MB-OFDM超宽带系统的数据辅助同步估计算法研究
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摘要
同步技术是基于MB-OFDM的超宽带系统关键技术之一，贯穿于系统基带工作的整个过程中，其工作周期高于其他基带技术，因此同步问题被认为是高功率消耗的技术。此外，同步效率直接影响着整个系统的性能，同步过程中的任何错误与偏差，比如定时同步过程中的捕获与估计误差、载波估计误差等，都将严重影响系统性能。总体来说，在基于MB-OFDM的超宽带系统中，同步包括物理层同步与网络层同步两部分，物理层同步技术包括时域同步与频域同步两部分。在时域同步完成的基础上，系统可以完成FFT处理，实现频域的信道估计与量化，以及频偏的进一步校正与跟踪。只有实现有效同步，才可以保证无误进行时频转换，准确接收信息并进行相应的处理。因此可以说基于MB-OFDM的超宽带物理层同步技术是接收机准确接收信息的基础。
目前关于MB-OFDM超宽带同步系统的研究，虽然从不同方面解决了同步中的部分关键问题，然而仍然存在着诸多不完善之处，如在导频识别方面没有有效的解决方案，符号定时同步方面存在着门限受限性、定时结果不确定不唯一、低信噪比下性能不佳，频偏估计方面算法计算复杂度高等问题。本文通过对基于MB-OFDM的超宽带系统同步技术进行深入研究，针对研究中存在的问题，提出了基于能量峰值检测的导频识别方法、基于极性比对的符号定时同步方法、简化的极性比对唯一性定时同步法、非全周期相位旋转频偏估计法等同步方案，一方面为目前研究中存在的问题提供了解决方案，另一方面，也实现了能耗与同步性能的折中。本文主要创新点包括以下四个方面：
1.首次提出基于能量峰值检测的导频模式识别方法。接收机在接收到第一个导频符号后，计算该符号能量，并以该能量的70%作为门限，继而计算与第一个接收导频符号相隔多个符号间隔的一个符号周期内的符号能量，并与门限阈值相比对，判断所采取的发送导频模式。该方法的提出避免了相关峰值检测法的弱抗噪声性，为导频识别提供了可行的研究方案，在TFC1、TFC3、TFC5的导频模式结构下，都可以有效识别当前采取的导频模式。
2.提出了基于导频极性比对的唯一性定时同步方法，利用导频符号极性的不变特性，在门限类同步方法的基础上，通过计算接收符号与预定义导频符号的极性异或运算，判断接收符号的相对极性，当所有通过门限的采样所在符号极性都与导频序列相同时，最后一个通过门限的采样的下一个采样即为同步估计点；当所有通过门限的采样所在符号极性都与导频序列不同时，第一个通过门限的采样即为同步估计点；如果通过门限的采样所在符号极性并不都相同，则需要进行互相关峰值检测，以保证定时同步结果的唯一性，实现符号定时同步。仿真表明，本文所提的基于极性判断的符号定时同步方案与CBTS相比，不仅可以保证定时同步结果的唯一性，而且估计结果的MSE与同步概率都有明显改善，尤其是在低信噪比环境下，本文所提方法表现出了更高的稳定性。此外，本方案的提出可以适用于一般性门限类同步算法，提升其性能，保证结果唯一性，同时降低算法的门限依赖性。
3.提出了简化的导频极性比对定时同步法，在极性比对时，不再比对通过门限的所有采样所在符号的极性，而只比较第一个通过门限的采样所在符号的极性，如果该符号极性与PS序列相同，则还需要进行互相关峰值检测；如果该符号极性与FS序列一致，则第一个通过门限的采样即为定时同步估计值。仿真与分析表明该方法在保证定时同步唯一性的同时，定时同步的MSE与同步概率性能都接近于本文所提出的基于导频极性比对的唯一性定时同步方法，优于CBTS算法。在算法复杂度方面，相对于导频极性比对法，减少了M-1次异或运算与加和运算。
4.首次提出基于相位旋转的频偏估计算法，以非全周期估计代替全符号周期进行基于MB-OFDM的超宽带系统频偏估计，通过非全周期频偏估计结合相位旋转，充分考虑了多径信道的衰落特性，降低算法复杂度近50%，同时保持了算法的频偏估计性能；提出并验证了频偏估计的CRLB不仅与估计运算的符号长度有关，还受相位旋转角度的影响，当符号长度固定时，相位旋转角度越大，CRLB越小，算法性能越优越。将CAZAC序列应用于基于MB-OFDM的超宽带系统，以之代替系统原有的导频序列，不改变导频结构，不影响系统性能，可灵活定义单符号周期内的符号组成。
本文通过对基于MB-OFDM的超宽带系统同步技术的研究，在导频模式判断、符号定时同步、载波频偏估计方面提出了有效的同步方案，突破了门限依赖性、定时同步不确定等局限性。研究成果将推动同步技术的发展。
Synchronization is one of key technologies, particully with base-band singals, forMB-OFDM based UWB systems. Varieties of applications based on MB-OFDM UWBtechnology, such as wireless home environment systems, are power sensitive systems,which have demanding requirement, like low power and low complexity. Synchronizationissue goes through receiving process, which has much more duration than other basebandtechniques. As a result, synchronization is of high power consumption, generally. Inaddition, synchronization effectiveness directly affects performance of the system. Anyerrors in synchronization processes, such as timing shift in the capture and carrier estimationerror would rapidly degrade system performance. Overall, the system synchronizationcovers physical layer and network layer synchronization. The physical layer basedsynchronization technologies are with time domain and frequency domain synchronization.Only if time domain synchronization is completed, does the system could implement FFTprocessing, further correction of frequency domain channel estimation and quantification, aswell as frequency offset and tracking. With effective synchronization, time frequencyconversion could be worked out, as long as receiving information and appropriate treatment.Therefore physical layer synchronization is the foundation of receiver performance forMB-OFDM based UWB systems.
Current researches have solved synchronization key issues for MB-OFDM based UWBsystems from different aspects. However, there are still many imperfections, such asineffective pilot identification schemes, threshold restriction and timing uncertain outcomein symbol timing synchronization, high computational complexity in frequency offsetestimation, and so on. In this paper, a preamble identification method based on derivative ofenergy peak detection, a symbol timing synchronization method based on preamblepolarities comparison, a simplifying timing synchronization approach guaranteeing theuniqueness of synchronization sample, and a synchronization solution of non-full-cycle phase rotation frequency offset estimation method are proposed based on deep study ofsystem requirements. The research provides a solution to get a compromise between energyconsumption and synchronization performance. The main innovations of this article are asfollows.
1. A pattern recognition method was first proposed based on derivative of energy peakdetection. Receiver calculates energy of the first received pilot symbol.70%of the energy istaken as the threshold. Then energy of a symbol period of several symbol periods distanceaway from the first pilot symbols is computed and compared with the threshold. Matchingthe output of pilot symbols energy with different patterns, preamble modes could beestimated. The proposed method avoids weak noise resistance of peak detection, whichcould effectively distinguish preamble modes of pilots, no matter with TFC1, TFC3, orTFC5.
2. A new approach for timing synchronization estimation with polarity comparison forMB-OFDM based UWB systems is proposed. We attempt to locate the start sample of framesequences (FS) by calculating difference of the two cross correlation functions amongreceived symbols, the successive received symbols and predefined preamble sequence. Itmakes sense to propose polarity comparison and identification ideas to the scenario, thecross correlation difference exceeding predefined threshold is not unique. If polarities ofselected symbols are not all the same, the estimator is put forward to find out a peak ofcorrelation summation to figure out the unique timing point and promote synchronizationaccuracy. Uniqueness and accuracy of timing synchronization, therefore, could beguaranteed. The performance of the proposed estimator is evaluated by MSE andsynchronization probability. The proposed estimator could carry out timing synchronizationfor MB-OFDM based UWB systems and make the uniqueness of timing index for sure. TheMSEs of the proposed estimator are evidently lower than the reference method for a greatdeal. Total synchronization probability could reach as much as99%.
3. A simplified polarity comparing and uniqueness guarantee (PCUG) timingsynchronization estimation approach for MB-OFDM based UWB systems is proposed. Pilotpolarities comparison is carried out for the first exceeding threshold sample instead of allsamples exceeding threshold. If the polarity of the first exceeding threshold sample is thesame with PS polarity, a further step is needed; otherwise, it is a FS sequence, the currentsample is the estimated timing point. The proposed algorithm could make the uniqueness oftiming index for sure. Simulations indicate that this proposal could get a performanceapproximating with polarity comparing method. Meanwhile, a computation complexity ofM-1XOR and adding operation are reduced.
4. In view of the complexity and high accuracy requirement of frequency offset estimatealgorithm for MB-OFDM based UWB system,1/2and1/4times of pilot symbol cycleCAZAC sequences are defined as pilot frequency sequence. Estimation within one symbol cycle is carried out through averaging samples of two neighboring symbol cycles, afterwhich the operation expands to all the symbol cycles in one band group. Taking multipatheffect into account, the concept of phase rotation is proposed for a further step. Adjustingthe phase difference of estimated symbols by phase rotation, cross estimation could be done.Theoretical and simulation analysis indicate that CRLB does not only relate to estimationsymbol length, but also be influenced by phase difference of estimation symbols seriously.In the condition that the length of estimation is fixed, the bigger the phase rotation angle is,the smaller the CRLB is. The algorithm performs well. The complexity of proposedalgorithm is less than full cycle average estimation method for almost50%.
目前关于MB-OFDM超宽带同步系统的研究，虽然从不同方面解决了同步中的部分关键问题，然而仍然存在着诸多不完善之处，如在导频识别方面没有有效的解决方案，符号定时同步方面存在着门限受限性、定时结果不确定不唯一、低信噪比下性能不佳，频偏估计方面算法计算复杂度高等问题。本文通过对基于MB-OFDM的超宽带系统同步技术进行深入研究，针对研究中存在的问题，提出了基于能量峰值检测的导频识别方法、基于极性比对的符号定时同步方法、简化的极性比对唯一性定时同步法、非全周期相位旋转频偏估计法等同步方案，一方面为目前研究中存在的问题提供了解决方案，另一方面，也实现了能耗与同步性能的折中。本文主要创新点包括以下四个方面：
1.首次提出基于能量峰值检测的导频模式识别方法。接收机在接收到第一个导频符号后，计算该符号能量，并以该能量的70%作为门限，继而计算与第一个接收导频符号相隔多个符号间隔的一个符号周期内的符号能量，并与门限阈值相比对，判断所采取的发送导频模式。该方法的提出避免了相关峰值检测法的弱抗噪声性，为导频识别提供了可行的研究方案，在TFC1、TFC3、TFC5的导频模式结构下，都可以有效识别当前采取的导频模式。
2.提出了基于导频极性比对的唯一性定时同步方法，利用导频符号极性的不变特性，在门限类同步方法的基础上，通过计算接收符号与预定义导频符号的极性异或运算，判断接收符号的相对极性，当所有通过门限的采样所在符号极性都与导频序列相同时，最后一个通过门限的采样的下一个采样即为同步估计点；当所有通过门限的采样所在符号极性都与导频序列不同时，第一个通过门限的采样即为同步估计点；如果通过门限的采样所在符号极性并不都相同，则需要进行互相关峰值检测，以保证定时同步结果的唯一性，实现符号定时同步。仿真表明，本文所提的基于极性判断的符号定时同步方案与CBTS相比，不仅可以保证定时同步结果的唯一性，而且估计结果的MSE与同步概率都有明显改善，尤其是在低信噪比环境下，本文所提方法表现出了更高的稳定性。此外，本方案的提出可以适用于一般性门限类同步算法，提升其性能，保证结果唯一性，同时降低算法的门限依赖性。
3.提出了简化的导频极性比对定时同步法，在极性比对时，不再比对通过门限的所有采样所在符号的极性，而只比较第一个通过门限的采样所在符号的极性，如果该符号极性与PS序列相同，则还需要进行互相关峰值检测；如果该符号极性与FS序列一致，则第一个通过门限的采样即为定时同步估计值。仿真与分析表明该方法在保证定时同步唯一性的同时，定时同步的MSE与同步概率性能都接近于本文所提出的基于导频极性比对的唯一性定时同步方法，优于CBTS算法。在算法复杂度方面，相对于导频极性比对法，减少了M-1次异或运算与加和运算。
4.首次提出基于相位旋转的频偏估计算法，以非全周期估计代替全符号周期进行基于MB-OFDM的超宽带系统频偏估计，通过非全周期频偏估计结合相位旋转，充分考虑了多径信道的衰落特性，降低算法复杂度近50%，同时保持了算法的频偏估计性能；提出并验证了频偏估计的CRLB不仅与估计运算的符号长度有关，还受相位旋转角度的影响，当符号长度固定时，相位旋转角度越大，CRLB越小，算法性能越优越。将CAZAC序列应用于基于MB-OFDM的超宽带系统，以之代替系统原有的导频序列，不改变导频结构，不影响系统性能，可灵活定义单符号周期内的符号组成。
本文通过对基于MB-OFDM的超宽带系统同步技术的研究，在导频模式判断、符号定时同步、载波频偏估计方面提出了有效的同步方案，突破了门限依赖性、定时同步不确定等局限性。研究成果将推动同步技术的发展。
Synchronization is one of key technologies, particully with base-band singals, forMB-OFDM based UWB systems. Varieties of applications based on MB-OFDM UWBtechnology, such as wireless home environment systems, are power sensitive systems,which have demanding requirement, like low power and low complexity. Synchronizationissue goes through receiving process, which has much more duration than other basebandtechniques. As a result, synchronization is of high power consumption, generally. Inaddition, synchronization effectiveness directly affects performance of the system. Anyerrors in synchronization processes, such as timing shift in the capture and carrier estimationerror would rapidly degrade system performance. Overall, the system synchronizationcovers physical layer and network layer synchronization. The physical layer basedsynchronization technologies are with time domain and frequency domain synchronization.Only if time domain synchronization is completed, does the system could implement FFTprocessing, further correction of frequency domain channel estimation and quantification, aswell as frequency offset and tracking. With effective synchronization, time frequencyconversion could be worked out, as long as receiving information and appropriate treatment.Therefore physical layer synchronization is the foundation of receiver performance forMB-OFDM based UWB systems.
Current researches have solved synchronization key issues for MB-OFDM based UWBsystems from different aspects. However, there are still many imperfections, such asineffective pilot identification schemes, threshold restriction and timing uncertain outcomein symbol timing synchronization, high computational complexity in frequency offsetestimation, and so on. In this paper, a preamble identification method based on derivative ofenergy peak detection, a symbol timing synchronization method based on preamblepolarities comparison, a simplifying timing synchronization approach guaranteeing theuniqueness of synchronization sample, and a synchronization solution of non-full-cycle phase rotation frequency offset estimation method are proposed based on deep study ofsystem requirements. The research provides a solution to get a compromise between energyconsumption and synchronization performance. The main innovations of this article are asfollows.
1. A pattern recognition method was first proposed based on derivative of energy peakdetection. Receiver calculates energy of the first received pilot symbol.70%of the energy istaken as the threshold. Then energy of a symbol period of several symbol periods distanceaway from the first pilot symbols is computed and compared with the threshold. Matchingthe output of pilot symbols energy with different patterns, preamble modes could beestimated. The proposed method avoids weak noise resistance of peak detection, whichcould effectively distinguish preamble modes of pilots, no matter with TFC1, TFC3, orTFC5.
2. A new approach for timing synchronization estimation with polarity comparison forMB-OFDM based UWB systems is proposed. We attempt to locate the start sample of framesequences (FS) by calculating difference of the two cross correlation functions amongreceived symbols, the successive received symbols and predefined preamble sequence. Itmakes sense to propose polarity comparison and identification ideas to the scenario, thecross correlation difference exceeding predefined threshold is not unique. If polarities ofselected symbols are not all the same, the estimator is put forward to find out a peak ofcorrelation summation to figure out the unique timing point and promote synchronizationaccuracy. Uniqueness and accuracy of timing synchronization, therefore, could beguaranteed. The performance of the proposed estimator is evaluated by MSE andsynchronization probability. The proposed estimator could carry out timing synchronizationfor MB-OFDM based UWB systems and make the uniqueness of timing index for sure. TheMSEs of the proposed estimator are evidently lower than the reference method for a greatdeal. Total synchronization probability could reach as much as99%.
3. A simplified polarity comparing and uniqueness guarantee (PCUG) timingsynchronization estimation approach for MB-OFDM based UWB systems is proposed. Pilotpolarities comparison is carried out for the first exceeding threshold sample instead of allsamples exceeding threshold. If the polarity of the first exceeding threshold sample is thesame with PS polarity, a further step is needed; otherwise, it is a FS sequence, the currentsample is the estimated timing point. The proposed algorithm could make the uniqueness oftiming index for sure. Simulations indicate that this proposal could get a performanceapproximating with polarity comparing method. Meanwhile, a computation complexity ofM-1XOR and adding operation are reduced.
4. In view of the complexity and high accuracy requirement of frequency offset estimatealgorithm for MB-OFDM based UWB system,1/2and1/4times of pilot symbol cycleCAZAC sequences are defined as pilot frequency sequence. Estimation within one symbol cycle is carried out through averaging samples of two neighboring symbol cycles, afterwhich the operation expands to all the symbol cycles in one band group. Taking multipatheffect into account, the concept of phase rotation is proposed for a further step. Adjustingthe phase difference of estimated symbols by phase rotation, cross estimation could be done.Theoretical and simulation analysis indicate that CRLB does not only relate to estimationsymbol length, but also be influenced by phase difference of estimation symbols seriously.In the condition that the length of estimation is fixed, the bigger the phase rotation angle is,the smaller the CRLB is. The algorithm performs well. The complexity of proposedalgorithm is less than full cycle average estimation method for almost50%.
引文
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