Toward the Energy Efficiency of Resource Allocation Algorithms for OFDMA Downlink MIMO Systems

The problem of the simultaneous multi-user resource allocation algorithm in orthogonal frequency division multiple access (OFDMA) based systems has recently attracted significant interest. However, most studies focus on maximizing the system throughput and spectral efficiency. As the green radio is essential in 5G and future networks, the energy efficiency becomes the major concern. In this paper, we develop four resource allocation schemes in the downlink OFDMA network and the main focus is on analyzing the energy efficiency of these schemes. Specifically, we employ the advanced multi-antenna technology in a multiple input-multiple output (MIMO) system. The first scheme is based on transmit spatial diversity (TSD), in which the vector channel with the highest gain between the base station (BTS) and specific antenna at the remote terminal (RT) is chosen for transmission. The second scheme further employs spatial multiplexing on the MIMO system to enhance the throughput. The space-division multiple-access (SDMA) scheme assigns single subcarrier simultaneously to RTs with pairwise “nearly orthogonal” spatial signatures. In the fourth scheme, we propose to design the transmit beamformers based on the zero-forcing (ZF) criterion such that the multi-user interference (MUI) is completely removed. We analyze the tradeoff between the throughput and power consumption and compare the performance of these schemes in terms of the energy efficiency.     

A simple iterative carrier frequency synchronization technique for OFDMA uplink transmissions

This paper examines the carrier frequency offset (CFO) estimation problem in the tile‐based orthogonal frequency division multiple access (OFDMA) uplink systems, which is very challenging due to the presence of multiple CFOs. The existing solutions to this problem are either too complex to implement or not flexible in subcarrier allocation. To solve these problems, this paper proposes a tile‐structure based iterative multi‐CFO estimation technique. The proposed method is developed based on a special training sequence with repetitive structure. The inherent multi‐user interference (MUI) compression provided by the tile structure allows us to utilize the repetitive property of the training sequence to jointly estimate the CFOs in the frequency domain with low complexity. Combining the CFO estimation with an interference cancellation scheme and performing iteratively, the algorithm achieves high estimation accuracy and fast convergence. The proposed algorithm is suitable for any subcarrier assignment schemes. In addition, as compared with other existing time domain based algorithms, which achieve the Cramer Rao Bound (CRB) at the price of unaffordable complexity, it closely approaches their performance with over 70% computational saving, which is significantly important for practical implementation. Copyright © 2010 John Wiley & Sons, Ltd.     

Low-Complexity Orthogonal Spectral Signal Construction for Generalized OFDMA Uplink With Frequency Synchronization Errors

In orthogonal frequency-division multiplexing, the total spectral resource is partitioned into multiple orthogonal subcarriers. These subcarriers are assigned to different users for simultaneous transmission in orthogonal frequency-division multiple access (OFDMA). In an unsynchronized OFDMA uplink, each user has a different carrier frequency offset (CFO) relative to the common uplink receiver, which results in the loss of orthogonality among subcarriers and thereby multiple access interference. Hence, OFDMA is very sensitive to frequency synchronization errors. In this paper, we construct the received signals in frequency domain that would have been received if all users were frequency synchronized. A generalized OFDMA framework for arbitrary subcarrier assignments is proposed. The interference in the generalized OFDMA uplink due to frequency synchronization errors is characterized in a multiuser signal model. Least squares and minimum mean square error criteria are proposed to construct the orthogonal spectral signals from one OFDMA block contaminated with interference that was caused by the CFOs of multiple users. For OFDMA with a large number of subcarriers, a low-complexity implementation of the proposed algorithms is developed based on a banded matrix approximation. Numerical results illustrate that the proposed algorithms improve the system performance significantly and are computationally affordable using the banded system implementation     

Joint Estimation of the Timing and Frequency Offset for Uplink OFDMA

Inheriting all the advantages of Orthogonal Frequency Division Multiplexing (OFDM), plus the ability to offer a fine level of bit granularity and dynamic subcarrier allocation for multiuser diversity, the Orthogonal Frequency Division Multiple Access (OFDMA) has emerged as a potential candidate for multiple access technique for future broadband wireless networks. However, the benefits of OFDMA come with stringent requirements on synchronization, especially in the uplink. Unless the timing offsets (TOs) and carrier frequency offsets (CFOs) among users in the uplink are kept under tolerable ranges, inter-symbol interference (ISI), inter-channel interference (ICI) and multi-user interference (MUI) will occur, which degrade the overall system performance severely. Accurate estimation of TOs and CFOs is required for each user, so that they can be accounted for at the user’s side or compesated for at the base station. This paper proposes a novel method to estimate jointly TOs and CFOs in the time-domain for multi-user in the OFDMA uplink. The method is shown to offer good accuracy, while maintaining a reasonable complexity compared to conventional estimation schemes.     

SIR analysis and interference cancellation in uplink OFDMA with large carrier frequency/timing offsets

In uplink orthogonal frequency division multiple access (OFDMA), large timing offsets (TO) and/or carrier frequency offsets (CFO) of other users with respect to a desired user can cause significant multiuser interference (MUI). In this letter, we analytically characterize the degradation in the average output signal-to-interference ratio (SIR) due to the combined effect of both TOs as well as CFOs in uplink OFDMA. Specifically, we derive closed-form expressions for the average SIR at the DFT output in the presence of large CFOs and TOs. The analytical expressions derived for the signal and various interference terms at the DFT output are used to devise an interference cancelling receiver to mitigate the effect of CFO/TOinduced interferences.     

多小区5G认知无线电网络能量有效资源优化

为提高5G通信系统中能量利用效率,本文提出一种资源配置算法来解决多小区5G认知无线电网络中资源配置问题。针对需要优化的载波分配变量和功率变量,该算法采用交替优化的方式分别对上述变量进行优化。对于载波分配,所提算法依据最大化信噪比原则来分配载波;对于功率分配,本文将其转变为另外一个等效问题,然后利用连续凸近似方法求解。由于传统正交频分复用调制（OFDM）具有严重的频谱泄露,其他几种具有较低频谱泄露特性的5G候选调制方式,例如滤波器组多载波调制（FBMC）、通用滤波多载波调制（UFMC）、广义频分复用调制（GFDM）等,也被分析比较。仿真结果表明本文所提算法相比干扰受限算法具有更高的能量效率,并且证明具有较低频谱泄露的调制方式能取得更高的能量效率。      

OFDMA系统基于QoS保证和最大最小公平性准则下的动态资源分配

正交频分多址(OFDMA)技术以其更高的频谱效率和抗多径衰落特性成为高速无线通信网络的候选标准。兼顾效率和公平性是OFDMA系统资源分配亟待解决的问题。本文研究了OFDMA系统中的无线资源分配问题,既要保证QoS用户的最小速率要求,同时“尽力而为”用户之间必须满足最小速率最大化公平性(max-min fairness)准则;该资源分配问题可以表述为一个系统总功率约束下的子载波分配和功率控制的混合离散型优化模型,这是难解的NP-hard问题,穷举搜索的代价是极其巨大的。针对该非凸模型,本文设计一个拉格朗日松弛的优化算法,该算法中采用修正的椭球算法求解对偶问题。算法具有多项式时间复杂度,且与子载波数目呈线性增长关系。仿真结果表明,该算法能近似最优地满足用户QoS及最大最小公平性要求。      

Carrier Frequency Offset estimation for interleaved OFDMA uplink based on subspace processing

This paper investigates Carrier Frequency Offset （CFO）, estimation in the uplink of the Orthogonal Frequency-Division Multiple Access （OFDMA） systems with the interleaved subcarrier assignment. CFOs between the transmitters and the uplink receiver will destroy orthogonality among different subcarriers, hence resulting in inter-carrier interference and multiuser interference. A two-stage frequency offset estimation algorithm based on subspace processing is proposed. The main advantage of the proposed method is that it can obtain the CFOs of all users simultaneously using only one OFDMA block. Compared with the previously known methods, it not only has a relatively low implementation complexity but is also suitable for random subchannel assignment.     

Low‐complexity inter‐carrier interference cancelation scheme for SC‐IFDMA system

After the standardization of SCFDMA as the uplink transmission scheme for LTE, frequency synchronization and resulting interference cancelation received considerable attention. In this paper, mathematical modeling of uplink SCFDMA system with interleaved subcarrier assignment scheme (SC‐IFDMA) is carried out in the presence of carrier frequency offset, and the results were utilized in framing the concept of effective interference matrix, which efficiently represents the interference cancelation problem. We propose two schemes to mitigate the effects of interference and channel based on linear filtering approach, and the typical structure of effective interference and channel matrices in SC‐IFDMA were utilized in formulating a low‐complexity implementation model for the proposed compensation schemes. Even though many works have been reported in the field of interference cancelation of SCFDMA system, majority of them were extension of the interference compensation schemes proposed for OFDMA system, whereas schemes proposed in this paper utilize the typical characteristics of the SC‐IFDMA system. The proposed schemes were simulated using MATLAB, and the performance is compared with existing schemes. Copyright © 2015 John Wiley & Sons, Ltd.     

Green Resource Allocation for Multiple OFDMA Based Networks: A Survey

Orthogonal frequency division multiple access (OFDMA) is a popular and widely accepted multiple access technique to provide high data rate services in a mobile environment in the area of wireless communications. OFDMA can provide better flexibility in allocating the radio spectra by utilizing subcarrier allocations, scheduling, and energy control to obtain multi-dimension diversity gains. Due to its resource allocation flexibility, OFDMA has been widely used as a green air interface technology for the emerging broadband wireless access networks. This paper extensively addresses the integration of green OFDMA to the future air interface technologies, for instance:two-tier cellular, multi radio access technologies (RATs), FemtoCell, and relay networks. The main focus of the paper is to review and analyze the current OFDMA techniques to address the green resource allocation in multiuser diversity, where the critical constraints are the computational complexity, energy efficiency, and the sub-channel assignment. The future trend of OFDMA based networks will aim to maximize the energy efficiency of the exclusive channel assignment through a joint sub-channel and power allocation to accommodate high data traffic networks specially the relay based 5G cellular networks.     

Performance analysis of load balancing in OFDMA cellular networks with inter‐cell relay

To balance and improve the resource utilization of the orthogonal frequency division multiple access (OFDMA) cellular networks, we switch parts of the edge users in overloaded cell to the adjacent light‐loaded cells by using the inter‐cell relay. The efficiency of traditional load balancing method based on the cell switching is low and in some cases, will occur the collision load problem. In this paper, we propose a new load balancing scheme based on inter‐cell relay in downlink OFDMA cellular networks. Besides, we put forward a new spectrum division scheme to reduce interference and improve spectrum efficiency. According to the formulas derived from the model we established, we can obtain the signal interference ratio and further to calculate the amount of remaining subcarrier and the throughput of the system. Finally, through numerical calculation and simulation, the result shows that the load balancing and spectrum division schemes can remit the resource stress of the overloaded cell and improve the spectrum utilization in adjacent cells.     

Group-orthogonal multicarrier CDMA

In the presence of frequency-selective multipath fading channels, code-division multiple access (CDMA) suffers from multiuser interference (MUI) and intersymbol interference (ISI); but when properly designed, it enjoys multipath diversity. Orthogonal frequency-division multiple access (OFDMA) is MUI-free, but it does not enable the available channel diversity without employing error-control coding. On the other hand, coded OFDMA may achieve lower diversity than a CDMA system employing the same error-control codes. In this paper, we merge the advantages of OFDMA and CDMA to minimize MUI effects, and also enable the maximum available diversity for every user. In our group orthogonal multicarrier CDMA (GO-MC-CDMA) scheme, groups of users share a set of subcarriers. By judiciously choosing group subcarriers, we guarantee that every user transmits with maximum diversity. MUI is only present among users in the same group, and is suppressed via multiuser detection, which becomes practically feasible because we assign a small number of users per group. Performance is analyzed, and simulations are carried out to illustrate the merits of GO-MC-CDMA relative to existing alternatives.     

OFDMA系统存在定时偏移条件下的信干比分析

在OFDMA(正交频分多址)系统的上行链路,单个用户的定时偏移也会导致多用户干扰(MUI).实际的OFDMA标准利用接收机与发射机之间复杂的闭环修正来实现高精度定时,从而将多用户干扰控制在可接受的范围内.文中考虑并分析了OFDMA系统存在定时偏移的各种可能情况,以及每种情况下的信号与干扰的功率比(信干比).Matlab仿真证实了理论分析结果的正确性.     

Loading Schemes for Downlink OFDMA Systems

In this paper we study the subcarrier and bit allocation strategies for downlink OFDMA system.Our opti-mization objective is to find optimum subcarrier and bit assignment minimizing the total transmitted power with the con-straints on BER and data rate for all users.We divide this problem into three steps:resource allocation,subcarrier assign-ment and single-user power and bit allocation.For the first two steps we propose new algorithms.Various loading schemesconstitute by combining these algorithms as well as algorithms proposed in Ref.[6].Simulation results demonstrate thatour proposed suboptimal loading scheme can achieve performance closer to the near optimal algorithm in Ref.[8]withmuch lower complexity than schemes in Ref.[6].     

Energy efficiency and spectral efficiency tradeoff in downlink OFDMA systems with imperfect CSI

In this paper, we investigate the tradeoff between energy efficiency (EE) and spectral efficiency (SE) in downlink orthogonal frequency division multiplexing access (OFDMA) systems, whilst considering the channel estimation cost and the corresponding effect of imperfect channel state information (CSI) on SE and EE. The problem is formulated as a multi-objective optimization to determine the optimal pilot transmission power, data transmission power and subcarrier assignment, and then transformed into a single-objective optimization problem, which is a non-convex mixed-integer nonlinear programming (NCMINP) and NP-hard. To address it, we propose an efficient algorithm by adopting alternating optimization and convex optimization methods in lower power region as well as approximate conversion and branch-and-bound methods in high power region. Simulation results analyze and validate the performance of EE-SE tradeoff.     

Joint subcarrier and power allocation for downlink cellular OFDMA systems by intercell interference limitation

This paper addresses resource allocation for sum throughput maximization in a sectorized two-cell downlink orthogonal frequency-division multiple-access (OFDMA) system impaired by multicell interference. It is well known that the optimization problem for this scenario is NP-hard and combinational, which is here converted to a novel sum throughput maximization problem in cellular OFDMA systems based on the intercell interference limitation. Then, three subclasses of this new problem are solved. By the first subclass, on the assumption that subcarrier allocation parameters are fixed, an algorithm for optimal power allocation is obtained. However, the optimal resource allocation requires an exhaustive search, including the optimal power allocation which cannot be implemented in practice due to its high complexity. By the second subclass, the problem is reduced to a single cell case where the intercell interference in each subcarrier is limited to a certain threshold. Based on the solution of the single cell problem, a distributed resource allocation scheme with the aim of small information exchange between the coordinated base stations is proposed. By the third subclass, the centralized resource allocation for two adjacent cells as a general problem is solved. Here, the algorithm allocates simultaneously the subcarriers and the power of the considered two cells while the resource allocation parameters of both cells are coupled mutually. The present study shows that distributed and centralized resource allocation algorithms have much less complexity than the algorithm used in the exhaustive search and can be used in practice as efficient multicell resource allocation algorithms. Simulation results illustrate the performance improvements of the proposed schemes in comparison to the schemes with no intercell interference consideration.     

Dynamic hybrid duplex for rate maximization in OFDMA

Orthogonal frequency division multiple access (OFDMA) is considered as one of the technologies for 4th generation (4G) due to its robustness against multi‐path environment. OFDMA also has the advantage of flexibility in resource allocation, which cannot be supported by existing duplex schemes. In this paper, we propose dynamic hybrid duplex (DHD) which will enhance the efficiency and flexibility of the OFDMA system. To establish the framework of resource allocation for the DHD, we formulate DHD resource allocation problem (DRAP) which maximizes the total users' data rate with power, rate, and subcarrier allocation constraints. A heuristic procedure is developed to solve the problem. The procedure finds the best mode and the best amount of resources allocated to downlink (DL) and uplink (UL). Simulation is performed with five scenarios to evaluate the DHD. Its results show that the proposed DHD outperforms other duplex schemes in various environments. Copyright © 2010 John Wiley & Sons, Ltd.     

A Novel OFDMA Cellular System Based on Multiple FrFT Angles Reuse Scheme

In conventional Orthogonal Frequency Division Multiple Access (OFDMA) cellular systems, the fundamental cause of inter-cell interference (ICI) is that the signals from serving Base Station (BS) and interference BSs, are modulated by same exponential bases, at same subcarrier. In this letter, an effective OFDMA cellular mobile system based on multiple fractional Fourier transform (FrFT) angle reuse scheme is proposed. In this system, FrFT angle is regarded as new resource, which can be optimally allocated to each BS of cellular system. Because chirp bases, generated by different FrFT with optimally allocated angle, are mutually approximately orthogonal at same subcarrier, ICI can be perfectly suppressed. Both theory and simulations show the essential advantages over conventional OFDMA cellular systems for ICI suppression and bit error rate.     

OFDMA中继通信系统中非对称的资源分配

传统的OFDMA协作通信系统,中继经过一一对应的两跳子载波接收和发送数据信息,这一对称的资源分配模式限制了系统吞吐量的提高;本文放宽对称约束,研究了非对称模式下的资源分配问题。自由度的增加,增大了系统资源分配的复杂度,这是一个结合中继选择、子载波分配和功率控制等多维自由度的混合离散型优化问题,很难得到满足实时性要求的近似最优算法。针对该模型,本文提出基于拉格朗日松弛的联合优化算法,算法的关键在于OFDMA系统的渐进强对偶性,这个性质使得多项式时间算法的设计成为可能。算法的复杂度与子载波数目成线性关系。仿真结果表明,该算法的性能是近似最优的,而且与对称模式相比,非对称模式下的资源分配进一步提升了系统的吞吐量性能。      

Two novel iterative joint frequency-offset and channel estimation methods for OFDMA uplink

We address joint estimation of frequency offsets and channel responses in OFDMA uplink. A cyclically equal-spaced, equal-energy interleaved pilot preamble is proposed by which two iterative estimators are developed. In the first estimator, we develop a modified SAGE (space alternating generalized expectation-maximization) method by incorporating multiuser interference cancellation in both time and frequency domains into the SAGE method. The proposed modified SAGE method yields a faster convergence rate, a better estimation performance, and a lower complexity than the existing conventional SAGE method from [10]. In the second method, we propose a very low complexity ad hoc method to replace the high complexity frequency offset estimator of the first method. The proposed ad hoc method is developed based on the time-domain characteristics of the pilot preamble and the frequency-domain power concentration property of OFDMA systems. It achieves almost the same estimation performance as the first proposed method but requires significantly lower complexity. Both proposed methods are not only better in estimation performance, convergence rate, and complexity, but also more robust to the number of active uplink users than the existing method while requiring only one OFDM training symbol and no restriction of the data subcarrier assignment scheme.