衰落信道下基于正交频率码分复用的精确时间测量

针对衰落信道下的精确时间测量,给出基于正交频率码分复用技术的高精度时间测量算法。在不降低最大不模糊时间的前提下,通过时间延时、信道衰落、相位噪声参数的联合迭代估计,有效克服了频率选择性信道的衰落与相位噪声对时间测量的影响。仿真结果表明:联合迭代算法提高了时间测量的归一化均方根误差性能。

OFDMA系统中基于LMMSE信道估计算法的改进及其性能分析

OFDM调制技术用于移动通信领域时,其信道估计技术是一项关键技术.分析了一种基于频域相关的线性MMSE算法,并根据理论模型的假设,给出了最大时延、噪声功率等参数的近似估计方法,确定了一种可实用的LMMSE信道估计表达式.在此基础上,分析了算法复杂度和进一步简化的可能性.最后给出了不同信道条件下,改进的MMSE算法的性能仿真,并对仿真结果进行了分析.

OFDM调制的FPGA实现

提出了一种切实可行的 OFDM调制方式的 FPGA实现方法与结构 ,并介绍了其中

窄带干扰条件下水声OFDM系统LMMSE信道估计算法

因为正交频分复用(OFDM)系统中强窄带干扰引起水声信道估计性能退化,因此,提出了一种可实用的线性最小均方误差(LMMSE)信道估计方法.该方法结合最小二乘(LS)信道估计结果对受窄带干扰(NBJ)和加性高斯白噪声(AWGN)噪声干扰的导频子载波进行辨别,从而获得最有效信道抽头;根据导出的理论模型,给出了信道自相关、窄带干扰信号和噪声功率等参数的近似估计方法;并采用傅里叶变换以避免矩阵的求逆运算,减少了计算复杂度.仿真结果表明,在强干扰信号条件下,文中提出的方法接近理想状态的LMMSE方法.

运用仿真进行OFDM性能的研究

给出了 OFDM(正交频率分割复用 )的调制原理及其用傅立叶变换实现的具体方法 ,介绍了格形编码 TCM在 OFDM系统中所起的作用 ,并用 Matlab进行了 OFDM系统仿真 .

用于扩展目标检测的OFDM-MIMO雷达波形设计

为了检测扩展目标,该文提出一种已知统计特性的杂波和噪声背景下提高正交频率分集复用-多输入多输出(OFDM-MIMO)雷达检测性能的波形设计方法。利用扩展目标一维距离像的频率特性,基于匹配照射理论给出了雷达回波向量模型,在此基础上建立最大化检测信杂噪比的目标函数;通过交替迭代搜索获得优化发射波形和最优接收滤波器。仿真结果表明,经调制的OFDM-MIMO雷达发射波形具有稳健的抗杂波性能,可提高检测信杂噪比,改善OFDM-MIMO雷达检测性能。

基于多载波的无人机探测与通信技术研究

基于正交频率复用技术,提出了无人机(UAV)多载波雷达探测与正交频率复用高速通信联合设计方法。设计了探测与通信功能模块,阐述了发射和接收状态的工作方式,给出了同时极化频率捷变探测波形设计。性能仿真结果验证了多载波探测与同时极化和频率捷变技术联合设计的可行性,综合宽带探测、同时极化、频率捷变、多载波通信、自适应通信等,在实现快速高精度探测和高速通信双任务的同时,能满足无人机的系统多功能整合、小型化、低功耗、低成本的需求。

Polynomial rooting based frequency offset estimation for MIMO OFDM systems

Based on the frequency domain training sequences, the polynomial-based carrier frequency offset （CFO） estimation in multiple-input multiple-output （ MIMO ） orthogonal frequency division multiplexing （ OFDM ） systems is extensively investigated. By designing the training sequences to meet certain conditions and exploiting the Hermitian and real symmetric properties of the corresponding matrices, it is found that the roots of the polynomials corresponding to the cost functions are pairwise and that both meger CFO and fractional CFO can be estimated by the direct polynomial rooting approach. By analyzing the polynomials corresponding to the cost functions and their derivatives, it is shown that they have a common polynomial factor and the former can be expressed in a quadratic form of the common polynomial factor. Analytical results further reveal that the derivative polynomial rooting approach is equivalent to the direct one in estimation at the same signal-to-noise ratio（SNR） value and that the latter is superior to the former in complexity. Simulation results agree well with analytical results.

Correlation-based subblock partition design for PTS approach

In order to exploit the capability of the peak-to-average power ratio（PAPR）reduction afforded by the partial transmit sequences （PTS）approach in orthogonal frequency division multiplexing（OFDM）systems, subblock partition schemes for the PTS approach are studied. The motivation is to establish the relationship between the subblock partition and the capability of PAPR reduction through the periodic autocorrelation functions （ACFs）of partial transmit sequences and the periodic cross-correlation functions（CCFs）of signal candidates.Let Q represent the variation of the square magnitudes of ACFs.It is found that the lower the Q-value is, the better PAPR performance can be achieved, which is introduced as a design criterion for subblock partition.Based on this criterion, four common partition methods are compared and an efficient partition strategy is proposed. It is shown that structured partition schemes with low computational complexity have a large Q-value, leading to a poor PAPR performance.The new strategy can be regarded as a trade-off between PAPR performance and computational complexity.The simulation results show that the strategy can achieve an optimal performance with a relatively low complexity and, moreover,does not increase the amount of side information.

MIMO-OFDM通信系统研究

采用多入多出(MIMO)技术可以提高信道容量和信道可靠性,降低误码率。正交频分复用(OFDM)是一种特殊的多载波传输方案,各子载波在整个符号周期上正交,各子载波信号子频谱可以互相重叠,提高频带利用率。MIMO-OFDM技术是OFDM与MIMO技术结合形成的一种新技术,该技术是在OFDM传输系统中采用阵列天线实现空间分集,提高了信号质量。本文全面介绍了MIMO技术和OFDM技术及两者的结合,分析实现MIMO-OFDM技术的框架。

Efficient integer frequency offset estimation for OFDM systems

An efficient scheme of integer frequency offset estimate for orthogonal frequency division multiplexing （OFDM） systems is proposed based on a training symbol with several identicalparts. In this scheme, the received training symbol is first reshaped into several sub-symbols.It shows that the reshaping process in-troduees time diversity multiplexing.After a special fast Fourier transform （FFT） algorithm is applied to the sub-symbol,the integer frequency is estimated by finding the maximum magnitude of the resulting fre-quency domain signal.To improve the estimate performance,diversity combining methods are presented to makefull use of the multiple frequency domain sub-symbols.Compared to the traditional scheme, theproposed one has an improved estimate performance demonstrated by the computation simulation, while maintaining a very low complexity.

Joint Estimation of Carrier Frequency and Phase Offset Based on Pilot Symbols in Quasi-Constant Envelope OFDM Satellite Systems

Spectral efficiency and energy efficiency are two important performance indicators of satellite systems. The Quasi-Constant Envelope Orthogonal Frequency Division Multiplexing(QCE-OFDM) technique can achieve both high spectral efficiency and low peak-to-average power ratio(PAPR). Therefore, the QCE-OFDM technique is considered as a promising candidate multi-carrier technique for satellite systems. However, the Doppler effect will cause the carrier frequency offset(CFO), and the non-ideal oscillator will cause the carrier phase offset(CPO) in satellite systems. The CFO and CPO will further result in the bit-error-rate(BER) performance degradation. Hence, it is important to estimate and compensate the CFO and CPO. This paper analyzes the effects of both CFO and CPO in QCE-OFDM satellite systems. Furthermore, we propose a joint CFO and CPO estimation method based on the pilot symbols in the frequency domain. In addition, the optimal pilot symbol structure with different pilot overheads is designed according to the minimum Cramer-Rao bound(CRB) criterion. Simulation results show that the estimation accuracy of the proposed method is close to the CRB.

BLOCK CODING SCHEME FOR REDUCING PAPR IN OFDM SYSTEMS WITH LARGE NUMBER OF SUBCARRIERS

The major drawback in Orthogonal Frequency Division Multiplexing (OFDM) system is due to the high Peak-to-Average Power Ratio (PAPR), so the performance of the system is significantly degraded by the nonlinearity of a High Power Amplifier (HPA) in the transmitter.In order to mitigate distortion, a block coding scheme for reducing PAPR in OFDM systems with large number of subcarriers based on complementary sequences and predistortion is proposed,which is capable of both error correction and PAPR reduction. Computer simulation results show that the proposed scheme significantly improves Bit Error Rate(BER) performance as compared to an uncoded system when an HPA is employed or a coded system without predistortion.

Independent Component Analysis Based Blind Adaptive Interference Reduction and Symbol Recovery for OFDM Systems

To overcome the inter-carrier interference （ICI） of orthogonal frequency division multiplexing （OFDM） systems subject to unknown carrier frequency offset （CFO） and multipath, this paper develops a blind adaptive interference suppression scheme based on independent component analysis （ICA）. Taking into account statistical independence of subcarriers＇ signals of OFDM, the signal recovery mechanism is investigated to achieve the goal of blind equalization. The received OFDM signals can be considered as the mixed observation signals. The effect of CFO and multipath corresponds to the mixing matrix in the problem of blind source separation （BSS） framework. In this paper, the ICA- based OFDM system model is built, and the proposed ICA-based detector is exploited to extract source signals from the observation of a received mixture based on the assumption of statistical independence between the sources. The blind separation technique can increase spectral efficiency and provide robustness performance against erroneous parameter estimation problem. Theoretical analysis and simulation results show that compared with the conventional pilot-based scheme, the improved performance of OFDM systems is obtained by the proposed ICA-based detection technique.

REDUCING THE PEAK-TO-AVERAGE POWER RATIO OF OFDM SYSTEM WITH LOW COMPLEXITY

This paper proposes a companding scheme, where small signals are enlarged and large signals are reduced, to reduce the Peak-to-Average Power Ratio(PAPR). Computer simulation results show that the proposed technique has two advantages at least when compared with the conventional methods such as partial transmit sequence, selective mapping and the previous companding. First, it gets better PAPR performances with a lower complexity. Second, the scheme achieves greater performances gain with hardly any damnification of OFDM signals in some degree.

Fractional Frequency Offset Estimation for OFDM Systems in Non-Cooperative Communication

The traditional fractional frequency offset(FFO) estimation schemes for orthogonal frequency division multiplexing(OFDM) in non-cooperative communication have the problems of susceptible performance with the frequency offset values and the number of OFDM symbols,a novel fractional frequency offset blind estimation scheme based on EKF for OFDM systems is conceived.The nonlinear function of the frequency offset is calculated by employing the correlation.And then the frequency offset is estimated by means of the iterative algorithm of EKF.The finally fractional frequency offset is estimated by adopting repeated the above process.Simulation results demonstrate that the proposed scheme is robust to the frequency offset values without any requirements of a prior knowledge.

Resource Allocation for Two-Way Amplify and Forward OFDM Relay Networks

Physical-layer network coding(PNC) promises substantial theoretical gain to achieve the maximum system throughput in cooperative relay transmission. However, with the increasing global warming, how to reduce power consumption while satisfy system throughput requirement is becoming a vital issue. In this paper, we investigate energy-efficiency resource allocation(RA) based on PNC with amplify-and-forward(AF) protocol in orthogonal frequency division multiple(OFDM) bidirectional transmission. To minimize the overall transmit power consumption with required system throughput requirement, we consider joint subcarriers and power allocation and formulate the objective task into a constrained optimization problem where the best relay node is selected to minimize total transmit power. The closed form optimization power allocation solutions are acquired by analytical derivation. Based on derivation, we propose a novel optimal energy-efficient power allocation(OE-PA). Numerical results are given to evaluate the performance of the derived scheme as compared to other schemes and show that our scheme has signifi cant improvement to energy saving.

Does Amplify-and-Forward Cooperative Relay Improve OFDM/OFDMA Ergodic Capacity？

The ergodic information rate for Orthogonal Frequency-Division Multiplexing / Orthogonal Frequency-Division Multiple Access with amplify-and-forward （AF） relaying systems in the presence of frequency offsets is evaluated. Unlike previous work, per-subcarrier adaptive power allocation is performed on each relay to optimize the system ergodic information rate. For a given frequency offset and total number of relays M, the AF ergodic information rate is proven to be a monotonically increasing function of α （the ratio of the power allocated to the source node and the total transmit power）, implying that the maximum ergodic information rate can be obtained at α=1 （i.e., there is no cooperative relay）. Furthermore, the proof of ＂cooperative relays cannot improve the AF ergodic information rate in a quasi-static wireless channel＂ is also provided in this letter.

CARRIER FREQUENCY OFFSET ESTIMATION FOR INTERLEAVED OFDMA UPLINK BASED ON SUBSPACE PROCESSING

This paper investigates Carrier Frequency Offset (CFO) estimation in the uplink of the Orthogonal Frequency-Division Multiple Access (OFDMA) systems with the interleaved subcarrier assignment. CFOs between the transmitters and the uplink receiver will destroy orthogonality among different subcarriers, hence resulting in inter-carrier interference and multiuser interference. A two-stage frequency offset estimation algorithm based on subspace processing is proposed. The main advantage of the proposed method is that it can obtain the CFOs of all users simultaneously using only one OFDMA block. Compared with the previously known methods, it not only has a relatively low implementation complexity but is also suitable for random subchannel assignment.

基于OFDMA子载波功率分配的FFR广义分析模型

基于正交频分多址接入(OFDMA)系统对分数阶频率复用(FFR)技术进行了研究。通过引入功率比值和软频率复用因子的概念,提出了FFR广义分析模型。目前现存的FFR方案均可作为特例统一在该广义分析模型的框架之下。该模型从OFDMA子载波功率分配的角度出发,为分析FFR提供了一种新的视角。理论分析表明,可以通过调整功率比值连续地调整软频率复用因子。对于基于FFR的OFDMA蜂窝系统,通过该模型可以有效地平衡小区容量和覆盖之间的矛盾。最后,基于全球互操作性微波接入(WiMAX)系统,从计算机仿真的角度证实了该结论,同时给出了功率比值的合理取值范围,这对于实际系统的设计颇具指导意义。