多小区系统中基于信漏噪比最大化的分布式协作预编码

为了能够减少多小区基站协作时的回传开销和预编码复杂度,研究了基于信漏噪比(SLNR)最大化准则的协作多小区传输分布式预编码算法。多个小区协作传输数据,仅仅需要局部的信道状态信息和用户的数据信息,减少了无线回传开销,得到基站协作带来的性能提高。仿真表明,与协作多小区传输的迫零(ZF)算法和协作单小区传输的SLNR算法相比,协作多小区传输SLNR预编码在开销和性能之间取得了良好的折衷。     

3G LTE系统基于码本的自适应预编码方案

第三代移动通信系统长期演进(3G LTE)系统的下行链路传输中,基站端无法获知完全的信道状态信息(CSI),需要根据终端反馈的预编码序号从已有的码本中挑选合适的预编码矩阵进行信号处理。研究了多小区场景下基于有限信息反馈的码本选择算法,提出了一种单双流自适应的码本选择方案,分析了2×2天线情况下所提预编码算法的性能,通过计算机仿真给出了不同多天线预编码方案的性能对比。     

网络MIMO中的基站联合传输技术

协作多点（CoMP）是未来无线通信中的关键技术之一。多个小区基站通过对信道状态信息（CSI）和用户数据信息的不同程度共享,实现多基站协作传输,将传统意义上的干扰转化为对用户有用的信号,从而提高频谱效率,极大改善小区边缘用户的性能。块对角（BD）算法通过在干扰信道的零空间上设计合适的预编码向量,完全抵消干扰。本文将BD算法用于多小区联合传输,从空间维的角度剖析了其干扰抵消和宏分集增益的工作原理,并将其与迫零（ZF）算法,最小均方误差（MMSE）算法,时分多址（TDMA）以及单用户特征值波束成形算法比较,分析了各种算法的优劣。同时将协作BD算法与非协作BD算法比较。仿真结果表明BD算法优于上述预编码算法,协作传输优于非协作传输。
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LTE—A中协同多点传输的联合处理预编码方法

协同多点传输（CoMP）是围绕LTE—A的目标而提出的通过基站内不同远程射频单元（RRU）协作、基站和其所属中继协作和基站间协作等多种多点协作方式,减小小区边缘干扰、提高小区边缘频谱效率、增加有效覆盖的技术措施。CoMP中的联合处理技术（JP）对系统性能的提升最大,JP对性能提升的主要途径是基于信道信息的预编码技术。在不同的CoMP场景下,各种预编码方式各有优劣。越来越多的研究更集中于从压缩反馈量和优化码本设计两个角度来实现预编码的优化。     

反馈时延下LTE链路自适应算法性能研究

介绍了一种LTE系统的快速链路自适应算法,算法基于瞬时信道信息计算预编码矩阵指示、秩指示和信道质量指示3种反馈信息。当前对快速链路自适应算法的研究大部分都假设无反馈时延,因此研究了反馈时延对算法性能的影响,针对发射分集和闭环空间复用两种传输模式在低速和较高速场景下进行了算法性能评估。     

一种基于码本的SLNR算法

在有限反馈系统中,针对用户端向基站反馈完整的信道状态信息对上行链路传输能力要求较高、设备实现较为复杂的问题,给出一种基于码本的最大化SLNR及SFR的PF调度算法。该算法通过码本量化预编码矩阵,基站仅需用户端反馈码本编号即可获得预编码矩阵,从而大大降低了信息反馈量。仿真结果表明,算法保证了系统和边缘用户性能,但是,作为降低反馈量的代价,与引入码本前相比,算法性能略微下降。     

基于用户协作的小区间干扰抑制方案

以长期演进（LTE）上行链路单载波频分多址（SC-FDMA）传输方案为基础,提出了一种基于用户协作的小区间干扰抑制传输方案。该方案采用译码转发中继工作方式,位于相邻小区边缘的两用户互为中继相互协作,在3个时隙内完成用户终端到基站的信息传输。仿真研究表明：该方案能够有效抑制小区间用户共信道干扰,同时充分利用用户协作所提供的空间分集增益提高小区边缘用户链路传输可靠性。此外,该方案所采用的块空时分组编码（B-STBC）能够很好地保持用户终端发射信号的低峰均比特性。     

基于多点协作联合传输的超密集组网性能分析

超密集组网的基站高密度特性会带来严重的小区间干扰,多点协作联合传输应用于超密集组网进行干扰管理是目前的研究热点,该文对多点协作联合传输时基站密度对网络性能的影响进行了分析。首先采用随机几何方法推导了3维空间基站与用户距离的概率密度函数,为选取距离用户最近的多个基站联合传输的协作机制提供了基础;然后结合有界双斜率路径损耗模型,进行用户下行链路的干扰建模,进一步推导出用户下行链路覆盖率和网络区域频谱效率的表达式,并分析了协作基站数、基站密度等参数对网络性能的影响。数值仿真表明:协作基站数为2时就可使下行链路覆盖率增加10%,且实现2到3倍的频谱效率的增益,当协作基站数为3时,费效比更优,同时可得到多点协作下的基站密度极限使区域频谱效率最高。该文工作可为下一代移动通信网络的基站部署提供理论支持。     

协作多点传输系统有限反馈下容量分析

协作多点传输（Coordinated Multiple Point,CoMP）是围绕IMT Advanced的目标而提出的。它通过基站内不同射频接入点（RRU）协作、基站和其所属Relay协作及基站间协作等多种多点协作方式,以减小小区边缘干扰,提高小区边缘频谱效率,增加有效覆盖。这里在有限反馈信道下研究了如何使频谱效率最大化,并分3种情况进行容量分析,包括了在最大SNR波束成型、波束成型信道容量的上界,以及极端情况下的波束成型。     

分布式阵列系统中反馈比特的分配方法

在采用频分双工的通信系统中,基站通常需要用户反馈信道状态信息以进行预编码。对于分布式阵列系统,由于多个接入节点布置在小区的不同位置,用户与小区内不同接入节点之间的信道条件不同。当用于反馈信道状态信息的资源受限时,需要对反馈比特的分配方法进行优化,从而提高系统的整体性能。在多用户分布式阵列系统中,采用基于距离阈值的接入节点选择方法,并在此基础上结合随机矢量量化码本的量化特性以及泰勒展开法,对系统量化容量损失进行推导,并给出了近似表达式。基于该表达式,提出了反馈比特分配方法,与其他的分配方法相比,不限制用户选择的接入节点数目使本文的方法更加具有普遍性。仿真结果显示,在反馈资源有限的情况下,提出的策略优于传统的等比特分配方案,可以获得较好的性能。     

Exploiting non‐orthogonal multiple access in downlink coordinated multipoint transmission with the presence of imperfect channel state information

In this paper, the impact of imperfect channel state information (CSI) on a downlink coordinated multipoint (CoMP) transmission system with non‐orthogonal multiple access (NOMA) is investigated since perfect knowledge of a channel cannot be guaranteed in practice. Furthermore, the channel estimation error is applied to estimate the channel information wherein its a priori of variance is assumed to be known. The impact of the number of coordinated base stations (BSs) on downlink CoMP NOMA is investigated. Users are classified into one of two groups according to their position within the cell, namely, cell‐center user (CCU) and cell‐edge user (CEU). In this paper, ergodic capacity and sum capacity for both CCU and CEU are derived as closed forms. In addition, various experiments are conducted with different parameters such as SNR, error variance, and power allocation to show their impact on the CoMP method. The results show that CoMP NOMA outperforms the CoMP orthogonal multiple access (OMA) wherein the condition of the channel impacts the performance of CoMP NOMA less. It is worth noting that a higher number of coordinated BSs enhances the total capacity of CoMP NOMA. Finally, the performance analysis is validated due to the close accordance between the analytical and simulation results.     

Adaptive bit allocation scheme in multi-cell cooperative block diagonalization systems with delayed and limited feedback

In multi-cell cooperative multi-input multi-output(MIMO) systems,base station(BS) can exchange and utilize channel state information(CSI) of adjacent cell users to manage co-channel interference.Users quantize the CSIs of desired channel and interference channels using finite-rate feedback links,then BS can generate cooperative block diagonalization(BD) precoding matrices using the obtained quantized CSI at transmitter to supress co-channel interference.In this paper,a novel adaptive bit allocation scheme is proposed to minimize the rate loss due to imperfect CSI.We derive the closed-form expression of rate loss caused by both channel delay and limited feedback.Based on the derived rate loss expression,the proposed scheme can adaptively allocate more bits to quantize the better channels with smaller delays and fewer bits to worse channels with larger delays.Simulation results show that the proposed scheme yields higher performance than other allocation schemes.     

基于泊松点过程分布的多蜂窝协作系统中干扰对齐技术研究

在多蜂窝MIMO(Multiple-Input Multiple-Output)协作通信系统中,该文研究了基站站点服从泊松点过程(PPP)分布时,协作基站(BSs)和用户对采用干扰对齐技术的中断概率和网络吞吐量性能,推导了完全信道状态信息(CSI)和部分CSI两种情况下的上述性能的解析表达式,并分析了系统性能与协作参数的单调关系。仿真分析发现:在完全CSI情况下,网络吞吐量随着基站密度、协作基站数、天线数的增加而增加;在部分CSI情况下,由于综合考虑了信道训练和有限反馈带来的资源开销以及量化CSI引入的信道失真,存在一个使网络吞吐量最大的最优协作基站数,当天线数较少或用户移动速度较小时,应有较多的基站参与协作,当天线数较多或用户移动速度较大时,应适当减少协作基站数。     

Dynamic Cooperative Base Station Selection Scheme for Downlink CoMP in LTE-Advanced Networks

Coordinated Multi-Point (CoMP) transmission is a technique proposed to enhance the spectral efficiency and system throughput in an interference limited cellular networks. In CoMP joint processing (JP) scheme multiple base stations (BSs) are coordinately transmit data streams to each user. As more than two base stations are involved, abundant spatial resources are exploited and more backhaul spectrum for JP cooperation is required. The backhaul architecture for CoMP JP is crucial to provide low latency, unlimited capacity, less power consumption, and perfect synchronization among the BSs. However, satisfying all these constraints is impossible as the number of cooperative BSs increases for each user. In this paper, a dynamic cooperative base station selection scheme is proposed to reduce the backhaul load for CoMP user by selecting the appropriate number of coordinated BSs from the CoMP cluster to ensure the certain quality of service (QoS). In particular, for cell edge user the number of cooperative BSs per user has been selected in order to achieve reduced overhead and the allocation of backhaul capacity is performed under the max–min fairness criterion. Simulation results show that the proposed selection scheme achieves significant performance improvement than other transmission modes in terms of the average sum rate per backhaul use and minimal total power consumption.     

Spectrum efficiency sub-carrier and energy efficiency power allocation for downlink multi-user CoMP in multi-cell system

Coordinated multi-point transmission/reception （CoMP） was proposed currently as an effective technology to improve cell-edge throughput in next-generation wireless systems. Most of the existing work discussed clustering methods mostly to maximize the edge user throughput while neglecting the problem of energy efficiency, such as those algorithm clustering base stations （BSs） of better channel condition and BSs of worse channel condition together. In addition, BSs usually increase the transmit power to achieve higher throughput without any considering of interference caused to other users, that may result in energy waste. The authors focus on the throughput maximizing problem while fully considering energy saving problem in CoMP systems. A coefficient is defined to describe the fitness of clusters. Then a sub-carrier allocation algorithm with clustering method is put forward for CoMP downlink, which can save the transmit power of BS and increase the throughput. Furthermore a power allocation scheme is proposed based on non-cooperation game; in which the transmit power is decreased by BSs generally to reach the Nash equation （NE）. Simulation shows that the proposed sub-carrier allocation scheme and power allocation algorithm are better than the existing ones on users＇ throughput while consumes much less energy.     

Limited feedback schemes for downlink OFDMA based on sub-channel groups

In a downlink orthogonal frequency division multiple access (OFDMA) system, optimally allocating sub-channels across mobile users can require excessive feedback of channel state information (CSI). We consider an OFDMA model in which the feedback overhead is explicitly taken into account, given a fixed feedback rate and finite coherence time. The tradeoff between feedback rate and sum capacity is studied for two limited feedback schemes: a sequential scheme in which the users send compressed feedback bits over consecutive time slots, and a contention scheme in which users send their feedback via a random access protocol. For both schemes each feedback bit indicates a request for a group containing multiple subchannels. We show that the sum capacity for both schemes with optimized sub-channel groups grows linearly with the number of sub-channels N, and that the associated constant increases as the log of the normalized feedback rate measured in bits per coherence time per sub-channel. We also compare the asymptotic (large N) performance of the two limited feedback schemes as a function of the feedback rate and load (users per sub-channel). The sequential scheme performs best with moderate to large feedback rates, or small loads, whereas the contention scheme performs best with small feedback rates or large loads.     

Unquantized and uncoded channel state information feedback in multiple-antenna multiuser systems

We propose a channel state information (CSI) feedback scheme based on unquantized and uncoded (UQ-UC) transmission. We consider a system where a mobile terminal obtains the downlink CSI and feeds it back to the base station using an uplink feedback channel. If the downlink channel is an independent Rayleigh fading channel, then the CSI may be viewed as an output of a complex independent identically distributed Gaussian source. Further, if the uplink feedback channel is an additive white Gaussian noise channel, and the downlink CSI is perfectly known at the mobile terminal, it can be shown that UQ-UC CSI transmission (that incurs zero delay) is optimal in that it achieves the same minimum mean-squared error distortion as a scheme that optimally (in the Shannon sense) quantizes and encodes the CSI, while theoretically incurring infinite delay. Since the UQ-UC transmission is suboptimal on correlated wireless channels, we propose a simple linear CSI feedback receiver that can be used to improve the performance of UQ-UC transmission while still retaining the attractive zero-delay feature. We provide bounds on the performance of such UQ-UC CSI feedback and study its impact on the achievable information rates. Furthermore, we explore its application and performance in multiple-antenna multiuser wireless systems, and also propose a corresponding pilot-assisted channel-state estimation scheme.     

多基站协作系统中有限反馈策略及其码本设计

该文针对基于中央控制单元和大容量光纤骨干网连接而成的多基站集中调度协作系统所引起的高系统复杂度和高实现成本等问题,分析了一种可由现存蜂窝无线系统平滑过渡的多基站分布式协作MIMO系统结构及其信号传输特性。在此基础上,给出了该系统结构中基于信道量化码字索引信息的低速率有限反馈策略,讨论了对应的码本设计与构造准则,并提出了一种基于弦距离矢量量化准则的量化码本空间构造与实现算法。通过性能分析表明,该有限反馈策略获得的系统性能可明显优于模拟的导频重传方法,同时也好于基于欧几里德距离量化准则构造的码本空间性能。     

Regional Planning of Coordinated Multi-Point Transmission with Perfect Feedback

This paper studies regional planning issues of coordinated multi-point (CoMP) transmission with perfect feedback under single-user scenario. To determine a reasonable regional planning based on semi-dynamic cooperation idea, the net ergodic capacity optimization problem is derived when given the downlink received signal expression and the definition of penalty factor. The variables that should be optimized are the number of coordinated base stations (BSs) and the dividing-area radius. In order to solve this joint optimization problem, the capacity of non-CoMP and CoMP is analyzed and simplified with knowledge of index random distribution. Again on the basis of the above simplified expression, approximate results of two plans that determine a certain relationship between the number of cooperative BSs and the cell radius are given by using the method of fixed variable and calculus formula. Simulation results show that the two approximate curves are consistent with the results obtained by Monte Carlo simulation.     

一种改进的DL-CoMP-MU有限反馈方案

为提高现有的 CoMP下行链路的反馈能力,降低反馈开销,利用匹配滤波器原理、采用认知的思想方法来判断使用合理的协作传输结构,同时利用基于信号干扰噪声比(SINR)反馈开销方案代替CQI信息以降低信息的反馈量,从而进一步减少反馈信息量化误差。仿真结果证明,改进后的反馈开销方案相比传统方案,在提升反馈信息精确度的基础上,进一步减小了反馈开销。