Multiuser Orthogonal Frequency-division Multiple-access Systems Research Articles

Relay Selection and Resource Allocation for SWIPT in Multi-User OFDMA Systems

We investigate the resource allocation and relay selection in a two-hop relay-assisted multi-user orthogonal frequency division multiple access (OFDMA) network, where the end-nodes support the simultaneous wireless information and power transfer (SWIPT) employing a power splitting (PS) technique. Our goal is to optimize the end-nodes’ PS ratios as well as the relay, carrier, and power assignment so that the sum-rate of the system is maximized subject to harvested energy and transmitted power constraints. Such joint optimization with mixed-integer non-linear programming structure is combinatorial in nature. Due to the complexity of this problem, we propose to solve its dual problem, which guarantees asymptotic optimality and less execution time compared to a highly-complex exhaustive search approach. Furthermore, we also present a heuristic method to solve this problem with lower computational complexity. The simulation results reveal that the proposed algorithms provide significant performance gains compared to a semi-random resource allocation and relay selection approach and is close to the optimal solution when the number of OFDMA sub-carriers is sufficiently large.

Energy-efficient resource allocation for multiuser OFDMA system based on hybrid genetic simulated annealing

Orthogonal frequency division multiple access (OFDMA) is adopted in 4G wireless communication standard, where bandwidth resources can be split into smaller granular units. Tailored for slow adaptive OFDMA system, we study the energy-efficient resource allocation problem based on chance-constrained programming. The optimization objective is to minimize the power consumption over subcarrier and power allocation. The constraints include the outage probability constraint and target bit error rate (BER) constraint. Because the optimization problem contains the probabilistic constraint, it is a chance-constrained programming problem. Hence, support vector machine (SVM) is adopted to compute the outage probability constraint. Then, we integrate SVM and genetic simulated annealing (GSA) to develop hybrid genetic simulated annealing (HGSA). Simulation tests verify that HGSA not only has the lower consumption power, but also satisfies the outage probability constraint.

An Improved Frequency Offset Cancellation for Uplink Multi-User OFDMA System Based on TF-RLS Algorithm

Time and frequency recursive least squares algorithm (TF-RLS) is proposed to cancel the interference due to the frequency offset (FO) in uplink Orthogonal Frequency Division Multiple Access (OFDMA) system. TF-RLS algorithm is composed of two stages, which is TD-RLS scheme and FD-RLS scheme. In the first stage, TD-RLS scheme is selected to pre-cancel the frequency offset in the time domain, and then the interference induced by residual frequency offset has been eliminated by the FD-RLS scheme in the frequency domain. The result of bit error rate (BER) shows that its performance is robust for cancellation as comparison criteria, even though the frequency offset is 0.45. The 16QAM constellation figures are also simulated to observe the improvements from the proposed suppression schemes.

On the Complexity of Joint Subcarrier and Power Allocation for Multi-User OFDMA Systems

Consider a multi-user Orthogonal Frequency Division Multiple Access (OFDMA) system where multiple users share multiple discrete subcarriers, but at most one user is allowed to transmit power on each subcarrier. To adapt fast traffic and channel fluctuations and improve the spectrum efficiency, the system should have the ability to dynamically allocate subcarriers and power resources to users. Assuming perfect channel knowledge, two formulations for the joint subcarrier and power allocation problem are considered in this paper: the first is to minimize the total transmission power subject to quality of service constraints and the OFDMA constraint, and the second is to maximize some system utility function (including the sum-rate utility, the proportional fairness utility, the harmonic mean utility, and the min-rate utility) subject to the total transmission power constraint per user and the OFDMA constraint. In spite of the existence of various heuristics approaches, little is known about the computational complexity status of the above problem. This paper aims to fill this theoretical gap, i.e., characterizing the complexity of the joint subcarrier and power allocation problem for the multi-user OFDMA system. It is shown in this paper that both formulations of the joint subcarrier and power allocation problem are strongly NP-hard. The proof is based on a polynomial time transformation from the so-called 3-dimensional matching problem. Several subclasses of the problem which can be solved to global optimality or $\epsilon$-global optimality in polynomial time are also identified. These complexity results suggest that there are not polynomial time algorithms which are able to solve the general joint subcarrier and power allocation problem to global optimality (unless P$=$NP), and determining an approximately optimal subcarrier and power allocation strategy is more realistic in practice.

Impact of CSI on Radio Resource Management Techniques for the OFDMA downlink

Adaptive resource allocation can drastically increase the throughput of an Orthogonal Frequency Division Multiple Access (OFDMA) system when the Channel State Information (CSI) is accurately known. Unfortunately, in practice, perfect CSI is rarely possible. In this paper, we consider adaptive sub-carrier assignment for downlink multiuser OFDMA systems, where the transmitter has no knowledge of the instantaneous channel realizations. The problem we address is maximizing the sum-capacity of the system subject to user Quality of Service (QoS) requirements. A heuristic algorithm presented in \cite{Zhang06} is modified in order to provide an enhanced sub-optimal solution. Numerical results show that resources can be adaptively allocated using statistical CSI (SCSI) and that such an approach allows for an important number of user QoS requirements to be met. Comparisons between the instantaneous CSI (ICSI) and SCSI based resource allocation schemes demonstrate that their performance difference is highly dependent on the number of active users present in the cell, the QoS constraint, and the transmit power.

Fair multiuser channel allocation for OFDMA networks using Nash bargaining solutions and coalitions

In this paper, a fair scheme to allocate subcarrier, rate, and power for multiuser orthogonal frequency-division multiple-access systems is proposed. The problem is to maximize the overall system rate, under each user's maximal power and minimal rate constraints, while considering the fairness among users. The approach considers a new fairness criterion, which is a generalized proportional fairness based on Nash bargaining solutions and coalitions. First, a two-user algorithm is developed to bargain subcarrier usage between two users. Then a multiuser bargaining algorithm is developed based on optimal coalition pairs among users. The simulation results show that the proposed algorithms not only provide fair resource allocation among users, but also have a comparable overall system rate with the scheme maximizing the total rate without considering fairness. They also have much higher rates than that of the scheme with max-min fairness. Moreover, the proposed iterative fast implementation has the complexity for each iteration of only O(K/sup 2/Nlog/sub 2/N+K/sup 4/), where N is the number of subcarriers and K is the number of users.