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正交频分复用(OFDM)可以有效地将频率选择性衰落信道转化成多个并行平坦衰落信道,从而大大提高传输效率。多输入多输出正交频分复用(MIMO-OFDM)技术可以大幅度地提高无线通信系统的信道容量与传输速率,其信道估计精度对系统性能有着直接的影响。研究了基于导频插入的SISO-OFDM和MIMO-OFDM最小二乘法估计(LS)信道估计方法。采用广义平稳非相关散射下的多径时变瑞利信道模型,对各个算法进行了仿真试验,得出不同算法下信道估计的误比特曲线,比较了各个算法的性能。仿真结果表明:MIMO-OFDMLS算法在误码率性能上优于SISO-OFDM。 相似文献
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协作分集技术利用其他用户进行协作传输,可以获得一定的分集增益,但是在频率选择性衰落信道下性能不很乐观。为了对抗频率选择性衰落,采用正交频分复用(ODFM)技术与协作分集技术相结合的方法,研究了基于OFDM技术的协作通信,对在不同调制技术、不同信道估计方法下系统的性能进行了比较,为了进一步提高系统性能,对传统S&C算法进行了改进并应用于该系统中,仿真实验结果表明,改进的定时同步算法性能优于传统的S&C算法。 相似文献
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OFDM(Orthogonal Frequency Division Multiplexing,正交频分复用)系统中,信道的时间选择性快衰落会破坏子载波间的正交性,导致系统中出现载波间干扰(Inter-carrier Interference,ICI),降低OFDM系统性能.论文使用信道函数泰勒展开式的前两项近似信道函数,并以此为基础分析了OFDM在时间选择性快衰落信道中的性能,给出了ICI功率及其上界与信道最大多普勒扩展的关系式.仿真分析表明论文给出的关系式和ICI功率上界能够较为准确的描述OFDM系统在快衰落信道中ICI功率及其上界与信道最大多普勒扩展的关系. 相似文献
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文章以2×2多输入多输出正交频分复用(MIMO-OFDM)系统的信道估计技术为研究对象,分析了该系统中结合空时编码的判决反馈信道估计方法和二维导频辅助MMSE信道估计方法,并通过仿真比较其性能.结果表明,对于慢衰落信道,可采用判决反馈方法进行信道估计,而对于快衰落时变信道,宜采用二维导频辅助MMSE方法进行信道估计,但要付出频率利用率下降的代价. 相似文献
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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 相似文献
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LTE下行信道估计算法研究 总被引:2,自引:2,他引:2
近年来,正交频分复用(OFDM)技术以其在高速数据传输中具有的良好的抗多径衰落的特性以及较高的频谱利用率等优点得到了广泛的应用。在OFDM通信系统中,信号在带宽小于信道相干带宽的多个正交子载波上发射和传输,以克服多径衰落信道所带来的信号频率选择性衰落。为了补偿各个子载波上的多径衰落,这就需要一个性能良好的信道估计方案。对LTE中基于导频的下行信道估计算法进行研究,首先在导频子载波处采用LS(最小二乘法)算法估计出信道参数,然后通过内插的方法在数据子载波处内插得到信道参数。频域内插主要采用拉格朗日内插或是LMMSE(线性最小均方误差)内插;时域内插采用拉格朗日内插。 相似文献
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Yu Zhang Huaping Liu 《Wireless Communications, IEEE Transactions on》2006,5(11):3017-3022
Orthogonal frequency division multiplexing (OFDM) is robust against frequency selective fading, but it is very vulnerable to time selective fading. In quasi-orthogonal space-time coded OFDM (ST-OFDM) systems, channel variations cause not only inter-carrier interference among different subcarriers in one OFDM block, but also inter-transmit-antenna interference (ITAI). When applied in fast fading channels, common ST-ODFM receivers usually suffer from an irreducible error floor. In this letter, we apply frequency-domain correlative coding combined with a modified decision-feedback detection scheme to effectively suppress the error floor of quasi-orthogonal ST-OFDM over fast fading channels. The effectiveness of the proposed scheme in mitigating the effects of channel time selectivity is demonstrated through comparison with existing schemes such as zero-forcing, two-stage zero-forcing, and sequential decision feedback estimation 相似文献
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Orthogonal frequency division multiplexing (OFDM) has become a very popular method for high data rate wireless communications
because of its advantages over single carrier modulation schemes on multipath, frequency selective fading channels. However,
intercarrier interference, due to Doppler frequency shifts, and multipath fading severely degrade the performance of OFDM
systems. Estimation of channel parameters is required at the receiver. In this paper, we present a channel modeling and estimation
method based on the time-frequency representation of the received signal. The discrete evolutionary transform provides a time-frequency
procedure to obtain a complete characterization of the multipath, fading, and frequency selective channels. Simulations are
used to illustrate the performance of the proposed procedure and to compare it with other time-varying channel estimation
techniques. 相似文献
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分析了无人机测控信道模型以及由小规模衰落带来的非频率选择性衰落、频率选择性衰落与时间选择性衰落的产生机理及特性,并针对这3种衰落进行了抑制方法分析,指出OFDM、交织与编码以及空间分集是无人机测控中值得重点研究的抗衰落技术. 相似文献
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《Communications, IEEE Transactions on》2005,53(5):905-906
Bit Interleaved Time-Frequency Coded Modulation for OFDM Systems Over Time-Varying Channels Orthogonal frequency-division multiplexing (OFDM) is a promising technology in broadband wireless communications with its ability in transforming a frequency selective fading channel into multiple flat fading channels. However, the time-varying characteristics of wireless channels induce the loss of orthogonality among OFDM sub-carriers, which was generally considered harmful to system performance. In this paper, we propose a bit interleaved time–frequency coded modulation (BITFCM) scheme for OFDM to achieve both time and frequency diversity inherent in broadband time-varying channels. We will show that the time-varying characteristics of the channel are beneficial to system performance. Using the BITFCM scheme and for relatively low maximum normalized Doppler frequency, a reduced complexity Maximum Likelihood (ML) decoding approach is proposed to achieve good performance with low complexity as well. For high maximum normalized Doppler frequency, the inter-carrier interference (ICI) can be large and an error floor will be induced. To solve this problem, we propose two ICI mitigation schemes by taking advantage of the second order channel statistics and the complete channel information, respectively. It will be shown that both schemes can reduce the ICI significantly. 相似文献
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WuLili LiaoGuisheng BaoZheng ShangYong 《电子科学学刊(英文版)》2005,22(3):209-219
The paper investigates the problem of the design of an optimal Orthogonal Frequency Division Multiplexing (OFDM) receiver against unknown frequency selective fading. A fast convergent Monte Carlo receiver is proposed. In the proposed method, the Markov Chain Monte Carlo (MCMC) methods are employed for the blind Bayesian detection without channel estimation. Meanwhile, with the exploitation of the characteristics of OFDM systems, two methods are employed to improve the convergence rate and enhance the efficiency of MCMC algorithms.One is the integration of the posterior distribution function with respect to the associated channel parameters, which is involved in the derivation of the objective distribution function; the other is the intra-symbol differential coding for the elimination of the bimodality problem resulting from the presence of unknown fading channels. Moreover, no matrix inversion is needed with the use of the orthogonality property of OFDM modulation and hence the computational load is significantly reduced. Computer simulation results show the effectiveness of the fast convergent Monte Carlo receiver. 相似文献
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正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)是应用于高速率无线传输的一项有效技术。标准的OFDM系统的时频格是方形的,因此当信道在格点频率附近存在深度衰落时,未编码的OFDM系统面临着严重的符号恢复问题。为克服上述问题,在符号复用之前,可采用频谱利用率较高的Lattice编码。本文推导了针对衰落信道的编码优化性能指标、相应的编码设计标准,并提供了代数设计方法和实例。数值仿真结果验证了Lattice编码的OFDM系统相对于未编码系统有着更好的抗频率选择性衰落性能。 相似文献
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Exploiting Multipath Diversity in Multiple Antenna OFDM Systems With Spatially Correlated Channels 总被引:1,自引:0,他引:1
《Vehicular Technology, IEEE Transactions on》2005,54(4):1372-1378
Multiple antennas are useful in orthogonal frequency division multiplexing (OFDM) systems for providing transmit and receive diversity to overcome fading. Typically, these designs require considerable separation between the antennas. Spatial correlation is introduced when antennas are not well separated, and it often leads to performance degradation in a flat fading environment. However, in frequency selective fading channels with rich multipath diversity, OFDM receivers can overcome this performance degradation due to antenna correlation. This is due to transformation of a highly spatially correlated channel impulse response to a less spatially correlated channel frequency response inherently by an OFDM system in the presence of rich multipath diversity. We illustrate this for a simple receive diversity OFDM system and hence introduce the concept of space sampling at the receiver where antennas are placed relatively close to each other. The minimum separation required between the antennas under such circumstances is derived analytically, and it is shown that even with a separation of only$0.44lambda$ , the required spatial correlation in the channel frequency response becomes sufficiently low. Simulated performance results with such spacing for various multiple antenna OFDM systems corroborate the analytical results. 相似文献