Multiuser Orthogonal Frequency Division Multiplexing Systems Research Articles

Resource allocation based on subcarrier exchange in multiuser OFDM system

Multi-user orthogonal frequency division multiplexing (OFDM) is one of the most important techniques for next-generation wireless communication systems. The sum capacity and proportional fairness index are two important metrics in OFDM adaptive resource allocation. In existing researches, the two metrics have individually yielded approximately optimal solutions. However, as the two goals seem to contradict each other, it means that they cannot achieve optima simultaneously. This paper presents a resource allocation algorithm based on subcarrier exchange. By relaxing the fairness index, the proposed algorithm can dynamically adjust the fairness requirements. Furthermore, By considering a criterion that minimizes the loss of sum capacity, the proposed algorithm can balance the tradeoff between fairness and sum capacity. In contrast to existing algorithms, our proposal improves performance by approximately 30% in terms of sum capacity, at the expense of approximately 10% reduction in proportional fairness. Moreover, if a malicious user exists, each user in the system can obtain higher ergodic capacity compared with the conventional algorithms. To decrease the complexity, a low-complexity simplified alternative is presented, which outperforms the conventional algorithm with an increase in the number of users. Four numerical simulations are carried out to verify the efficiency and effectiveness. Finally, we discuss in detail the computational complexity of the proposed algorithm.

Joint subchannel and power allocation in multiuser OFDM systems with minimal rate constraints

SUMMARYIn this paper, we study the adaptive resource allocation in multiuser orthogonal frequency division multiplexing (OFDM) systems. We try to maximize the sum capacity of an OFDM system with given transmission power budget, while meeting users' minimal rate requirements. Unlike other resource allocation schemes, which generally separate subchannel allocation and power distribution into independent procedures, our proposed algorithm implements joint subchannel and power allocation. Given a set of subchannels, the required power to satisfy a user's minimal rate constraint is calculated by water‐filling policy. Then, the user who requires the maximum power to meet the rate requirement has a priority to obtain an additional subchannel. The procedure continues until all subchannels are consumed, by which time the consumed power to meet all users' rate requirements is also worked out. Finally, the margin power is allocated among all subchannels in an optimal manner to maximize the sum capacity of the OFDM system. Simulation results show that our proposed algorithm performs better than other existing ones. The solution produced by our proposed algorithm is close to the upper bound, while its complexity is relatively lower compared with other methods, which makes it attractive for applications. Copyright © 2012 John Wiley & Sons, Ltd.

Cross-layer scheduling with fairness for multi-user OFDM system: a restless bandit approach

In OFDM-based System such as long term evolution (LTE), the scheduling scheme plays an essential role in not only improving the capacity of system, but also guarantee the fairness among the user equipments (UEs). However, most existing work about scheduling only considers the current throughput in physical layer. Thus in this paper, a cross-layer scheduling with fairness based on restless bandit (CSFRB) scheme with the ‘indexability’ property is proposed for the multi-user orthogonal frequency-division multiplexing (OFDM) system to minimize the distortion in the application layer, to maximize the throughput and to minimize the energy consumption in the physical layer. The scheduling problem is firstly established as a restless bandit problem, which is solved by the primal-dual index heuristic algorithm based on the first order relaxation with low complexity, to yield the CSFRB scheme. Additionally, this scheme is divided into offline computation and online selection, where main work will be finished in former one so as to decrease the complexity further. Finally, extensive simulation results illustrate the significant performance improvement of the proposed CSFRB scheme compared to the existing one in different scenarios.

Performance Evaluation of a Joint Precoding and Channel Prediction scheme for Multiuser MIMO OFDM Systems in Time Varying Wireless Channel

Multiple-input, multiple-output (MIMO) communications have attracted considerable attention in the last years. In particular, multiuser MIMO (MU MIMO) systems are able to offer higher link capacity of MIMO systems as long as channel state information (CSI) is available at the transmitter, so that the spatial properties of the channel can be exploited by precoding. However, the time varying nature of the channel may cause CSI to be outdated, thus degrading the overall performance of the system. Within this context, channel prediction can provide up-to-date channel CSI and reduce the performance loss. This paper presents a channel predictor based on the setmembership partial update affine projection (SM-PUAP) filtering as way to compensate the outdated CSI caused by the temporal variations of the communication channel in a multiuser MIMO OFDM (orthogonal frequency division multiplexing) system. A realistic standardized channel model is considered to evaluate the predictor and its influence on the broadcast channel of a multiuser MIMO scenario. The performance of the predictor is also compared to the well-known adaptive algorithm recursive least squares (RLS). Simulation results show that the SM-PUAP predictor has lower computational cost and superior performance when compared to the classical algorithm.

Adaptive resource allocation for multi-user multi-server power-line communication OFDM systems

The bits and power allocation model of adaptive power-rate mixture for multi-user multi-server power-line communication systems was analyzed with the restrictions of maximal total power, fixed rate for each real time (RT) user, minimal rate for each non-real time (NRT) user, maximal bits and power for each subcarrier in each orthogonal frequency division multiplexing (OFDM) symbol. An algorithm of resource dynamic allocation in the first OFDM symbol of each frame and resource optimal adjustment in the latter OFDM symbol of each frame was proposed. In the first OFDM symbol of every frame, resource is firstly assigned for RT users so as to minimize their total used power until satisfying their fixed rates; secondly the remainder resource of power and subcarriers are assigned for NRT users so as to minimize their total used power until satisfying their minimal rates also; lastly the remainder resource is again assigned for NRT users according to the proportional fairness strategy so as to maximize their total assigning rate. In the latter OFDM symbol of each frame, bits are swapped and power is adjusted for every user based on the resource allocation results of anterior OFDM symbol. The algorithm is tested in the typical power-line channel scenarios and the simulation results indicate that the proposed algorithm has better performances than the classical multi-user resource allocation algorithms and it realizes the multiple aims of multi-user multi-server resource allocation for power-line communication systems.

Traffic‐aware downlink power allocation in multiuser OFDM system

AbstractMultiuser orthogonal frequency division multiplexing (OFDM) is a promising technology to achieve high uplink/downlink (DL) capacities in the next generation broadband wireless networks such as WiMAX (Worldwide Interoperability for Microwave Access). In this paper, we investigate the DL adaptive power allocation (APA) in multiuser OFDM system from the perspective of cross‐layer design. Specifically, we formulate APA as an optimization problem with the traffic profile of each user asit a priori knowledge. To solve the optimization problem, we develop a fairness‐constrained optimal prioritized effective throughput (PET) strategy and the corresponding iterative algorithms, aiming at balancing the prioritized effective throughput and the linear or logarithmic user satisfaction‐based fairness. Simulation results show that our proposed APA optimization approach can achieve satisfying performance. Copyright © 2011 John Wiley & Sons, Ltd.

Low complexity cross-layer design with packet dependent scheduling for heterogeneous traffic in multiuser OFDM systems

We propose an adaptive cross-layer design for the downlink multiuser orthogonal frequency division multiplexing (OFDM) systems, to maximize the weighted sum capacity of all users, where each user has multiple heterogeneous traffic queues simultaneously. A packet dependent (PD) scheduling scheme is employed at the medium access control (MAC) layer, which determines the packet transmission order by assigning different weights to different packets, and is shown by simulations more efficient than the previous methods where all packets in a queue have the same weight. The weight design in PD scheduling considers the delay, size and quality of service (QoS) priority level of packets. Each user weight employed in resource allocation at the physical (PHY) layer is obtained by summing up the weights of selected packets for the user. By properly choosing the number of packets used for weight calculation, the proposed cross-layer design requires a much lower overall complexity than the conventional queue based designs. It is also proven to achieve the maximum system stability region. Furthermore, simulation results show that the proposed suboptimal resource allocation algorithm provides a near-optimal performance while requiring a relatively low complexity.

Channel Estimation in Wireless OFDM Systems With Irregular Pilot Distribution

This paper addresses pilot-assisted channel estimation for wireless orthogonal frequency division multiplexing (OFDM) systems with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">irregular</i> pilot arrangements. Using nonuniform sampling techniques, we propose a novel channel estimator that is based on conjugate gradient iterations and features very low computational complexity. We investigate the impact of the pilot arrangement on the channel estimation mean square error and we provide a heuristic stopping criterion for early termination of the conjugate gradient iterations. One-dimensional and two-dimensional channel estimator implementations and applications of our method to multiple-antenna and multiuser OFDM systems are discussed. Extensive numerical simulations corroborate that the proposed method performs similarly to computationally much more expensive minimum mean square error estimators.

다중 사용자 OFDM 시스템에서 간략화된 채널 정보를 이용한 부반송파 할당 알고리즘

고속의 멀티미디어 서비스를 요구하는 추세에 따라 대역폭 효율이 높은 다중 사용자 OFDM(Orthogonal Frequency Division Multiplexing) 시스템에서 자원 할당 방법에 관한 많은 연구가 이루어지고 있다. 본 논문에서는 다중 사용자 OFDM 시스템에서 제한된 자원을 이용하는 자원 할당 방식을 제안한다. 제안된 방식은 기존 알고리즘에 비하여 간략화된 채널 상태 정보(CSI: Channel State Information)를 사용하면서 전체 데이터 전송률을 최대화하는 부반송파 할당 알고리즘이다. 간략화된 CSI 구조를 사용하기 때문에 각 사용자의 정확한 정보를 알 수 없지만, 사용자의 최소 요구 데이터 전송률, 평균 채널 이득 정보를 이용하여 QoS를 만족시킬 수 있는 사용자의 후보를 정하여 부반송파를 할당한다. 결과적으로 기존의 방식에 비해 적은 채널 정보를 사용하여 더 많은 사용자에게 QoS를 만족시킬 수 있다. The Orthogonal Frequency Division Multiplexing(OFDM) has been strongly recommended as a transmission technique in order to satisfy requests of high speed and high quality multimedia information. This paper considers resource allocation algorithm which supports the user Quality of Service(QoS) for multi-user OFDM system with simple CSI (Channel State Information) structure. After users eligible for services and the number of subcarrier are determined by minimum request data rate and average channel gain, subcarriers are allocated to increase total transmission rates. Although ideal CSI is not reported to base station, compared to conventional algorithm, we have obtained better result with simple CSI structure.

Estimation algorithms of multiple channels and carrier frequency offsets in application to multiuser OFDM systems

This paper introduces new joint estimation methods of multiple carrier frequency offsets (CFOs) and channel impulse responses (CIR) in application to multiuser orthogonal frequency division multiplexing (OFDM) systems. The estimators are derived from the optimal maximum-likelihood (ML) principle. Complexity reductions are achieved by exploiting the correlation properties of the training sequence (TS). The grid search algorithm is converted into a polynomial root finding procedure which leads to low-complexity closed-form estimators for moderate CFOs. Furthermore, iterative estimators for larger CFOs are proposed. Numerical results confirm that the performance degradation due to the approximations compared to the Cramer-Rao bound (CRB) is small and may be negligible in practice.

Linear prediction based semiblind channel estimation for multiuser OFDM with insufficient guard interval

To meet the demand of high data rate transmissions for multimedia wireless communications, orthogonal frequency division multiplexing (OFDM) systems in conjunction with multiple-input multiple-output (MIMO) signal processing have been considered one of the central techniques in advanced wireless communications. In the paper, two semiblind channel estimation algorithms are proposed for the uplink multiuser OFDM systems with insufficient guard interval, in contrast to sufficient guard interval assumed in most of the prior works. A zero-padding OFDM system, which zero-pads rather than cyclicly prefixing each block, is considered in this paper. By utilizing the relation between the linear prediction error filters (LPEFs) of the received signal with multiple prediction orders and the transmitted data sequence, the first proposed algorithm, namely the multistage LP (MLP) based algorithm, can estimate the MIMO channel coefficients, with only a single pilot OFDM block used. To reduce the sensitivity of the proposed algorithms to the channel order overestimation, it is proposed to implement the LPEFs with a QR-decomposition based approach. This QRdecomposition based approach alternatively computes the LPEFs without direct inversion of the received signal correlation matrix, thus exhibiting robustness against channel order overestimation. Some simulation results are presented to demonstrate the effectiveness and robustness of the proposed algorithms.

Cross-layer packet scheduling for downlink multiuser OFDM systems

This paper proposes a channel and queue aware fair (CQAF) packet scheduling scheme for the downlink packet transmission in multiuser orthogonal frequency division multiplexing (OFDM) systems. By making use of the information on the channel conditions and the queue lengths, the proposed CQAF packet scheduling scheme efficiently allocates the subcarriers, transmission power and modulation level to users under the constraints of total transmission power, the number of subcarriers, bit-error-rate (BER) requirement and generalized processor sharing (GPS)-based fairness requirement. The numerical results show that the proposed CQAF packet scheduling scheme can reduce the transmission delay and queue length significantly while maximizing system throughput and maintaining fairness among users.

Cross-Layer Resource Allocation for Mixed Services in Multiuser OFDM-Based Cognitive Radio Systems

We study the resource-allocation problem in a multiuser orthogonal frequency-division multiplexing (OFDM)-based cognitive radio (CR) system using a cross-layer approach. The goal is to provide satisfactory quality of service (QoS) to both real-time and non-real-time applications, despite the rapid variations in available resources caused by the activities of the primary users. The dynamic nature of the available spectrum gives rise to two resource allocation issues: 1) problem feasibility and 2) false urgency. To solve the problem-feasibility issue, which arises when resources are insufficient to meet all user QoS requirements, we adopt a goal-programming approach. The false-urgency issue that was caused by variations in available system resources is effectively avoided by a proposed rate-requirement calculation mechanism based on the status of the packets in queue and system resource availability. Simulation results show that the proposed cross-layer resource-allocation algorithm for CR systems performs better than existing algorithms that were designed for multiuser OFDM systems.

Resource Allocation for Multiuser Cooperative OFDM Networks: Who Helps Whom and How to Cooperate

Cooperative transmissions have been shown to be able to greatly improve system performance by exploring the broadcasting nature of wireless channels and cooperation among users. We focus, in this paper, on leveraging cooperation for resource allocation among users such that the network performance can be improved. Two important questions are answered-who should help whom among the distributively located users, and how many resources the users should use for cooperation to improve the performance. To answer these questions, a power-optimization, subcarrier-allocation, and relay-selection problem is formulated over a multiuser orthogonal frequency-division multiplexing (OFDM) network, which is applicable to systems such as wireless local area networks (WLANs). In the multiuser OFDM network, cooperation among different users is conducted by assigning the subcarriers of the helping users to relay a certain part of the helped users' data, while maintaining the desired rates of both helping users and helped users by means of power control and rate adaption. This way, the bandwidth efficiency of the multiuser OFDM system with cooperation is the same as that of the noncooperative OFDM system. The formulated optimization problem is an assignment problem for subcarrier usage and corresponding bit loading as well as power control. We provide an approximate closed-form solution for a two-user two-subcarrier case. Then, a suboptimal heuristic algorithm for a multiple-user multiple-subcarrier case is proposed and implemented in the base station to solve the formulated NP-hard problem. From the simulation results, the proposed scheme achieves up to 50% overall power saving for the two-user system and 19%-54% overall power saving for the multiuser case with random locations, compared with the current multiuser OFDM system without cooperative diversity. The proposed scheme is also compared to a much more complicated orthogonal frequency-division multiple access (OFDMA) system.

Feedback threshold with guaranteed QoS in multiuser OFDM systems

A threshold setting scheme is proposed based on the resource management and limited feedback theory in multiuser orthogonal frequency division multiplexing (OFDM) systems. In adaptive resource allocation, the factors denoting the quality of service (QoS) and fairness are both considered as the user weight. From the aspect of feedback outage probability, the proposed algorithm sets the threshold for each user related to its weight, which brings enough feedback to the user with greater weight. Analysis and simulation results show that, compared with the threshold ignoring weights, the proposed scheme has much lower feedback load while with better QoS.

An ISI Suppressing Method for SIMO-OFDM System Utilizing the Characteristic of the ETP-OFDM Model

This paper proposes a Inter-symbol Interference (ISI) suppressing method For Orthogonal Frequency Division Multiplexing (OFDM) system. In the environments where the delay spread exceeds the guard interval, the occurrence of ISI results in a degradation of system performance. A receiving method based on the Equivalent Transmission Path (ETP) model is proposed for Single-Input Multiple-Output (SIMO) OFDM system. Compared to the receiving scheme using Maximal Ratio Combining (MRC) method, the proposed one shows better performance in suppressing errors due to ISI. The paper also points out that the ETP-based model can be used for detecting the desired signal in the multi-user OFDM system. Simulation results of the system performance show the effectiveness of the proposed method over the conventional OFDM system in suppressing ISI.

A Low-Complexity QoS-Aware Proportional Fair Multicarrier Scheduling Algorithm for OFDM Systems

Orthogonal frequency-division multiplexing (OFDM) systems are the major cellular platforms for supporting ubiquitous high-speed mobile applications. However, a number of research challenges remain to be tackled. One of the most important challenges is the design of a judicious packet scheduler that will make efficient use of the spectrum bandwidth. Due to the multicarrier nature of the OFDM systems, the applicability and performance of traditional wireless packet-scheduling algorithms, which are usually designed for single-carrier systems, are largely unknown. In th.is paper, we propose a new quality-of-service (QoS)-aware proportional fairness (QPF) packet-scheduling policy with low complexity for the downlink of multiuser OFDM systems to allocate radio resources among users. Our proposed algorithm is based on a cross-layer design in that the scheduler is aware of both the channel (i.e., physical layer) and the queue state (i.e., data link layer) information to achieve proportional fairness while maximizing each user's packet-level QoS performance. The simulation results show that the proposed QPF algorithm is efficient in terms of average system throughput, packet-dropping probability, and packet delay, while maintaining adequate fairness among users with relatively low scheduling overhead.

Improved Subcarrier Allocation in Multi-User OFDM Systems

In a multi-user orthogonal frequency division multiplexing (OFDM) system, efficient resource allocation is required to provide service to more users. In this letter, we propose an improved subcarrier allocation algorithm that can increase the spectral efficiency and the number of total transmission bits even if the number of users is too large. The proposed algorithm is divided into two stages. In the first stage, a group of users who are eligible for services is determined by using the bit error rate (BER), the users' minimum data rate requirement, and channel information. In the second stage, subcarriers are first allocated to the users on the basis of channel state, and then the reallocation is performed so that resource waste is minimized. We show that the proposed algorithm outperforms the conventional one on the basis of outage probability, spectral efficiency, and the number of total transmission bits through a computer simulation.

A low-complexity bandwidth allocation algorithm for frequency-selective multiuser OFDM systems

This paper addresses the problem of efficient bandwidth allocation for multiuser orthogonal frequency division multiplexing (OFDM) systems over frequency-selective channels. For a given coding and medium access scheme and for specific channel gains, the achievable data rates depend on the assignment of the available subchannels to the different users. However, in frequency-selective channels, the various users experience different channel conditions, thus the application of effective bandwidth allocation rules results in enhanced multiuser diversity. This paper presents a computationally efficient solution for dynamic subchannel assignment. The proposed algorithm provides enhanced system performance compared with the fixed time division multiple access (TDMA) solution and also results in proportional data rates according to channel quality of each user. As a case study, we demonstrate the performance of the proposed algorithm in the indoor power line network with multiple OFDM links between a central station and a number of user devices.

Adaptive Subcarrier Assignment and Bit Allocation for Multiuser OFDM System Using Ordinal Optimization Approach

In this paper, we propose an ordinal optimization (OO) theory-based four-stage approach to deal with the subcarrier assignment and bit allocation problem of a multiuser orthogonal frequency-division multiplexing (OFDM) system. The four-stage OO approach ensures the quality of the obtained solution, however, at the cost of solving a continuous version of the considered problem in the first stage. To resolve this computational complexity problem, we propose a hardware-implementable dual projected gradient (DPG) method to exploit deep-submicrometer technology. Compared to some existing methods using numerous simulation cases with randomly generated parameters, our approach is excellent in the aspect of power consumption. In the meantime, the estimated computation time of our approach can meet the real-time application requirement. Furthermore, we use extensive simulations to compare our good-enough solutions with the true optimal solutions, and the results show that the objective value obtained by our approach deviates from the optimal objective value around 1% on average.