单载波频域均衡在地空高速数据链的应用

地空数据链的信道属于低仰角的多径信道,存在较严重的符号间干扰。正交频分复用(OFDM)是得到认可的有效抗多径方案,单载波频域均衡(SC-FDE)采用与OFDM相似的频域均衡方式,具有与OFDM相近的性能和复杂度,并克服了OFDM的一些不足。在比较SC-FDE和OFDM相似性、分析SC-FDE的信道估计与均衡基础上,提出了采用SC-FDE的地空高速数据链解调器结构,该结构在保留单载波处理的优点条件下实现高效的频域均衡。     

Cyclic Intersymbol Interference of Frequency-Domain Decision-Feedback Equalizer: Effect and Mitigation

This paper investigates an inherent but adverse phenomenon called cyclic intersymbol interference (cyclic ISI) occurred in frequency-domain decision-feedback equalizer (FD-DFE). The cyclic ISI is due to the cyclic-prefix insertion and modulo operation of the time-domain feedback filter. To analyze and further mitigate this problem, we first formulate the FD-DFE signal model in a compact and insightful vector-matrix form, which allows an efficient derivation of the MMSE receiver coefficients. Then, by viewing the bit-error-rate profile (BERP) with respect to different starting symbol position within a symbol block, it is shown that those leading data symbols suffer more from the cyclic-ISI effect and lead to a high error floor. Seeing this, a novel FD-DFE receiver structure, called P2B-FD-DFE (parallel-two-branch FD-DFE) receiver, is proposed to overcome this problem without sacrificing spectral efficiency. It consists of two parallel cooperating branches of almost identical FD-DFE receivers designed for different cyclic decision delays. Under severe cyclic-ISI circumstances, the receiver performance can be further improved by using (1) a larger block size, and/or (2) a cascade of multiple feedback stages. Simulation results demonstrate that, in severe ISI channel, while the conventional FD-DFE exhibits a very high error floor, our proposed receiver can almost completely suppress the cyclic-ISI effect. Rih-Lung Chung was born in Taoyuan, Taiwan R.O.C. He received the B.S. degree and finished the M.S. courses both in electrical engineering from Yuan-Ze University, Taiwan R.O.C., in 1998 and 1999, respectively. Now he is working on his doctoral degree. His research interests include digital communication, statistical signal processing, and channel estimation/equalization, especially for wireless communication system. Jeng-Kuang Hwang was born in Taipei, Taiwan, in 1962. He received the Ph.D. degree in electrical engineering from the National Tsing-Hua University, Hsin-Chu, Taiwan, in 1991. Currently, he is an associate professor with the Communication Engineering Department at Yuan-Ze University, Chung-Li, Taiwan. In 1997, he had been a visiting professor at AT&T Labs/Research, Redbank, NJ, doing researches on smart antennas for wireless communications. He has published over 70 journal and conference papers, and hold one US patent. His current research interests include communication and statistical signal processing, smart antennas, software-defined radios, VLSI signal processing, and power line communications.     

Receiver Design for Single-Carrier Block Transmission Over Doubly Selective Channels

Single-carrier block transmission is an alternative scheme to orthogonal frequency-division multiplexing (OFDM) for wireless broadband communications. In this paper, a receiver is designed for single-carrier block transmission with cyclic prefix for mobile broadband communications. As the wireless transmission is over doubly selective channels, a basis expansion model is used to capture both the time- and frequency-selectivity of the channel and is parameterized for the receiver design. The receiver estimates the channel model coefficients in the time domain and uses these coefficients for equalization in the frequency domain. The channel estimation is assisted by time-domain pilot insertion. The structure of the frequency-domain channel matrix is exploited and a linear minimum mean-square error equalizer is used for the equalization. When the basis expansion model well matches the physical channel, simulation results show superior receiving performance of the proposed system compared with the OFDM system with a similar complexity.     

Differential modulation techniques for a 34 MBit/s radio channel using orthogonal frequency division multiplexing

The orthogonal frequency division multiplexing (OFDM) technique has been proposed for terrestrial digital transmission systems due to its high spectral efficiency, its robustness in different multipath propagation environments and the ability of avoiding intersymbol interference (ISI). Our studies consider a radio channel bandwidth of 8 MHz and a data rate of 34 Mbit/s.In the case of the OFDM transmission system a coherent 64-QAM requires a channel estimation process and a channel equalization in frequency-selective interference situations [4]. The equalization process can be realized by a multiplier bank at the FFT output in the receiver, a so-called frequency-domain equalizer. Alternatively, a multilevel differential modulation technique, the so-called differential amplitude and phase shift keying (64-DAPSK) considering the phase and simultaneously the amplitude for differential modulation, is proposed and presented in this paper. Differential modulation/demodulation techniques do not require any explicit knowledge about the radio channel properties in the differential channel equalization. It is therefore not necessary to implement a frequency-domain equalizer in an OFDM/64-DAPSK receiver, which reduces the computation complexity. The performance of both modulation techniques has been analysed in the uncoded and coded case referring to Gaussian and frequency-selective Rayleigh fading channels. Simulation results are presented in this paper.The OFDM signal has a non-constant envelope with large instantaneous power spikes possible primarily resulting in an overdriving of the high power amplifier (HPA) at the transmitter. This leads to nonlinear distortion causing intermodulation noise and spectral spreading. Both effects can be limited by introducing an appropriate input backoff (IBO). In this paper the performance of OFDM signals in the presence of nonlinearities is analysed quantitatively.     

Transmission techniques for digital terrestrial TV broadcasting

The authors discuss the potential of OFDM signaling, with its limitations and inherent problems, as well as another potential technique that has so far been overlooked: single-carrier transmission with frequency-domain equalization. The carrier synchronisation issue is dealt with before the authors introduce coded-OFDM (COFDM), which makes use of channel coding and frequency-domain interleaving     

OFDM系统频偏自适应跟踪算法的研究

为了减小正交频分复用系统受频率偏差的影响,在OFDM系统中设计了一种新的载波频偏估计算法.频偏估计包括跟踪和捕获两个阶段,文中采用变步长自适应算法来进行跟踪,这个跟踪算法的捕捉范围是整个信号带宽的一半.通过此方法,不但可以利用前向纠错的方式来避免反馈,而且可以缩减复数乘法的次数以降低计算复杂度和系统功耗.该方法运算复杂度低,并具有较好的收敛性,能改善ODFM的解调性能.从计算机的仿真效果可以看出无论是在AWGN信道上还是在多径信道上都表现出了卓越的性能.     

A Comparison of Frequency-Domain Block MIMO Transmission Systems

Block transmission techniques, with appropriate cyclic prefix and frequency-domain processing schemes, have been shown to be excellent candidates for digital transmission over severely time-dispersive channels, allowing good performance with implementation complexity that is much lower than traditional time-domain processing schemes. Orthogonal frequency-division multiplexing (OFDM) modulation is the most popular block transmission technique. Single-carrier (SC) modulation using frequency-domain equalization (FDE) is an attractive alternative approach based on this principle. In this paper, we propose two new receiver structures for multiple-input-multiple-output (MIMO) channels employing SC (MIMO-SC) modulation and FDE schemes. These receivers have a hybrid structure with frequency-domain feedforward and time-domain feedback filters for intersymbol interference (ISI) and interference cancellation. The proposed schemes are compared with different MIMO systems employing OFDM modulation (MIMO-OFDM) receivers in terms of performance [bit error rate (BER) and throughput] and complexity. Our performance results show the superiority of MIMO-SC approaches relative to MIMO-OFDM in terms of the BER performance for the simulated scenarios. Also, the simulation results show that the proposed hybrid MIMO-SC receivers yield a higher throughput than a MIMO-OFDM system.     

Blind Receiver Design for OFDM Systems Over Doubly Selective Channels

We develop blind data detectors for orthogonal frequency-division multiplexing (OFDM) systems over doubly selective channels by exploiting both frequency-domain and time-domain correlations of the received signal. We thus derive two blind data detectors: a time-domain data detector and a frequency-domain data detector. We also contribute a reduced complexity, suboptimal version of a time-domain data detector that performs robustly when the normalized Doppler rate is less than 3%. Our frequency-domain data detector and suboptimal time-domain data detector both result in integer least-squares (LS) problems. We propose the use of the V-BLAST detector and the sphere decoder. The time-domain data detector is not limited to the Doppler rates less than 3%, but cannot be posed as an integer LS problem. Our solution is to develop an iterative algorithm that starts from the suboptimal time-domain data detector output. We also propose channel estimation and prediction algorithms using a polynomial expansion model, and these estimators work with data detectors (decision-directed mode) to reduce the complexity. The estimators for the channel statistics and the noise variance are derived using the likelihood function for the data. Our blind data detectors are fairly robust against the parameter mismatch     

16DAPSK+OFDM及其在数字调幅广播中应用

本文提出一种新的频域16DAPSK＋OFDM。并讨论它的频域差分调制和解调,然后在白高斯信道分析它的误比特性能。同时域16DASK＋OFDM相比,频域16DAPSK＋OFDM可用于信道特性随时间变化更快的应用。最后,在典型的调幅（AM）波段信道对两者误比特性能进行了仿真。结果表明,时域和频域16DPSK＋OFDM都可用于中短波信道。在中波信道,时域16DAPSK比例域16DAPSK性能要好,在短波信道,频域16DAPSK比时域16DAPSK性能要好。另外,两者调制和解调的计算复杂度基本一样,且都不需要信息道均衡。     

Accurate error-rate performance analysis of OFDM on frequency-selective Nakagami-m fading channels

Error rates of orthogonal frequency-division multiplexing (OFDM) signals in multipath slow fading Nakagami-m fading channels are considered. The exact probability density function of a sum of Nakagami-m random phase vectors is used to derive a closed-form expression for the error rates of OFDM signals. The precise error-rate analysis is extended to a system using multichannel reception with maximal ratio combining. An asymptotic error-rate analysis is also provided. For a two-tap channel with finite values of Nakagami-m fading parameters, our analysis and numerical results show that the asymptotic error-rate performance of an OFDM signal is similar to that of a single carrier signal transmitted over a Rayleigh fading channel. On the other hand, our analysis further shows that a frequency-selective channel that can be represented by two constant taps has similar asymptotic error-rate performance to that of a one-sided Gaussian fading channel. It is observed that, depending on the number of channel taps, the error-rate performance does not necessarily improve with increasing Nakagami-m fading parameters.     

Non-cooperative signal modulation recognition algorithm based on joint feature parameter extraction

Aiming at the problem that in the current electromagnetic environment,the modulation method is complicated,the frequency-consuming equipment increases,the spectrum is congested,and the electromagnetic environment interference increases,the algorithm of OFDM signal detection and subcarrier identification in the background of non-cooperative communication were deeply studied.Using the different distribution states of OFDM signals and single carrier signals in the time domain,a joint characteristic parameter was proposed to solve the existence problem of OFDM in the received signal.For the phase shift and frequency offset problems caused by the channel transmission to the signal,by using the periodic stability the blind parameter estimation was performed to obtain the signal prior information.On the basis of the obtained signal prior information,a multi-level classification and recognition method for non-cooperative OFDM signal sub-carrier signals was proposed.Therefore,a model based on non-cooperative communication system OFDM signal detection and subcarrier modulation identification was designed,and finally modulation identification of unknown signals was completed.Simulation experiments show that in non-cooperative communication systems,OFDM signals and single-carrier signals can be accurately identified,and ideal modulation recognition effects can be achieved on empty subcarriers,QPSK,and 16QAM in the receiver OFDM signal subcarriers,overcoming the channel transmission band The problems of phase shift and frequency offset have improved the accuracy of modulation mode identification.     

Cyclic prefixed single carrier transmission in ultra-wideband communications

This letter proposes cyclic prefixed single carrier transmission with frequency domain equalization (SC-FDE) as an alternative physical layer solution for UWB communications. The performance of SC-FDE over IEEE 802.15.3a UWB channel models is analyzed, simulated and compared with that of impulse based single carrier UWB (SC-UWB) and multicarrier UWB employing orthogonal frequency division multiplexing (OFDM-UWB). The impact of channel coding on the performance of OFDM and SC-FDE in UWB is also studied. Our results demonstrate performance advantage of the SC-FDE scheme, especially when implementation issues such as low complexity and low power consumption for UWB are taken into consideration. The performance of SC-FDE with diversity combining using oversampling is also investigated.     

一种基于基扩展模型的OFDM频域快时变信道估计方法

现有的OFDM系统快时变信道估计方法均为时域估计方法,假定信道为整数倍采样信道,利用理想的等间隔均匀导频恢复信道时域响应函数,进而采用基扩展模型拟合并估计其时变特性。而实际的信道通常为非整数倍采样信道,频带内可用的导频也并非假定的理想模式。此时,恢复的信道时域响应出现能量泄漏,导致算法性能大大受限。本文提出一种频域快时变信道估计方法,利用信道频域时变传输函数辅助估计,从而估计得到信道频域响应矩阵。该方法在频域实现,性能不受信道时域响应能量泄露影响。仿真实验从误码率、均方误差两方面分别验证了该方法的有效性。     

基于数据样本方差的正交频分复用水声通信多普勒频移估计方法

针对正交频分复用(OFDM)水声移动通信易受时变多普勒频移影响的缺点,该文提出一种基于数据样本方差的多普勒频移估计方法。利用前序符号的信道估计值恢复当前符号的有效数据序列及其频域分集副本,计算分集副本与数据序列的比值并搜索该比值序列在不同多普勒补偿因子下的方差,选取方差最小时对应的补偿因子作为多普勒频移估计值,利用稀疏贝叶斯学习和判决反馈信道估计算法获得修正后的信道频域响应并传递给后序符号,实现对多普勒频移的实时跟踪。数值仿真验证了该方法的可行性和优越性,海上试验证明,该方法实现了基于无人水下航行器的OFDM水声移动通信,能够对时变多普勒频移进行有效估计。     

Frequency-Domain Channel Estimation and Equalization for Continuous-Phase Modulations With Superimposed Pilot Sequences

This paper proposes a new frequency-domain channel-estimation and equalization method for continuous-phase modulation (CPM) block transmissions with superimposed pilot signals. Our method provides spectral and power-efficient broadband CPM wireless communications with less complexity than previous methods. The proposed frequency-domain channel estimation uses the superimposed pilot sequence as a reference signal to reduce the throughput loss caused by traditionally multiplexed pilots. The proposed CPM frequency-domain decision feedback equalizer (DFE) eliminates the complexity overhead of conventional decomposition-based CPM receivers.      

A subspace blind channel estimation method for OFDM systems without cyclic prefix

We propose a subspace based blind channel estimation method for orthogonal frequency-division multiplexing (OFDM) systems over a time-dispersive channel. Our approach is motivated by the resemblance of the multichannel signal model resulting from oversampling (or use of multiple receive sensors) of the received OFDM signal to that in conventional single carrier system. The proposed algorithm distinguishes itself from many previously reported channel estimation methods by the elimination of the cyclic prefix, thereby leading to higher channel utilization. Comparison of the proposed method with other two reported subspace channel estimation methods is presented by computer simulations to support its effectiveness.     

Multicarrier modulation with blind detection capability using cosine modulated filter banks

The cosine modulated filter bank (CMFB) is introduced as a multicarrier modulation (MCM) technique for wideband data transmission over wireless channels. Under the name discrete wavelet multitone modulation, CMFB has been considered for data transmission over digital subscriber lines. We propose a new receiver structure that is different from those proposed previously. The new structure simplifies the task of channel equalization, by reducing the number of equalizer parameters significantly. We also propose a novel blind equalization algorithm that fits very nicely in the proposed structure. Moreover, we discuss the bandwidth efficiency of the proposed CMFB-MCM system and show that it is superior to the conventional (single carrier) quadrature amplitude modulation (QAM) and orthogonal frequency-division multiplexing (OFDM). The CMFB is found to be a signal processing block that stacks a number of vestigial sideband modulated signals in a number of overlapping subchannels in the most efficient way. The proposed CMFB-MCM is also compared to OFDM with respect to bit-error rate performance. Under the conditions that the channel impulse response duration remains less than the length of cyclic prefix, OFDM is found marginally superior to CMFB-MCM. However, OFDM degrades very fast when the channel impulse response duration exceeds the length of the cyclic prefix. CMFB-MCM, on the other hand, is found less sensitive to variations in channel impulse response duration.     

基于正交频分复用的频域差分幅度相位调制

该文提出一种基于频域的幅度两电平的DAPSK+OFDM,并讨论该调制方式在频域的差分调制和解调,然后给出它在白高斯信道的误比特性能公式。最后在典型的调幅(AM)波段信道对它和基于时域的DAPSK+OFDM的误比特性能进行了仿真。结果表明:在短波信道,频域的DAPSK+OFDM比时域的DAPSK+OFDM性能好;在中波信道,时域的DAPSK+OFDM比频域的DAPSK+OFDM性能好。因此,数字调幅广播系统在短波信道可采用频域DAPSK+OFDM,在中长波信道可采用时域DAPSK+OFDM     

Fundamental Limit of OFDM Range Estimation in a Separable Multipath Environment

In this paper, orthogonal frequency division multiplexing (OFDM) for time-based range estimation (TBRE) in a separable multipath channel is investigated and analyzed with respect to its accuracy. First, the Cramer–Rao lower bound (CRLB) in a separable multipath channel is theoretically derived, and indicates a similar expression to that for a single path channel. The CRLB for non-data-bearing (NDB) OFDM transmission is compared to that for pseudo-noise (PN) transmission, demonstrating a large performance gap in favor of the NDB OFDM. Furthermore, the maximum likelihood estimator (MLE) for TBRE in a separable multipath channel is theoretically derived, also demonstrating a similar expression to that in a single path channel, except that several peaks instead of one peak are expected in a separable multipath channel corresponding to all arrival paths. The MLE for TBRE is then compared to the commonly used MLE for channel estimation, showing an equal performance in terms of mean square error when using an NDB OFDM transmission. Simulation results demonstrate a good agreement with our proposed theory.     

OFDM系统中时变信道下信道估计方法的研究

为减少OFDM系统中子信道间干扰(ICI)的影响,分析了分段频域均衡方法、近似分段线性的相邻符号ICI消除方法和卡尔曼滤波方法等几种快衰落环境下的信道估计方法的优劣,在此基础上提出了一种基于梳状导频的卡尔曼滤波信道估计方法.对滤波的结果,利用信道的频域相关函数和估计误差的协方差矩阵,在频域进行最小均方误差意义上的优化.仿真结果表明,该方法具有良好的估计性能.