采用低密度校验码的多天线OFDMA系统

在本文中针对频率选择性衰落信道提出了一种采用低密度校验（LDPC）码的多天线（MIMO）正交频分多址（OFDMA）系统。在这一多用户／多速率系统中，首先通过OFDMA消除用户之间的干扰，从而将多用户环境转化为单用户环境；然后通过联合MIMO软解调与LDPC码解码的Turbo迭代算法，消除发射天线之间的干扰并改善系统的性能。仿真结果表明，该方案可以有效地消除多用户干扰和发射天线之间的干扰，在通过利用空分复用以提高频谱效率的同时，也获得了由LDPC码提供的编码增益。由于该方案对于星座图没有任何限制，因此可以通过采用高效的调制方式进一步提高频谱效率。     

一种自适应跨层空间子信道分配算法——基于多用户MIMO／OFDM系统

MIMO／OFDM是未来宽带无线通信接口的有效架构。本文提出了一种自适应跨层空间子信道分配算法，在多用户MIMO／OFDM系统中，联合链路层截短ARQ（T-ARQ）技术，以获取物理层最大的系统吞吐量为目标，推导了子载波分配准则，并给出了相应的算法流程。仿真结果表明，该算法具有良好的性能，有效地提高了系统的传输速率。     

多用户 MIMO 多载波系统空时预编码技术的研究磁

多天线空时预编码技术利用信道状态信息能够有效地改善无线通信系统性能与吞吐量,成为未来无线移动通信领域研究的热点。根据发射端利用信道状态信息情况,预编码技术可分为完全信道预编码技术和部分信道预编码技术。文章主要研究了多用户 MIMO 多载波无线通信系统中空时预编码技术,研究了多用户 MIMO 多载波系统中空时预编码技术,文中还重点对基于码本的多用户 MIMO 预编码进行了研究。     

非理想CSI下MIMO MRC系统多用户分集性能分析*

针对在无线多输入—多输出（MIMO）通信系统的发射端采用波束成形、接收端采用最大比合并的MIMO MRC系统,考虑收发端仅能获得存在信道估计误差的非理想信道信息（CSI）且系统受到共信道干扰这一条件,分析系统的多用户分集性能。导出封闭形式的信号干扰噪声比的概率分布函数表达式和以平均容量的形式给出的系统多用户分集性能表达式。数值仿真证实了理论分析的结果。研究表明,信道估计误差和共信道干扰均会降低系统的多用户分集性能,而信道估计的精度是影响系统多用户分集性能的关键因素。     

基于合作似然检测和球形译码的多用户多输入输出多跳中继系统

针对多输入输出(MIMO)系统在传输数据过程中的误码率(BER)和信道容量优化问题,提出一种基于合作似然检测和球形译码的多用户MIMO多跳中继系统。首先,该系统构建二阶协作的MIMO中继系统模型对信道数据的中继传输过程以及路径损耗、阴影衰落情况进行分析;接着,通过球形译码来推导出等效的最大似然规则进行对数正态阴影衰落检测;最后提出最大调和平均信道功率选择策略,基于相关性链路度量和最大信道功率阈值为用户选择误码率更小的接入链路,从而改善多用户MIMO系统的性能。实验仿真结果显示,与基于互信息最大化的多用户MIMO多跳中继系统以及基于解码转发和多入多出技术正交空时分组编码(STBC)的中继系统相比,该系统在数据传输过程中的平均比特误码率分别降低了27.4%和32.6%,平均信道容量分别提高了9.5%和12.7%,因此在减少误码率和提高信道容量上具有较好的效果。     

基于格基缩减的MU-MIMO下行传输策略

块对角化（Block diagonalization, BD）预编码算法通过两次奇异值分解实现多用户间干扰消除并将下行多用户多输入多输出（Multi-user multiple-input multiple-output, MU-MIMO）信道解耦成多个独立 的单用户MIMO（Single-user MIMO, SU-MIMO）信道,但其计算复杂度也随着用户数量和矩阵维数的增加而增加 。在MU-MIMO下行系统中,提出基于格基缩减的改进块对角化传输策略,将BD算法的第2次奇异值分解替换为基于格基缩减的线性检测,可得到比传统BD传输策略更好的误码率性能以及更低的计算复杂度。     

一种应用于ORBF算法的多权重矩阵选择策略

为了提高多用户MIMO系统下行信道的总速率,提出一种基于正交随机波束形成的多权重矩阵选择算法.在每个时隙中,该算法利用多个权重矩阵生成随机波束,基站利用部分信道信息反馈,依据总速率最大的原则选择本时隙的最优权重矩阵并进行数据发送.该算法可以显著提高多用户MIMO系统的总速率,仿真结果也证明了该算法的有效性.     

同频干扰下MIMO信道容量研究

针对多用户环境下的多输入多输出（MIMO）系统容量问题,分别分析了同频干扰功率一定时,多干扰用户,小功率发射和少干扰用户,高功率发射对系统容量的影响,给出了两种条件下,系统容量与干扰用户数目以及干扰噪声比的函数关系。最后用蒙特卡洛方法仿真了系统的中断容量,并与不同情况下中断容量的理论曲线相比较,得出了干扰用户数目,干扰用户天线数目以及INR与系统中断容量的关系,用以指导MIMO系统级设计。     

基于压缩感知的多小区MASSIVE MIMO信道估计

针对多小区多用户大规模多输入多输出（MASSIVE MIMO）系统信道估计在低信噪比情况下估计精度较差的问题,提出了一种基于群智能搜索的果蝇分段正交匹配追踪（FF-StOMP）压缩感知算法。该算法在分段正交匹配追踪（StOMP）求解不同阈值下的信道矩阵参数与归一化最小均方误差的基础上,采用果蝇优化算法动态搜索出最小归一化均方误差与其对应的阈值,达到自适应参数设定的目的。仿真结果表明,与StOMP算法相比,信噪比在0~10 dB情况下,所提出的FF-StOMP算法信道估计性能能够提升0.5~1 dB;信噪比在11~20 dB时,信道估计性能能够提升0.2~0.3 dB。当小区用户数发生变化时,所提出的算法能实现自适应信道估计,能够有效提升MASSIVE MIMO系统低信噪比情况下的信道估计精度。     

MIMO系统在CDMA2000中的应用

在CDMA2000中应用MIMO，有效地利用随机衰落和多径延迟扩展，多数量级地提高了无线通信系统的传输速率，改善了系统的性能。     

Low-complexity approximate iterative LMMSE detection for large-scale MIMO systems

This paper deals with iterative detection for uplink large-scale MIMO systems. The well-known iterative linear minimum mean squared error (LMMSE) detector requires quadratic complexity (per symbol per iteration) with the number of antennas, which may be a concern in large-scale MIMO. In this work, we develop approximate iterative LMMSE detectors based on transformed system models where the transformation matrices are obtained through channel matrix decompositions. It is shown that, with quasi-linear complexity (per symbol per iteration), the proposed detectors can achieve almost the same performance as the conventional LMMSE detector. It is worth mentioning that the linear transformations are also useful to reduce the complexity of downlink precoding, so the relevant computational complexity can be shared by both uplink and downlink.     

Energy- and spectral-efficiency of zero-forcing beamforming in massive MIMO systems with imperfect reciprocity calibration: bound and optimization

In a time-division duplex (TDD) system with massive multiple input multiple output (MIMO), channel reciprocity calibration (RC) is generally required in order to cope with the reciprocity mismatch between the uplink and downlink channel state information. Currently, evaluating the achievable spectral efficiency (SE) and energy efficiency (EE) of TDD massive MIMO systems with imperfect RC (IRC) mainly relies on exhausting Monte Carlo simulations and it is infeasible to precisely and concisely quantify the achievable SE and EE with IRC. In this study, a novel method is presented for tightly bounding the achievable SE of massive MIMO systems with zero-forcing beamforming under IRC. On the basis of the analytical results, we demonstrate key insights for practical system design with IRC in three aspects: the scaling rule for interference power, saturation region of the SE, and the bound on the SE loss. Finally, the trade-off between spectral and energy efficiencies in the presence of IRC is determined with algorithms developed to optimize SE (EE) under a constrained EE (SE) value. The loss of optimal total SE and EE due to IRC is also quantified, which shows that the loss of optimal EE is more sensitive to IRC in a typical range of transmit power values.     

全双工MIMO中继系统中一种高性能波束成形算法

全双工中继系统相比于半双工中继系统可以极大地提高频谱利用率,但是中继收发端之间的信号泄漏严重影响全双工系统的性能。为了抑制基于译码转发的全双工多输入多输出中继系统的自干扰,提高信息传输速率,提出了一种波束成形算法。该算法在中继站采用基于最小均方误差的接收与发射波束成形,并联合两个波束成形矩阵建立迭代结构以得到最优解。仿真表明,同传统的零空间投影与最大化信干比算法相比,提出的算法能够有效提高系统性能。在中高信噪比时,该算法较最大化信干比算法获得0.8 bit/(s·Hz)左右的速率增益;当误码率达到10-3以及更低时,该算法相比于最大化信干比算法能获得1.5 dB左右的信噪比增益。     

Complementary meander‐line‐inspired dielectric resonator multiple‐input‐multiple‐output antenna for dual‐band applications

The communication presents a simple dielectric resonator (DR) multiple‐input‐multiple‐output (MIMO) dual‐band antenna. It utilizes two “I”‐shaped DR elements to construct an “I”‐shaped DR array antenna (IDRAA) for MIMO applications. The ground plane of the antenna is defected by two spiral complementary meander lines and two circular ground slots. In the configuration, two “I”‐shaped DR elements are placed with a separation of 0.098λ. The antenna covers dual‐band frequency spectra from 3.46 to 5.37 GHz (43.26%) and from 5.89 to 6.49 GHz (9.7%). It ensures the C‐band downlink (3.7‐4.2 GHz), uplink (5.925‐6.425 GHz), and WiMAX (5.15‐5.35 GHz) frequency bands. Each DR element is excited with a 50‐Ω microstrip line feed with aperture‐coupling mechanism. The antenna offers very high port isolation of around 18.5 and 20 dB in the lower band and upper band, respectively. The proposed structure is suitable to operate in the MIMO system because of its very nominal envelope correlation coefficient (<0.015) and high diversity gain (>9.8). The MIMO antenna provides very good mean effective gain value (±0.35 dB) and low channel capacity loss (<0.35 bit/s/Hz) throughout the entire operating bands. Simulated and measured results are in good agreement and they approve the suitability of the proposed IDRAA for C‐band uplink and downlink applications and WiMAX band applications.     

Investigation on Key Technologies in Large-Scale MIMO

Large-scale MIMO (multiple-input multiple-output) systems with numerous low-power antennas can provide better performance in terms of spectrum efficiency, power saving and link reliability than conventional MIMO. For large-scale MIMO, there are several technical issues that need to be practically addressed (e.g., pilot pattern design and low-power transmission design) and theoretically addressed (e.g., capacity bound, channel estimation, and power allocation strategies). In this paper, we analyze the sum rate upper bound of large-scale MIMO, investigate its key technologies including channel estimation, downlink precoding, and uplink detection. We also present some perspectives concerning new channel modeling approaches, advanced user scheduling algorithms, etc.     

A compact wide band textile MIMO antenna with very low mutual coupling for wearable applications

This communication presents a compact wide band wearable MIMO antenna with very low mutual coupling (VLMC). The proposed antenna is composed of Jeans material. Two “I” shaped stubs are connected in series and are employed on the ground plane between the two patches separated by 0.048 λ to increase isolation characteristics of the antenna‐port. The antenna covers frequency spectrum from 1.83 GHz to 8 GHz (about 125.5%) where the minimum port isolation of about 22 dB at 2.4 GHz and maximum of about 53 dB at 5.92 GHz are obtained. The envelope correlation coefficient (ECC) of the MIMO antenna is obtained to be less than 0.01 with a higher diversity gain (DG > 9.6) throughout the whole operating band. The proposed MIMO antenna is cost effective and works over a wide frequency band of WLAN (2.4‐2.484 GHz/5.15‐5.35 GHz/5.72‐5.825 GHz), WiMAX (3.2‐3.85 GHz) and C‐band downlink‐uplink (3.7‐4.2 GHz/5.925‐6.425 GHz) applications. Simulation results are in well agreement with the measurement results.     

NOMA双连接异构网络条件解耦设计架构及性能分析

为了改善异构网络上行链路的性能及提高网络覆盖概率,提出了采用解耦上下行链路级联和双连接方案的NOMA异构网络模型,设计了一种条件解耦上下行链路级联分析框架。同时,考虑了3种可能的上行链路级联场景,在给定的下行链路级联条件下对解耦上行链路级联进行设计和分析,推导了解耦上行链路级联概率表达式,获得了主、次接入距离统计描述。数值和仿真分析首先研究了系统参数对级联概率的影响,分别研究了解耦上下行链路级联和耦合上下行链路级联方案下NOMA异构网络的条件覆盖概率和频谱效率。实验结果表明,在NOMA双连接异构网络中,所提出的DUDA方案的覆盖概率性能优于传统的CUDA方案的。此外,2种方案的频谱效率性能好坏与系统参数相关。     

Slot allocation algorithms in centralized scheduling scheme for IEEE 802.16 based wireless mesh networks

In IEEE 802.16 based wireless mesh networks (WMNs), TDMA (Time Division Multiple Access) is employed as the channel access method and only TDD (Time Division Duplex) is supported and there are no clearly separate downlink and uplink subframes in the physical frame structure. As the uplink and downlink traffic has different characteristics in that the uplink traffic decentralizes in each MSS (Mesh Subscriber Station) and the downlink traffic centralizes in the MBS (Mesh Base Station), different scheduling methods should be taken in the uplink and downlink. This paper presents a uniform slot allocation algorithm which is suitable for both uplinks and downlinks. To achieve higher spatial reuse and greater throughput and to avoid switching frequently between receiving and transmitting within two adjacent time slots when a relay node forwards traffic, different link selection criteria are taken into account when allocating slots for uplinks and downlinks. A combined uplink and downlink slot allocation algorithm is proposed for further improving the spatial reuse and network throughput. The proposed algorithms are evaluated by extensive simulations and the results show that it has good performance in terms of spatial reuse and network throughput. To the best of the authors’ knowledge, this work is the first one that considers combined uplink and downlink slot allocation on the centralized scheduling scheme in IEEE 802.16 based WMNs.     

Performance analysis of multicell multiuser MIMO MC DS/CDMA system with MMSE user-ordered SIC technique

In this paper, we study the performance of co-channel interference (CCI) cancellation technique based on minimum mean square error (MMSE) user-ordered successive interference cancellation (UOSIC) for multi input multi output (MIMO) multicarrier (MC) direct sequence code division multiple access (DS/CDMA) system for multi cell setting. Though MC DS/CDMA possesses several advantages, interference limits the capacity of the system. Mitigating the interference can directly swell the system capacity. In this contribution, we consider K co-channel users arbitrarily distributed in an L cell uplink and downlink communication system with both the base stations (BSs) and the mobile stations (MSs) equipped with two antennas each. Also, we assume that, both the BSs and MSs employ space time block code (STBC) based on the transmission matrix (G2). Our simulation study shows that, MMSE UOSIC provides better achievable bit-error rate (BER) than MMSE maximum likelihood (ML) and MMSE successive interference cancellation (SIC) technique due to optimal ordering among users and iterative interference cancellation (IC).     

Enhanced handoff scheme based on efficient uplink quality estimation in LTE-Advanced system

Asymmetric traffics cause downlink–uplink asymmetric interference. It can lead to a critical unbalance between a downlink and an uplink channel qualities in cell edge areas. This paper proposes an enhanced handoff scheme including an efficient uplink channel estimation method. The proposed handoff scheme determines an appropriate handoff-timing and handoff-direction according to an estimated uplink channel quality and a measured downlink channel quality. In the proposed scheme, an uplink or downlink, whose quality dominantly affects a link failure, becomes the main handoff-criterion. An efficient uplink channel estimation method is also proposed to exploit an uplink channel quality in handoff. The proposed method estimates an uplink signal strength from the measured downlink signal strength and predicts an uplink interference based on the interference-level information from neighbor base stations. We propose two different uplink estimation modes such as the simple and the precise modes. The simple estimation mode calculates the uplink channel quality for the overall bandwidth for a general handoff process. The precise estimation mode finds the best uplink band for the handoff user who wants an elaborate handoff process. In the elaborate handoff, a target base station allocates the best uplink band to a handoff user to provide the better uplink channel quality. Simulation results show that the proposed uplink estimation method can accurately compute the uplink channel quality of neighbor cells where the estimation error rate is less than 0.7% The simulation results also show that the proposed handoff scheme reduces handoff-call-dropping probability by up to 69% compared to LTE-Advanced system. In addition, the end-to-end delay of the proposed scheme can be better than that of LTE-Advanced system by 26%.