OFDM系统中频偏估计的研究

发布时间：2018-06-23 01:20

本文选题：正交频分复用 + 多用户系统　；参考：《复旦大学》2014年博士论文

【摘要】：正交频分复用(OFDM)技术具有良好的抗多径干扰能力以及很高的频谱效率,现已在无线通信系统中得到广泛应用。但与单载波系统相比,OFDM系统对频偏误差比较敏感：载波频偏与采样频偏都会破坏系统的正交性,从而引起子载波间干扰(ICI)。而在多用户OFDM系统中,不同用户的信号经过无线信道传播后在接收端相互叠加,又引入了其它用户的ICI,使得系统性能进一步下降。本文考虑了OFDM系统中载波频偏和采样频偏的估计问题,先后讨论了单用户系统中初始频偏的捕捉以及多用户系统中残留频偏的跟踪,最后利用载波频偏估计的思想解决了相位模糊的问题。本文首先讨论了整数载波频偏的估计。现有文献在估计整数频偏时假设小数频偏已经被事先完全补偿。但是实际中,由于噪声的缘故,小数频偏的估计会有一定的误差,所以在补偿之后系统中仍然会有一些残留小数频偏,并会对整数频偏的估计产生较大的影响。本文分析了传统算法有残留小数频偏时性能下降的原因,并提出了一种鲁棒的整数频偏与信道的联合ML估计算法。仿真结果表明,本文算法的性能优于传统算法。对于小数载波频偏与采样频偏,现有文献中的联合ML估计算法具有较高的估计精度,但是需要进行二维穷举式搜索,计算量较大。本文提出了一种低复杂度的小数频偏与采样频偏的联合ML估计算法：该算法首先证明小数频偏可以直接求解,从而将原来的二维搜索变为一维搜索；进而在二阶泰勒展开后,提出了一种对采样频偏的近似ML估计。仿真结果表明,本文算法的性能与传统算法几乎一样,但计算量大大降低。针对多用户OFDM系统,由于各个用户分别使用不同的晶振,所以频偏误差各不相同,接收端需要同时估计多个载波频偏和采样频偏。现有文献对多用户频偏的估计采用了迫零准则,因而有噪声增强问题。本文提出了一种新的多用户OFDM系统中载波频偏与采样频偏联合估计算法。新算法利用QR分解完成用户分离,可以消除已估用户对剩余用户的干扰。为了进一步提高估计精度,新算法还对接收信号进行了加权平均。仿真结果表明,新算法的性能要优于现有文献中的算法。考虑到晶振的温漂会使得频偏随时间发生变化,本文还进一步讨论了时变多用户系统中载波频偏与信道的联合跟踪问题：将频偏作为隐变量,采用EM算法求解最大后验估计；为了能够求得E步中隐变量的条件期望,本文利用变分推断来近似原来的后验概率。仿真结果表明,本文算法可以在较少的迭代次数下达到较高的跟踪精度。最后,本文利用载波频偏估计的思想解决了相位模糊的问题。在跟踪频偏与信道变化的过程中,需要大量的导频开销。盲信道估计不依赖导频信号,但辨识结果存在相位模糊的问题。最近的研究表明,相位模糊是由于以前的算法忽略了发送信号中先验的星座图信息。本文采用载波频偏估计的思想,证明了模糊相位可以分解为小数相位和整数相位两部分,而且它们的可辨识性也与小数频偏、整数频偏相类似。该结论可以进一步拓展到有任意多个星座图同时使用的情况。仿真结果表明,本文算法的性能与利用导频消除相位模糊的性能不差上下。
[Abstract]:Orthogonal frequency division multiplexing (OFDM) technology has good anti multipath interference ability and high spectral efficiency. It has been widely used in wireless communication system. But compared with single carrier system, OFDM system is more sensitive to frequency offset error: carrier frequency offset and sampling frequency offset can break the orthogonality of the system and cause inter carrier interference. (ICI). In the multiuser OFDM system, the signal of different users superimposition each other at the receiving end after the wireless channel is propagated and the ICI of other users is introduced. The performance of the system is further reduced. This paper considers the estimation of the carrier frequency offset and sampling frequency offset in the OFDM system, and discusses the capture of the initial frequency offset in the single user system. As well as the tracking of residual frequency offset in multiuser system, the problem of phase ambiguity is solved by using the idea of carrier frequency offset estimation. First, the estimation of integer carrier frequency offset is discussed. There will be some error in the estimation, so there will still be some residual decimal frequency offset in the system after compensation, and it will have a great influence on the estimation of integer frequency offset. In this paper, the reason for the performance decline of the traditional algorithm with residual decimal frequency offset is analyzed, and a robust combined ML estimation algorithm for the integer frequency offset and the channel is proposed. The real results show that the performance of the proposed algorithm is superior to the traditional algorithm. For the decimal carrier frequency offset and sampling frequency offset, the combined ML estimation algorithm in the existing literature has a high estimation accuracy, but it needs a two-dimensional exhaustive search and a large amount of computation. This paper presents a combined ML estimation of the low complex number frequency offset and sampling frequency offset. Algorithm: this algorithm first proves that the frequency offset of the decimal can be solved directly, and then turns the original two-dimensional search into one dimension search. Then, after two order Taylor expansion, an approximate ML estimation for sampling frequency offset is proposed. The simulation results show that the performance of this algorithm is similar to that of the traditional algorithm, but the computation is greatly reduced. OFDM system, because each user uses the different crystal oscillator respectively, the frequency offset error is different, the receiver needs to estimate multiple carrier frequency offset and sampling frequency offset at the same time. The existing literature has adopted the zero forcing criterion for the estimation of multiuser frequency offset. Therefore, there is a noise enhancement problem. In this paper, a new carrier frequency offset in the multi-user OFDM system is proposed. The new algorithm uses the sampling frequency offset estimation algorithm. The new algorithm uses the QR decomposition to complete the user separation, which can eliminate the interference of the estimated user to the remaining users. In order to further improve the estimation precision, the new algorithm also carries out weighted average of the received signal. The simulation results show that the performance of the new algorithm is better than the existing algorithms. Temperature drift will change frequency offset with time. This paper further discusses the joint tracking problem of carrier frequency offset and channel in time-varying multiuser systems: using frequency offset as implicit variable and using EM algorithm to solve the maximum posterior estimation. In order to find the conditional expectation of hidden variables in E step, this paper uses variational inference to approximate the original The simulation results show that this algorithm can achieve higher tracking accuracy under less iterative times. Finally, the problem of phase ambiguity is solved by the idea of carrier frequency offset estimation. In the process of tracking frequency offset and channel change, a large number of pilot open pins are needed. Blind channel estimation is not dependent on pilot signal, but identification is not dependent on the pilot signal. The result shows that there is a problem of phase ambiguity. The recent study shows that the phase ambiguity is due to the previous algorithm neglecting the prior constellation information in the transmitted signal. In this paper, the idea of carrier frequency offset estimation proves that the fuzzy phase can be decomposed into two parts of the decimal phase and the integer phase, and their identifiability is also associated with the decimal frequency offset. The integer frequency offset is similar. This conclusion can be further extended to the situation that any multiple constellation diagrams are used simultaneously. The simulation results show that the performance of the proposed algorithm is not very different from the performance of using pilot to eliminate phase ambiguity.
【学位授予单位】：复旦大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TN929.53

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