Robust Multiuser Interference Cancellation for OFDM Systems With Frequency Offset

In orthogonal frequency division multiple access systems clusters of subcarriers are assigned to different users for parallel data transmissions. The subcarriers are overlapped, but orthogonal to each other such that there is no intercarrier interference (ICI). However, synchronization errors among users cause the loss of the orthogonality and introduce ICI resulting in multiple-access interference. Synchronization between users is particularly difficult in the uplink channel where the user signals are potentially asynchronous and affected by different frequency offsets due to misalignment in carrier frequencies and Doppler shifts. This paper proposes a method to lower the effects of different frequency offsets among user signals in an OFDMA uplink system. The multiple access interference due to the user frequency misalignments is reduced by reconstructing and removing the interfering signals in the frequency domain. An approach based on the selective cancellation method, is proposed and its performance is analyzed by means of theoretical analysis and computer simulations. The effectiveness of the proposed system has been evaluated in the case of ideal and no-ideal frequency offset estimation and has been compared with that of the classical successive and parallel cancellation schemes. Simulation results show that the proposed approach allows performance close to the ideal case, i.e., with ideal frequency synchronization among users, with a low increase of the implementation complexity. Moreover, it is also highlighted here, that the successive cancellation method slightly outperforms the selective scheme, at the expense of a higher computational complexity and processing delay     

Alternating Multiuser Detection with Soft Interference Cancellation for Heterogeneous-Signaling MIMO CDMA Systems

This paper presents an effective alternating multiuser detector for the uplink of heterogeneous-signaling multiple-input multiple-output code division multiple access systems over multipath fading channels, where the data are transmitted using either spatially multiplexing for high transmission rate or space-time block code for transmit diversity gains. The new MUD first separates users into two groups according to their transmission signaling schemes and then detects the transmitted symbols in each group alternately with the removal of iteratively refined soft information-assisted multiple access interference to enhance the interference cancellation capability. Moreover, for practical low-complexity implementations, the users in each group are further partitioned into smaller subgroups based on their effective channel correlations and then detected in parallel by a bank of minimum mean-squared error soft detectors to further reduce the computational load. Conducted simulations show that the proposed MUD can render superior performance compared with previous approaches, especially in highly loaded scenarios.     

Space-Time Adaptive MMSE Multiuser Decision Feedback Detectors With Multiple-Feedback Interference Cancellation for CDMA Systems

In this paper, we propose a novel space-time minimum mean square error (MMSE) decision feedback (DF) detection scheme for direct-sequence code-division multiple access (DS-CDMA) systems with multiple receive antennas, which employs multiple-parallel-feedback (MPF) branches for interference cancellation. The proposed space-time receiver is then further combined with cascaded DF stages to mitigate the deleterious effects of error propagation for uncoded schemes. To adjust the parameters of the receiver, we also present modified adaptive stochastic gradient (SG) and recursive least squares (RLS) algorithms that automatically switch to the best-available interference cancellation feedback branch and jointly estimate the feedforward and feedback filters. The performance of the system with beamforming and diversity configurations is also considered. Simulation results for an uplink scenario with uncoded systems show that the proposed space-time MPF-DF detector outperforms existing schemes such as linear, parallel DF (P-DF), and successive DF (S-DF) receivers in terms of bit error rate (BER) and achieves a substantial capacity increase in terms of the number of users, compared with the existing schemes. We also derive the expressions for MMSE achieved by the analyzed DF structures, including the novel scheme, with imperfect and perfect feedback and expressions of signal-to-interference-plus-noise ratio (SINR) for the beamforming and diversity configurations with linear receivers.     

An Ordered Successive Interference Cancellation Scheme in UWB MIMO Systems

In this letter, an ordered successive interference cancellation (OSIC) scheme is applied for multiple‐input multiple‐output (MIMO) detection in ultra‐wideband (UWB) communication systems. The error rate expression of an OSIC receiver on a log‐normal multipath fading channel is theoretically derived in a closed form solution. Its bit error rate performance is analytically compared with that of a zero forcing receiver in the UWB MIMO detection scheme followed by RAKE combining.     

空分复用多输入多输出系统中的分组多用户分集

该文针对基于空分复用的多用户多输入多输出系统下行链路,研究了空间衰落相关和信道估计误差对系统容量的影响,提出了两种新的分组多用户分集方案。所提出的方案通过合理选择共道用户,能有效提升系统容量,同时兼顾公平性。分析和仿真结果表明,所提出的方案对衰落相关性以及信道估计误差具有较好的稳健性;其中,第2种方案与第1种相比较,容量性能更优,但具有较高的计算复杂度。     

非精确信道状态信息下MIMO系统中的干扰删除

复杂的多天线环境中,未知的干扰和噪声无处不在,这些因素不但引起了多输入多输出(MIMO)系统信号检测的偏差,还加大了获取精确信道状态信息(CSI)的难度。因此,为了删除MIMO系统中的未知干扰,该文建立了非精确CSI下的系统模型,模型中的非线性函数表示MIMO系统中的干扰。基于非参数理论提出了针对这种干扰的干扰删除方法。与以往方法不同的是,模型中的干扰是完全未知的。进一步地,从理论上证明了干扰删除的有效性,即证明了估计函数的收敛性。仿真结果表明,所提出的方法能有效地删除系统中的干扰。     

Optimal Lattice-Reduction Aided Successive Interference Cancellation for MIMO Systems

In this letter, we investigated the optimal minimum-mean-squared-error (MMSE) based successive interference cancellation (SIC) strategy designed for lattice-reduction aided multiple-input multiple-output (MIMO) detectors. For the sake of generating the MMSE-based MIMO symbol estimate at each SIC detection stage, we model the so-called effective symbols generated with the aid of lattice-reduction as joint Gaussian distributed random variables. However, after lattice-reduction, the effective symbols become correlated and exhibit a non-zero mean. Hence, we derive the optimal MMSE SIC detector, which updates the mean and variance of the effective symbols at each SIC detection stage. As a result, the proposed detector achieves a better performance compared to its counterpart dispensing with updating the mean and variance, and performs close to the maximum likelihood detector.     

多用户检测的串行干扰抵消器算法

CDMA由于其独特的性能成为第三代移动通信中标准多址方式,传统的CDMA信号解调方法是求信号与本用户PN码的相关运算,而此时其它用户的CDMA信号就成为干扰信号.本文提出了一种串行干扰相减式抵消器法,实验表明,这种方法在对功率不相等的多用户进行信号检测时性能优于传统方法.     

CDMA系统中的多用户干扰抵消

本文介绍在多步检测结构中把干扰对消和利用天线阵的自适应MMSE检测器［1］结合起来,并介绍一种新的基于信号子空间估计的检测器。使用这种新的检测器可以使当子空间的维数减少时,干扰就从接收到的信号中抵消,并用正交投影子空间跟踪估计（OPAST）［2］来自适应的估计检测器和空间特征。与MMSE检测器相比,这种检测器在性能上更优。     

Performance Analysis of Scheduling in Multiuser MIMO Systems with Zero-Forcing Receivers

Despite its low-complexity, the zero-forcing receiver is known to suffer from noise enhancement to restore the spatially multiplexed data in a single-user MI MO system. Nevertheless, in the multiuser system, the poor-channel avoidance property of the scheduling technique provides a natural way to overcome the drawback of noise enhancement (Heath et al., 2001). In this paper, we present an analytical framework to evaluate the performance of the zero-forcing receiver operating in the multiuser MIMO system with user scheduling. Using the order statistics technique, we derive closed-form expressions for the sum-rate capacity of the multiuser MIMO system that employs the simple spatial multiplexing at the transmitter and zero-forcing processing at the receiver with a number of scheduling algorithms. These closed-form expressions hold for an arbitrary finite number of users and facilitate efficient numerical evaluations for cases of practical interest. In addition, the tractable analysis provides insight into how the scheduling technique affects the performance of the multiuser MIMO system under scalar feedback and vector feedback. The results are also extended to the case of heterogonous users with unequal average SNR.     

Opportunistic Feedback for Multiuser MIMO Systems With Linear Receivers

A novel multiuser scheduling and feedback strategy for the multiple-input multiple-output (MIMO) downlink is proposed in this paper. It achieves multiuser diversity gain without substantial feedback requirements. The proposed strategy uses per-antenna scheduling at the base station, which maps each transmit antenna at the base station (equivalently, a spatial channel) to a user. Each user has a number of receive antennas that is greater than or equal to the number of transmit antennas at the base station. Zero-forcing receivers are deployed by each user to decode the transmitted data streams. In this system, the base station requires users' channel quality on each spatial channel for scheduling. An opportunistic feedback protocol is proposed to reduce the feedback requirements. The proposed protocol uses a contention channel that consists of a fixed number of feedback minislots to convey channel state information. Feedback control parameters including the channel quality threshold and the random access feedback probability are jointly adjusted to maximize the average throughput performance of this system. Multiple receive antennas at the base station are used on the feedback channel to allow decoding multiple feedback messages sent simultaneously by different users. This further reduces the bandwidth of the feedback channel. Iterative search algorithms are proposed to solve the optimization for selection of these parameters under both scenarios that the cumulative distribution functions of users are known or unknown to the base station     

空时编码的多用户MIMO系统用户调度性能分析

文中针对正交空时编码的多用户MIMO系统,分析并比较了两种多用户调度方案的性能.第一种方案是基站在一个时隙内选择信噪比最大的用户进行通信,第二种方案是基站在一个时隙同时选择多个信噪比大的用户进行通信.文中对两种方案的容量都作了理论推导和仿真比较.仿真结果显示,前一种方案性能好于第二种方案,但是第二种方案可以同时服务多个用户.     

Parallel Interference Cancellation in Multiuser CDMA Channel Estimation

Parallel interference cancellation (PIC) based channel parameter estimators for frequency selective fading channels are proposed for the uplink in code-division multiple-access (CDMA) mobile communication systems. The performance of PIC based algorithms depends heavily on the quality of the multiple-access interference estimates, which can be improved by using adaptive channel estimation filters. The performance of two adaptive complex channel coefficient estimation filters has been verified in a fading channel by computer simulations. According to the results, the PIC based adaptive channel estimators outperform clearly conventional, successive interference cancellation, and decorrelation based adaptive channel estimators. The PIC method is also used in delay tracking. By using the principles of sample-correlate-choose-largest (SCCL) delay trackers, a robust algorithm for multiuser delay tracking in fading channels is obtained.     

Cross-Layer Design for Multiuser MIMO MRC Systems With Feedback Constraints

Cross-layer techniques represent efficient means to enhance throughput and increase the transmission reliability of wireless communication systems. In this paper, a cross-layer design of aggressive adaptive modulation and coding (A-AMC), truncated automatic repeat request (T-ARQ), and user scheduling is proposed for multiuser multiple-input–multiple-output (MIMO) maximal ratio combining (MRC) systems, where the impacts of feedback delay (FD) and limited feedback (LF) on channel state information (CSI) are also considered. The A-AMC and T-ARQ mechanism selects the appropriate modulation and coding schemes (MCSs) to achieve higher spectral efficiency while satisfying the service requirement on the packet loss rate (PLR), profiting from the feasibility of using different MCSs to retransmit a packet, which is destined to a scheduled user selected to exploit multiuser diversity and enhance the system's performance in terms of both transmission efficiency and fairness. The system's performance is evaluated in terms of the average PLR, average spectral efficiency (ASE), outage probability, and average packet delay, which are derived in closed form, considering transmissions over Rayleigh-fading channels. Numerical results and comparisons are provided and show that A-AMC combined with T-ARQ yields higher spectral efficiency than the conventional scheme based on adaptive modulation and coding (AMC), while keeping the achieved PLR closer to the system's requirement and reducing delay. Furthermore, the effects of the number of ARQ retransmissions, numbers of transmit and receive antennas, normalized FD, and cardinality of the beamforming weight vector codebook are studied and discussed.      

Inter-Relay Interference Cancellation Using MIMO Detection

Recent mobile applications have required high data rate. In metropolitan areas where data traffic is higher than other areas, heterogeneous networks (HetNet) are used to meet the traffic demand (Hoadley and Maveddat in IEEE Commun Mag 19(2):4–5, 2012). The HetNet has several low power base stations or remote radio heads in a macro cell coverage and sometimes coordinates the low power transmission points forming virtual multiple-input and multiple-output system. In this paper, inter-relay interference (IRI) cancellation for two-path relay system is considered. In existing schemes, IRI cancellation is performed at a destination node but proposed method performs the cancellation at relay nodes to reduce the complexity of detection algorithm for the destination node. Simulation results also show that the method of this paper achieves better BER performance than the existing schemes.     

Interference Cancellation in Body-Area Networks Using Linear Multiuser Receivers

The problem of interference cancellation may arise in the nearest future for body-area networks (BANs) whenever a certain number of BANs are led to operate in the vicinity of each other under the same spectrum band. In this paper, using the data obtained from the measurements, the performance in terms of bit error rate (BER) of three linear CDMA receiver structures is investigated for BANs: the conventional receiver, the minimum mean square error receiver, and the decorrelator receiver. We show that in synchronous scenario, the three receivers exhibit the same performance regardless of the number of supported users and SNR level. The BER performance of the linear receivers in on-body channels is better than in Rayleigh channel with the belt-chest channel exhibiting the best gain due to the presence of a stronger LOS component. Furthermore, with orthogonal codes, these receivers are shown to be robust to the MAI level increase and do not require a perfect power control.     

基于迭代并行干扰消除的低复杂度大规模MIMO信号检测算法

基于干扰消除思想该文提出一种适用于大规模MIMO系统上行链路的低复杂度迭代并行干扰消除算法,在算法实现中避免了线性检测算法所需的高复杂度$({\cal O}({K^3}))$矩阵求逆运算,将复杂度保持在$({\cal O}({K^2}))$。在此基础上,引入噪声预测机制,提出一种基于噪声预测的迭代并行干扰消除算法,进一步提高了硬判决检测性能。考虑天线间残留干扰,将干扰消除思想运用到软判决中,最后提出一种基于迭代并行干扰消除的低复杂度软输出信号检测算法。仿真结果表明：提出的信号检测方法的复杂度优于MMSE检测算法,经过几次简单的迭代,算法即快速收敛并获得接近甚至优于MMSE检测算法的误码率性能。     

Design of Multiuser Downlink Linear MIMO Precoding Systems With Quantized Feedback

We treat the problem of sum-rate maximization via scheduling and linear precoding in multiuser (MU) multiple-input–multiple-output (MIMO) systems with quantized feedback. We formulate the optimal quantized linear precoder design problem and provide the numerical procedure for finding the solution. We also provide a simple scheduling scheme that exploits the MU diversity gain. With the aim of sum-rate maximization, we further address the quantization codebook design based on the capacity measure by introducing a new distance metric. It is demonstrated that, with or without scheduling, the proposed optimal linear precoding scheme achieves significant gain over the conventional linear precoder designs with similar feedback overhead, such as the zero-forcing precoder. Moreover, although the proposed quantization codebook design improves the performance of other existing MU-MIMO precoding schemes, the performance gain offered by it is typically much higher when it is coupled with the proposed linear precoding design.