A novel transmit scheme for OFDM systems with virtual subcarriers

Based on the double repetition structure in the frequency domain and the proper pulse shaping filter in the time domain, a novel transmit scheme for orthogonal frequency division multiplexing （OFDM） systems with virtual subcarriers is proposed. Our proposed scheme with comb-type pilots can provide feasibility to reduce the channel estimation errors at the band edges with conventional inverse fast Fourier transform （IFFT）-based channel estimator. Also this transmit scheme can provide additional diversity compared to the conventional OFDM scheme. Meanwhile, the receiver schemes corresponding to the proposed transmit scheme are presented to obtain the advantages mentioned above. All these efficacies are confirmed by simulations.     

Exploiting cooperative multipath –Doppler diversity in relay-assisted high-speed train communications

Severe doubly selective channel fading, or the time- and frequency-selective fading, prevailing in high- speed train （HST） communications, has been a critical issue that hinders the improvement of system efficiency and reli- ability. In this paper, a relay-assisted HST communication system is considered, and a low-complexity but effective scheme is proposed to cooperatively exploit the joint mul- tipath-Doppler diversity of both the direct link and the relay link and thus to mitigate the negative effect of channel fading and improve the system reliability. In particular, using a special precoding structure followed by two-dimensional Fourier transform, the transmit signal is generated which in effect parallelizes the doubly selective fading channel. As a result, in the transform domain, pointwise combining and equalization, which are of linear complexity, become feasi- ble and diversity gain is obtained. The maximal diversity order is then analyzed under the amplify-and-forward relay settings, which reveals that by cooperative relaying the joint multipath-Doppler diversity of both links can be well exploited. Simulation results verified our analysis.     

Differential space-time modulation in rapidly fading channels

Previously proposed differential modulation schemes for time-varying channels may not achieve the full transmit diversity and the maximum Doppler diversity simultaneously. Based on an existing basis expansion model, a new differential space-time code, which wisely combines interleaver/de-interleaver with traditonal space-time transmitting technique to overcome such limitation, .is presented. Two noncoherent differential decoders, named decision-feedback differential detector （DF-DD） and Viterbi-algorithmbased multiple-symbol-detection differential detector （ MSD-DD）, are also derived. We show that our design may recover data symbols with full antenna diversity and the maximum Doppler diversity at high signal-to-noise ratio. System performance is evaluated with simulations.     

Blind channel estimation through redundant linear precoding for OFDM systems

A novel approach of blind channel estimation through redundant linear precoding for orthogonal fre-quency-division multiplexing （OFDM） is proposed. A redundant linear frequency-domain preceder is ap-plied to each pair of blocks before they enter the OFDM system. With the aid of the introduced structure, the frequency-selective channel can be identified at the receiver through auto-correlation operations, sin-gular value decomposition （SVD） and scalar ambiguity resolution. The proposed blind channel estimation method has low computation complexity and requires no prior statistical information of channel or noise. The redundant linear frequency-domain precoder is employed to identify the frequency-selective fading channels. And the proposed blind channel estimation method has high spectral efficiency because it re-quires no training sequence. Computer simulations have proved that this proposed blind channel estima- tion method can identify the frequency-selective channels perfectly and have a good performance.     

A simple channel estimator for space-time coded OFDM systems in rapid fading channels

A simple channel estimator for space-time coded orthogonal frequency division multiplexing (OFDM) systems in rapid fading channels is proposed. The channels at the training bauds are estimated using the EM (expectation-maximization) algorithm, while the channels at the data bauds are estimated based on the method for modelling the time-varying channel as the linear combination of several time-invariant “ Doppler channels“. Computer simulations showed that this estimator outperforms the decision-directed tracking in rapid fading channels and that the performance of this method can be improved by iteration.     

Comparison of Achievable Rates of OFDM and Single Carrier Communication Systems

This paper presents a theoretical comparison of a single carrier system and a multi-carrier system through an analysis of the achievable rate under frequency selective fading with channel state information at the receiver. A scheme was designed to compare the achievable rates of a single carrier system and an Or-thogonal Frequency Division Multiplexing (OFDM) system. A thorough theoretical analysis of the two-path channel was conducted, and simulations were also used to analyze practical stochastic channels. Analysis and simulation results show that the achievable rates of the two approaches are comparable when the channel is flat fading. However, when the channel is frequency selective fading, the single carrier system outperforms the OFDM system. The achievable rate of the OFDM system is about 10% lower than that of the single carrier system at higher SNRs.     

The investigation on two-dimensional pilot-symbol-aided channel estimation method for OFDM system

Channel estimation for orthogonal frequency division multiplexing （OFDM） system has attracteo widespread attention. In this paper, a novel efficient two-dimensional （2-D） channel estimation algorithm based on fast Fourier transform （FFT） is proposed for a time-variant, frequency-selective wideband wireless channel. Both theoretical analysis and simulation results are addressed in the paper. The simulation results prove that the proposed algorithm has simpler implementation, better performance and wider application than other traditional deeision-direeted algorithms.     

Multi-user performance analysis of differential frequency hopping system over Rayleigh-fading channel

A novel non-coherent detection scheme for differential frequency hopping （DFH） system is proposed in asynchronous multi-user environments over Rayleigh-fading channels. The synchronous and asyn- chronous multi-user performances of DFH with the conventional detection scheme and this novel detection scheme are analyzed, respectively. The performance results are validated with simulation. The results of analyses and simulations prove two conclusions. Firstly, the performance of asynchronous multi-user DFH system overcomes that of synchronous multi-user DFH system over Rayleigh-fading channel. Secondly, the novel detection scheme can achieve better performance than the conventional non-coherent detection scheme in asynchronous multi-user environments.     

A novel adaptive classification scheme for digital modulations in satellite communication

To make the modulation classification system more suitable for signals in a wide range of signal to noise ratios （SNRs）, a novel adaptive modulation classification scheme is presented in this paper. Differ-ent from traditional schemes, the proposed scheme employs a new SNR estimation algorithm for small samples before modulation classification, which makes the modulation classifier work adaptively according to estimated SNRs. Furthermore, it uses three efficient features and support vector machines （SVM） in modulation classification. Computer simulation shows that the scheme can adaptively classify ten digital modulation types （i.e. 2ASK, 4ASK, 2FSK, 4FSK, 2PSK, 4PSK, 16QAM, TFM, π/4QPSK and OQPSK） at SNRS ranging from 0dB to 25dB and success rates are over 95% when SNR is not lower than 3dB. Accuracy, efficiency and simplicity of the proposed scheme are obviously improved, which make it more adaptive to engineering applications.     

Optimal power allocation for amplify-and-forward in single relay non-cooperative systems

<正> For a single-relay amplify-and-forward (AF) non-cooperative system,an optimal power proportionbetween source and relay is considered.Aiming to minimize end-to-end bit error rate (BER) and maximizeattainable rate,both large-scale path loss and small-scale Rayleigh fading are taken into account.Aclosed form expression to allocate power in optimal proportion at source is obtained.Simulation resultsshow that the proposed scheme to distribute power can minimize BER under any channel conditions.     

一种在快衰落空间相关环境下特征向量调制的非相干通信方法

利用信道的统计相关信息,提出一种在快衰落相关信道中非相干通信的新方法.发射端将信息比特1调制为发射相关矩阵的最大特征向量,将信息比特0调制为0特征向量.接收端将每根天线上的信号功率合并,并通过与一预设阈值比较来检测比特信息流.分析表明,这种方法实现了最大发射相关增益与完全接收分集增益.进一步将这种特征向量调制扩充到M进制调制的环境下,分析可得此时的系统分集增益下降2M-1倍.通过与差分酉空时调制(differential unitary space time modulation)的仿真比较,在块衰落(block fading)准静态信道中,与差分酉空时调制有相当的性能;在快衰落的信道中,差分酉空时调制已无法工作,而该方法仍可以实现完全接收分集增益.     

频率选择性衰落无线信道下的发送分集技术

通过研究平坦衰落信道下空时分组码(STBC)发送分集技术，结合正交频分复用(OFDM)技术，提出适用于频率选择性衰落信道的STBC-OFDM发送分集方法．然后对STBC-OFDM系统在随机信道和恒参信道中的性能进行仿真．研究和仿真结果表明，STBC-OFDM技术是一种非常有效的抗频率选择性随机衰落技术．与OFDM系统比较，STBC-OFDM系统误码性能得到很大改善，系统的频带利用率通过采用多电平调制方式得到提高．STDC-OFDM方法适用于数据高速传输的宽带无线系统的下行链路．     

级联低密度校验码的差分酉空频调制技术

提出了一种新的级联低密度校验码的“差分酉空频调制”方案,该方案适用于收发端均未知信道状态信息的多输入多输出(OFDM)系统.“差分酉空频调制”是在发送天线与OFDM子载波之间进行“空频”编码,而在相邻两个OFDM码字之间进行差分调制,它可以充分利用多径衰落信道所提供的频率分集资源,不需要任何信道状态信息就能够获得最大的频率分集增益.由于其具有最大化的频率分集增益和较高的编码增益,故该方案的系统误码率性能远远优于新近研究文献中所提出的“差分酉空时调制”OFDM技术.在接收端提出了非相干迭代译码方案,其性能良好,同时译码复杂度相对较低.相应的仿真研究表明,该方案在“时频双选”信道下具有很强的鲁棒性.     

一种适用于快速双选衰落信道的时频双差分空时编码正交频分复用技术

为了保证在快速双选衰落信道下的有效数据传输,提出了一种新颖的时频双差分空时编码正交频分复用(OFDM)技术.该技术通过分别在OFDM系统的子载波之间和OFDM符号之间引入差分空时编码,实现了在系统时域和频域上的差分调制.文中同时给出了相应的准最优译码算法,并对系统性能进行了分析和计算机仿真.结果表明,该技术在快速双选衰落信道下能够保持良好的误码率性能.     

基于导频辅助的单载波空时分组编码传输技术

研究了频率选择性衰落信道中采用2个发送天线1个接收天线的单载波Alamouti空时分组编码数据块传输技术．在传输系统发射端利用导频符号辅助的思想设计出伪噪声PN导频序列和信息数据的排布结构;接收端一方面利用导频PN序列循环相关的信道估计算法获得信道冲激响应估值,另一方面采用干扰抵消技术消除导频序列对接收信息数据的影响,之后引入单载波频域均衡技术对接收数据进行处理以克服信道的频率选择性衰落．仿真结果表明,这种基于导频辅助的单载波空时分组编码传输系统相对于单天线传输系统获得了空时结构所带来的全速率下的最大分集增益,具有比Alamouti空时分组编码与OFDM相结合的传输系统更高的数据传输效率和更好的系统性能．     

基于多项式模型和卡尔曼滤波器的正交频分复用自适应信道估计算法

在正交频分复用(OFDM)系统中,为了准确地检测接收信号并实现解调,必须对信道的传输函数进行准确估计.本文提出了一种适用于OFDM通信系统,基于二阶多项式模型和卡尔曼滤波器(Kalman filter)的自适应信道估计算法.利用二阶多项式模型对无线时变信道进行建模,通过合理安排插入导频结构,使用块状导频,可以使得原本二维的时频多项式模型减少到一维,降低信道估计算法的复杂度.同时,在多项式模型的基础上,将时变无线信道等效为一autoregressive(AR)过程,从而建立起卡尔曼滤波器的过程方程与测量方程,通过卡尔曼滤波器的递推公式,即可实现对多项式模型参数的自适应估计.本文提出的算法可以仅使用最少的导频(即与多项式模型参数相同个数的导频)实现准确的信道估计.由于每次所需导频个数的减少,接收端所需存储的未解调信号的数量也随之降低,大幅减少了接收端所需的存储空间.计算机仿真证明,与线性插值算法和基于二阶多项式模型算法的信道估计相比,在相同输入信噪比和归一化多普勒频移条件下,本文提出的算法具有较低的误码率.     

一种具有可分级转码能力的无线视频鲁棒传输方法

结合空时正交频分复用(OFDM)和转换编码技术,提出一种无线衰落信道下具有可分级转码能力的鲁棒视频传输方法.该方法可将高质量的MPEG-2压缩视频转换为低码率、低分辨率H.264可分级码流,以满足网络带宽和终端设备显示的要求.同时,考虑分级码流的重要性,转码输出的可分级码流经过分层空时编码的OFDM系统实现不对等保护传输.实验结果表明:在典型的随机突发错误的无线环境下,本文可分级转码能力的视频传输系统性能优于非分级转码的视频传输系统.     

频率选择性信道下空时分组码直接盲信号检测算法

针对空时分组码系统中训练序列的引入会降低系统传输效率的问题,提出了一种在频率选择性衰落信道下无需信道估计直接对空时分组码进行盲信号检测的算法.该算法在发射端采用线性预编码并且在接收端采用子空间方法进行解码.和已提出的其他盲信号检测算法不同,它可以在未知信道信息的情况下以满速率发送和接收信号并获得满的分集增益.该算法可以被推广到有任意发射天线数的空时编码结构.Monte Carlo仿真实验表明,该算法和使用准确信道信息的解码算法性能相近.     

抗信道快变的酉空时星座设计

分析了移动台以不同大小速率移动时,信道衰落系数在同一分组长度内的快变性。通过计算机仿真分析发现：酉空时码的误码率随着信道衰落系数自相关函数的减小而恶化,且随分组长度增大而加深恶化。在系统化酉空时星座设计方法的基础上,提出了一种抗信道快变的酉空时星座设计方案。仿真结果表明：该方案可有效地缓解由于信道快变而引起的误码率恶化问题。     

一种基于检测转发方式的酉空时协作分集技术

对一种基于检测转发方式的酉空时协作分集技术进行了研究,该技术可以在发送端和接收端都不知道信道信息的情况下使数据可靠地传输.在该协作分集技术下,通过对2个用户协作系统进行仿真分析,结果表明,其误码率性能与每个用户的信道和用户间的信道相关.