多用户MIMO-OFDM系统功率最小化算法

针对多用户总速率和最大化算法,很少考虑用户之间公平性的不足,提出一种多用户功率优化算法,在满足每个用户速率要求的前提下,使多用户MIMO-OFDM系统的发射功率最小.仿真结果表明,该算法可在10-6>的精度内满足不同用户速率要求,同时所需的下行系统发射功率与理论上最少的基于循环迭代注水算法的DPC传输方式接近,具有较高的实用价值.     

多用户大规模MIMO自适应安全传输策略

大规模MIMO系统采用空分多址可以提高系统吞吐量,同时利用多用户下行信号的相互协作可以对窃听者造成叠加干扰,带来了天然的安全增益。但目前该系统的物理层安全研究仍采用传统的人工噪声方案,忽略了多用户信号干扰带来的安全增益,造成严重的功率浪费。针对这一问题,该文分析了多用户信号干扰对系统可达平均安全速率和平均安全能效的影响,给出了系统的最佳接入用户区间。研究发现,在系统接入用户数较少和用户数较多时,系统安全能力较弱,针对此分别提出了N波束加扰和基于用户位置的用户调度的自适应安全传输策略。最后通过仿真验证了理论推导和所提策略的有效性,利用该文所提策略,能够保证系统天然安全能力不足时的安全通信。     

On the interference management in wireless multi-user networks

A study on interference management schemes in wireless multi-user networks is presented. We analyze the interference management problem in cellular networks and show that interference management is an optimization problem, for which we propose a general formulation. Using this general formulations we show that different interference management approaches are either exact or approximated solutions to this optimization problem. For each radio resource management technique, we provide a general overview and discuss its relation vis-a-vie other interference management techniques. As a case study, we then apply the proposed general formulation on the interference management in OFDM wireless networks and show that it results in a joint transmit scheduling and dynamic sub-carrier and power allocation scheme. A polynomial-time heuristic algorithm is also proposed to solve the formulated optimization problem. The distinguishing feature of the proposed scheme is that it gives in one shot, the transmission scheduling, the sub-carriers assigned to each user, and the power allocated to each sub-carrier, based on a fair and efficient framework while satisfying the delay requirements of real-time users.     

Scheduling Algorithms and Throughput Maximization for the Downlink of Packet-Data Cellular Systems with Multiple Antennas at the Base Station

The capacity-achieving coding scheme for the multiple-input multiple-output (MIMO) broadcast channel is dirty-paper coding. With this type of transmission scheme the optimal number of active users that receive data and the optimal power allocation strategy are highly dependent on the structure of the channel matrix and on the total transmit power available. In the context of packet-data access with adaptive transmission where mobile users are equipped with a single receive antenna and the base station has multiple transmit antennas, we study the optimal number of active users and the optimal power allocation. In the particular case of two transmit antennas, we prove that the optimal number of active users can be a non-monotonic function of the total transmit power. Thus not only the number of users that should optimally be served simultaneously depends on the user channel vectors but also on the power available at the base station transmitter. The expected complexity of optimal scheduling algorithms is thus very high. Yet we then prove that at most as many users as the number of transmit antennas are allocated a large amount of power asymptotically in the high-power region in order to achieve the sum-capacity. Simulations confirm that constraining the number of active users to be no more than the number of transmit antennas incurs only a marginal loss in spectral efficiency. Based on these observations, we propose low-complexity scheduling algorithms with sub-optimal transmission schemes that can approach the sum-capacity of the MIMO broadcast channel by taking advantage of multiuser diversity. The suitability of known antenna selection algorithms is also demonstrated. We consider the cases of complete and partial channel knowledge at the transmitter. We provide simulation results to illustrate our conclusions.     

Hierarchical constellation for multi-resolution data transmission over block fading channels

We propose a new technique for multi-resolution video/image data transmission over block fading channels. The proposed scheme uses an adaptive scheduling protocol employing a retransmission strategy in conjunction with a hierarchical signal constellation (known also as nonuniform, asymmetric, multi-resolution constellation) to give different transmission priorities to different resolution levels. Transmission priorities are given in terms of average packet loss rate as well as average throughput. Basically, according to the transmission scheduling and channel state (acknowledgment signal) of the previous transmission, it dynamically selects packets from different resolution levels to transmit for the current transmission. The bits from the selected packets are assigned to different hierarchies of a hierarchical 4/16-quadrature amplitude modulation to transmit them with different error protections. The selection of packets for transmission and the assignment of these selected packets to different hierarchies of the hierarchical constellation are referred to as the scheduling protocol in our proposed scheme. We model this protocol by a finite state first order Markov chain and obtain the packet loss rate and the packet transmission rate over Nakagami-m block fading channel in closed-form. Some selected numerical results show that the proposed scheme can control the relative packet loss rate and the packet transmission rate of different resolution levels by varying the priority parameter (or equivalently, the asymmetry) of the hierarchical constellation and the maximum number of allowed retransmissions.     

多用户MIMO系统最优发送策略研究

研究一个收发双方都采用多天线的K用户MIMO系统的前向链路的几种最优发送策略.利用矢量广播信道和矢量多接入信道之间的对偶性交换两类信道的发送协方差矩阵以达到快速优化;分析了在各用户功率受限时总的信道容量最大的算法;研究在假定基站采用理想的线性多用户MMSE接收时的最优发送.在总功率受限时通过调整各用户的协方差矩阵实现平均标准MSE最优,采用自适应功率分配可以进一步优化MSE.分析最优化问题与KKT条件的关系,通过迭代计算单用户平均最小均方误差,利用内点法计算互协方差矩阵优化问题.     

Practical Multiuser Diversity With Outdated Channel Feedback

Inspired by the information theoretic results concerning multiuser diversity, we address practical issues in implementing multiuser diversity in a multiple access wireless setting. Considering a channel-assigning strategy that assigns the channel only to the user with the best instantaneous SNR [3], our emphasis is on the effects of channel feedback delay in downlink transmissions. A finite set of M-ary quadrature amplitude modulation (M-QAM) constellations is adopted and a constant transmit power is assumed in this practical multiuser adaptive modulation scheme. Based on the closed-form expressions for average bit error rate (BER) and average data rate, we illustrate the impact of channel feedback delay on the achievable multiuser diversity gain with the number of users. Simple and accurate asymptotic approximations are also provided in the limit of large numbers of users. Focusing on different applications, we propose two optimization criteria for the switching thresholds, based on either an average BER, or an outage probability constraint. Two novel constant power, variable rate M-QAM schemes that are less sensitive to feedback delay are proposed using the optimal switching thresholds, which are derived to maximize the average data rate subject to these two constraints, respectively. To obtain a certain degree of fairness among the users, we also consider a fair channel-assigning strategy that assigns the channel to only the user with the greatest normalized SNR.     

基于SumTree采样结合Double DQN的非合作式多用户动态功率控制方法

为了保证认知无线网络中次用户本身的通信服务质量,同时降低次用户因发射功率不合理而造成的功率损耗,提出了一种基于SumTree采样结合深度双Q网络（Double Deep Q Network,Double DQN）的非合作式多用户动态功率控制方法。通过这种方法,次用户可以不断与辅助基站进行交互,在动态变化的环境下经过不断的学习,选择以较低的发射功率完成功率控制任务。其次,该方法可以解耦目标Q值动作的选择和目标Q值的计算,能够有效减少过度估计和算法的损失。并且,在抽取经验样本时考虑到不同样本之间重要性的差异,采用了结合优先级和随机抽样的SumTree采样方法,既能保证优先级转移也能保证最低优先级的非零概率采样。仿真结果表明,该方法收敛后的算法平均损失值能稳定在0.04以内,算法的收敛速度也至少快了10个训练回合,还能提高次用户总的吞吐量上限和次用户功率控制的成功率,并且将次用户的平均功耗降低了0.5 mW以上。     

802.11ax系统中基于传输延迟的多用户调度与资源分配算法

IEEE 802.11ax系统中站点(Station,STA)数量众多和潜在的高数据包冲突率导致无线局域网通信效率显著降低,本文针对上行多用户传输中的无效帧填充问题,以每轮传输中用户组的传输延迟为优化目标,提出一种多用户调度和资源分配算法.基于OFDMA上行调度接入中动态传输时间的帧交互方案,接入点(Access Po...     

Optimized layered multicast with superposition coding in cellular systems

We consider the problem of optimal power allocation and optimal user selection in a layered multicast transmission over quasi‐static Rayleigh fading channels. A scheme based on superposition coding is proposed in which basic multicast streams and enhanced multicast streams are superimposed and transmitted by a base station, while users with worse channel conditions can only decode basic multicast streams, and users with better channel conditions can decode both basic and enhanced multicast streams. In this paper, subject to fixed user selection ratios, the optimal power allocation for each stream that maximizes average throughput is investigated, and the impact of power allocation on average outage probability is discussed. Finally, subject to fixed transmit power and power allocation, the optimal user selection ratio for enhanced multicast streams is also studied. Numerical results show that the optimized layered multicast scheme outperforms the conventional multicast scheme in terms of average throughput. Copyright © 2010 John Wiley & Sons, Ltd.     

Coordinated User Scheduling Based on Hybrid CSI in Multi-User MIMO Relay System with Limited Feedback

This paper considers coordinated user scheduling in a multi-user two-hop multi-input multi-output relay system with limited feedback. The proposed scheme utilizes a quantized transmit correlation and channel quality information, enabling to achieve both interference mitigation and multi-user diversity (MUD) gain. To this end, we first investigate the effect of quantization error on the statistical characteristics of co-channel interference (CCI) cased by the relay. Then, the coordinated user-scheduling strategy is designed with the use of eigen-beamforming to maximize the desired signal power in an instantaneous manner while minimizing the CCI in an average sense. Analytic and numerical results show that the proposed scheme can maximize the achievable sum-rate by handling a tradeoff between interference mitigation and MUD gain according to the number of quantization bits, providing a large sum-rate performance in the presence of quantization error.     

Utility-based adaptive radio resource allocation in OFDM wireless networks with traffic prioritization

In this letter, a joint transmit scheduling and dynamic sub-carrier and power allocation method is proposed to exploit multi-user diversity in downlink packet transmission in an OFDM wireless network with mixed real-time and non-real-time traffic patterns. To balance efficiency and fairness and to satisfy the QoS requirements of real-time users, we utilize a utility-based framework and propose a polynomial-time heuristic algorithm to solve the formulated optimization problem. The distinguishing feature of the proposed method is that it gives in one shot, the transmission scheduling, the sub-carriers assigned to each user, and the power allocated to each sub-carrier, based on a fair and efficient framework while satisfying the delay requirements of real-time users.     

Some results on the sum-rate capacity of MIMO fading broadcast channels

We study the ergodic sum-rate capacity of the fading MIMO broadcast channel which is used to model the downlink of a cellular system with N/sub t/ transmit antennas at the,base and K mobile users each having N/sub r/ receive antennas. Assuming perfect channel state information (CSI) for all users is available at the transmitter and the receivers, we evaluate the sum-rate capacity numerically using the duality between uplink and downlink. Assuming Nt K, we also derive both upper and lower bounds on the sum-rate capacity to study its increase rate due to multi-user diversity. Finally, we compare three transmission schemes which use the single-user-MIMO scheme (SU-MIMO), ranked known interference (RKI) and zero-forcing beamforming (ZFB), respectively, to transmit to a selected set of users in order to approach the sum-rate capacity. We show that both ZFB and RKI outperform SU-MIMO in a cellular downlink scenario. when many mobile users are present.     

Optimum scheduling for smart antenna systems in Rayleigh fading channels

Optimum and suboptimum scheduling schemes are proposed in the uplink of array antenna systems. We assume that once a user is permitted to transmit the data, the user transmits the data using the maximum power, and adjusts the transmission bit rate so that the energy-per-bit-to-interference-plus-noise-density ratio should be equal or higher than a threshold. In this system model, we consider an optimization problem: How many and which users should be selected to transmit their data at a time in order to maximize the throughput? Based on the analysis on the complexity of the optimum scheme, we propose another optimum scheme having reduced complexity. To reduce the complexity further, we also propose three suboptimum scheduling schemes by limiting the number of users simultaneously transmitting, by choosing the users with higher channel gains, and by partitioning a cell into multiple parts. In circular and linear arrays, the performance results of all the proposed schemes are presented and compared.     

Joint power control and intracell scheduling of DS-CDMA nonrealtime data

The performance of DS-CDMA systems depends on the success in managing interference arising from both intercell and intracell transmissions. Interference management in terms of power control for real time data services like voice has been widely studied and shown to be a crucial component for the functionality of such systems. In this work we consider the problem of supporting downlink nonreal time data services, where in addition to power control, there is also the possibility of controlling the interference by means of transmission scheduling. One such decentralized schedule is to use time division so that users transmit in a one-by-one fashion within each cell. We show that this has merits in terms of saving energy and increasing system capacity. We combine this form of intracell scheduling with a suggested distributed power control algorithm for the intercell interference management. We address its rate of convergence and show that the algorithm converges to a power allocation that supports the nonreal time data users, using the minimum required power while meeting requirements on average data rate. Numerical results indicate a big potential of increased capacity and that a significant amount of energy can be saved with the proposed transmission scheme     

Non-Linear Pre-Coding for Multiple-Antenna Multi-User Downlink Transmissions with Different QoS Requirements

An efficient non-linear pre-filtering technique based on Tomlinson-Harashima pre-coding (THP) has recently been proposed by Liu and Krzymien in the context of multiple-antenna multi-user systems. The algorithm is based on the zero-forcing (ZF) criterion and is derived under the assumption that the number of users is equal to the number of transmit antennas. In contrast to other existing methods, it ensures the same signal-to-noise-ratio (SNR) at each mobile terminal so as to guarantee a fair treatment of all active users. In multimedia applications, however, several types of information with different quality-of-service (QoS) requirements must be sent simultaneously on different subchannels. Motivated by the above problem, in the present work we design a THP-based pre-filtering algorithm for multiple-antenna multi-user systems in which the base station allocates the transmit power according to the QoS requirement of each active user. Compared to existing alternatives, the proposed scheme is simpler to implement and suited for practical situations where the number of active users may be less than the number of transmit antennas     

Performance analysis of wireless multi-user VoIP system with adaptive modulation and coding

In this paper, we propose a new framework to analyze performance considering finite-length queuing and adaptive modulation and coding for multi-user Voice over IP (VoIP) services in wireless communication systems. We formulate an uplink VoIP system as a two-dimensional discrete-time Markov chain (DTMC) based on a Markov modulated Poisson process traffic model for VoIP services and modulation and coding scheme (MCS)-level set transition reflecting users’ channel variations. We extend the transition modeling of the MCS-level for a single-user to the transition modeling of the MCS-level set for multiple users. Since the users can have various MCS combinations in the case of a multi-user system, the MCS-level set transitions are more complicated than the MCS-level transitions of the single-user case. Throughout our DTMC formulation, we present various performance metrics, such as average queue-length, average throughput, packet dropping probability, packet loss probability, and so on. By using the results of the packet loss probability, we can find an optimum packet error rate value that minimizes the total packet loss probability.     

Performance analysis of an opportunistic multi‐user cognitive network with multiple primary users

Consider a multi‐user underlay cognitive network where multiple cognitive users concurrently share the spectrum with a primary network with multiple users. The channel between the secondary network is assumed to have independent but not identical Nakagami‐m fading. The interference channel between the secondary users (SUs) and the primary users is assumed to have Rayleigh fading. A power allocation based on the instantaneous channel state information is derived when a peak interference power constraint is imposed on the secondary network in addition to the limited peak transmit power of each SU. The uplink scenario is considered where a single SU is selected for transmission. This opportunistic selection depends on the transmission channel power gain and the interference channel power gain as well as the power allocation policy adopted at the users. Exact closed form expressions for the moment‐generating function, outage performance, symbol error rate performance, and the ergodic capacity are derived. Numerical results corroborate the derived analytical results. The performance is also studied in the asymptotic regimes, and the generalized diversity gain of this scheduling scheme is derived. It is shown that when the interference channel is deeply faded and the peak transmit power constraint is relaxed, the scheduling scheme achieves full diversity and that increasing the number of primary users does not impact the diversity order. Copyright © 2014 John Wiley & Sons, Ltd.     

Performance of Primary Users in Spectrum Sharing Cognitive Radio Environment

This paper investigates the performance of the primary user in a multiuser cognitive radio environment. Using spectrum sharing method, multiple cognitive users compete to share a channel dedicated to the primary user in order to transmit their data to a common receiver unit. Only one cognitive user is scheduled to share the channel, and to do so, its transmit power should not increase the primary user’s outage probability above a certain limit. The average bit error rate and the average channel capacity of the primary user are derived in this setup for different cognitive scheduling schemes. The performance measures of the primary user are found to depend on the cognitive user’s scheduling criterion and on the parameters of the communication environment.     

一种选择最大多用户分集MIMO信道调度方法及性能分析

提出了一种选择最大多用户分集MIMO信道调度方法,能在不降低性能的基础上减少反馈负载：每个用户将最大的信干噪比值与设定的门限比较,只有大于门限的值及对应的发射天线序号返回给基站,基站分配独立的信道给最大信干噪比用户。给出了所提方案的系统平均容量和反馈负载量的分析,仿真结果与分析一致。