In this study we introduce Logarithmic Frequency Shift Keying (Log-FSK), a novel frequency modulation for over-the-air computation (AirComp). Log-FSK leverages non-linear signal processing to produce AirComp in the frequency domain, this is, the maximum frequency of the received signal corresponds to the sum of the individual transmitted frequencies. The demodulation procedure relies on the inverse Discrete Cosine Transform (DCT) and the extraction of the maximum frequency component. Log-FSK enables the computation of functions beyond the sum by incorporating nomographic function representation. Furthermore, unlike existing AirComp modulations, Log-FSK allows to compute several functions in a single transmission. We evaluate the capabilities of the scheme in an additive white Gaussian noise (AWGN) and flat-fading channels. To demonstrate its practicality, we present specific applications and experimental results showcasing the effectiveness of Log-FSK AirComp within linear Wireless Sensor Networks (WSN). Our numerical results show that Log-FSK outperform linear analog modulations in terms of MSE and power consumption.

The increasing demand for bandwidth has led fifth generation (5G) mobile communication systems to adopt the use of millimeter waves (mmWaves), to enable higher data rates. However, operating at such high frequencies poses technological challenges, one of the most prominent being the phenomenon known as phase noise (PN), which degrades the system, especially when combined with the orthogonal frequency-division multiplexing (OFDM) waveform. Traditionally, this effect is corrected by using exclusively dedicated pilots for PN tracking, which severely degrade the data rate. This paper presents pilotless machine learning (ML)-based methods for estimating and compensating PN with the goal of eliminating the need for pilots, leading to a substantial increase of the data rates. In addition, this paper demonstrates the feasibility of these solutions for practical mmWaves deployments through tests with real measurements in an experimental setup using an mmWave 5G-developed platform.

The low-Earth orbiting (LEO) satellite constellations in the 90s (such as Iridium and Globalstar), although representing a major technical breakthrough, were not able to achieve the initial goal of complementing the second-generation mobile terrestrial networks due to the rapid worldwide adoption of GSM and other standards. However, the decline of conventional linear television and the persisting need to mitigate the digital divide still affecting billions of people recently generated a renewed interest by private investors for Low-Earth Orbiting (LEO) satellite mega-constellations. The mega-constellations under deployment can provide lower delays versus geostationary (GEO) satellites, broadband access anywhere, anytime leveraging the low-cost series production of small satellites, and more affordable launch solutions. At the end of the last decade, key industrial players realized the potential complementary role satellites can play to extend the 5G terrestrial network coverage over low-density populated areas, oceans, or similar. This has led to great technological developments and a sharp reduction in LEO satellite volume, weight, and, ultimately, manufacturing and launching costs. Also, it has triggered the inclusion of a non terrestrial network (NTN) component in the latest 5G 3GPP standard releases. Yet, there are many technological challenges remaining to ensure that both the quality and cost of the NTN services are comparable to the terrestrial counterpart. This is mainly due to the constraints in satellite payload power, mass, and antenna size. This comes in addition to the stringent power flux density limitations on the ground, in particular in the below 6 GHz satellite bands. The required order of magnitude increase of the effective delivered throughput and service cost reduction can only be achieved by a mix of system architectures and innovative technologies for both the space and ground segments. Space communication systems and technologies will also play a key role in human return to the Moon planned for mid-2020, to prepare for human exploration of Mars in the more distant future and further cosmic exploration. As missions voyage further from Earth, it is important to consider how we can continue to reliably communicate with them and how they will accurately navigate through space when they are so far from home.

Federated edge learning (FEEL) is a framework for training models in a distributed fashion using edge devices and a server that coordinates the learning process. In FEEL, edge devices periodically transmit model parameters to the server, which aggregates them to generate a global model. To reduce the burden of transmitting high-dimensional data by many edge devices, a broadband analog transmission scheme has been proposed. The devices transmit the parameters simultaneously using a linear analog modulation, which are aggregated by the superposition nature of the wireless medium. However, linear analog modulations incur in an excessive power consumption for edge devices and are not suitable for current digital wireless systems. To overcome this issue, in this paper we propose a digital frequency broadband aggregation. The scheme integrates a Multiple Frequency Shift Keying (MFSK) at the transmitters and a type-based multiple access (TBMA) at the receiver. Using concurrent transmission, the server can recover the type (i.e., a histogram) of the transmitted parameters and compute any aggregation function to generate a shared global model. We provide an extensive analysis of the communication scheme in an additive white Gaussian noise (AWGN) channel and compare it with linear analog modulations. Our experimental results show that the proposed scheme achieves no drop in performance up to -10 dB and outperforms the analog counterparts, while requiring 14 dB less in peak-to-average power ratio (PAPR) than linear analog modulations.

In high-resolution Earth observation imagery, Low Earth Orbit (LEO) satellites capture and transmit images to ground to create an updated map of an area of interest. Such maps provide valuable information for meteorology and environmental monitoring, but can also be employed for real-time disaster detection and management. However, the amount of data generated by these applications can easily exceed the communication capabilities of LEO satellites, leading to congestion and packet dropping. To avoid these problems, the Inter-Satellite Links (ISLs) can be used to distribute the data among multiple satellites and speed up processing. In this paper, we formulate a satellite mobile edge computing (SMEC) framework for real-time and very-high resolution Earth observation and optimize the image distribution and compression parameters to minimize energy consumption. Our results show that our approach increases the amount of images that the system can support by a factor of$12\times $and$2\times $when compared to directly downloading the data and to local SMEC, respectively. Furthermore, energy consumption was reduced by 11% in a real-life scenario of imaging a volcanic island, while a sensitivity analysis of the image acquisition process demonstrates that energy consumption can be reduced by up to 90%.

The integration of non-terrestrial networks into the 5G ecosystem is mainly driven by the possibility of provisioning service in remote areas. In this context, the advent of flying base stations at the low Earth orbit (LEO) will enable anywhere and anytime connectivity. To materialize this vision, it is of utmost importance to improve radio protocols with the aim of allowing direct satellite access. Bearing this aspect in mind, we present a new random access signal, which is based on the filter bank multicarrier (FBMC) waveform, and a computationally efficient detection scheme. The proposed solution outperforms the standardized access scheme based on single-carrier frequency division multiplexing (SC-FDM), by reducing out-of-band (OOB) emissions and reducing the missed detection probability in presence of very high carrier frequency offset (CFO), which is inherent to LEO satellite systems. The improvement is related to the fine frequency resolution of the detector and the use of pulse shaping techniques. Interestingly, the FBMC-based random access signal achieves a high level of commonality with 5G new radio, as the preamble generation method and the time-frequency allocation pattern can be kept unchanged. Concerning the practical implementation aspects, the complexity of the detector is similar in both SC-FDM and FBMC.

The low earth orbit (LEO) satellite system is one of the promising solutions to provide broadband services to a wide-coverage area for future integrated LEO-6G networks, where users' demands vary with time and geographical locations. Conventional satellites with fixed beam pattern and footprint planning may not be capable of meeting such dynamic requests and irregular traffic distributions. As the development of flexible satellite payload with beamforming capabilities, spot beams with flexible size and shape are considered potential solutions to this issue. As an early investigation, in this paper, we consider the scenarios where satellite payloads are equipped with multiple beam patterns and study the optimal beam pattern selection. We exploit the potential synergies of joint resource optimization between adaptive beam patterns and non-orthogonal multiple access (NOMA) in a LEO satellite system, where NOMA is employed to reduce intra-beam interference and flexible beam pattern is adopted to mitigate inter-satellite interference. The formulated problem is to minimize the capacity-demand gap of terminals, which falls into mixed-integer nonconvex pro-gramming (MINCP). To tackle the discrete variables and non-convexity, we design a joint approach to allocate power and select beam patterns. Numerical results show that the proposed scheme achieves capacity-demand gap reduction of 37.8% over conventional orthogonal multiple access (OMA) and 42.5% over the fixed-beam-pattern scheme.

Next-generation satellite systems require more flexibility in resource management such that available radio resources can be dynamically allocated to meet time-varying and non-uniform traffic demands. Considering potential benefits of beam hopping (BH) and non-orthogonal multiple access (NOMA), we exploit the time-domain flexibility in multi-beam satellite systems by optimizing BH design, and enhance the power-domain flexibility via NOMA. In this paper, we investigate the synergy and mutual influence of beam hopping and NOMA. We jointly optimize power allocation, beam scheduling, and terminal-timeslot assignment to minimize the gap between requested traffic demand and offered capacity. In the solution development, we formally prove the NP-hardness of the optimization problem. Next, we develop a bounding scheme to tightly gauge the global optimum and propose a suboptimal algorithm to enable efficient resource assignment. Numerical results demonstrate the benefits of combining NOMA and BH, and validate the superiority of the proposed BH-NOMA schemes over benchmarks.

This work presents an orthonormal transform that splits the Orthogonal Frequency Division Multiplex (OFDM) symbol into slices with ranked rate and decoding complexity. The advantage over the existing carrier or time segmentation is that the proposed technique does not depend on the frequency channel to produce slices of equal rate. Also, the encoding and the decoding complexity is kept simple.

In this paper, we investigate potential synergies of non-orthogonal multiple access (NOMA) and beam hopping (BH) for multi-beam satellite systems. The coexistence of BH and NOMA provides time-power-domain flexibilities in mitigating a practical mismatch effect between offered capacity and requested traffic per beam. We formulate the joint BH scheduling and NOMA-based power allocation problem as mixed-integer non-convex programming. We reveal the exponential-conic structure for the original problem, and reformulate the problem to the format of mixed-integer conic programming (MICP), where the optimum can be obtained by exponential-complexity algorithms. A greedy scheme is proposed to solve the problem on a timeslot-by-timeslot basis with polynomial-time complexity. Numerical results show the effectiveness of the proposed efficient suboptimal algorithm in reducing the matching error by 62.57% in average over the OMA scheme and achieving a good trade-off between computational complexity and performance compared to the optimal solution.

The use of multi-antenna transmitters is emerging as an essential technology of the future wireless communication systems. While Zero-Forcing Beamforming (ZFB) has become the most popular low-complexity transmit beamforming design, it has some drawbacks basically related to the effort of "trying" to invert the channel coefficients towards the interfered users. In particular, ZFB performs poorly in the low Signal-to-Noise Ratio (SNR) regime and does not work when the interfered users outnumber the transmit antennas. In this paper, we study in detail an alternative transmit beamforming design framework, where we allow some residual received-interference power instead of trying to null it completely out. Subsequently, we provide a close-form non-iterative optimal solution that avoids the use of sophisticated convex optimization techniques that compromise its applicability onto practical systems. Supporting results based on numerical simulations show that the proposed transmit beamforming is able to perform close to the optimal with much lower computational complexity.

This paper tackles the problem of user scheduling in satellite content delivery networks with precoding. The clustering process has to consider two crucial and independent characteristics of the user terminals. On the one hand, users belonging to the same group shall have a reduced Euclidean norm between their channel vectors in order to obtain the maximum precoding gain. On the other hand, with the aim of exploiting the multicast capabilities of the system, user terminals grouped in the same cluster shall have requested the same content. The resulting clustering problem is formulated as a multigraph (also known as multiview) spectral clustering problem. The paper shows that this unsupervised learning framework is able to capture the different peculiarities of the mentioned problem. Two different techniques are introduced and validated in a close-to-real numerical simulation.

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.

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 been partially funded by the Spanish MECD FPU fellowship program granted to the author Borja Genovés Guzmán, the Catalan Government under Grant 2017-SGR-1479, and the Spanish Government under the national project ’TERESA-ADA’ with ID no. TEC2017-90093-C3-2-R and TEC2017-90093-C3-1-R (MINECO/AEI/FEDER, UE).

Index Modulations, in the form of Spatial Modulation or Polarized Modulation, are gaining traction for both satellite and terrestrial next generation communication systems. Adaptive Spatial Modulation based links are needed to fully exploit the transmission capacity of time-variant channels. The adaptation of code and/or modulation requires a real-time evaluation of the channel achievable rates. Some existing results in the literature present a computational complexity which scales quadratically with the number of transmit antennas and the constellation order. Moreover, the accuracy of these approximations is low and it can lead to wrong Modulation and Coding Scheme selection. In this work we apply a Multilayer Feedforward Neural Network to compute the achievable rate of a generic Index Modulation link. The case of two antennas/polarizations is analyzed in depth, showing not only a one-hundred fold decrement of the Mean Square Error in the estimation of the capacity as compared with existing analytical approximations, but also a fifty times reduction of the computational complexity. Moreover, the extension to an arbitrary number of antennas is explained and supported with simulations. More generally, neural networks can be considered as promising candidates for the practical estimation of complex metrics in communication related settings.

This paper studies the End-to-End (E2E) performance of a High Throughput Satellite (HTS) system that utilizes a transparent (non-regenerative) optical feeder link to transport the DVB-S2 signals associated with the large number of spot-beams in the radio access link. Two main sources of non-linearity are considered to characterize the effect that the satellite forward link has in the DVB-S2 signal, namely the optical Mach-Zehnder Modulator (MZM) in the gateway and the High Power Amplifier (HPA) in the satellite. Digital Pre-Distortion (DPD) and linear equalization are also implemented to mitigate the impact of the non-linear distortion and the inter-symbol interference on the Packet Error Rate (PER) of the received DVB-S2 signal. The performance analysis is carried out using the Total Degradation (TD) as performance indicator. As expected, the Input Back-Off (IBO) of the HPA and the modulation index of the MZM should be jointly optimized according to the DVB-S2 MODCOD that is selected for transmission. Therefore, the stronger is the Forward Error Correction (FEC) code that is used, the lower is the IBO and the higher is the modulation index that can be utilized to minimize the impact of non-linearities in the E2E performance.

In a multi-beam satellite communication system, traffic requests are typically asymmetric across beams and highly heterogeneous among terminals. In practical operations, it is important to achieve a good match between the offered and requested traffic, i.e., to improve the performance of Offered Capacity to requested Traffic Ratio (OCTR). Due to satellites’ payload constraints and limited flexibilities, it is a challenging task for resource optimization. In this paper, we tackle this issue by formulating a max-min resource allocation problem, taking fairness into account such that the lowest OCTR can be maximized. To exploit the potential synergies, we introduce Non-Orthogonal Multiple Access (NOMA) to enable aggressive frequency reuse and mitigate intra-beam interference. Although NOMA has proven its capabilities in improving throughput and fairness in 5G terrestrial networks, for multi-beam satellite systems it is unclear if NOMA can help to enhance the OCTR performance, and hence is worth quantifying how much gain it can bring. To solve the problem, we design a suboptimal algorithm to firstly decompose the original problem into multiple convex subproblems by fixing power allocation for each beam, and secondly adjust beam power to improve the minimum OCTR in iterations. Numerical results show the convergence of the proposed algorithm and the superiority of the proposed NOMA scheme in max-min OCTR.

The Centralized Radio Access Network (C-RAN) architecture has potential to achieve the target goals that have been set for 5G. However, as the number of radio signals to be transported from the Centralized Base Station (CBS) to the distributed Remote Antenna Units (RAUs) grows, the use of contemporary digital fronthaul technologies (such as CPRI) complicates. One simple way to address this problem consists in multiplexing the baseband signals intended to the different RAUs in the frequency domain and, after that, use the resulting analog signal to modulate the intensity of the optical carrier. This way, there is no bandwidth expansion and the processing delay is kept low; however, as the ideal fronthaul assumption does not hold any more in this case, the impairments that the analog optical fronthaul introduces should be taken into account when performing the digital signal processing in transmission. In this paper, we focus on the non-linear distortion that the external Mach-Zehnder (optical) modulator introduces and, through it, estimate the End-to-End (E2E) performance when different intensity modulation indexes (or clipping ratios) are used. As expected, moderate levels of non-linear distortion are beneficial if properly selected according to the C-RAN configuration.

Mutual Information is the metric that is used to perform link adaptation, which allows to achieve rates near capacity. The computation of adaptive transmission modes is achieved by employing the mapping between the Signal to Noise Ratio and the Mutual Information. Due to the high complexity of the computation of the Mutual Information, this process is performed off-line via Monte Carlo simulations, whose results are stored in look-up tables. However, in Index Modulations, such as Spatial Modulation or Polarized Modulation, this is not feasible since the constellation and the Mutual Information are channel dependent and it would require to compute this metric at each time instant if the channel is time varying. In this paper, we propose different approximations in order to obtain a simple closed-form expression that allows to compute the Mutual Information at each time instant and thus, making feasible the link adaptation.

This paper deals with the problem of precoding, scheduling, and link adaptation in next generation mobile interactive multibeam satellite systems. In contrast to the fixed satellite services, when the user terminals move across the coverage area, additional challenges appear. Due to the time varying channel, the gateway has only access to a delayed version of the channel state information (CSI), which can eventually limit the overall system performance. However, in contrast to general multiuser multiple-input multiple-output terrestrial systems, the CSI degradation in multibeam mobile applications has a very limited impact for typical fading channel and system assumptions. Under realistic conditions, the numerical results show that precoding can offer an attractive gain in the system throughput compared with the conservative frequency reuse allocations.

In this paper, we present a novel modulation technique for dual-polarization communication systems, which reduces the error rate compared with the existent schemes. This modulation places the symbols in a 3-D constellation, rather than the classic approach of 2-D. Adjusting the phase of these symbols depending on the information bits, we are able to increase the bit rate. Hence, the proposed scheme conveys information by selecting both polarization state and the phase of radiated electromagnetic wave. We also analyses the performance of the 3-D polarized modulation (PMod) for different constellation sizes and we obtain a curve of rate adaptation. Finally, we compare the proposed 3-D PMod with the other existing schemes, such as single polarization phase shift keying and double polarization vertical Bell Laboratories layer space-time, both carrying the same number of information bits. The results show that the 3-D PMod always outperforms all other schemes, except for low-order modulation. Therefore, we can conclude that the 3-D PMod is an excellent candidate for medium- and high-modulation-order transmissions.

Owing to the rapidly increasing traffic demands on satellite connectivity, the current exclusive frequency allocation is becoming obsolete. Instead, aggressive frequency reuse and interference mitigation techniques are promising ideas that both industry and academia are investigating. This paper proposes an optimization precoding technique for dealing with the multibeam interference due to the aggressive frequency reuse. In contrast to general multiuser multiple input multiple output (MIMO) schemes, multibeam satellite precoding techniques call for frame-by-frame quadratically constrained quadratic optimization of a large number of variables. We focus on the multigroup multicast beamforming optimization problem, and we propose to adopt a consensus-based alternating direction method of multipliers (C-ADMM) approach, in order to mitigate complexity. The proposed C-ADMM approach is shown to exhibit comparable optimization performance at considerably lower complexity relative to the prior state-of-art for the formulation considered.

This paper studies the sum data rate that a Visible Light Communication (VLC) system with multiple users can achieve when phosphor-converted white LED panels are used to provide illumination and communication simultaneously. Three different transmission schemes based on Asymmetrically Clipped Optical Orthogonal Frequency Division Multiplexing (ACO-OFDM) are considered to allocate the communication resources in each transmission point: Frequency Reuse (FR), Joint Transmission Coordinated Multi-Point (JT-CoMP), and a Hybrid combination of them. Since phosphor-converted white LEDs have a larger modulation bandwidth for blue optical wavelengths rather than green-yellow-red ones, the performance of each transmission method is also evaluated using a visible and/or blue light optical filter in front of the Photodetector (PD). When the transmission points apply FR, strong inter-cell interference results in high variability of the achievable data rate at different locations. On the other hand, when JT-CoMP is applied, a more homogeneous data rate coverage is achieved, reducing the peak data rate in the inner parts of the cells but improving notably the data rate in cell-edge areas.

Determining the capacity of a modulation scheme is a fundamental topic of interest. Index modulations (IM), such as spatial modulation (SMod), polarized modulation (PMod), or frequency index modulation (FMod), are widely studied in the literature. However, finding a closed-form analytical expression for their capacity still remains an open topic. In this paper, we formulate closed-form expressions for the instantaneous capacity of IM, together with its second- and fourth-order approximations. We show that, in average, the second-approximation error tends to zero for low signal to noise ratio (SNR) and is o(SNR). Also, a detailed analysis of the ergodic capacity over Rayleigh, Rice, and Nakagami-m channel distributions is provided. As application of the capacity analysis, we leverage the proposed expressions to compute the ergodic capacities of SMod for different antenna configuration and correlations, PMod for different channel components and conditions, and FMod for different frequency separations.

This paper deals with the problem of precoding in multibeam satellite systems. In contrast to general multiuser multiple-input-multiple-output cellular schemes, multibeam satellite architectures suffer from different challenges. First, satellite communications standards embed more than one user in each frame in order to increase the channel coding gain. This leads to the different so-called multigroup multicast models, whose optimization requires computationally complex operations. Second, when the data traffic is generated by several Earth stations (gateways), the precoding matrix must be distributively computed and meet additional payload restrictions. Third, since the feedback channel is adverse (large delay and quantization errors), the precoding must be able to deal with such uncertainties. In order to solve the aforementioned problems, we propose a two-stage precoding design in order to both limit the multibeam interference and to enhance the intra-beam minimum user signal power (i.e., the one that dictates the rate allocation per beam). A robust version of the proposed precoder based on a first perturbation model is presented. This mechanism behaves well when the channel state information is corrupted. Furthermore, we propose a per beam user grouping mechanism together with its robust version in order to increase the precoding gain. Finally, a method for dealing with the multiple gateway architecture is presented, which offers high throughputs with a low inter-gateway communication. The conceived designs are evaluated with a close-to-real beam pattern and the latest broadband communication standard for satellite communications.

This paper aims at designing an onboard beam generation process for a hybrid onboard on-ground multibeam satellite architecture. The proposed method offers a good tradeoff between total throughput and feeder link bandwidth requirements compared with pure on-ground systems. Full frequency reuse among beams is considered, and the beamforming at the satellite is designed for supporting interference mitigation techniques. In addition, in order to reduce the payload cost and complexity, this onboard beamforming is assumed to be constant and the same for forward and return link transmissions so that the same array-fed reflector can be used for forward and return links, leading to a substantial reduction of the payload mass. To meet all these requirements, a novel robust minimum mean square error optimization is conceived. The benefits of the considered scheme are evaluated with respect to the current approaches both analytically and numerically. Indeed, we show that with the DVB-RCS and DVB-S2 standards, our proposal allows increasing the total throughput within a range between 6% and 15% with respect to other onboard processing techniques in the return and forward link, respectively.

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%.

This paper evaluates the capacity of weighted circularly convolved FBMC/OQAM systems. A rigorous mathematical model is derived to calculate the increase of capacity that can be obtained thanks to the lattice structure of the modulation and the exploitation of the intrinsic interference. The numerical results reveal that a signal to noise ratio gain of 2dB is obtained in one resource block of 12 subcarriers, translating into a capacity increase of 11% with respect to OFDM, which nuances some previous results on this topic in the literature.

In this paper, we consider the cognitive satellite uplink where satellite terminals reuse frequency bands of Fixed-Service (FS) terrestrial microwave links which are the incumbent users in the Ka 27.5–29.5 GHz band. In this scenario, the transmitted power of the cognitive satellite terminals has to be controlled so as to satisfy the interference constraints imposed by the incumbent FS receivers. We investigate and analyze a set of optimization frameworks for the power and rate allocation problem in the considered cognitive satellite scenario. The main objective is to shed some light on this rather unexplored scenario and demonstrate feasibility of the terrestrial-satellite co-existence. In particular, we formulate a multi-objective optimization problem where the rates of the satellite terminals form the objective vector and derive a general iterative framework which provides a Pareto-optimal solution. Next, we transform the multi-objective optimization problem into different single-objective optimization problems, focusing on popular figures of merit such as the sum-rate or the rate fairness. Supporting results based on numerical simulations are provided which compare the different proposed approaches.

This paper considers a multigateway multibeam satellite system with multiple feeds per beam. In these systems, each gateway serves a set of beams (cluster) so that the overall data traffic is generated at different geographical areas. Full frequency reuse among beams is considered so that interference mitigation techniques are mandatory. Precisely, this paper aims at designing the precoding scheme which, in contrast to single gateway schemes, entails two main challenges. First, the precoding matrix shall be separated into feed groups assigned to each gateway. Second, complete channel state information (CSI) is required at each gateway, leading to a large communication overhead. In order to solve these problems, a design based on a regularized singular value block decomposition of the channel matrix is presented so that both intercluster (i.e., beams of different clusters) and intracluster (i.e., beams of the same cluster) interference is minimized. In addition, different gateway cooperative schemes are analyzed in order to keep the inter-gateway communication low. Furthermore, the impact of the feeder link interference (i.e., interference between different feeder links) is analyzed and it is shown both numerically and analytically that the system performance is reduced severely whenever this interference occurs even though precoding reverts this additional interference. Finally, multicast transmission is also considered. Numerical simulations are shown considering the latest fixed broadband communication standard DVB-S2X so that the quantized feedback effect is evaluated. The proposed precoding technique results to achieve a performance close to the single gateway operation even when the cooperation among gateways is low.

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 ST, 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 ST, 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 presents the novel application of polarized modulation (PMod) for increasing the throughput in mobile satellite transmissions. One of the major drawbacks in mobile satellite communications is the fact that the power budget is often restrictive, making it unaffordable to improve the spectral efficiency without an increment of transmitted power. By using dual polarized antennas in the transmitter and receiver, the PMod technique achieves an improvement in throughput of up to 100% with respect to existing deployments, with an increase of less than 1 dB at low Eb/N0regime. Additionally, the proposed scheme implies minimum hardware modifications with respect to the existing dual polarized systems and does not require additional channel state information at the transmitter; thus it can be used in current deployments. Demodulation (i.e., detection and decoding) alternatives, with different processing complexity and performance, are studied. The results are validated in a typical mobile interactive scenario, the newest version of TS 102 744 standard [Broadband Global Area Network (BGAN)], which aims to provide interactive mobile satellite communications.

In this paper, we consider Analog Network Coding (ANC) in the Multiple Access Relay Channel (MARC) with multiple relays, and provide the following three-fold contribution: 1) we introduce a tractable mathematical framework for computing the Symbol Error Rate (SER) of Maximum-Likelihood (ML), Zero-Forcing (ZF), and Minimum Mean Square Error (MMSE) receivers; 2) by capitalizing on this tractable mathematical framework, we formulate a power allocation problem that is proved to be convex for ML, ZF and MMSE receivers; and 3) we provide closed-form expressions of the optimal power to be allocated to the sources and the relays for ZF and MMSE receivers. With the aid of Monte Carlo simulations, we validate the accuracy of the proposed mathematical framework for various network topologies and channel conditions, as well as study the effectiveness of optimal power allocation. It is shown, in particular, that power optimization is beneficial as the number of sources increases and if the quality of the source-relay links is better than the quality of the relay-destination links.

In this paper, we examine multi-source multi-relay systems that employ Analog Network Coding for which we provide a two-fold contribution: i) We derive a closed-form upper bound of the average symbol error rate (SER) per source of the system, which is shown to be tight in the high-Signal-to-Noise Ratio (SNR) region, especially for an adequate number of relays, and ii) based on this bound, for a given total power budget to be distributed among the source and relay nodes we formulate the power allocation optimization problem with the aim of minimizing the SER per source. Results show that for a common target SER among the sources, an increase in their number results in an increase in the expected energy gains over the equal power allocation for all the nodes policy.

Next generation high throughput satellite systems are expected to rely on the deployment of a high number of beams in the coverage area. To this end, the payload is equipped with an array fed reflector antenna whose elements/feeds are more than the number of users. Under this context, as the number of beam increases the ground and space units must employ an extensive communication effort on the feeder link (i.e. the link between gateway and satellite) to exchange all of the feed signals yet maintaining a low processing responsibility to the payload. Since the feeder link spectral resources are scarce, it is desirable to use an on-board beam generation process which leads to a significant feeder link bandwidth reduction. In this context, a space/ground processing (i.e. both the gateway and the satellite digitally process the transmitted/received data) must be employed. This implies to implement an on-board beam generation process and, when a full frequency pattern among beams is used, on-ground linear processing techniques are required in order to reduce inter user/beam interference. In this paper, we restrict ourselves to design an on-board beam generation process in the forward link of a space/ground architecture satellite system. Moreover, we focus on linear minimum mean square error precoding strategy at a ingle gateway architecture. In addition, in order to more reduce payload complexity the beam generation process is assumed to be non channel adaptive (fixed) even if the user link (i.e. the link between satellite and users) channel appears as a random variable. This is done by means of considering robust optimization approach. Then, our design is evaluated both analytically and numerically, and it outperforms the current beam generation process. The provided simulation results quantify the performance gain obtained by our method.

In this paper, we provide a theoretical analysis of the optimal Maximum-Likelihood (ML) detector for the recently proposed multistream Spatial Modulation (SM) concept for multiple-input-multiple-output (MIMO) systems. A Union Bound that is based on closed-form formulas is derived for the average bit error probability (ABEP) of the ML detection, which shows an excellent fit with respect to Monte Carlo simulations for the high signal-to-noise ratio (SNR) region. Furthermore, numerical results for different rates are provided regarding the comparison of the ABEP of multistream SM with ML detection with its recently proposed counterpart with suboptimal detection and with the ABEPs of other well-known MIMO schemes, such as conventional Spatial Multiplexing, Alamouti, and SM, which show its advantage over them. In addition, for the examined MIMO configuration and rates, the relative transmit energy efficiency gain of multistream SM with ML detection over the aforementioned methods is calculated for a low target ABEP.

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.

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.

In this paper, we propose a closed-loop and single RF multiple-input-multiple-output (MIMO) method, particularly suitable for the Uplink of cellular systems, which is based on the low-complexity and recently devised method of Spatial Modulation. By selecting a subset of the available transmit antennas to implement Spatial Modulation based on the instantaneous Bit Error Rate (BER), the proposed method achieves both multiplexing and transmit-diversity gains by utilizing only one RF chain. By taking into account the total power consumption at the transmit side, we numerically show that our proposed method is more energy efficient than the single RF transmit antenna selection method for the same target performance and several MIMO configurations and correlation values among the transmit antennas.

In this paper we address the problem of margin adaptive scheduling in the downlink of an orthogonal frequency division multiple access (OFDMA) multiple-input multiple-output (MIMO) system. Optimal resource allocation in MIMO systems requires the joint optimization of: a) linear transmit and receive spatial filters, b) channel assignment and c) power allocation. This problem is not convex and its complexity becomes thus intractable already for small sets of users and subcarriers. To reduce the complexity of the problem at hand, we propose a novel heuristic strategy that partitions the users in different groups according to their average channel quality and addresses the original problem by solving a succession of lower-complexity allocation problems. The spatial dimension is employed to prevent multiple access interference from hindering the performance of the sequential allocation. To further reduce the complexity burden we introduce a linear programming formulation in combination with a waterfilling-based strategy to allocate channels and power to the groups of users. Numerical results and evaluation of the computational complexity show that, though suboptimal, in most cases the proposed algorithm manages to exploit in an original way the inherent multi-user diversity of multi-carrier systems to ease the task of resource allocation with a very limited performance loss from the theoretic optimum.

Over the last years, cooperative communications have been studied from both fundamental and practical points of view. However, most of the existing studies of cooperative communications rely on math or computer simulation. In this paper, we describe the implementation of a cooperative communication strategy for wireless networks in a testbed based on Click Modular Router. We describe the methodology we have followed to reprogram the drivers controlling the data layer functions of off-the-shelf commercial Wireless Network Interface Cards (NICs). More precisely, we have implemented a Cooperative Automatic Retransmission (C-ARQ) scheme to show that the performance of wireless communications can be boosted by using an intermediate relay when the wireless channel between the source and the destination is in bad conditions. The results presented in this paper are promising, as they show that previous theory and simulation results presented in the literature can become true in a real implementation.

This paper proposes a transmit beamforming technique to cope with the problem of the Multiple Input Single Output Interference Channel (MISO IC). In this scenario, the achievable sum-rate is highly conditioned by the inter-user interference. Furthermore, optimizing the sum-rate is known to be a cumbersome problem. In order to deal with this problem, we approximate the sum-rate when the desired received signal power level is higher than the multiuser interference power level (i.e. when the Signal-to-Interference Ratio (SIR) is high). For that case, the sum-rate expression can be optimized distributively and it leads to a transmit beamformer that outperforms the existing decentralized techniques when the channel gains are asymmetric. Numerical simulations show the performance of the proposed technique which improves the existing decentralized designs for the MISO IC.

This paper addresses the problem of transmit beam-forming in open spectrum communication systems. In these scenarios, the operator or the data communication provider delivers a set of base stations to the final users under no radio planning strategy. In order to deal with such an uncoordinated communication system, this work presents a centralized algorithm for calculating the optimal precoding strategy the via semidefinite relaxation technique. The problem to be solved is a novel design of minimum norm beamformers under QoS and received power constraints. Numerical results will show the performance of the method.

Generalized Space Shift Keying (GSSK) modulation is a recently proposed low-complexity concept for Multiple-Input- Multiple-Output (MIMO) wireless systems. GSSK modulation is a generalized version of Space Shift Keying (SSK) modulation, which provides a better spectral efficiency through multiple active antennas at the transmitter. An apparent weakness of GSSK modulation is that it does not exploit the transmit-antennas to achieve transmit-diversity. In this paper, we propose a precoding method for GSSK modulation, which simultaneously achieves high diversity and coding gains. The solution is based on: i) \emph{co-phasing} the active antennas of each spatial-constellation point; and ii) properly \emph{rotating} the phases among spatial-constellation points. The new scheme requires Channel State Information at the Transmitter (CSIT), i.e., the channel phases of each wireless link, which can be obtained through a feedback channel. For the case of a perfect feedback channel, we analytically show that for three and four antennas at the transmitter a full transmit diversity can be achieved without reducing the achievable rate. Furthermore, for various MISO configurations and achievable rates we show through Monte Carlo simulations that our proposed scheme outperforms state-of-the-art open- loop GSSK schemes, in terms of both diversity and coding gain, when the number of bits allocated for the quantization of each channel phase is between 2 and 4.

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.

Signal processing in satellite applications is usually performed either on-ground or on-board, i.e. at the gateway station or in the payload. Within the framework of the European Space Agency (ESA) SatNEx III study, a hybrid approach has been considered by splitting the processing between the satellite and the gateway, aiming to strike a better balance between performance and payload complexity. The design of a high-capacity multi-beam system has been carried out, to assess the potential applicability of a hybrid space-ground processing architecture (Digisat) for satellite broadband systems; this is achieved via hybrid space-ground beamforming, MIMO and MIMO-MUD, Precoding, as well as Digital Feeder link techniques.

Next generation high throughput satellite systems are expected to rely on the deployment of a high number of beams in the user link, and therefore operate in an interference limited regime. This paper builds on the combination of advanced interference mitigation techniques and ground based beamforming to cope with this increased level of interference among users. Focussing on the forward link of a multi-beam broadband satellite system, we consider the joint design of linear precoding and ground based beamforming at the gateway. This joint design is modeled, and different linear precoding techniques are considered for comparison. The provided simulation results quantify the performance gain generated by this joint design with respect to considering precoding for a fixed on-board beamforming. The robustness of the considered scheme to channel estimation errors is also analyzed.

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.

Radio Resource Management with Inter-cell Interference Coordination (ICIC), is a key issue under investigation for next generation wireless systems such as Long Term Evolution (LTE). Although centralized resource allocation (RA) and collaborative processing can optimally perform ICIC, the overall required complexity suggests the consideration of distributed techniques. In this paper we propose and compare a centralized RA strategy aimed at maximizing the sum-rate of a multi-cell clustered system in presence of power and fairness constraints, and a distributed RA strategy where inter-cell interference is partially coordinated through power planning schemes with preassigned power (an example is fractional frequency reuse). The distributed RA strategy reduces both signaling and feedback requirements, preserves intra-cell fairness and jointly works with a load balancing algorithm to support inter-cell fairness. We show in the results that the distributed RA with aggressive frequency reuse is able to approach the performance of the centralized RA when the number of users is large, while preserving the same level of fairness.

In this paper, we describe GREENET (an early stage training network in enabling technologies for green radio), which is a new project recently funded by the European Commission under the auspices of the 2010 Marie Curie People Programme. Through the recruitment and personalized training of 17 Early Stage Researchers (ESRs), in GREENET we are committed to the development of new disruptive technologies to address all aspects of energy efficiency in wireless networks, from the user devices to the core network infrastructure, along with the ways the devices and equipment interact with one another. Novel techniques at the physical, link, and network layers to reduce the energy consumption and carbon footprint of 4G devices will be investigated, such as Spatial Modulation (SM) for Multiple-Input-Multiple-Output (MIMO) systems, Cooperative Automatic Repeat reQuest (C-ARQ) protocols, and Network Coding (NC) for lossy networks. Furthermore, cooperation and cognition paradigms will be exploited as additional assets to improve the energy efficiency of wireless networks with the challenging but indispensable constraint of optimizing the system capacity without degrading the user's Quality-of-Service (QoS).

Physical-layer network coding over wireless networks can provide considerable throughput gains with respect to traditional cooperative relaying strategies at no loss of diversity gain. In this paper, a novel cooperation protocol is developed based on complex-field wireless network coding. Sources transmit efficiently information symbols linearly combined with symbols from other sources. Different from existing wireless network coding protocols, transmissions are not restricted to binary symbols, and do not have to be received simultaneously. In a network with N sources, the developed protocol can achieve throughput up to approximately 1/N symbols per source per channel use, as well as diversity of order N. To deal with decoding errors at sources, selective- and adaptive-forwarding protocols are also developed at no loss of diversity gain. Analytical results corroborated by simulated tests show considerable performance gains with respect to distributed space-time coding, and bit-level network coding protocols.

This Letter addresses the statistical modeling of dual-polarized MIMO-LMS fading channels. In the absence of accurate experimental results, a statistical model for the characterization of MIMO-LMS channels is proposed based on consolidation of available experimental results for SISO-LMS and MIMO wireless channels as well as on their extrapolation to the MIMO-LMS case of interest. Moreover, a step-by-step methodology for the simulation and time-series generation of the proposed MIMO-LMS channel model is provided, which is useful for the design and performance assessment of MIMO-LMS transmission systems. The proposed model incorporates the effects of all relevant critical channel aspects in a flexible and fully-parameterized way.

This paper addresses the energy efficiency analysis of the relay channel under additive white Gaussian noise. We consider the rate bounds given by decode and forward and the cut set bound and assume that resources are optimally allocated to maximize the spectral efficiency according to the channel information and the sum network energy. The low energy analysis tools are used to compute the maximum rate per energy (RPE) and the slope of the spectral efficiency as a function of the energy per bit. Using these metrics, the energy efficiency benefit of several capabilities at terminals is investigated. Specifically, we take into account: (i) the phase synchronization between transmitters, (ii) the full duplex capability at the relay and (iii) the channel access via superposition.

In this paper, we address the problem of user scheduling in downlink coordinated multi-point transmission (CoMP) systems, where multiple users are selected and then served with zero forcing beamformer simultaneously by several cooperative base stations (BSs). To reduce the enormous overhead led by obtaining full channel state information at the transmitter, a low-feedback user scheduling method called channel norm-based user scheduler (NUS), is proposed by exploiting the asymmetric channel feature of CoMP systems. Simulation results show that the channel norm provides sufficient information for user scheduling when each BS has one antenna, where the performance gap between the NUS and the greedy user selection (GUS) is negligible with respect to both the cell average throughput and the cell edge throughput. When each BS has multiple antennas, NUS is inferior to GUS, but still significantly outperforms the uncoordinated systems.

This paper provides details and performance evaluation of the implementation of the Persistent Relay Carrier Sensing Multiple Access (PRCSMA) protocol for Cooperative ARQ (C-ARQ) scenarios using off-the-shelf wireless cards. The underlying idea of PRCSMA is to modify the basic rules of the IEEE 802.11 MAC protocol to execute a distributed C-ARQ scheme in wireless networks in order to enhance their performance and to extend coverage. For the implementation, we modify the HostAP driver for off-the-shelf air-interfaces. HostAP is a Linux based driver for 802.11b WLAN cards based on Intersil's Prism 2.5. Performance evaluation tests provide proof-of-concept of the efficiency, in terms of mean delay, of the PRCSMA in realistic conditions.

A multisource cooperative protocol is developed capable of achieving diversity order up to the number of cooperating users at a high throughput. In this design each source jointly encodes its own new information symbol with the information symbols received from other sources at past instants. Joint encoding is done using linear complex-field coefficients. Throughput analysis shows gains with respect to existing multi-source protocols and approaches the throughput of non-cooperative schemes. Diversity analysis shows that full spatial diversity is achievable. Simulations confirm the analytically established assessments.

A cross-layer based dynamically tuned queue length scheduler is presented in this paper, for the Downlink of multiuser and multiantenna WLAN systems with heterogeneous traffic requirements. An opportunistic scheduling algorithm is applied, while real time classes are prioritized. A trade-off between the throughput maximization of the system and the guarantee of the QoS requirements is obtained. Therefore the length of the queue is dynamically tuned to select the appropriate conditions based on the operator requirements.

Scheduling in a downlink channel based on partial channel state information at the transmitter, is carried out through a multibeam opportunistic beamforming technique. Within a more practical perspective, this paper first presents a transmission strategy where a minimum rate per user is required, which in a wireless fading scenario, can be only guaranteed under a certain system outage constraint. An automatic repeat request (ARQ) is proposed to combat the system outage, so that it increases the system reliability, which stands as a possible Quality of Service (QoS) indicator for the system behaviour. Closed form expressions for the ARQ gain are obtained together with its maximum scheduling delay performance. The derived expressions are then tested via simulations in several transmission scenarios.

This paper investigates the impact of usual practical constraints for the relay channel in the energy efficient regime. Specifically, we take into account constraints on the transmission/reception duplexing and on the synchronization between the source and the relay. New and already known bounds for the maximum rate-per-energy that depend on the constraints considered, are presented in a unified manner. Lower bounds are with decode-and-forward and upper-bounds with the cut-set bound. It is shown that while synchronism provides gains on the maximum RPE, duplexing does not. To see the gain by duplexing the slope of the spectral efficiency with respect the energy-per-bit is also derived.

Scheduling in a Broadcast (BC) channel based on partial Channel State Information at the Transmitter (CSIT) is carried out in an opportunistic way, where several orthogonal beams are randomly generated at the Base Station transmitter to simultaneously deliver several users with their intended data. The paper presents a power allocation over the transmitting beams, where a minimum rate per user restriction is required for each scheduled user, standing as a potential Quality of Service (QoS) indicator for the system behaviour. However, in practical wireless scenarios the CSIT is imperfect due to non-accurate estimation, so that robust schemes are required to meet the system demands. Based on the allowed system outage in the QoS achievement, different robust power allocation schemes are proposed, which are efficiently solved through convex optimization tools. The presented strategies are later compared via simulations for the different scenarios and the system specifications.

This paper presents two lower bounds and one upper bound on the maximum Rate-Per-Energy (RPE) that can be achieved over the orthogonal, additive white Gaussian noise, multi-hop relay channel. The study of the maximum RPE for relay networks not only determines the energy efficiency of a communication system but can also define efficient energy allocation, relay selection, and routing solutions. For the three bounds studied here, these solutions have an attractive distributed structure. The optimal relaying strategy remains unknown, even for the most simple one-relay network and, thus, only bounds on the maximum RPE can be obtained. Here, lower bounds are obtained by considering relays that decode a previously transmitted message (regenerative) from just one of its received signals (non-accumulative). The first lower bound is obtained from the analysis of the traditional multi-hop network, where each relay is required to decode and retransmit the complete source message. Then, this lower bound is tightened, by considering relays that, instead of trying to retransmit the source message, facilitate the transmission between the previous relay and the destination. The destination decodes the source message by using every transmitted signal. Finally, an upper bound valid for any relaying strategy is derived by solving the max-flow min-cut bound.

Scheduling in a broadcast channel based on partial channel-state-information at the transmitter is carried out in an opportunistic way. In this paper, the number of generated beams is larger than the number of available base station antennas, where the beams are obtained through Grassmannian line packing to guarantee the largest orthogonality among them. Approximate expressions and bounds are derived for the sum capacity of the proposed approach together with its scaling law. For practical systems, we show that this scheme can increase the number of serviced users under a minimum-rate-per-user requirement.

In this paper, the multi-hop relay network with transmissions over orthogonal AWGN channels is optimized in terms of maximum rate-per-unit-energy. For such a scenario, the optimal regenerative-forwarding strategy is still unknown. Furthermore, among the known ones any outperforms, in all situations, the others. The results for four different cases and discussion on the energy allocation and routing requirements are provided. First, as a comparative example, a lower bound is obtained by considering the traditional multi-hop relaying network where all communication are point-to-point reliable. The solution for this scenario is shown to have good distributed properties. Then, an accumulative strategy which is known to achieve the capacity forallcastcommunication is solved. In this case, the solution requires exhaustive computation. To maintain the point-to-point distributed properties a unicast strategy is investigated, with relays not decoding the source message but a bin index transmitted by the previous relay. Finally, an upper- bound on the above regenerative techniques is derived and analyzed.

Spatial scheduling in a heterogeneous WiMax Downlink channel, based on partial channel state information at the transmitter (CSIT), is carried out through a multibeam opportunistic beamforming technique. In contrast to existing literature, this paper considers a more practical perspective and presents a strategy where a minimum rate per user is required, which can only be guaranteed under a certain system outage restriction. Within this scenario, the paper then formulates the minimum rate and maximum/mean delay expressions in closed form expressions. Furthermore, in opportunistic schemes a high number of users is required to improve the sum rate system performance, but for practical considerations, a large number of available users induces a large service delay. A dynamic call admission control (CAC) is then proposed to regulate the number of users, and to guarantee the service to all the users in the cell under rate and delay requirements. The derived expressions are then tested via simulations in several transmission scenarios, where users running delay-constrained (DC) and delay-tolerant (DT) applications coexist.

Scheduling in a broadcast (BC) channel based on partial channel state information at the transmitter (CSIT) is carried out in an opportunistic way, where several orthogonal beams are randomly generated at the base station transmitter to simultaneously deliver several users with their intended data. Within a more practical perspective of the opportunistic systems, this paper first presents a transmission scheme where a minimum rate per user restriction is required for each scheduled user. This minimum rate is demanded by each user to properly decode and manage its received signal, which stands as a possible Quality of Service (QoS) indicator for the system behaviour. Then, the paper considers an imperfect CSIT situation where robust schemes are required to meet the QoS restrictions. Two robust opportunistic transmission philosophies are presented through a power allocation over the transmitting beams, and they are efficiently solved via convex optimization tools.

We consider a cooperative network where the communication between a single source-destination pair is assisted by the transmission of relays. In the considered cooperative model, once a node requires cooperation, it broadcasts a claim for cooperation message. Under high mobility conditions or in some special scenarios this is the natural procedure to discover the potential relays. However, when there is a large number of nodes that can potentially serve as a relay, a Multiple Relay Access Control (MRAC) problem turns up and the relay transmissions must be organized in an effective manner. We introduce this problematic and propose three different strategies to manage this severe competitive channel. In these strategies either the source, the destination or both trigger the cooperative process. The methodology employed makes the results and conclusions valid for a wide range of cooperative scenarios, and hence, useful for a wide range of applications.

Traditional works rely on the use of complete channel state information (CSI) at the base station to take space-time scheduling decisions in the uplink of multi-user MIMO wireless systems. However, this might be a complex and costly solution. On the other hand, the simplest scheduler is the Round Robin (RR) scheduler that schedules terminals one after the other without using CSI. However, when multiple antennas are used, many terminals could be simultaneously scheduled because of the additional spatial dimension. The optimal number of terminals to be scheduled by a space-time scheduler with no CSI in a multi-user SIMO system is investigated in this paper. Furthermore, we also study how spatial diversity and multiplexing gains are translated into throughput gains. In particular, when a ZF receiver is used, we show that the benefits of using multiple antennas increase with the minimum SNIR requirements and that, when the number of antennas is much higher than the number of terminals, the throughput gains due to multiplexing increase linearly with the number of terminals and exponentially with the minimum SNIR requirements.

Scheduling in a broadcast (BC) channel based on partial channel state information at the transmitter (CSIT) is carried out through a multibeam opportunistic beamforming technique. Within a more practical perspective, this paper first presents a transmission strategy where a minimum rate per user is required, which in a wireless fading scenario, can only be guaranteed under a certain system outage constraint. This minimum rate is demanded within a given time interval to satisfy maximum delay restrictions for the user application. Closed form expressions for maximum scheduling delay and maximum jitter are obtained, standing as possible quality of service (QoS) indicators for the system behaviour. The derived expressions are then tested via simulations in several transmission scenarios.

This paper provides useful insights into the practical design of bit allocation algorithms in multiantenna multiuser orthogonal frequency-division multiplexing (OFDM) systems. With the degrees of freedom obtained with multiple antennas and multiple subcarriers, the performance might be enhanced at the expense of a higher complexity. Since the scheduling with realistic integer mappings is an NP-complete combinatorial problem, suboptimum solutions based on the scalar product are shown to be good candidates for yielding a realizable scheduler at a wireless physical layer. Additionally, a power reuse strategy is proposed to lower the computational requirements of such a system. Besides the tradeoff between performance and complexity, there exists the tradeoff between performance and signaling. Therefore, we show that the required signaling might be reduced either by a user-subcarrier clustering or by using a scheme that forces an equal mapping for all the users at the same subcarrier, which might be well-suited if instantaneous fairness is required. The proposed strategies are evaluated for typical OFDM-based wireless LAN scenarios.

In this letter, we study spatial Tomlinson-Harashima precoding (STHP) from an information theoretic point of view encompassing both the single and the multiuser scenarios. Since the performance of any communications preceding scheme relies on the quality of the channel state information (CSI) that is made available at the transmitter side, in this work we focus our attention on the analysis of the robustness of STHP in terms of rate loss when the CSI is imperfect. Precisely, we present a robust design of the set of moduli used in STHP and, consequently, of the corresponding power allocation, that maximizes the achievable rates for the worst-case errors in the CSI in the small errors regime.

Multi-user diversity (MUD) scheduling is based on the idea that access to the channel should be given to users with better channel state. In a system with many users whose channels vary independently, such strategy maximizes the probability that the channel of the scheduled user is near its peak. Consequently, the total ergodic capacity of the system is maximized. However, from a network point of view, performance is usually evaluated in terms of average throughput and average delay rather than ergodic capacity and fairness. Furthermore, information-theoretic rates are not achievable at the PHY layer and more practical solutions must be considered. In this paper the MUD problem is approached from a MAC layer perspective and the throughput maximal MUD scheduling policy, i.e., the policy that maximizes the total average throughput is investigated. We also study the average throughput region and show the suitability of optimal information-theoretic approaches in practical environments. The average throughput region is also contemplated in wireless networks where Heterogeneous multi-user diversity (HMUD) can be exploited. Finally, the average throughput region is related to the stability region and some average delay bounds for different ARQ mechanisms are provided.

In this paper, the problem of resource allocation in multiuser single-antenna wideband OFDM(A) systems is considered from a cross-layer point of view. The main motivation is to show advantages of such systems with respect to narrowband systems. Despite the maximum normalized average throughput is not increased with respect to that in a narrowband system, a more efficient use of resources is possible by considering frequency as an additional resource to be allocated. It is shown that when the bandwidth is considerably larger than the coherence bandwidth of the channel and the channel is varying slowly with respect to the scheduling period, average delay can significally be improved. In that case, the average delay is only proportional to the scheduling period and not to the channel coherence time (as it is the case in narrowband systems). Furthermore, different reasource allocation policies are analyzed to show average delay improvements when buffer occupancy information is used.

In this paper repetition-based cooperative diversity technique has been taken into consideration. For this technique there exists a trade off between the gains achieved by diversity reception and the decrease in bandwidth efficiency. Both effects have a hard dependence on the number of cooperating terminals. Here an analysis based on the average bit error rate (BER) of multilevel signaling MQAM modulation is presented to characterize the optimal achievable end-to-end throughput. In this way it is shown that threshold decisions can be taken to find out the optimal choice for the number of relays and modulation size. Thanks to that, efficient protocols can be designed using this information.

It has been shown that the cooperation among nodes can improve the performance of a network under certain considerations. So far, research focus has been mainly put on cooperation and not on the coordination required to obtain this cooperation. In this paper we discuss how the MAC layer plays a key role in determining the effectiveness of a cooperative orthogonal multiple relay channel. We propose and analyze the performance of a novel MAC protocol based on the legacy IEEE 802.11 standard in a representative and general study case scenario. The main conclusion of the study is that a proper selection of the MAC protocol being used in a cooperative system is highly relevant in order to actually achieve the benefits of this kind of systems

In multi-user communications, the access point (AP) has several alternatives for distributing the scarce resources among users. Since there exists a trade-off between the global performance and the individual needs, an analytical framework to study fairness is derived, which completes the scope given by the existing fairness indexes in the literature. The framework proposed in this paper is a way to interpret fairness that has been inspired by portfolio selection; basically, it analyzes the mean vs. standard deviation trade-off. In this work, the target application is a multi-antenna AP transmitting simultaneously to several single-antenna terminals, although this framework is valid to analyze other procedures in multi-user communications.

Given a zero forcing transmit beamforming, we focus on how the multi-antenna access point distributes the scarce resource (power) among the single-antenna terminals. Since there is a clear trade-off between the satisfaction of the individual needs and the global performance of the cell, several criteria are proposed, ranging from a classical physical layer point of view of capacity (rate) maximization to bit error rate (BER)-based cost functions, which are closer to the second layer of the protocol stack. Between two traditional techniques, namely the uniform power allocation and the equal BER and rate, a new one is proposed, which ultimately provides an intermediate performance. Then, we add BER (or signal to noise ratio) constraints so that the admission control problem has to be solved. Among traditional options, we propose a new mechanism to balance the above-mentioned trade-off between the total performance and the particular user behavior. The results in terms of fairness are presented by a mean vs. variance plot and by the Gini plot.

The performance of any communications precoding scheme relies on the quality of the channel state information that is made available at the transmitter side. In this paper, we study the robustness of spatial Tomlinson-Harashima precoding (STHP) in terms of rate loss when errors are considered in the feedback link. We will evaluate the performance degradation of the uniform power allocation with antenna selection strategy for STHP (which is quasioptimal when there are no feedback errors) and compare it with a robust power allocation designed to maximize the achievable rates for the worst-case feedback noise. It is shown that the performance difference between both approaches is very small. Consequently, uniform power allocation with antenna selection arises as a good solution for STHP even in the presence of feedback errors.

Research on multi-user diversity (MUD) is traditionally carried out from an information-theoretic point of view. Such information-theoretic approaches are feasible in homogeneous systems where all terminals use the same modulation, channel coding, transmission rate, and packet length. In homogeneous systems, the selection of the terminal with highest SNR leads to the highest system goodput. Unfortunately, in heterogeneous systems, choosing the user with highest SNR does not lead to highest system goodput in general. In this paper, we propose a crosslayer approach to the MUD scheduling problem in heterogeneous systems. More specifically, we propose using a maximum expected goodput criterion for MUD scheduling. Furthermore, contrary to the traditional approach where link adaptation is not considered in MUD scheduling studies, we combine MUD scheduling with a link adaptation strategy based on the same maximum expected goodput criterion. Such techniques are analyzed and simulated in a IEEE802.11e environment.

We consider the problem of resource allocation in multiuser single-antenna wideband OFDM(A) systems. The key advantage of such systems with respect to narrow band systems is the possibility of considering frequency as an additional resource to be allocated. Although the maximum normalized average throughput is not increased with respect to that in a narrowband system, a more efficient use of resources is possible especially if the bandwidth is considerably larger than the coherence bandwidth of the channel and the channel is varying slowly with respect to the scheduling updates. This is mainly because randomness in the system is increased by the wideband resources. The work presented in this paper analyzes the effects of bandwidth on the delay characteristics. To this end, the relationship between ergodic information rates, stability and delay in multiuser communications systems is studied and candidate resource allocation policies are presented and simulated

Theoretical investigation have shown that multiple antenna schemes can improve link reliability and provide high spectral efficiency. Space-time block coding (STBC) and vertical Bell Labs layered space time (V-BLAST) systems are capable of meeting these requirements by using multiple antenna strategies,respectively. However, in order to achieve largely improved performance they need precise channel information at the receiver, which is difficult to be guaranteed in some specific scenarios. In this paper, independent component analysis (ICA) technique is exploited to detect the transmitted signals blindly, which needs not the channel estimation. By analyzing the essential structures of STBC and V-BLAST systems, the specific models suitable to ICA are established. The robustness against channel estimation error and flexibility of system design can be acquired by using the ICA based scheme. Simulation results demonstrate the validity of proposed approach and the feasibility of ICA based scheme is also discussed.

The spread spectrum modulation presents a more robust interference rejection capability than non-code–based modulations. However, the increase of the communication services has augmented the power of the interference signals. One example is unlicensed bands such as the 2.4–2.5 GHz one, where the DS-CDMA communications of the IEEE 802.11b standard are interfered with by other Industrial Scientific and Medical (ISM) services. One way of improving the interference robustness of the DS-CDMA receivers is introducing an interference canceler (IC) before the despreading stage. The optimum criterion to estimate or detect the interference is the maximum-a-posteriori (MAP) one. However, it requires a high computational burden and a complete knowledge of the statistical model, which reduce its feasibility. To overcome these drawbacks this article proposes an interference canceler based on Fuzzy Logic that allows (1) suppressing both analog and digital interferences, and (2) easily controlling the complexity versus performance trade-off. In other words, the design of the fuzzy canceler follows a down/top strategy; that is, a first low complex design is possible just departing from linguistic information and, in a second stage, we refine it, incorporating the available statistical information into its structure. Finally, to assess the proposed fuzzy canceler we have derived asymptotic performance bounds. © 2005 Wiley Periodicals, Inc. Int J Int Syst 20: 253–274, 2005.

In this paper, we present a unified view of both, PHY and MAC layer visions for a multiple input multiple output (MIMO) multiple access channel. First, we evaluate the impact in terms of throughput of a MAC scheduling strategy, based on throughput maximization and compare it with a PHY scheduling strategy based on capacity maximization. Second, a MAC scheduler with QoS requirements is presented. The particularity of such a MAC scheduler is that it combines PHY level instantaneous optimization with MAC level average maximization. We present mathematical expressions to evaluate the schedulers. We also present our design as an extension of the multiuser diversity scheduler for the multi-user detector case.

At a multi-antenna base station, the multiple antennas are used to enhance the scheduling task. After applying a zero forcing transmit beamforming, the scheduling distributes the resources among the users. Several criteria are presented and their performance in the high signal-to-noise ratio (SNR) range is analyzed. The next issue is to take into account the SNR requirements from the users and perform admission control accordingly. An algorithm that yields the optimum performance is proposed, together with a new criterion that falls between the optimization of the best global performance and the satisfaction of the individual needs. Simulation results are provided to show the performance of the techniques.

We address the problem of the design of a transmitter in a multi-input-multi-output (MIMO) space diversity system. We consider a design which is robust to errors in the channel estimate available at the transmitter. The design is based on the minimization of the necessary transmit power while guaranteeing a minimum and specified quality of service (QoS) in terms of signal to noise ratio (SNR) with a certain probability. According to this objective, the transmitter exploits all the eigenmodes of the MIMO channel estimate, distributing the transmit power in a robust way among the channel modes by using orthonormal temporal signatures. In order to guarantee a full rate transmission, the orthonormal temporal signatures designed for real constellations in orthogonal space-time block codes (OSTBC) are used, leading to a system structure based on a combination of OSTBC and orthogonal weighted beamforming.

Orthogonal frequency division multiplexing (OFDM) has been recently established for several systems such as HiperLAN/2 and Digital video/audio broadcasting, due the easy implementation of the modulator/demodulator and the equalizer. Moreover, also increasing interest is currently being put on multiple-input multiple-output (MIMO) channels, based on the use of antenna arrays at both the transmitter and the receiver. Here, we propose two joint beamforming strategies of low computational load for systems combining OFDM and MIMO. The ultimate objective is the maximization of the signal-to-noise and interference ratio (SNIR) over the carriers subject to a total transmit power constraint. Specifically, the maximization of the harmonic SNIR mean and the minimum SNIR over the subcarriers are proposed. The asymptotic behavior of the proposed methods is analyzed to provide a complete comparative and general view of the most relevant and already known transmit power allocation strategies. Finally, a theoretical analysis of the performance degradation of these techniques is carried out for the case in which the channel state information (CSI) is not perfect. Monte Carlo simulation results for the system bit-error rate and performance degradation with imperfect CSI are provided.

The spread spectrum modulation presents a more robust interference rejection capability than the non code-based modulations. DS-SS communications are unlicensed and quite often used to support missions on mobile platforms. In particular, the unlicensed bands such as the Industrial Scientific and Medical (ISM) can seriously saturate the interference rejection of the spread spectrum systems. To overcome this drawback it is possible to use an interference canceller (IC) before the despreading stage. In order to design an optimum IC to suppress analog and digital interferences, we have to implement the Conditional Mean (CM) and the Maximum-A-Posteriori criteria. However, both filters present a high computational burden which reduces their real feasibility. In addition, the communication scenario could be corrupted by Gaussian or/and Non-Gaussian noise, which makes more complex the structure of the optimum detector. This paper proposes an interference canceller based on Fuzzy Logic that allows to suppress either analog or digital interferences and decreases the computational burden of the CM estimator and MAP detector.

This paper addresses the problem of the spatial scheduling of users in a cell for simultaneous downlink transmission from a base statimultiple antennas on (BS) having multiple antennas under a perspective of joint Physical and Medium Access Control (PHY-MAC) design. First of all, we compute the transmit beamvectors for each group according to a zero forcing (ZF) criterion, which gives a simple closed-form solution. We show first that it is equivalent to the minimization of the maximum bit error rate (BER). In this paper, the main contribution lies on the resolution of the NP-complete combinatorial problem that comes up as a cost function if we want to minimize the total transmit power. The solution of the NP-complete problem is performed by the stochastic technique simulated annealing (SA). Additionally, we present two heuristic algorithms that may enable a real-time implementation of this scheduling approach.

In this paper we address the design of a multiple transmit antenna system in which the channel state information (CSI) at the transmitter is not perfect. Two different approaches are analyzed: one based on the minimization of the mean square error (MMSE) and the other based on the application of the maximum likelihood sequence estimation (MLSE). In both cases a Bayesian criterion is used in order to take into account the error between the CSI and the real channel. Finally, some simulation results and conclusions are provided, showing the gain of these approaches when the error between the CSI and the real channel is either Gaussian or uniform, where this last case corresponds to a quantization of the channel time response in order to transmit the CSI through a feedback channel from the receiver to the transmitter.

In this paper we address the problem of the design of a power allocation strategy for an orthogonal frequency division multiplexing (OFDM) system. We assume no spatial diversity at the transmitter and receiver, that is, a single-input-single-output (SISO) channel is considered. The goal is to allocate all the available power at the transmitter among the different carriers according to a minimum mean bit error rate (BER) criterion. When designing this strategy, we assume that channel state information (CSI) is available at the transmitter. Two different approaches are taken. The first one considers that the channel estimates are perfect, whereas the second approach assumes that there is some estimation error, and therefore, this approach is robust in nature. Finally, we propose an efficient algorithm for computing the power allocated to the carriers for both approaches, and show some simulation results in order to compare them.

In ad-hoc radio networks, mechanisms on how to access the radio channel are extremely important in order to impure network efficiency and, when needed, to guarantee QoS. Traditionally, medium access control (MAC) protocols in ad hoc networks have been designed to face off the well known collision resolution problem. However, when using advanced signal processing techniques, general assumptions on collisions and packet loss are no longer valid. Besides, little has been reported about MAC algorithms dealing with multiaccess channels in ad hoc networks. In this paper, we present a novel decentralized multiaccess MAC protocol for ad hoc networks This MAC protocol is a hybrid CDMA-TDMA in which a cross layer approach has been followed to dynamically adapt to the traffic load. Closed expressions for the throughput and delay of the network are presented as a function of the multipacket reception capability of the receiver, the number of codes and the packet retransmission probability.

This paper studies the application of the minimum mean square error (MMSE) beamformer to orthogonal frequency division multiplexing (OFDM)-based wireless local area networks. The questions here addressed are mainly the design with finite-length data and the choice of the OFDM signal domain where the beamformer is applied, either frequency or time. As OFDM signals need more samples than other modulations to stabilize the estimation of the signal statistics, how to exploit the finite-length training sequence provided for the design of equalizers becomes an important issue. The paper also shows that the usual frequency processing in OFDM is not always the best choice for the spatial beamforming, mainly for channels with a very high delay spread. Then, time processing turns out to be the best suited approach in terms of the tradeoff between performance and complexity. Additionally, novel modifications of the MMSE spatial filter are proposed to improve the raw bit-error rate performance: 1) a temporal semiblind approach that exploits the cyclic prefix and 2) windowing in the frequency domain.

Adaptive antenna arrays can be used to improve the performance of Orthogonal Frequency Division Multiplexing systems. They are usually applied in the frequency domain, after the FFT demodulation. In this paper, the selected algorithms are applied in time domain. A blind method is firstly proposed for the maximization of the temporal Signal to Noise plus Interference Ratio (SNIR) in frequency selective channels exploiting the temporal redundancy introduced by the cyclic prefix (CP). This algorithm may cope well with non-stationary interference that appears during the burst transmission. This blind technique is extended so as to take advantage of the preamble that precedes the useful data in the HIPERLAN/2 (HL/2) specification. Simulations show that this latter semiblind approach improves the overall performance of the system with respect to the traditional Sample Matrix Inversion (SMI) algorithm even in interference stationary scenarios. Additionally, a moving pilot structure is proposed to perform more accurate channel estimation, which further improves the performance of the semiblind technique.

Orthogonal Frequency Division Multiplexing (OFDM) has been recently established for several systems such as HIPERLAN/2 and Digital Video and Audio Broadcasting. The success of this modulation is due to the easy implementati9n of the modulator and demodulator, including the equalization process. Moreover, in last years also increasing interest has been put on the study of Multi-Input-Multi-Output (MIMO) channels, based on the use of arrays of antennas at both the transmitter and the receiver. In this paper we propose two new and low computational cost joint beamforming strategies for communication systems that combine OFDM and MIMO channels. An asymptotic analysis of the proposed methods is carried out, providing a complete comparative study among other classical strategies. To demonstrate the potential of the proposed techniques, we present MonteCarlo simulation results of the system BER and make comparisons with other methods.

This paper proposes and analyzes two different spatial filter architectures for the reception of OFDM signals: the classical sample matrix inversion (SMI) algorithm and the' recently proposed Matched Desired Impulse Response (MDIR) beamformer. We derive the asymptotic output signal to interference plus interference ratio (SINR) provided by the two spatial filters and compare their performance in different scenarios. The results are useful in the sense that they provide the asymptotically optimum number of adjacent carriers to be processed by a single beamformer in a finite sample size situation.

The performance of very high data rate wireless local area networks (WLAN) is degraded by several problems such as long delay spread in outdoor environments or interference that may arise both from other WLAN or from other systems sharing the same band. The current standards allow the use of multiple antennas at the receiver in a single input multiple output (SIMO) configuration. Traditionally, algorithms have been applied in the frequency domain, but this paper presents a comparison of three approaches to be applied also in the time domain. It is shown that a significant uncoiled bit error rate (BER) performance improvement can be achieved by working before the FFT at the receiver side, especially for channels with large delay spreads.

The sample matrix inversion (SMI) algorithm has been widely applied in the filed of adaptive antenna arrays. However, in time division multiple access (TDMA) systems with known data sequences, a low cost solution would imply the computation of the beamformer based exclusively on this data. In some cases, this might not be possible since the known data could not be sufficient to get a stable estimate of both the sample covariance matrix and the steering vector needed for the SMI spatial filter. In this paper, windowing techniques are applied in order to calculate a beamformer per subcarrier for orthogonal frequency division multiplexing (OFDM) systems with very limited known training. This approach is compared to the subcarrier grouping approach, and the presented results show that the former method provides a further performance gain, even in channels with high delay spread.

The Bayesian or maximum a posteriori (MAP) symbol-by-symbol detector allows minimum BER single user detection in CDMA systems with less memory requirements than the multiuser maximum likelihood sequence estimator. This work proposes a further complexity reduction by developing a fuzzy recursive implementation of the Bayesian detector. Simulation studies demonstrate the almost optimal performance of the developed fuzzy detector. The resulting system offers a performance v.s. complexity trade-off very appealing for detection in a 3/sup rd/ generation mobile handset.

This work studies the usefulness of fuzzy rank ordering for multiuser code division multiple access (CDMA) detection. The robust performance of the proposed receiver is compared with that of two existing detectors.

The maximum likelihood sequence estimator (MLSE) and the Bayesian detector provide the best performance (at BER sense) under a data sequence or a symbol-by-symbol detection philosophy, respectively. However, these detectors present a high computational burden and are quite unaffordable for a mobile terminal that exploits also the spatial diversity. To overcome this problem fuzzy logic has been applied to design a new recursive space-time detector. Simulation results show that the proposed techniques reduce the computational burden and offer similar performance to that of the optimal one.

The well-known structure of an array combiner along with a maximum likelihood sequence estimator (MLSE) receiver is the basis for the derivation of a space-time processor presenting good properties in terms of co-channel and intersymbol interference rejection. The use of spatial diversity at the receiver front-end together with a scalar MLSE implies a joint design of the spatial combiner and the impulse response for the sequence detector. This is faced using the MMSE criterion under the constraint that the desired user signal power is not cancelled, yielding an impulse response for the sequence detector that is matched to the channel and combiner response. The procedure maximizes the signal-to-noise ratio at the input of the detector and exhibits excellent performance in realistic multipath channels.

The use of spatial diversity at the receiver front-end together with a sequence detector implies a joint design problem of the spatial combiner and the sequence detector impulse response. This joint design is usually faced under the constraint that the impulse response of the sequence detector is matched to the channel plus combiner response. This procedure maximizes the signal to noise ratio at the input of the detector but does not guarantee that the so-called effective signal to noise ratio is maximized. This work presents a procedure that, starting from the matched criteria, faces directly the maximization of the effective signal to noise ratio, yet preserving all the features of the spatial processor in terms of co-channel and high order intersymbol interference rejection.

This work develops a novel dynamic fuzzy logic system that, based on a fuzzy basis function expansion, successfully solves the non-linear problem of narrowband interference prediction and rejection in DS-SS. A fuzzy basis function representation provides a natural framework for combining both numerical and linguistic information in a uniform fashion. The result is a low complexity non-linear adaptive line enhancer, which offers a faster convergence rate and an overall better performance over other well-known non-linear line enhancers.

The use of spatial diversity at the receiver front-end together with a sequence detector implies a joint design problem of the spatial combiner and the sequence detector impulse response. This joint design is usually faced under the constraint that the impulse response of the sequence detector is matched to the channel and combiner response. This procedure maximizes the signal to noise ratio at the input of the detector but, as it is well known, this does not guarantee a minimum probability of error, which is more related to the so-called effective signal to noise ratio. This work presents a procedure that, starting from a simple structure for the space-time receiver aims directly at the maximization of the effective signal to noise ratio, yet preserving all the features of the spatial processor in terms of co-channel and high order intersymbol interference rejection.

An algorithm for the joint tracking of source DOAs and sensor positions is presented to address the problem of DOA tracking in the presence sensor motion. Initial maximum likelihood estimates of source DOAs and sensor positions are refined by Kalman filtering. Spatio-temporally correlated array movement is considered. Source angle dynamics are used to achieve correct data association. The new technique is capable of performing well for the difficult cases of sources that cross in angle, fully coherent sources, as well as sources of identical or vastly different (possibly time-varying) power. Computer simulations show that the approach is robust in the presence of array motion modeling uncertainty and effectively reduces dependence on expensive and possibly unreliable hardware.

This work addresses the problem of spatial reference estimation in mobile scenarios. Novel techniques based on fuzzy logic are introduced to enhance the performance of a tracking system. Specifically, the model-free function approximation capability of fuzzy logic is used to obtain high resolution angle estimates from the spatial spectral density. These estimates are used to improve the resolution of the tracker. To the authors knowledge it is the first time that fuzzy logic is introduced in array spectral estimation. This work also develops a fuzzy controller for acting as an interpolative supervisor of different trackers that apply in different operating conditions of the dynamic nonlinear system. The result is a localization and tracking system that attains a resolution comparable to that of high resolution techniques as the minimum variance or Capon estimator. One of the main features of the proposed technique is its low computational burden. In summary, the presented system supports the expectation of adaptive arrays for obtaining a communication front-end of affordable complexity, developing cost and good performance.

This paper addresses the problem of simultaneous multiuser service. To find a feasible solution to the collision problem, two time reference multibeamformer systems are developed (BPSK TRMB and frequency framing TRMB). Based on a two-stage architecture, they enlarge the potential of existing separating neural networks and the algorithm (EM). Their goal is to perform a low computational simultaneous digital demodulation of users. The results support the frequency framing TRMB as the more robust system, which is able to give service to users in very degradated SNR scenarios; only the decoupling weights and their corresponding High Order Statistics learning is the overhead from a pure superposition of single channel receivers.>