LTE-A中自适应多流波束赋形算法研究

多流波束赋形技术是LTE-Advanced中采用的多天线增强技术。采用单用户多流波束赋形技术,可以使得单个用户在某一时刻可以进行多个数据流传输,在提升系统性能上表现出极大优势。提出一种LTE-Advanced中下行单用户自适应多流波束赋形算法,基站端基于接收模型对下行链路容量进行预估计,在一定信噪比要求的条件下,以信道容量最大化准则选择波束赋形流数。仿真结果表明,自适应算法增加了波束赋形系统的灵活性,能够有效提高LTE-Advanced系统中下行链路性能。     

LTE系统中的双流波束赋形技术研究

通过洋细分析波束赋形算法及其在LTE系统甲的实现方式,对波束赋形性能以及发射天线数对波束赋形系统性能的影响进行了仿真,仿真结果表明,双流波束赋形适用于小区中心区域SINR高的区域：增加发射天线的数量能够改蓠波束赋形系统的SINR性能,从而提升系统的容量。最后指出了波束赋形技术进一步的研究方向。     

TD-SCDMA下行波束成形技术研究

讨论了TD-SCDMA系统的下行信号模型,并根据TD-SCDMA系统上行信道估计可以直接用于下行波束赋形的特点,研究了特征值分解方法下的下行波束成形算法的性能并仿真比较了其与基于最大径准则等下行波束形成算法在不同环境下的性能。     

一种自适应阵列天线波束赋形合成算法

自适应阵列天线中的波束赋形技术是智能天线数字信号处理部分的核心,在介绍和比较常用波束赋形算法以及RLS算法的基础上,提出了-种改进的RLS算法以提高系统的性能。对于用自适应阵列天线改善GPRS基站系统,提高其容量、减少干扰信号、提高信噪比、提高系统频谱利用率有很大的现实意义。     

智能天线在TD-LTE中的应用分析

文章从技术层面介绍了智能天线的基础技术、波束赋形技术和自适应算法,介绍了TD-LTE中智能天线的单流波束赋形、双流波束赋形技术及相关算法,分析了智能天线在TD-LTE中的应用情况,最后简述了智能天线技术的发展态势。     

TD-SCDMA系统中基于上行参数的下行波束赋形算法

在TD－SCDMA系统中，我们在基站发送端进行波束赋形来提高下行链路的性能。以最优准则－最大信干噪比特征分解算法为例，我们分析基于上行参数进行下行赋形的基本原理，模拟了不同环境下赋形算法的性能，并与全向情况进行比较，得出了相应的结论。     

MISO系统下行波束赋形及仿真

研究了基于TD-SCDMA系统的下行波束赋形,利用最大信干噪比准则计算出天线的加权系数,在发送端进行赋形,并做了仿真,结果表明下行波束赋形能明显降低接收端误码率,进而降低了基站发射功率。     

基于种群技术的数字波束赋形算法分析

在移动通信中采用波束赋形算法,可以更好地利用天线的空分性能和发射功率,但是需要考虑算法的收敛速度和计算速度。如果移动用户穿过蜂窝小区时速度很快,那么即使是短时间内位移量也很大。而赋形算法如果收敛慢或者没有检测到这个变化,就会仍然针对原来的问题进行计算,从而跟踪不到用户,影响通话。介绍了移动通信对波束赋形算法的要求,分析了两种基于种群技术的数字波束赋形算法。     

一种基于特征值分布的波束赋形方案

该文分析了在不同角度扩展和DOA情况下的特征值分布,研究了在不同角度扩展环境下不同波束赋形算法的性能。在此基础上,提出了一种基于特征值分布的波束赋形方案。该方案根据信道的特征值分布自适应地采用不同波束赋形算法。仿真表明,该方案与PPPW(Per-Path Per-Weight)波束赋形算法性能相近,同时大大降低计算复杂度。     

蜂窝网络下行链路中基于干扰图的干扰对齐算法

针对多用户蜂窝网络的下行链路,提出了基于干扰图的干扰对齐算法。首先用干扰图表示蜂窝网络的小区间干扰（ICI）分布,每五个相邻小区划分为一簇。然后在簇内先根据对称信道和不对称信道,分别设计接收波束赋形矩阵将四个ICI对齐到两个干扰子空间,再设计发送波束赋形矩阵与干扰子空间正交。最后通过簇的平移实现蜂窝网络内所有活动小区的干扰对齐。该算法减少了基站和用户的天线数及计算复杂度。仿真结果表明所提算法消除了ICI、用户间干扰（IUI）及簇间干扰,有效提高了蜂窝网络容量。      

多小区蜂窝网络波束成形优化策略

为了克服多小区的邻小区干扰问题,该文在多小区蜂窝网络中,研究了多小区多用户下行协同传输技术。该文提出一种联合优化基站和中继的波束成形权重的迭代算法,在基站和中继总功率限制下最大化最差用户接收信干噪比(Signal-to-Interference-and-Noise Ratio, SINR)。该文提出的联合优化波束成形策略,可以应用半正定松弛技术(Semi-Definite Relaxation, SDR)得到有效的解决。仿真表明,在多小区通信系统中,该文提出的迭代算法只需要少量的迭代次数就可以达到近似最优性能,并且在性能和资源消耗之间给出一种折中的传输策略。     

Multi-cell uplink-downlink beamforming throughput duality based on Lagrangian duality with per-base station power constraints

Despite significant research efforts in beamforming, the maximum achievable downlink throughput with beamforming in a multi-cell environment is not available due to difficulty in finding optimal downlink beamforming. Thus, to reformulate the problem into a more solvable form, we derive dual uplink throughput optimization problem to multi-cell downlink beam- forming throughput maximization with per-base station (BS) power constraints based on Lagrangian duality. The optimal downlink beamforming is shown to be a minimum mean squared error (MMSE) beamforming in the dual uplink. It is also shown that the dual uplink problem achieves the same optimal throughput as the primal downlink problem.     

Optimal designs of collaborative relay‐assisted multiuser beamforming for cellular systems

With careful calculation of signal forwarding weights, relay nodes can be used to work collaboratively to enhance downlink transmission performance by forming a virtual multiple‐input multiple‐output beamforming system. Although collaborative relay beamforming schemes for single user have been widely investigated for cellular systems in previous literatures, there are few studies on the relay beamforming for multiusers. In this paper, we study the collaborative downlink signal transmission with multiple amplify‐and‐forward relay nodes for multiusers in cellular systems. We propose two new algorithms to determine the beamforming weights with the same objective of minimizing power consumption of the relay nodes. In the first algorithm, we aim to guarantee the received signal‐to‐noise ratio at multiusers for the relay beamforming with orthogonal channels. We prove that the solution obtained by a semidefinite relaxation technology is optimal. In the second algorithm, we propose an iterative algorithm that jointly selects the base station antennas and optimizes the relay beamforming weights to reach the target signal‐to‐interference‐and‐noise ratio at multiusers with nonorthogonal channels. Numerical results validate our theoretical analysis and demonstrate that the proposed optimal schemes can effectively reduce the relay power consumption compared with several other beamforming approaches. Copyright © 2012 John Wiley & Sons, Ltd.     

Downlink channel covariance matrix (DCCM) estimation and itsapplications in wireless DS-CDMA systems

The downlink channel covariance matrix (DCCM) is of vital importance in determining downlink beamforming weights for base station (BS) antenna array systems. For the frequency-division-duplex (FDD) mode, DCCM is difficult to obtain due to a lack of direct measurement of downlink channel responses. In this paper, a novel technique is proposed for estimating DCCM using uplink channel responses only, which does not need direction-of-arrival (DOA) estimation and its association. The downlink beamforming scheme is then proposed for wireless DS-CDMA systems, using the obtained DCCM information together with the so-called virtual uplink beamforming and power control technique. Computer simulations show that using the BS antenna array together with this new beamforming technique can provide larger system capacity than traditional DOA-based approaches, which just direct the main beam toward the desired user     

Predictive downlink beamforming for wideband CDMA over Rayleigh-fading channels

A new approach to adaptive downlink beamforming to combat fast Rayleigh fading is presented. In this approach, the antennas at the base transceiver station form transmit beam patterns according to the prediction of downlink channels. The channel prediction is a linear prediction based on the autoregressive model, which is downsampled to extend the memory span given fixed model order. For a wideband code-division multiple-access downlink, pre-RAKE transmission is employed to achieve the multipath diversity gain. In particular, we combine pseudoinverse directions of arrival beamforming with pre-RAKE transmission to alleviate self-interference. The beamforming weights are adjusted within a downlink frame to compensate the predicted fading. We give measures of the prediction and beamforming performance and evaluate the impact of prediction errors on the downlink. Ray tracing simulations in a three-dimensional urban physical model show that the predictive downlink beamforming outperforms the conventional beamforming over Rayleigh-fading channels.     

Two suboptimal algorithms for downlink beamforming in FDD DS-CDMAmobile radio

Two suboptimal algorithms are proposed for downlink beamforming in FDD DS-CDMA mobile radio by using uplink beamforming weights. One is a null-constrained method, which maintains the same null positions for both uplink and downlink beam patterns; the other is a frequency-calibrated method which constrains the same main beam positions for both patterns. We also evaluate the multicell downlink capacity of DS-CDMA systems using a per-user-per-weight beamforming scheme. Outer cell interference is modeled as an AWGN process whose variance is proportional to the average intracell total transmitted power. Computer simulations are given to compare the single cell and multicell capacities using different downlink beamforming weight generation algorithms     

System evaluation of optimal downlink beamforming with congestion control in wireless communication

We investigate the use of congestion control and joint optimal downlink beamforming, power control, and access point allocation, in a multi-cell wireless communication system. The access points of the system employ smart antennas and single antennas are used at the terminals. The possibility to send messages to multiple terminals at the same frequency in the same time slot is exploited. We show how previously proposed algorithms for optimal downlink beamforming easily can be extended to determine also the optimal access point for each mobile terminal. In order to assign resources, optimal beamforming requires a feasible set of mobiles, i.e. that all admitted users can be offered the required signal-to-interference-and-noise ratio. Therefore, an algorithm for deciding which mobile terminals to admit or reject from a congested system is proposed and evaluated. Using the proposed congestion algorithm, joint optimal downlink beamforming is evaluated and the throughput increase as compared to decentralized beamforming algorithms and other congestion control strategies is assessed from a system point of view. The results show that the proposed strategy can almost double the throughput compared to decentralized beamforming algorithms and give a fivefold increase in throughput compared to conventional beamforming without any interference suppression.     

Intelligent antenna solutions for UMTS: algorithms and simulation results

This article outlines the development of intelligent antenna (IA) solutions for UMTS, a third-generation W-CDMA system. Since the selection of an antenna configuration paired with realizable uplink/downlink algorithms that can satisfy all operating environments is a broad task, this article focus is on cost-effective antenna arrays for macrocells. Algorithms that exploit the antenna configurations and act at both the physical and MAC layers are highlighted and supported by simulation results. Two solutions stand out for UMTS: a universal beamforming algorithm that unifies user-specific and fixed beamforming under one framework, and multibeam scheduling (MBS) that significantly increases downlink packet data throughput using the concept of code reuse in conjunction with beamforming. The article summarizes the critical issues that were faced in the development of an IA solution capable of delivering the theoretically promised benefits to end users.