重叠复用技术应用在OFDM系统及其Turbo码技术的研究

【作者】 王颖；

【导师】 李道本；

【作者基本信息】 北京邮电大学 ， 信号与信息处理， 2013， 博士

【摘要】 为了提高无线通信系统的传信率,频谱利用率,以及功率效率,本论文引入重叠复用理论和技术,提出了相应的创新性解决方案,具体包括时间域、编码域等多种实现方式,包括对系统结构的改进,复用波形的选择,最佳接收机,与多载波、多天线技术的结合,采用空间零相关窗连续导频方案进行信道估计,与LTE/LTE-A参考信号方案的对比等方面,并通过理论分析和计算机仿真,证明了所提出的技术和方案,对系统性能,以及频谱利用率有明显提升。针对时间域重叠复用(OVTDM)系统,研究发现,OVTDM技术采用波形分割的思想,通过缩小波形信号的时间间隔,引入不同波形的重叠复用,从而获得编码约束关系,并有效提高系统频谱效率。相比传统的高阶调制技术,具有更好的误码性能,更少的输出电平数,以及更高的频谱利用率；在高频谱效率下,一定的观察误码率下(如￡=10-),OVTDM技术的谱效率与归一化信噪比门限,大致呈线性关系。编码的OVTDM系统,其性能超过传统二元域编码配合高阶调制的方案,且性能增益随着采用的前向纠错码的码率的增加而增大。引入采用Turbo码并行级联结构的Turbo-OVTDM技术,相比单路传输的OVTDM技术,系统的抗噪声能力得到增强。在高频谱效率的OVTDM系统设计中,引入没有移位叠加的纯OVTDM技术(重叠重数K,),即POVTDM,将POVTDM与移位OVTDM技术(重叠重数K2)采用串行级联结构,并在同相、正交支路并行复用传输,实现的总谱效率为2K1K2。接收端先分离并行支路的数据,然后分别检测。相比同谱效率的编码调制技术,其性能增益能进一步提高,且译码复杂度从22K1k2降为2×2K1K2。当POVTDM技术与Turbo-OVTDM技术串行级联传输时,接收端引入迭代译码,典型参数下,迭代性能在4次迭代后基本收敛,同样的谱效率下,在观察的误码率内,该系统对应的门限信噪比更低。为将OVTDM技术进一步推向实用化,研究了限带条件下的OVTDM系统,即对OVTDM输出信号进行滤波。重点研究了采用不同的复用波形,如矩形波,具有根升余弦频谱的波形(即工程中常用的RRC滤波器),升余弦波形的OVTDM技术,在AWGN信道的理论性能,以及在时变信道中对抗随机衰落的性能。通过理论推导,OVTDM技术采用波形分割的编码方式,其信道容量超过奈奎斯特准则下的香农容量。传统的对抗衰落的分集技术,需要额外的收发设备,或占用额外的时频资源,而OVTDM技术能够获得一定的隐分集增益,在时变信道中能获得良好的分集性能。针对限带的OVTDM系统,采用升余弦复用波形时,可以实现良好的限带性能,观察滤波器的功率谱和相位谱。信号频谱的第一旁瓣,相比频谱主瓣的峰值,约降低了-35dB,第二旁瓣相比主瓣,降低了-45dB。当时域波形的冲激相应范围为{0,2T}时,主瓣所占的带宽范围为{-1/T,1/T},对应的相位特性在该区间内呈线性分布。观察滤波器的噪声功率谱密度,可知,此时的噪声已是非白的,其功率谱包络基本与采用的复用波形的频谱一致,讨论了OVTDM系统的最佳接收机结构,接收端对接收到的符号序列,先匹配滤波,然后将输出波形白化,再进行最大似然序列检测。通过计算机仿真,验证了这种结构的合理性。由于OFDM/OFDMA技术应用的广泛性,考虑将重叠复用技术与多载波技术结合,对于OVTDM技术,因为其输出的信号包络是非平坦的,用于多载波系统中会引入子载波间干扰,设计了一种联合检测的多载波OVTDM系统,结果表明,子载波数目增加时,该系统能够获得一定的编码增益；采用Turbo码并行级联结构的OVTDM技术,应用在多载波系统中,能够进一步的降低归一化信噪比门限。对于编码域重叠复用(OVCDM)技术,可以灵活地设计OVCDM编码矩阵,从而获得更好的误码性能。着重研究了OVCDM技术构建的系统,与LTE标准中物理层相关技术的对比,并将OVCDM和OFDM技术结合,引入多天线技术,进一步提高系统容量。实际的通信系统中,信号经过无线信道传输时,会发生一定的时间,频率,空间扩散现象,导致接收端信号特性产生变化,信道估计是通信系统设计的一个重要方面。本文中,研究了一种新型的应用在OFDM-MIMO系统的,空间零相关窗连续导频方案,该方案采用具有良好相关特性的互补码组,作为基本的导频序列。同时,采用四色原理组网,在19小区组网的环境下,只需要4组互补码组,艮『J可规避小区内的同色干扰。设计的导频方案能够适用于LTE/LTE-A的资源块中传输,通过调整导频占用的时域OFDM符号长度,以及插入密度,可实现各种场景下的应用。相比LTE/LTE-A系统的参考信号方案,该方案能够获得更高的估计精度,且能有效的对抗同色干扰,且高信噪比下的估计性能基本不受天线数目和小区数目的影响。为进一步研究连续导频方案的理论性能,推导了该方案,与常用的频域估计方法,如LS估计,LMMSE估计的性能对比,连续导频方案采用线性相关器,获得信道冲击响应的时域估值,在一般车速下,估计精度较高；相比LS估计器,该方案能获得较大的信噪比增益,.特定参数下,该增益能达到13.8dB；在高信噪比区域,该方案能逼近LMMSE估计的性能,且只需要信道长度这一先验信息,不需要矩阵求逆和协方差矩阵等外信息,能够实现信道估计复杂度和性能的折中。更多还原

【Abstract】 The overlapped multiplexing theory and technology is introduced and analyzed, so as to improve the data signaling rate, spectrum utilization and power efficiency for the wireless communication system. The overlapped multiplexing theory and technology includes several implementation models, such as OVTDM technology and OVCDM technology. Several innovative solutions, including improvement of the structure for the system, the optimal selection of the waveforms, the optimal receiver, and combination with the multi-carrier and multi-antenna techniques are proposed. Besides, the channel estimation approach using zero correlation window continuous pilot scheme are presented, and the approach is compared with reference signal approach in LTE/LTE-A system. Through theoretical analysis and computer simulation, it’s turned out that the proposed technologies and solutions, can improve the system performance and spectrum efficiency significantly.For the OVTDM systems, it’s found that, using waveform segmentation approach, the technology introduces the waveform overlapped multiplexing idea by narrowing the time interval of waveform signals, thereby it can obtain encoding constraints relationship between adjacent waveform samples, and improve the spectrum efficiency of system effectively. Compared with the traditional high-order modulation technology, OVTDM shows better error performance, less output levels, and higher spectrum efficiency. With high system spectral efficiency, the normalized SNR threshold of OVTDM technology turns out to approximately linear with the spectrum efficiency, under the observed BER. The performance of coded OVTDM system is preferable, than traditional binary field coding scheme together with higher order modulation scheme. In particular, the performance gain increases with the increase of code rate for the FEC code. The Turbo-OVTDM technology, using the parallel concatenated structure of Turbo code, is turned out that, it can enhance the communication reliability of system than OVTDM technology with single branch for data transmission.In the system design for realizing high spectral efficiency, the pure OVTDM (POVTDM) with no shift superimposition is introduced (overlapped multiplicity is K1), and a serial concatenated structure is used between POVTDM and OVTDM (overlapped multiplicity is K2). The data symbols are transmitted and multiplexed in the in-phase branch and orthogonal branch, whose spectrum efficiency is2K1K2. In the receiver, data in the parallel branches are separated firstly, and then the two branches are detected respectively. Compared to the coded modulation scheme with the same spectral efficiency, the performance gain can be further improved, while the decoding complexity is reduced from22k1k2to2×2k1k2. When the OVTDM module in the sending side is replaced with Turbo-OVTDM scheme, the iterative decoding is introduced in the receiver. With typical parameters, the serial concatenated structure between POVTDM and Turbo-OVTDM is turned out to have lower SNR threshold under the observed BER level, and the iterative performance is converged after4iterations.The overlapped multiplexing technology is researched and analyzed in theoretical significance so far. For its further application in the practical systems, OVTDM is supposed to work effectively under band-limited conditions, i.e. the output signal of OVTDM is filtered before sending through the channel. Several multiplexing waveforms for OVTDM is analyzed, such as the rectangular wave, root raised cosine (RRC) spectrum waveform (it is commonly used in engineering known as RRC filter), and time domain raised cosine waveform. The theoretical performances in AWGN channel, as well as time-varying channel performance against random fading are studied intensively. Through theoretical derivation. OVTDM technology uses waveform segmentation idea to obtain the encoding constraint, and its capacity outperforms Shannon capacity under the Nyquist criterion. The traditional diversity techniques to against fading, needs additional transceiver, or extra time-frequency resources. The OVTDM technology can employ the hidden diversity gain in time-varying channel, and get good diversity performance.For band-limited OVTDM system, it can realize good band-limiting performance with the raised cosine multiplexing waveforms. Through the observation of power spectrum and phase spectrum of the waveforms, the power spectrum of the first side lobe attenuates approximately-35dB, compared with the spectrum of the main lobe. And the spectrum of the second side lobe attenuated about-45dB. The time interval of impulse response for raised cosine waveform is [0,2T], and the frequency range of the main lobe of power spectrum is [-1/T,1/T], the corresponding phase spectrum is distributed linearly in the frequency range. It’s also found that the noise power spectral density of the filter is non-white, whose power spectrum envelope is consistent with that of waveforms substantially. The optimal receiver structures of the system are discussed. The received symbol sequences are firstly sent through matched filter, and then they are processed with the maximum likelihood sequence detection using Ungerboeck metric. The structure is verified to be reasonable by computer simulation.As the multi-carrier technique, such as OFDM/OFDMA is widely used in wireless communication systems, the combination of overlapped multiplexing technology with multi-carrier technique is considered in the paper. When the OVTDM technology is used in multi-carrier systems, it would cause inter carrier interference, as the signal envelop of OVTDM output symbols is not flat. A multi-carrier OVTDM system is designed with joint detection algorithm, which is turned out that it can obtain certain performance gain when the number of subcarrier is increased. When the Turbo-OVTDM scheme is combined with multi-carrier technique, the SNR threshold can be decreased to a certain extent. For the OVCDM technology, the coding matrix can be designed flexibly, and it’s easily to apply OFDM and multi-antenna techniques in the OVCDM system. The system performance is compared with LTE PHY layer coded modulation scheme. The performance of OVCDM-OFDM systems, can hold the line, even outperform with LTE scheme in the ITU channel model, such as ITU-Vehicle A, and ITU-Urban Macro scenario.Channel estimation is an important aspect for system design, as the signal characteristics would change when it is send through the radio channel, when certain time, frequency, spatial diffusion phenomena occurs. In the paper, a novel ZCW continuous pilot scheme for OFDM-MIMO system is proposed, and the pilot sequence is generated on the base of complementary code group with good correlation characteristics. With the use of four-color principle for networking, it only needs4complementary code groups under the19cell networking environment, where the interference from the same color cell can be avoided with certain design. The pilot scheme can be applied in LTE/LTE-A system, by adjusting the length of pilot sequence, and the pilot insertion density in the resource blocks. Compared with the reference signal program in LTE/LTE-A standard, the proposed scheme is able to obtain higher estimation accuracy, and work effectively against the interference from the same color cells, and in the high SNR region, the estimation performance is substantially independent of the number of antennas, as well as the number of cells.For further study of the theoretical performance of the continuous pilot, the performance comparison with typical frequency domain schemes are analyzed. The proposed scheme is turned out to have relatively high estimate accuracy with low calculation complexity, and the estimate performance is unaffected with number of transmit antennas. Compared with LS estimator, the present scheme can obtain large gains with specific system parameters, with simple linear correlation under the same mean square estimation level. With certain parameters, the performance gains can be13.8dB. Compared with LMMSE estimator in FD, the scheme approaches LMMSE’s favorable performance in high signal to noise ratio region, with knowledge of channel taps length only and no more a priori information in the estimate process. The trade-off between complexity and estimation performance can be obtained for the continuous pilot scheme.更多还原