OFDM系统中的子载波分配策略

正交频分复用(OFDM)技术是未来宽带无线通信中最流行的技术之一。研究了适用于多用户(OFDM)系统中的多业务间子载波自适应资源分配机制。考虑了每个用的不同业务需求,资源根据每个用户不同的服务质量要求来分配。提出了基于OFDM系统的自适应子载波重分配算法,该算法能够最大程度地灵活分配所有子载波。仿真表明,使用此算法能在很大程度上改善OFDM系统的性能。     

一种针对双衰落信道OFDM的新均衡算法

在OFDM系统中，子载波间的正交性是保证OFDM性能的重要保障。针对双选择性衰落信道下的OFDM系统，该文在分析载波间干扰（ICI）的基础上，提出了一种采用频域迭代消除ICI的均衡算法。分析和仿真结果表明此方法能有效地保证载波间的正交性和改善了OFDM系统的误码率（BER）性能。     

一种适用于快衰落信道的ICI抑制方案

针对高速移动环境下多普勒频偏造成信道的快衰落和正交频分复用(OFDM)系统中子载波间干扰(ICI)的问题,提出了一种适合快衰落环境的OFDM系统子载波间干扰抑制算法。此算法用线性变化模型来近似一个OFDM符号周期内的信道冲激响应,并以此为基础采用迭代MMSE均衡方法抑制载波间干扰。分析和仿真结果表明,此方法能有效地保证载波间的正交性,从而改善了OFDM系统的误码率(BER)性能。     

一种改进的OFDM/OQAM系统信道估计算法

基于块状导频的信道估计方法可以克服OFDM/OQAM(OFDM/Offset QAM)系统所固有的符号间和载波间干扰,从而成为该类系统通用的信道估计方法。该文基于块状导频结构和OFDM/OQAM的系统特点,分析了系统相邻子载波之间的相关性,并在此基础上提出一种改进的信道估计算法,通过计算相邻子载波的相关系数,在频域进行有效的加权运算来降低干扰和噪声对信道估计的影响。分析和仿真结果表明,该算法能够有效地提高传统算法的信道估计精度和系统性能。     

时间选择性快衰落信道中OFDM系统的ICI功率分析

OFDM(Orthogonal Frequency Division Multiplexing,正交频分复用)系统中,信道的时间选择性快衰落会破坏子载波间的正交性,导致系统中出现载波间干扰(Inter-carrier Interference,ICI),降低OFDM系统性能.论文使用信道函数泰勒展开式的前两项近似信道函数,并以此为基础分析了OFDM在时间选择性快衰落信道中的性能,给出了ICI功率及其上界与信道最大多普勒扩展的关系式.仿真分析表明论文给出的关系式和ICI功率上界能够较为准确的描述OFDM系统在快衰落信道中ICI功率及其上界与信道最大多普勒扩展的关系.     

OFDM系统中的迭代并行子载波间干扰消除技术

由于实际信道的时变性，OFDM系统中每一子载波上的接收信号受到其它子载波上所传输信号的影响，形成载波间干扰(ICI)并造成误码性能降低。本文提出了适用于OFDM系统的迭代并行子载波间干扰消除接收技术，分析和仿真结果表明此方法可以有效地消除载波间干扰，在归一化多普勒频移为0．1和0．001时，分别提高接收机的性能0.8dB和0．5dB。     

一种COFDM系统抗频率选择性衰落的方法

在频率选择性衰落信道中,宽带OFDM信号的部分子载波可能受到衰落的影响,造成调制到子载波上的数据位出现误码。提出一种COFDM系统对抗频率选择性衰落的方法,在发送端通过对受衰落子载波的预处理结合LDPC差错控制编码;在接收端,对受衰落子载波的预处理进行逆映射和差错控制译码,通过对错误图样的统计即可得到受衰落子载波信息。文中方法运算简单、复杂度低、系统额外开销小。     

频率选择性衰落信道下OFDM子载波数盲估计

提出了一种在频率选择性衰落信道下的基于高阶循环累积量的OFDM信号子载波数盲估计方法,扩展了该方法的适用范围,并进行了理论推导和试验仿真。结果表明OFDM信号的特定高阶循环累积量仅在子载波位置存在,通过检测循环频率的个数可以估计出子载波数。高阶累积量具有良好的抗噪性能,在低信噪比下具有良好的估计性能。     

OFDM中基于信道变化载波分配算法

OFDM系统利用用户的多样性,根据信道状态对系统中的子载波资源进行动态的分配,有效的提高了系统的容量。最大化系统容量的子载波分配算法,忽略了用户之间的公平性。使某些信道状态不好的用户由于没有被分配子载波,不能进行数据的传送。针对这一问题提出一种新的子载波的分配方法,在有效保证系统容量的前提下,通过确保每个用户分配到的子载波数目,兼顾了用户之间资源分配的公平性。     

DF-OFDM多中继系统中的子载波匹配

DF-OFDM技术必将成为现代无线通信的核心技术,但合理的资源分配仍然是必须要解决的问题之一.在移动通信系统抗信道衰落和提高信道容量方面,协作通信是一种很好的解决方案.针对改进型的多中继DF-OFDM系统中子载波匹配的问题,应用了固定子载波匹配和自适应子载波匹配两种方法,后者是以匈牙利算法为基础的.通过仿真比较了这两种方法对系统信道容量性能的影响,结果表明自适应子载波匹配方法更能有效提高信道容量.     

An effective suboptimal power loading scheme in OFDM-based cognitive radio systems

Orthogonal frequency division multiplexing (OFDM) is an attractive modulation candidate for Cognitive Radio (CR) networks. Effective and reliable subcarrier power allocation in OFDM-based Cognitive Radio (CR) networks is a challenging problem. This paper focuses on the power allocation for OFDM-based Cognitive Radio (CR) networks. Our objective is to maximize the total transmission rates of Secondary Users (SU) by adjusting the power of subcarrier while the interference introduced to the Primary User (PU) is within a certain range and the total power of subcarrier is not beyond the total power constraint. We investigate the optimal power allocation algorithm for OFDM-based Cognitive Radio (CR) based on convex optimization theory. Then, because of high complexity of the optimal power allocation algorithm, we propose an effective suboptimal power loading scheme. Theory analysis and simulation results show that the performance of the suboptimal power allocation algorithm is close to the performance of the optimal power allocation algorithm, while the complexity of the suboptimal power allocation algorithm is much lower.     

Worst subcarrier avoiding water-filling subcarrier allocation scheme for OFDM-based CRN

Efficient and reliable subcarrier power joint allocation is served as a promising problem in cognitive OFDM-based Cognitive Radio Networks (CRN). This paper focuses on optimal subcarrier allocation for OFDM-based CRN. We mainly propose subcarrier allocation scheme denoted as Worst Subcarrier Avoiding Water-filling (WSAW), which is based on Rate Adaptive (RA) criterion and three constraints are considered in CRN. The algorithm divides the assignment procedure into two phases. The first phase is an initial subcarrier allocation based on the idea of avoiding selecting the worst subcarrier in order to maximize the transmission rate; while the second phase is an iterative adjustment process which is realized by swapping pairs of subcarriers between arbitrary users. The proposed scheme could assign subcarriers in accordance with channel coherence time. Hence, real time subcarrier allocation could be implemented. Simulation results show that, comparing with the similar existing algorithms, the proposed scheme could achieve larger capacity and a near-optimal BER performance.     

Resource Allocation of Limited Feedback in Clustered Wireless Mesh Networks

This paper focuses on resource allocation problem of limited feedback in OFDM-based clustered wireless mesh networks. Firstly, the channel quantization codebook is designed according to the equal probability quantizer and joint power, rate and channel quantization threshold codebook scheme (PRCQT) is proposed. Secondly, we deduce the subcarrier allocation criterion utilizing the Lagrange multiplier method. Finally, stochastic resource allocation scheme of limited feedback (SRALF) utilizing the stochastic optimization tools for OFDM-based clustered WMNs is proposed. Simulation results demonstrate SRALF+PRCQT scheme not only improves the network throughput, but also has advantage of limited feedback overhead.     

Opportunistic power allocation strategies and fair subcarrier allocation in OFDM-based cognitive radio networks

In this paper we have studied the subcarrier and optimal power allocation strategy for OFDM-based cognitive radio (CR) networks. Firstly, in order to protect the primary user communication from the interference of the cognitive user transmissions in fading wireless channels, we design an opportunistic power control scheme to maximize the cognitive user capacity without degrading primary user’s QoS. The mathematical optimization problem is formulated as maximizing the capacity of the secondary users under the interference constraint at the primary receiver and the Lagrange method is applied to obtain the optimal solution. Secondly, in order to limit the outage probability within primary user’s tolerable range we analyze the outage probability of the primary user with respect to the interference power of the secondary user for imperfect CSI. Finally, in order to get the better tradeoff between fairness and system capacity in cognitive radio networks, we proposed an optimal algorithm of jointing subcarrier and power allocation scheme among multiple secondary users in OFDM-based cognitive radio networks. Simulation results demonstrate that our scheme can improve the capacity performance and efficiently guarantee the fairness of secondary users.     

Practical Implementation of Bit Loading Schemes for Multiantenna Multiuser Wireless OFDM Systems

This paper provides useful insights into the practical design of bit allocation algorithms in multiantenna multiuser orthogonal frequency-division multiplexing (OFDM) systems. With the degrees of freedom obtained with multiple antennas and multiple subcarriers, the performance might be enhanced at the expense of a higher complexity. Since the scheduling with realistic integer mappings is an NP-complete combinatorial problem, suboptimum solutions based on the scalar product are shown to be good candidates for yielding a realizable scheduler at a wireless physical layer. Additionally, a power reuse strategy is proposed to lower the computational requirements of such a system. Besides the tradeoff between performance and complexity, there exists the tradeoff between performance and signaling. Therefore, we show that the required signaling might be reduced either by a user-subcarrier clustering or by using a scheme that forces an equal mapping for all the users at the same subcarrier, which might be well-suited if instantaneous fairness is required. The proposed strategies are evaluated for typical OFDM-based wireless LAN scenarios.     

一种基于子载波合并的多播资源调度算法

针对无线OFDM多播系统,该文提出一种基于子载波合并的多播资源调度算法。该算法通过提前将子载波分组,避免了不必要的子载波配对;自适应地选择子载波合并\非合并,在分集和复用两种模式中选取最优方案;同时根据子载波功率分配的特点,将其解耦为配对子载波集合内功率分配和集合间功率分配两个子问题,进一步优化了算法性能。仿真结果表明,与现有方案相比,所提方案能够在复杂度较低的情况下,较好地提升系统性能。     

Robust channel estimation for the OFDM-based WLAN systems with imperfect synchronization

As an effective technique for combating multipath fading and for high data rate transmission over wireless channels, orthogonal frequ- ency division multiplexing (OFDM) is extensively used in wireless local area network (WLAN) systems to support high-performance bandwidth- efficient multimedia services. In this paper, a robust channel estimation scheme is proposed for the OFDM-based WLAN systems with imperfect synchronization. The frame structure information, the preamble information, the pilot information are efficiently utilized in the proposed channel estimation scheme. Simulation results are used to illustrate the performance of the proposed scheme.     

Reducing feedback load for OFDM-based wireless system

In this paper, the feedback load reduction problem in wireless systems based on orthogonal frequency division multiplexing (OFDM) is investigated and an opportunistic feedback scheme (OFS) is proposed. The key idea behind OFS is that only the key channel gains which can significantly affect the system throughput are fed back to the BS. Firstly, the key channel gains are proved to belong to a channel gain interval. Secondly, a statistical method is proposed to estimate the channel gain interval. Thirdly, the opportunistic feedback scheme is formulated and the feedback load of OFS is analyzed. The advantage of OFS is threefold: the first is OFS can work in both OFDM-based multicast system and OFDM-based unicast system. The second is the channel fading type of the BS-user link is not required, which is more realistic. The third is OFS can get better feedback load performance compared with other schemes, while achieving almost the same throughput performance compared with that of full feedback scheme.