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Fractional Pilot Reuse in Massive MIMO Systems  [PDF]
Italo Atzeni,Jesús Arnau,Mérouane Debbah
Mathematics , 2015,
Abstract: Pilot contamination is known to be one of the main impairments for massive MIMO multi-cell communications. Inspired by the concept of fractional frequency reuse and by recent contributions on pilot reutilization among non-adjacent cells, we propose a new pilot allocation scheme to mitigate this effect. The key idea is to allow users in neighboring cells that are closest to their base stations to reuse the same pilot sequences. Focusing on the uplink, we obtain expressions for the overall spectral efficiency per cell for different linear combining techniques at the base station and use them to obtain both the optimal pilot reuse parameters and the optimal number of scheduled users. Numerical results show a remarkable improvement in terms of spectral efficiency with respect to the existing techniques.
Low-Complexity Design of Generalized Block Diagonalization Precoding Algorithms for Multiuser MIMO Systems  [PDF]
K. Zu,R. C. de Lamare,M. Haardt
Computer Science , 2013,
Abstract: Block diagonalization (BD) based precoding techniques are well-known linear transmit strategies for multiuser MIMO (MU-MIMO) systems. By employing BD-type precoding algorithms at the transmit side, the MU-MIMO broadcast channel is decomposed into multiple independent parallel single user MIMO (SU-MIMO) channels and achieves the maximum diversity order at high data rates. The main computational complexity of BD-type precoding algorithms comes from two singular value decomposition (SVD) operations, which depend on the number of users and the dimensions of each user's channel matrix. In this work, low-complexity precoding algorithms are proposed to reduce the computational complexity and improve the performance of BD-type precoding algorithms. We devise a strategy based on a common channel inversion technique, QR decompositions, and lattice reductions to decouple the MU-MIMO channel into equivalent SU-MIMO channels. Analytical and simulation results show that the proposed precoding algorithms can achieve a comparable sum-rate performance as BD-type precoding algorithms, substantial bit error rate (BER) performance gains, and a simplified receiver structure, while requiring a much lower complexity.
多用户MIMO系统中连续块对角化预编码算法
Successive Block Diagonalization Precoding for Multi-User MIMO System
 [PDF]

王敏,王杰令,周斌,王棚
- , 2017,
Abstract: 块对角化(BD)预编码是多用户MIMO(MU-MIMO)系统下行链路中广泛采用的线性预编码方案。当用户有多路数据流时,传统的BD预编码存在数据流间等效信道增益不平衡的问题。该文基于BD预编码算法,提出连续BD预编码(SBD)算法。新算法考虑天线间干扰提出新的干扰矩阵构造方法,连续地计算每根接收天线所对应的预编码矩阵。SBD预编码减小了由BD预编码引起的每个用户多路数据流之间有效信道增益的差距,提高了用户的平均误码性能。发射机采用空间分集的发送策略时,接收端进行合并,与BD算法相比,能够获得更多的分集增益。计算机仿真表明,在{2,2}*4的MU-MIMO系统中,当误码率为10^(-3)时,所提算法与现有BD算法相比,获得约2dB的性能增益。
Block Diagonalization (BD) is a common linear precoding technique for the downlink of multi-user MIMO(MU-MIMO) systems. A drawback of conventional BD is that the effective channel gain for each data stream can be severely unbalanced when each user has multiple data streams. In this paper, a new precoding scheme called successive BD (SBD) is proposed based on BD precoding by introducing a new construction scheme of interference matrices considering inter-antenna interference. The precoding matrix responding to each of the receive antennas is successively calculated. SBD precoding reduces the difference in effective channel gain between each user’s multiple data streams caused by the original BD precoding and improves the average bit error performance of the user. More spatial diversity gain can be obtained compared with BD precoding when precoding and combing are employed and spatial diversity transmission strategy is adopted. Simulations show that when the bit error rate is 10^(-3), the proposed method has about 2 dB gain over the conventional BD precoding in {2,2}*4 MU-MIMO systems
Pilot Reuse for Massive MIMO Transmission over Spatially Correlated Rayleigh Fading Channels  [PDF]
Li You,Xiqi Gao,Xiang-Gen Xia,Ni Ma,Yan Peng
Mathematics , 2015, DOI: 10.1109/TWC.2015.2404839
Abstract: We propose pilot reuse (PR) in single cell for massive multiuser multiple-input multiple-output (MIMO) transmission to reduce the pilot overhead. For spatially correlated Rayleigh fading channels, we establish a relationship between channel spatial correlations and channel power angle spectrum when the base station antenna number tends to infinity. With this channel model, we show that sum mean square error (MSE) of channel estimation can be minimized provided that channel angle of arrival intervals of the user terminals reusing the pilots are non-overlapping, which shows feasibility of PR over spatially correlated massive MIMO channels with constrained channel angular spreads. Regarding that channel estimation performance might degrade due to PR, we also develop the closed-form robust multiuser uplink receiver and downlink precoder that minimize sum MSE of signal detection, and reveal a duality between them. Subsequently, we investigate pilot scheduling, which determines the PR pattern, under two minimum MSE related criteria, and propose a low complexity pilot scheduling algorithm which relies on the channel statistics only. Simulation results show that the proposed PR scheme provides significant performance gains over the conventional orthogonal training scheme in terms of net spectral efficiency.
Optimal Multiuser MIMO Linear Precoding with LMMSE Receiver  [cached]
Shu Fang,Gang Wu,Shao-Qian Li
EURASIP Journal on Wireless Communications and Networking , 2009,
Abstract: The adoption of multiple antennas both at the transmitter and the receiver will explore additional spatial resources to provide substantial gain in system throughput with the spatial division multiple access (SDMA) technique. Optimal multiuser MIMO linear precoding is considered as a key issue in the area of multiuser MIMO research. The challenge in such multiuser system is designing the precoding vector to maximize the system capacity. An optimal multiuser MIMO linear precoding scheme with LMMSE detection based on particle swarm optimization is proposed in this paper. The proposed scheme aims to maximize the system capacity of multiuser MIMO system with linear precoding and linear detection. This paper explores a simplified function to solve the optimal problem. With the adoption of particle swarm optimization algorithm, the optimal linear precoding vector could be easily searched according to the simplified function. The proposed scheme provides significant performance improvement comparing to the multiuser MIMO linear precoding scheme based on channel block diagonalization method.
Optimal Multiuser MIMO Linear Precoding with LMMSE Receiver  [cached]
Fang Shu,Wu Gang,Li Shao-Qian
EURASIP Journal on Wireless Communications and Networking , 2009, DOI: 10.1155/2009/197682
Abstract: The adoption of multiple antennas both at the transmitter and the receiver will explore additional spatial resources to provide substantial gain in system throughput with the spatial division multiple access (SDMA) technique. Optimal multiuser MIMO linear precoding is considered as a key issue in the area of multiuser MIMO research. The challenge in such multiuser system is designing the precoding vector to maximize the system capacity. An optimal multiuser MIMO linear precoding scheme with LMMSE detection based on particle swarm optimization is proposed in this paper. The proposed scheme aims to maximize the system capacity of multiuser MIMO system with linear precoding and linear detection. This paper explores a simplified function to solve the optimal problem. With the adoption of particle swarm optimization algorithm, the optimal linear precoding vector could be easily searched according to the simplified function. The proposed scheme provides significant performance improvement comparing to the multiuser MIMO linear precoding scheme based on channel block diagonalization method.
Limited Feedback Precoding for Massive MIMO  [PDF]
Xin Su,Jie Zeng,Jingyu Li,Liping Rong,Lili Liu,Xibin Xu,Jing Wang
International Journal of Antennas and Propagation , 2013, DOI: 10.1155/2013/416352
Abstract: The large-scale array antenna system with numerous low-power antennas deployed at the base station, also known as massive multiple-input multiple-output (MIMO), can provide a plethora of advantages over the classical array antenna system. Precoding is important to exploit massive MIMO performance, and codebook design is crucial due to the limited feedback channel. In this paper, we propose a new avenue of codebook design based on a Kronecker-type approximation of the array correlation structure for the uniform rectangular antenna array, which is preferable for the antenna deployment of massive MIMO. Although the feedback overhead is quite limited, the codebook design can provide an effective solution to support multiple users in different scenarios. Simulation results demonstrate that our proposed codebook outperforms the previously known codebooks remarkably. 1. Introduction High-rate data demand increases faster and faster with the new generation of devices (smart phones, tablets, netbooks, etc.). However, the huge increase in demand can be hardly met by current wireless systems. As is known to us, MIMO channels, created by deploying antenna arrays at the transmitter and receiver, promise high-capacity and high-quality wireless communication links by spatial multiplexing and diversity. Basically, the more antennas the transmitter or the receiver equipped with, the more degrees of freedom that the propagation channel can provide, and the higher data rate the system can offer. Therefore, there is significant effort within the community to research and develop massive MIMO technology, which is a hot topic nowadays [1]. For multiuser MIMO systems, we can utilize precoding to explore massive MIMO potentials. The essence of precoding techniques is to mitigate the interuser interference and to improve the effective received SNR. Herein, channel state information at the transmitter (CSIT) is an essential component when trying to maximize massive MIMO performance via precoding. In time division duplexing (TDD) system, channel reciprocity can be utilized for pilot training in the uplink to acquire the complete CSIT, but the pilot contamination and imperfect channel estimation based on uplink pilots would lead to inaccuracy of the CSIT. In frequency division duplexing (FDD) system, the CSIT shall be acquired via the feedback channel, which is usually limited in practice. Hence, a finite set of precoding matrices, named codebook, known to both the receiver and the transmitter should be predesigned. The receiver selects the optimal precoding matrix from the
MIMO Precoding Using Rotating Codebooks  [PDF]
C. Jiang,M. Wang,C. Yang
Mathematics , 2010,
Abstract: Next generation wireless communications rely on multiple input multiple output (MIMO) techniques to achieve high data rates. Feedback of channel information can be used in MIMO precoding to fully activate the strongest channel modes and improve MIMO performance. Unfortunately, the bandwidth of the control channel via which the feedback is conveyed is severely limited. An important issue is how to improve the MIMO precoding performance with minimal feedback. In this letter, we present a method that uses a rotating codebook technique to effectively improve the precoding performance without the need of increasing feedback overhead. The basic idea of the rotating codebook precoding is to expend the effective precoding codebook size via rotating multiple codebooks so that the number of feedback bits remains unchanged. Simulation results are presented to show the performance gain of the proposed rotating codebook precoding over the conventional precoding.
大规模MIMO系统中多小区导频重用对上行可达速率的影响
Impact of multi-cell pilot reuse on the achievable uplink rate for massive MIMO systems
 [PDF]

何强,张秀军,肖立民,周世东
- , 2015,
Abstract: 在大规模多入多出(MIMO)系统中, 导频在多小区间重用是影响系统可达速率的一个重要因素。该文选取一般化的信道模型, 考虑系统采用功率控制, 导出了最小二乘(LS)和最小均方误差(MMSE)这2种信道估计方法下上行可达速率的下界。利用该下界对基站天线数无穷多时导频重用的影响进行分析, 结果表明: 在非相关信道下, 用户上行极限可达速率仅仅受限于导频污染, 且功率控制的引入使得该速率部分地由导频干扰用户到各自小区基站的大尺度衰落系数决定; 在相关信道下, 导频污染不是制约系统极限可达速率的唯一因素, 在使用LS信道估计时导频污染会始终存在, 而MMSE信道估计下导频污染可能会消失。
Abstract:In massive multiple-input multiple-output (MIMO) systems, multi-cell pilot reuse is an important factor affecting the achievable system uplink rate. A generalized channel model with adaptable power control was used to derive the lower bound on the achievable uplink rate for both least square (LS) and minimum mean square error (MMSE) channel estimations. Then, the impact of pilot reuse as the number of base station (BS) antennas tends to infinity is analyzed based on these bounds. For uncorrelated channels, the ultimate achievable uplink rate of a user is mainly limited by pilot contamination and partially by the large-scale fading coefficients from a pilot interfering with users communicating to their base stations of their own cells due to power control. For correlated channels, pilot contamination is not the only factor that constrains the ultimate achievable system uplink rate, but always exists when LS channel estimation is used, but may vanish when MMSE is used.
大规模MIMO系统中多小区导频重用对上行可达速率的影响
Impact of multi-cell pilot reuse on the achievable uplink rate for massive MIMO systems
 [PDF]

何强,张秀军,肖立民,周世东
- , 2015,
Abstract: 在大规模多入多出(MIMO)系统中, 导频在多小区间重用是影响系统可达速率的一个重要因素。该文选取一般化的信道模型, 考虑系统采用功率控制, 导出了最小二乘(LS)和最小均方误差(MMSE)这2种信道估计方法下上行可达速率的下界。利用该下界对基站天线数无穷多时导频重用的影响进行分析, 结果表明: 在非相关信道下, 用户上行极限可达速率仅仅受限于导频污染, 且功率控制的引入使得该速率部分地由导频干扰用户到各自小区基站的大尺度衰落系数决定; 在相关信道下, 导频污染不是制约系统极限可达速率的唯一因素, 在使用LS信道估计时导频污染会始终存在, 而MMSE信道估计下导频污染可能会消失。
Abstract:In massive multiple-input multiple-output (MIMO) systems, multi-cell pilot reuse is an important factor affecting the achievable system uplink rate. A generalized channel model with adaptable power control was used to derive the lower bound on the achievable uplink rate for both least square (LS) and minimum mean square error (MMSE) channel estimations. Then, the impact of pilot reuse as the number of base station (BS) antennas tends to infinity is analyzed based on these bounds. For uncorrelated channels, the ultimate achievable uplink rate of a user is mainly limited by pilot contamination and partially by the large-scale fading coefficients from a pilot interfering with users communicating to their base stations of their own cells due to power control. For correlated channels, pilot contamination is not the only factor that constrains the ultimate achievable system uplink rate, but always exists when LS channel estimation is used, but may vanish when MMSE is used.
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