一种新型的基于OFDM系统的自适应HARQ方案

基于正交频分复用(OFDM)系统提出一种自适应混合自动重传请求(HARQ)方案,它充分利用发送端获得的信道状态信息对每次重传数据符号进行重组并且采用星座重组方案平均调制符号内各比特间的可靠性差异。仿真分析表明,自适应HARQ方案与星座重组方案相结合大大提高了系统通过率、降低误块率和传输时延。     

自由空间光通信中的带符号光正交空间调制

【目的】针对光正交空间调制(OQSM)中频谱效率较低和系统吞吐量受限的问题,文章在自由空间光通信(FSOC)中提出了一种带符号的光正交空间调制(SOQSM)方案;另外,针对OQSM性能分析时所用信道不适合所有湍流的情况,文章采取适合所有湍流强度的Málaga信道对系统的误码性能进行分析。【方法】首先,该方案在发送端将输入的二进制比特流划分为5个部分,第1个部分用于星座符号映射,后4个部分用于激光器映射;然后,星座符号映射用于传输星座符号及其逆符号的实部和虚部;激光器映射分为同相相位与正交相位两部分,用于激活激光器的序列映射;最后,通过信道进行传输,在接收端进行信号的接收和处理。【结果】相比OQSM,SOQSM在一个传输周期内空间域额外携带了2log₂N_t比特的信息,并在符号域和空间域分别计算出平均误码率上界,最终得到SOQSM方案的平均误码率上界,其中计算平均误码率上界的方法具有较好的收敛性。该方案较大程度地提高了系统的频谱效率和传输速率,拥有较好的误码性能。【结论】蒙特卡洛仿真结果表明：在频谱效率相同时,相比光空间调制(OSM)和OQSM,...     

OFDM系统中HARQ方案与性能仿真

研究了正交频分复用(OFDM)系统中采用的混合自动重传请求(HARQ)技术,对其性能和实现方案进行了分析;在采用高阶调制时,为均衡同一映射符号内比特可靠性差异采用星座重排方案,并对星座重排技术进行了研究;在典型城区多径信道环境下,对不采用HARQ、采用Type I HARQ、采用Type Ⅲ chase合并的HARQ和采用带有星座重排的Chase合并的HARQ4种方案的误码率和吞吐量性能分别进行了仿真;仿真结果表明,星座重排技术结合Chase合并的HARQ机制是无线通信系统中提高传输质量和吞吐量的一种高效可行的方案.     

一种基于准正交空时码的低复杂度MIMO差分检测方法

针对可实现全速率传输的准正交空时码,提出了一种低复杂度的准正交MIMO差分检测方法.该方法在发射端对数据比特进行联合星座映射,构造准正交空时码进行差分编码;接收端采用最大似然准则对两组星座符号对(symbol pair)并行差分检测.本文提出的星座集合及联合星座映射方法简化了接收端检测算法,降低了检测计算复杂度.     

一种多符号网格码与差分空时分组码级连方法

我们提出了一种多符号网格码(MTCM)与差分空时分组码级连的传输方法,研究了在传输信道是慢时变平坦瑞利衰变的条件下,并采用理想交织时系统的性能。导出了这个级连系统的成对错误概率上限表达式,分析表明,系统的性能主要受到不同信号序列间的最小汉明矩离和信号星座间的平方欧几里德矩离的影响。仿真结果表明,在相同的频谱有效性和信号星座条件下,这种级连的系统比非级连差分空时编码系统有更好的性能。     

基于遗传算法的全索引调制的物理层加密算法

基于带指数调制的OFDM系统(Orthogonal Frequency Division Multiplexing with Index Modulation,OFDM-IM)以其潜在的分集增益而被人们所熟知。随后,一种基于遗传算法的全索引调制的OFDM(GA-OFDM-AIM)方案被提出,与传统的OFDM-IM相比,其采用遗传算法辅助子块设计,通过交叉、变异和选择过程,得到了低误码率的子块实现,但由于窃听者可以通过搜索算法或检测信道中的子块,传输安全无法得到保证,为此提出了基于遗传算法的全索引调制的物理层加密算法。利用无线信道特点,采用密钥生成技术提取初始密钥并且作为混沌发生器的初始值产生混沌序列,再利用该混沌序列对GA-OFDM-AIM中的子块进行星座点映射和星座旋转,打乱了星座阶数和调制类型的特征,使得窃听者难以破解系统的参数。为验证该算法的有效性和安全性,分析了不同的攻击模型,并比较了加密前后星座的误码性能。安全分析和仿真结果表明,该方案能有效地提高系统的安全性和传输性能。     

基于Rayleigh衰落信道下LDPC编码的星座成形

针对消息在信道传输过程中的成形损耗问题,提出了一种在Rayleigh衰落信道下基于低密度奇偶校验(Low-Density Parity-Check, LDPC)编码的星座成形系统。该方法基于脉幅调制(Pulse Amplitude Modulation, PAM)下的星座图,通过在发送端加入一个成形编码器(作用是提高输出比特为0的概率),使得星座图中低能量的符号比高能量的符号更容易被选中,从而提高成形增益。并且在解调器和译码器之间进行迭代译码,进一步提高了系统的性能。理论分析和仿真结果表明,在独立Rayleigh衰落信道下,与不采用星座成形的系统相比,该方案大大提高了系统的误差性能和成形增益。     

非频率选择性衰落MIMO信道上基于序列蒙特卡罗的数字调制识别方法

该文针对非频率选择性衰落多输入多输出(MIMO)信道提出了一种基于序列蒙特卡罗(SMC)方法的幅度-相位调制方式识别方法。首先将MIMO系统等效为一个动态状态空间模型,然后利用序列重要性采样和模式转移步骤估计每根发送天线采用的各种可能调制方式的概率,最后利用各个信道上发送符号的不相关性在长为N的观测信道上进行噪声平均。该方法能够在识别数字调制方式的同时估计发送数据符号。其复杂度是信道观测长度、发送天线数、采样大小、调制星座大小的线性函数。仿真结果表明提出的数字调制识别方法在各种调制星座上具有良好的性能。     

一种采用自适应星座重组的新型HARQ方案

为了使无线通信系统中数据的传输快速可靠,需要采用自动重发请求(ARQ)和前向纠错码(FEC)相结合的混合自动重发请求(HARQ)技术。针对星座映射中比特符号产生可靠性差异,提出一种自适应星座重组的新型HARQ方案。仿真结果表明,该新型HARQ方案具有降低系统误码率和时延,提高系统性能的优点。     

瑞利衰落信道下SCMA码本设计

为了设计近似最优的稀疏码本,以多用户叠加星座点的误符号率性能为评价指标,分别针对上行、下行传输信道,设计了适用于瑞利衰落信道的近似最优稀疏码本.基于叠加星座点与误符号率的数学模型,提出了一种近似最优稀疏码本的设计方案,码本的设计步骤包括因子图矩阵的设计、码字对的匹配以及最优旋转角度的确定.仿真结果表明,所提码本设计方案...     

基于预编码的全码率准正交空时分组码

针对传统八天线全码率准正交空时分组码解码复杂度高的问题,该文提出了两种基于预编码的传输方案。利用四天线正交码扩展得到新的八天线准正交码,结合两个反馈相位信息构成的预编码矩阵,使信道矩阵正交化,消除码间串扰,实现码元独立最大似然解码。发送端采用交织技术,进一步提高了性能,实现了双码元联合最大似然解码。和获得满分集增益的星座图旋转方案不同,预编码方案最大程度上减少了码间串扰,提高准正交码的性能。仿真结果表明,基于预编码的两种准正交码性能好于星座旋转准正交码,而且降低了解码复杂度。     

采用相位判决的低复杂度广义空间调制检测算法

针对广义空间调制(GSM)系统中信号检测复杂度过高的问题,提出了一种基于相位判决的低复杂度检测算法.首先根据一种排序准则对天线组合进行排序,然后将排序后的天线组合中的符号向量依次通过基于相位判决的迫零(ZF)均衡器进行检测,最终得到星座调制符号和激活天线组合.分析和仿真结果表明,该检测算法可以有效缩小接收端的搜索范围,在提供与最大似然(ML)检测算法相近的误比特率(BER)性能的同时,计算复杂度降低了98％.     

Detection of OFDM Signals in Fast-Varying Channels With Low-Density Pilot Symbols

In fast-varying channels, an orthogonal frequency-division multiplexing system needs to insert denser pilot symbols among transmitted symbols in tracking the variation of a channel. However, using denser pilot symbols reduces transmission throughput. In this paper, we propose a pseudopilot algorithm for data detection in fast-varying channels without increasing the pilot density. Our algorithm is based on a regressional model-based least-squares-fitting approach. Within a block of received symbols, we select some data symbols and regard them as pseudopilot symbols. The receiver considers all the possible patterns of the pseudopilots and associates each of them with a data sequence and a corresponding metric. The associated data sequence, whose metric is minimum, is selected as the detected data sequence. Our algorithm is not based on a decision-directed or decision-feedback architecture because the pseudopilots do not come from any detected symbols. The proposed algorithm needs to search all the possible patterns of the pseudopilots, and the complexity may increase with the number of pseudopilots and constellation size. To reduce the number of search, we further propose two modified approaches. The simulation results show that the performance of the proposed algorithms could approach a bit-error probability lower bound that is obtained by letting the receiver know the true values of the pseudopilots. Compared with the linear interpolation method, the proposed algorithm shows obvious improvement in fast-varying channels. The proposed modified approaches could also effectively reduce the number of search while maintaining the performance. We also give the complexity analysis of the proposed algorithm and an approach to determine the degree of the regression polynomial.     

Mixed-Q linear space-time codes

A new modulation method for linear space-time codes is proposed based on using constellations of different sizes for different symbols. It is shown that the proposed method significantly reduces the complexity of the sphere decoding algorithm. The complexity reduction is more pronounced in high-rate codes, where each code matrix carries a large number of symbols. We also show that the choice of constellation size provides a tradeoff between performance and complexity. Using this, some guidelines for choosing constellation size are presented. As one introduces more constellation disparity in the code, the complexity is further reduced, while the performance loss grows. Typically, a complexity reduction of one to two orders of magnitude can be achieved at the expense of about 3 dB coding gain. We suggest a simple modification in our design to reduce this loss to about 2 dB.     

Signal constellations for quasi-orthogonal space-time block codes with full diversity

Space-time block codes (STBCs) from orthogonal designs proposed by Alamouti, and Tarokh-Jafarkhani-Calderbank have attracted considerable attention lately due to their fast maximum-likelihood (ML) decoding and full diversity. However, the maximum symbol transmission rate of an STBC from complex orthogonal designs for complex signals is only 3/4 for three and four transmit antennas, and it is difficult to construct complex orthogonal designs with rate higher than 1/2 for more than four transmit antennas. Recently, Jafarkhani, Tirkkonen-Boariu-Hottinen, and Papadias-Foschini proposed STBCs from quasi-orthogonal designs, where the orthogonality is relaxed to provide higher symbol transmission rates. With the quasi-orthogonal structure, the quasi-orthogonal STBCs still have a fast ML decoding, but do not have the full diversity. The performance of these codes is better than that of the codes from orthogonal designs at low signal-to-noise ratio (SNR), but worse at high SNR. This is due to the fact that the slope of the performance curve depends on the diversity. It is desired to have the quasi-orthogonal STBCs with full diversity to ensure good performance at high SNR. In this paper, we achieve this goal by properly choosing the signal constellations. Specifically, we propose that half of the symbols in a quasi-orthogonal design are chosen from a signal constellation set A and the other half of them are chosen from a rotated constellation e/sup j/spl phi// A. The resulting STBCs can guarantee both full diversity and fast ML decoding. Moreover, we obtain the optimum selections of the rotation angles /spl phi/ for some commonly used signal constellations. Simulation results show that the proposed codes outperform the codes from orthogonal designs at both low and high SNRs.     

GA-PTS Using Novel Mapping Scheme for PAPR Reduction of OFDM Signals Without SI

In recent time orthogonal frequency division multiplexing (OFDM) has proved its mettle as a preferred choice for high speed transmission in wireless applications due its efficient mechanism to combat the inter symbol interference. However sudden high peaks in OFDM signal envelope lead to high peak to average power ratio (PAPR). Enhanced values of PAPR result into complex RF amplifier circuit with reduced efficiency. There are various methods available for PAPR reduction, out of them partial transmit sequence (PTS) is most effective proven choice for PAPR reduction. However PTS technique requires cumbersome searching of all possible phase factors to find optimal phase factor which produces lowest PAPR, the information regarding optimal phase set used at the transmitter need to be sent to the receiver as side information (SI) for decoding purpose, however transmitting SI requires additional transmission bandwidth thus reducing overall bandwidth efficiency. To reduce the exhaustive searching genetic algorithm (GA) could be used with PTS leading to GA-PTS system. In this paper a GA-PTS system is proposed which uses novel octagonal geometry for constellation extension purpose. This scheme does not require transmission of any side information for decoding purpose at receiver and at the same time GA-PTS system reduces the required number of searches. Simulations are presented to show that proposed scheme provides similar PAPR performance as conventional PTS but without need of SI transmission at reduced number of searches.     

A full-rate, full-diversity four-antenna quasi-orthogonal space-time block code

We present a complex, full-rate quasi-orthogonal space-time block code for four transmit antennas. Using carefully tailored constellation phase rotations, we show that this code achieves full diversity for specialized PSK-based constellations. The optimal receiver for the new code decouples the symbol detection problem into pairs of symbols, thus greatly reducing complexity. Finally, we present and compare performance of the new code with several other codes in the literature. The new code is shown to perform as well as the best known code of its class.     

FrFT-OFDM系统的低复杂度峰均功率比抑制技术研究

针对基于分数阶Fourier变换的OFDM系统(简称FrFT-OFDM系统)的高峰均功率比(PAPR)问题,该文提出一种低复杂度的峰均比抑制算法。通过对随机相位序列采用周期延拓至FrFT-OFDM符号长度,相位因子加权后与子载波调制前的数据相乘的方式,实现对高峰均比的有效抑制。该算法只需要一次逆离散分数阶Fourier变换(IDFrFT),所有备选信号直接通过时域chirp圆周移位的加权和得到。仿真结果表明,当备选信号个数相同时,该算法与选择映射(SeLecting Mapping, SLM)算法的PAPR抑制性能相当,比部分传输序列(Partial Transmit Sequence, PTS)算法具有更好的PAPR抑制性能,同时,该算法较SLM和PTS算法的运算量降低。     

广义正交码索引调制系统的性能分析

为了提高码索引调制（code index modulation,CIM）系统的传输效率,提出了一种具有更低复杂度的单输入单输出（single input single output,SISO）的广义正交码索引调制（generalized orthogonal code index modulation,GQCIM）系统。CIM系统使用扩频码和星座符号传输信息,但只能激活两个扩频码索引和一个调制符号。而GQCIM系统以一种新颖的方式克服了只激活一个调制符号的限制,同时充分利用了调制符号的正交性,增加扩频码索引以传输更多的额外信息位,提高了系统的传输效率。此外,分析了GQCIM系统的理论性能,推导了误码率性能的上界。通过蒙特卡罗仿真验证了GQCIM系统的性能,对比发现GQCIM系统的理论和仿真性能一致。而且在相同的传输效率下,结果显示GQCIM系统的性能优于同样具有正交性的调制系统,如广义码索引调制（generalized code index modulation,GCIM）系统、CIM系统、码索引调制-正交空间调制（code index modulation aided quadrat...     

Errors and erasures decoding of BCH and Reed-Solomon codes for reduced M-ary orthogonal signaling

The paper presents a comparison of communication systems using different signal constellation sizes and Reed-Solomon or Bose-Chaudhuri-Hocquengem codes with different rates so that the overall required bandwidth is the same for each system. In these comparisons, the channel symbol size is smaller than the code symbol size, so that a code symbol contains parts of multiple channel symbols. Thus, the normal assumption of independent code symbols does not apply. Instead, consideration must be taken to obtain the best arrangement of channel symbols in each code symbol. Analytical expressions are developed to compare the bit error probability performance of comparable systems, based on individual codewords using errors-only decoding and errors and erasures decoding with transmission over a Rayleigh fading channel.