Subcarrier Pairing Research Articles

Optimal subcarrier pairing scheme for maximal ratio combining in OFDM power line communications

Power line communication (PLC) networks for a smart grid require high reliability for data communications. In order to improve the reliability of PLC networks, we consider a maximal ratio combining (MRC) diversity scheme for a power line orthogonal frequency division multiplexing (OFDM) system. An optimal subcarrier pairing scheme is proposed to maximize the MRC gain. Numerical results are presented to verify that the proposed scheme provides enhanced performance.

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.

Optimal and sub‐optimal resource allocation in multiple‐input multiple‐output‐orthogonal frequency division multiplexing‐based multi‐relay cooperative cognitive radio networks

The authors consider the resource allocation issue in multiple-input multiple-output-orthogonal frequency division multiplexing-based cooperative cognitive radio networks with the decode-and-forward (DF) as the cooperation strategy. The dual decomposition technique is recruited to obtain an optimal subcarrier pairing, relay selection and power allocation. The optimal resource allocation is performed subject to the individual power constraints in source and relays so that the sum-rate is maximised while the interference induced to the primary system is kept below a pre-specified interference temperature limit. Moreover, because of the high computational complexity of the optimal approach, a sub-optimal algorithm is further proposed. In the sub-optimal scheme, the joint allocation of the resources is performed taking into account the channel qualities, the DF cooperation strategy, the interference induced to the primary system and the individual power budgets. The performance of different approaches and the impact of the constraint values and deploying multiple-antennas at users are explored through numerical simulations.

Sum rate maximization for multi-cell downlink OFDMA with subcarrier pair-based opportunistic DF relaying

This paper considers multi-cell decode-and-forward (DF) relay-aided orthogonal frequency division multi-access (OFDMA) downlink systems, in which all sources and relays are coordinated by a central controller for resource allocation (RA). The improved subcarrier pair-based opportunistic DF relaying protocol proposed and studied in the IEEE International Conference on Communications, Beijing, 3795–3800, 2008 and IEEE Trans. Signal Process. 61:2512–2524, 2013 is applied. This protocol has a high spectrum efficiency (HSE) in the sense that all unpaired subcarriers are utilized for data transmission during the second time slot (TS). In particular, the sum (over all cells and all destinations) rate maximized problem with a total power constraint in each cell+ is formulated. To solve this problem, an iterative RA algorithm is proposed to optimize mode selection (decision whether the relay should help or not), subcarrier assignment and pairing (MSSAP) and power allocation (PA) in an alternate way. As for the MSSAP stage of each iteration, the formulated problem is decoupled into subproblems with the tentative PA results. Each subproblem can be easily solved by using the optimal results of a linear assignment problem (LAP), which is then solved by the Hungarian Algorithm in polynomial time. As for the PA stage of each iteration, an algorithm based on single-condensation and geometric programming (SCGP) is proposed to optimize PA in polynomial time with the tentative MSSAP results. The proposed algorithm is coordinate ascent (CA)-based and therefore can reach a local optimum in polynomial time. Finally, the convergence and effectiveness of the proposed algorithm, the impact of relay position and total power on the system performance, and the benefits of using subcarrier pairing (SP) and the HSE protocol are illustrated through numerical experiments.

Joint Resource Allocation for Lifetime-aware Cooperative Multi-carrier DF System

In this paper, we study the problem of optimal resource allocation for lifetime maximization in an orthogonal-frequency-division multiplexing (OFDM) system with decode-and-forward relay. The goal is to minimize total energy cost of the system by jointly optimizing power allocation, subcarrier pairing and relay selection. We present a heuristic solution that is composed of two parts. The first part is an optimal power allocation approach to allocate power to a subcarrier pair of the source and the relay. The second part is a modified Hungarian algorithm to make subcarrier pairing and relay selection. Evaluations show that the presented scheme outperforms other schemes in the total transmitted data and the network lifetime.

Performance Analysis of Two-Way OFDM Relay Systems with Ordered Subcarrier Pairing

In this paper, we first prove the optimality of ordered subcarrier pairing (OSP) in two-way orthogonal frequency division multiplexing (OFDM) relay systems. The probability distribution function (PDF) and the cumulative distribution function (CDF) of the signal-to-noise ratio (SNR) is then derived for each subcarrier pair of the two-way links over Rayleigh fading channels. Furthermore, we analyze the performance of the systems including outage probability, symbol error rate (SER) and capacity. Monte Carlo simulations validate our analysis.

Optimally Joint Subcarrier Pairing and Power Allocation for OFDM System with Multihop Symbol Level DF Relaying

Subcarrier pairing (SP) and power allocation (PA) can improve the channel capacity of the OFDM multi-hop relay system. Due to limitations of processing complexity and energy consumption, symbol-level relaying, which only regenerates the constellation symbols at relay nodes, is more practical than code-level relaying that requires full decoding and encoding. By modeling multi-hop symbol-level relaying as a multi-staged parallel binary symmetric channel, this paper introduces a jointly optimal SP and PA scheme which maximizes the end to end data rate. Analytical arguments are given to reveal the structures and properties of the optimal solution, and simulation results are presented to illustrate and justify the optimality.

Relay Selection and Subcarrier-Pair Based Energy-Efficient Resource Allocation for Multirelay Cooperative OFDMA Networks

Energy-efficient resource allocation is investigated for a relay-based multiuser cooperation orthogonal frequency division multiple access (OFDMA) uplink system with amplify-and-forward (AF) protocol for all relays. The objective is to maximize the total energy efficiency (EE) of the uplink system with consideration of some practical limitations, such as the individual power constraint for the users and relays and the quality of service (QoS) for every user. We formulate an energy-efficient resource allocation problem that seeks joint optimization of subcarrier pairing, relay selection, subcarrier assignment, and power allocation. Unlike previous optimization throughput models, we transform the considered EE problem in fractional form into an equivalent optimal problem in subtractive form, which is solved by using dual decomposition and subgradient methods. To reduce computation costs, we propose two low-complexity suboptimal schemes. Numerical studies are conducted to evaluate the EE of the proposed algorithms.

Joint sub-carrier pairing and resource allocation for cognitive networks with adaptive relaying

Relayed transmission in a cognitive radio (CR) environment could be used to increase the coverage and capacity of communication system that benefits already from the efficient management of the spectrum developed by CR. Furthermore, there are many types of cooperative communications, including decode-and-forward (DAF) and amplify-and-forward (AAF). In this paper, these techniques are combined in an adaptive mode to benefit from its forwarding advantages; this mode is called adaptive relaying protocol (ARP). Moreover, this work focuses on the joint power allocation in a cognitive radio system in a cooperative mode that operates ARP in multi-carrier mode. The multi-carrier scenario is used in an orthogonal frequency division multiplexing (OFDM) mode, and the problem is formulated to maximize the end-to-end rate by searching the best power allocation at the transmitters. This work includes, besides the ARP model, a sub-carrier pairing strategy that allows the relays to switch to the best sub-carrier pairs to increase the throughput. The optimization problem is formulated and solved under the interference and power budget constraints using the sub-gradient algorithm. The simulation results confirm the efficiency of the proposed adaptive relaying protocol in comparison to other relaying techniques. The results show also the consequence of the choice of the pairing strategy.

Resource Allocation in Relay-Aided OFDM Cognitive Radio Networks

In this paper, we investigate the resource allocation problem in a single-user relay-aided cognitive radio (CR) underlay network. Both the CR network and the primary network operate under the orthogonal frequency-division multiplexing (OFDM) scheme. Different from the conventional resource allocation problem, the relay node here is capable of performing subcarrier permutation over two hops such that the signal received over a particular subcarrier is forwarded via a different subcarrier. The objective is to maximize the throughput of the CR network subject to a limited power budget at the secondary source and the relay node and to interference constraints at the primary receiver. Optimization is performed under a unified framework where power allocation at the source node, power allocation at the relay node, and subcarrier pairing at the two hops are jointly optimized. The joint resource-allocation scheme yields an asymptotically optimal solution. We further design a suboptimal algorithm that sacrifices little on performance but could significantly reduce computational complexity. Finally, numerical examples are provided to corroborate the proposed studies.

A Multidimensional Resource-Allocation Optimization Algorithm for the Network-Coding-Based Multiple-Access Relay Channels in OFDM Systems

Joint scheduling and resource allocation in uplink orthogonal frequency-division multiplexing (OFDM) systems is complicated and even becomes intractable with a large subcarrier and a large user number. This paper investigates the resource allocation for OFDM-based multiuser multiple-access relay channels (MARCs) with network coding. We formulate a joint optimization problem considering source-node pairing, subcarrier assignment, subcarrier pairing, and power allocation to maximize the sum rate under per-user power constraints. The problem is solved in polynomial time by optimizing three separate subproblems. To further reduce the complexity, three low-complexity suboptimal algorithms are then proposed when fixing partial resource. The simulation results show the performance gains of the proposed algorithms versus per-node transmit power and source-node number, and the impact of relay location is also evaluated.

Proof of Optimality of Ordered Subcarrier Pairing in Amplify-and-Forward OFDM Relay Systems without Diversity

Amplify-and-forward (AF) orthogonal frequency division multiplexing (OFDM) relay systems without diversity are studied, with emphasizing on the subcarrier pairing at the relay node. Using the subcarrier pairing based on the signal-to-noise power ratios (SNRs) of subcarriers, it is proven that the ordered subcarrier pairing (OSP), according to normalized channel gains of the subcarriers of OFDM at two hops, is optimal in terms of system capacity, without any assumption for SNRs of subcarriers and types of power constraints. The proof about the optimality of OSP shows that in this kind of relay systems, the subcarrier pairing and power allocation are completely decoupled.

Optimal resource allocation for opportunistic spectrum access in multiple‐input multiple‐output –orthogonal frequency division multiplexing based cooperative cognitive radio networks

In this study, the problem of resource allocation in multiple-input multiple-output-orthogonal frequency division multiplexing-based cooperative cognitive radio networks is considered. The cooperation strategies between the secondary users is the decode-and-forward (DF) strategies. In order to obtain an optimal subcarrier pairing, relay assignment and power allocation in the system, the dual decomposition technique is recruited. The optimal resource allocation is realised under the individual power constraints in source and relays, so that the sum rate is maximised, whereas the interference induced to the primary system is kept below a pre-specified interference temperature limit. Moreover, because of the high computational complexity of the optimal approach, a suboptimal algorithm is further proposed. The joint allocation of the resources in suboptimal algorithm is carried out taking into account the channel qualities, the DF cooperation strategy, the interference induced to the primary system and the individual power budgets. The performance of the different approaches and the impact of the constraint values and deploying multiple antennas at users are discussed through the numerical simulation results.

QoS‐driven jointly optimal subcarrier pairing and power allocation for OFDM amplify‐and‐forward relay systems

SUMMARYIn this paper, we investigate the quality‐of‐service (QoS) driven subcarrier pairing and power allocation for two‐hop amplify‐and‐forward OFDM relay systems. By integrating the concept of effective capacity, our goal is to maximize the system throughput subject to a given delay QoS constraint. We propose a jointly optimal subcarrier pairing and power allocation scheme, which can be implemented with two separate steps. First, pair the subcarriers over the source‐relay channel and relay‐destination channel by the descending order of the subcarriers’ channel gains. Second, by making use of the derived equivalent end‐to‐end channel gains of the subcarrier pairs, optimally allocate power over the subcarrier pairs, and then optimally partition the power of the subcarrier pairs between the source and the relay. The simulation results show that our proposed scheme can efficiently provide different levels of delay QoS guarantees, even if under stringent delay QoS constraints. The simulation results also verify that our proposed scheme shows significant superiorities over the other existing schemes. Copyright © 2013 John Wiley & Sons, Ltd.

Joint Power Allocation and Subcarrier Pairing for Cooperative OFDM AF Multi-Relay Networks

For conventional subcarrier pairing scheme 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 this letter, we propose to assign each SP to all the relays. Thus, over a specific subcarrier, the destination receives signals transmitted from all the relays. We propose a joint power allocation and subcarrier pairing scheme which maximizes the transmission rate subject to total network power constraint. The problem is simplified and solved by using dual method.

Power Minimization for OFDM Transmission with Subcarrier-Pair Based Opportunistic DF Relaying

This paper develops a sum-power minimized resource allocation (RA) algorithm subject to a sum-rate constraint for cooperative orthogonal frequency division modulation (OFDM) transmission with subcarrier-pair based opportunistic decode-and-forward (DF) relaying. The improved DF protocol first proposed in [1] is used with optimized subcarrier pairing. Instrumental to the RA algorithm design is appropriate definition of variables to represent source/relay power allocation, subcarrier pairing and transmission-mode selection elegantly, so that after continuous relaxation, the dual method and the Hungarian algorithm can be used to find an (at least approximately) optimum RA with polynomial complexity. Moreover, the bisection method is used to speed up the search of the optimum Lagrange multiplier for the dual method. Numerical results are shown to illustrate the power-reduction benefit of the improved DF protocol with optimized subcarrier pairing.

A Network Flow Approach to Throughput Maximization in Cooperative OFDMA Networks

In wireless cooperative orthogonal frequency-division multiple-access (OFDMA) networks, it is important to adapt the transmission strategies for each user according to the network channel dynamics in order to optimize the overall system performance. The adaption involves transmission mode selection (a user can choose from direct or cooperative transmission), subcarrier assignment, subcarrier pairing (the incoming and outgoing subcarriers at the relay for cooperative transmission need to be matched), relay selection, as well as power allocation and hence is highly challenging. Many previous works only consider a subset of the adaptation. In this paper, we tackle the joint optimization problem using a network flow approach. Specifically, we first show that for given power allocation, the combinatorial optimization problem of transmission mode selection, subcarrier assignment, relay selection and subcarrier pairing for the system total throughput maximization can be transformed into a minimum cost network flow (MCNF) problem with integer solutions. The linear optimal distribution (LOD) algorithm is applied to find the optimal solution in polynomial time. We then solve the mixed integer programming problem of the joint assignment and power allocation in an asymptotically optimal way in the dual domain. Simulation results show that the proposed algorithms can significantly enhance the overall system throughput.

Feedback-Aided Pilot Placement and Subcarrier Pairing for AF OFDM Relay Channels

This paper examines the use of channel feedback to adaptively determine pilot placement among subcarriers in amplify-and-forward (AF) orthogonal frequency-division multiplexing (OFDM) relay systems. With feedback of the previous channel estimate, pilot subcarriers at the source and relay can be reassigned to reduce channel estimation error and ensure that good subcarriers are made available for data transmission. The minimum effective signal-to-noise ratio (SNR) among data subcarriers is utilized as the performance metric for the optimization of the pilot locations. The effective SNR is defined as the receive SNR at the destination, taking into consideration the source-to-relay (SR) and relay-to-destination (RD) channels, as well as their respective channel estimation errors. The feedback gain, which is defined as the ratio between the minimum effective SNR achieved with the optimal feedback-aided pilot placement and that achieved with the equal-spaced pilot placement, is used to measure the gains achieved through feedback. In this paper, we first derive an analytic lower bound on the optimal feedback gain and show its rate of increase with respect to the number of subcarriers. These studies are performed for systems both without and with subcarrier pairing (SP) at the relay. Specifically, in systems without SP, the relay forwards each datum on the same subcarrier on which it is received. In systems with SP, the relay is allowed to reassign the subcarriers used to forward the data received from the source. With the goal of maximizing the minimum effective SNR, the optimal SP scheme is shown to be an inverse mapping between the SR and RD subcarriers with respect to their channel gains. Due to the complexity of finding the optimal placement, a suboptimal but efficient algorithm called the iterative pilot relocation (IterPR) scheme is then proposed. The effectiveness of the proposed IterPR schemes is demonstrated through numerical simulations.

Energy-efficient power allocation for non-regenerative OFDM relay links

We consider a two-hop non-regenerative (amplify-and-forward) relay link with one source, one destination and one relay, in which orthogonal frequency division multiplexing (OFDM) is used. The power allocation (PA) to maximize the energy efficiency for the whole link is investigated under quality of service (Qos) requirement and separate power constraints. An approximation method with low complexity is first proposed to convert the PA problem into a quasi-concave problem, which can obtain a tight lower bound in high SNR region. Furthermore, a more accurate approximation method with higher complexity is raised, which can approach the actual performance both in the low SNR and high SNR regions. We also introduce subcarrier pairing scheme to further enhance the energy efficiency for the link. Simulation results are shown to compare the performance of the two proposed methods.

Weighted Sum Rate Maximization for Downlink OFDMA With Subcarrier-Pair Based Opportunistic DF Relaying

This paper addresses a weighted sum rate (WSR) maximization problem for downlink OFDMA aided by a decode-and-forward (DF) relay under a total power constraint. A novel subcarrier-pair based opportunistic DF relaying protocol is proposed. Specifically, user message bits are transmitted in two time slots. A subcarrier in the first slot can be paired with a subcarrier in the second slot for the DF relay-aided transmission to a user. In particular, the source and the relay can transmit simultaneously to implement beamforming at the subcarrier in the second slot. Each unpaired subcarrier in either the first or second slot is used for the source's direct transmission to a user. A benchmark protocol, same as the proposed one except that the transmit beamforming is not used for the relay-aided transmission, is also considered. For each protocol, a polynomial-complexity algorithm is developed to find at least an approximately optimum resource allocation (RA), by using continuous relaxation, the dual method, and Hungarian algorithm. Instrumental to the algorithm design is an elegant definition of optimization variables, motivated by the idea of regarding the unpaired subcarriers as virtual subcarrier pairs in the direct transmission mode. The effectiveness of the RA algorithm and the impact of relay position and total power on the protocols' performance are illustrated by numerical experiments. The proposed protocol always leads to a maximum WSR equal to or greater than that for the benchmark one, and the performance gain of using the proposed one is significant especially when the relay is in close proximity to the source and the total power is low. Theoretical analysis is presented to interpret these observations.