Pilot Position Selection and Detection for Channel Estimation of SC-FDE

In this letter, we propose a simple pilot position selection/detection technique for channel estimation of single- carrier frequency domain equalization (SC-FDE). Unlike the conventional channel estimation techniques such as frequency domain superimposed pilot technique (FDSPT), the proposed scheme selects the pilot positions to minimize the distortion of original signals caused by the loss of useful data tones in frequency domain. The corresponding receiver structure is also presented, where the pilot positions are blindly detected and the distorted data symbols are iteratively reconstructed. Simulation results show that the proposed system gives better BER performance than the FDSPT and approaches the lower bound of SC-FDE.     

Iterative channel estimation and pilot design rules for high‐mobility comb‐pilot OFDM system

With wireless communications in high‐mobility environment becoming popular, this poses a big challenge for communication systems based on the comb‐pilot OFDM, such as IEEE 802.11p, since it has not the enough pilots to estimate the time‐ and frequency‐selective channel accurately. In this paper, several comb‐pilot schemes and three comb‐pilot design rules are proposed to meet the Nyquist criterion for sampling the vehicle‐to‐vehicle (V2V) channel and the requirements of second‐order statistic of V2V channel. Based on the proposed pilot schemes, an iterative channel estimation method from the CE‐BEM model is proposed, together with three ICI cancellation methods. After thorough simulation, the effectiveness of the comb‐pilot design rules, the proposed channel estimation method, and intercarrier interference (ICI) cancellation methods is verified. Compared with other channel estimation methods, the proposed method performs better. The simulation results also reveal that the channel order L+1 has a great impact on the performance of the comb‐pilot OFDM system.     

无CP的OFDM联合信道估计和干扰抵消

无循环前缀OFDM无线通信系统可以提高频谱利用效率。提出一种基于无循环前缀(CP)OFDM系统的联合信道估计和干扰抵消算法。采用最小二乘法(Least Square)估计信道时域矢量,通过离散傅里叶变换(DFT)得到相应的OFDM信道的频域特性,由迭代算法对信号进行联合检测和干扰抵消。仿真结果表明,信道估计结果能够达到较高的精度,误码率也接近有循环前缀的OFDM无线通信系统。     

An Adaptive Channel Estimation Technique in MIMO OFDM Systems

In this paper, an adaptive channel estimation for MIMO OFDM is proposed. A set of pilot tones first are placed in each OFDM block, then the channel frequency response of these pilot tones are adaptively estimated by reeursive least squares （RLS） directly in frequency domain not in time domain. Then after the estimation of the channel frequency response of pilot tones, to obtain the channel frequency response of data tones, a new interpolation method based on DFT different from traditional linear interpolation method according to adjacent pilot tones is proposed. Simulation results show good performance of the technique.     

Joint Frequency Offset and Channel Estimation for OFDM Systems Using Pilot Symbols and Virtual Carriers

We consider joint estimation of carrier frequency offset and channel impulse response (CIR) for orthogonal frequency division multiplexing (OFDM) systems with pilot symbols and virtual subcarriers (VCs). We derive the receive-signal correlation structure due to the pilots and VCs, give the evidence of joint multivariate Gaussian distribution of the received samples, and derive an approximate maximum likelihood (ML) frequency offset estimator. We also derive the asymptotic mean-square error (MSE) and an approximate Cramer-Rao bound (CRB) and establish the asymptotic unbiasedness. Without pilots, in high signal-to-noise ratio, our estimator is equivalent to Liu and Tureli's estimator with N_v virtual carriers. When the pilot number (Np) is greater than the channel length L, our estimator acts as a subspace-based estimator with N_v + N_p - L virtual carriers. A decision-directed joint ML estimator is derived to iteratively update the estimates of frequency offset, data symbols and CIR. The optimal pilot and virtual carrier placement strategies are also discussed. The resulting decision-directed joint estimator performs within 0.8 dB of the ideal case even when the frequency offset is as large as 20%     

Clipping noise mitigation for channel estimation in OFDM systems

This letter provides a new technique to mitigate the clipping noise on pilot symbols in clipped and pilot-aided orthogonal frequency division multiplexing (OFDM) systems. The basic principle is to directly filter the clipping noise on the positions of pilots in the frequency domain before the insertion of pilots. Simulation results show that the new technique can effectively improve channel estimation and system performance.     

OFDM系统基于导频的联合信道估计与干扰抵消算法

对OFDM(orthogonal frequency-division multiplexing)系统中由于时频双选信道产生的载波间干扰进行了分析,并在此基础上提出了一种新的OFDM导频符号结构及信道估计方法,从而实现了在一个OFDM符号内时域信道估计和干扰抵消。仿真结果表明:提出的信道估计与干扰抵消相结合的联合算法不仅可以给出精度较高的信道信息,而且可以明显提高系统的性能。     

Robust channel estimation and ISI cancellation for OFDM systems with suppressed features

A feature-suppressed orthogonal frequency-division multiplexing (OFDM) system and the corresponding channel estimation and intersymbol interference (ISI) mitigation techniques are investigated in this paper. Cyclic prefix (CP) and pilot tones, which are commonly used in civilian OFDM systems for ISI mitigation and channel estimation, create distinctive waveform features that can be easily used for synchronization and channel estimation purposes by intercepting receivers. As a result, CP and pilot tones are eliminated in the proposed feature suppressed OFDM system to reduce the interception probability. Instead, a set of specially designed OFDM symbols, driven by different pseudorandom sequences, are employed as preambles to avoid unique spectral signature. These preambles are inserted into the OFDM data symbol stream periodically and in a round-robin manner. In addition, a random frequency offset is introduced to each preamble to further mask the multicarrier signature. New challenges arising from these feature suppression efforts are studied, including robust channel estimation and demodulation techniques in the presence of frequency offset and severe interference. Based on our interference analysis, an iterative ISI and intercarrier interference (ICI) estimation-cancellation-based technique is proposed for both channel estimation and OFDM data demodulation. Our channel estimator performs joint frequency offset and channel impulse response estimation based on the maximum-likelihood (ML) principle. To reduce its complexity, we employ a number of techniques, which include approximation of the ML metrics, as well as fast Fourier transform pruning. The performances and feasibility of the proposed feature suppressed OFDM system and the channel estimator are analyzed and verified through numerical simulations.     

时频双选信道OFDM系统的ICI消除与均衡

在正交频分复用（OFDM）系统中,因高速移动造成的多普勒效应导致子载波间正交性的破坏并产生载波间干扰（ICI）。为了消除ICI,本文通过分析ICI分布特性及带状矩阵特性,提出了低复杂度的迭代干扰抵消算法和基于最小均方误差准则的排序串行干扰抵消(MMSE-OSIC)算法。仿真结果表明,与传统子载波间干扰频域均衡算法相比,新算法在计算复杂度和性能之间取得了良好的平衡,且MMSE-OSIC算法可以利用时变信道的时间分集特性在高信噪比情况下有效地消除“地板效应”。      

Channel estimation in space and frequency domain for MIMO-OFDM systems

Multiple-input multiple-output (MIMO) systems can be combined with orthogonal frequency division multiplexing (OFDM) systems to improve the capacity and quality of wireless communications. In this article, a channel estimation technique in both space and frequency domain for MIMO-OFDM systems is proposed. It is shown that the proposed scheme with space-frequency pilot tones achieve optimal minimum mean square error (MMSE) channel estimation. Simulation results indicate that the proposed method achieves good performance.     

Adaptive crosstalk cancellation and laser frequency driftcompensation in dense WDM networks

Two variations of the LMS algorithm are proposed to cancel received linear crosstalk in dense WDM networks. Analysis and simulations show that with the addition of a few photodetectors, channel spacing requirements can be reduced by over 50 percent. Simulations using a demultiplexer with Gaussian bandpass characteristics show that a 2.5 dB signal-to-crosstalk-plus-noise ratio can be increased to over 35 dB. The decision-directed algorithm will work with OOK data or any intensity modulation scheme which uses the absence of light as one symbol. The decision-directed algorithm makes assumptions on the desired laser frequency so it can accommodate only limited laser drift. A second cancellation algorithm uses pilot tones added to each laser signal in order to cancel the crosstalk. It will work with analog or digital intensity-modulated data and will automatically configure itself to account for laser drift. Both algorithms are blind in that they do not require training sequences for initialization. Analysis shows that weights derived from pilot tones are nearly optimum for canceling crosstalk for the data. Simulations of both algorithms are presented. Finally both algorithms are shown to be capable of canceling nonlinear beating terms     

Decision-directed maximum-likelihood estimation of OFDM framesynchronisation offset

The authors propose a method to estimate the synchronisation offset for orthogonal frequency division multiplexing (OFDM) frame alignment without resort to pilot tones. A decision-directed maximum-likelihood estimation of frame synchronisation offset is derived, and the performance of the proposed scheme is confirmed by computer simulation for QAM systems     

一种有效的MIMO OFDM信道估计方法

传统的MIMO OFDM信道估计方法总是假设信道在由若干OFDM符号组成的每一帧的传输过程中保持不变,但这个假设在快变信道的条件下就不成立了.本文提出了一种适合快变信道的有效的估计方法.该方法通过在每个OFDM符号中设置若干训练子载波来跟踪快变的信道.首先讨论了在每个OFDM符号中所需的训练子载波的数目及其相应的位置,给出了在多发射天线多接收天线条件下的训练子载波的正交结构,然后在估计出训练子载波处的信道频率响应后.提出了一种不同于根据邻近训练子载波进行线性插值的以获得用于传输数据的子载波的信道频率响应的方法.仿真结果表明了我们所提出的方法的有效性.     

Subspace-based blind and semi-blind channel estimation for OFDMsystems

This paper proposes a new blind channel estimation method for orthogonal frequency division multiplexing (OFDM) systems. The algorithm makes use of the redundancy introduced by the cyclic prefix to identify the channel based on a subspace approach. Thus, the proposed method does not require any modification of the transmitter and applies to most existing OFDM systems. Semi-blind procedures taking advantage of training data are also proposed. These can be training symbols or pilot tones, the latter being used for solving the intrinsic indetermination of blind channel estimation. Identifiability results are provided, showing that in the (theoretical) situation where channel zeros are located on subcarriers, the algorithm does not ensure uniqueness of the channel estimation, unless the full noise subspace is considered. Simulations comparing the proposed method with a decision-directed channel estimator finally illustrates the performance of the proposed algorithm     

Comb type pilot aided channel estimation in OFDM systems with transmit diversity

Transmit diversity can be applied to OFDM systems by adopting space time code. Since the received signal is the overlapped signals transmitted from different transmit antennas, channel estimation is a rather challenging task for space time coded OFDM (ST-OFDM) systems. Pilot structure can help the receiver to effectively separate the overlapped signals and perform accurate channel estimation. In this paper, we propose three different channel estimation algorithms based on specially designed comb type pilots inserted in frequency domain. One of our proposed algorithms is performed in frequency domain and the other two are performed in time domain. Such comb type pilot based algorithms can provide higher bandwidth efficiency than common significant-tap-catching algorithm using training block pilots. Numerical analyzes and computational simulation show that our proposed estimation schemes have the same good performance while the time domain methods have relatively simple structure.     

OFDM system channel estimation using time-domain training sequencefor mobile reception of digital terrestrial broadcasting

A channel estimation technique using time-domain training sequence is investigated. It is demonstrated that this technique can estimate the channel status more accurately, under low C/N situations, than the conventional frequency domain technique using in-band pilots. A novel channel estimation and compensation scheme for a fast fading channel is developed for an OFDM system using time-domain training sequence. It is compared with the frequency domain pilot channel estimation scheme     

Performance evaluation of OFDM and single‐carrier systems using frequency domain equalization and phase modulation

In this paper, we study the performance of the continuous phase modulation (CPM)‐based orthogonal frequency division multiplexing (CPM‐OFDM) system. Also, we propose a CPM‐based single‐carrier frequency domain equalization (CPM‐SC‐FDE) structure for broadband wireless communication systems. The proposed structure combines the advantages of the low complexity of SC‐FDE, in addition to exploiting the channel frequency diversity and the power efficiency of CPM. Both the CPM‐OFDM system and the proposed system are implemented with FDE to avoid the complexity of the equalization. Two types of frequency domain equalizers are considered and compared for performance evaluation of both systems; the zero forcing (ZF) equalizer and the minimum mean square error (MMSE) equalizer. Simulation experiments are performed for a variety of multipath fading channels. Simulation results show that the performance of the CPM‐based systems with multipath fading is better than their performance with single path fading. The performance over a multipath channel is at least 5 and 12 dB better than the performance over a single path channel, for the CPM‐OFDM system and the proposed CPM‐SC‐FDE system, respectively. The results also show that, when CPM is utilized in SC‐FDE systems, they can outperform CPM‐OFDM systems by about 5 dB. Copyright © 2010 John Wiley & Sons, Ltd.     

时变信道下OFDM系统中基于载频间干扰抑制的迭代检测算法

为了有效地抑制OFDM系统载频间干扰（ICI），提出一种并行干扰消除的迭代检测算法。基于检测信号的信干噪比最大化的准则，通过对接收信号进行判决、重构、干扰抵消处理，建立了ICI抑制的迭代检测算法，给出了信号检测流程。通过对时变信道的分析，研究了时变信道中归一化频偏对系统陛能的影响，给出了性能仿真结果。仿真结果表明所建议的算法可有效地利用时变信道的时间分集特性来提高系统性能，计算复杂度从O（N^3）降低到O（N）。     

Optimizing Pilot Locations Using Feedback in OFDM Systems

Pilot-aided orthogonal frequency-division multiplexing (OFDM) channel estimation has, so far, been optimized only for open-loop systems for which the uniform spacing of pilots is optimal. In this paper, we examine closed-loop OFDM systems and show how uniform pilots may no longer be optimal. In doing so, we propose a feedback technique in which the pilot allocation mechanism is adapted to the mobile channel. After deriving a general expression for the symbol error rate as a function of pilot allocation, we optimize the pilot locations and propose two practical solutions using bounds on the objective function. We proceed to formulate pilot allocation based on the maximal average OFDM channel capacity. We examine the feedback overhead that is associated with our method and make comparisons with the conventional uniform framework. Finally, we suggest the use of vector quantization in the context of the generalized Lloyd algorithm to reduce feedback and extend our method to a multiple-input–multiple-output (MIMO)-OFDM system based on the Alamouti space–time block code. Monte-Carlo simulations illustrate the error-rate/capacity performance gains via nonuniform pilots and illustrate the effects of Doppler frequency and carrier frequency offset.      

OFDM系统的迭代联合信道估计与符号检测算法

提出了通过降低导频功率来提高OFDM传输效率,同时采用迭代的联合信道估计和符号检测算法来保证较好误码性能的接收方案。理论分析与仿真结果表明,当数据信息与导频信息的功率比不高于l0dB时,本文提出的算法能够有效地降低系统误码率,同时该方法还具有算法简单,收敛速度快的特点。