Spectral Efficiency Enhancement Research Articles

Energy efficiency maximization of full-duplex two-way relay-assisted device-to-device communications underlaying cellular networks

With the substantial progress in self-interference (SI) cancelation, the full-duplex (FD) technique, which allows the communication user to transmit and receive signals over the same frequency band simultaneously, enables a significant enhancement of spectral efficiency (SE) in comparison with the traditional half-duplex (HD) technique. Recently, relay-assisted device-to-device (D2D) communications underlaying cellular networks have aroused a great deal of research interests due to its high SE. For the new meaningful paradigm of the combination of the FD and the amplify-and-forward (AF) relay-assisted D2D communications, analyzing the SE and energy efficiency (EE) is crucial, which have not been investigated in the existing works. In this paper, we focus on the EE of the FD two-way (FDTW) relay-assisted D2D communications with uplink channel reuse by considering the residual SI at the D2D users and compare it with the HD counterpart. Our goal is to find the optimal transmission powers and amplification gain to maximize the system EE while guaranteeing SE requirements and maximum transmission power constraints. A new two-tier alternative iteration optimization algorithm is proposed to solve the optimization problem. Simulation results show that (1) the results obtained by the proposed algorithm is very close to those obtained by the exclusive searching method, (2) smaller residual power of SI leads to better performance of EE and SE, (3) the SE obtained by FDTW relay-assisted D2D networks is higher than the SE obtained by the HD counterpart, and (4) the EE comparison of FDTW relay-assisted D2D networks and its HD counterpart depends on the residual power of SI. The EE obtained by FDTW relay-assisted D2D is higher than the EE obtained by HD counterpart only when the residual power of SI is sufficiently small.

VCM-OFDM technique for advanced space communications system with high spectral efficiency

The development of precise scientific payloads brings higher demand on the efficiency of space communications system to transmit the increasing volume of scientific data. Aiming to this issue, Orthogonal Frequency Division Multiplexing (OFDM) is chosen for its inherent capability of high-rate data transmission. Further, considering the dynamic link condition due to satellite orbital motion, we propose a new technique which combines Variable Coding Modulation (VCM) with OFDM to enhance the communication link spectral efficiency with required transmission reliability. With VCM-OFDM technique, the channel coding and modulation mode can be variable with time according to the link conditions, in order to fit the link budget curve and maintain a relatively fixed link margin. Hence, link resource waste can be reduced and throughput can be remarkably improved. Considering that OFDM-based systems are sensitive to Doppler shifts/spread, the coding and modulation mode (CODMOD) selection should be optimized subject to this scenario. This paper introduces the architecture of near-earth space data transmission system based on VCM-OFDM technique. The Doppler influence is analyzed through simulation and the CODMOD selection algorithm is discussed. The results prove the high performance on spectral efficiency enhancement of VCM-OFDM by comparison with several existing alternative methods.

Performance analysis of multi-hop full-duplex decode-and-forward relaying

Full-duplex relaying (FDR), which can receive and transmit simultaneously over the same frequency band, enables a significant enhancement of spectral efficiency and has attracted much attention. In this paper, we investigate the performance of multi-hop decode-and-forward (DF) FDR systems, in which the relay nodes suffer not only from self-interference but also from inter-relay interference (IRI). Two cases are considered for the IRI, i.e., the mth relay only knows the channel state information (CSI) from the (m−1)th relay or knows perfect CSI from all other relays. Assuming that all the channels including the residual self-interference (RSI) and IRI are subject to Rayleigh fading, we first determine the cumulative distribution function (CDF) of the end-to-end signal-to-interference-plus-noise ratio (SINR) for each case. Then based on these results, we derive the outage probability, symbol error probability and ergodic capacity, for each case respectively. Finally, we compare the performance of multi-hop FDR with multi-hop half-duplex relaying (HDR) and validate our analysis by the simulation results.

Performance Enhancement of Spectral efficiency and throughput with Distributed Dynamic channel allocation using genetic algorithm

The efficiency associated with mobile systems will depend on a good channel allocation method. Some sort of genetic algorithm (GA) dependent on distributed dynamic channel allocation model is offered. Mobile service stations (MSS) help to make every one of the channel allocation decision intended for mobile hosts in that cell based on the local information. Proposed model which reuses available channels more efficiently. A reserved pool associated with channels in each and every cell to make the model fault tolerant which allows any mobile or portable to continue communication with its mobile hosts within the absence of adequate channel within the cell. Therefore, the offered GA intended for distributed dynamic channel allocation to reinforce the spectral efficiency along with throughput of the cell network. Simulation results evaluate the performance of the distributed dynamic channel allocation based on genetic algorithm.

Multi-User Detection for Radar and Communication Multifunction System

A multifunctional system that performs synthetic aperture radar (SAR) and communication tasks is presented. The system depends on orthogonal frequency division multiplexing (OFDM) waveform shaping and multiple input multiple output (MIMO) configuration to achieve enhancement in the spectral efficiency of communication operation and provide interference rejection in SAR imaging. The SAR interval idle time is utilized to enhance the communication signal through diversity. Adaptive system features are introduced to achieve multi-user detection and improve the communication reliability, by means of space division multiple access (SDMA) and space–time block coding (STBC). Investigations of linear and nonlinear multi-user detection (MUD) techniques are provided. Moreover, a genetic algorithm-based system is developed to optimize the MUD problem and to provide near-optimal performance even for overloaded scenarios. A gain of more than 4 dB is achieved at bit error rate of 10-2.

Wireless Femto-Relays

The continuous increase in mobile data traffic creates the need for radical innovations in the mobile broadband system design. Heterogeneous networks (HetNets) deployment paradigm is an emerging research trend considered as the most significant aspect for meeting the mobile data challenge, mainly by proposing spectral efficiency enhancements and cell capacity gains from an overall system perspective. Two are the main challenges that dense small cell networks are facing: a) various types of interference mainly caused by unplanned deployments, and b) limited and Quality of Service (QoS)-unreliable wired backhaul links. In this paper, the authors propose a model where femtocells' and relays' complementary characteristics are effectively exploited into femto-relays. More specifically, the authors describe four main functionalities of this model: a) interference protection, b) two-fold backhaul alternatives (i.e. wired and wireless backhaul), c) opportunistic femto-relay selection, and d) full-duplex in-band relaying. To evaluate the anticipated technical impact of the proposed model, targeted simulation results are presented for a campus topology where femto-relays are compared with classic femtocells. Finally, related open issues, such as signaling overheads management, context-aware resource management, and business logic aspects are presented, with emphasis on the femto-relay's real market penetration opportunities in the mid and long-term future.

Towards 1 Gbps/UE in Cellular Systems: Understanding Ultra-Dense Small Cell Deployments

Todays heterogeneous networks comprised of mostly macrocells and indoor small cells will not be able to meet the upcoming traffic demands. Indeed, it is forecasted that at least a 100x network capacity increase will be required to meet the traffic demands in 2020. As a result, vendors and operators are now looking at using every tool at hand to improve network capacity. In this epic campaign, three paradigms are noteworthy, i.e., network densification, the use of higher frequency bands and spectral efficiency enhancement techniques. This paper aims at bringing further common understanding and analysing the potential gains and limitations of these three paradigms, together with the impact of idle mode capabilities at the small cells as well as the user equipment density and distribution in outdoor scenarios. Special attention is paid to network densification and its implications when transitioning to ultra-dense small cell deployments. Simulation results show that network densification with an average inter site distance of 35 m can increase the cell- edge UE throughput by up to 48x, while the use of the 10GHz band with a 500MHz bandwidth can increase the network capacity up to 5x. The use of beamforming with up to 4 antennas per small cell base station lacks behind with cell-edge throughput gains of up to 1.49x. Our study also shows how network densifications reduces multi-user diversity, and thus proportional fair alike schedulers start losing their advantages with respect to round robin ones. The energy efficiency of these ultra-dense small cell deployments is also analysed, indicating the need for energy harvesting approaches to make these deployments energy- efficient. Finally, the top ten challenges to be addressed to bring ultra-dense small cell deployments to reality are also discussed.

Quadrature spatial modulation–performance analysis and impact of imperfect channel knowledge

AbstractIn this paper, a new multiple‐input multiple‐output transmission technique called quadrature spatial modulation (QSM) is proposed and analyzed in the presence of imperfect channel estimation at the receiver. In QSM, conventional spatial constellation diagram of spatial modulation (SM) system is expanded to include both in‐phase and quadrature components. As such, significant enhancement in the overall spectral efficiency is achieved while retaining all inherent advantages of SM technique, such as inter‐channel interference avoidance, single radio frequency chain transmitter and low receiver complexity. It is shown that significant performance enhancements can be achieved as compared with SM, Alamouti, and spatial multiplexing systems. Besides, the impact of Gaussian imperfect channel estimation on the performance of QSM system is studied. A closed‐form expression for the pairwise error probability of generic QSM system is derived and used to calculate a tight upper bound of the average bit error probability over Rayleigh fading channels with perfect and imperfect channel knowledge. Also, simple asymptotic expression is derived and analyzed. Obtained Monte Carlo simulation results highlight the accuracy of the conducted analysis. Copyright © 2014 John Wiley & Sons, Ltd.

Improving the performance of cell edge users with fractional coordinated fair scheduling

In this study, the authors propose a fractional coordinated fair scheduling (FCFS) scheme for improving the performance of cell edge users in long term evolution (LTE-advanced) networks. To enhance the network spectral efficiency and reduce co-channel interference, they extend the inter-cell interference coordination technique for multi-sector cells. The sectors are coordinated in the sense that they are allowed to transmit simultaneously, if the neighbouring sectors co-channel interference is less than a threshold value. Accordingly, a coordinated scheduling scheme is proposed to transmit to a fraction of users, located in cell edges, with good channel conditions. The proposed scheme maintains fairness among the users using alpha-fair criterion. Furthermore, to reduce the computational complexity of the scheduling scheme, they propose a heuristic algorithm based on choosing an appropriate subset of users for scheduling computations. Performance evaluation of FCFS shows that the cell edge users’ throughput improves because of the spectral efficiency enhancement and interference reduction. Moreover, FCFS achieves a higher fairness index with respect to a non-coordinated fair scheme.

On the Capacity of Downlink Multi-Hop Heterogeneous Cellular Networks

Multi-hop heterogeneous cellular networks (MHCNs) consist of conventional macro cellular networks overlaid with an irregular deployment of low-power base stations (BSs), where the communication between BSs and mobile users can be established through a single hop or multiple hops. By modeling different kinds of randomly located BSs as K tiers of independent homogeneous Poisson Point Processes, we first explore the capacity of downlink MHCNs and derive the expression of capacity under Rayleigh fading channels. Particularly, the capacity gain achieved by cell splitting and multi-hop relaying is quantified for the first time. We then study the effects of BS density, transmit power, and signal-to-interference-plus-noise-ratio (SINR) threshold on the capacity of MHCNs. More importantly, we obtain the spectral efficiency enhancement condition under which the increase of BS density and transmit power improve the spectral efficiency, thereby enhancing the capacity. One interesting observation is that at a given SINR threshold, the capacity increases with BS density when all the tiers have the same SINR threshold. Moreover, the capacity of some special networks (i.e., heterogeneous cellular networks, multi-hop cellular networks, and conventional cellular networks) are derived directly by specializing some system parameters in our results. Finally, numerical studies and simulations are conducted to validate our analysis.

Incidence Coloring-Assisted Frequency Assignment in Two-Hop OFDMA-based Cellular Networks

In this paper, we consider frequency assignment for orthogonal frequency division multiple access-based cellular networks. We develop a new framework for the mathematical modeling of the frequency assignment problem, which aims at suppressing multi-links interference and enhancing spectral efficiency. The concept of incidence coloring from modern graph theory is utilized to recast the original frequency assignment problem and guide the solution from graph theory perspective. In addition, optimal position of relay stations in a hierarchical cluster based two-hop cellular networks is investigated, and proposes an efficient frequency assignment scheme based on the incidence coloring. System-level simulation results demonstrate that the frequency assignment scheme can effectively mitigate inter-cell interference and improve signal-to-interference-plus-noise ratio. Compared with existing frequency assignment schemes, better coverage performance is obtained and throughput of cell-edge mobile stations is greatly improved.

Energy-Efficient Resource Allocation in Downlink OFDM Wireless Systems With Proportional Rate Constraints

Orthogonal frequency-division multiplexing (OFDM), with its own advantages of spectral efficiency (SE) enhancement and flexible resource allocation, is a promising basic technique for fulfilling the ever increasing demand for high-data-rate transmission and various service type support for mobile multimedia communications. In the meantime, energy efficiency (EE) has now become a critical metric for green system design. In this paper, energy-efficient and fair resource allocation is investigated in a downlink OFDM-based mobile communication system. Given a subcarrier assignment, the bisection-based optimal power allocation (BOPA) is proposed at first, which achieves the maximum EE and guarantees proportional data rates for users. Then, a two-step subcarrier assignment is designed to avoid unaffordable computational complexity of exhaustive search. In the first step, the estimated energy-efficient transmit power is found via the assumptions on flat fading and subcarrier sharing. In the second step, the traditional spectral-efficient subcarrier assignment (SESA) is introduced to complete the bandwidth resource allocation among users. Although the two-step subcarrier assignment is suboptimal due to the fact that the optimization is done independently in two separate steps, numerical results demonstrate that its performance is very close to the optimum. This paper also studies the difference between the energy-efficient solution and the traditional spectral-efficient policy and observes that they are similar with each other in the low channel-gain-to-noise ratio (CNR) regime. This observation is helpful and could enable that the energy-efficient design can be turned into the relatively simpler spectral-efficient policy when the CNR is low.

Enhancing spectral-energy efficiency forLTE-advanced heterogeneous networks: a users social pattern perspective

The development of LTE-Advanced and beyond cellular networks is expected to offer considerably higher data rates than the existing 3G networks. Among the many potential technologies in LTE-Advanced systems, users? characteristics and social behavior have been studied to improve the networks? performance. In this article we present the concept of user social pattern (USP), which characterizes the general user behavior, pattern, and rules of a group of users in a social manner, and utilize USP as an optimization basis for network performance enhancement. From large-scale traffic traces collected from current mobile cellular networks, the USP model is evaluated and verified. Furthermore, to evaluate the potential of spectral efficiency and energy efficiency enhancement based on USP in LTE-A HetNets, we establish a complete system and link-level HetNet simulation platform according to 3GPP LTE-A standards. Then, based on the platform, simulations are performed to evaluate the impact of USP on spectral and energy efficiency in an LTE-A network, and a USP-based spectral efficiency and energy efficiency enhancement scheme is proposed for a HetNet of the LTE-A system. Simulation results validate that USP can be used as an effective concept for network performance optimization in an LTE-A system.

Tradeoff between energy-efficiency and spectral-efficiency by cooperative rate splitting

The trend of an increasing demand for a high-quality user experience, coupled with a shortage of radio resources, has necessitated more advanced wireless techniques to cooperatively achieve the required quality-of-experience enhancement. In this study, we investigate the critical problem of rate splitting in heterogeneous cellular networks, where concurrent transmission, for instance, the coordinated multipoint transmission and reception of LTE-A systems, shows promise for improvement of network-wide capacity and the user experience. Unlike most current studies, which only deal with spectral efficiency enhancement, we implement an optimal rate splitting strategy to improve both spectral efficiency and energy efficiency by exploring and exploiting cooperation diversity. First, we introduce the motivation for our proposed algorithm, and then employ the typical cooperative bargaining game to formulate the problem. Next, we derive the best response function by analyzing the dual problem of the defined primal problem. The existence and uniqueness of the proposed cooperative bargaining equilibrium are proved, and more importantly, a distributed algorithm is designed to approach the optimal unique solution under mild conditions. Finally, numerical results show a performance improvement for our proposed distributed cooperative rate splitting algorithm.

Enhancing spectral efficiency for lte-advanced and beyond cellular networks [Guest Editorial

The rapidly growing demands for bandwidth-intensive mobile broadband services have triggered tremendous efforts to develop the long-term-evolution (LTE)-Advanced and beyond cellular networks, which are widely deemed as a major advancement of the existing LTE networks. Constrained by limited spectral resources, wireless communication researchers and engineers have proposed a number of technologies to efficiently utilize spectral resources in multiple dimensions. Specifically, with a large number of transmit antennas deployed at a base station, massive multiple-input multiple- output (MIMO) (also called full-dimension MIMO [FD-MIMO] in the Third Generation Partnership Project [3GPP] community) is expected to achieve enormous spectral efficiency by exploiting degrees of freedom in the spatial domain. Recent theoretical and experimental findings highlight that massive MIMO is a promising solution to improve data rates , link reliability, and power savings for cellular networks. Another attractive technology, small cells, has also shown great potential for enhancing system throughputs of LTE-Advanced and beyond cellular networks. In the small cell solution, lower-cost base stations are densely deployed, and the size of a cell is substantially reduced, thus enabling great improvement of frequency reuse ratios in the geographical domain and significant enhancement of the spectral efficiency of cellular networks. Also, other technologies such as deviceto-device (D2D) and cognitive radio have also emerged as promising solutions to boost the spectral efficiency of cellular networks in various scenarios.

A Novel Transposed Uniform Cross‐Correlation Modified Prime Code for Enhancement of Capacity and Spectral Efficiency of Networks

ABSTRACTA novel codes set is proposed based on the Uniform Cross‐Correlation Modified Prime Code (UCMPC), hereby referred to as Transposed UCMPC (T‐UCMPC). This code set offers higher capacity, better spectral efficiency, lower bit error rate and higher security as compared with the other existing prime codes. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2952–2955, 2013

Spectral efficient compressive transmission framework for wireless communication systems

Increasing demand of high-speed data rate is leading to a challenging task to provide services to the users within exponentially growing market for wireless multimedia services. Subsequently, the available radio resources are becoming scarce because of different factors such as spectrum segmentation and dedicated frequency allocation to existing wireless standards. Exploring new techniques for enhancing the spectral efficiency in wireless communication has been an important research challenge. In this study, the enhancement of spectral efficiency of wireless communication systems is considered. A framework is proposed to implement the concept of compressive sampling (CS) for compressing the natural random signals. The performance of proposed framework is evaluated in the context of multiple input multiple output orthogonal frequency division multiplexing system. Simulation-based results show that 25% of resources can be saved by marginal trade-off with the quality of service (QoS) requirement applying CS to the natural random signals. Furthermore, it can be claimed that this QoS trade-off can be optimised with dynamic selection of random measurement matrices.

A Distributed Approach to Interference Alignment in OFDM-Based Two-Tiered Networks

In this paper, we consider a two-tiered network and focus on the coexistence between two tiers at the physical layer. We target our efforts on a Long-Term Evolution Advanced (LTE-A) orthogonal frequency-division multiple-access (OFDMA) macrocell sharing the spectrum with a randomly deployed second tier of small cells. In such networks, high levels of cochannel interference between the macrocell base station and the small-cell base station (SBS) may largely limit the potential spectral efficiency gains provided by frequency reuse 1. To address this issue, we propose a novel cognitive interference alignment-based scheme to protect the macrocell from cross-tier interference while mitigating the co-tier interference in the second tier. Remarkably, only local channel state information (CSI) and autonomous operations are required in the second tier, resulting in a completely self-organizing approach for the SBSs. The optimal precoder that maximizes the spectral efficiency of the link between each SBS and its served user equipment is found by means of a distributed one-shot strategy. Numerical findings reveal nonnegligible spectral efficiency enhancements with respect to traditional time division multiple access (TDMA) approaches at any signal-to-noise ratio (SNR) regime. Additionally, the proposed technique exhibits significant robustness to channel estimation errors, achieving remarkable results for the imperfect CSI case and yielding consistent performance enhancements to the network.

Multiuser Resource Allocation Optimization Using Bandwidth-Power Product in Cognitive Radio Networks

In this paper, the problem of resource allocation optimization is studied for a single-cell multiuser cognitive radio network in the presence of primary user networks. The spectral access of the cognitive radio network is based on Orthogonal Frequency Division Multiple Access (OFDMA). A joint bandwidth and power allocation is performed so that users' rate requirements are satisfied, and the integrity of primary user communication is preserved. In this work, two unique challenges are addressed. The first is the incorporation of primary user activity in the design of resource allocation technique, and the second is the limited hardware capabilities of cognitive terminals compared to those available at the cognitive base station. To address these problems, a novel resource allocation framework is proposed based on the bandwidth-power product minimization, which is an effective metric in evaluating the spectral resource consumption in a cognitive radio environment. The framework takes into consideration the challenges aforementioned. The results show significant enhancement in spectral efficiency by using our framework compared to classical power adaptive optimization using iterative waterfilling scheme.

A Survey on Beamforming Techniques for Wireless MIMO Relay Networks

One of the major challenges the mobile broadband community faces is the exponential increase in mobile data traffic, even more so, for cell-edge users. Thus, in a multitier network, the demand for high-speed and interference-free transmission and reception is inevitable. Beamforming (BF) is an advanced technology that offers a significantly improved solution to reduce the interference levels and improve the system capacity. Accordingly, the establishment of relays in mobile data networks has emerged spectral efficiency enhancements and cell capacity gains from an overall system perspective. This paper provides a comprehensive survey focused on the performance of adopted beamforming technique on MIMO relay networks that is expected to overcome crucial obstacles in terms of capacity and interference. The main objective is to point out the state-of-the-art research activity on BF techniques in MIMO relay networks, under various network performance challenges. Thereby, it focuses on recently developed procedures for interference modeling and mitigation, BF channel modeling, channel estimation and feedback, complexity and power consumption, adaptive BF for multiuser relaying, degrees of freedom, diversity issues, and spectral efficiency, in cooperative and opportunistic systems. Different network topologies have been considered and categorized, pertaining the challenges of BF implementation in MIMO relay networks.