| openaire: EC/HE/101169042/EU//FOCAL

Publisher Copyright: © 2026 IEEE. | openaire: EC/HE/101192080/EU//6G-LEADER

We consider a pull-based status update system consisting of a finite-state Markov source, an energy-harvesting-enabled transceiver, and a sink. Both data and command channels are error-prone. The sink is interested in real-time tracking of the source and sends commands for updates to the transceiver. However, due to noise in the command channel, the transceiver may experiencefalse alarmsormiss-detections. We study the problem of minimizing the long-term time average of a (generic) distortion subject to a constraint on the average number of commands. Due to the pull-based nature of the system, the controller (i.e., sink) has only partial observability of the source state and the available energy at the transceiver. Therefore, we model the problem as a constrained partially observable Markov decision process (C-POMDP), which is then cast as a constrained belief-MDP (C-belief-MDP) problem. The infinite belief-state space makes solving the C-belief-MDP difficult. Thus, we effectively truncate the belief-state space and formulate a finite-state C-belief-MDP problem. We develop a stationary randomized policy (SRP) to solve the problem by employing Lagrangian relaxation, the relative value iteration algorithm (RVIA), and bisection search. Due to the curse of dimensionality in deriving the SRP, we also develop a low-complexity policy applying the drift-plus-penalty method, which transforms the C-belief-MDP problem into a sequence of per-slot optimization problems. Simulation results show the effectiveness of the proposed policies and their superiority compared to baseline policies.

Advancements in high-frequency communication technologies, including millimeter-wave (mmWave), terahertz (THz), and optical wireless bands, play a crucial role in extending wireless connectivity beyond 5 G. These bands provide ultra-wide bandwidths that enable very high data rates, support dense device deployments, and precise positioning. However, their performance critically depends on maintaining clear Line-of-Sight (LoS) conditions, since Non-Line-of-Sight (NLoS) components suffer from strong attenuation and reflection losses. While mmWave links may provide limited connectivity through NLoS reflections, LoS propagation remains the main factor governing the link budget and reliability of the target applications. In contrast, THz and optical wireless links are almost completely blocked by opaque materials, making LoS assurance essential. This paper tackles the issue of LoS coverage by determining the minimum number and optimal placement of Access Points (APs) required to ensure LoS connectivity in stochastic environments with random obstacles. The environment is modeled as a visibility graph whose nodes represent sub-polygons and edges denote visibility overlaps. Using maximal-clique clustering and maximum-clique packing algorithms, the proposed deterministic framework ensures LoS coverage under all realizations within the modeled stochastic ensemble, achieving up to a 50% reduction in the number of required APs while maintaining over 90% LoS coverage for every realization of obstacle locations.

In the last years, it has been shown that nonlinear (NL) orthogonal frequency division multiplexing (OFDM) can outperform linear OFDM because the nonlinear distortion has useful information about the transmitted signals. However, only a maximum likelihood (ML) receiver can exploit this information. Despite its potential, the optimal ML receiver for NL OFDM is highly complex, and its performance is difficult to simulate. Moreover, although some theoretical performance bounds exist for OFDM transmissions involving many subcarriers and high signal-to-noise ratio (SNR), the behavior of NL OFDM under more practical SNR conditions remains insufficiently explored. This paper focuses on optimal detection methods for NL OFDM systems. We present a comprehensive analysis of how the distortion received on different subcarriers contributes to the signal of a specific subcarrier and derive a performance bound for the ML detection of NL OFDM that is applicable across a broad range of SNR values. Furthermore, we propose a practical iterative decision-directed receiver that achieves significant performance improvements over linear OFDM in both uncoded and coded setups.11This work supported by Fundação para a Ciência e Tecnologia and Instituto de Telecomunicações under the project UIDB/50008/2020, DOI:10.54499/UIDB/50008/2020, and Universidade Lusófona.

Optical Wireless Communication (OWC) can complement wireless access services provided over Radio-Frequency (RF) bands by offering abundant additional unlicensed spectrum while enhancing privacy, security, and sustainability. This technology, however, requires Line-of-Sight (LoS) links with no obstructions between transmitter and receiver. In addition, the range of Visible Light Communication (VLC) for wireless access is limited to a few meters when using Light Emitting Diodes (LEDs). Hence, deploying VLC Access Points (APs) to prevent indoor coverage gaps is a challenging task. In this paper, we first characterize the VLC cell range in terms of the three-dimensional (3D) position, orientation, and parameters of the LEDs and the photodetector (PD). Secondly, we propose a graph model to represent the floor plan and, based on it, a clique clustering method that determines the minimum number of APs (and their locations) to ensure LoS coverage. Then, we transform this graph into a connectivity tree, such that the APs can communicate with each other via LoS infrared wireless. Based on our findings, the proposed deployment approach significantly outperforms conventional methods in terms of coverage area and required number of APs, improving the minimum/mean data rate of the VLC network when deployed to give service indoors.

We focus on stochastic geometry analysis of a low Earth orbit (LEO) narrowband terrestrial-satellite uplink with satellite base stations (SBSs) in a uniform constellation equipped with narrow Gaussian beams. The served and interfering omnidirectional user equipments (UEs) are distributed on the Earth’s surface according to a homogeneous Poisson point process (HPPP) with Nakagami faded signals. This study presents a detailed but comprehensive mathematical analysis of several key metrics: the signal-to-interference ratio (SIR), the SIR meta distribution (MD), the signal-to-interference-plus-noise ratio (SINR), and the average throughput. Many results are presented in simple analytical and closed forms containing more insight than the expressions proposed in prior works. The results indicate an optimal UE density depending on the altitude, elevation angle, and the width of the antenna gain, maximizing the average throughput. However, this optimal density leads to a significant variance in the user experience regarding link quality (i.e., the users are not treated fairly).

In 5G, the analog beamforming architecture, typically used in millimeter wave (mmWave) bands, operates on a pre-defined beam set relying on grid-of-beams (GoB) approach. Therefore, efficient beam selection considering dynamic wireless environments incorporating the available measurements from the user equipment (UE) to enhance the overall throughput is of paramount importance. In typical beam selection scheme selecting a beam out of candidate beams would be to choose the beam that has the strongest reference signal received power (RSRP). This approach may not yield the optimal outcome. For example, selected beams might be reserved for UEs that cannot utilize all the available resource blocks (RBs) efficiently. To this end, a deep reinforcement learning (DRL)-based beam selection framework is introduced for maximizing the throughput of the users. We first formulate the proposed beam selection framework by utilizing the RSRP measurements, and time domain activation information of the beams to group the UEs in a way that the time and frequency resources are utilized more efficiently resulting in better observed throughput. The formulated problem is complex as the optimized policy for beam selection is to find a balance between signal quality and RB optimization simultaneously. Therefore, we employ recent advances in DRL to solve the formulated problem with an optimized policy. Specifically, proximal policy optimization (PPO) method incorporating recurrent neural networks (RNNs) for capturing the temporal properties of the wireless channel is exploited for training. Simulations demonstrate that up to 10% throughput gains can be achieved compared with baseline approaches.

This paper proposes a three-dimensional (3D) satellite-terrestrial communication network assisted with reconfigurable intelligent surfaces (RISs). Using stochastic geometry models, we present an original framework to derive tractable yet accurate closed-form expressions for coverage probability and ergodic capacity in the presence of fading. A homogeneous Poisson point process models the satellites on a sphere, while RISs are randomly deployed in a 3D cylindrical region. We consider nonidentical channels that correspond to different RISs and follow the $\kappa-\mu$ fading distribution. We verify the high accuracy of the adopted approach through Monte Carlo simulations and demonstrate the significant improvement in system performance due to using RISs. Furthermore, we comprehensively study the effect of the different system parameters on its performance using the derived analytical expressions, which enable system engineers to predict and optimize the expected downlink coverage and capacity performance analytically.

In this article, we review the key concepts and the progress in the design of physical-layer aspects of in-band full-duplex (IBFD) communications. One of the fundamental challenges in realizing IBFD is self-interference that can be up to 100 dB stronger than signals of interest. Thus, we start by reviewing state-of-the-art research in self-interference cancellation, addressing both model-based and emerging machine learning-based methods. Then, we turn our attention to new wireless systems with many degrees of freedom for which the traditional IBFD designs do not gracefully scale and, hence, require many innovations to enable IBFD. We provide an extensive review of basic concepts and state of the art in massive multiple-input–multiple-output IBFD. Then, we consider the mmWave band IBFD and review advanced physical-layer architectures. The above review provides the proper context to discuss IBFD innovations and new challenges for sixth-generation networks and beyond, where wireless networks are envisioned to be multifunctional, combining communications with functions such as sensing, cognitive radios, physical-layer security, and wireless power transfer. We conclude this article with a status update on the adoption of IBFD in communication standards.

As wireless information transmission (WIT) progresses into its sixth generation (6G), a challenge arises in sustaining terminal operations with limited batteries for Internet-of-Things (IoT) platforms. To address this, wireless power transfer (WPT) emerges as a solution, empowering battery-less infrastructures and enabling nodes to harvest energy for sustainable operations. Thus, the eclectic integration of WPT with WIT mechanisms becomes crucial to mitigate the need for battery replacements while providing secure and reliable communication. A novel protocol that amalgamates WIT and WPT calledInformation Harvesting (IH)has recently been proposed to effectively handle challenges in wireless information and power transfer (WIPT) by employing index modulation (IM) techniques for data communication atop the existing far-field WPT mechanism. This paper presents a unified framework for IM-based IH mechanisms and evaluates their energy harvesting capability, bit error rate (BER), and ergodic secrecy rate (ESR) performance for diverse IM schemes. The findings indicate the significant potential of the IM-based IH mechanism in facilitating reliable data communication within existing far-field WPT systems while underscoring promising refinements in green and secure communication paradigms for next-generation IoT wireless networks.

Considering the current capability in hardware design, the wireless power transmission (WPT) enables the next stage in the current consumer electronics revolution by prolonging the lifetime of devices powered by batteries. Recently, Information Harvesting (IH) introduced a novel mechanism for wireless power and information transfer which differs from existing far-field wireless power transfer and simultaneous transmission of power and data protocols. In this paper, a new IH mechanism which relies on quadrature spatial modulation (QSM) is proposed and the simulation results demonstrate that the proposed mechanism improves the secrecy at information receiver and provide more harvested energy to the harvester.

Wireless Inertial Measurement Units provide motion capture data with a low hardware cost while offering a lot of mobility for the user. The current solutions rely on Wi-Fi or custom radio protocols, which are usually access point-centre having all the traffic routed through a single access point , limiting direct interaction capabilities with the surrounding devices. Bluetooth Low Energy (BLE) forms point-to-point networks directly between two devices without additional networking, thus, a BLE based motion capture suit could enable seamless direct cooperation between two robots or a human and a robot. In this paper, the feasibility of BLE 5 for motion capturing is investigated by designing and testing an implementation of such a motion capture system using existing commercial hardware. More specifically, a mobile phone was utilized as the receiver device for real-time visualization, whereas BLE sensors were used in order to test the feasibility of the technology for actual use and to identify the bottlenecks in the system for future optimizations. The designed prototype system achieved similar or better performance for the static accuracy and delays in the system when compared to the currently existing commercial suits, while providing a feasible throughput for real-time motion capturing.

We studied power splitting-based simultaneous wireless information and power transfer (PS-SWIPT) in multiple access channels (MACs), considering the decoding cost and non-linear energy harvesting (EH) constraints at the receiving nodes to study practical limitations of an EH communication system. Under these restrictions, we formulated and analyzed the achievable rate and maximum departure regions in two well-studied scenarios, i.e., a classical PS-SWIPT MAC and a PS-SWIPT MAC with user cooperation. In the classical PS-SWIPT MAC setting, closed-form expressions for the optimal values of the PS factors are derived for two fundamental decoding schemes: simultaneous decoding and successive interference cancellation. In the PS-SWIPT MAC with user cooperation, the joint optimal power allocation for users as well as the optimal PS factor are derived. This reveals that one decoding scheme outperforms the other in the classical PS-SWIPT MAC, depending on the function type of the decoding cost. Finally, it is shown that the cooperation between users can potentially boost the performance of a PS-SWIPT MAC under decoding cost and non-linear EH constraints. Moreover, effects of the decoding cost functions, non-linear EH model and channel quality between the users are studied, and performance characteristics of the system are discussed.

We propose a Partition-based Visibility (PV) graph modeling to find the minimum number of Visible Light Communications (VLC) nodes and their locations for reliable indoor coverage. VLC network offers a low-cost technology on a license-free spectrum to complement the contemporary mobile network services offered on Radio Frequency (RF) bands. However, VLC suffers from propagation limits: Firstly, in presence of opaque obstacles such as walls, doors, and even curtains, strong link blockage is experienced as the power of reflections is much weaker than the power of the Line-of-Sight (LoS) link. Secondly, the received optical power at users drops as the angle of irradiance between a LED lamp and the user increases, imposing a range constraint on the VLC nodes. So, inspired by the Art Gallery Problem, we optimize the number and the locations of VLC nodes by characterizing the PV graph as a dual presentation of the floor plan and a Maximal Clique Clustering algorithm, which is able to solve not only the art gallery problem but also to extend the approach for the range constrained case.

Full-duplex relaying is an enabling technique of sixth generation (6G) mobile networks, promising tremendous rate and spectral efficiency gains. In order to improve the performance of full-duplex communications, power control is a viable way of avoiding excessive loop interference at the relay. Unfortunately, power control requires channel state information of both source-relay and relay-destination channels, as well as of the loop interference channel, thus resulting in increased overheads. Aiming to offer a low-complexity alternative for power control in full-duplex relay networks, we adopt reward-based learning in the sense of multi-armed bandits. More specifically, we provide bandit-based power control algorithms, relying on acknowledgements/negative-acknowledgements observations by the relay. The proposed algorithms avoid the need for channel state information acquisition and exchange, and can be employed in a distributed manner. Performance evaluation results in terms of outage probability, average throughput and accumulated regret over time highlight an interesting performance-complexity trade-off compared to optimal power control with full channel knowledge and significant performance gains over the cases without power control and random power level selection.

Visible Light Communications (VLC) rely on LED light sources to transmit information wirelessly, offering an alter-native air interface to contemporary mobile networks using Radio Frequency bands for wireless connectivity. Most commercial LEDs found in the market nowadays have been designed for illumination, showing a notable non-linear response when per-forming the Electrical-to-Optical (E/O) conversion, particularly when using deep Intensity Modulation (IM) indexes for increased useful radiated power. Several different OFDM-based waveforms, adapted to the unipolarity requirement that IM demands, have been proposed in the literature. However, the performance of most of these Optical OFDM waveforms is seriously affected by the non-linear magnitude distortion of the low-cost white LEDs, particularly when the Peak-to-Average Power Ratio (PAPR) is high. To tackle this issue, this paper analyzes the performance of Constant-Envelope (CE) OFDM, a waveform that modulates the phase of an Intermediate Frequency (IF) carrier with a real-valued OFDM signal before its E/O conversion. The Bit Error Rate of CE-OFDM is compared to the one of DC-biased Optical OFDM and passband OFDM using a software-defined VLC demonstrator. CE-OFDM allows the VLC system to work in energy-efficient regimes, thanks to its robustness to the non-linear magnitude distortion introduced by the LED.

Millimeter-wave (mmWave) massive multiple input multiple input (MIMO) has shown great potential in user equipment (UE) localization of 5G wireless communication systems. However, mmWave signals usually suffer from non-line-of-sight (NLOS) propagation, which will affect mmWave MIMO-based UE localization performance. Hence, it is non-trivial to reveal how NLOS propagation affect mmWave-based UE localization performance. In this paper, we give a unified analysis framework for UE localization performance gain from harnessing NLOS propagation. Firstly, a closed-form Cramer-Rao lower bound on mmWave MIMO-based UE localization is derived to shed lights on its performance limit. Secondly, NLOS propagation-caused localization error for conventional UE localization methods without harnessing multipath effect is analysed. Finally, the information contribution from NLOS channel is quantified, which sheds light on how to smartly harness NLOS propagation and the associated UE localization performance gain.

In this paper, we are interested in the joint estimate of user equipment (UE) location and orientation for millimeter-wave multi-input-multi-output (mmWave MIMO) systems. In practice, mmWave signals suffer from small-scale fading, which degrades UE localization. Moreover, mmWave MIMO-based UE localization is a non-convex optimization problem, and the bruteforce application of conventional optimization methods will result in a poor solution or lead to large computational cost. In order to address the above challenges, we propose a novel simultaneous localization and channel estimate (SLCE) algorithm, where the UE location parameters and small-scale fading coefficients are jointly optimized. In such a case, the disturbance of small-scale fading on UE localization is alleviated. Thanks to our problem-specific update rule design, the proposed SLCE algorithm achieves a large performance gain over existing baseline methods.

We propose a cell planning method for wireless cellular networks to simultaneously balance the volume shares of the cells and to preserve their regularity. In this framework, we first optimize the base station locations to minimize the coefficient of variation (CoV)-based service demand share metric using the power Voronoi algorithm. Then, we maximize the downlink coverage probability by defining a local regularity optimization problem and solve it through particle swarm, farthest point, average point, and spatial median optimization methods. Finally, we apply the proposed methods on a real network to show how the proposed methods simultaneously minimize both CoV-based service demand share and local regularity metrics.

Visible Light Communication (VLC) systems provide not only illumination and data communications, but also indoor monitoring services if the effect that different events create on the received optical signal is properly tracked. For this purpose, the Channel State Information (CSI) that a VLC receiver computes to equalize the OFDM subcarriers can be reused to train an unsupervised learning classifier. This way, different clusters can be created on the collected CSI data, which could be then mapped into relevant events, such as the position of an object in the sensing area. When compared to supervised learning algorithms, the proposed approach does not need tags in the training data, simplifying notably the implementation. The validation of this monitoring approach was done using a software-defined VLC transmission based on OFDM, in which a reflected copy of the intensity modulated signal coming from a white LED was captured by a pair of photodetectors. The experimental evaluation of the VLC-based monitoring demo achieved a positioning accuracy in the few-centimeter-range, without the necessity of deploying a large number of sensors and/or adding a sensor on the object to-be-tracked.

Estimation methods for passive near-field localization have been studied to an appreciable extent in signal processing research. Such localization methods find use in various applications, for instance in medical imaging. However, methods based on the standard near-field signal model can be inaccurate in real-world applications, due to deficiencies of the model itself and hardware imperfections. It is expected that deep neural network (DNN) based estimation methods trained on the nonideal sensor array signals could outperform the model-driven alternatives. In this work, a DNN based estimator is trained and validated on a set of real world measured data. The series of measurements was conducted with an inexpensive custom built multichannel software-defined radio (SDR) receiver, which makes the nonidealities more prominent. The results show that a DNN based localization estimator clearly outperforms the compared model-driven method.

We propose the use of adaptive optimized irregular constellation to enhance the performance of full duplex (FD) wireless radio systems which suffer from residual self-interference (RSI). Specifically, an error performance expression which takes into account the RSI level and signal-to-noise ratio is first derived for a single FD relay link. During the search for optimal constellation for a given parameter set, there are no predefined assumptions on symbol point locations. The optimization framework yields adaptive irregular constellations depending on RSI levels and power budget values. It is shown that the proposed adaptive transmission along with optimized irregular constellations achieves better error performance than the conventional$M$-QAM constellations and other$M$-ary improper Gaussian signaling (IGS) constellations.

This paper studies a novel way to estimate the position of an object in an indoor environment, using the Channel State Information (CSI) that a Visible Light Communication (VLC) system collects to maintain the link-level connectivity. First, supervised learning is applied to characterize, the effect that an object in variable but known positions has on the received optical wireless signal. Second, the trained classifier is used to estimate the new unknown positions that the object may take, making use of the instantaneous CSI that is used to equalize the data-carrying signal samples in reception. The practical validation of the proposed positioning approach was done with the aid of a software-defined VLC link based on OFDM, in which a copy of the intensity modulated signal coming from a Phosphor-converted LED is captured by Photodetectors (PDs) in different room locations. Then, the CSI of the VLC receiver is used to train a Random Forest classifier, which will predict the position of the object during the assessment phase. The performance evaluation of our experimental setting shows that the proposed VLC-based positioning approach can reach a few centimeter accuracy, provided that a proper training is executed, without the necessity of deploying a large number of PDs in the room, or adding a VLC receiver on the object to be tracked.

The integration of satellite-terrestrial networks is beneficial in terms of the increase of the network capacity and coverage. In such a heterogeneous network, highly efficient spectrum utilization is extremely important. This could be achieved by the single frequency reuse which allows increasing the capacity at the cost of increased interference. Interference is one of the main parameters that limits the link-level performance in such a network. In this paper, we examine the frequency reuse scenario by analyzing the impact of terrestrial interference to the uplink of a low Earth orbiting (LEO) satellite constellation in the high International Mobile Telecommunications (IMT) frequency bands. To this end, we propose a novel stochastic geometry based analytical framework that is able to accommodate various aspects of realistic satellite networks. The accuracy of the analysis is verified by using advanced simulation tools.

We analyze the performance of amplify-and-forward (AF) automatic repeat request (ARQ) for a two-hop cooperative system with reliability constrains. For this setup, we first derive the closed-form outage probability expression. Next, we present a power allocation scheme that allows us to achieve a target outage probability, while minimizing the outage-weighted average power expenditure for asymmetric power allocation between the source and relay. This is cast as an optimization problem, and the optimal power allocation (OPA) is obtained in closed form by invoking the Karush-Kuhn-Tucker (KKT) conditions. We evaluate numerically the OPA strategy between different AFARQ transmission rounds and we show that the proposed scheme provides large power gains with respect to the optimized point-to-point ARQ scheme, as well as with respect to the equal power allocation (EPA) strategy.

We study low-overhead uplink multi-access algorithms for massive Internet-of-Things (IoT) that can exploit the MIMO performance gain. Although MIMO improves system capacity, it usually requires high overhead due to Channel State Information (CSI) feedback, which is unsuitable for IoT. Recently, a Pseudo-Random Beam-Forming (PRBF) scheme was proposed to exploit the MIMO performance gain for uplink IoT access with uniform channel and load, without collecting CSI at the BS. For non-uniform channel and load, new adaptive beamselection and random-access algorithms are needed to efficiently utilize the system capacity with low overhead. Most existing algorithms for a related multi-channel scheduling problem require each node to at least know some information of the queue length of all contending nodes. In contrast, we propose a new Low-overhead Multi-Channel Joint Channel-Assignment and Random-Access (L-MC-JCARA) algorithm that reduces the overhead to be independent of the number of interfering nodes. A key novelty is to let the BS estimate the total backlog in each contention group by only observing the random-access events, so that no queue-length feedback is needed from IoT devices. We prove that L-MC-JCARA can achieve at least `0.24`` of the capacity region of the optimal centralized scheduler for the corresponding multi-channel system.

We study the optimization of base station (BS) localization in cellular networks with non-uniform service demand distribution taking into account a variety of server-side and user-side requirements. To this end, we propose a power Voronoi algorithm (PVA) to obtain uniform traffic volume shares per cell while simultaneously meeting the user-side and server-side requirements of the heterogeneous devices. We describe a numerical method to solve the optimization problem by updating the weights associated to different cells in the PVA. Then, we take into account minimum signal to interference plus noise ratio at all devices as a user-side requirement. It is observed that BSs are concentrated in the areas with condensed demand and high SINR requirements. To show the advantages of the proposed method, the results are compared for different SINR requirement distributions giving insight on the performance of the algorithm.

As low Earth orbit (LEO) satellite communication systems are gaining increasing popularity, new theoretical methodologies are required to investigate such networks' performance at large. This is because deterministic and location-based models that have previously been applied to analyze satellite systems are typically restricted to support simulations only. In this paper, we derive analytical expressions for the downlink coverage probability and average data rate of generic LEO networks, regardless of the actual satellites' locality and their service area geometry. Our solution stems from stochastic geometry, which abstracts the generic networks into uniform binomial point processes. Applying the proposed model, we then study the performance of the networks as a function of key constellation design parameters. Finally, to fit the theoretical modeling more precisely to real deterministic constellations, we introduce the effective number of satellites as a parameter to compensate for the practical uneven distribution of satellites on different latitudes. In addition to deriving exact network performance metrics, the study reveals several guidelines for selecting the design parameters for future massive LEO constellations, e.g., the number of frequency channels and altitude.

A pseudo-random beamforming (PRBF) based random access (RA) system is proposed to enable uplink (UL) machine-type communications (MTC) with ultra low signaling overheads. Specifically, a pseudo random (PR) sequence is used as public information to coordinate the beamforming vectors used at the base station (BS) and the devices. Within the coherence time window, each device distributively determines in advance the ''good'' time slots and receiving beams for transmission. This UL protocol reduces the overheads due to the feedback of channel state information and the control signals for centralized scheduling. This paper derives the throughput and user scaling of the proposed M- PRBF-CA protocol for achieving spatial multiplexing gain, under both an i.i.d. slow fading channel and a correlated slow fading channel. Our simulation results confirm the analysis in both fading channel models.

Cellular vehicle-to-network (V2N) communication will be the backbone of the connected vehicles of future. One of the key requirements of the connected vehicles is a near universal coverage on the streets. Traditional radio network planning for cellular coverage is done with raster format whereby all pixels in a network area have equal weighting. Whereas, for V2N communication the target is to primarily ensure continuous network coverage on the streets. In this work, a route-based methodology which is pertinent for V2N coverage analysis is presented. This method adds another key parameter into consideration, namely, the base stations (BS) deployment schema. Existing cellular networks, whereby, small base stations (BSs) are deployed at the macro BS cell-edge, at traffic hotspots or to compensate for coverage holes, may not be sufficient for V2N coverage especially at millimeter wave (mmWave) carrier frequencies. Herein, we perform the coverage analysis for an existing small BS deployment as well as for an ultra-dense deployment at 2 GHz, 5 GHz and 28 GHz carriers. The statistics for signal-to-noise-plus-interference ratio (SINR) and achieved rate are aggregated by a large number of realistic vehicular routes from Google Directions application programming interface (API).

We consider measures of time-frequency localization (TFL) for stochastic signals. The approach is complementary to the use of TFL in prototype filter design; here, TFL is instead applied to multiplexed waveform packets with the objective to evaluate multi-user interference in a multiple access scenario rather than combat channel dispersion. We show that a generalization of the Heisenberg parameter to N-D stochastic signals directly characterizes the localization of the inter-user interference in the time-frequency phase space. A tight bound is provided, which shows the fundamental tradeoff between the TFL of a packet and the orthogonality among the multiplexed waveforms inside the packet. The Hermite-Gauss waveforms are optimally localized with regard to this measure. We also derive the expressions for the TFL of a Gabor system consisting of Nttime and Nffrequency shifts of a prototype, on the conventional and staggered lattices. In the limit of large N, the particular properties of the prototype yield diminishing returns to the overall localization. Finally, we compare the performance of waveforms in a connectionless and asynchronous random access scenario. At lower access intensities, where the out-of-band emissions are the significant limiting factor, the outage probability for smaller access packets is shown to vary significantly between the modulations. This variability diminishes when N is increased, which is consistent with the presented theory.

In the modeling of full duplex (FD) transceivers using multicarrier modulations, it is usually assumed that the intercarrier interference of the coupling path is negligible. Therefore, the system can be analyzed on a subcarrier basis, because the model reduces to a set of orthogonal flat channels. However, for more general FD transceivers, or when hardware components introduce nonlinear distortion, the orthogonality is lost and a more rigorous model is needed. In this paper, we introduce a model for the coupling interference of FD systems, which allows the uplink and the downlink to have different time and frequency offsets, and different parameters such as the number of subcarriers and symbol length. In addition, we present a linear approximation for the nonlinear coupling based on a generalization of the Bussgang's theorem, for the case of transmitters with nonuniform power allocation. We validate the proposed model with measurements and show numerically that it improves the accuracy of the signal to interference plus noise ratio expression, and the formulation of optimization problems. Therefore, the proposed model paves the way for a more realistic performance analysis and the derivation of more accurate power allocation strategies than is possible with the classic model.

Genuine full-duplex operation requires effective mitigation of self-interference (SI) due to simultaneous transmission and reception at the same frequency band. In addition to its well-known harmful effect on signals of high peak-to-average power ratio, nonlinear behavior of a power amplifier (PA) complicates SI cancellation and induces spectral regrowth. We introduce a digital predistortion architecture that linearizes the response of the PA in a full-duplex transceiver. Specifically, we propose a two-step procedure to estimate the predistorter parameters and the SI canceller coefficients. Different from a direct application of conventional predistorters, the proposed architecture does not need an extra RF chain to estimate the PA response and exploits the inherent SI signal instead. Finally, simulation results show that the proposed scheme is able to increase significantly the signal-to-interference-plus-noise ratio at the transceiver output and to reduce out-of-band emissions when compared to linear and nonlinear cancellation without predistortion.

We study the physical-layer performance of a wideband multihop system in which a long chain of amplify-and-forward relays is used for routing orthogonal frequency division multiplexing (OFDM) symbols from a source to a destination. Especially, the symbols are modulated in the source and demodulated in the destination while the relays simply forward respective waveforms with scalar amplification. We first conclude that the power-delay profile (PDP) of the effective end-to-end channel in such a system converges to a specific bell-shaped function when each per-hop channel in the link is modelled with a uniform or exponential PDP. We then derive new closed-form expressions for evaluating the signal-to-interference-and-noise ratio (SINR) of the system in the case of an asymptotically large hop count.

We describe a new approach to improve the success rate of symbol decoding with very high-order modulations at receivers equipped with low-resolution analog-to-digital converters (ADC). The proposed method is based on generating self-inflicting pseudo-random white noise signal at the receiving antenna array in order to improve the performance of spatial oversampling. As the optimal amount of this artificially induced noise strongly depends on the ADC resolution, transmit symbol modulation, and the number of receive antennas, we design an adaptation mechanism to render an acceptable balance for successful decoding between the inner- and outer-region symbols of the constellation. Numerical findings demonstrate, for the first time, that the feasibility of high-order symbol modulations (e.g., 64/256-QAM) is within reach by low-resolution ADCs.

Evolving 5G cellular communication is envisioned to enable connectivity for a wide range of new use cases. The focus on mission-critical communications, such as factory automation, public safety and vehicular safety applications presses the demand for ultra-reliable communication. In this paper, we investigate by various means of multi-hop relaying schemes to improve the reliable communication in a factory environment. Impacts due to half- and full-duplex operation at relay nodes are studied together with resource allocation. No retransmissions are considered and the relaying schemes are limited to two hops to meet the low latency constraint. Performance evaluations in a factory scenario are shown. We illustrate that in a network of uncoordinated cells one can exploit the varying fading profiles and can improve the offered reliable rate using multi-hop communication.

In this paper, we investigate a codeword refinement technique called double codeword coding in the context of MIMO feedback. In the proposed method, the second codeword is fed back in addition to the best codeword as their interpolation at the transmitter improves the channel knowledge. The derivation of the optimal interpolation parameter is presented together with the derivation of an upper-bound to the interpolation parameter based on channel quality feedback available at the transmitter. We show that double codeword feedback refinement has slightly lower complexity than alternative techniques and still achieves similar performance. Furthermore, double codeword coding can be generalized to higher transmission ranks. Using extended multiuser MIMO link simulations, we verify that the double codeword feedback is able to provide significant gains in 3GPP LTE context.

Machine-to-machine (M2M) communication represents a new paradigm for mobile cellular networks, where a massive number of low-cost devices request the transfer of small amounts of data without human intervention. One option to tackle this problem is obtained by combining random beamforming (RBF) with opportunistic scheduling. RBF can be used to induce larger channel fluctuations, and opportunistic scheduling can be used to select M2M devices when their overall channel quality is good. Traditional RBF does not fulfill M2M requirements, because overall channel quality needs to be tracked continuously. In order to tackle this limitation, a novel codebook-based RBF architecture that identifies in advance the time instants in which overall channel quality should be reported, within a coherence time window, is proposed. This opportunistic feedback mechanism reduces signaling overhead and enables energy saving at M2M devices. A simplified methodology is presented to evaluate the system mean data rate, using for this purpose closed form formulas derived from SNR distribution approximations. Results reveal that the performance loss that is experienced for introducing the proposed modifications to traditional RBF scheme is negligible. The concepts analyzed in this paper provide useful insights, and show that codebook-based RBF with simplified opportunistic scheduling algorithms is an excellent combination to provide wide-area M2M services with low-cost devices and limited signaling overhead.

Unlocking the true potential of massive antenna arrays for cellular networks operating in frequency division duplex (FDD) mode is a challenging task as inadequate channel state information (CSI) by conventional limited-size UE feedback forces crude beamforming in downlink. This paper develops a new technique for the CSI acquisition problem of FDD massive multiple-input multiple-output (MIMO) networks based on estimating the reciprocal characteristics of the multipath channel directly from uplink, while merely the nonreciprocal properties of the dominant propagation paths are estimated and fed back from UE. The main benefits of this technique over compressive-sensing methods include allowing single-beam pilot transmission during downlink training and relaxed UE-side computational complexity, as the full knowledge of CSI vector is no longer required at UE. The design can be accomplished by measuring the direction-of-arrival information and average powers of the dominant paths from uplink, indicating the target dominant paths to corresponding UEs, estimating and subsequently signaling the random phase information of the target paths from each UE, and reconstructing the aggregate CSI vector at eNB. The numerical evaluation results demonstrate that the proposed mechanism offers promising potential for reliable FDD operation in next generation cellular networks equipped with massive antenna arrays.

In this paper, we consider the problem of channel and phase noise estimation for an orthogonal frequency division multiplexing (OFDM) radio link. We solve this problem by first investigating the subspace in which the phase noise spectral vector lies and then exploiting this information during estimation. Building upon earlier works, the phase noise spectral estimate is obtained by minimizing a homogeneous quadratic cost function and the channel estimate in turns depends on the obtained phase noise estimate. We show that, at infinite signal-to-noise ratio, the true phase noise spectral estimate lies in the null space of the matrix associated with the cost function. We utilize this knowledge by imposing constraints that adhere to this null space when minimizing the cost function. In addition, we also propose constraints based on knowledge of the covariance matrix of the phase noise process. Through simulations, we demonstrate lower phase noise mean-square error (MSE) and consequently lower channel MSE when incorporating the subspace information.

Hardware non-idealities in wireless transmitter electronics cause distortion that is not captured by conventional linear channel models; in fact, error-vector magnitude (EVM) measurements in conformance testing conceptually reduce their collective effect to an additive noise component at each subcarrier. Motivated by the EVM, the present paper considers a `binoisy' multiple-input multiple-output (MIMO) channel model where the additional non-idealities manifest themselves as an additive distortion noise term at the transmit side. Through this extended MIMO relation, the effects of hardware impairments on the achievable rates of different digital modulation schemes are studied via large system analysis. The numerical results illustrate how tolerable EVM levels depend non-trivially on various factors, including the signal-to-noise ratio, modulation order and the level of asymmetry in antenna array configurations.

We propose a new type of UE feedback report for future frequency division duplex (FDD) systems that relaxes the acquisition problem of downlink channel state information (CSI) at eNB with massive antenna arrays. The CSI acquisition is based on estimating the reciprocal characteristics of the multipath propagation channel at eNB during uplink transmission, whereas the non-reciprocal fast-fading characteristics of the downlink channel is estimated at UE and subsequently fed back in the form of multipath phase indication (MPI) feedback report. Such an approach offers several critical benefits over the compression-based feedback schemes, such as its overhead being immune to the number of antenna elements and avoiding the time consuming training-based channel estimation at UE; hence this new CSI acquisition should be considered a viable candidate for future massive multi-input multi-output (MIMO) systems, particularly when operating at higher frequency bands that will allow densely packed antenna structures.

Hardware impairments in radio-frequency components of a wireless system cause unavoidable distortions to transmission that are not captured by the conventional linear channel model. In this paper, a “binoisy” single-user multiple-input multiple-output (SU-MIMO) relation is considered where the additional distortions are modeled via an additive noise term at the transmit side. Through this extended SU-MIMO channel model, the effects of transceiver hardware impairments on the achievable rate of multi-antenna point-to-point systems are studied. Channel input distributions encompassing practical discrete modulation schemes, such as, QAM and PSK, as well as Gaussian signaling are covered. In addition, the impact of mismatched detection and decoding when the receiver has insufficient information about the non-idealities is investigated. The numerical results show that for realistic system parameters, the effects of transmit-side noise and mismatched decoding become significant only at high modulation orders.

Spatial multiplexing is one of the multi-antenna techniques employed by current generation wireless systems. Based on existing research, spatial multiplexing is known to be susceptible to interference. However, it is not well known what the effect of spatial multiplexing on other transmissions is, i.e., how does user performance depend on whether a neighboring cochannel interferer applies a single (spatial) stream or a multi stream transmission. This work attempts to alleviate this shortcoming by analyzing the impact of interference rank (number of spatial streams) on a beamforming and orthogonal space-time block coded user transmission. We generalize existing analytical results on signal-to-interference-plus-noise-ratio distribution and outage probability under arbitrary number of unequal power interferers. Our results are in close form and include interference rank as a parameter, allowing for a thorough study of its impact. Analysis shows that higher rank interference causes lower outage probability, and can support higher outage threshold especially in the case of beamforming.

Spatially distributed transmissions in coordinated multipoint (CoMP) systems can lead to mean channel power imbalance (CPI) at the receiver. Similar imbalance also occurs in distributed antenna and co-located multi-antenna systems due to inaccurate antenna calibration. This paper studies performance impact of power imbalance on some practical CoMP methods with limited feedback. We derive approximate analytical expressions of asymptotic capacity, optimal amplitude weights, as well as signal-to-noise ratio gain for a few methods under analysis. Numerical results validate the analysis and show impacts of erroneous feedback under CPI. Results demonstrate that CPI has significant negative impact on the CoMP performance. Furthermore, our results reveal that amplitude information at transmitter is crucial and detrimental effect of CPI can be effectively compensated by using long-term amplitude information at transmitter. Moreover, additional short-term amplitude feedback shows insignificant gain when a large number of diversity antennas in base stations or CoMP suffer from feedback errors. In fact, a sparsely quantized phase and long-term power information feedback can lead to performance very close to the use of full channel state information at the transmitter.

The ideal radio waveform for an upcoming 5th Generation (5G) radio access technology should cope with a set of requirements such as limited complexity, good time/frequency localization and simple extension to multi-antenna technologies. This paper discusses the suitability of Orthogonal Frequency Division Multiplexing (OFDM) and its recently proposed enhancements as 5G waveforms, mainly focusing on their capability to cope with our requirements. Significant focus is given to the novel zero-tail paradigm, which allows boosting the OFDM flexibility while circumventing demerits such as poor spectral containment and sensitivity to hardware impairments.

This paper studies a relative antenna calibration scheme for time division duplex (TDD) systems, which relies on user equipment (UE) for the estimation of transceiver response via multiple forward and reverse link channel measurements. Since the downlink channel knowledge at the base station (BS) needs to be accurate, UE feeds the quantized channel coefficients back to BS. Once accurate channel estimation is available at BS within a calibration phase, the transceiver response can be approximated by total least squares method. We propose an adaptive quantization method for downlink channel state information, which selects the number of feedback bits based on the estimation quality of the instantaneous uplink channel. We also discuss two reference UE selection methods based on channel quality indication feedback and UE-BS channel signal-to-noise ratio measurement.

This paper considers low-complexity multi-user scheduling algorithms for multi-carrier multiple-input multiple-output systems. With large number of active users and transmit antennas, the optimal scheduler is computationally too demanding, therefore more efficient sub-optimal schedulers are of interest. We propose genetic and greedy-based iterative algorithms with successive zero forcing for the multi-user multi-carrier scheduling problem, where at each iteration, the proposed algorithms schedule one additional user on every resource unit. In system-level simulations, we analyze their performance and complexity requirements, and demonstrate that the genetic-based iterative scheduler performs well when the network is dense with many users, whereas the greedy-based iterative scheduler can be preferable when there are less users in the network.

This paper studies the design of a linear precoder and receiver for the downlink of multi-user multiple-input multiple-output (MIMO) systems with imperfect channel state information (CSI). First, we modify the regularized block diagonalization scheme for the case of imperfect CSI, where we assume the variance of the channel estimation error is known. Next, we propose a novel joint design for the optimized precoder and receiver pair based on the minimum mean square error (MMSE) criterion, where we introduce two transmit channel matrices designed for high signal-to-noise ratio conditions. We compare the performance of these schemes with the generalized MMSE channel inversion filtering via numerical analysis. The results demonstrate the superiority of our joint precoder and receiver design.

This paper studies the impact of two implementation imperfections, namely, transceiver frequency-response non-reciprocity and antenna mutual coupling mismatches at the base-station and user equipment on the channel reciprocity assumption in TDD systems. Comprehensive signal models are first developed to analyze the joint effects of these two imperfections in the TDD multi-user MIMO-OFDM downlink transmission system context, covering both zero-forcing (ZF) and eigenbeamforming based transmitter processing. The corresponding performance degradation is then evaluated in terms of SINRs and maximum achievable sum-rate. The analysis shows that during downlink transmission, the transceiver non-reciprocity and antenna mutual coupling mismatches at the base-station introduce inter-user interference (IUI) and are the major causes for the resulting performance degradation. Implementation imperfections at user equipment side, in turn, only introduce inter-stream interference (ISI) that can be fairly easily suppressed in detector processing as part of effective precoded channels. In order to achieve throughputs close to the ideal case, transceiver frequency-response mismatches and antenna mutual coupling at the base-station side need to be extremely well calibrated, generally below 35-40dB in terms of relative mismatch levels.

The articles in this special issue focus on the technology and applications supported by in-band full duplex wireless services.

In-band full-duplex (IBFD) operation has emerged as an attractive solution for increasing the throughput of wireless communication systems and networks. With IBFD, a wireless terminal is allowed to transmit and receive simultaneously in the same frequency band. This tutorial paper reviews the main concepts of IBFD wireless. One of the biggest practical impediments to IBFD operation is the presence of self-interference, i.e., the interference that the modem's transmitter causes to its own receiver. This tutorial surveys a wide range of IBFD self-interference mitigation techniques. Also discussed are numerous other research challenges and opportunities in the design and analysis of IBFD wireless systems.

Despite the intensive recent research on wireless single-channel full-duplex communications, relatively little is known about the transceiver chain nonidealities of full-duplex devices. In this paper, the effect of nonlinear distortion occurring in the transmitter power amplifier (PA) and the receiver chain is analyzed, beside the dynamic range requirements of analog-to-digital converters (ADCs). This is done with detailed system calculations, which combine the properties of the individual electronics components to jointly model the complete transceiver chain, including self-interference cancellation. They also quantify the decrease in the dynamic range for the signal of interest caused by self-interference at the analog-to-digital interface. Using these system calculations, we provide comprehensive numerical results for typical transceiver parameters. The analytical results are also confirmed with full waveform simulations. We observe that the nonlinear distortion produced by the transmitter PA is a significant issue in a full-duplex transceiver and, when using cheaper and less linear components, also the receiver chain nonlinearities become considerable. It is also shown that, with digitally intensive self-interference cancellation, the quantization noise of the ADCs is another significant problem.

Simultaneous reception and transmission in the same frequency, so-called full-duplex operation, causes an infinite feedback loop in an amplify-and-forward relay. The unwanted echoes may result in oscillation at the relay, making it unstable, and distorting the spectrum. This paper presents an adaptive MIMO filtering method for full-duplex amplify-and-forward relays that aims at solving the joint problem of self-interference mitigation and equalization of the source-relay channel. The scheme exploits the knowledge of the autocorrelation of the transmitted signal as the only side information while allowing the relay, in the best case, to implement precoding as if there was not any self-interference or frequency selectivity in the source-relay channel. Finally, the proposed adaptation algorithm is investigated by determining its stationary points and by performing simulations in a MIMO-OFDM framework.

We develop and analyze a time-domain bi-level power control scheme using Almost Blank Subframe (ABS) for downlink cross-tier interference mitigation. Even for only one femto cell and macro cell, the optimization problem given rate constraint is nonconvex. We prove that by relaxing the integer requirement for subframe division, the optimal strategy is using either power control or ABS individually, and a joint scheme is not needed in the optimal solution. Imposing the integer subframe number requirement, this conclusion is no longer true. However, we show that the optimal solution also assumes simple structure by choosing from one of three feasible solutions. Simulation results show that the joint power control and ABS scheme outperforms traditional single-level power control scheme for downlink cross-tier interference mitigation with low computational complexity.

We consider full-duplex multi-hop forwarding in a Beyond 4G local area network. In the network, there is a high density of self-backhauling relay nodes that simultaneously act as access points towards the users, in addition to few nodes with wired backhaul. The access is framed and synchronized along the multi-hop flow, and the nodes apply time division duplexing. Interference cancelation as well as power optimization is performed within the multihop route. Simulations are carried out in a local area network consisting of multiple multi-floor buildings. The propagation channel is modelled using static pathloss, log-normal distributed random variable, or static pathloss with Rayleigh fading. The simulation results indicate that full-duplex relaying improves the network performance over half-duplex relaying, if self-interference channel attenuation is kept over 80 dB. The means of achieving tolerable self-interference levels in full-duplexing relays via physical design of the relay, and analog and digital interference cancellation are discussed.

In a cognitive radio system, awareness of the prevailing radio environment of the primary network, such as digital terrestrial television, is important since it governs the emission limits for the secondary system devices. Field strength estimates of the primary network are typically computed using terrain based radio propagation models, or by using univariate interpolation techniques to approximate the field strength from measurement samples. However, the limited geographical information or small numbers of measurement samples sets a restriction on the accuracy of the field strength estimates. In this paper, we propose a multivariate interpolation method for incorporating field measurements into radio propagation models. The proposed method shows considerable increase in estimation accuracy compared to previously presented methods. The model is verified using field measurement data from an operational digital terrestrial television network.

In this paper we address the design of MIMO double codebook structure for cross-polarized antenna arrays. Double codebooks consist of two codebook components, a wide-band long-term component and a short-term sub-band component. Herein we study particularly the two layer short-term sub-band component and provide the complete set of structure types leading to orthogonal constant modulus codewords and cross-polarization phase of M-PSK alphabet. By 3GPP LTE system simulations we show that a novel, two layer structure type, provides significant improvements in channels with large angular spreads.

Bidirectional wireless communication between two multi-antenna terminals that operate in a full-duplex mode is considered. The effects of loopback self-interference and transmit-side noise due to radio front-end imperfections are analyzed in the asymptotic regime where the number of antennas at both terminals grows large. The study compares two transparent mitigation schemes by which conventional spatial multiplexing can operate without being aware of the self-interference issue: spatial-domain null-space projection and time-domain subtractive cancellation. Both schemes eliminate the information-bearing part of the self-interference perfectly, but due to transmitter imperfections, the cancellation approach suffers from residual self-interference. The receive-side null-space projection, on the other hand, suppresses also the transmit-side distortion, but at the cost of reduction in the degrees of freedom available for data reception. The results indicate that suppression becomes preferred when the level of transmit-side noise increases and the receiver does not have instantaneous knowledge about the spatial structure of the self-interference. Exhaustive Monte Carlo simulations also show that the asymptotic analytical results are useful for evaluating the performance of practical not-so-large systems.

In heterogeneous cellular scenarios with macrocells, femtocells or picocells users may suffer from significant co-channel cross-tier interference. To manage this interference 3GPP introduced almost blank subframe (ABSF), a subframe in which the interferer tier is not allowed to transmit data. Vulnerable users thus get a chance to be scheduled in ABSFs with reduced cross-tier interference. We analyze downlink scenarios using stochastic geometry and formulate a condition for the required number of ABSFs based on base station placement statistics and user throughput requirement. The result is a semi-analytical formula that serves as a good initial estimate and offers an easy way to analyze impact of network parameters. We show that while in macro/femto scenario the residue ABSF interference can be well managed, in macro/pico scenario it affects the number of required ABSFs strongly. The effect of ABSFs is subsequently demonstrated via user throughput simulations. Especially in the macro/pico scenario, we find that using ABSFs is advantageous for the system since victim users no longer suffer from poor performance for the price of relatively small drop in higher throughput percentiles.

A new approach to relay and user selection is proposed using threshold-based transmission to reduce outage probability and save transmit power. By exploiting properties of dual-hop transmission using amplify-and-forward (AF) relaying, new threshold-based relay and user (TRU) selection protocols that employ one-bit channel state information (CSI) feedback are proposed for multiple-relay, multiple-user systems. These new protocols have the advantages of (1) economical feedback, and (2) optional adaptive power-saving transmission. Centralized selection (CS) and distributed selection (DS) policies as well as lower-complexity suboptimal selection schemes are investigated for downlink transmission. Exact expressions for outage probability, outage diversity and feedback requirement are provided to quantify the complexity/performance trade-offs. The analysis shows that selection threshold(s) can be chosen such that transmission is interrupted when system outage is certain, which allows power saving without impairing outage performance. These protocols are compared in terms of performance, energy-efficiency and system requirement to others that trade outage performance for additional energy saving and/or reduced complexity.

We derive the average capacity for an OFDM radio link that is impaired by phase noise and carrier frequency offset at both transmitter and receiver side. It is well known that, for OFDM, phase noise and frequency offset cause intercarrier interference (ICI). We first notice the dependency of the instantaneous capacity on the ICI power and on the fading channel. Using a Taylor series approximation, we show that the ICI power can be well represented in a quadratic form whose probability density function we derive. Using the derived distribution and assuming a Rayleigh-fading channel, we derive analytical expressions of average capacity. Different test cases are presented to study the sensitivity of the capacity on phase noise and frequency offset. The main observations suggest that, for reasonable phase noise levels, the capacity is practically insensitive to the frequency offset.

We propose a discrete-time model for phase noise processes occurring in phase locked loop (PLL) based oscillators. By using linear time-invariant analysis in the phase domain, we arrive at a parallel auto-regressive moving-average phase noise model that captures the major noise sources and corresponding loop transfer functions of the PLL circuitry. The acquired model allows us to derive analytically the distribution of the inter-carrier interference (ICI) power in the receiver, shown to be a sum of correlated gamma random variables with a model-specific covariance matrix. Finally, we utilize the derived ICI distribution and proceed to analyze the effects of phase noise and multipath fading on the performance of OFDM radio links. For example, we evaluate the rate loss without ICI compensation or knowledge at the receiver, and illustrate the trade-off introduced by phase noise between the OFDM symbol length and the overhead caused by the cyclic prefix. Throughout the paper, simulations confirm the accuracy of the analytical expressions.

Predistortion techniques are implemented in OFDM systems to improve power amplifier efficiency while keeping in-band and out- of-band distortion at acceptable levels. Predistorter (PD) design for broadband MIMO-OFDM systems introduces several implementation challenges. Coupling effects cannot be avoided in MIMO transceivers where the paths are implemented on the same chipset. Moreover, baseband PDs are affected by imbalances in the IQ modulator. This paper proposes a MIMO-PD that linearizes the power amplifier (PA) response and compensates for both crosstalk and IQ imbalance. Error vector magnitude (EVM) and adjacent channel power ratio (ACPR) results show a significant improvement compared with conventional PD systems.

Modern cellular communication systems are based on frequency reuse 1. Although reuse 1 promises high peak data rates and significantly simplified network planning, the cell edge or cell overlap areas suffer from excessive inter-cell interference. In this work we address interference management for downlink part of these systems. Part of the radio resources is primarily reserved for protected access of vulnerable users. The protected access is based on a novel sounding/silencing protocol that takes advantage of instantaneous signal to interference plus noise ratio prediction and has power to silence strong interferers with low priority. The accompanied signaling scheme is developed for time division duplex systems. Our results in an indoor scenario with partly overlapping closed subscriber groups show that even with realistic control signal reception model the proportion of zero throughput users can be reduced by more than 50%.

Mobile communication systems have evolved over the past decades and each new generation brought new experience to the users enabled by technology innovations, while keeping some well established principles from previous generations. This trend continued up to LTE (Long Term Evolution) Advanced, the predominant 4thgeneration system which has just been standardized in 3GPP and is being rolled out soon. How will this trend continue to future systems which will be deployed in some 10 years from now which will be advanced enough to be called “Beyond 4G” (B4G)? This article presents how such B4G systems will look like and some key technologies they will rely on including versatile numerology, massive virtual MIMO from many base stations, both centralized and distributed architectures using fiber optics as backbone, advanced interference mitigation, cognitive self organization, and wideband RF radios.

We analyze the effect of Wiener phase noise on the capacity and signal-to-interference-plus-noise (SINR) ratio. Our analysis includes phase noise at the transmitter and receiver end of an OFDM communication link. We see that the capacity and SINR are random variables whose distribution depends on the phase noise and the fading channel. Using a Taylor series approximation, we show that the random variable, characterizing the phase noise, in the performance metrics can be expressed as a sum of correlated gamma variables with rank-deficient square-root normalized covariance matrix. The approximation holds well when the ratio between the subcarrier spacing and the 3dB bandwidth of the oscillator power spectral density is at least one order of magnitude, which for most practical oscillators is the case. In earlier literature, the probability density function of a sum of correlated gamma variables with full-rank square-root normalized covariance matrix was derived. We extend these results to the rank-deficient case and apply them to the random variable of our case. With the probability density functions characterizing the phase noise and the fading channel at hand, we proceed to obtain closed-form statistical measures of capacity and SINR. The simulations show the good agreement with our analytical expressions.

Precoding codebook design for limited feedback MIMO systems is known to reduce to a discretization problem on a Grassmann manifold. The case of two-antenna beamforming is special in that it is equivalent to quantizing the real sphere. The isometry between the Grassmannian G2,1ℂand the real sphereS2shows that discretization problems in the Grassmannian G2,1ℂare directly solved by corresponding spherical codes. Notably, the Grassmannian line packing problem in ℂ2, namely maximizing the minimum distance, is equivalent to the Tammes problem on the real sphere, so that optimum spherical packings give optimum Grassmannian packings. Moreover, a simple isomorphism between G2,1ℂandS2enables to analytically derive simple codebooks in closed-form having low implementation complexity. Using the simple geometry of some of these codebooks, we derive closed-form expressions of the probability density function of the relative SNR loss due to limited feedback. We also investigate codebooks based on other spherical arrangements, such as solutions maximizing the harmonic mean of the mutual distances among the codewords, which is known as the Thomson problem. We find that in some special cases, Grassmannian codebooks based on these other spherical arrangements outperform codebooks from Grassmannian packing.

Focusing on two-antenna infrastructure relays employed for coverage extension, we develop hybrid techniques that switch opportunistically between full-duplex and half-duplex relaying modes. To rationalize the system design, the classic three-node full-duplex relay link is first amended by explicitly modeling residual relay self-interference, i.e., a loopback signal from the transmit antenna to the receive antenna remaining after cancellation. The motivation for opportunistic mode selection stems then from the fundamental trade-off determining the spectral efficiency: The half-duplex mode avoids inherently the self-interference at the cost of halving the end-to-end symbol rate while the full-duplex mode achieves full symbol rate but, in practice, suffers from residual interference even after cancellation. We propose the combination of opportunistic mode selection and transmit power adaptation for maximizing instantaneous and average spectral efficiency after noting that the trade-off favors alternately the modes during operation. The analysis covers both common relaying protocols (amplify-and-forward and decode-and-forward) as well as reflects the difference of downlink and uplink systems. The results show that opportunistic mode selection offers significant performance gain over system design that is confined to either mode without rationalization.

Nonlinear effects of a power amplifier (PA) in communications systems include spectral spreading, which causes adjacent channel interference. For predistorter design, it is crucial to employ a model that can sufficiently model the intermodulation distortion (IMD) caused by the PA. The order of IMD generated by a polynomial is determined by its highest order basis. For a piecewise linear function, the order of nonlinearity cannot be directly deduced from its basis function. This paper investigates how IMD is introduced by the simplicial canonical piecewise linear (SCPWL) function. We examine the operation imposed by the SCPWL basis function on sinusoid and multi-tone signal in the time domain and frequency domain. Our analysis results in expressions that can be used to predict the output spectrum of the SCPWL basis. The SCPWL basis operation involves spreading of the input signal spectrum. The output spectrum of each SCPWL basis includes infinite IMD components. The IMD order of the SCPWL model is not determined by the highest order basis. Instead, the number of SCPWL parameters, i.e., the number of linear affine segments that defines the function, determines how accurately it can model the IMD components.

We study the fundamental rate-interference trade-off between full-duplex and half-duplex transmission modes in a two-hop infrastructure-based decode-and-forward relay link. First, we derive closed-form expressions for average end-to-end capacities. Then, we show that it may be better to tolerate some loop interference in the full-duplex mode than to halve the end-to-end symbol rate by allocating two orthogonal time slots in the half-duplex mode. Furthermore, we evaluate the loop interference power levels below which the full-duplex mode achieves better capacity than the half-duplex mode. Our results indicate that the full-duplex mode is preferable in terms of capacity for many practical signal-to-noise ratios, e.g., above 3-10dB depending on the setup. Finally, we show that full-duplex relays can further improve the end-to-end capacity by adjusting transmit power.

Direct-conversion OFDM transceivers are seriously affected by front-end distortions like IQ imbalance and phase noise. Moreover, OFDM systems are sensitive to nonlinear distortions in power amplifiers and A/D converters, and to carrier frequency offset (CFO). We present a novel baseband compensation technique that compensates these main impairments in OFDM systems. Unlike compensation scheme that target a subset of impairments, our method jointly mitigates the effects of IQ imbalance, phase noise, carrier frequency offset and nonlinear distortion. Simulations of the proposed scheme with large levels of CFO, IQ imbalance and phase noise show an excellent performance in terms of bit error rate.

This paper proposes new selection schemes based on one-bit feedback for a system with multiple amplify-and-forward relays and users over Rayleigh fading channels. We introduce new distributed selection (DS), simplified DS (DSS), and centralized selection (CS) schemes that reduce outage and save transmit power using one-bit feedback. The CS scheme is optimal by selecting only the dual-hop link with equivalent end-to-end SNR above the outage threshold. DS and DSS schemes, on the other hand, require less feedback at the BS and employ a decentralized selection policy. The feedback bit in those schemes render the comparison result between the first- and second- hop SNR and an appropriate threshold. The chosen selection threshold in DS and DSS schemes allows the dual-hop links for which the system outage is certain to be discarded. In the three selection schemes, transmission is switched off to save power when there is no dual-hop link meeting the selection requirement. Closed-form expressions for the DS, DSS, and CS outage probabilities are derived. Numerical results reveal that, even though DSS is a simplified version of the DS scheme, they have similar outage performance. Additionally, DS and DSS are quite competitive with CS for a small number of relays relative to the number of users.

We investigate the impact of fixed relay station deployment in a single frequency network (SFN) using orthogonal frequency-division multiplexing (OFDM). We provide semi-analytical methods for relay-based SFN performance evaluation. Monte Carlo simulations are performed to provide numerical calculation of multi-cell network performance. The performance metric is the cumulative distribution function of the signal-to-interference-plus-noise-ratio (SINR) for different user positions in the network. Common relaying methods for two-hop amplify-and-forward (A&F) relay networks are evaluated. The impact on the SINR is compared for different relaying gains, locations and densities of relays, as well as different transmission protocols, such as full-duplex (FD) and half-duplex (HD). Overall, taking into account their different rates and physical layer performances, FD appears to outperform HD. In addition to showing the benefit of relay deployment for enhancing network performance, our simulations show that the fixed and variable gains perform equally well.

This paper studies the performance of a two-hop infrastructure-based relay link. The relay operates in a half-duplex mode using an amplify-and-forward protocol. Considering transmission of OFDM signals with nonlinear power amplifiers (PAs), we determine exact closed-form expressions for end-to-end bit error probabilities (BEPs). The expressions allow us to analyze the effect of nonlinear distortion as well as the special case of linear PAs. Furthermore, we derive asymptotic BEP floor values which illustrate how the end-to-end performance is limited by both the nonlinear PA distortion and the weakest hop. Our discussion indicates that the influence of the nonlinear distortion can be mitigated by properly adjusting the PA back-offs.

We study the physical-layer performance of a wideband multihop system in which a chain of amplify-and-forward relays is deployed for routing data from a source to a destination. The novelty of the system model w.r.t. prior work is in assuming frequency-selective multipath channels for all hops. We propose a new approach in which the channels are specified in terms of clustered exponentially decaying power-delay profiles (PDPs). Focusing on relays that apply time-domain signal processing, the distinctive form of the end-to-end source-destination PDP motivates us to introduce a new hypoexponential PDP and study it in detail. Then, we analyze the performance of orthogonal frequency division multiplexing (OFDM) transmission over the multihop relay link. The type of the adopted analysis is suitable mainly for investigating the typical time-domain issues of OFDM transmission such as excessive multipath delay spread, non-ideal time syncronization, and interference due to insufficient cyclic prefix duration. In particular, we derive new closed-form expressions for evaluating the signal-to-interference and noise ratio. Our analysis confirms that multihopping increases considerably the delay spread of the end-to-end PDP and makes the channel more frequency selective. This imposes significant effect on the design of receiver time synchronization and pilot structures, and on the choice of OFDM physical layer parameters.

We consider a downlink OFDMA transmission system in which an infrastructure-based relay node is deployed for extending the coverage of a base station. Focusing on decode-and-forward (DF) operation, we study how different relay functionalities affect the system performance. The functionalities include fixed or adaptive subcarrier pairing, information redistribution, buffering, and adjustment of the time shares allocated for the two hops. In particular, it is of great interest for system design to be able to evaluate the performance gains due to each functionality, because they come at the expense of increased complexity and cost of the relay architecture. The main contribution of the paper is to analyze the performance of the proposed protocols by deriving new closed-form expressions for the average end-to-end capacity. The calculated expressions facilitate the performance comparison between the different functionalities.

We study the performance of a two-hop full-duplex OFDM relay link, where the relay, or in-band repeater, amplifies and forwards its input signal on the same channel as the main transmitter. We derive new closed-form expressions for the end-to-end signal-to-interference and noise ratio (SINR) when the multipath channels are modeled with clustered exponential power-delay profiles. Our system model incorporates the deleterious effect of loop interference associated with full-duplex relays due to signal leakage from transmission to reception. In particular, our analysis shows that the relay gain is a central parameter in the mitigation of residual loop interference and for the optimization of the end-to-end performance.

This paper presents a new low complexity method of providing improved channel state information at the transmitter within closed-loop MIMO systems. We propose to feedback the second best codeword in addition to the best codeword. At the transmitter, these two precoding codewords are interpolated using geodesic. The optimal interpolation parameter is derived in closed-form for single-stream transmission. The performance of the proposed method with b-bit codewords is shown to be close to a codebook with 2b-bit codewords, while our method has significantly lower computational complexity.

Current broadband wireless networks are characterized by large cell sizes. Yet, even in advanced networks, users on the cell edge will face relatively low Signal-to-Interference-plus-Noise-Ratio (SINR). An attractive solution for this problem is provided by multi-hop technologies. In this paper, we consider the performance of full duplex Amplify-and-Forward (AF) and half duplex Decode-and-Forward (DF) Relay Nodes (RNs) from 3G LTE-Advanced perspective. The comparison between AF and DF relaying is important because both approaches are currently under consideration in LTE-Advanced study item in 3GPP. Performance evaluation considers AF RN loop back signal interference and concurrent DF RN transmissions on the access link. Results show that the concurrent transmissions improve the spectral efficiency for DF RN over performing AF RN.

MIMO-OFDM systems with channel reciprocity allow spatial multiplexing using eigenbeamforming. However, eigenbeamforming requires singular value decomposition, which is phase ambiguous and causes the precoded channel to become phase discontinuous in frequency domain. This changes statistical properties of channel components, which degrades the performance of channel estimation. In this paper we propose phase unifying method to improve channel statistics and consequently decrease the error in channel estimation. In addition, we investigate the 2D-Wiener channel estimation with respect to different channel frequency correlation functions. The performance is evaluated using LTE link simulator.

This paper studies receiver techniques for nonlinear amplifier distortion compensation in an OFDM relay-assisted cooperative communication system. The system model includes a nonlinear amplifier at the amplify-and-forward relay, modeled as a solid state power amplifier (SSPA). A maximum ratio combiner (MRC) is introduced that includes the effects of the amplifier distortions. The MRC is obtained by a proper modeling of the nonlinear distortion noise. Furthermore, we introduce a power amplifier nonlinearity cancellation (PANC) technique that is suitable for cooperative systems. Our simulation results confirm that the new MRC and PANC techniques offer substantial performance improvements if the relayed signal is subject to nonlinear distortion.

We study the fundamental capacity trade-off between full-duplex and half-duplex transmission modes in a two- hop communication system with a fixed infrastructure-based amplify-and-forward relay. First, we derive closed-form expressions for the average end-to-end capacity in the relay link. We show that it may be better to tolerate some loop interference with the full-duplex mode than to consume channel resources by allocating two orthogonal channels with the half-duplex mode. Furthermore, we evaluate the maximum loop interference power levels that still allow the full-duplex mode to achieve the same capacity as the half-duplex mode. Our results indicate that with practical signal-to-noise ratio values, the full-duplex mode is preferable in terms of capacity.

This paper considers usage of a single-frequency mode for cell coverage extension in infrastructure-based relay links. We provide new closed-form expressions for outage probability by taking into account practical constraints such as interference due to frequency reuse and signal leakage between the relay transmitter and receiver. The analysis covers both decode-and-forward and amplify-and-forward protocols both in downlink and in uplink. For amplify-and-forward relaying, variable gain and fixed gain methods for transmit power normalization are discussed. Simulations show excellent agreement with theory and confirm that the single-frequency mode can be applied at the cost of tolerable signal degradation.

We study unitary precoding for multistream MIMO systems with partial channel state information at the transmitter. We introduce a quantization scheme in which the full space of non-equivalent precoding matrices is partitioned into Grassmannian and orthogonalization parts. The Grassmannian part is used for maximizing the power after precoding and the orthogonalization part is used for removing cross talk between the data streams. We show that orthogonalization improves the attainable capacity when the receiver is linear. We give a parametrization for the non-equivalent orhogonalization matrices and a metric which measures the orthogonality of the transmission. Optimal orthogonalization codebooks for two-stream transmission are presented. When feedback is limited, the optimal partitioning of feedback bits between Grassmannian and orthogonalization parts becomes an issue. In correlated scenarios, the number of feedback bits may be significantly reduced by investing bits into the orthogonalization part.

This letter derives new gain control schemes for an amplify-and-forward single-frequency relay link in which loop interference from the relay transmit antenna to the relay receive antenna has to be tolerated. The proposed gain control schemes take into account the effect of residual loop interference that remains after imperfect loop interference cancellation. As a result of our gain control strategy, the signal-to-interference and noise ratio can be maximized while, at the same time, transmit power is decreased. Finally, we evaluate system performance by deriving closed-form outage probability expressions for the gain control schemes.

We propose a closed-loop scheme that combines opportunistic beamforming (OBF) and closed-loop (CL) transmit-diversity (TD) techniques. This quantized OBF scheme is compatible with CL TD algorithms, and allows to adaptively control the amount of feedback overhead according to the number of participating users. Closed-form expressions for the SNR gain of the quantized OBF proposal are derived. Important reductions in the amount of feedback overhead are observed without affecting the SNR gain considerably, when compared to CL TD techniques.

We study the performance of fair physical layer schedulers that select a pair of orthogonal users for simultaneous transmission. The schemes rely on partial channel state information compatible with 3GPP specifications and differ with respect to their usage of channel quality indicator in scheduling decisions. Closed-form expressions for the achievable sum-rate of three practical scheduling algorithms are derived.

We study a wireless two-hop relaying network where a destination node receives a superposition of direct link and relayed signals, because the same physical channel is employed for both transmissions. We assume fixed-gain amplify-and-forward operation in the relay and develop a power allocation technique that requires only slowly varying channel state information. The optimal power allocation maximizes the end-to-end signal- to-noise ratio under total transmit power constraint. Furthermore, we propose a nearly optimal power allocation that can be determined using closed-form expressions.

This work analyzes the outage probability of fair physical layer schedulers that select a pair of orthogonal users for simultaneous transmission. These schemes rest on finite-rate channel state information compatible with 3GPP specifications and differ with respect to their usage of channel quality indicator in scheduling decisions. Closed-form approximations for the outage probability of these practical transmission schemes are derived. Important outage capacity gains are observed when these multiuser scheduling strategies are compared to the scheme that transmits sequentially to only one user at a time.

This paper proposes a novel split predistorter structure to remove nonlinear distortion caused by nonlinear power amplifier with memory. Unlike conventional techniques, the new technique does not require an estimation of the memory model at the transmitter leading to a reduced implementation complexity. Simulations have been carried out using the SUI3 channel model developed for IEEE802.16 standard. The results verify the good performance of the proposed predistorter for several relevant power amplifier models.

Assuming a single bounce scattering geometric channel model with circularly symmetric scatter density we deduce a general integral relation between angle of arrival (AOA) and scatter density function. Then we show that this relation can be expressed in terms of an integral equation that admits an accurate numerical solution provided that AOA is known. Furthermore, we show that similar integral equation relation between time of arrival (TOA) and scatter density function can be efficiently solved numerically for a given TOA. Results indicate that the scatter density can be numerically computed for a given AOA or TOA. Since these marginal distributions can be directly fit in channel measurements, the method provides means to adjust the applied geometrical channel model through field measurements. Hence, we follow the idea of inverse scattering theory where the far field pattern of scattered waves is known and the aim is to define the structure of the scattering medium. We consider two example AOA distributions, Gaussian and Student's t-distribution, found in literature. Our results show that scatter densities as well as the corresponding TOAs greatly vary depending on the assumed AOA

This work proposes a new partial update filtering technique tailored for adaptive transmit beamforming with low feedback rate. A signal dependent selection rule is derived that singles out one component of the beamforming vector to be updated. This provides an efficient way to perform the update, which escalates the performance with the number of available feedback bits without increasing the computational complexity. An operation count for the proposed algorithm and existing solutions is provided. Simulations show that the proposed scheme outperforms the tracking capabilities of existing codebook solutions in low mobility scenarios, while having comparable bit error probability performance and a computational complexity reduction of 90%.

This work proposes a novel adaptive transmit beamforming algorithm, suitable for single-beam multiple-input multiple-output (MIMO) systems employing closed-loop feedback channels with low rate. A low computational complexity solution is achieved by means of a partial update strategy, in which only one of the complex baseband weights of the transmit antenna array is modified at each instance. The new algorithm can operate sequentially over the antennas, or employ a signal-dependent criterion that determines which antenna coefficient to update. The solution for the weight under consideration maximizes the signal-to-noise ratio at the receiver, given the current channel state and beamforming weights. Simulations show that the proposed algorithm reaches near optimal bit error probability (BEP) performance in low mobility scenarios, and offers advantages over non-recursive codebook solutions on a wide range of fading rates. The formulas presented in this paper specialize to a new solution in the single receive antenna multiple-input single-output (MISO) case, which allows an intuitive geometrical interpretation. The new MISO formulation is shown to outperform a recently proposed partial update beamforming scheme for single receive antenna systems.

We consider a Gaussian parallel relay network where multiple wireless relays forward cooperatively so that a destination receives a superposition of signals propagated through multiple distinct relay paths. Previous research often assumes that the relayed signals arrive synchronously in the receiver. This is not feasible in practical wireless networks, because spatial separation causes different propagation delays for the signal components and the destination observes a multipath channel. We model the effect of signal delays by introducing spatial separation to the network geometry and present trade-off analysis between diversity, signal bandwidth, relay density, and the number of relays for both narrowband and wideband reception at the destination

We show that channel-aware scheduling can significantly improve the capacity of CDMA uplink when compared to round robin scheduling. Using both analysis and simulations we quantify the capacity increase due to channel-aware scheduling in terms of reduction in intercell interference. The scheduling approaches can be used, e.g., within the current cdma2000 and WCDMA systems. Finally, we demonstrate that soft handover reduces the benefit from channel-aware scheduling and base station antenna gain properties are important for the scheduling performance

Previous works have shown that in case of multiple transmit antenna broadcast channels, the total system throughput can be greatly improved by combining spatial prefiltering (SP) with multiuser scheduling (MS) [G. Caire et al., 2003], [D. Samardzija et al., 2003]. However, most of the studies assume full channel state information at the transmitter (CSIT). In this paper, we study the achievable data rates within the current WCDMA HSDPA specification and present different schemes based on the WCDMA closed-loop transmit diversity (CLTD) modes. Although the achievable throughput is interference limited at high SNR due to partial CSIT, we show that at moderate power regimes a significant performance gain can be achieved by the proper combination of SP and MS concepts, compared to the systems that only transmit to the user with the best channel condition in each transmission time interval (TTI).

We compare the performance of three physical layer scheduling algorithms: round robin; transmit time selection diversity (TTSD); maximum SNR. With TTSD, downlink transmission is suspended if the instantaneously received SNR in the mobile station falls below a given threshold, which requires only one feedback bit per user; this considerably decreases the overhead of the feedback signaling when compared with a maximum SNR scheduler. In the case of FDD systems, TTSD and maximum SNR schedulers require a fast feedback channel from mobile to base station, like the one in the FDD WCDMA system. It is shown that, while the maximum SNR scheduler outperforms the other two schedulers when the feedback signaling is error-free, TTSD is better in the case of feedback bit errors. Furthermore, while orthogonal space-time coding does not improve the performance with channel aware schedulers when compared to single antenna transmission, it is shown that space-time coding makes the system more robust and gives gain in the case of feedback errors.

Several diversity techniques employing multiple antennas in the base station have been developed for mobile communication systems. However, the performance of diversity methods strongly depends on the correlation between antenna elements. When multiple copolarized antennas are used in base station, the practical factor limiting the number of diversity branches is the required antenna separation, because large antenna separation makes the antenna array unfavourable large while small antenna separation may result in high correlations between the antennas. Therefore, dual-polarized antennas are commonly used to implement two diversity branches. If more than two diversity branches are needed, spatially separated dual-polarized antennas provide an attractive solution alternative. In this paper we propose a model for calculating the correlations between the cross and copolarized antennas when employing spatially separated dual-polarized antennas with nominal /spl plusmn/45/spl deg/ inclination to vertical linear polarization. Results are of theoretical nature and show the relationship between base station antenna correlations and the properties of the physical channel, base station, and mobile station antennas.

We examine a two-rate communication system where the transmission is suspended if the received SNR falls below a given threshold. We assume an FDD system and derive novel closed-form bit error probabilities in the presence of feedback errors in flat Rayleigh fading environment for single antenna transmission, orthogonal space-time block coding and transmit antenna selection. A single-user two-rate system would result in an infrequent use of the channel and long delays, but combined with a physical layer multiuser scheduler like in the high speed downlink packet access scheme included in the 3GPP WCDMA Release 5 specification the overall channel utilization can be improved.

We examine site selection diversity transmission (SSDT) for 3GPP WCDMA forward link by means of analytical tools. Hard handover (HHO), soft handover (SHO) and SSDT are compared by using the receiver bit error probability as a performance measure taking into account the effect of feedback bit errors as well as the shadow fading. Results show that without fast transmission power control the performance gain from SSDT can be seriously degraded by feedback bit errors.

Transmit diversity techniques have received a lot of attention recently, and open-loop and closed-loop downlink transmit diversity modes for two transmit antennas have been included in the 3GPP WCDMA Release 5. System capacity can be increased from that of open-loop modes if the transmitter can be provided additional side information of the downlink channel. In a frequency division duplex (FDD) system this requires a separate feedback channel. Quantization of the feedback messages in closed-loop transmit diversity schemes reduces the performance of the system. However, this effect has not been usually taken into account in the analysis. In this paper we analyze the effect of quantization and deduce an analytic formula for the asymptotic bit error probability of the closed-loop transmit diversity schemes defined in 3GPP WCDMA specification.

Transmit diversity techniques have received a lot of attention recently, and open-loop and closed-loop downlink transmit diversity modes for two transmit antennas have been included in 3GPP WCDMA release 4. System capacity can be increased from that of open-loop modes if the transmitter is equipped with additional side information of the downlink channel. In a frequency division duplex system, this means that the receiver has to provide the information through a feedback mechanism. We study the performance loss caused by feedback errors using the bit error probability as a performance measure. It is shown that the asymptotic diversity of closed-loop schemes is reduced to one in the presence of feedback errors, i.e., the performance is similar to a single transmit antenna system with some SNR gain. The proof is given in a very general manner and it is valid for a large variety of feedback schemes. Two example schemes, selection and cophasing algorithms, are discussed in more detail to verify the results.

Transmit diversity techniques provide attractive solutions for increasing downlink capacity in 3G systems within low-mobility environments. Open-loop and closed-loop transmit diversity modes with two transmit antennas have already been included in 3GPP WCDMA Release 4, and at the moment schemes exploiting a larger number of transmit antennas are being developed. This paper analyzes the performance of a simple closed-loop transmit diversity scheme suitable for 3GPP WCDMA. The goal is to study the effect of different feedback delays on the performance of the given closed-loop scheme. Although the analysis applies only in the case of the specific algorithm (phase-only feedback) it is expected that the performance of the more general magnitude and phase feedback shows similar properties. We use signal-to-noise ratio (SNR) gain as a performance measure, which corresponds to the gain in expected received SNR in a mobile terminal when feedback information from the receiver is applied in the transmitter.

The authors introduce two classes of finite impulse response (FIR)-median hybrid (FMH) filters called in-place growing FIR-median hybrid (IPG-FMH), and recursive in-place growing FIR-median hybrid (RIPG-FMH) filters. This extended family of filters exhibits a small bias error in the vicinity of a step edge and yet provides good noise attenuation. The new filters are compared with standard median, recursive median, FIR-median hybrid, and recursive FIR-median hybrid filters. Examples of each filter type are given and the behavior of the filters is examined by means of simulations. It is shown that the IPG-FMH filter can be expressed as a weighted FMH (WFMH) filter.>