OFDM快时变信道下的相位旋转调制技术

由于发射端和接收端振荡器之间的频率偏差以及无线信道的时变性,OFDM系统各个子载波之间的正交性将会被破坏,从而产生载波间干扰(ICI)。该文利用发送端的多天线分集,提出了一种新型的相位旋转调制技术。它通过在频域对不同发送天线上的发送信号进行不同角度的旋转,使接收端等效信道频域响应产生的载波间干扰最小,达到抑制载波间干扰的作用。实验证明,这种新型的相位旋转调制技术不会改变OFDM系统的发送功率,能够在不占用额外系统带宽的情况下有效地消除快时变信道环境下系统的载波间干扰,达到较好的误码率性能。     

一种适用于快衰落信道的ICI抑制方案

针对高速移动环境下多普勒频偏造成信道的快衰落和正交频分复用(OFDM)系统中子载波间干扰(ICI)的问题,提出了一种适合快衰落环境的OFDM系统子载波间干扰抑制算法。此算法用线性变化模型来近似一个OFDM符号周期内的信道冲激响应,并以此为基础采用迭代MMSE均衡方法抑制载波间干扰。分析和仿真结果表明,此方法能有效地保证载波间的正交性,从而改善了OFDM系统的误码率(BER)性能。     

OFDM系统中成形脉冲抑制ICI方法研究

OFDM系统面临的一个主要问题是对频率偏移的敏感性,频率偏移将导致子载波间干扰(ICI)的产生,从而大大降低系统性能.本文主要分析频偏存在时脉冲成形与子载波间干扰的关系,给出各种成形脉冲性能的评价标准,并提出在此标准下性能最佳的成形脉冲.     

OFDM系统中提高吞吐量的均衡算法

文章分析了在OFDM系统中具有不足循环前缀的时域均衡,提出了一种基于最大系统吞吐量新均衡算法的改进。首先对符号间干扰(ISI)和载波间干扰(ICI)进行了分析,并在此基础上给出了实现最大化吞吐量均衡算法的三个步骤。仿真结果显示该算法对吞吐量的改进超过了一些已存在的算法。最后与其它均衡算法比较给出结论     

移动OFDM CFOs估计的步长算法实现

为了保持移动中正交频分复用(OFDM)技术子载波间的正交性,避免载波间的干扰(ICI)引起的系统性能退化,建立了载波频率偏移(CFOs)的步长估计模型,对模型的稳定性、步长参数和CFOs的估计误差等进行研究,并进行了MATLAB仿真。VHDL程序在电路板上运行的实验结果表明:在±3σ估计变化区间内,移动速度为400和800 km/h时,CFOs的误差分别近似为0.010和0.017,符合CFOs估计误差必须精确的在子载波空间1~2%范围内的衡量标准,同时程序在电路板上能够正确执行。     

双选择信道中的时间偏移广义频分复用通信

当信道存在时间-频率双选择性时,严重的子载波间干扰和子符号间干扰将导致广义频分复用(GFDM)系统性能显著下降.为此,该文提出一种时间偏移GFDM系统(TO-GFDM),通过对传统GFDM系统的原型滤波器进行时间偏移,来提高双选择信道下GFDM系统的性能.该文推导了GFDM信号在双选择信道中的平均信干比公式,并提出基于...     

OFDM系统中载波间干扰抑制方法

在高速移动通信环境下,OFDM系统在传输过程中出现的多普勒频移和收发两端本地振荡器之间的频率偏差,形成子载波间干扰(ICI)并造成系统性能降低。在分析子载波间干扰机制的基础上,讨论了自消除方法和分段均衡方法,并提出一种利用加窗技术改进的分段均衡方法。仿真结果表明,采用该改进的分段均衡方法能更好地改善系统的性能,有约2 dB的信道估计增益。     

OFDM系统中成形脉冲抑制ICI方法研究

OFDM系统面临的一个主要问题是对频率偏移的敏感性,频率偏移将导致子载波间干扰（ICI）的产生,从而大大降低系统性能。本文主要分析频偏存在时脉冲成形与子载波间干扰的关系,给出各种成形脉冲性能的评价标准,并提出在此标准下性能最佳的成形脉冲。     

OFDMA系统中的干扰抵消技术研究

针对上行0FDMA系统中由于载波频率偏移所引起的子载波间干扰．提出了基于MMSE检测的串行干扰抵消技术。并对该方案与MMSE检测的误符号率性能以及抗远近效应的性能进行了比较。     

基于OSIC的MIMO SC-FDMA系统载波同步算法

针对MIMO SC-FDMA系统载波频率偏移敏感问题,提出一种基于OSIC的载波同步算法.该算法对存在载波频偏(CFO)的信号进行排序、零化、迭代均衡实现干扰消除.理论分析和仿真实验表明该算法实现了载波频偏干扰消除的目的,并且提高了系统性能.     

一种基于导频设计的MIMO-OFDM载波频偏估计方法

在多输入多输出-正交频分复用(MIMO-OFDM)系统中载频同步是个关键问题.考虑在该类系统中,存在着多个不同的载波频偏这个具有普遍适应性的模型,为了估计多个不同的频偏,提出了一种导频设计方案－子波段方法,将多个频偏的估计问题转换为一组单个频偏估计问题,从而降低了复杂度,同时可以有效地抑制载波间干扰和天线间干扰,提高载波频偏估计性能.     

A novel non-data-aided symbol timing recovery technique for OFDM systems

A new highly efficient non-data-aided technique to recover symbol timing of orthogonal frequency-division multiplexing systems is proposed. The algorithm in the proposed work exploits the interference that results due to the loss of orthogonality between subcarriers, where the second-order statistics of the resulting interference is proportional to the offset from the optimum sampling point. The presented technique does not require prior fine carrier synchronization, and it is capable of extracting symbol timing at low E/sub s//N/sub 0/ values with large carrier frequency offsets (CFOs). The system performance was investigated in multipath fading channels with large CFOs and additive white Gaussian noise.     

Performance of FHSS systems employing carrier jitter againstone-dimensional tone jamming

The performance of a frequency-hop spread-spectrum system employing carrier jitter against one-dimensional tone jamming (n=1 band multitone jamming) is investigated. First, noncoherent BFSK signaling under continuous-wave (CW) tone interference with arbitrary frequency offset is analyzed. A closed-form expression is derived for the error probability when there is one interfering CW tone and the background noise is negligible. When the background noise is significant, an expression involving one numerical integration is derived for the probability of error. It is shown that an interfering CW tone with power less than that of the signal can still cause errors with significant probability for certain ranges of carrier offsets. Next, the authors apply these results in analyzing the performance of a FHSS communications system under one-dimensional tone jamming when the communicator pseudorandomly jitters his carrier frequency from hop to hop. Two different methods of carrier jittering are considered. It is found that one of the schemes offers approximately a 3 dB gain in signal-to-noise ratio over a system without carrier jittering while the other scheme offers no significant gain     

The sensitivity of downlink MC-DS-CDMA to carrier frequency offsets

We investigate the sensitivity of a downlink MC-DS-CDMA system to carrier frequency offsets. It is shown that the resulting interference power depends only on the ratio of the frequency offset to the carrier spacing. Hence, for given a frequency offset and system bandwidth, the MC-DS-CDMA performance degradation is independent of the spreading factor but increases with the number of carriers; equivalently, for given frequency offset and symbol rate, the performance degradation increases with the ratio of the number of carriers to the spreading factor     

Decision Feedback Frequency Offset Estimation and Tracking for General ICI Self-Cancellation Based OFDM Systems

Inter-carrier interference (ICI) self-cancellation schemes were often employed in many OFDM systems as a simple and effective approach to suppress ICI caused by carrier frequency errors. The same procedure, however, can not perform very well at high frequency offsets. We propose a simple decision feedback scheme based on the general ICI self-cancellation scheme to perform estimation and tracking of the carrier frequency offsets. A system with the scheme does not consume additional bandwidth since it used the same data symbols employed for ICI cancellation for the estimation. After an initial estimation is completed, the scheme switches to the tracking mode to carry out the estimation of deviations in the frequency offsets. Finally this fine-tuned estimate is applied to the ICI self-cancellation scheme concurrently for frequency offset correction and hence improved the system performance greatly. Simulation results showed that our scheme allowed up to 9% of random variations in the frequency offset. The effectiveness of our scheme is further verified by calculating the bit error rate performance of various OFDM receivers.     

Iterative MMSE equalization and CFO compensation for the uplink SC‐FDMA transmission

Single‐carrier frequency division multiple access is greatly sensitive to carrier frequency offset (CFO) between transceivers. This leads to the destruction of orthogonality among subcarriers, which in turn leads to inter‐carrier interference and multiple access interference between different users. Minimum mean square error (MMSE) equalizer that uses an inverse operation on an interference matrix with a dimension equal to the number of subcarriers is normally used to invalidate CFO effects. Hence, the terminal processing complexity is very high. The proposed conjugate gradient method attempts to mitigate the higher computational complexity by iteratively evaluating the MMSE solution without direct matrix inverse operation. To further mitigate the multiple access interference, MMSE combined with parallel interference cancellation is also implemented. The analysis of the proposed method shows better performance and fast convergence in single‐carrier frequency division multiple access systems. The maximum iteration number to formulate an accurate solution is almost equal to the number of active users in the uplink access. Simulation results bring out the effectiveness of the present method compared with the existing CFO compensation schemes in terms of computational complication and system performance with large frequency offsets. Copyright © 2016 John Wiley & Sons, Ltd.     

SER performance evaluation and optimization of OFDM system with residual frequency and timing offsets from imperfect synchronization

This paper deals with the effects of residual timing and frequency offsets on the symbol error rate (SER) performance of an orthogonal frequency division multiplexing (OFDM) system. The synchronization of an OFDM system generally consists of a coarse frequency and timing acquisition stage and a refine stage. Due to the presence of Gaussian noise, channel distortions and implementation losses of synchronization and equalization algorithms, residual frequency and timing offsets always exist for an OFDM receiver. The residual frequency and timing offsets are proven to be Gaussian distributed, with their corresponding variances determined. The reception process of an OFDM signal with frequency and timing offsets is analyzed. A closed-form analytical result on the SER of an OFDM system with residual synchronization errors is derived. Computer simulations and analyses show that the frequency and timing offsets affect the OFDM subcarriers differently. With this observation, a new technique is proposed to minimize the SER of the OFDM systems by adjusting the distribution of transmission power among the subcarriers.     

Real-Time Frequency Estimation of Complex GMSK Signal of Green Communications Devices

Parameter estimation of signals of universal software radio peripheral (USRP) devices is crucial to solve the problem of phase offsets of received signals in distributed beamforming. For systems that will utilize the closed loop feedback algorithm where the receiver needs to send the received signal strength (RSS) values periodically to the beamforming node so as to take advantage of energy conservation, the frequency and phase of these signals should be estimated before smoothening by nonlinear filters. This article presents the estimation of the frequency offsets of a Gaussian minimum shift keying (GMSK) signal from N210 USRP devices in real time by using the Radix-2 fast Fourier transform (FFT) algorithm in GNURadio. For these green communications devices, most of the needed hardware parts have been software defined, thereby reducing the supposed energy consumption. The frequency offsets from reference carrier frequencies of 900 MHz and 2.4 GHz are less than 3 kHz each before the estimation, but the average offsets are 45 Hz and 100 Hz after the estimation, respectively. The high offset value experienced with the 2.4 GHz carrier was due to consistent interference from devices on that same frequency.     

Joint Estimation of the Timing and Frequency Offset for Uplink OFDMA

Inheriting all the advantages of Orthogonal Frequency Division Multiplexing (OFDM), plus the ability to offer a fine level of bit granularity and dynamic subcarrier allocation for multiuser diversity, the Orthogonal Frequency Division Multiple Access (OFDMA) has emerged as a potential candidate for multiple access technique for future broadband wireless networks. However, the benefits of OFDMA come with stringent requirements on synchronization, especially in the uplink. Unless the timing offsets (TOs) and carrier frequency offsets (CFOs) among users in the uplink are kept under tolerable ranges, inter-symbol interference (ISI), inter-channel interference (ICI) and multi-user interference (MUI) will occur, which degrade the overall system performance severely. Accurate estimation of TOs and CFOs is required for each user, so that they can be accounted for at the user’s side or compesated for at the base station. This paper proposes a novel method to estimate jointly TOs and CFOs in the time-domain for multi-user in the OFDMA uplink. The method is shown to offer good accuracy, while maintaining a reasonable complexity compared to conventional estimation schemes.     

Performance of a coded multi-carrier DS-CDMA system in multi-path fading channels

Multi-carrier DS-CDMA has been considered as an effective scheme for reducing multiple access interference in quasi-synchronous transmission. The scheme allows the reduction of multiple access interference by transferring the orthogonality property of the signals into the frequency domain where the orthogonality property is robust to relative chip offsets between the spreading codes of the various users. However in multi-path channels, the multi-carrier technique results in frequency non-selective fading in the sub-channels, due to the narrower bandwidth, hence a reduction of the capability of the spread spectrum signal to mitigate the effect of multi-path propagation. In this paper, we consider the use of a Reed-Muller code with soft decision decoding to regain the corresponding loss in performance, and compare the resulting system with a single carrier DS-CDMA system. The effect of system parameters such as the number of sub-channels is investigated through numerical calculation and simulation, from which a number of system design criteria are arrived at.