Robust channel estimation for OFDM systems with rapid dispersivefading channels

Orthogonal frequency-division multiplexing (OFDM) modulation is a promising technique for achieving the high bit rates required for a wireless multimedia service. Without channel estimation and tracking, OFDM systems have to use differential phase-shift keying (DPSK), which has a 3-dB signal-to-noise ratio (SNR) loss compared with coherent phase-shift keying (PSK). To improve the performance of OFDM systems by using coherent PSK, we investigate robust channel estimation for OFDM systems. We derive a minimum mean-square-error (MMSE) channel estimator, which makes full use of the time- and frequency-domain correlations of the frequency response of time-varying dispersive fading channels. Since the channel statistics are usually unknown, we also analyze the mismatch of the estimator-to-channel statistics and propose a robust channel estimator that is insensitive to the channel statistics. The robust channel estimator can significantly improve the performance of OFDM systems in a rapid dispersive fading channel     

基于粒子滤波的大气激光OFDM时变信道半盲估计

为了有效改善大气激光正交频分复用通信系统接收端的符号检测性能,采用现有的混合粒子滤波算法对大气激光正交频分复用时变信道进行半盲估计,并进行了理论分析与实验验证.与传统的基于导频的时变信道估计方法相比,该方法可有效改善接收端的符号检测性能,并通过MATLAB仿真结果验证了该方法的有效性.结果表明,在相同的信噪比下,所用方...     

一种循环前缀为PN序列的OFDM信道估计算法

与传统的正交频分复用（OFDM）频域信道估计方法不同，提出了一种循环前缀为PN序列的OFDM信道估计方法：在时域OFDM符号中不插入传统意义上的循环前缀，而是插入PN序列作为训练序列，将PN序列与对应的接收序列作圆周相关处理获得信道频率响应估计；而由于PN序列周期性出现，它也起到了循环前缀的作用。仿真结果表明：在多径瑞利衰落信道中，在相同信噪比和相同信道利用率的条件下，提出的算法比频域梳状导频插入的时域滤波算法具有更高的信道估计精度。     

能量泄漏对OFDM系统LS信道估计性能影响的分析

针对OFDM系统的无线信道中非整数倍采样间隔多径时延的LS信道估计性能及其误差对系统误码性能的影响进行了理论分析和仿真实验。结果表明：LS的信道估计误差功率和OFDM系统的误码性能与无线信道中多径时延是否为整数倍无关,只与信道噪声功率强相关,即能量泄漏并没有带来文献指出的系统性能恶化。     

Simplified channel estimation for OFDM systems with multipletransmit antennas

Multiple transmit-and-receive antennas can be used in orthogonal frequency division multiplexing (OFDM) systems to improve communication quality and capacity. In this paper, we present two techniques to improve the performance and reduce the complexity of channel parameter estimation: optimum training-sequence design and simplified channel estimation. The optimal training sequences not only simplify the initial channel estimation, but also attain the best estimation performance. The simplified channel estimation significantly reduces the complexity of the channel estimation at the expense of a negligible performance degradation. The effectiveness of the new techniques is demonstrated through the simulation of an OFDM system with two-transmit and two-receive antennas. The space-time coding with 240 information bits per codeword is used for transmit diversity. From the simulation, the required signal-to-noise ratio is only about 9 dB for a 10% word error rate for a channel with the typical urban- or hilly-terrain delay profile and a 40-Hz Doppler frequency     

OFDM系统中改进的16QAM软判决解调算法

针对OFDM信号在多径衰落信道中传输的特点,提出了一种改进的16QAM解调软信息计算方法,通过在均衡时利用信道信息改善信号总体的信噪比水平,并且使软判决结果包含更多的信道信息,从而提高软判决译码的性能。仿真结果表明,所提出的方法能有效地改善系统在多径衰落信道环境下的性能,且易于工程实现。     

An efficient data rate maximization algorithm for OFDM based wireless networks

In this paper we present a computationally efficient, suboptimal integer bit allocation algorithm that maximizes the overall data rate in multiuser orthogonal frequency division multiplexing (OFDM) systems implemented in wireless networks. Assuming the complete knowledge of a channel and allowing a subchannel to be simultaneously shared by multiple users we have solved this data rate maximization problem in two steps. The first step provides subchannel assignment to users considering the users’ requests on quality of service (QoS) expressed as the minimum signal-to-noise ratio (SNR) on each subchannel. The second step provides transmit power and bit allocation to subchannels in order to maximize the overall data rate. To reduce computational complexity of the problem we propose a simple method which assigns subchannels to users and distributes power and bits among them. We have analyzed the performance of our proposed algorithm by simulation in a multiuser frequency selective fading environment for various signal-to-noise ratios and various numbers of users in the system. We have concluded that our algorithm, unlike other similar algorithms, is suitable for OFDM wireless networks, especially when signal-to-noise ratio in the channel is low. Also, the results have shown that the total data rate grows with the number of users in the system.     

基于块状导频的OFDM信道估计

首先对OFDM系统中基于导频辅助的信道估计算法进行研究,为了降低复杂度,利用奇异值分解(SVD)给出一种低阶近似的信道估计器。仿真结果表明LMMSE算法利用信道相关特性增加了运算量,但可以获得较好的估计效果,其简化算法(SVD)的性能略有下降,且随着信噪比增大,会出现"地板效应"。     

Selective Time-Domain Filtering for Reduced-Complexity PAPR Reduction in OFDM

A new selective time-domain filtering scheme for reducing the peak-to-average power ratio (PAPR) in orthogonal frequency-division multiplexing (OFDM) systems is proposed. The proposed scheme not only relies on conventional pilot symbol channel estimation and demodulation techniques to recover the data in OFDM systems but also uses additional pilot symbols. The proposed scheme has greatly reduced complexity compared with the selective mapping (SLM) scheme and only slightly poorer performance than the SLM scheme. The new scheme achieves significant PAPR reduction at a cost of typically 0.1–0.75 dB in signal-to-noise ratio (SNR) relative to pilot symbol OFDM not having PAPR reduction.      

A Background Sample-Time Error Calibration Technique Using Random Data for Wide-Band High-Resolution Time-Interleaved ADCs

Sample-time error among the channels of a time-interleaved analog-to-digital converter (ADC) is the main reason for significant degradation of the effective resolution of the high-speed time-interleaved ADC. A calibration technique for sample-time mismatches has been proposed and implemented at a low level of complexity. The calibration method uses random data and is especially suitable for ADCs used in digital data communication systems. An 800-MS/s four-channel, time-interleaved ADC system has been implemented to evaluate the performance of the technique. The experimental results show that the spurious-free dynamic range of the ADC system is improved to 58.1 dB at 350 MHz. The ADC system achieves a signal-to-noise and distortion ratio of 59.6 dB at 5 MHz and 50.1 dB at 350 MHz after calibration.     

A 32-mW 320-MHz continuous-time complex delta-sigma ADC for multi-mode wireless-LAN receivers

We present an experimental continuous-time complex delta-sigma multi-bit modulator, implemented in standard 0.25-/spl mu/m CMOS technology and meeting all major requirements for application in IEEE 802.11a/b/g wireless LAN receivers. The clock frequency is 320 MHz, producing an oversampling ratio of 16 for 20 MHz channel bandwidths. The modulator supports two operation modes for zero-IF and low-IF receiver architectures respectively, requires a single 2.5-V power supply, and dissipates only 32 mW of power. The measured peak signal-to-noise ratio is 55 dB. Further experimental results using sine-wave and OFDM test signals are also presented.     

Capacitor-Swapping Cyclic A/D Conversion Techniques With Reduced Mismatch Sensitivity

This work proposes two capacitor-swapping techniques, random feedback-capacitor interchanging (RFCI) and averaging RFCI (ARFCI) techniques, for cyclic analog-to-digital converters (ADCs) to reduce the harmonic distortion caused by capacitor mismatch without trimming or calibration. The proposed RFCI and ARFCI techniques can be realized by simply rearranging the capacitor connections of the ADCs in different operation cycles. Hence, complicated circuits are not needed. The RFCI technique improves upon the spurious-free dynamic range (SFDR) of conventional ADCs without sacrificing the signal-to-noise-and-distortion ratio (SNDR). The ARFCI technique has better SNDR characteristics but less SFDR improvement than RFCI. With RFCI and ARFCI, the capacitor matching requirement is relaxed for high SFDR and the capacitance can then be reduced to meet the SNDR requirement, reducing the driving capability of the opamps, and thus reducing the total power and area of the ADCs. The prior commutated feedback-capacitor switching (CFCS) technique (see Yu , 1996) has less effect on the SFDR of cyclic ADCs but improves the signal-to-noise ratio (SNR). This work proposes a reconfigurable cyclic ADC architecture that can be easily reconfigured to operate with one of the RFCI, ARFCI, and CFCS techniques by a simple timing control circuit. This reconfigurable topology provides three conversion characteristics with one item of intellectual property (IP), rather than three separate IPs and thus greatly enhances the capabilities of cyclic ADCs.      

Block-Based Performance Measures for MIMO OFDM Beamforming Systems

In this paper, we consider an adaptive modulation system with multiple-input–multiple-output (MIMO) antennas in conjunction with orthogonal frequency-division multiplexing (OFDM) operating over frequency-selective Rayleigh fading environments. In particular, we consider a type of beamforming with a maximum ratio transmission/maximum ratio combining (MRT-MRC) transceiver structure. For this system, we derive a central limit theorem for various block-based performance metrics. This motivates an accurate Gaussian approximation to the system data rate and the number of outages per OFDM block. In addition to the data rate and outage distributions, we also consider the subcarrier signal-to-noise ratio (SNR) as a process in the frequency domain and compute level crossing rates (LCRs) and average fade bandwidths (AFBs). Hence, we provide fundamental but novel results for the MIMO OFDM channel. The accuracy of these results is verified by Monte Carlo simulations, and applications to performance analysis and system design are discussed.      

Model-based channel estimation for OFDM signals in Rayleigh fading

This paper proposes a robust pilot-assisted channel estimation method for orthogonal frequency division multiplexing (OFDM) signals in Rayleigh fading. Our estimation method is based on nonlinear regression channel models. Unlike the linear minimum mean-squared error (LMMSE) channel estimate, the method proposed does not have to know or estimate channel statistics like the channel correlation matrix and the average signal-to-noise ratio (SNR) per bit. Numerical results indicate that the performance of the proposed channel estimator is very close to the theoretical bit error propagation lower bound that is obtained by a receiver with perfect channel response information     

Performance analysis of adaptive loading OFDM under Rayleigh fading

In this paper, we investigate the performance of adaptive loading orthogonal frequency-division multiplexing (OFDM) under Rayleigh fading with maximal ratio-combining (MRC) diversity at the receiver. We assume that channel-state information is available at both the transmitter and the receiver. Closed-form expressions for the lower bound on the average capacity of OFDM transmission under Rayleigh fading are provided for ideal MRC diversity. Simple approximate expressions for the average capacity of the Rayleigh-fading channel are also provided for the high signal-to-noise ratio (SNR) case. In the second part of this paper, a maximum-rate adaptive-loading strategy is derived for uncoded quadrature-amplitude-modulation modulated OFDM. Simple lower bound expressions and high-SNR approximations are provided for the average spectral efficiency of the maximum-rate adaptive-loaded uncoded OFDM under Rayleigh-fading channel conditions. According to the results, the performance of the uncoded adaptive-loading OFDM is about 8.5 dB inferior to the capacity bound at 10/sup -5/ symbol error probability under frequency-selective Rayleigh fading.     

Estimation of channel statistics for iterative detection of OFDM signals

Maximum likelihood sequence estimation for orthogonal frequency division multiplexing (OFDM) transmissions over unknown multipath fading channels is analytically infeasible for lack of efficient methods to maximize the likelihood function. A practical solution to this problem has been recently proposed in the context of space-time block-coded OFDM by resorting to the expectation-maximization (EM) algorithm. The resulting detector operates iteratively, exploiting knowledge of the channel statistics and the operating signal-to-noise ratio (SNR). In this work, we address the problem of estimating the above quantities and propose a recursive solution based on ad hoc reasoning. Simulations indicate that the EM detector employing the estimated SNR and channel statistics has better performance than other schemes operating in a mismatched mode. Also, the performance loss with respect to a system with perfect channel knowledge is negligible at SNR values of practical interest.     

Effects of time selective multipath fading on OFDM systems forbroadband mobile applications

The effects of time selective multipath fading are investigated for orthogonal frequency-division multiplexing (OFDM) systems in broadband mobile applications. Although in a multipath environment an OFDM system is very robust against frequency selective fading, it is very sensitive to time selective fading characteristics of the mobile channel, causing inter-carrier-interference and degrading system performance. To achieve a signal-to-interference ratio of 20 dB, the OFDM symbol duration must be less than 8% of the channel coherence time     

Doppler Spread Estimation for Broadband Wireless OFDM Systems Over Rician Fading Channels

In this paper, we present a new Doppler spread estimation algorithm for broadband wireless orthogonal frequency division multiplexing (OFDM) systems with fast time-varying and frequency-selective Rayleigh or Rician fading channels. The new algorithm is developed by analyzing the statistical properties of the power of the received OFDM signal in the time domain, thus it is not affected by the influence of frequency-domain inter-carrier interference (ICI) introduced by channel variation within one OFDM symbol. The operation of the algorithm doesn’t require the knowledge of fading channel coefficients, transmitted data, or signal-to-noise ratio (SNR) at the receiver. It is robust against additive noise, and can provide accurate Doppler spread estimation with SNR as low as 0 dB. Moreover, unlike existing algorithms, the proposed algorithm takes into account the inter-tap correlation of the discrete-time channel representation, as is the case in practical systems. Simulation results demonstrate that this new algorithm can accurately estimate a wide range of Doppler spread with low estimation latency and high computational efficiency.     

OFDM channel estimation by singular value decomposition

We present and analyze low-rank channel estimators for orthogonal frequency-division multiplexing (OFDM) systems using the frequency correlation of the channel. Low-rank approximations based on the discrete Fourier transform (DFT) have been proposed, but these suffer from poor performance when the channel is not sample spaced. We apply the theory of optimal rank-reduction to linear minimum mean-squared error (LMMSE) estimators and show that these estimators, when using a fixed design, are robust to changes in channel correlation and signal-to-noise ratio (SNR). The performance is presented in terms of uncoded symbol-error rate (SER) for a system using 16-quadrature amplitude modulation (QAM)     

Optimum receiver design for wireless broad-band systems using OFDM.I

Orthogonal frequency-division multiplexing (OFDM) is the technique of choice in digital broad-band applications that must cope with highly dispersive transmission media at low receiver implementation cost. In this paper, we focus on the inner OFDM receiver and its functions necessary to demodulate the received signal and deliver soft information to the outer receiver for decoding. The effects of relevant nonideal transmission conditions are thoroughly analyzed: imperfect channel estimation, symbol frame offset, carrier and sampling clock frequency offset, time-selective fading, and critical analog components. Through an appropriate optimization criterion (signal-to-noise ratio loss), minimum requirements on each receiver synchronization function are systematically derived. An equivalent signal model encompassing the effects of all relevant imperfections is then formulated in a generalized framework. The paper concludes with an outline of synchronization strategies