Multiuser OFDM Research Articles

Low-Complexity Energy-Efficient Power and Subcarrier Allocation in Cooperative Networks

This letter addresses the joint subcarrier and power allocation problem in downlink multiuser OFDM cooperative networks. The sum of the logarithmic per user time-averaged bits-per-Joule is maximized. The optimization problem is decomposed into two subproblems. First, the solution for the subcarrier allocation considering the proportional fairness is obtained. Second, an alternating power allocation (PA) scheme with separate transmit power constraints at the source and relays is derived under frequency selective fading channels. Then the complexity of the proposed scheme is analyzed. Finally, numerical results present the convergence, energy efficiency and outage probability performance of the proposed scheme.

OFDM 시스템의 비트 및 부채널 할당을 위한 선형계획법 기반 휴리스틱

The advantages of the orthogonal frequency division multiplexing (OFDM) are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion. To further utilize vast channel capacity of the multiuser OFDM, one has to find the efficient adaptive subchannel and bit allocation among users. In this paper, we compare the performance of the linear programming dual of the 0-1 integer programming formulation with the existing convex optimization approach for the optimal subchannel and bit allocation problem of the multiuser OFDM. Utilizing tight lower bound provided by the LP dual formulation, we develop a primal heurisitc algorithm based on the LP dual solution. The performance of the primal heuristic is compared with MAO, ESA heuristic solutions, and integer programming solution on MATLAB simulation on a system employing M-ary quadrature amplitude modulation (MQAM) assuming a frequency-selective channel consisting of three independent Rayleigh multi-paths.

A Subcarrier-Pair Based Resource Allocation Scheme Using Proportional Fairness for Cooperative OFDM-Based Cognitive Radio Networks

The paper presents a joint subcarrier-pair based resource allocation algorithm in order to improve the efficiency and fairness of cooperative multiuser orthogonal frequency division multiplexing (MU-OFDM) cognitive radio (CR) systems. A communication model where one source node communicates with one destination node assisted by one half-duplex decode-and-forward (DF) relay is considered in the paper. An interference-limited environment is considered, with the constraint of transmitted sum-power over all channels and aggregate average interference towards multiple primary users (PUs). The proposed resource allocation algorithm is capable of maximizing both the system transmission efficiency and fairness among secondary users (SUs). Besides, the proposed algorithm can also keep the interference introduced to the PU bands below a threshold. A proportional fairness constraint is used to assure that each SU can achieve a required data rate, with quality of service guarantees. Moreover, we extend the analysis to the scenario where each cooperative SU has no channel state information (CSI) about non-adjacent links. We analyzed the throughput and fairness tradeoff in CR system. A detailed analysis of the performance of the proposed algorithm is presented with the simulation results.

Adaptive Subcarrier, Bit and Power Allocation Based on Hopfield Neural Network for Multiuser OFDM

A kind of adaptive subcarrier, bit and power allocation method utilizing Hopfield neural network (HNN) to minimize the overall transmit power of multiuser OFDM is studied in this paper. In order to find the power optimal subcarrier, bit and power allocation under the constraints that one subcarrier can only be allocated to one user and all users are allocated the same numbers of subcarrier, the number of bits of each subcarrier is finite, bit data can be allocated to each subcarrier, two kinds of new energy constrained functions are constructed for the HNN. It is shown through numerical simulations that the proposed methods can find the optimal allocation with less complexity compared with the exhaustive method.

Optimal Wireless Communications With Imperfect Channel State Information

This paper studies optimal transmission over wireless channels with imperfect channel state information available at the transmitter side in the context of point-to-point channels, multiuser orthogonal frequency division multiplexing, and random access. Terminals adapt transmitted power and coding mode to channel estimates in order to maximize expected throughput subject to average power constraints. To reduce the likelihood of packet losses due to the mismatch between channel estimates and actual channel values, a backoff function is further introduced to enforce the selection of more conservative coding modes. Joint determination of optimal power allocations and backoff functions is a nonconvex stochastic optimization problem with infinitely many variables that despite its lack of convexity is part of a class of problems with null duality gap. Exploiting the resulting equivalence between primal and dual problems, we show that optimal power allocations and channel backoff functions are uniquely determined by optimal dual variables. This affords considerable simplification because the dual problem is convex and finite dimensional. We further exploit this reduction in computational complexity to develop iterative algorithms to find optimal operating points. These algorithms implement stochastic subgradient descent in the dual domain and operate without knowledge of the probability distribution of the fading channels. Numerical results corroborate theoretical findings.

Uplink Resource Management for Multiuser OFDM Video Transmission Systems: Analysis and Algorithm Design

We consider a multiuser OFDM system in which users want to transmit videos via a base station. The base station knows the channel state information (CSI) as well as the rate distortion (RD) information of the video streams and tries to allocate power and spectrum resources to the users according to both physical layer CSI and application layer RD information. We derive and analyze a condition for the optimal resource allocation solution in a continuous frequency response setting. The optimality condition for this cross layer optimization scenario is similar to the equal slope condition for conventional video multiplexing resource allocation. Based on our analysis, we design an iterative subcarrier assignment and power allocation algorithm for an uplink system, and provide numerical performance analysis with different numbers of users. Comparing to systems with either only physical layer or only application layer information available at the base station, our results show that the user capacity and the video PSNR performance can be increased significantly by using cross layer design. Bit-level simulations which take into account the imperfection of the video coding rate control, the variation of RD curve fitting, as well as channel errors, are presented.

Energy and spectral efficiency trade‐off for the downlink OFDM‐distributed antenna systems

SUMMARYThis paper investigates an energy efficient optimization scheme for the downlink multiuser OFDM‐distributed antenna systems. We adopt a multicriteria optimization method to offer a systematic study on the relationship between spectral efficiency (SE) and energy efficiency (EE). First, we transform the energy efficient optimization problem with high complexity into a simpler downlink multiuser OFDM problem. Then, using the weighted sum method in multicriteria optimization, an optimal energy efficient scheme is presented to allocate the available power to balance the trade‐off between SE and EE efficiently. Simulation results demonstrate that the energy efficient scheme is effective, and there existed a trade‐off between SE and EE in the downlink multiuser OFDM‐distributed antenna systems. Copyright © 2013 John Wiley & Sons, Ltd.

Capacity analysis of threshold-based SNR scheduler in LTE systems

This paper presents the capacity analysis of a threshold-based SNR scheduler in the long-term evolution (LTE) cellular systems. LTE standard has adopted multiuser OFDMA, and stipulates adjacent subcarrier groupings for mapping the physical OFDM subcarriers into resource blocks that form the basic unit of radio resource management (RRM) in LTE network. The standard however did not specify the details of the RRM algorithm to be employed, leaving this aspect for vendors to differentiate their products. Popular RRM algorithms such as round-robin (RR), proportional fairness (PF), and maximum SNR (MaxSNR), have been implemented recently as operator-selectable options on LTE base station (BS). In this paper, we present a threshold-based SNR scheduler that has the capability of modeling all of the above-mentioned algorithms and thus allows vendors to combine the separate implementations of these algorithms into one generalized scheduling algorithm, where the threshold level used at any time instant defines the scheduling discipline to be realized. We derive the capacity enhancement achievable using the proposed scheduling scheme, and also present system-level simulations to corroborate the analysis. Our analytical and simulation results indicate that the proposed algorithm models the existing ones closely at different values of the threshold. The results also demonstrate the data rate enhancements, and the level of user fairness, achievable in the network for various levels of the threshold.

Analysis and Comparison of Compensation Techniques of HPA Nonlinearity in Multi-User MIMO OFDM Systems using MMSE Receiver

The combination of Multiple Input Multiple Output (MIMO) with Orthogonal Frequency Division Multiplexing (OFDM) is a promising technique for high performance with very high data rates in 4G broadband wireless communications. However, the non-linearities caused by the High Power Amplifier (HPA) degrade the performance of MIMO-OFDM systems. In this paper, we present a theoretical analysis of nonlinear (NL) distortion effects in a MIMO-OFDM system in terms of error probability and system Capacity. Then, two compensators for reducing the distortion are proposed. The first is based on neural networks and the second is an iterative technique called Power Amplifier Nonlinearty Cancellation (PANC). These techniques correct the nonlinear distortion effects on the received signal, accompanied with MMSE detector. This receiver has shown best performance comparing to other receivers [1]. Simulation results show the comparison between the theoretical and simulated of nonlinear distortion effects in MIMO-OFDM transmission systems, as well as the performance of different compensators.

Novel Finite Summation Expressions for Total Interference Power due to Carrier Frequency Offset in Uplink Multiuser OFDM Systems

Carrier Frequency Offset which poses adverse threats in uplink Multiuser OFDM systems induces Inter-Carrier Interference (ICI) and Multiuser Interference (MUI). It is very essential that the power of ICI and MUI is calculated precisely. In previous work, the total interference power was presented in an indefinite integral form or as an infinite series. Further, an ideal channel was the scenario under consideration in earlier work. In this paper, novel finite summation expressions for power of ICI and MUI are derived over slowly fading channels which make the calculation of total interference simpler for any number of users by just specifying the total number of users and the number of subcarriers allotted for each user. Based on the mean of the dominant signal and the total interference power experienced in the system, closed-form expressions for mean and variance of the desired signal in uplink multiuser OFDM system are obtained which provide an idea of the strength of the desired signal. Finite-sum expressions for the total interference power, and closed-form expressions for mean and variance of the desired signal under Rayleigh, Weibull and Nakagami fading channels are derived and plotted from which the response of the system can be understood perfectly.

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.

A novel transform domain processing based channel estimation method for OFDM radio-over-fiber systems

In this paper, a transform domain processing (TDP) based channel estimation method for orthogonal frequency-division multiplexing (OFDM) Radio-over-Fiber (RoF) systems is proposed. Theoretically investigation shows that TDP can greatly reduce the number of required training symbols. An 8 x 4.65 Gb/s multi-user OFDM RoF system over 40 km fiber link and 60 GHz wireless link is experimentally demonstrated utilizing TDP scheme. Compared with conventional time domain averaging (TDA) scheme, the overhead can be reduced from several tens of training symbols to merely one symbol and the receiver sensitivity has been improved by 1.8 dB at BER of 3.8 x 10(-3). The calculated BER performance for 8 wireless users clearly validates the feasibility of this TDP-based channel estimation method.

Performance of Subcarrier and Power Allocation Orthogonal Frequency-Division Multiplexing on Millimeter Wave

Local multipoint distribution system (LMDS) that operated in millimeter waves can be used to fulfill the need of bit rate higher than 40 Mbps. However it has problem when applied in tropic country such as Indonesia because of the high rainfall. Therefore LMDS system was developed by cross-layer mechanism. In this research we used joint subcarrier and power allocation (JSPA) technique in multi-user cross-layer OFDM. This technique is proposed to increase the performance of telecommunication system even there were disturbance of rain attenuation. The research is discussing the performance of transmission capacity, data rate, utility and fairness of JSPA algorithm that applied in rain attenuation measurement in Surabaya. The result shows the increment of performance system using JSPA technique. For 40 dB rain attenuation, JSPA can achieves respectively average capacity of transmission up to 173,3%, 189,9 % for data rate and 9,6% for fairness system. The application of JSPA technique improve the performance of utility 13,61-15,48 bps/Hz

Chunk-Based Energy Efficient Resource Allocation in OFDMA Systems

Energy efficiency (EE) capacity analysis of the chunk-based resource allocation is presented by considering the minimum spectrum efficiency (SE) constraint in downlink multiuser orthogonal frequency division multiplexing (OFDM) systems. Considering the minimum SE requirement, an optimization problem to maximize EE with limited transmit power is formulated over frequency selective channels. Based on this model, a low-complexity energy efficient resource allocation is proposed. The effects of system parameters, such as the average channel gain-to-noise ratio (CNR) and the number of subcarriers per chunk, are evaluated. Numerical results demonstrate the effectiveness of the proposed scheme for balancing the EE and SE.

PAR-Aware Large-Scale Multi-User MIMO-OFDM Downlink

We investigate an orthogonal frequency-division multiplexing (OFDM)-based downlink transmission scheme for large-scale multi-user (MU) multiple-input multiple-output (MIMO) wireless systems. The use of OFDM causes a high peak-to-average (power) ratio (PAR), which necessitates expensive and power-inefficient radio-frequency (RF) components at the base station. In this paper, we present a novel downlink transmission scheme, which exploits the massive degrees-of-freedom available in large-scale MU-MIMO-OFDM systems to achieve low PAR. Specifically, we propose to jointly perform MU precoding, OFDM modulation, and PAR reduction by solving a convex optimization problem. We develop a corresponding fast iterative truncation algorithm (FITRA) and show numerical results to demonstrate tremendous PAR-reduction capabilities. The significantly reduced linearity requirements eventually enable the use of low-cost RF components for the large-scale MU-MIMO-OFDM downlink.

QoS-Aware Energy-Efficient Radio Resource Scheduling in Multi-User OFDMA Systems

In this letter, the energy-efficient radio resource scheduling is studied with quality of service (QoS) guarantees in a multi-user OFDMA system. In contrast to existing works on QoS-aware scheduling, which mostly take sum throughput or total transmission power as optimization objectives, we formulate an optimization problem to maximize the energy efficiency subject to users' QoS requirements. Based on Lagrangian duality then, an effective algorithm named QoS-aware energy-efficient resource scheduling is proposed to address the problem. Simulation results show that our proposed algorithm can effectively achieve the optimal solution and bring remarkable improvements on energy efficiency with QoS guarantees.

Low Complexity Greedy Power Allocation Algorithm for Proportional Resource Allocation in Multi-User OFDM Systems

Low Complexity Greedy Power Allocation Algorithm for Proportional Resource Allocation in Multi-User OFDM Systems

Adaptive Resource Allocation for Improved DF Aided Downlink Multi-User OFDM Systems

In this letter, we propose a joint resource allocation algorithm for an OFDM-based multi-user system assisted by an improved Decode-and-Forward (DF) relay. We aim at maximizing the sum rate of the system by jointly optimizing subcarrier pairing, subcarrier pair-user assignment, and power allocation in such a single DF relay system. When the relay does not perform any transmission on some subcarriers in the second phase, we further allow the source to transmit new symbols on these inactive subcarriers. We effectively solve the formulated mixed integer programming problem by using continuous relaxation and dual minimization methods. Numerical results verify the theoretical analysis, and illustrate the remarkable gains resulted from the extra direct-link transmissions.

Optimization of Energy Efficient Subcarrier and Power Allocation in Multi User OFDMA Systems

Optimization of Energy Efficient Subcarrier and Power Allocation in Multi User OFDMA Systems

Non-Cooperative Multi-Cell Resource Allocation and Modulation Adaptation for Maximizing Energy Efficiency in Uplink OFDMA Cellular Networks

We address the optimization problem of maximizing uplink energy efficiency, defined as sum of users bits-per-joule (bpj), in a multi-cell multi-user OFDMA network. Firstly, we propose a non-cooperative distributed scheme, in which mobile stations (MS) in different cells with given subcarrier assignment (SA) adjust the modulation and transmit power non-cooperatively, to optimize its own bpj. An advanced non-cooperative BS-controlled resource allocation scheme, in which the base station in each cell jointly performs SA, modulation adaptation, and power control for MSs in a non-cooperative manner, is proposed. Numerical results illustrate the tradeoff between energy and spectral efficiency, and show the proposed scheme makes great improvement on bpj with marginal loss of throughput as compared to other conventional scheme.