一种新颖的多用户OFDM资源分配算法

在多用户正交频分复用(OFDM)系统中设计一种新颖的子载波-功率分配方法来最大化用户数据传输速率.这个方案分成两部分,子载波分配和功率分配.其中,子载波分配方法是在信道容量矩阵中将信道容量最好的子载波首先进行分配,功率分配采用注水方法.研究和模拟结果表明,该算法在只改变各个子载波增益系数的基础上,基本保持了较低的复杂度,并且极大地提高了用户数据传输速率.     

认知OFDM系统的一种公平有效的多用户资源分配方案

在可用子载波、干扰温度、次级用户功率预算及公平性等约束条件下,提出了一种适用于认知OFDM系统的公平有效的多用户资源分配方案.该方案将子载波和功率分配分为两个相继的步骤,以降低计算复杂度.第一步引入子载波分配的比例公平原则,保证每个用户都能满足一定的通信要求;第二步采用一种改进的快速迭代注水算法,通过选择适当的调整步长...     

协作分集OFDM系统的混合排序子载波配对算法

考虑到信源子载波和中继子载波的配对能在频率选择信道环境下提高协作分集系统的性能,对协作分集正交频分复用(OFDM)系统的子载波配对进行了研究,推导了2种特殊情况下的最佳子载波配对方法,即顺序配对和逆序配对,在此基础上,提出了一种基于混合排序的子载波配对算法.该算法根据信源-中继链路、信源-目标链路和中继-目标链路上的每个子载波的信噪比状态,自适应地选择顺序配对或逆序配对.仿真结果表明,提出的混合排序子载波配对算法能显著提高系统的平均可取得速率,性能明显优于顺序配对和逆序配对.     

A joint iterative channel estimation and symbol detection algorithm for OFDM systems over time-varying channels

针对正交频分复用(OFDM)系统中由于信道快时变导致的子载波间干扰(ICI),基于基扩展模型,提出了一种时变信道下OFDM系统联合迭代信道估计与符号检测算法.该算法将基于基扩展模型的最小均方误差信道估计算法与一种类似于串行干扰抵消的符号检测方式相结合,并利用插值和滤波过程进一步消除噪声影响,进而通过联合迭代获得较为精确的符号检测结果.同时,利用带状矩阵近似降低算法的计算复杂度,在复杂度与算法性能之间取得比较好的折中.理论分析和仿真结果表明,在快时变信道条件下,这种新的联合迭代信道估计与符号检测算法可有效地提高系统性能.     

802.20协议物理层关键技术OFDM和Flash-OFDM

就802.20协议物理层目前采用的OFDM/OFDMA技术和Flash-OFDM技术进行比较.OFDMA是一种多载波调制技术,它的原理是无线通信信道划分为多个窄带子信道同时传送,在每个了信道上使用一个子载波进行调制.Flash-OFOM技术采用FDD双工方式,上下行链路是数百个子信道组成的宽带载波,传输数据时给每个用户分配了信道.每个子信道采用了自适应调制和先进的编码技术,其频谱利用率比CDMA2000系统高3倍.Flash-OFDM在时间上以跳频方式使用OFDM的子载波,以实现信号扩频.它还有频率分集能力,减小了同一小区内的用户间干扰,同时具有OFDM和跳频扩频技术的优点,因而相邻节点可以使用相同频率的子载波.Flash-OFDM为解决小区间干扰,采用了功率控制,用户只发射能有效通信的功率.     

Comparison between time shifted non-orthogonal frequency division multiplexing and OFDM

通过将时移参数引入多载波调制,形成了一种具有全新思想的时移非正交频分复用(TS-NFDM)调制技术,并通过理论分析和仿真实验将该技术的性能与当前在无线通信中扮演重要角色的正交频分复用(OFDM)技术进行了比较.比较结果表明,TS-NFDM技术具有明显的优势,其时移特性使其表现出许多优良的性能,其中最突出的是频带利用率高,约为OFDM的4.29倍,另外,信号功率的峰值均值比也明显低于OFDM.     

基于循环延迟分集的差分空时频编码方案

结合正交空时码的分集优势,将正交空时码应用到差分空频系统中,提出了一种基于循环延迟分集的多流差分空时频编码新方法,在差分系统中同时获得了空间和频率分集.该差分编码方法在满足信道的时间和频率选择性前提下,只需要发送很少的参考矩阵用于非相干检测.仿真结果表明,正交空时码的引入,可以有效提高整个系统的分集增益,从而在很大程度上改善整个差分MIMO-OFDM系统的误码性能.     

基于子载波权衡的多用户OFDMA资源分配算法

在多用户OFDMA资源分配中提出一种基于子载波权衡的资源分配算法.资源分配过程中,在对公平性最差的用户进行子载波分配时,进行子载波权衡.根据权衡后子载波信道增益的变化情况,进行子载波分配.通过子载波权衡,将待分配的子载波分配给权衡用户;同时对于公平性最差的用户,从已分配给权衡用户的子载波中获得一个权衡子载波;使得权衡后分配的子载波较权衡之前获得更高的信道增益.仿真结果表明,算法在保证用户公平性的同时,提升了系统的频谱利用率.     

一个MTCM-OFDM系统模型及其性能的分析

提出了一种将多网格编码调制(MTCM)与正交频分复用(OFDM)系统相结合的方法,建立了一个MTCM-OFDM系统模型,并对其进行了性能分析和仿真.分析表明,MTCM可提高系统的误码率性能,将MTCM应用于OFDM系统,能增大OFDM符号内一组子载波上符号序列之间的欧氏距离,从而降低误码率,但不会造成信息速率和带宽效率的损失.根据MTCM网格图中不同状态下输出符号序列可以有较大欧氏距离的特点,给出了一种符号序列的两步检测算法:首先利用最大似然序列检测以较大概率检测出网格图的状态序列,然后在两个连续的状态之间检测判决符号序列.该算法利用了MTCM编码调制的特点,因而时延小、复杂度低,性能逼近直接的一步最大似然检测.理论分析和仿真结果均证明了MTCM-OFDM系统和这种检测算法的上述性能特点.     

OFDM系统在时变信道下的迭代干扰抵消算法

为克服正交频分复用(OFDM)系统中因时变多径衰落信道破坏子载波的正交性引起载波间干扰(ICI),从而导致系统性能下降的问题,提出了一种基于最小均方误差(MMSE)估计和迭代算法相结合的迭代干扰抵消算法.为了消除一般MMSE均衡方法将载波间干扰当作加性高斯白噪声(AWGN)所带来的误差,该算法先使用MMSE方法对传输信号进行初值估计,再通过迭代干扰抵消和MMSE均衡相结合,来消除由时变信道所带来的载波间干扰.多次仿真表明,在时变多径衰落信道中使用本算法能够有效地消除载波间干扰,使系统性能得到很好的改善.     

一种OFDM系统动态子载波比特和功率分配联合算法

提出一种多用户OFDM(orthogonal frequency division multiplexing)系统下行链路，具有信道变化实时性的动态子载波、比特和功率分配联合算法(UA)，在满足各个用户数据速率和BER要求的同时使总的发送功率最小。提出的算法与动态子载波分配算法(WSA)相比，计算复杂度相当，在移动信道环境下仿真结果表明性能有一定的改善。     

Reduced complexity detection technique for layered space time block coded multiple-input multiple-output orthogonal frequency division multiplexing

The combination of vertical Bell Labs layered space time (V-BLAST) and space time block coding (STBC), known as a Layered STBC (LSTBC) system, offers high spectral efficiency with a higher order of diversity. The system structure, computational complexity and error performance of the V-BLAST and LSTBC multiple-input multiple-output orthogonal frequency division multiplexing systems are analysed and compared. It is shown that, compared with V-BLAST, the overall diversity order of LSTBC increases two fold. This diversity gain is achieved at the expense of a four?fold increase in the computational complexity of the QR decomposition (QRD) algorithm, required at the receiver of both LSTBC and V-BLAST. The authors propose a modified QRD algorithm which reduces this four-fold complexity to two fold.     

Performance of spatially multiplexed MC-CDM with zero-forcing unified successive interference cancellation detection

Spatially multiplexed multicarrier code-division multiplexing (SM-MC-CDM) is a multiple-input multiple-output, orthogonal frequency division multiplexing (MIMO-OFDM) communication technique with multiple antennas used for spatial multiplexing and with frequency domain spreading on each antenna. Unified successive interference canceller (U-SIC) is an efficient detector recently introduced for SM-MC-CDM. This paper presents an analytical approach to the performance of zero-forcing (ZF) U-SIC for SM-MC-CDM communications. For a system with an equal number of transmit and receive antennas, an approximation for the probability density function of post-detection signal-to-noise ratio (SNR) is used to derive a closed-form analytical upper bound and approximations for the probability of error and ergodic capacity. It is shown that SM-MC-CDM with ZF U-SIC is able to achieve higher diversity order than that achieved by ZF and minimum mean squared error (MMSE) V-BLAST detectors used on each subcarrier of a MIMO-OFDM system with the same number of subcarriers. The diversity order obtained increases with the number of subcarriers. It is also shown that the ergodic capacity of the system decreases with increasing number of subcarriers.     

Cross-layer adaptive resource allocation for OFDM systems with hybrid smart antennas

Orthogonal frequency division multiplex (OFDM) has become one of the most popular air-link technologies for future broadband wireless communications. To further improve its bandwidth efficiency and system performance, adaptive resource allocation and smart antenna techniques have been widely used in the OFDM system. However, the use of fully adaptive beamforming in an OFDM system significantly increases the complexity of the medium access control layer design and thus affects the implementation of adaptive resource allocation. A novel cross-layer adaptive resource allocation strategy with hybrid adaptive array and switched- beam smart antennas suitable for the OFDM systems has been proposed. With the help of different smart antennas schemes based on different users' quality of service requirements, the strategy effectively reduces the complexity of adaptive resource allocation in an OFDM system, while still maintaining a satisfactory system performance.     

Interpolation-based precoding with limited feedback for MIMO-OFDM systems

The channel state information (CSI) at the transmitter can significantly improve the performance of multiple-antenna systems. However, providing full knowledge of CSI at the transmitter may not be affordable in many practical cases. Thus, exploiting the partial channel knowledge to improve system performance seems to be attractive. An interpolation based limited feedback precoding scheme (ILFP) for MIMO-OFDM systems is proposed. In this scheme, both the transmitter and the receiver store the codebook of precoding matrices constructed offline using two-variable joint vector quantisation. Considering the correlation between OFDM subcarriers, they are divided into subcarrier clusters. At the receiver, precoding is carried out on the clusters, and then the precoding information is conveyed to the transmitter by limited bits of feedback. At the transmitter, the precoding matrices for each subcarrier are obtained by interpolation according to the feedback. Simulation results show that the proposed scheme outperforms the existing schemes.     

On subchannel inversion for improvement of power efficiency in OFDM-based systems

The problem of power loading in orthogonal frequency division multiplexing (OFDM)-based systems that provides minimization of the bit-error rate (BER) performance under a fixed transmit power (or equivalently minimisation of the transmit power under a fixed value of the BER) is addressed in this work. A lemma about inversion of subchannel power gains is proved and examples that exhibit its utility are presented. As a result, a non-iterative adaptive power allocation technique is derived. The algorithm combines the recently proposed ordered subcarrier selection method with 'subchannel inversion'. Compared with the original technique, such a combination provides a power gain that depends on the channel statistics.     

OFDMA系统资源优化综述

在正交频分多址(OFDMA)系统中,有效的资源分配可以充分发挥系统性能.目前的资源优化主要分为频谱效率优化与系统能效优化两个方面.频谱效率优化即在给定的总功率下,最大化系统总容量;系统根据信道状态信息,分步对子载波和功率进行分配.根据不同的业务需求,频谱效率优化又细分为仅考虑频谱效率的优化以及考虑用户公平性与复杂通信环境下的优化.系统能效优化是在保证系统总容量的同时,降低系统的总功率消耗.能效优化算法采用了分式规划、约束条件松弛与拉格朗日对偶分解等最优化理论的方法进行资源分配的优化.在加入不同约束的考虑下,能效优化算法分为保证用户公平性、回程线路容量、通信安全等不同的能效优化方案.但是,这两方面的资源优化对不完整信道信息的情况考虑较少,对于多业务的资源分配,未能针对业务多样性进行高效资源优化.另外,资源优化在跨层资源分配上仍有不足.     

单载波频域判决反馈均衡水声通信技术研究

水声信道中多途干扰严重,由多途效应引起的码间干扰是影响水声通信系统的关键性因素。单载波频域均衡(Single-Carrier Frequency Domain Equalization,SC-FDE)技术基于正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)技术提出,能有效对抗水声信道中的多途干扰,同时能避免OFDM技术峰值平均功率比高的不足。文中先介绍判决反馈均衡算法,并与其他均衡算法的抗多途性能进行比较。然后为提高系统均衡的可靠性和水声信道带宽利用率,对传统数据帧结构进行改进。将独特字(Unique Words,UW)序列均分,提高水声信道估计的精度,进而降低误码率,增加传输的可靠性;增加数据帧中有用信息符号,系统误码性能几乎不变,水声信道频带有效利用率提高。最后开展水池试验,验证了算法的有效性和可靠性。