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

Publisher Copyright: © 2026 IEEE. EC/HE/101192113/EU//AMBIENT-6G

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.

With the commercialization of Fifth-Generation (5G) technologies, the emergence of data-intensive applications has driven the need for future-generation networks to support seamless, immersive and high bandwidth services while ensuring enhanced quality of service for mobile users. To address this, Millimeter Wave (mmWave) frequency bands offer substantial potential for delivering high data rates, though, prone to blockages due to clutter, particularly in dense urban environments. Thus, Integrated-Access-and-Backhaul (IAB) deployment enables network densification by improving the line-of-sight condition to the serving node, replacing part of the optical fibre backhaul links with mmWave wireless links. This paper evaluates the IAB-enhanced cellular network performance in a Manhattan-like urban setting, across four potential network deployment scenarios, comprising candidate locations for mobile User Equipment (UE) placement. The mobile UEs can connect to the serving node, either gNodeB (gNB) that may take the role of IAB donor or IAB node, enabling the best link performance. Simulation results demonstrate significant Signal-to-Interference-plus-Noise-Ratio (SINR) gains of 10dB at the 50th percentile across all IAB deployment scenarios when using the optimal path selection approach compared to forcing UEs to connect to a specific serving node, besides the best option. These SINR gains translate into improved spectral data rates, in the order of 0.11- 2.72 bps/Hz. This study emphasizes the strategic placement of IAB nodes and optimized path selection in dense urban mmWave networks to enhance the end-user experience.

5G New Radio (NR) is the de facto global standard for 5G mobile networks. 5G NR was developed by 3GPP and designed to provide new kinds of mobile services across very large geographical areas. One of the main challenges that evolutions of 5G NR must face is the limited radio frequency resources that are available for Beyond-5G mobile connectivity. Optical wireless bands have been already considered for Wireless Local Area Network (WLAN) technologies, where they are denoted as Light Fidelity (Li-Fi). However, and in order to make optical wireless an integral part of Beyond 5G mobile standards, it is convenient to first assess the performance that is feasible when an actual 5G NR waveform is transmitted over an optical wireless frequency band. This paper presents the first experimental proof-of-concept for the transmission of a 5G NR frame over an LED-based optical wireless link. The obtained results are very motivating, as the 5G NR data rates that would be feasible over optical wireless links are very similar to the ones observed nowadays over the n78 RF band (centered around 3.5 GHz, with 100 MHz bandwidth). However, the most important difference is that the spectrum of the optical wireless signal will be reusable in very small cells of radius not larger than few meters, with the possibility of deploying many remote optical units for ultra-densification in indoor service areas without notable inter-cell interference.

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.

Ambient backscatter communication (AmBC) is a key enabler for green Internet of Things (IoT) employing ambient radio frequency (RF) signals for low-power communication. The AmBC devices discussed in the literature so far harvest light to power their microcontrollers for controlling RF modulation and reflection circuits. The ubiquitous presence of configurable light-emitting diodes (LEDs) based luminaires can also leverage such control functionalities by involving visible light communication (VLC). In this paper, we propose a visible light-enabled AmBC system that integrates VLC and AmBC for future ambient power-enabled IoT. We design LiBD, a backscatter device (BD) that uses visible light signals for both modulation control and energy supply. We demonstrate the real-time end-to-end data transmission with a proof of concept experiment. Evaluation results show that the LiBD supports multiple sub-6 GHz RF bands and can receive modulated light at frequencies up to 250 kHz. The investigations verify the feasibility of integrating the VLC and AmBC for future green IoT systems.

In this paper, we propose and evaluate a hybrid visible light communication (VLC)/radio-frequency (RF) network architecture, where a VLC access point serves two user equipments (UEs), which also act as RF relays in order to extend the network’s coverage to a third UE outside the VLC cell. The proposed protocol is inspired by uplink rate-splitting multiple access to efficiently route the messages to the UEs. In more detail, the proposed protocol utilizes the nuances of the specific network architecture to efficiently utilize the wireless resource blocks for both coverage and throughput. The protocol is then optimized by maximizing the minimum achievable rate. Simulation results show that the proposed method achieves superior results compared with a more conventional benchmark scheme, that is also optimized under the same constraints.

Optimizing the energy efficiency (EE) of wireless networks is one of the key priorities in the design of beyond 5G mobile technologies. In this pursuit, the use of new frequency bands, in combination with advanced multiple access protocols and cooperative communications strategies, has recently shown promising results. To this end, this paper investigates an indoor wireless network that aggregates communication resources in visible light and radio-frequency (RF) bands, taking advantage of the complementary aspects of the two technologies. More specifically, a non-orthogonal multiple access (NOMA) scheme is introduced for the visible light communication (VLC) downlink, such that cell-edge users experiencing a weak VLC signal enhance their aggregated data rate with the aid of cooperative communications over RF sidelinks (i.e., device-to-device links). The optimal resource allocation strategy over both VLC and RF bands is derived aiming at EE maximization based on the Dinkelbach’s algorithm and successive convex approximation. Additionally, for the sake of flexibility, a weighted EE metric is proposed for the characterization of the aggregated VLC/RF network performance. Simulation results are provided to validate the proposed analysis, revealing the impact of various design and system parameters, such as the weighting factors, quality of service requirements, and channel conditions.

The integration of Radio Frequency (RF) and Visible Light Communication (VLC) technologies has been considered an enabler to achieving the Key Performance Indicators (KPIs) in Beyond 5G (B5G). Apart from higher data rates for enhanced Mobile Broadband applications, Ultra-Reliable and Low-Latency Communications and massive Machine-Type Communications must be also supported. This poses notable challenges in the design of a mobile communication system that relies exclusively on the use of licensed RF spectrum. In order to cope with the requirements of B5G services, the complementary benefits that RF and VLC bands have in terms of communication bandwidth, signal propagation characteristics, and ultra-densification feasibility, can be exploited. For this purpose, this paper studies the performance of two integration approaches, namely RF - VLC selection (Layer-3 or network-layer) and RF-VLC aggregation (Layer-2 or MAC-layer). Based on the obtained simulation results, it is possible to conclude that RF - VLC aggregation outperforms RF - VLC selection in terms of data rate performance, especially when ultra-reliable communication services are required to connect a large number of user terminals placed in an indoor scenario.

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.

Visible Light Communications (VLC) is a candidate technology that complements the benefits of radio communication networks, particularly in those situations where a low-cost solution using license-free spectrum is required to enable ultra-dense deployments of indoor small cells. However, the main drawback that VLC has when compared to wireless communications on RF bands is that, in presence of obstacles between the transmitter and the receiver, full-blockage events are likely to happen as the power received on reflections is much weaker than the power of the blocked line-of-sight link. In this paper, we take advantage of this phenomenon and study the effect that different activities performed by people in the service area to-be-monitored have on the Automatic Repeat reQuest (ARQ) signaling of the VLC link. Based on the presented experimental studies, it is possible to conclude that different relevant events are able to be detected correctly according to the statistics of the ARQ signaling that is collected from the ongoing VLC transmission.

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.

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.

This paper proposes a Machine Learning (ML) algorithm for hybrid beamforming in millimeter-wave wireless systems with multiple users. The time-varying nature of the wireless channels is taken into account when training the ML agent, which identifies the most convenient hybrid beamforming matrix with the aid of an algorithm that keeps the amount of signaling information low, avoids sudden changes in the analog beamformers radiation patterns when scheduling different users (flashlight interference), and simplifies the hybrid beamformer update decisions by adjusting the phases of specific analog beamforming vectors. The proposed hybrid beamforming algorithm relies on Deep Reinforcement Learning (DRL), which represents a practical approach to embed the online adaptation feature of the hybrid beamforming matrix into the channel states of continuous nature in which the multiuser MIMO system can be. Achievable data rate curves are used to analyze performance results, which validate the advantages of DRL algorithms with respect to solutions relying on conventional/deterministic optimization tools.

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.

Visible light communication (VLC) systems should be designed to provide illumination and wireless data services simultaneously. To achieve this goal at a reasonable cost, the use of Phosphor-Converted (PC) LEDs for indirect illumination should be favored to provide a homogeneous and reliable coverage in the whole service area. Unfortunately, PC-LEDs found in the market so far have not been designed for data transmission; moreover, the response of the other (electro-) optical components of the VLC link are far from ideal. In this paper, we estimate the data rate that is feasible with VLC when indirect illumination is used. For this purpose, the end-to-end response of the VLC link is first modeled using actual measurements of the spectral power distribution of a PC-LED lamp and the ceiling reflectance. Then, different power allocation schemes are studied assuming an optical OFDM waveform. As commercial LEDs have a relatively slow time response, the equivalent VLC channel that results has strong frequency selectivity; therefore, notable data rate gains are achievable when waterfilling power allocation is applied.

Visible Light Communication (VLC) systems are designed to provide illumination and data services simultaneously. To achieve this goal, LED lamps are usually deployed on the ceiling of the rooms, in order to maximize the chances of having Line-of-Sight (LoS) connectivity between the VLC transmitter and the random locations that the VLC receiver can take. In an early stage of adoption, where the cost of LED lighting fixtures incorporating VLC technology will be high to enable ultra-dense deployments, it is expected that only one VLC transmitter is placed per room. In this situation, the use of direct illumination may have serious problems to satisfy the illumination constraints and provide a homogeneous data rate coverage. Moreover, the use of a single powerful LED lamp per room may create discomfort glare effect to users and over-exposure problems in areas of the room at which the light beam is directed. In order to address these problems, this paper studies the data rate that VLC technology can achieve with indirect illumination. That is, when the LED lamp is pointing upwards, and the VLC user receives the optical signal that is reflected back from the ceiling. Obtained simulation results show that indirect illumination provides a more homogeneous data rate coverage when compared to the direct case, while simultaneously verifying the illumination constraints in most of the places that the VLC receiver may take in the room.

This paper demonstrates the feasibility of using supervised learning algorithms to identify the presence of different objects, taking advantage of the effect that they create on the VLC channel gains. For this purpose, a software-defined VLC link is implemented using a Phosphor-converted LED, whose light intensity is modulated by an Optical OFDM frame that includes synchronization words and pilot sequences for channel estimation. Actual estimated channel gains are collected in the receiver, which are used to train and assess the performance of the Random Forest classifier. The accuracy of the monitoring system is evaluated using three different objects, showing an accuracy in the order of 90% in detecting the objects, even when they take different positions when obstructing the VLC link.

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

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

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

Spatial modulation (SM) added to traditional OFDM communications has been intensively studied as a candidate transmission method to convey high-speed, low-delay, power-efficient and high-mobility 5G communications in a reliable basis. This approach implies the use of multiple antennas at the transmitter. Then, the fundamental aspect revised in this work takes into account that in a single-carrier SM system, the selection of the active transmit antenna according to (part of) the information bits makes it possible to use a single power amplifier (PA) that is switched among the available antennas. On the other hand, in a conventional SM-OFDM system, every antenna needs to be continuously active as the index information is typically different for each subcarrier. Consequently, we propose a transmission scheme that precodes the information symbols in frequency domain, such that the global symbol period is split into partitions that enable a sequential operation of antennas which can be fed by a single PA. In addition, it is possible to establish that the proposed approach tends to be more robust against disturbances observed in high mobility environments.

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

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.

This paper studies the gain that cooperative multi-hop transmission provides when used to boost the data rate in Device-to-Device (D2D) communication. Both D2D transmitter and receiver are located in the coverage area of the same Macro Base Station, who is in charge of the control signaling to construct the cooperative cluster(s) of low-cost Relay Nodes (RNs) that Decode-and-Forward information non-coherently from source to destination. Communication resources are divided into two or three equal orthogonal parts for two- and three-hop cooperative transmissions, respectively. For the three-hop cooperative case, backward Interference Cancellation (IC) is also considered in the RNs of the first cluster to reduce multiplexing loss (using two orthogonal portions of communication resources instead of three). The end-to-end data rate of different multi-hop cooperative transmission strategies is studied for different clustering algorithms and measurement reports (i.e., SNR and SINR). Based on obtained performance results, it is possible to conclude that three-hop cooperative transmission with backward IC provides better performance than its two-hop counterpart.

A simple quantization scheme is proposed to adjust separately the amplitudes and phases of signals in a codebook-based Transmit Beamforming (TBF) scheme. The proposed quantization scheme is valid for both desired signal power maximization (egoistic TBF) and co-channel interference mitigation (altruistic TBF), and does not require an exhaustive search to find the optimal TBF vector. Performance results are presented when variable numbers of bits for amplitude and phase feedback resolutions are used for both egoistic and altruistic TBF schemes.

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.

In this paper, we study the impact of cross-layer interference in a two-tier Heterogeneous Network (Het-Net), where the assumption of a single dominant interferer is not always valid. We investigate the improvements that can be achieved when Transmit Beamforming (TBF) is utilized to mitigate the interference originating from multiple interference sources. For this purpose, closed-form expressions are presented to model the outage capacity of a heavily interfered Macro User Equipment (MUE), when a variable number of Femto Base Stations (FBSs) substitute their current beamforming vectors that maximize own-user performance (i.e., egoistic TBF) with those that minimize emitted interference (i.e., altruistic TBF). Our analysis reveals that application of altruistic TBF is most efficient when performed in clusters of interferers, ranked from strongest to weakest according to the level of interference that is observed at the MUE. Determination of the optimal number of participating clusters is strongly connected to the type of feedback algorithm used, and the resolution of the feedback message to be exchanged.

Recently, two cooperative transmission strategies, Coordinated Multi-Point (CoMP) and wireless relaying, are widely expected to be deployed in future LTE-Advanced (LTE-A) systems to improve the coverage of high data rates and cell-edge throughput. Motivated by the common cooperative diversity feature, in this paper we propose a novel OFDM-based relay system with coordinated wireless backhaul (CoMP backhaul), in which two cooperative base stations (CBS) simultaneously transmit information to an infrastructure-based cell-edge relay node (RN), so as to enhance the rate of backhaul link, as well as improve the end to end (e2e) throughput. In order to maximize the achievable e2e rate, a new Co-Phasing waterfilling power allocation scheme is developed for the multi-carrier CoMP backhaul. Meanwhile, with Taylor series expansion, a simplified iterative Co-Phasing waterfilling (ICPWF) algorithm is proposed to seek the optimal power allocation solutions. Numerical results are presented to verify the optimality of the derived scheme and to show e2e throughput gains over non-coordinated schemes.

This work discusses a pricing-based distributed network power minimization approach for multicarrier networks. The aim is to allocate the powers of transmitters over available carriers, such that all Transmitter-Receiver (Tx-Rx) links meet a rate constraint and the total transmit power of the network is minimized. We seek to find a local optimum of the total transmit power by solving the Karush-Kuhn-Tucker (KKT) optimality conditions in a distributed way. Each transmitter minimizes the weighted sum of its powers over carriers, subject to a fixed rate constraint, by a weighted waterfilling principle. The weights consist of interference pricing terms received from interfered links. The exchange of information among the Tx-Rx links enables a non-selfish response that reduces mutual interference. Performance is evaluated in a Small Cell Network (SCN) and compared to a baseline non-cooperative approach. The results show that the proposed algorithm can guarantee a higher rate than the baseline approach, while reducing significantly the total transmit power of the network.

This paper presents a cooperative decentralized scheme for adjusting the Uplink (UL)-Downlink (DL) configuration of a Small Cell Network (SCN) with a TDD air interface. The goal of the cooperative decentralized scheme is to make a more efficient use of common wireless resources, selecting convenient TDD-frame configurations to maximize a sum utility function that takes into account both individual traffic demands and actual interference coupling situations. The cooperative decentralized scheme for UL/DL adaptation can be executed locally at each Base Station (BS), and relies solely on the exchange of low-rate signaling information among neighboring cells. Based on observed performance improvements, we conclude that cooperative decentralized schemes are a viable option for flexible-TDD implementation in SCN environments, particularly in presence of cells with strong interference coupling and low-rate signaling capabilities.

Green radio has attracted much attention currently, and among the many factors to be considered, power consumption reduction is of great significance. In this paper, we consider a power minimization problem with rate requirement and power constraints for downlink multi-cell cooperative MIMO(Co-MIMO) systems. A transmit signal covariance design scheme is proposed, and an integrated algorithm is developed to search the optimal covariance for power minimization. Numericalsimulations verify the optimality of our proposed transmit signal covariance scheme. A significant performance gain is observed over the uniform power allocation (UPA) and the noncooperative geometric mean decomposition (GMD)-based schemes.

Out-of-band power emissions impose stringent limits on the performance that new techniques, such as intercell interference coordination (ICIC) and cognitive radio (CR), provide over wireless systems with OFDM-based air interfaces. To tackle this problem, in this work we consider the use of a simple symbol merging (SM) approach, to enable the reduction of adjacent channel interference when generating the OFDM signal. The basic idea behind this proposal is simple: implement an inverse discrete Fourier transform (IDFT) calculation with double length, to merge OFDM symbols in pairs and reduce by half the number of points with discontinuities in time domain (compared to a conventional OFDM transmission). In addition, a spectral precoding technique is also introduced, to provide a continuous time behavior in the concatenation points that remain after the merging. Based on different performance analyses it is possible to conclude that, our proposed spectral precoding method guarantees a reduced distortion level in the frequency domain, when compared to traditional N-continuous OFDM techniques reported in the literature. Finally, the effect of avoiding the use of cyclic prefix (CP) in part of the OFDM symbols is also studied.

In small cell networks (SCNs) co-channel interference is an important issue, and necessitates the use of interference mitigation strategies that allocate resources efficiently. This work discusses a distributed utility-based algorithm for downlink resource allocation (i.e., power and scheduling weights per carrier) in multicarrier SCNs. The proposed distributed downlink resource allocation (DDRA) algorithm aims to maximize the sum utility of the whole system. To achieve this goal, each base station (BS) selects the resource allocation strategy to maximize a surplus function comprising both, own cell utility and interference prices (that reflect the interference that is caused to neighboring cells). Two different utility functions are considered: max-rate and proportional fair-rate. For performance evaluation, a SCN deployed in a single story WINNER office building is considered. Simulation results show that the proposed algorithm is effective in enhancing not only the sum data rate of a SCN, but also the degree of fairness in resource sharing among users.

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.

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.

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).