Cooperative Orthogonal Frequency Division Multiplexing Research Articles

Coded Assisted Transmit and Receive IQ Mismatch Compensation With Channel Estimation for AF Cooperative OFDM Systems

Amplify-and-forward (AF) orthogonal frequency division multiplexing (OFDM) transmissions encounter a significant difficulty in the form of in-phase and quadrature phase (IQ) mismatch. Previous reports on this problem have solely been discussed in the context of uncoded transmissions. In addition, in these precedent studies, IQ equalization must be conducted following the estimation stage for accurate detection of data symbols. This research delves into the issue of the IQ mismatch between transmission and reception in AF-OFDM systems in the context of channel coding. We design a magnificent code-aided approach to predict the overall channel impulse responses (CIRs), which encompass the actual CIRs and IQ mismatch originating at the source, relay, and destination. Instead of using a collection of algorithms, the proposed approach can be utilized to estimate nine parameters simultaneously. Due to the impractical nature of the precise maximum-likelihood (ML) strategy to this situation, we instead utilize an expectation-maximization (EM) process as a low complexity strategy to predict the parameters under consideration. The suggested estimation approach uses an iterative process to improve predictions exploiting the a priori knowledge gained from the soft information supplied by the channel decoder. In addition, we demonstrate how to carry out data detection by making use of the estimated parameters. The simulation results verify the effectiveness of the proposed estimator and detector for usage in real-world settings, with superiority over the conventional ones.

Capacity and outage performance of multiple AF relays based cooperative OFDM-QIM system

In this paper, we propose a multiple amplify-and-forward (AF) relays based cooperative orthogonal frequency division multiplexing (OFDM) with quadrature index modulation (C-OFDM-QIM) system to enhance the spectral efficiency and the transmission reliability, where two independent subcarrier subsets are activated to convey the real and imaginary symbol vectors, respectively, and the per-subcarrier relay selection (RS) scheme is adopted to select the relay with the maximum end-to-end signal-to-noise ratio (SNR) for each active subcarrier. Both fixed-gain AF and variable-gain AF protocols are investigated at the selected relays. Based on the order statistics and the max-flow min-cut theory, the closed-form average outage probability (AOP) and the average capacity are derived, respectively. Numerical results demonstrate that the theoretical AOP is correct and accurate and the proposed C-OFDM-QIM system outperforms the traditional cooperative OFDM with index modulation (C-OFDM-IM) system in the spectral efficiency, the outage performance, and the average capacity.

Joint Resource Allocation in Secure OFDM Two-Way Untrusted Relay System.

The security issue of wireless communication is a common concern because of its broadcast nature, especially when the relay becomes an eavesdropper. In the orthogonal frequency division multiplexing (OFDM) relay system, when the relay is untrusted, the security of the system faces serious threats. Although there exist some resource allocation schemes in a single-carrier system with untrusted relaying, it is difficult to apply them to the multi-carrier system. Hence, a resource allocation scheme for the multi-carrier system is needed. Compared to the one-way relay system, a two-way relay system can improve the data transmission efficiency. In this paper, we consider joint secure resource allocation for a two-way cooperative OFDM system with an untrusted relay. The joint resource allocation problem of power allocation and subcarrier pairing is formulated to maximize the sum secrecy rate of the system under individual power constraints. To solve the non-convex problem efficiently, we propose an algorithm based on the alternative optimization method. The proposed algorithm is evaluated by simulation results and compared with the benchmarks in the literature. According to the numerical results, in a high signal-to-noise ratio (SNR) scenario, the proposed algorithm improves the achievable sum secrecy rate of the system by more than 15% over conventional algorithms.

Distributed Polar Code Based on Plotkins Construction with MIMO Antennas in Frequency Selective Rayleigh Fading Channels

A Plotkin based polar coded orthogonal frequency division multiplexing (OFDM) with multiple input and multiple output (MIMO) antennas scheme is proposed and its performance over frequency selective Rayleigh fading channel has been evaluated. In the considered Plotkins construction, the longer length code word is formed on the basis of two shorter length code words. Due to the presence of a parallel split in the design of the considered Plotkins construction, the proposed scheme is efficiently extended to coded cooperative scenarios. As the relay always plays crucial role in the design of cooperative communication system, therefore an intelligent criteria for the selection of information bits is employed at the relay node. To check the efficacy and to maintain the complete fairness in the performance of the proposed coded cooperative scheme, the authors have also developed the traditional nested polar coded cooperative scheme in the context of OFDM with MIMO antennas. The Monte Carlo simulated bit error rate (BER) performance revealed that the proposed coded cooperative scheme outperforms the traditional nested polar coded cooperative OFDM with MIMO antennas scheme by gain of $$0.5 \;{\sim}\; 0.6$$ dBs. This significant gain in BER performance of the proposed coded cooperative scheme is made possible due to the efficient selection criteria and the joint successive cancellation (SC) decoding tecnique employed at relay node and at the destination node, respectively. Moreover, the proposed coded cooperative scheme outperforms its corresponding coded non-cooperative counterpart scheme by roughly a gain of 1 dB under identical conditions.

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.

Performance of multicarrier cooperative communication systems over underwater acoustic channels

Multicarrier transmission in the form of orthogonal frequency division multiplexing (OFDM) is an efficient method to handle inter-symbol interference resulting from the frequency selectivity of the underwater acoustic (UWA) channels. OFDM can be further combined with cooperative transmission to benefit from the spatial diversity advantages and improve the performance over conventional point-to-point UWA communication systems. In this study, the authors consider a cooperative OFDM UWA communication system and investigate the outage performance for both amplify-and-forward relaying and decode-and-forward (DF) relaying. They derive expressions for bounds on outage probability and outage capacity. Monte Carlo simulations corroborate and quantify the enhanced performance of cooperative OFDM UWA communication compared with direct transmission systems. Through numerical simulations, they investigate the impact of geometry on outage probability and signal-to-noise ratio requirements in cooperative OFDM UWA communication with DF relaying. Furthermore, the authors analyse the effects of relay location and carrier frequency on the outage probability for both relaying schemes.

Turbo Coded OFDM Combined with MIMO Antennas Based on Matched Interleaver for Coded-Cooperative Wireless Communication

A turbo coded cooperative orthogonal frequency division multiplexing (OFDM) with multiple-input multiple-output (MIMO) antennas scheme is considered, and its performance over a fast Rayleigh fading channel is evaluated. The turbo coded OFDM incorporates MIMO (2 × 2) Alamouti space-time block code. The interleaver design, and its placement always plays a vital role in the performance of a turbo coded cooperation scheme. Therefore, a code-matched interleaver (CMI) is selected as an optimum choice of interleaver and is placed at the relay node. The performance of the CMI is evaluated in a turbo coded OFDM system over an additive white Gaussian noise (AWGN) channel. Moreover, the performance of the CMI is also evaluated in the turbo coded OFDM system with MIMO antennas over a fast Rayleigh fading channel. The modulation schemes chosen are Binary Phase shift keying (BPSK), Quadrature phase shift keying (QPSK) and 16-Quadrature amplitude modulation (16QAM). Soft-demodulators are employed along with joint iterative soft-input soft-output (SISO) turbo decoder at the destination node. Monte Carlo simulated results reveal that the turbo coded cooperative OFDM system with MIMO antennas scheme incorporates coding gain, diversity gain and cooperation gain successfully over the direct transmission scheme under identical conditions.

Error performance and energy efficiency analyses of fully cooperative OFDM communication in frequency selective fading

This study devises for the first time a comprehensive analysis of decode-and-forward, space–time coded, fully cooperative orthogonal frequency division multiplexing (OFDM) systems from both error performance and energy efficiency perspectives, in either identically or non-identically distributed frequency selective Rayleigh fading channels. The authors’ analyses show the interesting result that the fully cooperative OFDM is better than direct OFDM transmission from both perspectives in many cases, especially at a low power and low signal-to-noise ratio regime, making it particularly useful for emerging low power applications, such as wireless body area networks.

Optimal resource allocation for cooperative orthogonal frequency division multiplexing‐based cognitive radio networks with imperfect spectrum sensing

This study investigates sensing-based spectrum sharing access (SSSA) and sensing-based spectrum opportunistic access (SSOA) schemes in cooperative orthogonal frequency division multiplexing (OFDM)-based cognitive radio networks with imperfect spectrum sensing. The optimal resource allocation strategy, including sensing time and transmit power, is designed to maximise the ergodic throughput of the secondary system. For a two-hop cooperative communication in the secondary system, the authors adopt the amplify-and-forward relay protocol and enable the source and relay to sense the state of the primary user jointly. To protect the PU effectively from harmful interference, they consider the average interference power constraint in each hop. The total average transmit power constraint of the source and relay is considered. Two simplified versions for the SSSA and SSOA schemes are employed because of the complexity of the problem. They then propose two algorithms that acquire the optimal sensing time and power allocation for both schemes. Finally, simulation results are presented to compare the performance of the two schemes.

Energy‐efficiency maximisation for cooperative and non‐cooperative OFDMA cellular networks—a survey

AbstractIn this survey, techniques to enhance energy efficiency (EE) in orthogonal frequency division multiple access (OFDMA) and orthogonal frequency division multiplexing (OFDM) systems, with or without the utilisation of the cooperative network paradigm, considering also the features provided in the standards of modern cellular wireless networks, such as LTE‐Advanced and WiMAX, are discussed. For the non‐cooperative EE maximisation case, we summarise resource allocation problems and also describe some techniques that can be combined with the basic power/subcarrier allocation problems. When considering the cooperative OFDM(A) case, we first discuss four basic variables that arise with the relay station implantation, and after that, other features are also listed, which can be combined with the previously discussed issues. Finally, we review some of the standardisation documents available for fourth‐generation systems in order to obtain system parameters and simulation scenarios, discuss some methods to analyse and solve the optimisation problems that can be proposed with the aforementioned techniques and then point out important open trends and research challenges in the EE maximisation problem considering OFDM(A) scenario. Copyright © 2014 John Wiley & Sons, Ltd.

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.

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.

Improvement in Channel Estimation and Error Rate Performance in Mobile Cooperative OFDM Systems with Intercarrier Interference

In mobile cooperative orthogonal frequency division multiplexing (OFDM) systems with amplify-and-forward (AF) relays, in which the source and relay terminals are moving, Doppler frequency might become very large due to the formation of double-hop channels, possibly causing very large intercarrier interference (ICI). Therefore, ICI mitigation technique capable of reducing the impact of high Doppler frequency is required. This paper reports the study of ICI mitigation technique that uses frequency domain equalizer (FDE) with improvements in three aspects, namely, better channel estimation using cubic-spline approximation, use of selection combining to utilize cooperative diversity in order to suppress ICI, and simple error correcting code with double interleaving to suppress errors due to residual ICI. By using the proposed ICI mitigation technique, error rate performance of mobile cooperative OFDM systems can be improved significantly. Even at high Doppler frequencies, error floors due to residual ICI can be decreased by up to two decades.

Subcarrier-pair Based Power Allocation for Cooperative OFDM AF Multi-Relay Networks

For conventional subcarrier pairing schemes in cooperative orthogonal frequency division multiplexing amplify and forward multi-relay networks, to avoid interference, each subcarrier pair (SP) is assigned to only one relay. Over a specific subcarrier, the destination receives signals transmitted from only one relay. In our subcarrier pairing scheme, we assign each SP to all the relays. Thus, over a specific subcarrier, the destination receives signals transmitted from all the relays. Since it is assumed that there exists the direct link from the source to the destination, we assume that the source also transmits signals during the second time slot for the direct transmission mode. We propose an enhanced joint subcarrier pairing and power allocation optimization scheme which maximizes the transmission rate subject to total network power constraint. The problem is simplified and solved by using dual method. It is shown from simulation results that our proposed scheme outperforms the other schemes.

Multiple carrier frequency offsets tracking in co‐operative space‐frequency block‐coded orthogonal frequency division multiplexing systems

This study addresses the problem of carrier frequency offset (CFO) tracking in co-operative space-frequency block-coded orthogonal frequency division multiplexing (OFDM) systems with multiple CFOs. Considering that the inserted pilot tones are decayed by data subcarriers in the presence of multiple CFOs, a novel recursive residual CFO tracking (R-RCFOTr) algorithm is proposed. This method first removes CFO-induced inter-carrier interference from data subcarriers, and then updates the residual CFO (RCFO) estimation of each OFDM block recursively. When used in conjunction with a multiple CFOs estimator, the proposed R-RCFOTr can effectively mitigate the impacts from the multiple RCFOs with affordable complexity. Finally, simulation results are provided to validate the effectiveness of our proposed R-RCFOTr algorithm, which has performance close to that of perfect CFO estimation at moderate and high signal-to-noise ratio, and significantly outperforms conventional CFO tracking algorithm for large CFOs.

An EM Algorithm-Based Disintegrated Channel Estimator for OFDM AF Cooperative Relaying

The cooperative orthogonal frequency-division multiplexing (OFDM) relaying system is widely regarded as a key design for future broadband mobile cellular systems. This paper focuses on channel estimation in such a system that uses amplify-and-forward (AF) as the relaying strategy. In the cooperative AF relaying, the destination requires the individual (disintegrated) channel state information (CSI) of the source-relay (S-R) and relay-destination (R-D) links for optimum combination of the signals received from source and relay. Traditionally, the disintegrated CSIs are obtained with two channel estimators: one at the relay and the other at the destination. That is, the CSI of the S-R link is estimated at relay and passed to destination, and the CSI of the R-D link is estimated at destination with the help of pilot symbols transmitted by relay. In this paper, a new disintegrated channel estimator is proposed; based on an expectation-maximization (EM) algorithm, the disintegrated CSIs can be estimated solely by the estimator at destination. Therefore, the new method requires neither signaling overhead for passing the CSI of the S-R link to destination nor pilot symbols for the estimation of the R-D link. Computer simulations show that the proposed estimator works well under the signal-to-noise ratios of interest.

Multiple CFO Mitigation in Amplify-and-Forward Cooperative OFDM Transmission

In cooperative orthogonal frequency division multiplexing (OFDM) systems, accurate frequency synchronization is critical to achieving any potential gains brought by the cooperative operation. The carrier frequency offsets (CFOs) present among multiple nodes (source, relays and destination) are more difficult to tackle than the single CFO problem in point-to-point systems. Multiple CFOs cause phase drift, inter-carrier interference (ICI) and inter-block interference (IBI) in the received signal. This paper deals with the CFO induced interference mitigation problem in distributed space time block coded (STBC) amplify-and-forward (AF) cooperative OFDM systems. We propose a two step approach to recover the phase distortion and suppress the ICI and IBI using low complexity methods to achieve high performance. The first step is time domain (TD) compensation and the second step is frequency domain (FD) decoding. Two TD compensation schemes are proposed, i.e., IBI-removal and ICI-removal. The IBI-removal scheme decouples the two blocks of one STBC codeword completely and then decodes the ICI degraded blocks individually. The ICI-removal scheme removes ICI first and the subsequent decoding requires joint decoding of the two blocks. Simulation results show that the IBI-removal scheme which is of lower complexity performs well with small CFO. For large CFO, the ICI-removal with modified iterative joint maximum likelihood decoding (MIJMLD) outperforms other schemes.

Achievable data rates and power allocation for frequency-selective fading relay channels with imperfect channel estimation

In this article, we investigate the information-theoretical performance of a cooperative orthogonal frequency division multiplexing (OFDM) system with imperfect channel estimation. Assuming the deployment of training-aided channel estimators, we derive a lower bound on the achievable rate for the cooperative OFDM system with amplify-and-forward relaying over frequency-selective Rayleigh fading channels. The bound is later utilized to allocate power among the training and data transmission phases. Numerical results demonstrate that the proposed power allocation scheme brings between 5 and 19% improvement depending on the level of signal-to-noise ratio and relay locations.

协作分集OFDM系统中的时频同步

In the cooperative relay system, different frames from different relays are not aligned in the time domain, and have different carrier frequency offsets (CFOs). Estimation of these time offsets and CFOs is a crucial and challenging task. In cooperative orthogonal frequency division multiplexing (OFDM) systems, a new preamble structure is designed to assist the destination node to distinguish the signals from different relays, and the corresponding time and frequency synchronization algorithms are also presented. In the aspects of the timing synchronization, the coarse timing offset estimation is accomplished by using the symmetric conjugate of the training symbol presented in this paper and the fine timing offset estimation is accomplished by segmented moving correlation. In the aspects of the frequency synchronization, the fractional frequency offset is estimated by using the phase difference of the received signal and the integral frequency offset is estimated by utilizing the good autocorrelation of the training symbol in frequency domain. The analysis and the simulation results show that, compared with the traditional scheme, the proposed scheme not only has a significantly smaller MSE of timing and frequency offset estimation but also has a lower complexity. Therefore, it is more suitable for the time and frequency synchronization of OFDM-based cooperative systems.

Optimal Bit and Power Loading for Amplify-and-Forward Cooperative OFDM Systems

In this paper, we investigate bit and power allocation strategies for an orthogonal frequency division multiplexing (OFDM) cooperative network over frequency-selective fading channels. We assume amplify-and-forward relaying and consider the bit error rate (BER) performance as our performance measure. Aiming to optimize the BER under total power constraint and for a given average data rate, we propose three adaptive algorithms; optimal power loading (OPL), optimal bit loading (OBL), and optimal joint bit and power loading (OBPL). Our Monte Carlo simulation results demonstrate performance gains through adaptive bit and power loading over conventional non-adaptive systems as well as currently available adaptive cooperative scheme in the literature. The impact of practical issues on the performance of proposed adaptive schemes such as imperfect channel estimation and limited feedback is further discussed.