Block differentially encoded OFDM with maximum multipath diversity

This paper proposes a novel block differentially encoded orthogonal frequency-division multiplexing for multicarrier transmissions over frequency-selective fading channels. Choosing appropriate system parameters, we divide the set of correlated subchannels into subsets of independent subchannels. Within each subset, differential unitary space-time modulation is performed by treating each subchannel as a transmit antenna. In addition to low complexity, the proposed system enjoys maximum multipath diversity and high coding advantages. Analytic evaluation and corroborating simulations reveal its performance merits.     

Bandwidth efficient transceiver design for differentially encoded OFDM system

In this paper, we present a bandwidth efficient non-coherent transceiver design for single input single output orthogonal frequency division multiplexing (SISO-OFDM) modulation with differential encoding. Under fast channel fading or in low signal-to-noise ratio (SNR) regime, pilot assisted channel estimation is not feasible. In such channel conditions, conventional non-coherent detection methods are not reliable resulting in poor throughput. We propose a frequency-spread time-encoded (FSTE) method for OFDM modulation, which exploits multipath diversity and achieves target energy-per-bit to noise spectral density \({E_b}/{N_0}\) in low SNR regime by spreading differentially encoded information symbols along OFDM sub-carriers. We investigate the impact of spreading on bit-error rate (BER) and throughput under relative mobility and multipath fading scenarios. In order to maximize the throughput of the proposed method, we also optimize spreading factor and modulation order. The simulation results demonstrate significant BER and throughput performance gain as compared to prevailing differential encoding methods.     

Interference mitigation for asynchronous cooperative STBC-OFDM systems

This paper addresses the problem of interference mitigation in cooperative Space Time Block Coded Orthogonal Frequency Division Multiplexing (STBC-OFDM) systems in the presence of asynchronism. This scheme first preprocesses the received ST codewords to convert the equivalent fading matrix into a suboptimal ordering upper triangular form based on low complexity permutation QR decomposition, and then suppresses the InterCarrier Interference (ICI) and InterSymbol Interference (ISI) by exploiting Successive Interference Cancellation (SIC) technique. Simulation results show that the performance of the proposed algorithm slightly outmatches or asymptotically approaches to that of the existing Minimum Mean Square Error (MMSE) detector depending on the magnitude of the Carrier Frequency Offsets (CFOs) but with less complexity.     

一种用于OFDM系统的频率分集ARQ方案

研究了一种适合高数据速率的正交频分复用OFDM传输的频率分集ARQ方案.通过采用等增益合并和子载波分配等技术,该方案可利用频率分集来有效减少慢衰落信道中的重传次数.计算机仿真显示在累计正确分组接收率为0.99时此方案的重传次数为3次,而普通的方案要10次以上.     

A new PTS OFDM scheme with low complexity for PAPR reduction

In this paper, we introduce a new partial transmit sequence (PTS) orthogonal frequency division multiplexing (OFDM) scheme with low computational complexity. In the proposed scheme, 2/sup n/-point inverse fast Fourier transform (IFFT) is divided into two parts. An input symbol sequence is partially transformed using the first l stages of IFFT into an intermediate signal sequence and the intermediate signal sequence is partitioned into a number of intermediate signal subsequences. Then, the remaining n-l stages of IFFT are applied to each of the intermediate signal subsequences and the resulting signal subsequences are summed after being multiplied by each member of a set of W rotating vectors to yield W distinct OFDM signal sequences. The one with the lowest peak to average power ratio (PAPR) among these OFDM signal sequences is selected for transmission. The new PTS OFDM scheme reduces the computational complexity while it shows almost the same performance of PAPR reduction as that of the conventional PTS OFDM scheme.     

CO-OFDM系统中一种改进的同步算法方案

对相干光正交频分复用(CO-OFDM)系统时序和频率 同步算法进行了改进,消除了Park的方法中由于时间标尺旁瓣引起 的定时模糊性,利用单个训练符号实现了和Schmidl方法一样宽的频偏估计范围,且保持 同等的估计精 度。在20Gb/s的CO-OFDM系统上对改进算法进行仿真测试的结果表 明,通过色散补偿的辅助,改进的同步 算法方案实现了精确的定时估计,和节省训练符号且宽范围的频差估计。此外,色散补偿的 辅助提升了子载波恢复的性能,使得误比特率(BER)为10－3时系统的光信噪比(OSNR)要求降低了3dB。     

多业务多用户OFDM系统中的资源快速分配方案

本文提出了多用户OFDM系统下行链路中的一种快速实用的资源分配算法.该算法以最大化系统吞吐量为目的,同时满足CBR(固定速率)和VBR(可变速率)用户的QoS(服务质量)要求.算法使用分组代替迭代计算,摒弃复杂的子载波交换操作,以极低的复杂度实现了等功率下的自适应子载波分配和比特加载.本文在选择性衰落信道下对算法进行评估,并与[6]中的算法比较.仿真表明,该算法可以在更低的复杂度D(KN log N 2KN)下实现与[6]同样的吞叶量.     

General ICI self-cancellation scheme for OFDM systems

One of the challenges in designing orthogonal frequency-division multiplexing (OFDM) systems is their inherent sensitivity to any frequency shift in the signal. A frequency offset between the local oscillators at the transmitter and receiver causes a single frequency shift in the signal, while a time-varying channel can cause a spread of frequency shifts known as the Doppler spread. Frequency shifts ruin the orthogonality of OFDM subcarriers and cause intercarrier interference (ICI); therefore, quickly diminishing the performance of the system. ICI self-cancellation schemes have been proposed to reduce the sensitivity of OFDM systems to frequency shifts. These schemes use signal processing and frequency domain coding to reduce the amount of ICI generated as a result of frequency shifts, with little additional computational complexity. These methods can be used as an alternative to the fine frequency-offset estimation methods to battle oscillator frequency offset or simply be used as an ICI mitigation technique when the system is operating over time-varying channels. We propose a general ICI self-cancellation scheme that can be implemented through windowing at the transmitter and receiver. We show that the previously proposed self-cancellation schemes are equivalent to special cases of this method. Through theoretical analysis of the signal-to-interference ratio and bit-error rate and the use of Monte Carlo simulations, we demonstrate that the proposed system considerably outperforms the existing systems in the presence of frequency offset or time variations in the channel. We consider both coherent and noncoherent systems.     

Frequency reuse scheme for cellular OFDM systems

A new frequency reuse scheme for coherent orthogonal frequency division multiplexing (OFDM) cellular systems using high level modulation schemes, whereby both data and pilot parts can have different frequency reuse factors, is described. The capacity increase is investigated by simulations     

A low complexity selected mapping scheme for peak to average power ratio reduction with digital predistortion in OFDM systems

One of the effective methods used for reducing peak‐to‐average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems is selected mapping (SLM). In this paper, a new SLM scheme called DSI‐SLM, which is a combination of dummy sequence insertion (DSI) and conventional selected mapping (C‐SLM) is proposed. Previous techniques have had some drawbacks. In DSI, increasing the number of dummy sequences to have better PAPR degrades transmission efficiency, and in C‐SLM, the complexity rises dramatically when the number of sub‐blocks increases. The proposed DSI‐SLM scheme significantly reduces the complexity because of the reduction in the number of sub‐blocks compared with the C‐SLM technique while its PAPR performance is even better. To enhance the efficiency of the OFDM system and suppress the out‐of‐band distortion from the power amplifier nonlinearity, a digital predistortion technique is applied to the DSI‐SLM scheme. Simulations are carried out with the actual power amplifier model and the OFDM signal based on the worldwide interoperability for microwave access standard and quadrature phase‐shift keying modulation. The simulation results show improvement in PAPR reduction and complexity, whereas the BER performance is slightly worse. Copyright © 2011 John Wiley & Sons, Ltd.     

超宽带低截获率扩频系统实现

在现有器件工作速度受限条件下,提出了一种超宽带扩频系统实现方案.通过对扩频后的信号频域进行多载波二次扩展,提高了扩频系统的可实现带宽,从而在保持高扩频增益的前提下获得了高传输速率,降低了有效信号的功率谱密度要求,达到了隐藏有效信号频谱、降低截获率的目的.最终系统实现结构统一,扩展形式灵活,解决了现有条件下宽带高速率扩频系统的实现困难.     

正交频分复用技术在协作通信下的应用

协作分集技术利用其他用户进行协作传输,可以获得一定的分集增益,但是在频率选择性衰落信道下性能不很乐观。为了对抗频率选择性衰落,采用正交频分复用(ODFM)技术与协作分集技术相结合的方法,研究了基于OFDM技术的协作通信,对在不同调制技术、不同信道估计方法下系统的性能进行了比较,为了进一步提高系统性能,对传统S&C算法进行了改进并应用于该系统中,仿真实验结果表明,改进的定时同步算法性能优于传统的S&C算法。     

一种基于分布式频域线性卷积码的低计算复杂度的均衡算法

在本文中,考虑了基于分布式频域线性卷积空频码(distributed frequency-domain linear convolutivespace-frequency codes,DFLC-SFC)的协同通信系统。该协同通信系统考虑了多个载波频率偏移(multiple carrierfrequency offsets,MCFOs)的影响,同时假定中继结点到目的结点的信道是平坦衰落的。通过数学推导,该系统模型得到了简化。因此,本文最终得到了等价的限带模型,同时也分析了关于最终等价信道矩阵的限带属性。在此限带模型的基础之上,本文提出了一种应用LDHH矩阵分解的"限带"块最小均方误差(minimum mean square error,MMSE)均衡方法。进一步地,为了实现相应的限带操作,本文还定义了一类特殊的掩码矩阵。相比较于那些传统的MMSE均衡方法,本文所提出的均衡方法在保持较满意的系统性能的前提下,具有相对较低的计算复杂度。     

Precise analysis of bit-error probability for asynchronous multicode DS-CDMA systems

A precise bit-error probability (BEP) analysis method is derived for a multicode direct-sequence code-division multiple-access (DS-CDMA) system in an additive white Gaussian noise channel. The method is applicable to a multicode DS-CDMA system with an arbitrary number of multiple code sequences, and any selection of multiple code sequences. The precise method gives results that discriminate the effect of the selection of different multiple code sequences on the BEP, whereas Gaussian approximations (GAs) do not. Thus, the new method can be used to select the best multicode set for a given system, a task that cannot be achieved using GAs. A two-step analytical procedure enables deriving an explicit, compact form for the CF of the receiver decision statistic in a DS-CDMA system with an arbitrary number of multiple code sequences, and for any selection of multiple code sequences.     

A distributed cooperative relaying optimization scheme for secondary transmission in Cognitive Radio networks

In Cognitive Radio (CR) networks, cooperative communication has been recently regarded as a key technology for improving the spectral utilization efficiency and ensuring the Quality of Service (QoS) for Primary Users (PUs). In this paper, we propose a distributed joint relay selection and power allocation scheme for cooperative secondary transmission, taking both Instantaneous Channel State Information (I-CSI) and residual energy into consideration, where secondary source and destination may have different available spectrum. Specifically, we formulate the cognitive relay network as a restless bandit system, where the channel and energy state transition is characterized by the finite-state Markov chain. The proposed policy has indexability property that dramatically reduces the compu-tation and implementation complexity. Analytical and simulation results demonstrate that our pro-posed scheme can efficiently enhance overall system reward, while guaranteeing a good tradeoff between achievable date rate and average network lifetime.     

A dual protection scheme for impulsive noise suppression in OFDM systems

Orthogonal frequency division multiplexing (OFDM) can be susceptible to impulsive noise arising from numerous sources in a noisy communications environment. Conventional Reed–Solomon (RS) codes are particularly useful for burst-error corrections and have been employed in OFDM systems to manage impulsive noise. The performance gains, however, have been somewhat limited given the sensitivity to other noise types typically present in a noisy channel. In this regard, a novel scheme utilizing a time-domain pre-processing mean filter in combination with RS coding is proposed for impulsive noise suppression in OFDM systems. This scheme is split into two stages. In the first stage, a proposed mean filter effectively detects and removes the impulsive noise using the measured statistics of the impulsive noise. In contrast to a conventional blanking type filter, the traditional mean replacement value is replaced by a composite comparison value (CCV). This principle creates a more accurate estimate of the original OFDM signal after impulsive noise removal. The residual impulsive noise is then managed by a RS decoder in the second stage. Our results show that this dual faceted approach improves OFDM performance when compared to filtering and coding techniques alone.     

基于前导序列的OFDM定时同步与信道估计算法

本文针对IEEE802.11a前导序列无法精确定时的缺点,设计了一种新的前导序列,能够通过平台检测实现数据到达预警,之后利用有尖锐峰值的共轭对称结构做精确定时,因此误捕获率低,且不受频率偏移的影响.针对LS算法未考虑噪声带来估计误差的缺点,改进算法将LS算法估计的频域信道传输函数转换到时域,在时域利用信道脉冲响应的有限性,消除一部分噪声,再将其变换到频域替代LS算法所估计的信道传输函数.仿真结果表明,改进算法的均方误差较LS算法约有8dB的性能增益.     

Intercarrier interference self-cancellation scheme for OFDM mobilecommunication systems

For orthogonal frequency-division multiplexing (OFDM) communication systems, the frequency offsets in mobile radio channels distort the orthogonality between subcarriers resulting in intercarrier interference (ICI). This paper studies an efficient ICI cancellation method termed ICI self-cancellation scheme. The scheme works in two very simple steps. At the transmitter side, one data symbol is modulated onto a group of adjacent subcarriers with a group of weighting coefficients. The weighting coefficients are designed so that the ICI caused by the channel frequency errors can be minimized. At the receiver side, by linearly combining the received signals on these subcarriers with proposed coefficients, the residual ICI contained in the received signals can then be further reduced. The carrier-to-interference power ratio (CIR) can be increased by 15 and 30 dB when the group size is two or three, respectively, for a channel with a constant frequency offset. Although the redundant modulation causes a reduction in bandwidth efficiency, it can be compensated, for example, by using larger signal alphabet sizes. Simulations show that OFDM systems using the proposed ICI self-cancellation scheme perform much better than standard systems while having the same bandwidth efficiency in multipath mobile radio channels with large Doppler frequencies     

A novel ICI self‐cancellation scheme for OFDM systems

We propose a novel intercarrier interference (ICI) self‐cancellation scheme for orthogonal frequency division multiplexing (OFDM) systems. The symmetric scheme is the best among all ICI self‐cancellation scheme in the literature. Its coefficient pair is (1, ? 1), and the loading subcarriers are the k_th and N?k ? 1_th subcarriers, where N is the number of subcarriers. We propose to modify the symmetric scheme and change the coefficient pair from (1, ? 1) to (1, ?µ) where µis between 0 and 1. The proposed modified symmetric scheme has better carrier‐to‐interference‐ratio (CIR) than all previous ICI self‐cancellation schemes by at least 1.7 dB when the normalized frequency offset is 0.5. Copyright © 2011 John Wiley & Sons, Ltd.