多径信道下TD-LTE系统中定时同步算法的研究

多径传播是无线信道所固有的特性,信道时延的存在将会导致符号间干扰(ISI),严重影响接收端信号的解调.本文重点研究了多径信道下TD-LTE系统中的定时同步模块,并对基于循环前缀(CP)的能量差分算法加以改进,联合主同步信号(PSS)精确估计定时同步位置.仿真结果表明,该算法在多径衰落信道下能获得较好的定时同步性能,即使在信道时延较大的情况下也能够稳定地工作.     

MSK信号的最大似然检测

最小频移键控(Minimum Shift Keying,MSK)是一种连续相位的频移键控。MSK信号包络恒定,相干检测时的误码率性能比一般的频移键控要好。MSK在引入倾斜相位的概念后,其相位格图类似于卷积码的时不变网格图,因此能用Viterbi算法实现最优解调。理论分析和数值模拟表明,在相位与时间严格同步的条件下,MSK基于Viterbi算法的最大似然检测法性能更好,是最优的解调方法。这里仅考虑在加性高斯白噪声信道下信号传输的相干检测。     

短波高速QAM信号解调算法的研究

文中针对短波高速QAM信号,提出并实现了一种全数字的解调方案。采用了最大平均功率定时同步算法和最大似然载波相位估计算法。利用同步导频使信号的眼图张开,进而对数据段进行信道均衡与相位补偿。采用判决反馈模式的非线性均衡器,更好地补偿了信道畸变。这种前向式解调结构减小了算法的复杂性,易于DSP实现。Matlab仿真结果表明该方案有效可行。     

两种定时同步算法的讨论与分析

介绍基于最大似然准则的定时同步算法,通过仿真发现该算法应用于AWGN信道效果很好,但是应用于平坦衰落信道效果相对较差。为了解决信号在平坦衰落信道传输过程中遇到的一系列问题,对基于循环平稳特性的定时同步算法进行了研究和改进。仿真结果表明该算法可以很好地适应于衰落信道。     

多径衰落信道MPSK信号超指数迭代盲解调算法

为了提高多径衰落信道下的盲解调性能,提出了一种结构简单的MPSK信号盲解调算法。首先利用超指数迭代分数间隔盲均衡器实现联合定时同步与均衡,然后对均衡器输出信号进行非线性变换实现载波频偏的估计,最后利用二阶数字判决锁相环跟踪相位变化纠正剩余频偏和相偏。仿真结果表明,在多径衰落信道条件下,与现有算法相比,基于超指数迭代分数间隔盲均衡器的盲解调算法实现简单,误码率低,而且具有收敛速度快、性能稳定等优点。     

一种短波衰落信道下FSK信号FFT解调方法

对传统FSK信号的多种解调方法进行了简单介绍,在短波衰落信道下,由于多径和衰落的影响,其解调性能严重恶化,甚至不能得到正确的比特流。提出了一种基于时频域的FFT的FSK信号解调方法,利用FSK信号不同频率点处的功率谱峰值进行检测判决,并详细介绍了其实现方法。对其在高斯白噪声下的性能进行了仿真,性能与非相干包络解调理论性能相当,而对衰落信号,相比其他解调方法,此解调方法也可以给出较好的解调效果,适用于工程应用。     

基于Chebyshev混沌序列的正交宽带调制解调方法

针对常规的相干解调和非相干解调在水声信道通信中可靠性差的问题,文中提出了一种基于Chebyshev混沌序列的正交宽带调制解调方法。该方法基于海洋水声信道环境与传播特点,采用统计特性接近环境噪声且具有尖锐自相关、宽带特性的Chebyshev混沌序列作为信息载体和同步信号,并利用欧氏距离分类器对接收信号进行非相干分类判决解调。该方法可有效抑制水声信道因多途传播而引起的频率选择性衰落和码间干扰,从而更好地实现低信噪比条件下可靠水声通信。仿真实验证明,在-10 dB以上信噪比环境下,文中所提通信方法在具有多途效应的水声信道上具有良好的通信性能。     

一种新型的基于块自适应滤波的MSK信号解调技术

本文针对MSK调制信号在军用和民用领域都得到了广泛应用的这一事实,首先简单地分析了MSK信号非相干解调性能和差分解调性能的不足.然后根据MSK数字调制信号的特点,将频域块自适应滤波技术和码元FFT求模判决算法相结合,提出了一种新型的MSK信号数字解调算法.最后在详细阐述新型MSK信号解调算法的基础上,利用计算机基于蒙特卡洛方法对新算法的解调性能进行了性能仿真,并对仿真结果进行了较为详细的分析.计算机仿真结果表明该新算法不仅简单有效,而且对噪声有很好的抑制作用.与经典的非相干解调和差分解调相比,该新算法具有较低的解调误比特率和较低的频偏敏感度,并且解调性能得到了明显的提高,具有一定的实际应用价值.     

短波低速率数据传输研究

以第三代短波通信网络标准中的低速数据传输规程为基础,设计了一种低信噪比且存在多径衰落的信道中数据传输的调制解调方案.方案中利用相关搜索、FFT捕获的方法捕获信号,利用滑动相关完成位同步和信道估计,利用RAKE接收机抗多径衰落,并用DSP实现了数据的低误码率传输.     

一种无需导频的适用于差分OFDM系统的符号与采样钟联合同步方法

该文提出了一种适用于OFDM系统的联合符号和采样钟同步校正方法,其中同步校正是在数字域通过改变对接收过采样信号的插值(interpolation)和抽取(decimation)实现的。这种方法在发送端相邻载波间采用差分QPSK调制,在接收端利用QPSK的差分解调信号获得同步误差信号,从而获得关于OFDM符号同步和采样钟同步调整的算法,其特点是无需专门的同步导频信号。所提出算法的同步性能在高斯白噪声信道和多径衰落信道均得到验证。     

Multifrequency minimum shift keying

A new modulation scheme that produces a constant envelope continuous phase signal set with a compact power spectrum and power efficiency better than that of MSK is proposed. The scheme can be implemented by quadrature-carrier multiplexing of two frequency/phase modulated signals of the type NFSK/2PSK, both with the same frequency in each transmission interval, and with sinusoidal symbol shapes. The generated signal can be viewed in each transmission interval as an MSK signal at one of the N frequencies and is referred to as multifrequency minimum shift keying (MF MSK). Modulation, demodulation, and synchronization circuits are described, and the spectral properties and power efficiency on the AWGN channel are analyzed. Combining a number of attractive attributes such as constant envelope, excellent spectral properties, high power efficiency, and self-synchronization capability, the proposed modulation format lends itself to a variety of applications, one of them being the digital satellite link     

Measurements and models of a land mobile satellite channel and their applications to MSK signals

The fading and shadowing effects observed on land mobile satellite signals are characterized in statistical terms. Models are developed in terms of probability distribution of the signal's envelope and phase as well as its rate of change with time. The multipath fading and shadowing effects modelled are typical of those encountered in a mobile-satellite link in rural and suburban areas. A comparison of the models with experimental data at 870 MHz and 1542 MHz is given. Applications of the models to predict performance of a minimum shift keying (MSK) signal at 2400 bits/s show that large margins are required to compensate for the effects of fading and shadowing. In addition, the results show that conventional coherent demodulation of a MSK signal may not be feasible due to phase variation caused by fading and shadowing. On the other hand, the results show that random FM has negligible effect on the probability of error of the MSK signal at 2400 bits/s when frequency demodulation is used.     

MSK信号的数字解调新方法

近年来通信信息的数字解调技术引起了人们的广泛关注，本文根据MSK信号的数学特征，提出了一种基于MSK信号特殊点FFT的实部或虚部比较判决的方法来实现MSK信号的数字解调。推导了在高斯白噪声背景下该方法的误码情况。计算机仿真结果表明该算法是简单、有效的。与传统的MSK最佳解调方法相比本算法对载波频偏不敏感，可以应用于一些通信侦察和截获的场合。     

Performance of sequence estimation scheme of narrowband digital FMsignals with limiter-discriminator detection

A communication scheme using binary FM with noncoherent limiter-discriminator detection has been well known. Up to now, the improvement of bit error rate at the receiver side has been carried out through the bandwidth optimization of the IF filter, the decision feedback equalization (DFE), or simple two-state maximum likelihood sequence estimator (MLSE). This channel is inherently the intersymbol interference (ISI) channel due to the premodulation baseband filtering as well as the narrowband IF filtering. So the sequence estimation scheme using the Viterbi algorithm can be applied successfully, although the channel is not additive white Gaussian and maximum likelihood in the strict sense. In this paper, through computer simulations, we examine the actual BER improvement of the sequence estimation scheme with multiple-state trellis especially for MSK and GMSK signals. We mainly consider static AWGN and frequency nonselective Rician fading channels. Consequently, by adjusting the IF filter bandwidth, very large estimation gains are obtained compared to the conventional DFE or MLSE detector for AWGN and Rician fading channels. This scheme does not produce large demodulation delay and is implemented only by adding the signal processing part to the final stage of the receiver. This scheme seems to be very useful for any applications including satellite mobile channels     

一种新的基于前导序列的MSK位定时同步方法

数字通信系统中,位定时同步是实现正常数字通信的必要条件.本文基于1bit差分解调算法,提出了一种基于前导序列的位定时同步方法.基于前导序列的自相关性,该方法首先将差分解调数据划分为若干并行支路,然后利用并行滑动相关法搜索各支路前导序列的自相关峰,最后根据大数判决方法完成位定时同步.通过Matlab仿真表明,该方法性能接近Monte Ca]or仿真.     

Preamble and embedded synchronization for RF carrier frequency-hopped OFDM

In this work, we apply RF carrier frequency hopping to orthogonal frequency-division multiplexing (FH-OFDM). Achievable hop rate and bandwidth efficiency are determined based on signal acquisition/synchronization and data demodulation performance in the presence of unknown time-frequency offsets, and channel gain/phase perturbations. We compare performance using two different data-aided synchronization approaches. The first method sends synchronization information in a preamble before the OFDM payload symbol, whereas the second method embeds the synchronization information directly into the OFDM symbol stream. In the embedded technique, superposition of the synchronization information causes interference onto the OFDM data information. Thus, the sync information must be removed before satisfactory bit-error rate (BER) performance can be achieved. Consequently, embedded interference cancellation (EIC) is utilized which requires accurate estimation of the synchronization offsets and channel perturbations. Using coherent quadrature phase-shift keying-OFDM modulation, performance comparisons are presented using the COST207 multipath fading channel model. Fading channel BER performance results indicate that the embedded technique incurs only a slight signal-to-noise ratio penalty (less than 1 dB) compared with the preamble method. However, the embedded method offers the potential for improved hop rate and bandwidth efficiency because no dedicated slot is required for a synchronization field.     

Inherent limitations in data-aided time synchronization ofcontinuous phase-modulation signals over time-selective fading channels

Time synchronization of continuous phase modulation (CPM) signals over time selective, Rayleigh fading channels is considered. The Cramer-Rao lower bound (CRLB) for this problem is studied for data-aided (DA) synchronization (i.e., known symbol sequence transmission) over a time-selective Rayleigh fading (i.e., Gaussian multiplicative noise) channel. Exact expressions for the bound are derived as well as simplified, approximate forms that enable derivation of a number of properties that describe the bound's dependence on key parameters such as signal-to-noise ratio (SNR), channel correlation, sampling rate, sequence length, and sequence choice. Comparison with the well-known slow fading (i.e., constant) channel bound is emphasized. Further simplifications are obtained for the special case of minimum phase keying (MSK), wherein it is shown how the bound may be used as a sequence design tool to optimize synchronization performance     

一种单天线同频混合信号幅度的估计算法

针对非协作通信中平坦衰落条件下单天线接收两路MSK混合信号分量的幅度估计问题,提出了一种基于max - min思想的幅度盲估计算法.该算法采用混合信号幅值序列中多个极大极小幅值的估计值来进行混合信号分量的幅度估计,并采用加窗局部极值平均的方法来获得多个极大极小幅值的估计值.仿真结果表明:在信噪比大于10 dB条件下,该...     

A low‐complexity MSK‐based M‐ary cyclic‐shift keying transceiver for direct sequence spread spectrum system over multipath fading channels

In this paper, a low‐complexity spread spectrum system with M‐ary cyclic‐shift keying (MCSK) symbol spreading is proposed. In addition, by using the minimum‐shift‐keying (MSK) as the chip‐level modulation, we obtain a high‐rate QPSK‐MCSK transceiver scheme which not only provides a constant‐envelop and continuous‐phase transmitted signal, but can also achieve a better performance than the conventional direct sequence spread spectrum (DSSS) system. At the transmitter, the data stream is first mapped into QPSK‐MCSK symbols in terms of orthogonal Gold code sequences, then followed by the cyclic prefix (CP) insertion for combating the interblock interference, and finally applying the MSK scheme to maintain the constant‐envelope property. The receiver first performs MSK demodulation, then CP removal, and finally the channel‐included MCSK despreading and symbol demapping. Furthermore, the single input single output (SISO) QPSK‐MCSK transceiver can be easily extended to the multiple input single output (MISO) case by incorporating the space–time block coding for high‐link quality. Simulation results show that the proposed SISO and MISO QPSK‐MCSK systems significantly outperform the conventional DSSS counterparts under the AWGN channel, and attain a more robust performance under the multipath fading channel. Copyright © 2011 John Wiley & Sons, Ltd.