Optimization Framework and Graph-Based Approach for Relay-Assisted Bidirectional OFDMA Cellular Networks

This paper considers a relay-assisted bidirectional cellular network where the base station (BS) communicates with each mobile station (MS) using OFDMA for both uplink and downlink. The goal is to improve the overall system performance by exploring the full potential of the network in various dimensions including user, subcarrier, relay, and bidirectional traffic. In this work, we first introduce a novel three-time-slot time-division duplexing (TDD) transmission protocol. This protocol unifies direct transmission, one-way relaying and network-coded two-way relaying between the BS and each MS. Using the proposed three-time-slot TDD protocol, we then propose an optimization framework for resource allocation to achieve the following gains: cooperative diversity (via relay selection), network coding gain (via bidirectional transmission mode selection), and multiuser diversity (via subcarrier assignment). We formulate the problem as a combinatorial optimization problem, which is NP-complete. To make it more tractable, we adopt a graph-based approach. We first establish the equivalence between the original problem and a maximum weighted clique problem in graph theory. A metaheuristic algorithm based on any colony optimization (ACO) is then employed to find the solution in polynomial time. Simulation results demonstrate that the proposed protocol together with the ACO algorithm significantly enhances the system total throughput.Comment: 27 pages, 8 figures, 2 table

Power Allocation and Cooperative Diversity in Two-Way Non-Regenerative Cognitive Radio Networks

In this paper, we investigate the performance of a dual-hop block fading cognitive radio network with underlay spectrum sharing over independent but not necessarily identically distributed (i.n.i.d.) Nakagami-$m$ fading channels. The primary network consists of a source and a destination. Depending on whether the secondary network which consists of two source nodes have a single relay for cooperation or multiple relays thereby employs opportunistic relay selection for cooperation and whether the two source nodes suffer from the primary users' (PU) interference, two cases are considered in this paper, which are referred to as Scenario (a) and Scenario (b), respectively. For the considered underlay spectrum sharing, the transmit power constraint of the proposed system is adjusted by interference limit on the primary network and the interference imposed by primary user (PU). The developed new analysis obtains new analytical results for the outage capacity (OC) and average symbol error probability (ASEP). In particular, for Scenario (a), tight lower bounds on the OC and ASEP of the secondary network are derived in closed-form. In addition, a closed from expression for the end-to-end OC of Scenario (a) is achieved. With regards to Scenario (b), a tight lower bound on the OC of the secondary network is derived in closed-form. All analytical results are corroborated using Monte Carlo simulation method

Multi-source in DF cooperative networks with the PSR protocol based full-duplex energy harvesting over a Rayleigh fading channel: performance analysis

Due to the tremendous energy consumption growth with ever-increasing connected devices, alternative wireless information and power transfer techniques are important not only for theoretical research but also for saving operational costs and for a sustainable growth of wireless communications. In this paper, we investigate the multi-source in decode-and-forward cooperative networks with the power splitting protocol based full-duplex energy harvesting relaying network over a Rayleigh fading channel. In this system model, the multi-source and the destination communicate with each other by both the direct link and an intermediate helping relay. First, we investigate source selection for the best system performance. Then, the closed-form expression of the outage probability and the symbol error ratio are derived. Finally, the Monte Carlo simulation is used for validating the analytical expressions in connection with all main possible system parameters. The research results show that the analytical and simulation results matched well with each other.Web of Science68327526

Dispensing with channel estimation: differentially modulated cooperative wireless communications

As a benefit of bypassing the potentially excessive complexity and yet inaccurate channel estimation, differentially encoded modulation in conjunction with low-complexity noncoherent detection constitutes a viable candidate for user-cooperative systems, where estimating all the links by the relays is unrealistic. In order to stimulate further research on differentially modulated cooperative systems, a number of fundamental challenges encountered in their practical implementations are addressed, including the time-variant-channel-induced performance erosion, flexible cooperative protocol designs, resource allocation as well as its high-spectral-efficiency transceiver design. Our investigations demonstrate the quantitative benefits of cooperative wireless networks both from a pure capacity perspective as well as from a practical system design perspective

Energy harvesting over Rician fading channel: A performance analysis for half-duplex bidirectional sensor networks under hardware impairments

In this paper, a rigorous analysis of the performance of time-switching energy harvesting strategy that is applied for a half-duplex bidirectional wireless sensor network with intermediate relay over a Rician fading channel is presented to provide the exact-form expressions of the outage probability, achievable throughput and the symbol-error-rate (SER) of the system under the hardware impairment condition. Using the proposed probabilistic models for wireless channels between mobile nodes as well as for the hardware noises, we derive the outage probability of the system, and then the throughput and SER can be obtained as a result. Both exact analysis and asymptotic analysis at high signal-power-to-noise-ratio regime are provided. Monte Carlo simulation is also conducted to verify the analysis. This work confirms the effectiveness of energy harvesting applied in wireless sensor networks over a Rician fading channel, and can provide an insightful understanding about the effect of various parameters on the system performance.Web of Science186art. no. 1781

Adaptive relay techniques for OFDM-based cooperative communication systems

Cooperative communication has been considered as a cost-effective manner to exploit the spatial diversity, improve the quality-of-service and extend transmission coverage. However, there are many challenges faced by cooperative systems which use relays to forward signals to the destination, such as the accumulation of multipath channels, complex resource allocation with the bidirectional asymmetric traffic and reduction of transmission efficiency caused by additional relay overhead. In this thesis, we aim to address the above challenges of cooperative communications, and design the efficient relay systems. Starting with the channel accumulation problem in the amplify-and-forward relay system, we proposed two adaptive schemes for single/multiple-relay networks respectively. These schemes exploit an adaptive guard interval (GI) technique to cover the accumulated delay spread and enhance the transmission efficiency by limiting the overhead. The proposed GI scheme can be implemented without any extra control signal. Extending the adaptive GI scheme to multiple-relay systems, we propose a relay selection strategy which achieves the trade-off between the transmission reliability and overhead by considering both the channel gain and the accumulated delay spread. We then consider resource allocation problem in the two-way decode-and-forward relay system with asymmetric traffic loads. Two allocation algorithms are respectively investigated for time-division and frequency-division relay systems to maximize the end-to-end capacity of the two-way system under a capacity ratio constraint. For the frequency-division systems, a balanced end-to-end capacity is defined as the objective function which combines the requirements of maximizing the end-to-end capacity and achieving the capacity ratio. A suboptimal algorithm is proposed for the frequency-division systems which separates subcarrier allocation and time/power allocation. It can achieve the similar performance with the optimal one with reduced complexity. In order to further enhance the transmission reliability and maintaining low processing delay, we propose an equalize-and-forward (EF) relay scheme. The EF relay equalizes the channel between source and relay to eliminate the channel accumulation without signal regeneration. To reduce the processing time, an efficient parallel structure is applied in the EF relay. Numerical results show that the EF relay exhibits low outage probability at the same data rate as compared to AF and DF schemes

Performance Analysis, Resource Allocation and Optimization of Cooperative Communication Systems under Generalized Fading Channels

The increasing demands for high-speed data transmission, efficient wireless access, high quality of service (QoS) and reliable network coverage with reduced power consumption impose demanding intensive research efforts on the design of novel wireless communication system architectures. A notable development in the area of communication theory is the introduction of cooperative communication systems. These technologies become promising solution for the next-generation wireless transmission systems due to their applicability in size, power, hardware and price constrained devices, such as cellular mobile devices, wireless sensors, ad-hoc networks and military communications, being able to provide, e.g., diversity gain against fading channels without the need for installing multiple antennas in a single terminal. The performance of the cooperative systems can in general be significantly increased by allocating the limited power efficiently. In this thesis, we address in detail the performance analysis, resource allocation and optimization of such cooperative communication systems under generalized fading channels. We focus first on energy-efficiency (EE) optimization and optimal power allocation (OPA) of regenerative cooperative network with spatial correlation effects under given power constraint and QoS requirement. The thesis also investigates the end-to-end performance and power allocation of a regenerative multi-relay cooperative network over non-homogeneous scattering environment, which is realistic case in practical wireless communication scenarios. Furthermore, the study investigates the end-to-end performance, OPA and energy optimization analysis under total power constraint and performance requirement of full-duplex (FD) relaying transmission scheme over asymmetric generalized fading models with relay self-interference (SI) effects.The study first focuses on exact error analysis and EE optimization of regenerative relay systems under spatial correlation effects. It first derives novel exact and asymptotic expressions for the symbol-error-rates (SERs) of M -ary quadrature amplitude and M -ary phase-shift keying (M -QAM) and (M -PSK) modulations, respectively, assuming a dual-hop decode-and-forward relay system, spatial correlation, path-loss effects and maximum-ratio-combing (MRC) at the destination. Based on this, EEoptimization and OPA are carried out under certain QoS requirement and transmit power constraints.Furthermore, the second part of the study investigates the end-to-end performance and power allocation of MRC based regenerative multi-relay cooperative system over non-homogeneous scattering environment. Novel exact and asymptotic expressions are derived for the end-to-end average SER for M -QAM and M -PSK modulations.The offered results are employed in performance investigations and power allocation formulations under total transmit power constraints.Finally, the thesis investigates outage performance, OPA and energy optimization analysis under certain system constraints for the FD and half-duplex (HD) relaying systems. Unlike the previous studies that considered the scenario of information transmission over symmetric fading conditions, in this study we considered the scenario of information transmission over the most generalized asymmetric fading environments.The obtained results indicate that depending on the severity of multipath fading, the spatial correlation between the direct and relayed paths and the relay location, the direct transmission is more energy-efficient only for rather short transmission distances and until a certain threshold. Beyond this, the system benefits substantially from the cooperative transmission approach where the cooperation gain increases as the transmission distance increases. Furthermore, the investigations on the power allocation for the multi-relay system over the generalized small-scale fading model show that substantial performance gain can be achieved by the proposed power allocation scheme over the conventional equal power allocation (EPA) scheme when the source-relay and relay-destination paths are highly unbalanced. Extensive studies on the FD relay system also show that OPA provides significant performance gain over the EPA scheme when the relay SI level is relatively strong. In addition, it is shown that the FD relaying scheme is more energy-efficient than the reference HD relaying scheme at long transmission distances and for moderate relay SI levels.In general, the investigations in this thesis provide tools, results and useful insights for implementing space-efficient, low-cost and energy-efficient cooperative networks, specifically, towards the future green communication era where the optimization of the scarce resources is critical

Spectrally Efficient Cooperative Relay Networks using Signal Space Diversity

Cooperative relaying has received widespread attention in recent years from both academic and industrial communities. It offers significant benefits in enabling connectivity as well as in increasing coverage, power saving, spatial diversity and channel capacity. However, one of the main limitations of the conventional cooperative relaying system is the repetition of the received data by the relays, which reduces the spectral efficiency and the data rate. In this thesis, signal space diversity (SSD) based technique is proposed to incorporate into the conventional relaying system to enhance spectral efficiency, data rate and system performance. Firstly, SSD is introduced into a two-way cooperative relaying system with three-phase two-way decode-and-forward (DF) protocol. In this system, four symbols are exchanged in three time slots, thereby doubling the spectral efficiency and the data rate compared to the conventional three-phase two-way DF relaying system that uses six time slots to exchange the same four symbols. Next, SSD is employed in a dual-hop relaying system using DF protocol without a direct link between the source and the destination. In this system, two symbols are transmitted in three time slots as compared to four time slots to transmit the same two symbols in the conventional dual-hop DF relaying system. These proposed systems are designed to exploit the inherent diversity in the modulation signal-space by rotating and expanding the ordinary constellation. The improvement in spectral efficiency is achieved without adding extra complexity, bandwidth or transmit power. A comprehensive analysis of these proposed systems is carried out over Rayleigh fading channels, and closed-form expressions for various performance metrics, including error probability, outage probability and channel capacity, are derived and illustrated. An asymptotic approximation for the error probability is obtained and is used to illustrate the impact of system parameters and diversity gain on the system performance. The optimization of relay location and power allocation in these systems is also examined. Extensive Monte Carlo simulations are performed to ascertain the accuracy of the analytical results presented in the thesis. Indeed, it is observed that the use of SSD in cooperative relaying can play a major role in the system design and performance improvement