Dynamic resource allocation in adaptive wireless OFDMA systems

Modern wireless orthogonal frequency division multiple access (OFDMA) systems incorporate dynamic resource allocation (DRA), adaptive modulation and coding (AMC), and power control (PC) to exploit multiuser diversity and achieve higher system throughput. In the literature, only a few proposed algorithms deal with the contiguous DRA problem according to which a contiguous collection of resources can be allocated to each user. This paper formulates this high complexity problem, provides a suitable decision metric and a simple yet efficient solution. The proposed algorithm allocates in each step a contiguous collection of resources to the pending user that leads to the highest estimated correctly received number of bits. Simulation results show that, in this way, considerably improved performance can be achieved in terms of overall system throughput, spectral efficiency, and served traffic. Copyright © 2010 John Wiley & Sons, Ltd.     

Tone diversity for OFDMA in broadband wireless communications

This paper presents two group‐based tone diversity schemes, namely, group‐orthogonal tone‐combining‐diversity (GO‐TCD) and group‐optimal tone‐selection‐diversity (GO‐TSD), for orthogonal frequency division multiple access (OFDMA) system in broadband wireless communications. In both schemes, the entire frequency band of N sub‐carriers is divided into N_G = N/L_f sub‐carrier groups with each having L_f sub‐carriers. When the channel knowledge is not available at the transmitter, GO‐TCD, which has a split‐and‐group structure to reduce the peak‐to‐average ratio (PAR) and employs the multi‐user detection for maximum‐likelihood (ML) estimation to increase the number of active users, is proposed and shown to provide a lower PAR and similar performance as compared with the group‐orthogonal multi‐carrier CDMA, and outperforms the random‐hopping (RH)‐OFDMA and matched‐filter based MC‐CDMA. When the channel knowledge is available at the transmitter, GO‐TSD can be used to select the best sub‐carrier in a given sub‐carrier group for any active user and shown to offer a throughput comparable to the optimal tone selection but with much less complexity. Copyright © 2006 John Wiley & Sons, Ltd.     

QoS aware adaptive resource allocation techniques for fair scheduling in OFDMA based broadband wireless access systems

A system based on orthogonal frequency division multiple access (OFDMA) has been developed to deliver mobile broadband data service at data rates comparable to those of wired services, such as DSL and cable modems. We consider the resource allocation problem of assigning a set of subcarriers and determining the number of bits to be transmitted for each subcarrier in OFDMA systems. We compare simplicity, fairness and efficiency of our algorithm with the optimal and proposed suboptimal algorithms for varying values of delay spread, number of users and total power constraint. The results show that the performance of our approach is appealing and can be close to optimal. We also consider another resource allocation scheme in which there is no fixed QoS requirements per symbol but capacity is maximized.     

Multichannel random access in OFDMA wireless networks

Orthogonal frequency-division multiple access (OFDMA) systems are considered promising candidates for implementing next-generation wireless communication systems. They provide multiple channels that can be accessed via random access schemes. However, traditional random access schemes could result in an excessive amount of access delay. To address this issue, we develop a fast retrial scheme that is based on slotted Aloha and exploits the structure of OFDMA. A salient feature of this scheme is that when collisions occur instead of retrials occuring randomly in time, they occur randomly in frequency, i.e., the scheme randomly selects the subchannels for retrial. To further achieve fast access, retrials are designed to follow the 1-persistent type, i.e., no exponential backoff. To achieve the maximum throughput, we limit the maximum number of allowed retrials according to the load condition. We also consider the issue of designing for an appropriate reuse factor for random access channels in order to overcome the intercell interference problem in OFDMA multicell environments. Our finding is that full sharing, i.e., a reuse factor of one, performs best for given random access channels. Through analysis and simulation, we confirm that our fast retrial algorithm has the advantage of high throughput and low access delay, and the full sharing policy for random access channels shows high throughput as well as low collision.     

Efficient scheduling algorithms for mixed services in wireless OFDMA system

Orthogonal Frequency Division Multiple Access (OFDMA) is an attracting system for combating the frequency selective fading. It’s widely considered as a promising candidate modulation and access scheme for next generation mobile communication systems. However, the explosive growth of multimedia applications raises higher performance requirements for Radio Resource Management (RRM) in OFDMA system. In particular, limited bandwidth and fierce resource competition impose a challenging issue on the design of packet scheduler. In this paper, we propose two scheduling algorithms applicable to mixed services (i.e., real-time and non-real-time services), that is QoS-oriented Dynamic Threshold Control (DTC) algorithm and fairness-oriented Fairness Aware and QoS Aware (FAQA) algorithm. These two algorithms are both composed of two stages, i.e., initial subcarrier allocation and subcarrier reallocation. For the DTC algorithm, in the stage of initial subcarrier allocation, based on the different metrics to weigh QoS between both services, we design a unique scheduling strategy for each type of service. A dynamic threshold is adopted to help users quickly recover from starvation, so that any one user in system can escape from severely degraded QoS. In the stage of subcarrier reallocation, we will reallocate the surplus subcarriers from the buffer-empty users to the buffer-nonempty users so as to increase spectrum efficiency. For FAQA algorithm, in the stage of initial subcarrier allocation, for the purpose of achieving Proportional Fairness (PF) with lower complexity, we deduce a simple scheduling strategy satisfying PF criterion by means of Greedy algorithm. In the stage of subcarrier reallocation, in order to increase the number of users satisfying QoS requirements, we’ll reallocate the surplus subcarriers from the QoS-satisfied users to the QoS-unsatisfied users. Simulation results demonstrate that, on one hand, the DTC algorithm not only guarantees the quality of both services, but also increases the spectrum efficiency; on the other hand, the FAQA algorithm well maintains the fairness among users, and increases the QoS satisfaction degree at the same time.     

基于天线波束成形的OFDMA系统的自适应资源分配

对正交频分复用接入系统在多用户多天线传输情况下的自适应资源分配策略进行了研究,提出了一种基于天线波束成形的,包括动态子载波分配、自适应调制、比特加载的无线资源分配方案.算法的优化设计目标是在满足总的恒定传输比特数和误比特率性能要求的情况下,使得系统总的发射功率最小.仿真结果表明,由于多天线阵列增益和多用户分集增益,系统整体性能得到了明显的优化.     

OFDMA上行无线网络中资源块和功率的分配

正交频分复用多址接入(OFDMA)技术已经广泛应用于宽带无线网络,比如IEEE 802.16(WiMAX)和3GPP长期演进技术(LTE)。现有的提高系统吞吐量的主要方法是通过增加移动台的发射功率来提高移动台的传输速率。本文研究了在OFDMA无线网络上行传输中的资源块和功率联合分配问题。目标是满足基本传输要求前提下减少移动台的功率损耗。由于优化方程是NP-hard模型,所以本文利用注水技术的优势提出了一个启发式算法。仿真结果表明启发式算法性能接近最优解,特别是网络处于非饱和条件下。     

子载波交织分配的OFDMA系统

近年来，正交频分多址接入系统做为一种新的多址接入技术受到了极大关注。在OFDMA中，相临的多个子载波被分配给不同的用户来为用户并行低速传输数据。子载波是相互间重叠且正交的，所以避免了载波间干扰的产生。文中介绍了向上链接子载波交织分配的OFDMA系统。     

Edge sidelobe suppressor scheme for OFDMA uplink systems

We propose an edge sidelobe suppressor (ESS) for the uplink of orthogonal frequency-division multiple-access (OFDMA) systems, which mitigates the interference introduced by different frequency offsets between a desired user and the other users. The simulation results confirm that the proposed ESS scheme improves system performance by approximately 5-dB signal-to-interference ratio as compared to conventional OFDMA systems, and without increasing the level of transmitter and receiver complexity.     

Two-phase downlink subcarrier allocation for multicell OFDMA systems

To increase spectrum efficiency, the radio resource management with frequency reuse factor = 1 is usually adopted in multi-cell orthogonal frequency division multiple access systems. Such a reuse-one strategy results in severe co-channel interference, especially penalizing mobile users near the cell border. This paper presents an effective subcarrier-assignment algorithm to mitigate the co-channel interference for multi-cell orthogonal frequency division multiple access downlink systems. The proposed algorithm consists of an initial assignment phase and a compensation phase. In the first phase, each subcarrier is assigned to mobile users with higher channel gains and lower mutual co-channel interferences. The second phase further compensates mobile users who do not get adequate resources to meet the requirement of quality of service in the first phase. System simulation results reveal that the proposed algorithm can significantly improve the number of QoS-satisfied users compared with previous used scheme.

     

Fairness based resource allocation for uplink OFDMA systems

This article investigates two fairness criteria with regard to adaptive resource allocation for uplink orthogonal frequency division multiple access （OFDMA） systems. Nash bargaining solution （NBS） fairness and proportional fairness （PF） are two suitable candidates for fairness consideration, and both can provide attractive trade-offs between total throughput and each user＇s capacity. Utilizing Karush-Kuhn- Tucker （KKT） condition and iterative method, two effective algorithms are designed, to achieve NBS fairness and proportional fairness, respectively. Simulation results show that the proposed resource allocation algorithms achieve good tradeoff between the overall rate and fairness, with little performance loss from the total capacity.     

Precise interference analysis of OFDMA time-asynchronous wireless ad-hoc networks

This paper presents a unified mathematical performance analysis of the physical layer in orthogonal frequency division multiple access (OFDMA) wireless ad hoc networks, where several independent transmitter-receiver pairs share a common wideband channel in a local area environment. Multiuser interference (MUI) occurs when the signals from different users arrive at a given receiver with arbitrary timing misalignments, leading to the destruction of the orthogonality between subcarriers. Precise interference analysis in white Gaussian noise and Rayleigh multipath fading is developed in a partially loaded OFDMA network. New exact expressions of symbol and bit error rates are given in the case of interleaved subcarrier assignment schemes. On the other hand, tight upper bounds and accurate improved Gaussian approximations are developed for arbitrary subcarrier assignment schemes. Furthermore, expressions of the cutoff rates are derived and employed to estimate the spectral efficiency in bits/sec./Hz. These are used to quantify the improvement in the spectral efficiency that can be achieved by a common MUI mitigating technique based on the extension of guard intervals and dynamic positioning of FFT windows.     

Adaptive radio resource allocation for multiple traffic OFDMA broadband wireless access system

1 Introduction Next-generation wireless communication systems (systems beyond 3G) will be required to provide flexible and easy deployment solution to high-speed communications and to support a variety of services utilizing advanced multiple access techni…     

Opportunistic multichannel Aloha: distributed multiaccess control scheme for OFDMA wireless networks

The authors consider multiaccess control for the uplink in orthogonal-frequency-division-multiple-access wireless networks, where subcarriers are grouped into clusters. A multichannel random access, based on local channel state information (CSI), was investigated, and an opportunistic multichannel Aloha (OMC-Aloha) was proposed accordingly. A key step is to build a mapping from a user's CSI to its transmission probability and subcarrier allocation. For the sake of comparison, the throughput of the optimal centralized scheduling was also characterized by using the Extreme-Value Theory of order statistics. The authors show that the OMC-Aloha is asymptotically order optimal, in the sense that the only performance loss compared to the optimal centralized scheduling is due to the contention inherent in random access. In addition, they examine the proportional fairness in heterogeneous systems.     

Asymmetric radio resource allocation scheme for OFDMA wireless networks with collaborative relays

This work addresses the radio resource allocation problem for cooperative relay assisted OFDMA wireless network. The relays adopt the decode-and-forward protocol and can cooperatively assist the transmission from source to destination. Recent works on the subject have mainly considered symmetric source-to-relay and relay-to-destination resource allocations, which limits the achievable gains through relaying. In this paper we consider the problem of asymmetric radio resource allocation, where the objective is to maximize the system throughput of the source-to-destination link under various constraints. In particular, we consider optimization of the set of cooperative relays and link asymmetries together with subcarrier and power allocation. We derive theoretical expressions for the solutions and illustrate them through simulations. The results show clear additional performance gains through asymmetric cooperative scheme compared to the other recently proposed resource allocation schemes.     

An ESPRIT-based approach for initial ranging in OFDMA systems

In this letter, an initial ranging scheme for orthogonal frequency-division multiple-access systems is proposed by which users that intend to establish a communication link with the base station (BS) perform spreading in both the time and frequency domains and their synchronization parameters are estimated at the BS in closed-form using the ESPRIT algorithm. Compared to existing alternatives, the resulting scheme exhibits increased robustness against residual frequency offsets without involving computationally demanding peak search procedures.     

降低OFDMA系统峰均功率比的编码-PTS方法

OFDMA系统的峰均功率比(PAPR)的抑制技术具有很大的工程实用价值。传统的降低OFDMA系统中峰均功率比的编码方法存在只适用于子载波数较小系统的问题,限制了其实用性。为了解决这一问题,借鉴采用部分传输序列(PTS)降低中峰均功率比的方法,将输入的数据符号等分成若干组,分别对每一分组进行编码,然后再通过辅助信息来合并这些分组,由此提出了一种适用于OFDMA系统降低峰均功率比的改进型的方法编码-PTS方法。同时,分析了利用迭代方法寻找次优辅助信息的方法,利用该方法能够降低系统的复杂度。仿真结果表明编码-PTS方法能够改善OFDMA系统的峰均功率比性能。     

A probabilistic model for sub-carrier allocation in OFDMA systems

Orthogonal frequency division multiple-access technique showed a successful utilization of channel features. It implements an orthogonal sub-carrier space to be shared among different users. The management of these sub-carriers in both power and frequency allocation is reflected on the systems performance as better utilization of bandwidth, and hence, better capacity is obtained. Sub-carrier allocation is used to avoid deep fading that might occur at some user’s locations but not at other user’s locations. In this paper, we devise an algorithm based on probabilistic model for sub-carrier allocation to avoid deep fading in some user’s signals. By controlling the sub-carrier allocation for each user, we can create a full rank channel for each user and hence, provide maximum capacity for the system. Simulation results showed that using the devised algorithm will avoid deep fading and utilize the bandwidth up to 40% more than localized allocation strategies.     

An irregular frequency reuse scheme for cellular OFDMA systems

While conventional frequency reuse techniques need to allocate mutually exclusive frequency resources to adjacent cells, this paper proposes a technique for OFDMA systems to use pseudo‐random subchannel allocation patterns based on the preamble number. Although sophisticated cell planning is not required for the proposed technique, irregular frequency patterns can allow efficient interference avoidance and high data throughput. Copyright © 2010 John Wiley & Sons, Ltd.