Doubly iterative receiver for block transmissions with EM-based channel estimation

Cyclic-prefix code division multiple access (CP-CDMA), multicarrier CDMA (MC-CDMA) and single carrier cyclic-prefix (SCCP) transmission are some schemes that could support the increasing demand of future high data rate applications. The linear and nonlinear equalizers used to detect the transmitted signal are always far from the Maximum-Likelihood (ML) detection bound. The block iterative generalized decision feedback equalizer (BI-GDFE) is an iterative and effective interference cancelation scheme which could provide near-ML performance yet with very low complexity. In order to deploy this scheme, the channel state information (CSI) must be available at the receiver. In practice, this information has to be estimated by using pilot and data symbols. This paper investigates the problem of channel estimation using the Expectation Maximization (EM) algorithm. The BI-GDFE provides the soft information of the transmitted signals to the EM-based algorithm in the form a combination of hard decision and a coefficient so-called the input-decision correlation (IDC). The resultant receiver becomes a doubly iterative scheme. To evaluate the performance of the proposed estimation algorithm, the Cramér-Rao Lower Bound (CRLB) is also derived. Computer simulations show that the bit error rate (BER) performance of the proposed receiver for joint channel estimation and signal detection can reach the performance of the BI-GDFE with perfect CSI.     

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.     

MLSE and MAP Equalization for Transmission Over Doubly Selective Channels

In this paper, equalization for transmission over doubly selective channels is discussed. The symbol-by-symbol maximum a posteriori probability (MAP) equalizer and the maximum-likelihood sequence estimation (MLSE) are discussed. The doubly selective channel is modeled using the basis expansion model (BEM). Using the BEM allows for an easy and low-complexity mechanism for constructing the channel trellis to implement the MLSE and the MAP equalizer. The MLSE and the MAP equalizer are implemented for single-carrier transmission and for multicarrier transmission implemented using orthogonal frequency-division multiplexing (OFDM). In this scenario, a complexity-diversity tradeoff can be observed. In addition, we propose a joint estimation and equalization technique for doubly selective channels. In this joint estimation and equalization technique, the channel state information (CSI) is obtained in an iterative manner. Simulation results show that the performance of the joint channel estimation and equalization approaches the performance when perfect CSI is available at the receiver.     

GMP-Based Channel Estimation for Single-Carrier Transmissions over Doubly Selective Channels

We present a graph-based channel estimation approach for SC-IFDE (single-carrier transmissions with iterative frequency domain equalization) without CP (cyclic prefix) over doubly selective channels using the recently developed Gaussian message passing (GMP) technique. A direct application of the GMP updating rules in the FFG (Forney-style factor graph) of the SC-IFDE system model incurs high complexity. Approximate updating rules are therefore developed to overcome this problem. The proposed GMP-based channel estimation approach has similar complexity as the low-complexity Kalman-filtering based frequency domain channel estimation approach in the literature, but significantly outperforms the latter due to its enhanced capability in capturing the time correlation information of doubly selective channels through bidirectional processing.     

在OFDM系统中用循环前缀对时变色散信道进行估计

目前，正交频分复用（OFDM）技术困能在无线时变信道中进行高速数据传输而受到广泛的关注。在相干的OFDM系统中，接收机能否获得准确的信道状态信息（CSI）是系统性能提高的关键。为了能在接收端获得准确的CSI和提高系统的传输速率，文中将文献[3]的算法由单路发射推广为I/Q两路发射，采用复抽头系数的FIR滤波器对多径衰落信道进行建模，并用通常被丢弃的循环前缀作为训练序列对信道进行估计和均衡，仿真结果表明改进的算法在相同的子数和子载波和比文献[3]传输效率提高一倍的情况下能有效地自动跟踪信道的变化。     

Turbo均衡在Non-CP SC-FDE系统中的应用

针对无循环前缀的单载波频域均衡(Non-CP SC-FDE)系统中的符号间干扰(ISI),提出了一种Turbo均衡与循环重构联合迭代算法。通过循环重构技术消除ISI,并采用频域均衡频域判决反馈(FDE-FDDF)方式,将Turbo均衡算法与循环重构算法结合在一起进行迭代。仿真结果表明,Non-CP SC-FDE系统使用该算法能实现与传统有CP系统近似的误码性能,在大幅提高传输效率的同时能够有效消除多径干扰。     

Frequency-Domain Block Turbo-Equalization for Single-Carrier Transmission Over MIMO Broadband Wireless Channel

This paper presents a new class of block turbo-equalizers for single-carrier transmission over multiple-input multiple-output (MIMO) broadband wireless channels. The key underlying idea consists in equalizing (nonoverlapping) groups of symbols and detecting their individual space-time components in a disjoint and iterative fashion. This functional split naturally induces new design options that have been accurately listed and described, i.e., choice of distinct criteria for intergroup interference (IGI) equalization and intragroup components detection, yielding hybrid structures, multiple iterative loops, and related scheduling variants. Selected algorithms in the proposed class are compared in terms of performance under various transmission scenarios. For all of them, minimum mean square error IGI equalization certainly occupies a central role (at least for the first iteration) and may be identified as the computational bottleneck. Fortunately, block spread transmission together with cyclic prefix operations make the channel matrix block circulant, thus allowing low-complexity inversion in the Fourier domain     

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.     

Interference-Resilient Block-Spreading CDMA With Minimum-MAI Sequence Design

Code-division multiple-access (CDMA) schemes based on block spreading implement the spreading of entire data blocks rather than single symbols, thus achieving a higher robustness against the frequency selectivity of the channel and allowing the use of efficient modulation/equalization schemes operating in the frequency domain (FD). In this paper, we present a new block CDMA (B-CDMA) system where a single cyclic prefix (CP) is used at the end of each spread block. This provides a higher spectral efficiency with respect to existing schemes. By observing that complete orthogonality among users is achievable only for half-loaded systems on dispersive channels, we introduce new criteria for the design of spreading and despreading sequences, which aim at minimizing the mean-square error at the output of the despreader. For the equalization of the received signal, we propose an iterative block decision feedback equalizer, which iterates between equalization and decoding. Equalization filters are designed to minimize the mean-square error and take into account the residual interference due to the nonorthogonality of the spreading sequences. The performance of B-CDMA is evaluated in an uplink wireless scenario and compared to existing CDMA schemes.     

Subblock processing in MMSE-FDE under fast fading environments

Frequency domain equalization (FDE) has been studied for reducing inter-symbol interference (ISI) caused by frequency selective fading in single carrier systems. When a high-mobility terminal exists in the system, the channel state may change within a DFT block. Then, the ISI reduction performance of FDE degrades since cyclicity of the channel matrix is lost. We propose to divide a received data block into multiple subblocks to decrease the channel transition within the DFT block in fast fading environments. Also, to satisfy periodicity of the received signal in each subblock, we introduce a pseudo cyclic prefix technique. The results of numerical analysis show that the proposed method can effectively decrease the error floor in fast fading environments.     

时域均衡在OFDM水声通信系统中的应用

为了对抗水声信道时延扩展大于循环前缀长度时引起的码间串扰问题,在基本的OFDM水声通信系统接收端进行DFT变换前加入相对较短的时域均衡器来限制信道冲激响应的长度。分析了基于MMSE准则的时域均衡算法的实现过程以及算法的复杂程度,对该算法在水声环境下的性能进行了仿真分析,并比较了影响系统性能的因素。结果表明通过加入时域均衡器能够有效对抗信道严重的时间离散性,改善OFDM水声通信系统在循环前缀不足时的系统性能。     

OFDM系统在循环前缀不足时的两种高效迭代均衡方法

本文提出两种新的用于循环前缀(CP)不足时正交频分复用(OFDM)系统的迭代均衡方法。首先,我们提出并行迭代均衡(PIE)方法,该方法分别使用时域判决反馈方法和频域并行迭代方法来消除符号间干扰(ISI)和子载频间干扰(ICI)。为了改进PIE的性能,提出基于高斯-塞德尔迭代的串行迭代均衡(SIE)方法。在不增加计算复杂度的情况下,SIE具有比PIE更快的收敛速度。仿真结果表明,新方法可以在几次迭代后得到接近CP足够情况下的系统性能,PIE的性能与传统的迭代干扰消除方法相同,而SIE则提供好得多的收敛性能。     

A general scheme for equalization of space-time block-coded systems with unknown CSI

Recently, space-time block codes (STBCs) have gained much attention as an effective transmit diversity technique to increase the capacity of wireless communication systems. In this paper, we consider a general technique for direct equalization of space-time block-coding systems with unknown channel state information (CSI). This technique is suitable for several existing hybrid STBC schemes, such as STBC/orthogonal-frequency-division multiplexing (STBC-OFDM) and STBC/code-division multiple access (STBC-CDMA). We show that by exploiting the redundancy in the structure of STBC, a zero-forcing equalizer can be constructed without channel estimation. The conditions for the identifiability of the zero-forcing equalizer are also derived to ensure correct equalization. To further improve the performance of the proposed method, a new iterative algorithm is developed by incorporating the finite alphabet property of information symbols. Simulation results show that the proposed algorithms can outperform comparative schemes in most simulation conditions.     

Space–Time Turbo Equalization With Successive Interference Cancellation for Frequency-Selective MIMO Channels

This paper addresses the issue of iterative space–time equalization for multiple-input–multiple-output (MIMO) frequency-selective fading channels. A new soft equalization concept based on successive interference cancellation (SIC) is introduced for a space–time bit-interleaved coded modulation (STBICM) transmission. The proposed equalizer allows us to separate intersymbol interference (ISI) and multiantenna interference (MAI) functions. Soft ISI is successively suppressed using a low-complexity suboptimum minimum mean square error (MMSE) criterion. The decoupling of ISI and MAI offers more flexibility in the design of the whole space–time equalizer. Different multiantenna detection criteria can be considered, ranging from simple detectors to the optimal maximum a posteriori (MAP) criterion. In particular, we introduce two soft equalizers, which are called SIC/SIC and SIC/MAP, and we show that they can provide a good performance-to-complexity tradeoff for many system configurations, as compared with other turbo equalization schemes. This paper also introduces an MMSE-based iterative channel state information (CSI) estimation algorithm and shows that attractive performance can be achieved when the proposed soft SIC space–time equalizer iterates with the MMSE-based CSI estimator.      

Semiblind Channel Estimation for MIMO Single Carrier With Frequency-Domain Equalization Systems

We propose a semiblind channel-estimation method for multiple-input multiple-output (MIMO) single carrier with frequency-domain equalization systems. By taking advantage of periodic precoding and the block circulant channel model after cyclic prefix removal, we obtain the channel-product matrices by solving a series of decoupled linear systems, which is gained from the covariance matrix of the received data. Then, the channel-impulse-response matrix is obtained by computing the positive eigenvalues and eigenvectors of a Hermitian matrix formed from the channel-product matrices. We also propose an optimal design of the precoding sequence, which minimizes the noise effect and numerical error in covariance matrix estimation, and discuss the impact of the optimal sequence on channel equalization. With the proposed framework, the method is shown to be robust with respect to channel-order overestimation, and the identifiability condition is simply that the channel-impulse-response matrix has full column rank. Due to the identifiability condition, the method is applicable to MIMO channels with more transmitters or more receivers. Simulations are used to demonstrate the performance of the proposed method.      

MIMO OFDM系统中的Turbo子载波均衡

在MIMO OFDM系统中,为了对抗同天线干扰及由于保护间隔不足而引起的码间干扰和载波间干扰,该文给出了一种基于MMSE的Turbo子载波均衡器。在该算法中,软输入软输出(SISO)的子载波均衡器与软输入软输出(SISO)解码器通过迭代进行软信息交换。仿真结果表明,与非迭代的子载波均衡器相比,该文给出的Turbo子载波均衡器能够有效利用时间和空间分集,使系统性能得到了改善。     

循环前缀块传输系统中的一种MIMO盲信道估计算法

信道估计一直是无线通信领域的研究热点之一,信道参数估计的好坏对系统的整体性能有着至关重要的影响。针对采用循环前缀的多输入多输出(MIMO-CP)系统,本文提出了一种基于子空间的盲信道估计方法,该算法利用了循环前缀所引起的冗余信息。基于子空间的盲信道估计算法都是通过对接收块的自相关矩阵进行奇异值分解(SVD)来实现信道估计的,因此需要利用尽可能多的接收块来得到准确的自相关矩阵的估计值,这就意味着会产生过久的判决延迟以及不能准确对快变信道进行跟踪。利用MIMO-CP系统中系统矩阵特有的块循环特性,对于连续的两个接收数据块以及对应的循环前缀部分组成的向量,可以重新构造一组新的向量而不改变系统的信道矩阵,因此可以通过较少的接收块来得到准确的自相关矩阵的估计值,该方法十分适用于对快变信道的盲估计。文章通过仿真分析了在不同的重复系数以及不同的接收块下该算法的性能并且比较了该算法与现有的“预编码”、“虚拟子载波”等盲信道估计算法的性能。仿真结果表明,提出的算法利用较少的数据块个数就得到了一个可靠的信道估计值和很好的误码率性能。      

Equalization for OFDM over doubly selective channels

In this paper, we propose a time-domain as well as a frequency-domain per-tone equalization for orthogonal frequency-division multiplexing (OFDM) over doubly selective channels. We consider the most general case, where the channel delay spread is larger than the cyclic prefix (CP), which results in interblock interference (IBI). IBI in conjunction with the Doppler effect destroys the orthogonality between subcarriers and, hence, results in severe intercarrier interference (ICI). In this paper, we propose a time-varying finite-impulse-response (TV-FIR) time-domain equalizer (TEQ) to restore the orthogonality between subcarriers, and hence to eliminate ICI/IBI. Due to the fact that the TEQ optimizes the performance over all subcarriers in a joint fashion, it has a poor performance. An optimal frequency-domain per-tone equalizer (PTEQ) is then obtained by transferring the TEQ operation to the frequency domain. Through computer simulations, we demonstrate the performance of the proposed equalization techniques.     

Optimal training sequences for channel estimation in bi-directional relay networks with multiple antennas

In this letter, we consider a bi-directional relay network in which two users, U _{and U, exchange their information via a relay station, RS. Multiple antennas are deployed at both users and at RS. Single carrier cyclic prefix (SCCP) is used to combat intersymbol interference (ISI) in frequency-selective fading channels. The transmission process is divided into two time slots. At the first time slot, both users send their information to RS concurrently. RS then amplifies and broadcasts its received signals at the second time slot. We propose an algorithm to estimate the channel information at end users based on the leastsquare (LS) principle. To further minimize the mean square error (MSE) of the estimate, a method to design the optimal training sequences is also proposed. Simulation results show that the performance achieved by our optimal design is close to that with perfect channel information.}     

Iterative Interference Cancellation and Channel Estimation for Mobile SC-FDE Systems

This letter presents a combination of iterative channel equalization and estimation algorithms for mobile users in an interference-prone single carrier (SC) space division multiple access (SDMA) uplink system. It is shown that the soft decision based iterative block decision feedback equalization (SD-IBDFE) with parallel interference cancellation (PIC) and a least squares (LS) adaptation algorithm give significant improvement in estimating the SDMA users' channels in an iterative manner. Also, iterative channel estimation over multiple frames to estimate the desired user's channels provides additional gain. Simulation results show that the combination of these low complexity algorithms gives reasonably good results in interference-dominated doubly selective channels.