【摘要】：正交頻分復(fù)用(Orthogonal Frequency Division Multiplexing, OFDM)系統(tǒng)在對(duì)抗多徑效應(yīng)時(shí)具有較高的可靠性,這使得OFDM在現(xiàn)代無線通信系統(tǒng)中得到了廣泛的應(yīng)用,例如基于長(zhǎng)期演進(jìn)(Long Term Evolution, LTE)的第四代移動(dòng)通信中正是采用了OFDM作為物理層的關(guān)鍵技術(shù)。在OFDM系統(tǒng)中,頻率選擇性信道被轉(zhuǎn)換為一組并行的頻率平坦信道,這組頻率平坦信道的信道響可以用一組具有單抽頭系數(shù)的均衡器來補(bǔ)償。這使得OFDM系統(tǒng)可以極大的簡(jiǎn)化均衡器設(shè)計(jì)同時(shí)維持較高的數(shù)據(jù)速率。正因如此,OFDM已經(jīng)在許多商業(yè)無線系統(tǒng)中被看作是一種標(biāo)準(zhǔn)的物理層技術(shù)。 為了進(jìn)行相干解調(diào),信道估計(jì)是OFDM接收機(jī)中必不可少的一個(gè)環(huán)節(jié)。傳統(tǒng)的信道估計(jì)技術(shù)采用基于導(dǎo)頻符號(hào)(Pilot)的方式進(jìn)行。為了改進(jìn)信道估計(jì)質(zhì)量,可以采用面向判決(Decision-Directed, DD)技術(shù),將判決后的數(shù)據(jù)符號(hào)反饋給信道估計(jì)器。假設(shè)判決結(jié)果是正確無誤的,這相當(dāng)于提高了導(dǎo)頻符號(hào)的密度,從而改善信道估計(jì)的性能。傳統(tǒng)的面向判決信道估計(jì)只能得到數(shù)據(jù)符號(hào)的硬判決結(jié)果,這種情況下的反饋方式也是唯一的,即將硬判決結(jié)果直接反饋給信道估計(jì)器。隨著Turbo接收機(jī)技術(shù)的采用,軟判決方式得到了廣泛應(yīng)用并顯著改善了接收機(jī)的性能。軟判決方式并不給出單一的判決結(jié)果,而是在獲得接收信號(hào)的條件下,給出星座圖中每個(gè)星座點(diǎn)的后驗(yàn)概率(A-posteriori Probability, AP)。相應(yīng)地,在進(jìn)行面向判決的信道估計(jì)時(shí),信道估計(jì)器從判決器獲得的也不再是硬判決結(jié)果,而是每個(gè)星座點(diǎn)的后驗(yàn)概率。與基于硬判決的面向判決不同,由于同時(shí)擁有每個(gè)星座點(diǎn)的出現(xiàn)概率,信道估計(jì)器可以有多種方法來利用這些后驗(yàn)概率進(jìn)行面向判決的信道估計(jì)。因此,反饋的方式不再唯一,而是有多種可能性。所以,需要研究最優(yōu)的反饋方式以達(dá)到最佳的信道估計(jì)性能,也就是說,需要解決軟判決方式下信道估計(jì)的最優(yōu)面向判決問題。 在一般的中低速移動(dòng)場(chǎng)景下,信道在一個(gè)OFDM符號(hào)內(nèi)產(chǎn)生的變化通�？梢院雎�,因此,傳統(tǒng)的OFDM系統(tǒng)中通常假設(shè)信道在一個(gè)OFDM符號(hào)持續(xù)時(shí)間內(nèi)不會(huì)發(fā)生改變。然而,隨著終端移動(dòng)性提高,多普勒頻移也相應(yīng)增加。在這種情況下,信道變化十分快速,在一個(gè)OFDM符號(hào)內(nèi)隨時(shí)間的變化已經(jīng)較為明顯。若仍然假設(shè)信道在一個(gè)OFDM符號(hào)內(nèi)不變,會(huì)帶來較大的估計(jì)誤差。因此,為了改進(jìn)快變信道條件下信道參數(shù)的估計(jì)性能,需要將信道在一個(gè)OFDM符號(hào)內(nèi)的變化考慮進(jìn)去。傳統(tǒng)的信道估計(jì)技術(shù)并不適用于這種情況,所以,需要研究快速時(shí)變信道條件下的信道估計(jì)問題。 提出了一種基于最大后驗(yàn)概率準(zhǔn)則(Maximum A-posteriori Probability, MAP)的迭代信道估計(jì)技術(shù),用以解決單發(fā)送天線、單個(gè)OFDM符號(hào)系統(tǒng)中的最優(yōu)面向判決問題。所得到的結(jié)果表明,應(yīng)該采用調(diào)和平均的方法來構(gòu)造信道估計(jì)時(shí)所用到的軟數(shù)據(jù)符號(hào)。雖然基于MAP準(zhǔn)則得到的方程沒有解析解,但可以通過不動(dòng)點(diǎn)迭代(Fixed Point Iteration, FPI)的方法迭代求解。進(jìn)一步發(fā)現(xiàn)所提出的基于不動(dòng)點(diǎn)迭代的MAP估計(jì)與期望-最大(Expection-Maximization, EM)算法具有緊密的聯(lián)系。這種聯(lián)系可以用來證明所提出的迭代信道估計(jì)算法的單調(diào)收斂特性。同時(shí),還證明了所提出的方法在大信噪比(Signal-to-noise ratio, SNR)條件下具有單步收斂特性。仿真結(jié)果表明,所提出的算法在大信噪比條件下可以達(dá)到Bayesian Cramer-Rao界(CRB)。 將基于MAP準(zhǔn)則的迭代信道估計(jì)技術(shù)一般化,解決了多發(fā)送天線、多OFDM符號(hào)系統(tǒng)中的最優(yōu)面向判決信道估計(jì)問題。有多根發(fā)送天線時(shí),期望發(fā)送天線上應(yīng)該采用調(diào)和平均的方式來構(gòu)造軟數(shù)據(jù)符號(hào),而干擾發(fā)送天線上則應(yīng)該采用算術(shù)平均的方式來構(gòu)造軟數(shù)據(jù)符號(hào)。同時(shí),當(dāng)考慮到時(shí)域上連續(xù)的多個(gè)OFDM符號(hào)后,信道參數(shù)在多個(gè)OFDM符號(hào)間的時(shí)變特性以及相關(guān)性也可以用來改進(jìn)信道估計(jì)的性能。與單發(fā)送天線、單OFDM符號(hào)的情況類似,所提出的迭代信道估計(jì)算法是基于MAP準(zhǔn)則推導(dǎo)得到的,并應(yīng)用不動(dòng)點(diǎn)迭代的方法來迭代求解。所提出的方法同樣可以保證收斂特性,同時(shí),當(dāng)信噪比足夠大時(shí),該算法仍然具有一步收斂特性。 研究了快速時(shí)變信道條件下OFDM系統(tǒng)中的信道估計(jì)技術(shù)。時(shí)變信道可以采用基展開模型(Basis Expansion Model, BEM)的方法來表示。對(duì)于BEM模型,基函數(shù)是已知的,只需要估計(jì)BEM模型的展開系數(shù)。本文中,考慮了連續(xù)的OFDM符號(hào)間的BEM模型系數(shù)的相關(guān)性,并基于最小均方誤差準(zhǔn)則(Minimum Means-Square-Error, MMSE)推導(dǎo)出了最優(yōu)的BEM系數(shù)估計(jì)器。采用本文所提出的方法,當(dāng)前OFDM符號(hào)的BEM系數(shù)可以通過對(duì)相鄰OFDM符號(hào)的BEM系數(shù)作線性組合的方式得到,因此不再需要為每個(gè)OFDM符號(hào)都插入導(dǎo)頻。本文中還考慮了實(shí)際的接收機(jī),包括基于限脈沖響應(yīng)濾波器的實(shí)現(xiàn)方案以及一種面向判決的迭代接收機(jī)結(jié)構(gòu)。
[Abstract]:Orthogonal Frequency Division Multiplexing (OFDM) systems have high reliability against multipath effects, which makes OFDM widely used in modern wireless communication systems. For example, in the fourth generation mobile communications based on Long Term Evolution (LTE), OFDM is used as physics. Key technologies of the layer. In OFDM systems, frequency selective channels are converted into a set of parallel frequency-flat channels whose channel responses can be compensated by a set of Equalizers with single-tap coefficients. This allows OFDM systems to greatly simplify equalizer design while maintaining high data rates. OFDM has been regarded as a standard physical layer technology in many commercial wireless systems.
In order to perform coherent demodulation, channel estimation is an indispensable part of OFDM receiver. Traditional channel estimation techniques are based on Pilot symbols. In order to improve the quality of channel estimation, decision-directed (DD) technique can be used to feedback the decision data symbols to the channel estimator. If the decision result is correct, it is equivalent to increasing the density of pilot symbols and improving the performance of channel estimation. Traditional decision-oriented channel estimation can only get the hard decision result of data symbols. In this case, the feedback method is also unique, that is, the hard decision result is directly fed back to the channel estimator. With the adoption of receiver technology, soft decision method is widely used and the performance of receiver is greatly improved. Soft decision method does not give a single decision result, but gives a posteriori probability (AP) of each constellation point in the constellation diagram under the condition of receiving the received signal. In channel estimation, the channel estimator obtains a posterior probability of each constellation point instead of a hard decision result. Unlike the hard decision-based decision-oriented channel estimator, which has the probability of each constellation point appearing simultaneously, the channel estimator can use these posterior probabilities for decision-oriented channel estimation in many ways. Therefore, the feedback method is no longer unique, but there are many possibilities. Therefore, it is necessary to study the optimal feedback method to achieve the best channel estimation performance, that is, to solve the problem of optimal decision-oriented channel estimation under soft decision mode.
In general low-speed mobile scenarios, the channel changes within an OFDM symbol are usually negligible. Therefore, in traditional OFDM systems, it is usually assumed that the channel does not change within the duration of an OFDM symbol. In order to improve the estimation performance of channel parameters under fast-varying channel conditions, it is necessary to take into account the channel changes within an OFDM symbol. The technique is not suitable for this case, so it is necessary to study the channel estimation problem under fast time-varying channel conditions.
An iterative channel estimation technique based on the Maximum A-posteriori Probability (MAP) criterion is proposed to solve the optimal decision-oriented problem for a single transmit antenna and a single OFDM symbol system. Although there is no analytical solution to the equation based on MAP criterion, it can be solved iteratively by Fixed Point Iteration (FPI). It is further found that the proposed MAP estimation based on fixed point iteration is closely related to Expection-Maximization (EM) algorithm. The monotone convergence property of the proposed iterative channel estimation algorithm is also proved. The proposed algorithm has one-step convergence property under the condition of large signal-to-noise ratio (SNR). Simulation results show that the proposed algorithm can reach the Bayesian Cramer-Rao bound (CRB) under the condition of large signal-to-noise ratio (SNR).
The iterative channel estimation technique based on MAP criterion is generalized to solve the problem of optimal decision-oriented channel estimation in multi-transmit antenna and multi-OFDM symbol systems. Similar to the case of single transmit antenna and single OFDM symbol, the proposed iterative channel estimation algorithm is based on MAP benchmark. The proposed method can also guarantee the convergence property. At the same time, the algorithm still has one-step convergence property when the signal-to-noise ratio is large enough.
Channel estimation techniques for OFDM systems with fast time-varying channels are studied. Time-varying channels can be represented by the Basis Expansion Model (BEM). For BEM models, the basis functions are known and only the expansion coefficients of BEM models are estimated. In this paper, the BEM model coefficients between consecutive OFDM symbols are considered. Based on the Minimum Means-Square-Error (MMSE), the optimal BEM coefficients estimator is derived. Using the proposed method, the BEM coefficients of current OFDM symbols can be obtained by linear combination of BEM coefficients of adjacent OFDM symbols, so it is no longer necessary to insert derivatives into each OFDM symbol. In this paper, we also consider the actual receiver, including the implementation scheme based on limited impulse response filter and a decision-oriented iterative receiver structure.
【學(xué)位授予單位】：北京交通大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2014
【分類號(hào)】：TN929.53

【共引文獻(xiàn)】

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本文編號(hào)：2184338