Cooperative OFDM Systems Research Articles

Subcarrier‐pair based joint resource allocation for secure cooperative OFDM system with multiple untrusted AF relays

In this paper, a cooperative orthogonal frequency division multiplexing system with multiple untrusted amplify-and-forward relays is considered. Since there exists no direct link between source and user because of the shadowing effect, a positive system secrecy rate cannot be obtained. Addressing this issue, a cooperative user-aided jamming technique to improve the secrecy performance is employed. With the aim of maximising the system secrecy rate, a joint resource allocation problem of power allocation, relay assignment and subcarrier pairing is formulated. By using the signal-to-noise ratio based approach and the Lagrange dual method, this nonconvex problem is solved efficiently. To reduce the calculation complexity, a suboptimal algorithm, which decouples the power allocation with the relay assignment and subcarrier pairing is further proposed. Numerical results are presented to demonstrate the advantage of the two proposed algorithms and the impact of relays' location and number of subcarriers on the secrecy performance. Moreover, it is shown that the secrecy performance of the proposed scheme is decreased with respect to the number of untrusted relays.

On the Effect of Relay Position in Carrier Interferometry OFDM for Cooperative Diversity by Wavelet Transform

In a wireless communication system, cooperative diversity can be used to combat fading effects. In cooperative diversity, the system uses idle antennas, which are located at different locations, as relays. Cooperative communication with OFDM is widely used in communication systems and it enhances communication performance of the system. There are different methods to enhance the performance of OFDM systems, carrier interferometry code is one of them. While, carrier interferometry code increase the performance of the system and also reduces PAPR level. Carrier interferometry codes can be implemented on an OFDM system by using FFT and wavelet. In this paper we investigate the effect of the relay position on the carrier interferometry cooperative OFDM system which uses wavelet transform. The effect of the relay position is investigated over different combiners and relaying strategies. The results show that, relay position effects the performance of the system and optimal location for the relay is right between source and destination.

Enhanced Performance of Amplify and Forward Cooperative OFDM System in a Hostile Environment

This paper introduces a cooperative orthogonal frequency division multiplexing (CO-OFDM) based on amplify and forward (AF) relay and utilizing a fast low complexity T-transform as a subcarriers modulation scheme. The proposed cooperative system is developed by changing the modulation/demodulation algorithms which significantly enhanced the immunity of the system to deep notches of the multipath channels. Thereafter, the paper presents a mathematical analytic model for the proposed amplify and forward CO-T-OFDM scheme. The use of T-transform in the transceiver of the proposed system enhances the signal diversity. Therefore, the presented AF cooperative system is efficient in enhancing the signal diversity, mitigating the effects of multipath fading channels and achieving more reliable bit-error-rate (BER) performance than the state-of-the-art cooperative system. It also moderates the peaks power of the signal. The BER of the developed AF cooperative system is evaluated by Monte Carlo simulation over frequency-selective multipath channels when AF relay is used, and compared with the BER performance of the standard AF cooperative system. The results show that the developed AF cooperative technology achieves better BER than the classic CO-OFDM under different transmission scenarios.

Efficient relay selection and subcarrier assignment in cooperative OFDMA systems with QoS guarantees

This paper investigates relay selection and subcarrier assignment in cooperative OFDMA systems with considerations of QoS requirements. The joint resource allocation problem is formulated as a 0-1 integer linear programming problem with the objective of maximizing the system throughput. The efficient joint optimal algorithm is proposed to tackle the problem. By using the penalty function, the complicating constraints are included in the objective function and the special model with a totally unimodular constraint coefficient matrix is constructed that could be solved rapidly as a linear programming with reduced computational complexity. Numerical results show that the proposed algorithm significantly outperforms previous works in terms of both heterogeneous services support and QoS guarantees.

Packet splitting and adaptive modulation based on CSI of time domain for decode‐and‐forward cooperative OFDM systems in different channel model

SummaryCooperative communications obtain the transmission and channel diversity gains by using the relay node. However, since cooperative communications transmit the redundancy signal to obtain the transmission diversity gain, the transmission rate is degraded. Moreover, since cooperative communications add the interference in the relay node, the diversity gain is also degraded. The packet splitting has been proposed based on the channel state information of the time domain to obtain the good system performance without the redundancy signal. Moreover, the adaptive modulation has been proposed to improve the transmission rate. In this paper, we propose the combination method with the packet splitting and the adaptive modulation based on the channel state information of the time domain to improve the bit error rate and throughput performances for decode‐and‐forward cooperative orthogonal frequency division multiplexing systems in the different channel model. From the computer simulation results, we determine the optimum weight and threshold for the proposed method. Moreover, the proposed method shows the good bit error rate and throughput performances.

Guest Editorial

Guest Editorial

On the Spectral-Energy Efficiency and Rate Fairness Tradeoff in Relay-Aided Cooperative OFDMA Systems

In resource constrained wireless systems, achieving higher spectral efficiency (SE) and energy efficiency (EE), and greater rate fairness are conflicting objectives. Here, a general framework is presented to analyze the tradeoff among these three performance metrics in cooperative OFDMA systems with decode-and-forward relaying, where subcarrier pairing and allocation, relay selection, choice of transmission strategy, and power allocation are jointly considered. In our analytical framework, rate fairness is represented utilizing the α-fairness model, and the resource allocation problem is formulated as a multi-objective optimization problem. We then propose a cross-layer resource allocation algorithm across application and physical layers, and further devise a heuristic algorithm to tackle the computational complexity issue. The SE-EE tradeoff is characterized as a Pareto optimal set, and the efficiency and fairness tradeoff is investigated through the price of fairness. Simulations indicate that higher fairness results in a worse SE-EE tradeoff. It is also shown imposing fairness helps to reduce the outage probability. For a fixed number of relays, by increasing circuit power, the performance of SE-EE tradeoff is degraded. Interestingly, by increasing the number of relays, although the total circuit power is increased, the SE-EE tradeoff is not necessarily degraded. This is thanks to the extra degree of freedom provided in relay selection.

Outage and SER analysis of DF cooperative OFDM systems with nonlinear power amplifiers

In this paper, a performance analysis of decode-and-forward (DF) cooperative OFDM systems is presented by considering the nonlinear distortions inserted by PAs at the source and relay. In particular, a theoretical formulation for the symbol error rate (SER) and for upper and lower bounds of the outage probability are developed considering fixed and selective DF relaying protocols. In the considered system model, the receiver exploits the signal received directly from the source as well as the signal transmitted through the relay using the maximum ratio combining (MRC) technique. The proposed analysis also studies the effects of the nonlinearities on the diversity order and it shows in what situations the PA nonlinearities affect significantly the system performance. The proposed theoretical expressions are validated by means of computer simulations.

Dynamic Optimization for Resource Allocation in Relay-Aided OFDMA Systems Under Multiservice

This paper investigates the dynamic resource allocation (RA) problem in cooperative OFDMA systems to maximize the average utility of all mobile stations (MSs) under different services. We propose a dynamic optimization framework for RA by considering three dynamic situations: time-varying fading channel, MSs' change of states, and relay stations (RSs)' change of states. Moreover, a dynamic RA algorithm based on discrete particle swarm optimization (DPSO) is proposed. The correlation between the adjacent frames is exploited to improve the performance of the dynamic RA algorithm. Simulation results show that the proposed dynamic algorithm achieves better performance at linear complexity, compared with the existing algorithms under different dynamic environments, while guaranteeing fairness to a proper level.

English

This project proposes the cooperative multiuser OFDMA systems with subcarrier resource allocation technique. A particular subcarrier resource allocation approach about which this paper proposes is a method, based on nodes that transmit and receive on adjacent OFDM subcarriers simultaneously. I propose a transceiver structure to reduce the interference between transmitting subcarriers and receiving subcarriers. The transceiver structure includes QPSK (Quadrature Phase Shift Keying).In this two sets of signals would be mixed; the required 90o of phase separation between the carriers allows the sidebands to be separated by the receiver using phase discrimination. Its performance in terms of signal to interference and noise ratio (SINR) is evaluated by simulation and is incorporated into a recently proposed cooperation strategy for OFDMA systems to examine its performance under the realistic structure.It is shown that SINR increased when QPSK modulation used.

Theoretical Outage Analysis of Nonlinear OFDM Systems with AF Relays

In this work, a performance analysis of the outage probability of a cooperative diversity OFDM system is presented, considering the distortions introduced by nonlinear power amplifiers (PAs) at the source and relay nodes. A closed form expression for the instantaneous SNR is developed and analytical expressions for the outage probability are derived, considering an amplify-and-forward (AF) relay and Rayleigh fading. Both the maximum ratio combing (MRC) and selection combining (SC) methods are considered. The derived expressions show how some PA parameters affect the outage probability and diversity order. Computer simulation results illustrate and confirm the proposed theoretical expressions under different scenarios.

Link adaptation algorithm for distributed coded transmissions in cooperative OFDMA systems

This paper proposes a link adaptation algorithm for cooperative transmissions in the down-link connection of an OFDMA-based wireless system. The algorithm aims at maximizing the spectral efficiency of a relay-aided communication link, while satisfying the block error rate constraints at both the relay and the destination nodes. The optimal solution would be to perform an exhaustive search over a high-dimensional space determined by all possible combinations of modulations, code rates and information block lengths on the individual channels of the cooperative link; clearly, such an approach has an intractable complexity. Our solution is to use a link performance prediction method and a trellis diagram representation such that the resulting algorithm outputs the link configuration that conveys as many information bits as possible and also fulfilling the block error rate constraints. The proposed link adaptation algorithm has linear complexity with the number of available resource blocks, while still provides a very good performance, as shown by simulation results.

Power Allocation Algorithms for Cooperative OFDM System with a Relay and BDPSK Modulation

In this paper, we investigate the Power Allocation (PA) problem for Cooperative Orthogonal Frequency Division Multiplexing (C-OFDM) communication system with a relay which Decodes and Forwards (DF) the data and with Binary Differential Phase Shift Keying (BDPSK) modulation. We propose the optimal power allocation algorithms to minimize Average Bit Error Rate (ABER). In the optimal algorithms, the system firstly allocates power for each sub-carrier pair, and then allocates power in every sub-carrier pair. In addition, the performance of optimal scheme with simple sub-carrier pairing proposed in this paper is better. Simulation results show that significant performance gains can be achieved by the proposed algorithms.

Relay selection for peer-to-peer cooperative OFDMA with channel distribution uncertainty

Peer-to-peer relay technique provides the opportunity of cooperative communication between users to achieve the transmission diversity. In this paper, the problem of relay selection and subcarrier allocation in peer-to-peer cooperative OFDMA system with channel distribution uncertainty is studied. The quality of service (QoS) requirements are characterized with the probabilistic constraints. The Kullback-Leibler (KL) divergence is adopted to describe the uncertainty of the channel state information (CSI), which follows a distribution deviated within a certain distance from the reference distribution. Since the probabilistic constraint is intractable, we resort to a tractable upper bound instead. The problem is solved by the interior point cutting plane (IPCP) method, which proceeds in round whereby an iterative method is designed to combine both golden section method and interior point method to find the saddle point of the constraint. Simulation results show the effectiveness of our algorithm.

An LP based Fair Subcarrier Allocation Algorithm for Cooperative OFDMA Systems with Grouped Users

Cooperative communication and orthogonal frequency division multiplexing (OFDM) technology are both promising candidates for next generation wireless communication systems. In this paper a Linear Programming (LP) based subcarrier allocation algorithm for cooperative multiuser OFDM system with grouped user is proposed. The proposed algorithm maximizes the data rate of all users over downlink under given constraints over non-sharing of carriers and fairness of data rate among all users. In this proposed algorithm the non sharing constraint of subcarriers have been relaxed initially into sharing constraint so that the non–convex optimization problem becomes a convex optimization problem, and thus, can solved efficiently using standard convex optimization tools. After maximizing total capacity of all active users in the system, the subcarrier allocation constraint is, then converted back to non-sharing constraint. The performance of proposed algorithm is studied and simulation results show that higher total capacity can be achieved using the proposed algorithm.

Extended joint channel estimation in cooperative multi-cell OFDM systems

Extended joint channel estimation in cooperative multi-cell OFDM systems

Realizing Cooperative Multiuser OFDMA Systems with Subcarrier Resource Allocation

Cooperative multiuser OFDMA is an important technology for current and future wireless communications systems. A critical part of this technology is performing multiuser subcarrier resource allocation and a wide variety of techniques have been suggested that can increase overall throughput, decrease total transmit power or improve other key performance measures. A particular subcarrier resource allocation approach that is of interest in this paper is a method based on nodes that transmit and receive on adjacent OFDM subcarriers simultaneously. This approach has shown significant promise, but its feasibility has not been fully investigated. In particular there will be a performance tradeoff in realistic systems as adjacent subcarriers will not be completely isolated and there will be interference on the receiving subcarriers that are adjacent to those that are transmitting. In this paper we look into the performance of such cooperative OFDMA systems under realistic conditions. We propose a transceiver structure to reduce the interference between transmitting subcarriers and receiving subcarriers. Its performance in terms of signal to interference and noise ratio (SINR) is evaluated by both analysis and simulation and is incorporated into a recently proposed cooperation strategy for OFDMA systems to examine its performance under the realistic structure. It is shown that although the cooperation strategy suffers from performance degradation due to the residual interference between the transmitting and receiving subcarriers, it still outperforms the conventional cooperation schemes.

Joint cooperative analog network coding and OFDM system for wireless communication networks

In wireless mobile communication networks, network coding and cooperation are the promising techniques to increase the spectrum utilisation efficiency. OFDM is also one of the most efficient modulation techniques to deal with multipath fading channel distortions. In this paper, a scheme of joint analog network coding and OFDM for cooperative communication systems is proposed, and the scheme suggests using network coding to improve the system spectral efficiency, and using OFDM to cope with channel distortion. The analysis of frame error probability for the proposed scheme is presented.

A Robust Coarse Timing Synchronizer Design for Cooperative Diversity OFDM System

In this paper, a robust coarse symbol timing offset synchronizer design for cooperative diversity OFDM system is proposed. Further, a preamble format suitable for cooperative diversity scenario is presented. Then frame structure, preamble and cooperative protocol for transmitter and relay is elaborated in context of deployment of proposed algorithm. The proposed algorithm shows good probability of synch detection even in a Rayleigh fading channel. Furthermore, two low complexity versions for coarse timing metric estimation are also described. The performance of these algorithms is evaluated for orthogonal, independent and proposed preamble using Monte Carlo simulations.

Ordering-based Iterative Maximum-likelihood Detector in Space-time Cooperative OFDM Systems

Ordering-based Iterative Maximum-likelihood Detector in Space-time Cooperative OFDM Systems