Blind MIMO Channel Estimation Based on Modulation-Induced Two-Cycle Cyclostationarity

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基于循环累量的多用户信道盲辨识

本文针对多用户系统提出了一种新颖的盲辨识方法。它首次利用了调制产生的高阶循环平稳性,可以识别出不同的用户各自的传输信道。最后文中提出了循环c(q,n)公式,给出信道的估计。方法简单明了,适合于多用户上行链路信道的盲辨识。文中还讨论了对算法进行的计算机模拟。     

Equalization and interference cancellation in linear multiusersystems based on second-order statistics

Potential applications of blind channel identification and equalization in data communication systems have recently been explored. For multiuser systems that are irreducible and column-reduced, second-order statistical methods normally can identify channel dynamics up to a unitary mixing matrix. Additional user separation (equalization) can rely on higher order statistics and other prior information. In this paper, we investigate the equalizability of user signals and the cancellation of unwanted interfering signals based only on second-order output statistics. We show that a user channel can be equalized if it has the longest memory. Furthermore, interfering user signals can be cancelled under a more relaxed multiuser channel condition     

利用调制引入循环平稳的CDMA盲信道估计

针对CDMA系统给出了一种新颖的盲辨识方法.它利用了恒模复序列调制产生的循环平稳性,其特色是,在接收机上可以根据循环统计量的支撑的不同,将各个用户的二阶循环统计量分离开来,每个用户形成自己的小规模子空间.采用已成熟的子空间方法,可以识别出不同的用户各自的片率传输信道,以便于均衡和检测.文中给出了自己的信道可辨识性的条件和证明,提出了辨识算法,并对算法进行了模拟.方法简单明了,适合于CDMA上行链路信道.     

Efficient channel blind estimator for multiuser system with conjugate cyclostationarity

The problem of blind channel identification in a multiuser system is considered in this article. For this purpose, a blind identification algorithm is proposed based on the conjugate cyclostationarity of the received signal. The new approach contains a two-stage identification procedure. First, the separation technique in the cyclic domain is used to separate the second-order cyclic statistics for each user. Second, a subspace algorithm based on the rational subspace theory is exploited to estimate the desired channel. Theoretical analysis and simulation results show that this algorithm is suitable for a multiuser system. Compared with other methods, the algorithm shows good performance even in a bad situation when the number of users is large and the diversity condition is unavailable.     

A low-complexity space-time OFDM multiuser system

Exploiting the Fourier basis structure both in the space and the time domains, we develop a low-complexity multiuser space-time coding scheme, multiuser (MU) angle-frequency coding scheme (MU-AFCS), to properly schedule the data streams of each user with respect to its corresponding angle-frequency channel structure for downlink wireless systems. With the proposed approach, a large amount of space resource left over by one user, in clustered multipath wireless channels, can be easily identified and used by the others without serious signal collision in the space domain. In doing so, low channel capacity resulting from poor channel structures in systems, allowing only single-user transmission at a time, can be greatly boosted. The key advantage of this approach is that only limited feedback of channel state information to the transmitter is required while multiuser macro-diversity is sufficiently exploited. In addition, the complexity of the proposed approach is much lower than that of the existing ones serving similar purposes. Through theoretical analyses and computer simulations, we demonstrate that the MU-AFCS can significantly increase the channel capacity as compared to the traditional orthogonal resource division MU multiple-input multiple-output (MIMO) systems.     

Noncoherent decorrelative detection for nonorthogonal multipulsemodulation over the multiuser Gaussian channel

This paper introduces the problem of noncoherent detection for nonorthogonal multipulse modulation in the context of the synchronous multiuser Gaussian channel. Each user sends an M-ary information symbol by transmitting one of M possibly nonorthogonal waveforms. Furthermore, the M signals of one user are allowed to be correlated with the signals of all other users. A key idea proposed here is that of a noncoherent decorrelative receiver front end. Like its counterpart in single-pulse modulation, this front end eliminates multiuser interference. It therefore reduces the multiuser detection problem into decoupled single-user problems over equivalent noise-enhanced single-user channels. Each equivalent single-user channel is rather general and can be described as one where the waveforms employed are not only correlated, but are also of unequal energies. Several new results pertaining to the design and analysis of optimum and suboptimum noncoherent detectors for this single-user channel are obtained. In the multiuser channel, these detectors constitute the post-decorrelative processing units for each user     

Performance of Primary Users in Spectrum Sharing Cognitive Radio Environment

This paper investigates the performance of the primary user in a multiuser cognitive radio environment. Using spectrum sharing method, multiple cognitive users compete to share a channel dedicated to the primary user in order to transmit their data to a common receiver unit. Only one cognitive user is scheduled to share the channel, and to do so, its transmit power should not increase the primary user’s outage probability above a certain limit. The average bit error rate and the average channel capacity of the primary user are derived in this setup for different cognitive scheduling schemes. The performance measures of the primary user are found to depend on the cognitive user’s scheduling criterion and on the parameters of the communication environment.     

MC-CDMA系统采用解相关-并行干扰抵消检测器的性能分析

该文分析了MCCDMA系统采用解相关-并行干扰抵消(DECPIC)多用户检测器的性能,把它与Hara提出的最小均方误差合并(MMSEC)单用户检测器在上行和下行信道加以比较.仿真结果显示下行信道应用MMSEC单用户检测性能较好,且能抵抗信道估计错误。而上行信道多用户检测器的性能也较好,但是多用户检测器会受较大的信道估计错误影响而性能降低.     

A joint-channel diagonalization for multiuser MIMO antenna systems

In this paper, we address the problem of improving the performance of multiuser space-division multiplexing (SDM) systems where multiple independent signal streams can be transmitted in the same frequency and time slot. The problem is important in multiuser multiple-input multiple-output systems where communication from one base station to many mobile stations can occur simultaneously. Our objective is to devise a multiuser linear space-time precoder for simultaneous channel diagonalization of the multiuser channels enabling SDM. Our new approach is based on diagonalizing the multiuser channel matrices and we use a variation of successive Jacobi rotations. In addition to the diagonalization, our approach attempts to optimize the resultant channel gains for performance enhancement. Our method is valid for both frequency-flat and frequency-selective fading channels but we assume that the base station knows all the channels and that they are quasi-stationary.     

Transmit power adaptation for multiuser OFDM systems

In this paper, we develop a transmit power adaptation method that maximizes the total data rate of multiuser orthogonal frequency division multiplexing (OFDM) systems in a downlink transmission. We generally formulate the data rate maximization problem by allowing that a subcarrier could be shared by multiple users. The transmit power adaptation scheme is derived by solving the maximization problem via two steps: subcarrier assignment for users and power allocation for subcarriers. We have found that the data rate of a multiuser OFDM system is maximized when each subcarrier is assigned to only one user with the best channel gain for that subcarrier and the transmit power is distributed over the subcarriers by the water-filling policy. In order to reduce the computational complexity in calculating water-filling level in the proposed transmit power adaptation method, we also propose a simple method where users with the best channel gain for each subcarrier are selected and then the transmit power is equally distributed among the subcarriers. Results show that the total data rate for the proposed transmit power adaptation methods significantly increases with the number of users owing to the multiuser diversity effects and is greater than that for the conventional frequency-division multiple access (FDMA)-like transmit power adaptation schemes. Furthermore, we have found that the total data rate of the multiuser OFDM system with the proposed transmit power adaptation methods becomes even higher than the capacity of the AWGN channel when the number of users is large enough.     

Exploiting Multiuser Diversity With Imperfect One-Bit Channel State Feedback

It has been well recognized that significant throughput gains can be leveraged in multiuser wireless communication systems by exploiting multiuser diversity with a smart scheduler. This scheduler collects channel state information (CSI) from all users and allocates the resources to the user(s) experiencing favorable channel conditions. However, for a frequency-division-duplex system with a large number of users, how to efficiently collect the required CSI will be a challenging task, especially when the feedback links are of limited capacity. In this paper, we propose a scheduling algorithm to exploit multiuser diversity with possibly imperfect one-bit channel state feedback. The basic idea is to define a threshold lambda and let each user report one-bit information to the scheduler about the comparison between its measured channel fading level and lambda. Correspondingly, the scheduler uses these feedback bits to classify all users into two sets and assigns the channel to one user belonging to the set experiencing favorable channel conditions. Several implementation schemes are developed by attacking the optimization of lambda under different system configurations, covering both the case when the one-bit feedback is perfect and those when the one-bit feedback is imperfect. Computer simulations show that when the user number is large, say, more than ten users, the proposed scheduling supports significantly larger data rate over the round-robin scheduling, while in comparison with the optimum scheduling with complete CSI, the performance loss is limited if the one-bit feedback is of high reliability. In addition, our studies show that we can effectively enhance the robustness against feedback imperfectness by incorporating the feedback reliability into optimization of lambda     

基于人工噪声的多用户MIMO系统加密算法

当多用户MIMO系统中的用户数多于或等于发射端天线数时,现有的基于人工噪声的物理层加密算法会导致合法用户无法正常接收。为提高此时的多用户MIMO系统的安全性,该文提出一种新的基于人工噪声的多用户MIMO系统加密算法。首先,发送端对多个合法用户进行联合处理,建立多用户联合信道状态矩阵;然后,将联合信道状态矩阵进行奇异值分解,并根据最小的奇异值进行预编码,以消除人工噪声对合法用户的影响;最后,提出一种优化功率分配的方案。仿真结果表明,该算法将多用户MIMO系统的保密容量平均增加了0.1 bit/(s?Hz),从而提高多用户MIMO系统的安全性。     

两种基于调制引入循环平稳的多用户系统盲检测研究

本文探讨了两种多用户系统的检测方法:第一种方法首先分别对各个用户的信道进行盲辨识,再进行多用户盲检测;第二种方法在收方利用频偏滤波(FRESH)的超强抗干扰能力,从收方混合多用户收信号中盲提取和盲分离出所需用户的收信号,再利用已辨识出的该用户的信道进行单用户盲检测。但这里的单用户检测不是普通的单用户检测,而是用了干扰抑制技术的新型检测,所以性能可以同多用户检测相当,克服了近远问题。     

Improved Robust Techniques for Multiuser Detection in Non-Gaussian Channels

In many physical channels where multiuser detection techniques are to be applied, the ambient channel noise is known through experimental measurements to be decidedly non-Gaussian, due largely to impulsive phenomena. This is due to the impulsive nature of man-made electromagnetic interference and a great deal of natural noise. This paper presents a robust multiuser detector for combating multiple access interference and impulsive noise in code division multiple access (CDMA) communication systems. A new M-estimator is proposed for "robustifying" the detector. The approach is corroborated with simulation results to evaluate the performance of the proposed robust multiuser detector compared with that of the linear decorrelating detector, and the Huber and the Hampel M-estimator based detectors. Simulation results show that the proposed detector with significant performance gain outperforms the linear decorrelating detector, and the Huber and the Hampel M-estimator based detectors. This paper also presents an improved robust blind multiuser detection technique based on a subspace approach, which requires only the signature waveform and the timing of the desired user to demodulate that user's signal. Finally, we show that the robust multiuser detection technique and its blind adaptive version can be applied to both synchronous and asynchronous CDMA channels.     

Design of Large Scale MU-MIMO System with Joint Precoding and Detection Schemes for Beyond 5G Wireless Networks

The large scale multiuser multiple input multiple output (MU-MIMO) is one of the promising communication technology for 5G wireless networks as it offers reliability, high spectral efficiency and high throughput. The lattice reduction (LR) precoding based user level local likelihood ascent search (ULAS) detection scheme is proposed in this paper for efficient signal detection in large scale MU-MIMO system. The initial solution of ULAS algorithm is obtained from the LR precoding assisted zero forcing detector. The LR precoding transforms the non-orthogonal channel matrix into nearly orthogonal channel, which helps to mitigate inter antenna interference (IAI) exists at each user. The remaining multiuser interference (MUI) imposed to each user from undesired users is cancelled by the proposed ULAS multiuser detection scheme. Thus, the proposed LR precoding assisted ULAS mitigates both IAI and MUI unlike the classical detector, those try to moderate either IAI or MUI. By contrast, the proposed ULAS detector provides performance close to optimal maximum likelihood detector with just a fraction of its complexity.

     

Performance of Orthogonal Beamforming for SDMA With Limited Feedback

On the multiantenna broadcast channel, the spatial degrees of freedom support simultaneous transmission to multiple users. The optimal multiuser transmission, which is known as dirty paper coding, is not directly realizable. Moreover, close-to-optimal solutions such as Tomlinson-Harashima precoding are sensitive to channel state information (CSI) inaccuracy. This paper considers a more practical design called per user unitary and rate control (PU2RC), which has been proposed for emerging cellular standards. PU2RC supports multiuser simultaneous transmission, enables limited feedback, and is capable of exploiting multiuser diversity. Its key feature is an orthogonal beamforming (or precoding) constraint, where each user selects a beamformer (or precoder) from a codebook of multiple orthonormal bases. In this paper, the asymptotic throughput scaling laws for PU2RC with a large user pool are derived for different regimes of the signal-to-noise ratio (SNR). In the multiuser interference-limited regime, the throughput of PU2RC is shown to logarithmically scale with the number of users. In the normal SNR and noise-limited regimes, the throughput is found to scale double logarithmically with the number of users and linearly with the number of antennas at the base station. In addition, numerical results show that PU2RC achieves higher throughput and is more robust against CSI quantization errors than the popular alternative of zero-forcing beamforming if the number of users is sufficiently large.     

Successive User Scheduling Schemes for a Multiuser MIMO System Employing Generalized Channel Inversion

Generalized channel inversion (GCI) is a precoding technique for multiuser multiple-input multiple-output system. While producing each user’s precoding matrix, GCI takes into account noise and thus it is more robust compared with alternative techniques such as block diagonalization technique in terms of sum rate capacity and frame error rate. In this paper, two suboptimal multiuser scheduling schemes for GCI are proposed that by scheduling a subset of mobile users nearly maximize the sum rate capacity. They employ an iterative approach involving a number of search steps. At each step, unselected mobile users are evaluated one by one, and only one of them is chosen according to given criteria. It is shown via computer simulations that the proposed schemes are capable of achieving a large portion of the sum rate capacity that is offered by an exhaustive search. The performance of the proposed multiuser scheduling schemes is evaluated when the antenna mutual coupling effects are taken into account at the mobile users’ sides. Numerical results reveal that the presence of antenna mutual coupling can result in an increased sum rate capacity when the array inter-element spacing is in the range of 0.3–0.4 wavelength.     

Blind multiuser detection for V-BLAST coded multicarrier CDMA system

Focusing on the space-time coded multiuser mobile communication systems in the frequencyselective fading environment, this paper proposes a Vertical Bell labs LAyered Space-Time （V-BLAST） coded Multicarrier Code-Division Multiple-Access （MC-CDMA） scheme and its blind channel identification algorithm. This algorithm employs an ESPRIT-like method and the singular value decomposition, and the channels between every transmit antenna of every user and every receive antenna of the base station are blindly estimated with a closed-form solution. Based on it, an equivalent Minimum Mean-Squared Error （MMSE） time-domain multiuser detector is derived. Moreover, the proposed scheme exploits the precoding in the transmitter in order to eliminate the constraint of more receive antennas than transmit ones, required by most conventional V-BLAST codec schemes. Computer simulation results demonstrate the validity of this proposed scheme.     

基于最小均方误差准则的盲多用户检测新算法

码分多址信号在无线多径衰落信道条件下的盲多用户检测具有很大的理论和实际意义。该文提出了一种直接基于最小均方误差(MMSE)准则的盲多用户检测算法。为保证算法收敛到目标用户,提出了一种基于MMSE准则的新的线性约束方法。该方法能保证检测器收敛到最优MMSE解。此外,该文还设计了一种形式简单、快速收敛的迭代算法。对比现有的最小输出能量(MOE)算法,仿真结果表明本文算法具有较好的性能。