In frequency-division duplex (FDD) massive multi-input multi-output (MIMO) systems, downlink channel state information (CSI) feedback from user equipment (UE) to the base station (BS) incurs significant overhead. Convolutional-based models are well-suited for capturing local CSI structures, while Transformer-based models specialize in global representations; however, existing approaches typically exploit only one of these properties, which limits reconstruction performance. We propose CsiFusionNet, a CSI feedback framework that integrates convolutional blocks with a Transformer decoder to jointly exploit local and global channel features. The encoder preserves low UE complexity, while the decoder refines local details and models global dependencies. Simulation results on the COST2100 dataset show that CsiFusionNet achieves superior normalized mean square error (NMSE) performance compared to existing approaches, while significantly reducing UE-side computational cost.

This article investigates the performance of a reconfigurable intelligent surface (RIS)-assisted full-duplex (FD) integrated satellite-terrestrial network (ISTN). Particularly, the RIS structure is incorporated into the ISTN topology to curb the effects of the undesirable self-interference (SI) resulting from the FD operation, cross-tier interference (CTI), and intracell interference (ICI). To examine this network performance, an optimization problem is formulated to maximize the sum rate of users by jointly optimizing the active beamforming and RIS reflection coefficients. To address the non-convexity of the optimization problem, it is decomposed into two subproblems: one which involves applying the fractional programming (FP) technique to find the optimal solution for the active beamforming variable, while the other subproblem constitutes the use of successive convex approximation (SCA) to optimize the passive beamforming variable. These subproblems are solved alternately until convergence. Numerical results presented herein confirm that the proposed algorithm outperforms the benchmark schemes.

The integration of a wireless backhaul network between a macro-cell base station (MBS) and multiple small-cell base stations (SBSs) with full-duplex (FD) technology at the SBSs offers significant potential to enhance spectrum efficiency in millimeter-wave (mmWave) communication systems. However, the FD mode also introduces an inherent trade-off between better time utilization and increased network interference, necessitating careful optimization of duplex modes at each SBS. In this paper, we propose a novel framework that jointly optimizes both the duplex mode selection for each SBS and the design of hybrid beamforming (BF) at the MBS and SBSs to maximize network throughput. Our framework alternately tackles two key subproblems: hybrid BF design and SBS duplex mode selection. For hybrid BF design, we employ a two-stage approach that combines a codebook-based radio frequency beam selection via a greedy algorithm with baseband BF optimization through a generalized power iteration method. For SBS duplex mode selection, we introduce the worst-mode switching (WMS) algorithm to efficiently find a near-optimal solution in polynomial time. Simulation results demonstrate that the proposed joint algorithm not only has low computational complexity but is also robust to various interference scenarios, including different ADC resolutions and varying levels of residual self-interference suppression. This adaptability results in superior sum-rate performance compared to conventional hybrid BF schemes and configurations where all SBSs operate exclusively in either full-duplex or half-duplex mode.

In this paper, we consider a low earth orbit satellite communication system with a legitimate user and an eavesdropper and investigate its secrecy performance. Considering the beam coverage of the satellites, we propose a system where a friendly jammer satellite transmits artificial noise signals only to eavesdroppers. We obtain the cumulative distribution functions (CDFs) of the signal-to-interference-plus-noise ratios taking into account the Nakagami-𝑚 fading and the positions of ground users and satellites. The CDFs are used to derive the analytical forms of the probability of non-zero secrecy capacity, secrecy capacity outage probability, and average secrecy capacity. In addition, for tractability and insight, asymptotic forms are also investigated at a high-power regime. Finally, we present the numerical results to validate the obtained expressions and show the diversity order of the secrecy capacity.

Amidst overwhelming demands for spectrum in the licensed wireless networks caused by the explosive breakthrough of the Internet of Things (IoT), the unlicensed band has been a tremendous resource in enhancing massive connectivity. This has also accelerated the creation of cutting-edge radio access technologies (RATs), such as 5G new unlicensed (NR-U) to support IoT networks. However, these efforts have been frustrated by several performance challenges, including increased interference which has led to the deterioration in the Quality-of-Service (QoS). Moreover, wireless engineers are tasked to develop RATs whose specifications comply with several inter-RAT coexistence regulations. In this article, we propose a joint IoT user selection and resource assignment algorithm to improve the performance of the 5G NR-U-enabled IoT system under QoS constraints with regulated interference to the Wi-Fi-enabled IoT system. Specifically, we formulate a mixed-integer nonlinear programming problem (MINLP) and decompose it into two subproblems, including user selection and power allocation. We adopt the difference of concave functions (DC) approach to solve the resulting optimization problems. We also exploit low-complexity algorithms based on matching theory for IoT user selection and dual-decomposition for power allocation. Especially, we derive a closed form of the power allocation expression using the Lagrangian method. Through various simulations, we demonstrate that the proposed algorithm enables multiuser diversity and significantly improves the spectral efficiency compared to the conventional scheme.

In this article, we study the optimization problem in low Earth orbit (LEO) satellite network scenarios. We present a 3D model of the LEO satellite utilizing a spherical coordinate system, allowing us to determine a practical elevation angle for the ground-to-satellite channel model. Our objective is to maximize the minimum average throughput by formulating an optimization problem that considers the association between the ground terminal (GT) and LEO satellite, as well as the transmit power of the GT. To address this problem, we propose an iterative algorithm that combines the block coordinate descent method with the successive convex approximation algorithm. Simulation results corroborate that our proposed optimal method achieves a substantial throughput gain of up to 59.63 % compared to conventional methods, while ensuring fairness. Additionally, we present two significant findings: 1) the efficiency of our method is demonstrated through Top-view 2D (and 3D) snapshots, showcasing the optimal decisions made, and 2) the impact of key parameters such as the number of resource blocks, interference, and network density on the network performance.

This paper investigates the framework of cross-entropy (CE) based clustering and beamforming for mmWave-non-orthogonal multiple access (NOMA) system taking into consideration the impact of imperfect successive interference cancellation (SIC). For the design of clustering and beamforming, we adopt CE based machine learning algorithm that has the objective to obtain the statistical parameters by minimizing the cross-entropy between optimal and sampling distributions. By using CE based clustering, the number of clusters can be adjusted to strike a balance between the inter-cluster interference and intra-cluster interference introduced by imperfect SIC. Furthermore, the inter-cluster interference induced by spatial beamforming is further reduced using CE based beamforming, which can significantly enhance the system performance of mmWave-NOMA. Based on the result, we compute the power allocation by dividing it into the intra-cluster and inter-cluster power allocation problems. In particular, we derive the optimal intra-cluster power allocation in a closed form and obtain the condition to guarantee the minimum rate requirements of all the users. We next solve the inter-cluster power allocation using convex optimization technique. Data-intensive simulation results illustrate that our proposed algorithm outperforms the conventional algorithm such as$K$-mean based clustering and the number of clusters can be controlled using CE based clustering algorithm.

A mega-constellation of low-altitude earth orbit (LEO) satellites (SATs) are envisaged to provide a global coverage SAT network in beyond fifth-generation (5G) cellular systems. LEO SAT networks exhibit extremely long link distances of many users under time-varying SAT network topology. This makes existing multiple access protocols, such as random access channel (RACH) based cellular protocol designed for fixed terrestrial network topology, ill-suited. To overcome this issue, in this paper, we propose a novel contention-based random access solution for LEO SAT networks, dubbed emergent random access channel protocol (eRACH). In stark contrast to existing model-based and standardized protocols, eRACH is a model-free approach that emerges through interaction with the non-stationary network environment, using multi-agent deep reinforcement learning (MADRL). Furthermore, by exploiting known SAT orbiting patterns, eRACH does not require central coordination or additional communication across users, while training convergence is stabilized through the regular orbiting patterns. Compared to RACH, we show from various simulations that our proposed eRACH yields 54.6% higher average network throughput with around two times lower average access delay while achieving 0.989 Jain’s fairness index.

A mega-constellation of low-altitude earth orbit (LEO) satellites (SATs) are envisaged to provide a global coverage SAT network in beyond fifth-generation (5G) cellular systems. However, such wide coverage rather makes it difficult to apply existing multiple access protocols, such as random access channel (RACH). To overcome this issue, in this paper, we propose a novel random access solution for LEO SAT networks, called as S-RACH. In contrast to existing standardized protocols, S-RACH is a model-free approach using deep reinforcement learning (DRL). Compared to RACH, we show from various simulations that our proposed S-RACH yields around 2x lower average access delay.

Integrating terrestrial networks with burgeoning high-altitude platform stations (HAPSs) will be a disruptive challenge for beyond-5G systems provisioning large-scale three-dimensional connectivity. Here, we study the problem of forwarding packets between terrestrial terminals and backhaul through multi-HAPS relaying. Considering the limited wireless backhaul, which is the practical constraint of HAPS relaying, dual-hop mixed radiofrequency/free-space optical (RF/FSO) networks are investigated, where backhaul-to-relay and relay-to-user communications employ FSO and RF links, respectively. To maximize the end-to-end network throughput, including downlink and uplink rate, we formulate the optimization problem for variables; the association between aerial and terrestrial terminals, transmit power, and deployment of multiple HAPSs, respectively. We tackle this problem using an iterative algorithm with proposed surrogate functions to efficiently obtain the locally optimal solution. Simulation results corroborate that our proposed optimal method achieves up to 11.3% spectral efficiency compared to the conventional heuristic method. Furthermore, we answer three questions; 1) what the wireless backhaul requirements are, 2) how the number of HAPSs and terrestrial terminals impact the network, and 3) what if a certain user terminal has a particular demand on data rate.

Satellite communication (SatCom) is an essential component of next-generation wireless communication to achieve a goal of ubiquitous connectivity on globe. The outage event of SatCom link connecting to a network is more critical in an infrastructure-deficient remote area. In this paper, we analyze outage probability (OP) and symbol error rate (SER) over SatCom downlink channels when the users are randomly located in single beam and multibeam area. The downlink beams will suffer from propagation loss and the shadowed-Rician fading depending on the user location which is assumed to follow a Poisson point process. For mathematically tractable, informative, and insightful interpretation, we obtain the asymptotic OP and SER expressions of user link under several channel conditions in the high power regime. Finally, numerical results are presented to verify the analysis and show the accuracy of the asymptotic results.

Terahertz (THz) communication is a promising technique for chip-to-chip communication and wireless personal area networks. In this paper, we present an experimental study and design to realize such THz communication systems. We develop two different chip sets for on-off-keying (OOK) modulation based THz transceivers which include carrier generators, modulators, THz amplifiers, and baseband amplifiers. Specifically, the first chip set integrates the circuit blocks for the OOK modulation without the THz amplifier for short-range communication. In addition, the second chip set design includes the THz amplifier modules to extend the coverage of transmission. For these two chip sets, we experimentally demonstrate the feasibility of the wireless communication at THz frequency bands and assess performance using the bit error rate (BER) analysis. We estimate the BER by calculating the signal-to-noise ratio (SNR) based on the eye diagram and compare with actual BER measurements and Monte Carlo simulations. We also address the impact of the distance, the transmit power, and the data rate for the proposed THz transceivers based on the link budget analysis, and confirm the accuracy of the derived BER expression.

In this paper, we investigate a mobile relaying system assisted by an unmanned aerial vehicle (UAV) with a finite size of the buffer. Under the buffer size limit and delay constraints at the UAV relay, we consider a dual-hop mixed free-space optical/radio frequency (FSO/RF) relaying system (i.e., the source-to-relay and relay-to-destination links employ FSO and RF links, respectively). Taking an imbalance in the transmission rate between RF and FSO links into consideration, we address the trajectory design of the UAV relay node to obtain the maximum data throughput at the ground user terminal. Specifically, we classify two relaying transmission schemes according to the delay requirements, i.e., i) delay-limited transmission and ii) delay-tolerant transmission. Accordingly, we propose an iterative algorithm to effectively obtain the locally optimal solution to our throughput optimization problems and further present the complexity analysis of this algorithm. Through this algorithm, we present the resulting trajectories over the atmospheric condition, the buffer size, and the delay requirement. In addition, we show the optimum buffer size and the throughput-delay tradeoff for a given system. The numerical results validate that the proposed buffer-aided and delay-considered mobile relaying scheme obtains 223.33% throughput gain compared to the conventional static relaying scheme.

A mega-constellation of low-earth orbit (LEO) satellites has the potential to enable long-range communication with low latency. Integrating this with burgeoning unmanned aerial vehicle (UAV) assisted non-terrestrial networks will be a disruptive solution for beyond 5G systems provisioning large-scale three-dimensional connectivity. In this article, we study the problem of forwarding packets between two faraway ground terminals, through an LEO satellite selected from an orbiting constellation and a mobile high-altitude platform (HAP) such as a fixed-wing UAV. To maximize the end-to-end data rate, the satellite association and HAP location should be optimized, which is challenging due to a huge number of orbiting satellites and the resulting time-varying network topology. We tackle this problem using deep reinforcement learning (DRL) with a novel action dimension reduction technique. Simulation results corroborate that our proposed method achieves up to 5.74x higher average data rate compared to a direct communication baseline without SAT and HAP.

In this work, we address the optimization of vertical backhaul framework where multiple aerial base station (ABS) are deployed to conFigure the intermediate backhaul links for terrestrial base stations (TBS) in wireless networks. Here, we focus on maximizing the downlink network throughput in mixed RF/FSO backhaul networks by optimizing resource allocation and placement. Specifically, the ABS-TBS association, transmit power, and positions of ABSs are alternately and iteratively optimized. To solve the corresponding mixed-integer non-convex optimization problem, we propose an efficient iterative algorithm by applying the block coordinate descent method and successive convex optimization techniques. Although we obtain suboptimal solutions due to the non-convexity of the problems, simulation results indicate that the proposed scheme demonstrates the significant network throughput gain compared with the conventional scheme.

In this paper, we investigate both the bit error rate (BER) and outage performance of free-space optical (FSO) links over strong turbulence combined with various pointing error conditions. Considering atmospheric turbulence and pointing errors as main factors that deteriorate the quality of an optical link, we obtain a unified finite series approximation of the composite probability density function, which embraces generalized pointing error models. This approximation leads to new unified formulas for the BER and outage capacity of an FSO link, which account for the two possible detection mechanisms of intensity modulation/direct detection and heterodyne detection. Selected simulation results confirm that the newly derived approximations can give precise predictions of both the average BER and the outage capacity of FSO communication that are generally applicable to all environments.

In this work, we address the trajectory optimization of a fixed-wing unmanned aerial vehicle (UAV) using free space optical communication (FSOC). Here, we focus on maximizing the flight time of the UAV by considering practical constraints for wireless UAV communication, including limited propulsion energy and required data rates. We find optimized trajectories in various atmospheric environments (e.g., moderate-fog and heavy-fog conditions), while also considering the channel characteristics of FSOC. In addition to maximizing the flight time, we consider the energy efficiency maximization and operation-time minimization problem to find the suboptimal solutions required to meet those constraints. Furthermore, we introduce a low-complexity approach to the proposed framework. In order to address the optimization problem, we conduct a bisection method and sequential programming and introduce a new feasibility check algorithm. Although our design considers suboptimal solutions owing to the nonconvexity of the problems, our simulations indicate that the proposed scheme exhibits a gain of approximately 44.12% in terms of service time when compared to the conventional scheme.

In this paper, we consider an unmanned aerial vehicle (UAV) aided mobile relaying system under a buffer constraint at the relay node. We propose a new relaying protocol employing mixed free-space optical/radio frequency (FSO/RF) communication, i.e., the source-relay and relay-destination link utilize FSO communication and RF communication, respectively, under the buffer constraint which is required to consider practical relay system. We study the trajectory optimization problem of buffer-constrained UAV-relaying in order to maximize the end-to-end data throughput. Taking the conditions of the mixed FSO/RF systems (e.g., a full-duplex relaying network that works in decode-and-forward, atmospheric attenuation, transmit power, and bandwidth in FSO and RF links) into consideration, we characterize the channel and throughput models. Furthermore, corresponding to the buffer-aided relaying, we derive a limited buffer constraint regarding the state of the queue in the buffer of relay. We solve the optimal trajectory problem of the UAV to maximize the throughput of user terminal using quadratically constrained programming. As a result, we propose an iterative algorithm that efficiently finds a local optimum solution for the throughput maximization problems. Our numerical results show that proposed buffer-aided mobile relaying achieves 161.3% throughput gains compared to a static relaying scheme.

In this paper, we deal with the performance analysis of a cumulative distribution function (CDF)-based multiuser scheduling scheme for downlink non-orthogonal multiple access (NOMA). With this scheme, the multiple users having relatively better channel gain are selected. We first statistically analyze the probability of user selection in terms of the weight of each user determining the time fraction. We confirm that equal weight ensures fair resource allocation. Under this equal weight condition for fairness, we derive the distribution of signal-to-interference-plus-noise ratio (SINR) when the specific user is selected. Simulation results show that with this scheme, the time resources can be allocated more fairly compared to the proportional fair scheduling scheme, especially under NOMA conditions. Further, by adjusting the weight of desired user, the average rate and selection probability can be controlled.

This paper presents novel ultrareliable and low-latency communication (URLLC) techniques for URLLC services, such as Tactile Internet services. Among typical use cases of URLLC services are teleoperation, immersive virtual reality, cooperative automated driving, and so on. In such URLLC services, new kinds of traffic such as haptic information including kinesthetic information and tactile information need to be delivered in addition to high-quality video and audio traffic in traditional multimedia services. Furthermore, such a variety of traffic has various characteristics in terms of packet sizes and data rates with a variety of requirements of latency and reliability. Furthermore, some traffic may occur in a sporadic manner but requires reliable delivery of packets of medium to large sizes within a low latency, which is not supported by current state-of-the-art wireless communication systems and is very challenging for future wireless communication systems. Thus, to meet such a variety of tight traffic requirements in a wireless communication system, novel technologies from the physical layer to the network layer need to be devised. In this paper, some novel physical layer technologies such as waveform multiplexing, multiple-access scheme, channel code design, synchronization, and full-duplex transmission for spectrally efficient URLLC are introduced. In addition, a novel performance evaluation approach, which combines a ray-tracing tool and system-level simulation, is suggested for evaluating the performance of the proposed schemes. Simulation results show the feasibility of the proposed schemes providing realistic URLLC services in realistic geographical environments, which encourages further efforts to substantiate the proposed work.

In uplink multiuser systems, different timing offset (TO) and/or carrier frequency offset (CFO) of multiple users cause multiuser interference (MUI) to a desired user. In this paper, we propose two different MUI cancellation methods, namely weighted parallel interference cancellation (WPIC) and adaptive interference cancellation filter (AICF), for uplink transmission with generalized frequency division multiplexing (GFDM) waveform over Rayleigh fading channel given the existence of TO and CFO of multiple users. The proposed WPIC performs MUI estimation using the detected data symbol of other users and obtains the optimal weight to maximize the signal-to-interference ratio (SIR) of the desired user. We also propose the AICF that aims to maximize the SIR of the desired user. Based on our SIR expression derived in closed-form in the presence of TO and CFO, the AICF is obtained by solving the Rayleigh quotient problem. Lastly, the impact of multiple antennas at the receiver on MUI cancellation is also analyzed to improve reliability. The simulation results show that the proposed WPIC and AICF effectively eliminate MUI and that the AICF can outperform the WPIC in terms of the SIR and spectral efficiency. In addition, the multiple receiver antennas effectively cancel MUI as the number of antennas goes to infinity.

In uplink multiuser systems, different timing offset (TO) and carrier frequency offset (CFO) of different users cause multiuser interference (MUI) to desired user. In this paper, we propose an optimal receiver filter for uplink transmission with generalized frequency division multiplexing (GFDM) when TO and CFO of different users exist. Proposed optimal receiver filter maximizes signal-to-interference ratio (SIR) of desired user with TO and CFO in Rayleigh channel. Based on SIR expression derived in closed form in the presence of TO and CFO, we obtain the optimal receiver filter by solving Rayleigh quotient problem. We show from the simulation that the proposed receiver filter effectively eliminates MUI caused by TO within cyclic prefix range and CFO compared to the conventional zero-forcing filter.

Order statistics find applications in various areas including communications and signal processing. In this paper, we introduce new results of the joint statistics of partial products of ordered random variables (RVs) based on a Mellin-transform-based unified analytical framework. With the proposed approach, we can systematically derive the joint statistics of any partial products of ordered statistics, in terms of the Mellin transform and the probability density function (PDF). Our Mellin-transform-based approach can apply when all the K-ordered RVs are involved even for more complicated cases, when only the Ks (Ks <; K) best RVs are also considered. In addition, the closed-form expressions for the exponential RV special case are presented. As an application example, these results can apply to the performance analysis of various wireless communication systems over fading channels.

In this paper, we propose a threshold-based-multiple optical signal selection scheme (TMOS) for free-space optical systems based on wavelength division multiplexing. With the proposed TMOS, we can obtain higher spectral efficiency while reducing the potential increase in complexity of implementation caused by applying a selection-based beam selection scheme without a considerable performance loss. Here, to accurately characterize the performance of the proposed TMOS, we statistically analyze the characteristics with heterodyne detection technique over independent and identically distributed Log-normal turbulence conditions taking into considerations the impact of pointing error. Specifically, we derive exact closed-form expressions for the average bit error rate, and the average spectral efficiency by adopting an adaptive modulation. Some selected results shows that the average spectral efficiency can be increased with TMOS while the system requirement is satisfied.

In this paper, we analyze the impacts of timing offset (TO), carrier frequency offset (CFO), and phase noise in orthogonal frequency division multiplexing (OFDM) and generalized frequency division multiplexing (GFDM) waveforms. As TO can be classified into four cases depending on the direction of offset, we provide the analysis of signal-to-interference ratio (SIR) for each of the cases when phase noise and synchronization errors of the desired user occur in frequency selective channel. We also propose the receiver filter for GFDM systems that is optimized to maximize SIR with CFO in additive white Gaussian noise channel. Simulation results show that GFDM is more sensitive to CFO than OFDM. We also confirm that GFDM systems using proposed receiver filter are robust against CFO compared with conventional systems.

Random unitary beamforming (RUB) achieves multiuser diversity gain over multiple-input multiple-output (MIMO) broadcast channels with channel state information (CSI). In this paper, we optimize RUB schemes in terms of energy efficiency (EE) which is defined as "capacity/power consumption". The key idea of our approach is to derive EE-optimal number of antennas and transmit power based on mathematical analysis. Using simple approximation expression, we derive efficient approach to find global optimum. Simulation results demonstrate that analytical results are accurate.

In this paper, we propose an interference alignment (IA) scheme based on limited feedback for the two-cell interfering multiple input multiple output multiple access channel (MIMOMAC), which obtains the precoding vectors in the codebook directly so that the inter-cell interference (ICI) channels are most aligned. With the quantized precoding vectors, the receive beamforming filters are designed to cancel the impact of interuser interference (IUI) and reduce the residual ICI. In addition, we characterize the complexity of our proposed IA scheme. From simulation results, we demonstrate that the proposed scheme can provide the better sum rate performance under the same system configurations than the existing schemes which mitigate the interference leakage by aligning the ICI channels under limited feedback.

Interference alignment (IA) needs multiple dimensions, which can be achieved by extending time, frequency or spatial domain. However, using time or frequency domain for alignment requires an exponential number of resources as the degrees of freedom increases. In addition, it is impractical to employ many number of antennas in the spatial-domain-based interference alignment system. In this paper, we consider MIMO-OFDM systems over K-user interference channels and present resource allocation criteria based on the IA scheme utilizing spatial and frequency domain simultaneously. Then, we propose a low complexity resource allocation scheme appropriate to both spatial and frequency domain IA. We show that a single additional subcarrier in the proposed resource allocation scheme is enough to improve the sum-rate performance close to the optimal performance.

In this paper, we propose a non-iterative transceiver design based on interference alignment (IA) for MIMO interfering broadcast channels with arbitrary number of cells and users per cell. We consider the case where a base station has more antennas than a user, which is a typical scenario of downlink transmission in the cellular network. By aligning the interference from other cells efficiently, we show that the proposed transceiver design can achieve the degrees of freedom (DOF) of$d$per user, i.e., total DOF of$\sum_{i=1}^{C}K_{i}d$, where$C$is the number of cells,$K_{i}$is the number of users for the$i$-th cell, and$d$is the number of streams per user. Accordingly, we investigate conditions for the antenna configuration to attain the total DOF of$\sum_{i=1}^{C}K_{i}d$. We also analyze the computational complexity and achievable DOF of the proposed transceiver design. Finally, simulation results are provided to assess the sum-rate performance and achievable DOF of the proposed transceiver design, which validates the advantage of our design over existing schemes.

In this paper, we consider an alternate transmission scheme for a multiple-input multiple-output interference relay channel where multiple sources transmit their own signals to their corresponding destinations via one of two relaying groups alternately every time phase. Each of the relaying groups has arbitrary number of relays, and each relay operates in half-duplex amplify-and-forward mode. In our scheme, the received signals at the relay nodes consist of desired signals and two different interference signals such as the inter-source interferences and the inter-group interferences which are caused by the phase incoherence of relaying. As such, we propose an iterative interference alignment algorithm to mitigate the interferences. We show that our proposed scheme achieves additional degrees of freedom compared to the conventional half-duplex relaying system in the interference relay channels.

In this paper, considering a two-cell interfering MIMO multiple access channel (MIMO-MAC), we propose an interference alignment scheme with an efficient limited feedback and investigate the conditions for the antenna configuration. In addition, we characterize the interference leakage power caused by quantization error as a function of feedback bits. From simulation results, we demonstrate that the proposed scheme can provide the better sum rate performance than the existing scheme under the same number of feedback bits.

In this paper, we consider resource allocation method in the visible light communication. It is challenging to achieve high data rate due to the limited bandwidth of the optical sources. In order to increase the spectral efficiency, we design a suitable multiple-input multiple-output (MIMO) system utilizing spatial multiplexing based on singular value decomposition and adaptive modulation. More specifically, after explaining why the conventional allocation method in radio frequency MIMO channels cannot be applied directly to the optical intensity channels, we theoretically derive a power allocation method for an arbitrary number of transmit and receive antennas for optical wireless MIMO systems. Based on three key constraints: the nonnegativity of the intensity-modulated signal, the aggregate optical power budget, and the bit error rate requirement, we propose a novel method to allocate the optical power, the offset value, and the modulation size. Based on some selected simulation results, we show that our proposed allocation method gives a better spectral efficiency at the expense of an increased computational complexity in comparison to a simple method that allocates the optical power equally among all the data streams.

In this paper, we propose two different node pair selection schemes to enhance the system performances such as the sum rate and the outage probability in multiple-input multiple-output (MIMO) interference channel where multiple transmitters and multiple receivers communicate simultaneously as a pair. Firstly, we propose the optimum node pair selection scheme in terms of the outage probability. Then the max-min node pair selection scheme is proposed to improve both the sum rate and the outage probability simultaneously. From some numerical results, we confirm that the proposed schemes provide significant performance enhancement compared to the system when node pairs are fixed.

Two-way relaying is the one of the solutions to mitigate the loss in spectral efficiency due to the half-duplex transmission in the multi-hop communication. In this paper we propose a simple two-way relaying scheme for the two-hop two-user multiple input multiple output (MIMO) system. In this system, a base station and a relay station (RS), both equipped with two antennas, form a point-to-point MIMO channel while RS and two single-antenna mobile users form a point-to-multipoint multiuser MIMO channel. Based on the proposed scheme, we also propose optimal power allocation algorithm at RS. Through the numerical examples, we show that our proposed scheme achieves significant sum rate gain compared to the one-way relaying scheme and its performance gain increases as the distance between RS and both users becomes closer.

In this paper, we develop a non-iterative transceiver design based on interference alignment (IA) for MIMO interfering broadcast channels with arbitrary number of cells and users per cell. By aligning the interference from other cells efficiently, we show that the proposed transceiver design can achieve the degrees of freedom (DOF) of d per user, i.e. total DOF of equation, where C is the number of cells, Kiis the number of users for the i-th cell, and d is the number of streams per user. We also investigate conditions for the antenna configuration in order to attain the total DOF of equation. From simulation results, we demonstrate that the proposed transceiver design can obtain higher DOF than the conventional scheme under equal antenna configuration.

In this paper, we propose a linear transceiver design based on interference alignment (IA) for multiple-input multiple-output (MIMO) heterogeneous networks, which consist of overlaying macrocell and picocells. By exploiting the concept of IA, we show that the proposed transceiver design can mitigate the effect of interference efficiently. We also provide a transceiver design based on IA for additional deployments of femtocells by little modification of the proposed design. From simulation results, we show that the proposed transceiver design can provide additional degrees of freedom (DOF) compared to the conventional interference coordination schemes. Furthermore, we also demonstrate that the proposed transceiver design can be easily generalized to the network with arbitrary number of cells and users per macrocell.

For multicell environment, relays introduce additional interferences at the cell edge. To overcome such intercell interferences, we propose the alternate relay transmission schemes. We quantify the ergodic rate performance of the proposed schemes over Nakagami-m fading channel. Numerical results show that the proposed schemes can effectively cancel the inter-cell interference and improve the sum rate performance for cell edge users.

In this paper, we propose an interference alignment scheme based on the random vector quantization codebooks for the constant MIMO interference channel. We quantify an upper bound of the rate loss of the proposed scheme and derive the scaling law of the feedback load to maintain the constant rate loss relative to the interference alignment with perfect channel knowledge. Moreover, we compare the proposed scheme with the interference alignment scheme using analog feedback and show the advantage of the proposed scheme under the constraint of the feedback channel uses. From simulation results, we demonstrate that the proposed scheme can provide the constant rate loss by imposing the feedback load according to the derived scaling law.

In a half-duplex relaying, the capacity pre-log factor 1/2 is a major drawback in spectral efficiency. This paper proposes a linear precoding/decoding scheme and an alternate relaying protocol in a dual-hop half-duplex system where three relays help the communication between the source and the destination. In our proposed scheme, we consider a phase incoherent method in relays in which the source alternately transmits message signals to the different relays. In addition, we propose a linear interference alignment scheme which can suppress the inter-relay interference resulting from the phase incoherence of relaying. Based on our analysis of degrees of freedom and our simulation results, we show that our proposed scheme achieves additional degrees of freedom compared to the conventional half-duplex relaying.

In this paper, we investigate a relay enhanced cellular system, where a relay station is located in the overlap area served by two base stations. We propose cooperative joint precoding schemes for the downlink transmission of such relay enhanced cellular system to maximize the system capacity while minimizing the interference at both the relay station and the mobile stations. We formulate the optimization problems to maximize the system capacity and design the multiuser precoding vectors at each base station and the relay station. We quantify the ergodic rate performance of the proposed multiuser precoding schemes through statistical analysis. The extensively derived ergodic expressions will facilitate the accurate performance evaluation of the proposed transmission schemes. Numerical results show that the proposed schemes can effectively cancel the interference and improve the sum rate and the outage performance for cell edge users.

In this paper, we propose a non-iterative symbol-wise beamforming scheme for MIMO-OFDM systems, which can provide the performance approaching that of the iterative symbol-wise beamforming scheme and can reduce the computational complexity significantly for channels with a small number of strong channel taps. Simulation results show that our proposed scheme leads to a negligible performance loss compared with the iterative symbol-wise beamforming scheme regardless of spatial correlation or presence of co-channel interference. For realistic situations, we also consider the system designs to manage imperfect channel knowledge at the transmitter or receiver. First, we propose a limited feedback technique and a codebook design algorithm for symbol-wise beamforming in a spatially correlated frequency selective channel. Simulation results show that the proposed limited feedback technique can provide insignificant outage capacity loss compared with the symbol-wise beamforming scheme with perfect channel knowledge when the proposed codebook is adopted with a moderate number of feedback bits. Second, we propose a robust transceiver optimization for symbol-wise beamforming under channel uncertainty. From simulation results, we confirm that our proposed robust transceiver optimization can incur less degradation than the iterative and proposed symbol-wise beamforming schemes under the assumption of a bounded channel uncertainty model.

In this paper, we consider a low complexity symbol-wise beamforming for MIMO-OFDM systems. We propose a non-iterative algorithm for the symbol-wise beamforming, which can provide the performance approaching that of the conventional symbol-wise beamforming based on the iterative algorithm. We demonstrate that our proposed scheme can reduce the computational complexity significantly. From our simulation results, it is evident that our proposed scheme leads to a negligible performance loss compared to the conventional symbol-wise beamforming regardless of spatial correlation or presence of co-channel interference.

In this paper, we propose a novel peak to average power ratio (PAPR) reduction scheme of orthogonal frequency division multiplexing (OFDM) signals. In our proposed scheme we exploit the repeated rotation and recombination of the in-phase and the quadrature components of an OFDM symbol in time domain. Our proposed scheme has two main advantages over the previously known scheme such as iterative partial transmit sequence (I-PTS), which are 1) remarkably lower computational complexity and 2) less required side information overhead than the I-PTS scheme. Based on our derived complementary cumulative distribution (CCDF) of PAPR, we show that our proposed scheme gives only 0.2 - 0.3 dB loss compared to the I-PTS scheme in terms of PAPR threshold while the complexity for the implementation is much reduced. Furthermore, the proposed scheme does not cause any loss of bit error rate performance over both the additive white Gaussian noise (AWGN) channel and the frequency selective channel even though the simulation results regarding the bit error rate (BER) performance is not shown in this paper due to the limited space of the pages.

In wireless X networks where all transmitters send the independent messages to all receivers, the theoretical degrees of freedom (DOF) and interference alignment (IA) scheme for its achievability are given by Cadambe and Jafar. However, IA scheme for wireless X network may be infeasible in practice unless the transmitters have the perfect channel information. In addition, if the transmitter with single antenna uses time-varying channel coefficients as a beamforming vector, the infinite channel extension is required to achieve the theoretical DOF of wireless X networks with perfect IA scheme. In this paper, we consider K-user MIMO X network where K transmitters and K receivers have M antennas each. While the beamforming vectors have been constructed with multiple channel uses over multiple time slot in the earlier work, we consider the beamforming vectors constructed only by a spatial signature over unit time. Accordingly the channel information at the transmitters can be available instantaneously. Then we propose the perfect IA scheme with no channel extension. Based on our sum-rate analysis and the simulation results, we confirm that our proposed scheme can achieve MK/2 DOF which is quite close to the theoretical degrees of freedom region of wireless X networks.

In this paper, we introduce the notion of a relay enhanced cellular system, where a relay station is located in the overlap area served by two base stations, and propose a cooperative joint precoding scheme for the relay enhanced cellular system. We consider a system in which two base stations apply linear precoding to minimize the interference at the relay station and the mobile stations and formulate the optimization problem to maximize the system capacity and design the multiuser precoding vector for each base station. We quantify the ergodic rate performance of the proposed multiuser precoding scheme through statistical analysis. Numerical results show that the proposed scheme can effectively cancel the interference and improve the sum rate performance for cell edge users.

Visible light communication (VLC) using optical sources which can be simultaneously utilized for illumination and communication is currently an attractive option for wireless personal area network. Improving the data rate in optical wireless communication system is challenging due to the limited bandwidth of the optical sources. In this paper, we design the singular value decomposition (SVD)- based multiplexing multiple-input multiple-output (MIMO) system to support two data streams in optical wireless channels. Noting that the conventional allocation method in radio frequency (RF) MIMO channels cannot be applied directly to the optical intensity channels, we propose a novel method to allocate the optical power, the offset value and the modulation size for maximum sum rate under the constraints of the nonnegativity of the modulated signals, the aggregate optical power and the bit error rate (BER) requirement. The simulation results show that the proposed allocation method gives the better performance than the method to allocate the optical power equally for each data stream.

In this paper, we consider the symbol-wise beamforming with limited feedback for MIMO-OFDM systems in the presence of co-channel interference. We propose a non-iterative algorithm for the symbol-wise beamforming with limited feedback, which provides the considerable performance with low computational complexity. We also propose the codebook for the symbol-wise beamforming with limited feedback under spatially correlated frequency-selective channel. It is clear from our simulation that our proposed scheme outperforms the symbol-wise beamforming with limited feedback based on the iterative algorithm. We also confirm that our proposed codebook shows a better performance compared to the random vector quantization (RVQ) codebook.

In this paper, we introduce the notion of a relay enhanced cellular system, where a relay station is located in the overlap area served by two base stations. We propose a cooperative joint precoding scheme in the downlink of the relay enhanced cellular system to maximize the system capacity while eliminating the interference at the mobile stations. We formulate an optimization problem to maximize the system capacity and design the multiuser precoding vectors for each base station and the relay station. We analyze the ergodic rate performance of the proposed multiuser precoding scheme over Nakagami-m fading channels. Numerical results show that the proposed scheme can effectively cancel the interference and improve the sum rate and outage performance for cell edge users.

In this paper, we propose an adaptive beam deactivation scheme for the coordinated multi-cell multiuser multiple-input multiple-output (MU-MIMO) downlink system with limited feedback. In the proposed scheme, each user feeds back the indices of a desired beam and multiple undesired beams from its codebook and some channel quality indicators to its base station (BS). Based on the feedback information and the proposed transmitting user set selection algorithm, the coordinating BSs select one user for each BS to serve. Through the numerical example, we show that the proposed scheme achieves comparable sum rate gain compared to the conventional zeroforcing based coordinated multi-cell system with reduced feedback load.

In this letter, we propose two-way amplify-and-forward relaying in conjunction with adaptive modulation in order to improve spectral efficiency of relayed communication systems while monitoring the required error performance. We also consider a multiple relay network where only the best relay node is utilized so that the diversity order increases while maintaining a low complexity of implementation as the number of relays increases. Based on the best relay selection criterion, we offer an upper bound on the signal-to-noise ratio to keep the performance analysis tractable. Our numerical examples show that the proposed system offers a considerable gain in spectral efficiency while satisfying the error rate requirements.

Random unitary beamforming (RUB) is an attractive transmission scheme for MIMO broadcast channels because of its ability to achieve high sum-rate capacity with limited feedback. In this letter, we derive the exact analytical expressions for the ergodic sum rate of MIMO broadcast channels with RUB. The analysis is facilitated by the development of the complete statistical characterization of ordered beam SINRs for a user, which can find their application in many related problems.

In this paper, we propose a new amplify-and-forward (AF) cooperative transmission scheme with adaptive modulation to maximize overall system spectral efficiency. We quantify the average spectral efficiency performance of the proposed scheme through statistical analysis based on an upper bound of the combiner output signal-to-noise ratio (SNR) and obtain the closed-form solutions for some practical scenarios of interest.

In this paper, we consider a beamforming system in millimeter-wave wireless personal area networks (WPAN), where transmit and receive weight vectors are jointly derived by exchanging a training sequence repeatedly with different combinations of transmit and receive weight vectors. We propose a low complexity codebook-based beamforming scheme that consists of multiple levels and level-adaptive antenna selection in order to reduce the beamforming setup time. For each level, 1) the transmit and receive antennas are selected according to pre-determined inter-element spacings, 2) the training sequences are sent with different weight vectors from a pre-defined codebook and 3) the receiver selects the best transmit and receive weight vectors in order to optimize an effective signal-to-noise ratio (SNR) and these vectors are used to determine the codebook for the next following level. Even with low complexity, we show from the numerical results that our proposed scheme can provide an effective SNR gain and an average spectral efficiency approaching those of the codebook-based beamforming with exhaustive search.

In this paper, we propose and study a cooperative transmission scheme for amplify-and-forward (AF) relaying system with adaptive modulation. Unlike the conventional incremental relaying with adaptive modulation (IR-AM) scheme, which improves the spectral efficiency with the minimum usage of relaying, our proposed scheme adaptively utilizes cooperative relaying to maximize the system spectral efficiency. We quantify the average spectral efficiency performance of the proposed scheme through statistical analysis and obtain the closed-form solutions for some practical scenarios of interest. Selected numerical results demonstrate that the spectral efficiency gain of the proposed scheme over the IR-AM scheme occurs mainly over the median signal-to-noise ratio range.

In this paper, we consider multi-carrier systems with multiple transmit antennas under a power balancing constraint. Applying transmit antenna selection and discrete rate adaptive modulation using M-ary quadrature amplitude modulation (QAM) according to the channel variation per subcarrier, we develop an optimal antenna selection scheme in terms of maximum spectral efficiency where all the possible grouping to send the same information bearing signals in a group of subcarriers are searched and the groups of subcarriers to provide the frequency diversity gain are formed. In addition, we propose a suboptimal method to reduce the computational complexity of the optimal method. The suboptimal scheme consider only the subcarriers under outage and those are combined sequentially until it meets a required SNR. Numerical results show that the proposed suboptimal method with diversity combining outperforms the optimal antenna selection without diversity combining, especially for low SNR region and offers the spectral efficiency close to that of the optimal method with diversity combining, while maintaining lower complexity.

In multicell wireless systems with insufficient frequency reuse, user transmission will suffer other-cell interference (OCI). Cell cooperation is an effective way to mitigate OCI and increase the system sum rate. An adaptive scheme for serving one user in each cell was proposed in. In this paper, we generalize that scheme by serving more than one user in each cell with adaptive zeroforcing beamforming (ZF) strategies. Based on our derived statistics of the signal-to-noise plus interference ratios, we choose the scheme to maximize the total ergodic sum-rate based on user locations. Through the numerical examples, we show that the total system sum rate can be improved by selecting appropriate transmitting strategy adaptively. As a result, our proposed system can explore spatial multiplexing gain without additional power and thus improves total system sum rate significantly.

We propose a new non-iterative coordinated beamforming scheme to obtain full multiplexing gain in 2-user MIMO systems. In order to find the beamforming and combining matrices, we solve a generalized eigenvector problem and describe how to find generalized eigenvectors according to the Gaussian broadcast channels. Selected simulation results show that the proposed method yields the same sum-rate performance as the iterative coordinated beamforming method, while maintaining lower complexity by non-iterative computation of the beamforming and combining matrices. We also show that the proposed method can easily exploit selective gain by choosing the best combination of generalized eigenvectors.

In this paper, we consider multi-carrier systems with multiple transmit antennas under the power balancing constraint, which is defined as the constraint that the power on each antenna should be limited under a certain level due to the linearity of the power amplifier of the RF chain. Applying transmit antenna selection and fixed-power variable-rate transmission per subcarrier as a function of channel variations, we propose an implementation-friendly antenna selection method which offers a reduced complexity in comparison with the optimal antenna selection scheme. More specifically, in order to solve the subcarrier imbalance across the antennas, we operate a two-step reallocation procedure to minimize the loss of spectral efficiency. We also provide an analytic lower bound on the spectral efficiency for the proposed scheme. From selected numerical results, we show that our suboptimal scheme offers almost the same spectral efficiency as the optimal one.

In this paper, We propose a new non-iterative coordinated beamforming scheme to obtain full multiplexing gain in 2-user MIMO systems. In order to find the beamforming and combining matrices, we solve a generalized eigenvector problem and describe how to find generalized eigenvectors according to the Gaussian broadcast channels. Selected simulation results show that the proposed method yields the same sum-rate performance as the iterative coordinated beamforming method with a lower complexity non-iterative computation of the beamforming and combining matrices. We also show that the proposed method can easily exploit selective gain by choosing the best combination of generalized eigenvectors.

Cooperative diversity has been recently presented as a new technique to improve the link performance of wireless communication systems. However, a loss in spectral efficiency due to half-duplex relaying can be a significant weak-point for high data rate communications. In this paper, we propose two-way amplify-and-forward relaying in conjunction with adaptive modulation in order to improve spectral efficiency of relayed communication systems while monitoring the required error performance. By using the ideal-scaled gain at the relay instead of the more practical power-scaled gain, we provide an approximate but a very accurate performance analysis of our proposed system. Our numerical examples show that the proposed system offers a considerable gain in the spectral efficiency and outage probability while satisfying the error rates requirements.

We consider, in this paper, channel allocation and rate adaptation scheme for relayed transmission over correlated fading channels via cross-layer design. Specifically, jointly considering the data link layer buffer occupancy and channel quality at both the source and relay nodes, we develop an optimal channel allocation and rate adaptation policy for a dual-hop relayed transmission. As such the overall transmit power for the relayed system is minimized while a target packet dropping rate (PDR) due to buffer over flows is guaranteed. In order to find such an optimal policy, the channel allocation and rate adaptation transmission framework is formulated as a constraint Markov decision process (CMDP). The PDR performance of the optimal policy is compared with that of two conventional suboptimal schemes, namely the channel quality based and the buffer occupancy based channel allocation schemes. Numerical results show that for a given power budget, the optimal scheme requires significantly less power than the conventional schemes in order to maintain a target PDR.

In this paper, we study two-way amplify-and-forward relaying in conjunction with adaptive modulation over a multiple relay network. In order to keep the diversity order equal to the number of relays and maintain a low complexity, we consider the best relay selection scheme in this work. Based on the proposed selection criterion for the best relay, we analyze the average spectral efficiency by its approximated upper bound. In addition, we extend the proposed scheme to the case where a direct path between source and destination exists. Our numerical examples show that the proposed system offers a considerable gain in the spectral efficiency while satisfying the error rates requirements.

In this paper, we consider two-way amplify-and-forward relaying communication over a multiple relays network in which the best relay node is selected among others. Our proposed scheme picks for duplex communication between the source and the destination only the relay which provides the best end-to-end performance based on the instantaneous channel information. Similar to the relay selection method proposed, our scheme also does not require any explicit communications among the relays. Based on the proposed relay selection criterion, we offer the performance bounds on the average sum-rate, average symbol error rate, outage probability over identically, independent distributed Rayleigh fading channels. Further, our numerical examples show that our proposed scheme provides considerable sum-rate gains and offers an increasing a diversity order as the number of relay candidates increases.

In this paper, we propose a low complexity cooperative communication systems with the amplify-and-forward protocol over a multiple distributed relay network where only one relay among relay candidates is selected according to a certain threshold. By setting different values of their threshold, we consider two different relay selection schemes: (i) achieving the maximum spectral efficiency and (ii) minimizing system complexity. From our provided statistics of the combined output signal-to-noise ratio and our analysis of the system complexity, we show that our proposed node selection schemes provide considerable spectral efficiency gain and outage performance while it has a relatively low complexity comparing to other schemes known in the literatures.

In this paper, we first present a new closed form approximation for the sum-rate of random beamforming. A generic curve fitting procedure is developed to make this approximation accurate for arbitrary signal-to-noise ratios (SNRs) and number of transmit antennas. Our analytical approach leads also to simple asymptotic formulas for high SNR or large number of antennas. Selected numerical results show that our proposed approximation leads to results that come very close to the exact sum-rate and that our asymptotic formulas converge to the exact values when the SNR or the number of antennas tend to high values.

In this paper, we consider an incremental relaying protocol based on an amplify-and-forward transmission in conjunction with the best relay selection scheme over non-identically distributed relay channels. In order to satisfy the spectral efficiency and the bit error rate (BER) requirements, adaptive modulation is applied to our proposed scheme. We derive the average spectral efficiency, average BER and outage probability for the performance analysis of our proposed scheme. For a more tractable analysis, we start with an upper bound for the combined signal-to-noise ratio at the destination and provide closed-form expressions for independent non-identically distributed Rayleigh fading conditions. Our analytical-based numerical results are validated by some computer-based simulations. These results show that our proposed scheme leads to a considerable improvement in the performance of cooperative diversity systems.

In this paper, we analyze the performance of a multi-channel system with the conventional adaptive modulation (AM) and two hybrid schemes, the adaptive modulation with diversity combining (AMDC) and the adaptive modulation with power loading (AMPL), respectively. In order to achieve higher spectral efficiency and more efficient usage of channel resources under the total transmit power constraint and the required bit error rate (BER), the AMDC jointly uses discrete-rate adaptation and diversity combining scheme, and the AMPL jointly uses discrete-rate adaptation and sub-optimal discrete-power loading. We derive exact closed-form expressions of the performance of a multi-channel system with the conventional adaptive modulation (AM) and these hybrid schemes, in terms of the spectral efficiency, the average BER and the outage probability. We also analyze the impact of imperfect channel estimation on the error performance and validate our analytical results by simulations. Numerical results show that both the AMDC and the AMPL offer higher spectral efficiency than the conventional AM, assuming the perfect channel estimation. However, AMDC offers better performance and reliability than AMPL, under imperfect channel estimation with complex Gaussian error.

In this paper we propose joint antenna selection MIMO and adaptive modulation scheme for high spectral efficiency and power efficiency wireless transceiver design. In our proposed scheme the number of active receive antennas for spatial multiplexing MIMO detection is determined at the rate of the large scale channel variation which is a function of the path loss and shadowing parameters, while the link adaptation is updated as fast as the coherence time. Hence, in our proposed scheme, the rate of antenna selection is slower than the link adaptation, which is motivated by the fact that the portion of the power consumption is dominated by RF chains during the initial period of the switched-on terminal and some wireless service does not require the large scale mobility. From our analysis we determine the number of the active receive antennas to achieve a certain target ergodic capacity as well as a target link performance and evaluate bit error rate, average spectral efficiency and the outage probability for two MIMO detection schemes such as maximum likelihood and zero-forcing. Some selected numerical examples are finally presented and discussed to illustrate the tradeoff offered by this scheme.

Random unitary beamforming (RUB) is an attractive low-complexity transmission scheme for multiuser MIMO broadcast channel. When there are a moderate number of users in the system, the sum rate performance of this signaling scheme becomes mainly interference-limited over high signal-to-noise ratio (SNR) region. In this paper, we study the performance of RUB-based multiuser MIMO systems when optimum combining is employed at mulitple-antenna mobile receivers to suppress multiuser interference. We adopt a geometric approach to derive the statistics of the output signal-to-interference ratio (SIR) of the optimum combiner, based on which a sum rate upper bound is obtained over high SNR region. We show through selected numerical examples that the proposed RUB with optimal combining achieves much better sum rate performance while maintaining low feedback load.

There is a continuing interest in low-complexity multiuser MIMO transmission techniques based on random unitary beamforming (RUB). Different RUB schemes mandate different amount of feedback to exploit the multiuser diversity gain for sum-rate capacity benefits. In this paper, we investigate the tradeoff of sum-rate capacity versus feedback load between different RUB schemes through accurate statistical analysis. Specifically, we derive the exact sum-rate expressions for two RUB schemes based on quantized signal-to-interference-plus- noise ratio (SINR) feedback. These analysis are accompanied by the development of the complete statistical characterizations of ordered SINRs for a user, which can find their application in many other related problems. We show through selected numerical examples that the additional feedback of the quantized values of best beam SINR from each user can help considerably improve the sum-rate performance of RUB systems.

In this paper, we consider an incremental relaying protocol based on an amplify-and-forward transmission in conjunction with a best relay selection scheme over non-identically distributed relay channels. In order to satisfy the spectral efficiency and the bit error rate (BER) requirements, adaptive modulation is applied to our proposed scheme. We derive the average spectral efficiency, average BER and outage probability for the performance analysis of our proposed scheme. For a more tractable analysis, we start with an upper bound for the combined signal-to-noise ratio at the destination and provide closed-form expressions for independent non-identically distributed Rayleigh fading conditions. Our analytical-based numerical results are validated by some computer-based simulations. These results show that our proposed scheme leads to a considerable improvement in the performance of cooperative diversity systems.

Transmit beamforming is simple method to achieve the full diversity gain that is available in multiple antenna (MIMO) wireless systems. Unfortunately, the prior condition to achieve this gain requires perfect channel knowledge at both transmitter and receiver, which is impractical on account of limited feedback link. Therefore, for the practical system, codebook based feedback scheme is often employed, where the beamforming vector is selected from the codebook to maximize the output signal-to-noise ratio (SNR) at receiver, and the receiver only sends back the index of the best beamforming vector to the transmitter. In this paper we derive analytical expression of average bit error rate (BER) for the codebook based transmit beamforming MISO system over the feedback error channel. Using this analytical result, we present optimum codebook indexing scheme to improve the performance of this system. From some selected numerical examples we show that our proposed codebook indexing scheme can provide nonnegligible performance improvements in terms of average BER over the severe feedback error channel.

Opportunistic relaying with cooperative diversity has been introduced in as a simple alternative protocol to the distributed space-time coded protocol while achieving the same diversity-multiplexing tradeoff performance as a point-to- point multiple-input multiple-output scheme. In this paper, we derive an upper and lower bound on the per hop output signal-to- noise ratio cumulative distribution function, probability density function and moment generating function. We also derive approximate but tight closed-form expressions for the lower and upper bounds of the outage probability of opportunistic relaying based on decode-and-forward transmission under the assumption of unbalanced/balanced hops and independent, identical distributed Rayleigh fading channels.

In this paper, we propose a hybrid adaptive modulation and power loading (AMPL) for wireless multi-channel system. While the hybrid adaptive modulation and diversity combining (AMDC) introduced in [1] exploits adaptive modulation and diversity combining scheme to achieve higher spectral efficiency, the proposed scheme in this paper jointly determines the transmit power and the constellation size of symbols for more efficient usage of channel resources, under the total transmit power constraint and the required bit error rate (BER). We derive exact closed-form expressions of the performance of the proposed scheme in terms of the spectral efficiency, the average BER and the outage probability. We also analyze the impact of imperfect channel estimation on the proposed scheme and validate our analytical results by simulations. Numerical results show that the hybrid AMPL offers higher spectral efficiency than conventional scheme, and is less sensitive to estimation errors than the hybrid AMDC as well.

In this paper, we present the exact sum-rate analysis of the multiuser MIMO system with random unitary beamforming (RUB). Specifically, with the derived statistics of ordered beam signal to interference and noise ratio, we obtain the analytical expression of system sum rate for arbitrary number of users. The effects of system parameters, including the number of users, the number of transmit antennas and the signal to noise ratio (SNR), on the sum rate are examined through numerical methods. Besides, we develop an upper bound for the sum rate, which reveals that the sum rate with RUB benefits mainly from multiuser directional diversity gain when SNR is high.

In this paper, we introduce a new zero-forcing beamforming (ZFBF) transmission scheme for multi-user MIMO Gaussian channel with finite rate feedback. With this transmission scheme, the base station adaptively utilizes transmit antennas according to the channel status and the number of active users to control the multiuser interference. We show through selected numerical results that our proposed scheme can achieve remarkable sum-rate improvement compared to other schemes with fixed transmit antennas.

Opportunistic relaying with cooperative diversity has been introduced in as a simple alternative protocol to the distributed space-time coded protocol while achieving the same diversity-multiplexing tradeoff performance as a point-to-point multiple-input multiple-output scheme. In this paper, we derive the upper and lower bounds for the cumulative distribution function of the signal-to-noise ratio per hop, and then provide their probability density function and moment generating function.We also derive approximate but tight closed-form expressions for the lower and upper bounds of the outage probability of opportunistic relaying based on decode-and-forward transmission under the assumption of unbalanced/balanced hops and independent, identically distributed Rayleigh fading channels.

We first consider conventional multi-channel systems with fixed-power variable-rate transmission of independent data streams as a function of channel variations. In these conventional systems, no data is transmitted through the sub-channels whose received signal strength is below a certain cutoff threshold. We then propose new hybrid adaptive modulation and diversity schemes for multi-channel transmission systems. The proposed hybrid schemes attempt to increase the overall spectral efficiency by (i) transmitting redundant information symbols through the deeply faded sub-channels that are not to be used in conventional multi-channel systems and then (ii) diversity combining these replicas at the receiver end. We evaluate the performance of the newly proposed hybrid schemes and compare their spectral efficiency with that of (i) the conventional scheme and (ii) a benchmark scheme which maximizes the overall spectral efficiency but is too complex for practical implementation. We show using some selected numerical examples that the proposed hybrid schemes offer spectral efficiency very close to that of the optimal scheme with much reduced complexity.

A simple and practical system based on a switched diversity scheme with adaptive modulation is presented. This system provides a reduced number of channel estimation while offering the optimum spectral efficiency given by a selection diversity system. In addition, the switching threshold is easily manipulated so as to make an efficient use of the tradeoff between spectral efficiency and channel estimation overhead. An extension of switched diversity into a multiuser scheduling is later also considered. This switch-based multiuser access scheme results in a lower average feedback load than that for the optimal selection-based multiuser scheme. Numerical results show that we can obtain a trade-off between spectral efficiency and the feedback load by choosing the switching threshold appropriately.

In this paper, a simple and practical multiuser scheduling based on switched diversity with adaptive modulation is presented. This switch-based multiuser scheduling scheme results in an average feedback load that is lower than using the optimal opportunistic multiuser scheduling scheme while offering the optimum overall system spectral efficiency when used with the highest threshold in the adaptive modulation. Numerical results show we can obtain a trade-off between spectral efficiency and the feedback load by choosing the user switching threshold appropriately. In addition, system operation can be adaptively altered based on the optimization goal and the given resource budget by a user switching threshold change.

The opportunistic relaying in [1] was proposed as an efficient cooperative protocol in the wireless multi-hop systems. However, the opportunistic relaying may still have high complexity as the number of relay nodes increases since the link quality of the paths generated by multiple relay nodes should be monitored. In this paper, we propose a novel multi-hop relay protocol named "switch and examine node selection"(SENS), where the relay node is selected if its link quality is above a certain threshold which is set to satisfy the required performance and the selected link is maintained unless its link quality does not fall below the threshold. From our derived statistics of the received output signal-to-noise ratio (SNR), we obtain the optimal threshold in terms of the link performance such as the average output SNR, the outage probability, and average error rate. Using the optimalthreshold, we show from the analytical and numerical results that the performance of our proposed SENS protocol provides the same performance as the opportunistic relaying protocol while the complexity of our proposed scheme is much lower than the one of opportunistic system.

The maximum likelihood detection with QR decomposition and M-algorithm (QRM-MLD) has been presented as a sub- optimum multiple input-multiple output (MIMO) detection scheme which can provide almost the same performance as the optimum maximum likelihood (ML) MIMO detection scheme but with the reduced complexity. However, due to the lack of parallelism and the regularity in the decoding structure, the conventional QRM-MLD which uses the tree-structure still has very high complexity for the very large scale integration (VLSI) implementation. In this paper, we modify the tree- structure of conventional QRM-MLD into trellis-structure in order to obtain high operational parallelism and regularity and then apply the Viterbi algorithm to the QRM-MLD to ease the burden of the VLSI implementation. We show from our selected numerical examples that, by using the QRM-MLD with our proposed trellis-structure, we can reduce the complexity significantly comparing to the tree-structure based QRM-MLD while the performance degradation of our proposed scheme is negligible.

Recently, the cooperative diversity systems, based on the distributed space-time coding (DSTC), have been presented to provide the spatial diversity gain in the wireless networks. Although the performance of DSTC cooperative diversity systems is quite attractive in terms of the link performance, the complexity and the power consumption at each node are the main practical issues for the deployment of the systems, in particular, when the number of nodes is large. In this paper, we propose the suboptimal relay selection algorithm where a predetermined threshold is set both at the relay node and the destination node, and the node selection is performed only when the signal quality of either relay node or destination node is below a predetermined threshold, which has less complexity than the opportunistic relaying in while satisfying the required performance. We provide the closed-form expression of statistics of the output SNR and study the complexity of the proposed scheme. Based on our analytical results we show several selected numerical examples to show the efficiency of our proposed schemes.

In this paper, we propose two different types of multiuser access scheme based on assembling the schemes of (i) the switch and examine scheduling in the network layer and (ii) the joint adaptive modulation and output-threshold maximal ratio combining (OT-MRC) in the physical layer and thus attempt to find an efficient cross-layer access scheme to exploit the user diversity as well as the high spectral efficiency and the antenna diversity gain. We consider the multi-user environment where base station equipped with single antenna attempts to communicate with each user equipped with L receive antennas based on switch-and-examine transmission (SET) as a user selection scheduling scheme. We propose two different SETs: (i) spectral efficient SET and (ii) feedback load efficient SET. We analyze the performance of our proposed schemes and compare them with the optimal scheduling scheme in terms of the average spectral efficiency and the average number of estimated paths per user to quantify the complexity.

In this paper we first consider conventional multichannel systems with fixed-power variable-rate transmission of independent data streams as a function of channel variations. In the conventional systems, no data is transmitted through the channels whose received signal strength is below a certain cut off threshold. We then propose a new hybrid adaptive modulation and diversity scheme for multi-channel transmission systems. The proposed hybrid scheme attempts to increase the overall spectral efficiency by (i) transmitting redundantly information symbols through the deeply faded channels that are not to be used in conventional multi-channel systems and then (ii) diversity combining these signals at the receiver end. We calculate the spectral efficiency of the newly proposed hybrid scheme and compare it to the spectral efficiency of (i) the conventional system and (ii) a benchmark optimal but complex joint adaptive and diversity scheme that maximizes the overall spectral efficiency. We show, with the help of some selected numerical examples, that the proposed hybrid scheme offers almost the same spectral efficiency as the optimal scheme with much reduced complexity.

In this paper, we propose a combined adaptive modulation transmission and output threshold MRC diversity reception scheme for high spectral and power efficiency wireless transceiver design. The modulation constellation size and the number of combined diversity paths are jointly determined to achieve the highest spectral efficiency but with low combining complexity, given the fading channel conditions and the required error rate performance. We derive the statistics of the resulting output signal-to-noise ratio (SNR) and then use it to analyze the proposed system (i) performance in terms of outage probability and average bit error rate, (ii) complexity in terms of the average number of path estimation and the average number of combined paths, and (iii) spectral efficiency. Some selected numerical examples are finally presented and discussed to illustrate the tradeoff offered by this scheme.

Abstract Accurate and fast algorithms for the local mean signal level estimation are very important for the successful implementation of many wireless communication enabling techniques such as handoff, power control, and adaptive gain control schemes. In this paper, we present on‐line simple‐to‐implement (i) minimum variance unbiased and (ii) maximum likelihood estimators for the local mean signal power estimation in the decibel domain. We consider a generalized Nakagami‐m fading environment and show that these estimators are asymptotically efficient in the number of samples and in the fading parameter m. Numerical and simulation results confirm that these estimators outperform the sample mean estimator and quickly approach the Cramer–Rao lower bounds. Copyright © 2006 John Wiley & Sons, Ltd.

In this paper, we propose a combined adaptive modulation transmission and output threshold MRC diversity reception scheme for high spectral and power efficiency wireless transceiver design. The modulation constellation size and the number of combined diversity paths are jointly determined based on the fading channel conditions and the required error rate performance. We derive the statistics of the resulting output signal-to-noise ratio (SNR) and then use it to analyze the proposed system in terms of performance, complexity, and spectral efficiency. Some selected numerical examples are finally presented to illustrate the mathematical formulism.

The performance analysis of dual (two-branch) maximal ratio combining (MRC) under imperfect weight, named imperfect MRC, due to the noise imbalance is derived over independent but non-identical Rayleigh fading channels. Considering the system (or channel) conditions that the noise level of each branch is different, we present the accurate performance analysis of imperfect dual MRC in terms of average combined signal-to-noise ratio (SNR), outage probability and average symbol error rate for a large class of modulations in closed-form and compare them with the performance of the perfect MRC and the perfect selection combining (SC) over non-identical but independent Rayleigh fading channels based on the interesting statistical results on the combined SNR. From the performance results we provide the criterion in choosing the imperfect MRC, perfect MRC, or SC depending on the degree of the difference of the noise level between branches.

In this paper, we propose some new statistical modeling of the residual frequency error when the classical open loop frequency estimator is employed. We obtain a closed-form for the cumulative density function (CDF) of the residual frequency error by using the method of convex combination. The simulation results show that the proposed statistical modeling is very accurate over a wide range of system parameters.

We propose a new switched transmit diversity technique in MIMO systems with an application to Alamouti's space time block coding. The proposed scheme is basically a closed loop system where the feedback bits are sent back to the transmitter to indicate the channel quality. Various switching strategies, depending on the size of feedback bits, are considered and optimized to compare the trade-off between the link performance and the number of antennas or feedback information. From our analytical and numerical results, we show that there is a considerable benefit in terms of the performance and practical use.

The statistics of the signal estimator for estimating the signal value in decibel (dB) domain proposed in T. Luo and Y.C. Ko (2004), is analyzed. The basic idea of the proposed estimator combines the linear averaging and log domain. Using the most frequently used averaging filter, i.e., sample mean averaging filter, we prove that the performance of the proposed method is very close to the one of the optimum estimator with an assumption of independent identically distributed signal samples under Nakagami fading channel model. Moreover, the proposed method does not need to explicitly estimate the Nakagami parameter, m, which is required for the optimum estimator. For the correlated samples to be used for the estimation, which is a more practical situation, we show from the simulation that the proposed estimator outperforms significantly the optimum minimum variance unbiased estimator obtained under the Rayleigh fading channels. We also show from the simulations that the proposed estimator works well for the multipath fading case and low signal strength.

In this paper, we consider a channel estimator in CDMA systems, where there exist both common pilot channel (CPICH) symbols, which is not power controlled signal, and the dedicated pilot symbols, which is power-controlled. Our proposed channel estimator estimates the channel by combining the CPICH symbols and dedicated pilot symbols. We derive the maximal likelihood (ML) estimator and present the approximate ML, which can be readily used for fast and accurate on-line channel estimation. We also derive the Cramer-Rao lower bound (CRLB) of the proposed method and compare it with the CRLB of the method using the common pilot channel symbols only. End-to-end full downlink SPW simulations have been run and we confirm that the significant gain can be achieved with a proposed estimator in a power-controlled CDMA system. The resulting algorithms can be applied to open loop and closed loop transmit diversity cases as well as single antenna case.

An optimum power loading algorithm for transmitter diversity systems over correlated and unbalanced Nakagami (1960) paths and its performance evaluation under perfect channel estimate conditions are derived. In addition, various online estimators of the required Nakagami channel parameters for optimized power loading and the comparison of their mean square error via Monte Carlo simulations are presented. Some of these estimators are used to obtain the performance of optimized transmitter diversity systems under imperfect channel estimation (ICE). These numerical results show that the diversity gain of these optimized systems compared with equipower systems increases as the severity of fading decreases and as the degree of branch imbalance increases even under ICE. On the other hand, in weakly correlated and (or) unbalanced branches, optimized transmitter diversity systems offer negligible gain or even losses compared with unoptimized systems because of the ICE.

Starting with a new expression for the probability density function of the signal-to-noise ratio at the output of dual-diversity equal-gain combining (EGC) over Rayleigh channels, we provide closed-form expressions for the average bit and symbol error rate of M-ary phase-shift keying signals. Numerical examples indicate that EGC maintains a good diversity gain as long as the degree of unbalance is not very low but suffers a sharp and significant degradation when the degree of unbalance approaches zero.

Accurate and fast algorithms for the local mean level estimation axe very important for the successful implementation of many wireless communication enabling techniques such as handoff, power control, and automatic gain control schemes. In this paper, we present maximum likelihood as well as variance unbiased estimators for the local mean signal decibel power. We consider a generalized Nakagami-m fading environment and show that these estimators are asymptotically efficient in the number of samples and in the fading parameter m. Numerical and simulation results confirm that these estimators clearly outperform the sample mean estimators and quickly approach Cramer-Rao lower bounds.

This paper relies on a characteristic function based approach to compute the level crossing rate and average outage duration of interference-limited wireless communication systems. The analytical results are quite general and are applicable to systems in which users are subject to nonidentically distributed Rician fading. These results are confirmed by Monte Carlo simulations and some numerical examples are discussed and interpreted.

A new technique based on the auto-correlation function is described for the estimation of the Doppler spread in mobile communication systems. We first propose to divide a uncertainty region of Doppler spread into multiple frequency bins. Based on the given multiple bins the correlator compares the estimated value at a certain time index to the theoretical exact value and then decides which bin the Doppler spread is estimated in. The certain time index can be optimized to give the largest decision region among multiple bins. We derive the optimum time index algorithm to give the largest decision region for each bin based on Rayleigh fading channel. We also apply the same Doppler spread estimator to the Rician case with the slight transformation of the received signal. We show that the proposed technique is not affected significantly by the Rician factor and the SNR degradation with the reasonable number of samples for estimation which is not the case of other estimators given in the literature.

Starting with a new expression for the probability density function of the signal-to-noise ratio at the output of dual-diversity equal-gain combining (EGC) over not identically distributed Rayleigh channels, we provide closed-form expressions for the outage probability as well as average bit and symbol error rate of binary and M-ary phase-shift keying signals. Numerical examples indicate that EGC maintains a good diversity gain as long as the degree of unbalance is not very low but suffers a sharp and significant degradation when the degree of unbalance approaches zero.

An optimum power loading algorithm for transmitter diversity systems over correlated and unbalanced Nakagami (1960) paths and its performance evaluation under perfect channel estimation conditions are derived. In addition, various on-line estimators of the required Nakagami channel parameters for optimized power loading and the comparison of their mean square error via Monte Carlo simulations are presented. These estimators are used to obtain the performance of optimized transmitter diversity systems under imperfect channel estimation (ICE). These numerical results show that the diversity gain achieved by these optimized systems increases as the severity of fading decreases and as the degree of branch unbalance increases even under ICE. On the other hand, in weakly correlated and/or unbalanced branches, optimized transmitter diversity systems offer negligible gain or even losses compared to unoptimized systems because of the ICE.

A "classical" probability density function (PDF)-based approach relying on the cumulative distribution function (CDF) of the combined output signal-to-noise ratio (SNR) as well as the joint PDF of the combined output SNR and its time derivative, is used to obtain exact closed form expressions for average outage duration (AOD) of maximal-ratio combiners (MRC) over independent and identically distributed (i.i.d.) Rayleigh and Rice fading channels. On the other hand, relying on numerical techniques for inverting Laplace transforms of CDFs and for the computation of the joint characteristic function (CF) of the combined output SNR random process and its time derivative, a CF-based approach is adopted to calculate the AOD of MRC over non-i.i.d. Rician diversity paths. The mathematical formalism is illustrated by presenting some numerical results/plots showing the impact of the power delay profile and the distribution of the angle of arrivals on the AOD of diversity systems operating over typical fading channels of practical interest.

Outage probability is an important performance measure of communication systems operating over fading channels. Relying on a simple and accurate algorithm for the numerical inversion of the Laplace transforms of cumulative distribution functions, we develop a moment generating function-based numerical technique for the outage probability evaluation of maximal-ratio combining (MRC) and postdetection equal-gain combining (EGC) in generalized fading channels for which the fading in each diversity path need not be independent, identically distributed, nor even distributed according to the same family of distributions. The method is then extended to coherent EGC but only for the case of Nakagami-m fading channels. The mathematical formalism is illustrated by applying the method to some selected numerical examples of interest showing the impact of the power delay profile and the fading correlation on the outage probability of MRC and EGC systems.