques (NOMA) to those satellite relayed communications for which a significant imbalance in the link quality of user terminals can be expected. The Signal-to-Interference and Noise Ratio (SINR) imbalance could be caused by the coexistence of different types of terminals, possibly with different antenna sizes, and offering different classes of service. This link SINR asymmetry can be exploited to outperform orthogonal access schemes under different rate metrics, paying special attention to fairness in the service provision. Both forward and asynchronous return link are addressed, with minimum signaling information and emphasis on some relevant implementation issues such as framing and synchronization.

Next generation satellite payload technology is expected to provide digital processing capabilities. This will pave the way to regard satellites as flying base stations. However, the initial access procedure must be improved. In this work we focus on non-geostationary earth orbit (NGEO) satellite networks. In this scenario, the random access preamble signal and the detection must be robust to large carrier frequency offsets (CFOs). Towards this end, we investigate the adoption of the pruned discrete Fourier transform spread filter bank multicarrier waveform. The proposed design is suitable for the access scheme of forthcoming 5G-based NGEO satellite communications. The reason is twofold. First, it improves the spectral confinement with respect to the standard single-carrier frequency-division multiplexing (SC-FDM) waveform. Second, it achieves a high level of commonality with 5G new radio, by keeping unchanged the subcarrier spacing, the slot duration and the preamble sequence. Remarkably, the new design allows the straightforward application of non-coherent post detection integration (NCPDI) techniques, which divide the correlation in blocks. Numerical results show that the proposed solution reduces out-of-band emissions and the missed detection probability in presence of CFO, with respect to the conventional approach based on SC-FDM and preamble detection with full-length correlation. This work has received funding from the ministry of Science, Innovation and Universities under project TERESA-TEC2017-90093-C3-1-R (AEI/FEDER,UE) and under grant RTI2018-099841-B-I00 and from the Catalan Government (2017-SGR-1479).

Non-Coherent Rate-Splitting (NCRS) was recently proposed as a practical multiuser coding and decoding scheme to increase the spectral efficiency of multibeam satellite communication systems. In this paper, we further study the practical realization of NCRS. We propose a modified coding scheme (${\mathrm {NCRS}}^{*}$) that is robust to a nonzero time offset among beams. In NCRS*, as opposed to NCRS, the beams send independently channel encoded and modulated waveforms.We assess the performance of NCRS* in terms of the achievable rate region. It is shown that NCRS* performs worse than NCRS, but better than or comparable to other competing schemes, which, as opposed to NCRS*, require flexible bandwidth allocation or perfect synchronization at the transmitter. We also propose a new N-MAP algorithm for the practical implementation of NCRS* receivers. Similar to the existing UMAP algorithm, N-MAP takes into account the modulation used by, and the time offset between, the signals received from the different beams. In most cases, however, N-MAP has a significantly lower complexity than U-MAP.

This paper investigates the application of multicast non-orthogonal multiple access (MC-NOMA) schemes to the forward link of a satellite communication system. In multicast transmission each frame contains information of multiple users. To benefit from the theory developed in NOMA, the proposed scheme creates two groups of users within each beam. The analysis conducted in this work reveals that the user grouping has an impact on the performance. In the light of this observation, power allocation and user clustering techniques have been derived to either maximize the sum-rate or achieve max-min fairness. The numerical simulation results show that MC-NOMA outperforms multicast orthogonal multiple access (MC-OMA) schemes, where different groups are served in orthogonal resources. Moreover, the gain of MC-NOMA over the MC-OMA becomes more prominent as number of users per group and the transmit power increases. The results show the minimum-rate and the sum-rate of MC-NOMA can be increased by a factor 2 and 1.45 with respect to MC-OMA, respectively.

© 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. Non-orthogonal transmission is a promising technology enabler to meet the requirements of 5G communication systems. Seminal papers demonstrated that non-orthogonal multiplexing techniques outperform orthogonal schemes in terms of capacity, latency, and user fairness. Since it is envisioned that satellites will be an integral component of the 5G infrastructure, it is worth studying how satellite communication systems can benefit from the application of non-orthogonal transmission schemes as well. Contrary to common perception, current communications through a satellite present a different architecture and face different impairments than those in the wireless terrestrial links. In particular, this work aims to describe different non-orthogonal schemes that are suitable for the forward link (i.e., satellite to user). In contrast with the return link of the satellite (i.e., user to satellite), where the use of non-orthogonal transmission schemes has been widely studied, less effort has been devoted to the forward link. In light of this, this article provides an overview and a novel taxonomy that is based on the forward link of different non-orthogonal multibeam transmission schemes. Finally, guidelines that open new avenues for research in this topic are provided.

The integration of satellite and terrestrial networks is a promising solution for extending broadband coverage to areas not connected to a terrestrial infrastructure, as also demonstrated by recent commercial and standardisation endeavours. However, the large delays and Doppler shifts over the satellite channel pose severe technical challenges to traditional terrestrial systems, as LTE or 5G. In this paper, two architectures are proposed for a LEO mega-constellation realising a satellite-enabled LTE system, in which the on- ground LTE entity is either an eNB (Sat-eNB) or a Relay Node (Sat-RN). The impact of satellite channel impairments as large delays and Doppler shifts on LTE PHY/MAC procedures is discussed and assessed. The proposed analysis shows that, while carrier spacings, Random Access, and RN attach procedures do not pose specific issues, HARQ requires substantial modifications. Moreover, advanced handover procedures will be also required due to the satellites' movement., Comment: Submitted to IJSCN Special Issue on SatNEx IV

We consider the forward link of a multibeam satellite system with high spectral reuse and the novel low-complexity transmission and detection strategies from [1]. More specifically, we study the impact of a time offset between the antenna beams that cooperate to simultaneously serve a given user. Assuming Gaussian signaling, we provide closed-form expressions for the achievable rate region. It is demonstrated that, in the absence of timing information at the gateway, this region is not affected by a time offset. Our numerical results further show that, in case timing is known at the gateway, an offset of half a symbol period at both user terminals is optimal in terms of spectral efficiency., Grant numbers : Satellite Network of Experts IV. © 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.

This work addresses the uneven traffic demand scenario in multi-beam satellite systems, in which a hot-spot beamis surrounded by cold beams. After partitioning the hotspot beam in different sectors, resource pulling from coldneighbouring beams is allowed following an aggressive frequency-reuse scheme. As a consequence, the level ofthe co-channel interference within the hot-spot beam increases. A scheme known as Non-Coherent Rate-Splitting (NCRS)is employed to cope with this interference, based on the exclusive use of magnitude channel state information at the transmitter (CSIT).The receiver complexity is increased with respect to full CSIT precoding schemes, which are considered for benchmarking purposes. Different NCRS strategies are analyzed and compared with several partial and full CSIT schemes. The proposed solution not only shows an improvement with respect to partial CSIT benchmarks, but also displays a competitive performance against full CSIT precoders.

This paper investigates a novel technique to deal with the interference in the forward link of multibeam satellite systems when aggressive frequency reuse schemes are employed. Taking into account only magnitude information about the forward channel, the gateway judiciously splits the messages to be transmitted into private and public parts. At the receive terminals, partial cancellation of the public messages is applied prior to private message detection. The practical significance of the absence of channel phase information is stressed and complemented by some additional insights on the implementation. Our numerical results show that, in terms of average total throughput, this technique combined with a 2-colour frequency reuse scheme can outperform a classic orthogonal system with a conservative 4-colour frequency reuse scheme, despite the additional co-channel interference.

Multi-beam satellite systems gain capacity from frequency reuse in the forward link, but they suffer from inter-system interference problems. Multi-user detection (MUD) is an advanced interference mitigation technique applied at the user terminal to cope with co-channel interferences. This paper complements theoretical studies on the efficiency of MUD by the development of a system performance demonstrator allowing the analysis of MUD performance limits under realistic channel conditions. A two-color system is assumed and compared to a traditional four-color application, which is to be used as a benchmark. By the application of MUD, a performance gain in peak and system throughput can be observed, while requiring higher complexity at the user terminal.

This paper analyzes frequency packing schemes applied to multibeam satellite systems. In order to simulate a realistic scenario we have taken into account the presence of co-channel interference and spectrum limitations. The results show that with low order modulations, e.g. QPSK and 8APSK, the frequency packing schemes outperform in terms of spectral eciency conventional single carrier schemes with high modulation orders up to 32APSK. However, the experimental validation carried out with the DVB-S2 modulation and coding (ModCod) schemes reveals that there is a gap between the maximum achievable rates and the throughput. This result highlights that new ModCod schemes have to be designed, so that frequency packing schemes perform closer to the theoretical bounds., Grant numbers : This work has also received funding from the Spanish Government under project TEC2014-59255-C3-1-R (ELISA) and from the Catalan Government under grant 2014SGR1567.

This paper characterizes the sources of distortion of offset-QAM based filter bank multicarrier systems when perfect channel state information at the receiver (CSIR) is not available. The statistical information of the distortion reveals that the mapping scheme, which determines how symbols are inserted in the frequency-time grid, can be optimized to reduce the impact of the estimation error. Moreover, the distortion analysis has been exploited to design variable incremental redundancy hybrid automatic repeat request (IR-HARQ) schemes. Numerical results show that the algorithm devised in this work guarantees the target block error rate (BLER) in low mobility scenarios. In addition, the proposed variable IR-HARQ scheme allows to relinquish up to 75 % of the resources to other users when the initial transmission fails.

The chapter shows that the FBMC/OQAM extension to MIMO architectures is not trivial since different multiantenna links are subject to independent fading conditions. As a consequence, the superposition of signals at the receiver does not preserve the characteristics of the modulation so that the intrinsic interference is not in quadrature with the desired data. Hence, the perfect reconstruction property cannot be exploited unless the channel is compensated. To get rid of the interference, it is desirable to have an equivalent response, which encompasses the precoder, the channel and the equalizer, with a diagonal structure. If the channel frequency response is sufficiently smooth in the pass band region of one subcarrier, OFDM-based solutions can be successfully applied. However, when the channel frequency selectivity is severe, new MIMO techniques have to be devised. With the aim of providing a concise overview and a valuable insight into the signal processing developments that allow the combination of FBMC/OQAM with MIMO, this chapter describes and assesses the state-of-the-art MIMO precoding and decoding designs for FBMC/OQAM systems in mild- and strong-frequency selectivity scenarios.

LTE-based satellite systems in LEO constellations are a promising solution for extending broadband coverage to areas not connected to a terrestrial infrastructure. However, the large delays and Doppler shifts over the satellite channel pose severe technical challenges to a traditional LTE system. In this paper, two architectures are proposed for a LEO mega-constellation realizing a satellite-enabled LTE system, in which the on-ground LTE entity is either an eNB (Sat-eNB) or a Relay Node (Sat-RN). Focusing on the latter, the impact of large delays and Doppler shifts on LTE PHY/MAC procedures is discussed and assessed. It will be shown that, while carrier spacings, Random Access, and RN attach procedures do not pose specific issues, HARQ requires substantial modifications. Moreover, advanced handover procedures will be also required due to the satellites' movement.

This paper considers interference mitigation techniques in the forward link of multibeam satellite systems. In contrast to previous works, either devoted to receiver interference mitigation (e.g. multiuser detection) or transmitter interference mitigation (precoding), this work evaluates the achievable rates of the joint combination of both techniques. On the one hand, precoding cannot properly mitigate all the inter- beam interference while maintaining a sufficiently high signal-to-noise ratio. On the other hand, the receiver cost and complexity exponentially increases with the number of signals to be simultaneously detected. This highlights that the receiver cannot deal with all the interferences so that in general only 2 signals are jointly detected. As a result, the use of precoding within a coverage area jointly with multiuser detection can both benefit from each other and extremely increase the achievable rates of the system. This is numerically evaluated in a close-to-real coverage area considering simultaneous non-unique decoding strategies. The results show the benefits of this joint scheme that eventually can increase the current precoding performance a 23%.

Next-generation communication systems have to comply with very strict requirements for increased flexibility in heterogeneous environments, high spectral efficiency, and agility of carrier aggregation. This fact motivates research in advanced multicarrier modulation (MCM) schemes, such as filter bank-based multicarrier (FBMC) modulation. This paper focuses on the offset quadrature amplitude modulation (OQAM)-based FBMC variant, known as FBMC/OQAM, which presents outstanding spectral efficiency and confinement in a number of channels and applications. Its special nature, however, generates a number of new signal processing challenges that are not present in other MCM schemes, notably, in orthogonal-frequency-division multiplexing (OFDM). In multiple-input multiple-output (MIMO) architectures, which are expected to play a primary role in future communication systems, these challenges are intensified, creating new interesting research problems and calling for new ideas and methods that are adapted to the particularities of the MIMO-FBMC/OQAM system. The goal of this paper is to focus on these signal processing problems and provide a concise yet comprehensive overview of the recent advances in this area. Open problems and associated directions for future research are also discussed.

This paper investigates the application of filter bank multicarrier modulation based on the OQAM (FBMC/OQAM) to multiple-input-multiple-output (MIMO) systems. Existing solutions guarantee satisfactory performance when the streams multiplexed on each subcarrier (S) and the number of transmit (NT) and receive (NR) antennas are related as S = min (NT, NR). When S T, NR), the techniques presented in previous works either exhibit an error floor or perform much worse than orthogonal frequency division multiplexing (OFDM). To make progress towards the combination of FBMC/OQAM with MIMO we propose a two-step approach and a coordinated beamforming algorithm to design the transmit and the receive processing. Numerical results show that the two-step method provides similar bit error rate (BER) as OFDM when S + 1 = NT = NR. Resorting to the coordinated beamforming solution, which is based on an iterative method, the application of FBMC/OQAM is extended to the general case S T, NR). Hence, the techniques presented in this paper demonstrate that FBMC/OQAM can achieve practically the same BER as OFDM with an increased spectral efficiency and a significantly decreased out-of-band radiation, which is an important advantage for non-contiguous spectrum allocations

This paper addresses the margin adaptive problem for the filter bank multicarrier (FBMC) modulation based on the offset QAM (OQAM), referred to as FBMC/OQAM. The analysis conducted in this paper reveals that due to the specific FBMC/OQAM transmission format the system has to deal with inter-user, inter-symbol and inter-carrier interference to solve the resource allocation problem. To manage the interference, the successive channel allocation method, originally proposed for the orthogonal frequency division multiplexing (OFDM), has been modified by organizing subcarriers in groups and assigning subcarriers to users in a block-wise fashion. Following this strategy and making some approximations we demonstrate that it is possible to pose a single problem that is valid for OFDM and FBMC/OQAM. In this case, numerical results show that FBMC/OQAM is more energy-efficient than OFDM because it requires less power to transmit the same information rate.

For single-frequency network (SFN) transmission, the echoes coming from different transmitters are superimposed at the reception, giving rise to a frequency selective channel. Although multicarrier modulations lower the dispersion, the demodulated signal is sensitive to be degraded by inter-symbol interference (ISI) and inter-carrier interference (ICI). In view of this, we use channel coding in conjunction either with filter bank multicarrier (FBMC) modulation or with orthogonal frequency division multiplexing (OFDM). To deal with the loss of orthogonality, we have devised an interference-aware receiver that carries out a soft detection under the assumption that the residual interference plus noise (IN) term is Gaussian-distributed. To keep the complexity low, we propose to estimate the variance of the IN term by resorting to data-aided algorithms. Experimental results show that regardless of the method, FBMC provides a slightly better performance in terms of coded bit error rate than OFDM, while the spectral efficiency is increased when FBMC is considered.

Several studies have revealed that OFDM is outperformed in several areas by the filter bank based multicarrier (FBMC) modulation thanks to pulse shaping techniques. Nevertheless, the combination of FBMC with multi-antenna architectures is nontrivial. Aiming at making progress towards this direction we study MIMO precoding techniques for the MISO broadcast channel (MISO-BC). Due to its superior performance over linear precoding techniques we have focused on the spatial Tomlinson Harashima precoder (STHP). The analysis carried out in this paper concludes that the conventional STHP performs poorly when it is combined with FBMC systems since it is not able to cope with ISI and ICI. To combat the interferences we propose a novel subband processing based on the STHP concept. The numerical results show that FBMC and OFDM almost give the same performance in terms of BER. However, FBMC is able to achieve spectral efficiency values that are 31% higher in comparison to the ones obtained in OFDM when 20% of the symbol duration corresponds to the cyclic prefix.

This paper deals with the design of MIMO pre-coding and decoding matrices for the filter bank multicarrier (FBMC) modulation. With the aim of increasing the system throughput we focus on the downlink of a multi-user communication system, where users share the same time slots and frequency resources. In this sense, we revisit the block diagonalization (BD) technique since it provides a good trade-off between complexity and performance. Based on the BD concept, we have formulated a new set of conditions that achieve interference-free data multiplexing in the FBMC context. In addition, we have carried out a theoretical analysis that reveals under what conditions MIMO-FBMC shows superior performance than MIMO-OFDM systems. Simulations results demonstrate that in the cases of study, the proposed solution slightly outperforms the classical BD technique, which can be combined with either OFDM or FBMC systems, in terms of bit error rate.

This paper addresses the joint design of MIMO precoding and decoding matrices for filter bank multicarrier (FBMC) systems based on OQAM, known as FBMC/OQAM. Existing solutions that support multi-stream transmission only give satisfactory performance in scenarios with high coherence bandwidth channels. To make progress towards the application of FBMC/OQAM to MIMO channels, we study the design of novel solutions that provide robustness against the channel frequency selectivity and support multi-stream transmission. To this end, two techniques have been devised under the criterion of minimizing the sum mean square error. The non-circular nature of the OQAM symbols has not been ignored, making evident the convenience of performing a widely linear processing. The first technique keeps the complexity at a reasonable level but in exchange the original problem is relaxed yielding a suboptimal solution. With the objective of performing closer to the optimum solution, the second option iteratively computes precoders and equalizers by resorting to an alternating optimization method, which is much more complex. We have demonstrated via simulations that the first technique nearly achieves the same results as the iterative design. Simulation results show that the proposed low-complexity solution outperforms existing MIMO-FBMC/OQAM schemes in terms of bit error rate. As for the comparison with OFDM, the numerical results highlight that FBMC/OQAM remains competitive, with and without perfect channel state information, while it provides spectral efficiency gains. Under highly frequency selective channels the proposed technique significantly outperforms OFDM.

This paper addresses the design of MIMO precoding and decoding techniques for the filter bank multicarrier (FBMC) modulation. Existing solutions give satisfactory performance in scenarios with high coherence bandwidth channels. With the aim of increasing the robustness against the channel frequency selectivity, we have rethought the problem, which results in a new subband processing. Simulation-based results show that the proposed solution can achieve similar bit error rates as the OFDMsolution, while the spectral efficiency is increased. These results are theoretically justified. As a conclusion, FBMC becomes attractive in frequency selective channels not only because it relaxes the frame synchronization with respect to OFDM, but also because it presents spatial multiplexing competitive results.

This paper investigates how to boost the forward link capacity in multibeam satellite systems. Since co-channel interference is the main inhibitor to achieve the full potential of frequency reuse, we propose to mitigate the negative impact of interference through cooperative transmission schemes. When desired information is shared among two beams, rates can be assigned to each user independently. This is crucial to benefit from the use of adaptive coding and modulation. In addition, the receiver can resort to multi-user detection, which is proven to give satisfactory performance when several signals are simultaneously received. Numerical results reveal that the proposed cooperative strategy brings substantial improvements, in terms of spectral efficiency, with respect to multibeam satellite systems, where receivers treat the interference as noise and data content is plainly delivered through the corresponding beam.

This paper tackles the sum-rate maximization in FBMC systems. It is well-known that multipath fading destroys the orthogonality of the subcarriers, which brings a non-convex problem that can be interpreted as the maximization of a difference of concave functions. This reformulation enables using DC programming to find the global optimal but the complexity becomes unaffordable in the FBMC context. Aiming at reducing the complexity we have devised a new suboptimal algorithm. The idea consists in smartly partitioning the band into three subsets so that subcarrier signals belonging to the same subset do not overlap in the frequency domain. If subsets maximize sequentially their own rate the original problem is turned into three simpler problems. Simulation-based results show that the sum-rate is in the same order as the water-filling used in OFDM and a suboptimal solution based on DC programming. However, the addressed technique enables saving up to 25% of the power at SNR=22 dB.

This paper studies the MIMO applicability to filter bank based multicarrier (FBMC) modulations for low coherence bandwidth channels. Under these conditions the channel frequency response cannot be modeled flat at a subcarrier level. This implies that the techniques originally devised for OFDM do not restore the orthogonality between subcarriers when they are directly applied to FBMC. Aiming at circumventing this problem we propose the design of two MIMO FBMC schemes, which are based on a new subband processing. The figures of merit that govern the design of the first and second scheme are the signal to leakage plus noise ratio (SLNR) and the signal to interference plus noise ratio (SINR), respectively. However, we do not restrict the analysis to the design of a new subband processing but we also carry out an asymptotic analysis of the complexity as well as tackle the problem of estimating the channel. Simulation-based results have demonstrated that the addressed solutions clearly outperform previous MIMO FBMC schemes in terms of BER. In comparison to OFDM, the devised techniques are able to remain competitive even if the knowledge of the channel state information is not perfect. In those scenarios where the cyclic prefix (CP) length is not sufficiently large to avoid inter block interference, the proposed designs are able to reduce the BER. However, the price that should be paid is the increase of the complexity.

In this letter the discrete rate maximization problem is investigated for FBMC/OQAM. The analysis reveals that if there is crosstalk certain bit allocations violate the power constraints. Aiming at ensuring the feasibility along with alleviating the complexity we have devised a novel iterative algorithm, which always converges. Simulation-based results show that the proposed algorithm performs close to the upper bound for high-coherence bandwidth channels. Under highly frequency selective channels the existing algorithms are not able to guarantee the target SER whereas our approach guarantees the QoS. This work was supported by the Spanish Ministry of Science and Innovation under Projects TEC2011-29006-C03-02 (GRE3N-LINK-MAC), TEC2011-29006-C03-03 (GRE3N-SYST), and TEC2008-06327-C03-01, and by the Catalan Government (2009SGR0891). Publicado

Build upon the OFDM/OQAM scheme this paper addresses the design of linear transmit and receive filters. Aiming at enhancing the robustness against multipath fading, two novel techniques are proposed. The first one jointly designs transmit and receive filters under the MMSE criterion. The second scheme follows a ZF approach. In this case transmit and receive filters are independently devised. Pre-equalizers obtained by both strategies are subjected to power constraints. Simulation-based results show that the two proposed techniques benefit from having perfect channel knowledge at both ends of the link to outperform the Alamouti scheme. The best performance in terms of BER is given when transmit and receive filters are jointly designed.

Multicell processing has been recognized as a solution to improve spectral efficiency in future wireless networks. However, the sharing of channel information and data among base stations (BSs) requires an higher backhaul throughput, which is not provided by current cellular systems. In this paper we consider a downlink multicell scenario with constraints on the backhaul throughput and BSs employing beamforming and QAM constellations. As for each mobile terminal (MT) the auxiliary BSs know only a partial version of data, we propose a transmission scheme where a quantized QAM symbol is transmitted cooperatively by all the BSs in the network, while the serving BS transmits an additional signal in order to allow the detection of the full QAM symbol at the receiver. We formulate the problem of maximizing the network spectral efficiency optimizing for each MT a) the QAM constellation size and b) the quantization rate. Numerical results provided for typical cellular scenarios show that the proposed practical approach outperforms in terms of network spectral efficiency both the case of single cell transmission and the case of full network coordination due to the backhaul constraints.

By including a cyclic prefix (CP), systems based on orthogonal frequency division multiplexing (OFDM) are capable of mitigating multipath with a single tap equalizer. Aiming at increasing the spectral efficiency, filter banks based multi carrier systems (FBMC) along with an offset quadrature amplitude modulation (OQAM) can be seen as a good alternative, although single tap equalizers may not provide enough robustness. In this regard we adapt to FBMC systems the structure consisting of a space-time processor together with a maximum likelihood sequence estimator (MLSE). Simulations show that in presence of large delay spreads the proposed scheme outperforms orthogonal frequency division multiplexing (OFDM) systems in terms of bit error rate (BER).

One of the phenomena that degrades the performance of multicarrier modulation (MCM) systems is the spectral nulls that multipath channel may have. As a result, the affected subcarrier signals may be severely degraded resulting in a loss of symbols. In this paper we have studied how to overcome this problem by taking advantage of spatial diversity along with the knowledge of channel state information at both ends of the link. To that end we devise broadband beamformers that work on a per-subcarrier basis for filter bank based multicarrier (FBMC) systems. In particular we have focused on the orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) scheme. The criterion that has been followed to design the transmit filters consists of minimizing the transmitted power while the quality of service (QoS) constraints are satisfied. Simulation-based results demonstrate the efficacy of the proposed optimal and suboptimal designs to reduce the transmitted power when compared to the Alamouti scheme at the same BER targets. Although the optimal design is more demanding in terms of complexity it yields 1dB of savings in the transmitted power in comparison to the sub-optimal alternative.

This paper evaluates the applicability of MIMO techniques to satellite networks in order to achieve diversity and multiplexing gain through dual polarized antennas. In single satellite scenarios the proposed STTC and OSTBC techniques proposed offer better BER than plain stream multiplexing along each polarizations and SISO transmissions. By adding a satellite with dual polarization antennas and performing a joint distributed OSTBC, the spectral efficiency increases as the satellites transmit in the same frequency band. Finally the hybrid satellite-terrestrial network has been considered for MIMO transmission. In this case, the spectral efficiency can be multiplied by 4 if a joint encoding of satellite and terrestrial signals is performed. In each scenario the broadcast signal follows the DVB-SH standard.

This paper investigates the application of non-binary low density parity check (NB-LDPC) codes to offset-QAM based filter bank multicarrier systems, known as FBMC/OQAM. The analysis conducted in the paper reveals that the mapping scheme can be optimized to simplify detection and minimize the correlation of the noise. Based on the proposed modifications, existing adaptive coding and modulation (ACM) algorithms can be easily tailored to accommodate the characteristics of NB-LDPC and FBMC/OQAM. In this paper, priority has been given to mutual information-based ACM algorithms. Numerical results verify that the proposed algorithm guarantees the target block error rate.

This paper deals with the problem of non-orthogonal multiple access (NOMA) in multibeam satellite systems, where the signals are jointly precoded. It is considered that the number of frames that are simultaneously transmitted is higher than the number of feeds, reducing the precoding interference mitigation capabilities as the system becomes overloaded. In order to solve this problem, we assume that the satellite user terminals are able to perform multi-user detection to mitigate the interference. In the current NOMA approach, it is assumed a successive interference cancellation (SIC) receiver. To increase the spectral efficiency, this paper investigates NOMA with simultaneous non-unique detection (SND). Compared to the case where user terminals perform single user detection (SUD), conventional scheduling heuristic rules do not longer apply in this scenario. Therefore, different scheduling algorithms are proposed considering both SIC and SND strategies. As the numerical evaluations show, SND yields larger average data rates than the SIC receiver. Concerning the scheduling, the best strategy is to pair users with highly correlated channels and the lowest channel gain difference. It is also shown that the sum-rate can be increased in overloaded satellite systems with respect to satellite scenarios, where the number of transmitted frames and feeds is the same.

The filter bank multicarrier modulation based on offset quadrature amplitude modulation (FBMC/OQAM) is being considered as an eligible technology for future wireless communications. However, the orthogonality may be destroyed in the presence of multipath fading, and thus, the demodulated data may be affected by inter-symbol and inter-carrier interference. To restore the orthogonality, it is deemed necessary to either precode the symbols to be transmitted or equalize the demodulated data. Under severe propagation conditions, both the precoders and equalizers should perform multi-tap filtering. In this paper, we try to determine whether it is the best choice to combat the channel at the transmit, or at the receive side. The answer to this question is not trivial, since with data precoding, the transmit power may increase, while equalization at the receive side may enhance the noise power. To cast light on this issue, this paper characterizes the average transmit and noise power to determine the influence of multi-tap filtering on the transmit signal power and on the noise variance. The analysis conducted in this paper reveals that if multi-tap precoding yields a power increase, then the noise variance would increase with the same magnitude if the same linear filter was moved from the transmit to the receive side. Therefore, if symbols are properly scaled when the transmit power increases, there is no degradation due to placing the complexity burden at the reception rather than at the transmission. To scale the symbols, it is mandatory to know the additional transmit power. In this sense, a low-complexity method to estimate the power increase is proposed. Simulation-based results confirm that the estimation is reasonably accurate.