CP-FREE OFDM系統(tǒng)MMSE均衡及信道估計(jì)
發(fā)布時(shí)間：2018-10-19 14:41
【摘要】：正交頻分復(fù)用(Orthogonal Frequency Division Multiplexing,OFDM)系統(tǒng)由于其高速率傳輸已經(jīng)在LTE系統(tǒng)中廣泛應(yīng)用了,而其性能的優(yōu)良和循環(huán)前綴的加入有很大關(guān)系,保證此系統(tǒng)可以在多徑信道下不受到碼間干擾的影響。不過在LTE系統(tǒng)中循環(huán)前綴的長度有時(shí)會(huì)占據(jù)20%的帶寬,隨著對(duì)頻譜利用率要求的提高,傳統(tǒng)OFDM系統(tǒng)中由于循環(huán)前綴的添加使得其利用率的提升比較有限。為此,許多學(xué)者展開了對(duì)無循環(huán)前綴OFDM系統(tǒng)(Cyclic Prefix-FREE OFDM,CP-FREE OFDM)系統(tǒng)的研究。現(xiàn)已有一個(gè)無循環(huán)前綴OFDM系統(tǒng),稱作符號(hào)循環(huán)移位均衡OFDM系統(tǒng)(Symbol Cyclic Shift Equalization OFDM,SCSE-OFDM),由Xiqing Liu提出,其中在接收端利用判決反饋均衡模塊將碼間干擾問題進(jìn)行消除,利用CP恢復(fù)過程實(shí)現(xiàn)了子載波干擾的消除。但是在此系統(tǒng)中,接收端僅考慮了迫零均衡從而產(chǎn)生了噪聲放大的問題,同時(shí)此系統(tǒng)中也沒有考慮信道估計(jì)的相關(guān)問題。本文為了提升無循環(huán)前綴OFDM系統(tǒng)的性能并完善此系統(tǒng)將從多個(gè)方面進(jìn)行研究。為了避免迫零均衡可能導(dǎo)致的噪聲放大的問題,本文首先考慮了僅平衡噪聲放大問題的最小均方誤差(Minimum Mean Square Error,MMSE)均衡器,接下來在Ghosh的判決反饋誤差為高斯分布的假設(shè)下設(shè)計(jì)了考慮判決反饋誤差的MMSE均衡器。為了了解判決反饋誤差的分布情況,對(duì)判決反饋誤差通過仿真分析首次觀察到其服從Alpha穩(wěn)定分布。為了減弱判決反饋誤差,本文通過設(shè)置反饋門限從而減少了將嚴(yán)重錯(cuò)判反饋的情況,系統(tǒng)的誤碼率性能得到了提升。由于接收機(jī)的硬判決,性能提升仍然受限。為此,本文將Turbo均衡技術(shù)和無循環(huán)前綴OFDM系統(tǒng)相結(jié)合。在接收機(jī)將MMSE均衡和Turbo解碼連接起來,利用Turbo譯碼輸出的外部信息在MMSE均衡中進(jìn)行估計(jì),然后利用估計(jì)出來的符號(hào)更新每一個(gè)比特的外部信息,通過多次循環(huán)性能有了很大的提升。為了使無循環(huán)前綴OFDM系統(tǒng)更加完整,本文最后對(duì)此系統(tǒng)下的信道估計(jì)進(jìn)行了研究。在添加導(dǎo)頻的情況下,設(shè)計(jì)了基于最小二乘(Least Square,LS)和最小均方(LMMSE)的兩種信道估計(jì)方案。此外,在不添加導(dǎo)頻的情況下基于子空間理論提出盲估計(jì)方案,利用過采樣及重復(fù)因子實(shí)現(xiàn)了信道估計(jì)。最后考慮在信道先驗(yàn)信息不足的情況下,利用自適應(yīng)濾波進(jìn)行信道估計(jì)。尤其研究了在Alpha穩(wěn)定分布噪聲下基于RLP(Recursive Least P-norm)和LMP(Linear Mean P-norm)的自適應(yīng)濾波技術(shù),信道估計(jì)準(zhǔn)確度得到了很好的提升。
[Abstract]:Orthogonal Frequency Division Multiplexing (Orthogonal Frequency Division Multiplexing,OFDM) system has been widely used in LTE system because of its high rate transmission, and its excellent performance is related to the addition of cyclic prefix. Ensure that the system can not be affected by inter-symbol interference in multipath channels. However, the length of cyclic prefix sometimes takes up 20% bandwidth in LTE system. With the increase of spectrum efficiency, the efficiency of cyclic prefix in traditional OFDM system is limited due to the addition of cyclic prefix. For this reason, many scholars have carried out researches on the system of OFDM system without cyclic prefix (Cyclic Prefix-FREE OFDM,CP-FREE OFDM). At present, there is a cyclic prefix OFDM system, called symbol cyclic shift equalization OFDM system (Symbol Cyclic Shift Equalization OFDM,SCSE-OFDM), which is proposed by Xiqing Liu, in which the decision feedback equalization module is used at the receiver to eliminate the inter-symbol interference (ISI) problem. The subcarrier interference is eliminated by using CP recovery process. However, in this system, the receiver only considers the problem of zero-forcing equalization to produce noise amplification, and the channel estimation is not considered in this system. In order to improve the performance and improve the performance of the OFDM system without cyclic prefix, this paper will study from many aspects. In order to avoid the problem of noise amplification caused by zero forcing equalization, the minimum mean square error (Minimum Mean Square Error,MMSE) equalizer for only balanced noise amplification is considered in this paper. Then, under the assumption that the decision feedback error of Ghosh is Gao Si distribution, a MMSE equalizer with decision feedback error is designed. In order to understand the distribution of the decision feedback error, the stable distribution of the decision feedback error from Alpha was observed through the simulation analysis for the first time. In order to reduce the decision feedback error, the feedback threshold is set to reduce the serious error feedback, and the BER performance of the system is improved. Performance improvements are still limited due to the hard decision of the receiver. Therefore, this paper combines the Turbo equalization technique with the OFDM system without cyclic prefix. The MMSE equalization is connected with the Turbo decoding in the receiver, and the external information output from the Turbo decoding is estimated in the MMSE equalization, and then the external information of each bit is updated with the estimated symbols. The performance has been greatly improved through multiple loops. In order to make the OFDM system with no cyclic prefix more complete, the channel estimation under this system is studied in the end. Two channel estimation schemes based on least square (Least Square,LS) and least mean square (LMMSE) are designed under the condition of adding pilot frequency. In addition, a blind estimation scheme based on subspace theory is proposed without adding pilot frequency. The channel estimation is realized by over-sampling and repetition factor. Finally, in the case of lack of prior channel information, adaptive filtering is used for channel estimation. In particular, adaptive filtering techniques based on RLP (Recursive Least P-norm and LMP (Linear Mean P-norm) are studied under Alpha stable distributed noise. The accuracy of channel estimation is improved well.
【學(xué)位授予單位】：哈爾濱工業(yè)大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：TN929.53
本文編號(hào)：2281443
[Abstract]:Orthogonal Frequency Division Multiplexing (Orthogonal Frequency Division Multiplexing,OFDM) system has been widely used in LTE system because of its high rate transmission, and its excellent performance is related to the addition of cyclic prefix. Ensure that the system can not be affected by inter-symbol interference in multipath channels. However, the length of cyclic prefix sometimes takes up 20% bandwidth in LTE system. With the increase of spectrum efficiency, the efficiency of cyclic prefix in traditional OFDM system is limited due to the addition of cyclic prefix. For this reason, many scholars have carried out researches on the system of OFDM system without cyclic prefix (Cyclic Prefix-FREE OFDM,CP-FREE OFDM). At present, there is a cyclic prefix OFDM system, called symbol cyclic shift equalization OFDM system (Symbol Cyclic Shift Equalization OFDM,SCSE-OFDM), which is proposed by Xiqing Liu, in which the decision feedback equalization module is used at the receiver to eliminate the inter-symbol interference (ISI) problem. The subcarrier interference is eliminated by using CP recovery process. However, in this system, the receiver only considers the problem of zero-forcing equalization to produce noise amplification, and the channel estimation is not considered in this system. In order to improve the performance and improve the performance of the OFDM system without cyclic prefix, this paper will study from many aspects. In order to avoid the problem of noise amplification caused by zero forcing equalization, the minimum mean square error (Minimum Mean Square Error,MMSE) equalizer for only balanced noise amplification is considered in this paper. Then, under the assumption that the decision feedback error of Ghosh is Gao Si distribution, a MMSE equalizer with decision feedback error is designed. In order to understand the distribution of the decision feedback error, the stable distribution of the decision feedback error from Alpha was observed through the simulation analysis for the first time. In order to reduce the decision feedback error, the feedback threshold is set to reduce the serious error feedback, and the BER performance of the system is improved. Performance improvements are still limited due to the hard decision of the receiver. Therefore, this paper combines the Turbo equalization technique with the OFDM system without cyclic prefix. The MMSE equalization is connected with the Turbo decoding in the receiver, and the external information output from the Turbo decoding is estimated in the MMSE equalization, and then the external information of each bit is updated with the estimated symbols. The performance has been greatly improved through multiple loops. In order to make the OFDM system with no cyclic prefix more complete, the channel estimation under this system is studied in the end. Two channel estimation schemes based on least square (Least Square,LS) and least mean square (LMMSE) are designed under the condition of adding pilot frequency. In addition, a blind estimation scheme based on subspace theory is proposed without adding pilot frequency. The channel estimation is realized by over-sampling and repetition factor. Finally, in the case of lack of prior channel information, adaptive filtering is used for channel estimation. In particular, adaptive filtering techniques based on RLP (Recursive Least P-norm and LMP (Linear Mean P-norm) are studied under Alpha stable distributed noise. The accuracy of channel estimation is improved well.
【學(xué)位授予單位】：哈爾濱工業(yè)大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：TN929.53
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