移动宽带MIMO-OFDM通信系统信道估计技术的研究
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摘要
大系统容量、高速数据传送和高频谱效率是现代无线通信的发展方向。多输入多输出(MIMO)和正交频分复用(OFDM)的结合是宽带无线通信最有发展前途的技术之一,由于其所具有的优势,MIMO-OFDM技术已经成为下一代移动通信系统下行高速数据传输的核心技术。MIMO-OFDM系统的空时解码和信号检测都需要精确的信道参数,因此信道估计是MIMO-OFDM无线通信系统的关键技术之一。由于接收天线上的导频接收信号是来自多个发射天线的导频信号经过各个独立信道后信号的叠加,这导致MIMO-OFDM系统信道估计的难度加大。信道估计技术通常有两类,一类是基于训练序列,一类是盲信道估计技术。目前通信系统主要应用基于训练序列的信道估计技术。论文主要研究移动宽带MIMO-OFDM系统基于训练序列的相关信道估计技术。
论文首先研究了MIMO-OFDM系统信道估计的最优导频序列。利用类循环Jacket矩阵的特性,提出了一种新的最优导频序列推导方法。基于最小平方(LS)信道估计的最小均方误差(MSE),通过构建类循环Jacket矩阵,分别推导出MIMO-OFDM系统分布在一个和多个OFDM符号上最优导频序列的表达式及其必要条件。与参考文献相比,论文的推导结果更具有一般性。推导结果表明：(1)最优导频序列在频域上是等功率、等间距和相移正交的；(2)最优导频序列的导频数量与发射天线的数量成正比,随着发射天线的增多,最优导频序列的导频数量成倍增加。为了提高传输效率和降低峰均比,MIMO-OFDM系统信道估计通常应用分布在多个OFDM符号上的最优导频序列。
MIMO-OFDM系统利用分布在多个OFDM符号上的最优导频序列进行信道估计,在在慢衰落信道能够保持较好的性能,但在快衰落信道其性能恶化严重,因而在实际系统中分布在多个OFDM符号上最优导频序列的应用受到限制。为了提高分布在多个OFDM符号上最优导频序列在快衰落信道中的性能,论文提出了一种基于虚拟导频的MIMO-OFDM信道估计改进方法。虚拟导频只有数学上的意义,实际上并不存在。如果在特定的子载波上数据位置上叠加虚拟导频,可得到由实际和虚拟导频组成的特殊梳状导频结构。根据最优导频序列在一个和多个OFDM符号上分布的特点可知,每个发射天线上的虚拟导频等于其他OFDM符号同一子载波上的实际导频。假定在所有发射天线上特定子载波上的数据位置发送叠加的虚拟导频,则在每个接收天线上相同的位置上就有相对应的叠加虚拟导频接收信号。虚拟导频接收信号可以利用同一子载波上相邻的实际接收导频信号通过维纳滤波或线性内插得到。然后,利用LS信道估计,每个OFDM符号的信道估计参数可以利用OFDM符号中的实际和虚拟导频接收信号进行计算得到。仿真结果表明,在快衰落信道,基于虚拟导频的信道估计改进方法比原来方法的性能明显提高了,其性能接近梳状结构最优导频序列的性能,而且导频数量远远小于梳状结构的导频数量。同时,虚拟导频接收信号只需要在时域上进行一维维纳滤波或者线性内插运算得到,计算复杂度很低。另外,论文还提出了一种基于连续滑窗平均的信道估计改进方法,它也能够大大提高分布在多个OFDM符号上最优导频序列信道估计的性能。
论文对推导出的MIMO-OFDM系统最优导频序列的性能进行了分析和评估,结果表明：(1)利用最优导频序列进行信道估计,不但能够提高系统性能,而且可以大大降低计算复杂度；(2)推导出的最优导频序列具有比较高的峰均功率比,导致其所在OFDM符号有较高的峰均比；(3)最优导频序列分布的OFDM符号数量越多,其峰均比越低。考虑到实际应用的限制,信道估计必须使用低峰均比的最优导频序列。为了得到低峰均比的最优导频序列,利用部分传送序列的思想,对导频序列乘以随机相位矢量。为了便于实现,选用PN序列作为随机相位矢量与推导出的最优导频序列相乘,能得到具有较低峰均比的改进最优导频序列。仿真结果证明了改进最优导频序列及其所在OFDM符号的峰均比都大大地下降,且性能仍保持最优。仿真结果还表明：最优导频序列分布在多个OFDM符号上比分布在一个OFDM符号上峰均比要低得多。
Large system capacity, high speed data rate and spectrum effciency are the tendencyof the modern wireless communications. The combination of multiple-input multiple-output(MIMO) and orthogonal frequency division multiplexing (OFDM) is one of the most promis-ing techniques for broadband wireless communications. For its advantage, MIMO-OFDMhas become a core technique in the high speed data down-link solution for the next gen-eration mobile wireless communication. Since both space-time decoding and data detec-tion require accurate channel parameters, channel estimation is one of the key techniquesfor MIMO-OFDM wireless communication systems. However, at the receiver antenna thereceived pilot signals are superposition of the pilot signals transmitted from different trans-mit antennas and experiencing the different independent channels respectively, which bringsmore diffculties for channel estimation in MIMO-OFDM systems.In general two classesof methods are available for channel estimation, one is based on training sequences, andthe other is blind channel estimation. Now the training sequences based channel estimationremains attractive in practice. This thesis mainly studies the correlative channel estimationtechniques based on training sequences in mobile broadband MIMO-OFDM communicationsystems.
This thesis frstly studies the optimal training sequences for channel estimation inMIMO-OFDM systems. By exploiting the properties of co-cyclic Jacket matrices, this thesisproposes a novel scheme for obtaining the optimal pilot sequences. Based on the minimummean square error (MSE) of the least squares (LS) channel estimation, the optimal pilot se-quences distributed over one and multiple OFDM symbols and their necessary conditionsare derived respectively by constructing the co-cyclic Jacket matrices. This derived resultsare more generalized than those in references. It is shown that (1) The derived optimal pi-lot sequences are equi-powered, equi-spaced and phase-shift orthogonal;(2) The number ofpilots in the optimal pilot sequences is proportionable to that of the transmit antennas, itmultiples with increasing the number of transmit antennas. To improve the data effciency and reduce the PAPR, the optimal pilot sequences over multiple OFDM symbols are oftenused for channel estimation in MIMO-OFDM systems.
In MIMO-OFDM systems, although the channel estimation scheme using the optimalpilot sequences over multiple OFDM symbols can achieve the perfect performance for slowlyfading channels, its performance deteriorates badly for very fast fading channels, which lim-its the use of the scheme in practical systems. To improve the performance of the optimalpilot sequences over multiple OFDM symbols in fast fading channels, this thesis proposes anovel enhanced channel estimation method using virtual pilot tones in MIMO-OFDM sys-tems. The virtual pilot tones only have the arithmetical meaning and don’t exist actually.If the virtual pilot tones are arithmetically superposed at the data locations over the specifcsub-carriers, then a special comb-type pilot structure is formed from the real and virtual pilottones. According to the characteristic of the optimal pilot sequences over one and multipleOFDM symbols, the virtual pilot tones are the same as the real pilot tones over the samesub-carriers in other OFDM symbols. Assuming that the superimposed virtual pilot tones atthe data locations over the specifc sub-carriers are transmitted from all transmit antennas,the corresponding virtual received pilot signals at the same locations are obtained from theneighboring real received pilot signals over the same sub-carriers by Wiener flter or linearinterpolation. Based on the LS channel estimation, the channel parameters can be obtainedfrom the combination of the virtual and real received pilot signals over each OFDM symbol.Simulation results show that the proposed channel estimation method greatly outperforms theprevious method for the optimal pilot sequences over multiple OFDM symbols in fast fadingchannels, as well as approaches the method for the comb-type optimal pilot sequences inperformance. Furthermore, the novel scheme requires much less pilot tones than comb-typeoptimal pilot sequences. Meanwhile, the virtual receive pilot tones only depend on one di-mensional Wiener flter or linear interpolation and has very low computational complexity.This thesis also proposes another enhanced channel estimation based on consecutive slid-ing windows and averaging, the scheme can also improve effciently the performance of theoptimal pilot sequences over multiple OFDM symbols in fast fading channels.
This thesis also analyzes and evaluates the performance of the derived optimal pilotsequences in MIMO-OFDM systems, the results show that (1) The channel estimation us-ing the optimal pilot sequences can not only get the optimal performance, but also havevery low computational complexity in MIMO-OFDM systems;(2) The derived optimal pilotsequences have very high peak-to-average power ration(PAPR), which results in the highPAPR for the OFDM symbols that include the optimal pilot sequences;(3) Increasing the number of the OFDM symbols where the optimal pilot sequences distributed, the PAPR be-come lower. Taking into account practical constraints, the optimal pilot sequences with lowPAPR must be used. To obtain the optimal pilot sequences with low PAPR, the pilot se-quences are weighted by random phase vectors according to the idea of the Partial TransportSequences(PTS). For simple implementation in practice, the PN sequences are selected asthe random phase vectors, the optimal pilot sequences with low PAPR are obtained. Thesimulation results show that the PAPR of the enhanced optimal pilot sequences is reducedgreatly, and the optimal performance remains.The results also show that the PAPR of theoptimal pilot sequences over multiple OFDM symbols is lower than those over one OFDMsymbol.
论文首先研究了MIMO-OFDM系统信道估计的最优导频序列。利用类循环Jacket矩阵的特性,提出了一种新的最优导频序列推导方法。基于最小平方(LS)信道估计的最小均方误差(MSE),通过构建类循环Jacket矩阵,分别推导出MIMO-OFDM系统分布在一个和多个OFDM符号上最优导频序列的表达式及其必要条件。与参考文献相比,论文的推导结果更具有一般性。推导结果表明：(1)最优导频序列在频域上是等功率、等间距和相移正交的；(2)最优导频序列的导频数量与发射天线的数量成正比,随着发射天线的增多,最优导频序列的导频数量成倍增加。为了提高传输效率和降低峰均比,MIMO-OFDM系统信道估计通常应用分布在多个OFDM符号上的最优导频序列。
MIMO-OFDM系统利用分布在多个OFDM符号上的最优导频序列进行信道估计,在在慢衰落信道能够保持较好的性能,但在快衰落信道其性能恶化严重,因而在实际系统中分布在多个OFDM符号上最优导频序列的应用受到限制。为了提高分布在多个OFDM符号上最优导频序列在快衰落信道中的性能,论文提出了一种基于虚拟导频的MIMO-OFDM信道估计改进方法。虚拟导频只有数学上的意义,实际上并不存在。如果在特定的子载波上数据位置上叠加虚拟导频,可得到由实际和虚拟导频组成的特殊梳状导频结构。根据最优导频序列在一个和多个OFDM符号上分布的特点可知,每个发射天线上的虚拟导频等于其他OFDM符号同一子载波上的实际导频。假定在所有发射天线上特定子载波上的数据位置发送叠加的虚拟导频,则在每个接收天线上相同的位置上就有相对应的叠加虚拟导频接收信号。虚拟导频接收信号可以利用同一子载波上相邻的实际接收导频信号通过维纳滤波或线性内插得到。然后,利用LS信道估计,每个OFDM符号的信道估计参数可以利用OFDM符号中的实际和虚拟导频接收信号进行计算得到。仿真结果表明,在快衰落信道,基于虚拟导频的信道估计改进方法比原来方法的性能明显提高了,其性能接近梳状结构最优导频序列的性能,而且导频数量远远小于梳状结构的导频数量。同时,虚拟导频接收信号只需要在时域上进行一维维纳滤波或者线性内插运算得到,计算复杂度很低。另外,论文还提出了一种基于连续滑窗平均的信道估计改进方法,它也能够大大提高分布在多个OFDM符号上最优导频序列信道估计的性能。
论文对推导出的MIMO-OFDM系统最优导频序列的性能进行了分析和评估,结果表明：(1)利用最优导频序列进行信道估计,不但能够提高系统性能,而且可以大大降低计算复杂度；(2)推导出的最优导频序列具有比较高的峰均功率比,导致其所在OFDM符号有较高的峰均比；(3)最优导频序列分布的OFDM符号数量越多,其峰均比越低。考虑到实际应用的限制,信道估计必须使用低峰均比的最优导频序列。为了得到低峰均比的最优导频序列,利用部分传送序列的思想,对导频序列乘以随机相位矢量。为了便于实现,选用PN序列作为随机相位矢量与推导出的最优导频序列相乘,能得到具有较低峰均比的改进最优导频序列。仿真结果证明了改进最优导频序列及其所在OFDM符号的峰均比都大大地下降,且性能仍保持最优。仿真结果还表明：最优导频序列分布在多个OFDM符号上比分布在一个OFDM符号上峰均比要低得多。
Large system capacity, high speed data rate and spectrum effciency are the tendencyof the modern wireless communications. The combination of multiple-input multiple-output(MIMO) and orthogonal frequency division multiplexing (OFDM) is one of the most promis-ing techniques for broadband wireless communications. For its advantage, MIMO-OFDMhas become a core technique in the high speed data down-link solution for the next gen-eration mobile wireless communication. Since both space-time decoding and data detec-tion require accurate channel parameters, channel estimation is one of the key techniquesfor MIMO-OFDM wireless communication systems. However, at the receiver antenna thereceived pilot signals are superposition of the pilot signals transmitted from different trans-mit antennas and experiencing the different independent channels respectively, which bringsmore diffculties for channel estimation in MIMO-OFDM systems.In general two classesof methods are available for channel estimation, one is based on training sequences, andthe other is blind channel estimation. Now the training sequences based channel estimationremains attractive in practice. This thesis mainly studies the correlative channel estimationtechniques based on training sequences in mobile broadband MIMO-OFDM communicationsystems.
This thesis frstly studies the optimal training sequences for channel estimation inMIMO-OFDM systems. By exploiting the properties of co-cyclic Jacket matrices, this thesisproposes a novel scheme for obtaining the optimal pilot sequences. Based on the minimummean square error (MSE) of the least squares (LS) channel estimation, the optimal pilot se-quences distributed over one and multiple OFDM symbols and their necessary conditionsare derived respectively by constructing the co-cyclic Jacket matrices. This derived resultsare more generalized than those in references. It is shown that (1) The derived optimal pi-lot sequences are equi-powered, equi-spaced and phase-shift orthogonal;(2) The number ofpilots in the optimal pilot sequences is proportionable to that of the transmit antennas, itmultiples with increasing the number of transmit antennas. To improve the data effciency and reduce the PAPR, the optimal pilot sequences over multiple OFDM symbols are oftenused for channel estimation in MIMO-OFDM systems.
In MIMO-OFDM systems, although the channel estimation scheme using the optimalpilot sequences over multiple OFDM symbols can achieve the perfect performance for slowlyfading channels, its performance deteriorates badly for very fast fading channels, which lim-its the use of the scheme in practical systems. To improve the performance of the optimalpilot sequences over multiple OFDM symbols in fast fading channels, this thesis proposes anovel enhanced channel estimation method using virtual pilot tones in MIMO-OFDM sys-tems. The virtual pilot tones only have the arithmetical meaning and don’t exist actually.If the virtual pilot tones are arithmetically superposed at the data locations over the specifcsub-carriers, then a special comb-type pilot structure is formed from the real and virtual pilottones. According to the characteristic of the optimal pilot sequences over one and multipleOFDM symbols, the virtual pilot tones are the same as the real pilot tones over the samesub-carriers in other OFDM symbols. Assuming that the superimposed virtual pilot tones atthe data locations over the specifc sub-carriers are transmitted from all transmit antennas,the corresponding virtual received pilot signals at the same locations are obtained from theneighboring real received pilot signals over the same sub-carriers by Wiener flter or linearinterpolation. Based on the LS channel estimation, the channel parameters can be obtainedfrom the combination of the virtual and real received pilot signals over each OFDM symbol.Simulation results show that the proposed channel estimation method greatly outperforms theprevious method for the optimal pilot sequences over multiple OFDM symbols in fast fadingchannels, as well as approaches the method for the comb-type optimal pilot sequences inperformance. Furthermore, the novel scheme requires much less pilot tones than comb-typeoptimal pilot sequences. Meanwhile, the virtual receive pilot tones only depend on one di-mensional Wiener flter or linear interpolation and has very low computational complexity.This thesis also proposes another enhanced channel estimation based on consecutive slid-ing windows and averaging, the scheme can also improve effciently the performance of theoptimal pilot sequences over multiple OFDM symbols in fast fading channels.
This thesis also analyzes and evaluates the performance of the derived optimal pilotsequences in MIMO-OFDM systems, the results show that (1) The channel estimation us-ing the optimal pilot sequences can not only get the optimal performance, but also havevery low computational complexity in MIMO-OFDM systems;(2) The derived optimal pilotsequences have very high peak-to-average power ration(PAPR), which results in the highPAPR for the OFDM symbols that include the optimal pilot sequences;(3) Increasing the number of the OFDM symbols where the optimal pilot sequences distributed, the PAPR be-come lower. Taking into account practical constraints, the optimal pilot sequences with lowPAPR must be used. To obtain the optimal pilot sequences with low PAPR, the pilot se-quences are weighted by random phase vectors according to the idea of the Partial TransportSequences(PTS). For simple implementation in practice, the PN sequences are selected asthe random phase vectors, the optimal pilot sequences with low PAPR are obtained. Thesimulation results show that the PAPR of the enhanced optimal pilot sequences is reducedgreatly, and the optimal performance remains.The results also show that the PAPR of theoptimal pilot sequences over multiple OFDM symbols is lower than those over one OFDMsymbol.
引文
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