【作者】 熊兴中；

【导师】 胡剑浩；

【作者基本信息】 电子科技大学 ， 通信与信息系统， 2009， 博士

【摘要】 多址技术一直是个人通信领域,尤其是基于蜂窝架构的无线移动通信系统中的关键技术之一。理论分析与实践经验均已证明了基于非正交分割时频资源的码分多址(CDMA)能够取得比正交分割时频资源的时分多址(TDMA)和频分多址(FDMA)更高的频谱效率,因此CDMA技术成为第三代移动通信系统(3G)的核心技术而得到了广泛的应用。然而CDMA系统由于多址干扰(MAI)的存在,在实际应用中并没有完全发挥出CDMA在容量上的潜在优势。随着CDMA系统容量的扩大,MAI问题日益严重,影响到3G以及未来移动通信系统容量和频谱利用效率的进一步提高。为了以较低的复杂度解决CDMA系统中日益严重的多用户干扰(MUI)问题,Liping教授提出了交织分多址(IDMA:Interleave-Division Multiple Access),简称为交织多址。在IDMA系统中,交织器作为区分用户的唯一手段,对不同的用户采用不同的交织图案。交织器输出序列的相邻码片之间近似无关,从而使逐码片(CBC:Chip-by-Chip)的迭代多用户检测(MUD:Multi-User Detection)得以实现,这也是IDMA的关键所在。IDMA作为一种新兴的无线多址接入技术,由于其独特的优势,使其在下一代移动通信中具有广泛的应用前景,已成为第四代移动通信(4G)的热门候选标准之一,并引起国内外研究人员的广泛关注。目前的研究表明,虽然IDMA系统的迭代多用户检测算法比传统CDMA系统简单,但是信号检测的收敛速度非常缓慢。而IDMA系统迭代检测的收敛速度直接影响着系统的数据传输速度,未来移动通信系统具有更高的数据传输速率,如何进一步提高IDMA系统迭代检测的收敛速度具有非常重要的意义。另外,在IDMA系统中,交织器作为区分用户的唯一手段,虽然上行链路与下行链路具有对称的特点,将上行链路的信号处理方法应用到下行链路中,从理论上是可行的,但实际上并不可取,毕竟下行链路中的用户端对复杂度、安全性、经济性等方面有很高的要求。基于上述问题,本课题着重研究了基于时分双工(TDD:Time-Division Duplexing)和时间反转(TR:Time Reversal)的IDMA系统的信号传输及检测技术,我们将这种系统称为时分双工时间反转IDMA,即TDR-IDMA。主要研究工作包括如下：1.针对IDMA迭代多用户检测的收敛速度随用户数增长而减慢的问题,本课题提出了一种有效的基于TR的IDMA迭代多用户检测算法。该算法通过上行链路时分双工获得信道冲激响应的时间反转,基站接收机利用信道冲激响应的时间反转预处理接收信号。借助于时间反转处理的时压缩特性以及不同用户信道冲激响应间的弱相关性和同一用户不同路径间的弱相关性,经过预处理,使IDMA迭代多用户检测的初始信干噪比(SINR:Signal to Interference and Noise Ratio)远高于未做时间反转预处理的IDMA的初始SINR。从而加快了IDMA迭代多用户检测的收敛速度,进而有效地解决了IDMA迭代多用户检测的速度瓶颈问题。2.针对IDMA下行链路中用户端接收设备复杂度比较高的问题,本课题提出了一种基于时分双工模式和时间反转技术的IDMA下行传输及简化接收方法。该方法通过时分双工获得信道冲激响应,基站发射端利用信道冲激响应的时间反转预处理发射信号。在时间反转及多输入单输出(MISO:Multiple Input-Single Output)技术作用下,利用不同用户信道冲激响应之间的弱相关性以及时间反转处理的空间和时间压缩特性,削弱多用户干扰、共道干扰(CCI)和符号间干扰(ISI)。从而使用户接收端只需一个简单的单径接收机即可完成信号的检测,避免了复杂的逐码片迭代多用户检测,同时将信道估计器也从用户端转移到基站端,进而使用户端接收机设备复杂度大大简化。另外,本课题还提出了一种IDMA/CDMA混合传输模式来克服IDMA下行链路中用户端接收设备复杂度比较高的问题。在这种混合模式中,上行链路采用IDMA传输及检测,下行链路采用IDMA/CDMA混合传输及检测。在下行传输及检测系统中,为了抑制或消除MUI,一方面,选用正交性能较好的码作为用户波形码,在用户接收端信号检测时,利用用户之间的波形码的互相关信息对接收信号进行处理；另一方面,将原来CDMA中各用户使用同一种交织器改为不同用户使用不同的交织器,进而可加强用户端的单用户信号检测。3.针对单载波IDMA系统频域均衡(FDE:Frequency Domain Equalization)中预编码的复杂度问题,本课题提出了基于时间反转预编码和迭代LMMSE频域均衡的传输检测算法,即单载波TDR-IDMA系统频域均衡的传输检测算法，从而使发射端预编码更简单,易于实现。最后给出了单载波TDR-IDMA系统的时间反转预编码与单载波IDMA系统的注水原理预编码、优化预编码和Proakis B信道未做预编码时在迭代LMMSE准则下的频域均衡算法的性能对比及分析。4.针对IDMA系统迭代检测的优势和最大期望(EM:Expectation Maximization)算法的特点,本课题提出了基于EM算法的IDMA系统的混合迭代信道估计。讨论了存在信道估计误差时的信号检测算法及评估方法,通过分析信道估计误差对IDMA系统性能的影响,提出了克服信道估计误差对IDMA系统性能影响的应对措施。更多还原

【Abstract】 Multiple access technique is one of the key techniques in cellular mobile communication systems,especially in personal communication systems.Theoretical research and practical experience show that code-division multiple-access (CDMA) systems based on non-orthogonal temporal or spectral resource have a higher spectral efficiency than time-division multiple-access (TDMA) systems based on orthogonal time-division or frequency-division multiple-access (FDMA) systems based on orthogonal frequency-division.Therefore, CDMA has been the key techniques in the third generation mobile communication system (3G), and has comprehensive applications in 3G systems.However, the capacity of CDMA systems is not maximized due to multiple access interference (MAI) in practical applications.MAI becomes severer with the increase of users.This affects the increase of capacity and spectral efficiency in 3G or future mobile communication systems.For addressing the complexity of multi-user detection (MUD) in CDMA systems, Prof. Liping proposes interleave-division multiple-access (IDMA).IDMA relies on interleaving as the only means to distinguish the signals from different users, different users have different interleavers. Thus the chip among the outputs of interleaver is approximately uncorrelated. This is the key of IDMA and this enables us to employ a very simple chip-by-chip (CBC) iterative MUD strategy in IDMA.IDMA is a new multiple access technique. IDMA has become one of the candidates for the air interface of fourth generation (4G) mobile communication systems due to its particular advantages.IDMA has attracted wide research attentions in China and abroad.The research shows that the iterative MUD algorithm in IDMA systems is much simpler than that used in traditional CDMA systems.Unfortunately, the convergence speed is very slow for the iterative MUD in IDMA systems and this directly affects the data transfer. The future mobile communication systems need high data rate.It is imperative for IDMA to enhance the convergence speed of iterative MUD.Interleaver is the only means to distinguish the signals from different users.According to the symmetrical feature between uplink and downlink, the processing method of uplink is directly used in downlink. This is feasible theoretically, but it is not optimal in practical. It does not meet the needs in system privacy, security, and economic costs for the receiver of downlink.For addressing above-mentioned problems, in this thesis,we propose a new transmission and receiving scheme for IDMA systems based on time-division duplexing (TDD) and time-reversal (TR) technique.We refer this system as time-division duplexing and time-reversal IDMA (TDR-IDMA).Our main contributions in this thesis research are as follows.1.According to the principle that the convergence of iterative MUD for IDMA degrades with the increase of user, we propose an efficient iterative MUD for IDMA systems based on TR technique. In this system, the time-reversed version of the channel impulse responses (TR-CIR) obtained from the uplink by TDD mode is used to process the received signal in the receiver at base station. By exploiting the weak correlations of channel impulse responses (CIR) for different users and multi-path, the signal to interference and noise ratio (SINR) at the beginning of turbo-like detection of IDMA becomes significantly higher than that of the traditional IDMA.As a result, considerably less number of iterations is needed for convergence in the proposed IDMA as compared with traditional IDMA.Our proposed technique effectively breaks the bottleneck of iterative MUD in IDMA receiver.2.To address the high complexity of user end receiver in IDMA,in this thesis,we propose a downlink transmission and receiving scheme for IDMA system based on time-division duplexing mode and time-reversal technique.The proposed scheme uses the time-reversed version of the channel impulse responses obtained from the uplink to pre-process the transmitted signal at base station. By exploiting the weak correlations of fading channels for different user ends (UEs),the spatial and temporal focusing of TR, as well as the multiple input-single output (MISO)configuration, it is helpful to alleviate the MAI, co-channel interference (CCI), and mitigate inter-symbol interference (ISI).It needs a simple one-finger receiver used at the UE.The UE can be simplified by canceling the iteration operation and removing the channel estimator from the receiver of UE to the receiver of the base station. Thus the data detection of the proposed scheme is rather simpler as compared with the traditional IDMA.In this thesis,we also propose a hybrid multiple access scheme which combines IDMA and CDMA to simplify the complexity of user end receiver. We employ IDMA technique as the uplink transmission and IDMA/CDMA technique for the downlink-transmission.For suppressing multiple user interference (MUI) in the downlink, the IDMA/CDMA hybrid scheme can cope with this issue with orthogonal spreading sequence as same as traditional CDMA systems.Thus iteration free receiving can be achieved at UEs.In the proposed scheme, random interleavers are used to distinguish different users in the downlink too.Each UE only requires the interleaver parameter and orthogonal sequences, which belongs to itself. The complexity of UEs in the proposed scheme is the same with that of the traditional CDMA systems.Thus the proposed scheme can share the advantage of IDMA and CDMA simultaneously.3.To resolve the high complexity of precoder in single-carrier IDMA frequency domain equalization (FDE), in this thesis,we propose a transmission and receiving algorithm for single-carrier IDMA system based on time reversal precoder and iterative LMMSE frequency domain equalization. That is transmission and receiving algorithm of single-carrier TDR-IDMA frequency domain equalization.Thus the complexity of transmitter is reduced and easy to implement. Finally, we give the performance comparison and analysis of single-carrier TDR-IDMA,single-carrier IDMA with water-filling precoder, single-carrier IDMA with optimal precoder, single-earrier IDMA with Proakis B channel based on iterative LMMSE frequency domain equalization algorithm.4.By exploiting the advantages of MUD in IDMA systems and Expectation Maximization (EM) algorithm, in this thesis,we develop a channel estimation approach for IDMA systems based on EM algorithm. The detection and evaluation algorithm of IDMA systems are also studied when there is the effect of channel estimation error. We design the method of overcoming the channel estimation error through the theoretic analysis of the effect of channel estimation error.更多还原