Capacities of Physical Layer Scheduling Strategies on a Shared Link

We compare capacities of three physical layer scheduling algorithms: on-off scheduling (OOS), maximum SNR, and round robin schedulers. With OOS, downlink transmission is suspended if the instantaneous received SNR in the mobile station falls below a given threshold requiring only one feedback bit per user, which considerably decreases the overhead of the feedback signaling when compared with maximum SNR scheduler. In case of FDD systems, OOS and maximum SNR schedulers require a fast feedback channel from mobile to base station like the one in the FDD WCDMA system, while round robin operates independently of the channel state. We derive asymptotic capacity gains of the three schedulers in high SNR region and study the effect of feedback bit errors to capacities. It is noticed that OOS has an optimal threshold, which depends on the number of users and their mean SNRs. As expected, maximum SNR scheduler provides the largest capacity among the three schedulers, but if additional spatial diversity, i.e., transmit antenna selection is available, the difference between the capacities of OOS and maximum SNR schedulers is small.     

Joint Subcarrier and Power Allocation in Channel-Aware Queue-Aware Scheduling for Multiuser OFDM

In an orthogonal frequency division multiplexing (OFDM) downlink scenario, we propose joint subcarrier and power allocation for channel-aware queue-aware scheduling while allowing multiple users to share a single OFDM symbol. Our approach is to combine subcarrier and power allocation by optimizing a user's power allocation immediately after the user has been allocated a subcarrier. Simulation results show that joint subcarrier and power allocation yields a significant performance improvement compared to other existing schemes which perform subcarrier allocation with a fixed (uniform) power allocation assumption. Joint subcarrier and power allocation is also extended to band-wise allocation of subcarriers in order to help reduce signaling overhead in time varying channels. We examine the trade-off between increasing the sub-band size and the corresponding degradation in system performance for different values of the channel multipath delay spread.     

WCDMA中变步长功率控制过程

本文提出了一种可应用于WCDMA中的变步长功率控制策略,这种功控策略根据TPC命令的"历史",将功率控制过程划分为Markov状态,分析信道衰落步长的概率密度,每一状态对应一个确定的步长值,从而可以确定功率控制步长集中的步长数,我们选用概率密度最大的数值,同时考虑硬件可以分辨的最小步长在0.5dB左右,确定步长集.再用仿真的方法,确定状态和步长的对应关系.发射机依据当前的TPC命令决定应该增大还是减小发射功率,依据当前TPC命令和"历史"上的TPC命令选择合适的功率步长,进而确定发射功率.     

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.     

基于改进图着色的D2D资源分配和功率控制

针对蜂窝网络中D2D（Device-to-Device）用户复用蜂窝信道带来的同频干扰问题,提出了一种基于改进图着色的资源分配和功率控制算法。首先通过构建干扰图和候选集进行用户之间干扰关系建模,并定义指数型累积因子改进图着色算法,为D2D用户分配蜂窝信道;再采用基于信干噪比的闭环功率控制算法动态调整D2D用户发射功率,减小由于信道复用产生的干扰。仿真结果表明,与现有算法相比,所提算法能够有效提升系统吞吐量和D2D用户接入率,实现信道资源的合理分配。     

无线网络中平均功率受限的延时确保调度机制的最优化研究

该文研究了在高斯信道下平均发射功率受限的延时确保调度器的最优化问题。文章首先证明了对于延时确保条件下平均发射功率最优的时不变调度器,其最优的平均发射功率为延时确保界Dmax的单调递减函数,并根据其单调性给出了平均发射功率最优调度器和延时确保最优调度器之间的对偶关系。基于该关系,给出了到达过程未知条件下平均功率受限的延时确保最优调度器的实现形式。该实现形式中参数的确定方法也在给定到达过程分布的条件下给出,并以泊松到达为例进行了分析。文章的最后还给出了该调度器的一种实际实现方案并进行了仿真,仿真结果表明该方案能够达到调度器的最优。     

Opportunistic beamforming using dumb antennas

Multiuser diversity is a form of diversity inherent in a wireless network, provided by independent time-varying channels across the different users. The diversity benefit is exploited by tracking the channel fluctuations of the users and scheduling transmissions to users when their instantaneous channel quality is near the peak. The diversity gain increases with the dynamic range of the fluctuations and is thus limited in environments with little scattering and/or slow fading. In such environments, we propose the use of multiple transmit antennas to induce large and fast channel fluctuations so that multiuser diversity can still be exploited. The scheme can be interpreted as opportunistic beamforming and we show that true beamforming gains can be achieved when there are sufficient users, even though very limited channel feedback is needed. Furthermore, in a cellular system, the scheme plays an additional role of opportunistic nulling of the interference created on users of adjacent cells. We discuss the design implications of implementing. this scheme in a complete wireless system     

一种适用于SC-FDMA多小区系统的协作调度和功率控制算法

该文针对长期演进(LTE)上行单载波频分多址(SC-FDMA)多小区系统的性能受限于小区间干扰的问题,提出一种综合考虑协作调度和功率控制的方案。该方法分步执行小区间的协作调度和功率控制,首先调度各小区中的用户,在此基础上优化用户的发射功率。调度时首先估计小区间的干扰信息并分配频率资源块给每个小区内的用户,在优化用户的发射功率时,同时考虑由于用户功率改变所造成的目标小区和其他干扰小区性能的变化。进一步提出一种低复杂度功率控制方案,在优化目标小区用户的功率时,只考虑受目标小区干扰影响最大的几个小区性能的变化,其他干扰小区性能的变化则通过引入补偿因子来估计。计算机仿真验证了该文所提方法在系统吞吐量和小区边缘吞吐量方面的性能优势显著。     

多用户MIMO系统下行链路的随机多波束复用技术

该文提出了一种随机多波束多用户复用技术,充分利用多用户分集以及基站多天线的空间自由度来提高系统吞吐量。不同于传统的随机波束形成技术,该技术首先在给定预编码码本内随机选取一个码字,然后调度多个空分复用用户以及其余预编码矩阵。该文采用了一种逐次调度的方式,第一次训练调度一个主发送用户并确定一个次发送预编码矩阵,通过第二次训练选择次发送用户,这种方式能以很小的反馈开销有效控制复用用户之间的相互干扰。同时,该文所提技术能进一步推广到用户具有不同天线配置的异构情形。仿真结果表明,该文技术在具有不同相关性的信道环境下都能获得较优的系统吞吐量。     

Subcarrier Wise Scheduling Methods for Multi-antenna and Multi-carrier Systems

In this paper, multiuser scheduling algorithms are evaluated for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) networks. These scheduling schemes allocate M [number of transmit antennas at base station (BS)] number of independent data streams from BS to the M most favourable users experiencing the highest signal-to-interference-plus-noise-ratio (SINR). Here, SINR is used to convey the channel state information (CSI) to the BS. We have investigated the system throughput and feedback overhead attained by these scheduling schemes for different scenarios as: (a) the maximum CSI is sent to the BS by every user and (b) the maximum CSI sent to the BS corresponding to every BS antenna. The overall feedback overhead incurred by MIMO-OFDM system increases linearly with number of users, number of subcarriers and number of transmit antennas. Hence, to reduce the feedback overhead, a scheme is proposed where users with SINR values greater than or equal to a predefined threshold value are only allowed to feedback the channel state information to BS. The relation between system throughput and various thresholds is also studied. The achievable system throughput results are validated by comparing the probability density function of achieved SINR values by different scheduling schemes.

     

The impact of antenna diversity in packet data systems with scheduling

It has been observed through simulations of some specific scheduling algorithms that multiuser diversity gains in packet data systems with channel-aware scheduling can be reduced in the presence of any form of link diversity, such as transmit antenna diversity or wideband multipath diversity. We establish that asymptotically, in the limit of large number of transmit antennas and users, the maximum throughput achieved by any optimal scheduling algorithm in the presence of transmit diversity under signal-to-noise-ratio-only feedback can be infinitely worse than that of a system with no diversity. Our results are general and are independent of any particular scheduling algorithm.     

Opportunistic eigenbeamforming: Exploiting multiuser diversity and channel correlations

Multiuser diversity is an inherent form of diversity present in any time-varying system with several users. An opportunistic scheduler has to be used in order to exploit this type of diversity. A scheme that increases the effective dynamic range of the channel by deploying multiple antennas at the transmitter is called opportunistic beamforming. Opportunistic beamforming increases the degree of multiuser diversity in several scenarios, including correlated channels. Nevertheless, multiuser diversity can also be combined with other transmit schemes that have proven to be effective in correlated channels, such as eigenbeamforming. Eigenbeamforming is a point-to-point link transmit technique that could easily be combined with an opportunistic scheduler to extract multiuser diversity. We refer to the joint use of eigenbeamforming with an opportunistic scheduler as opportunistic eigenbeamforming. In this work, we show that the available multiuser diversity with opportunistic eigenbeamforming is larger than the one achieved when opportunistic beamforming is employed using the proportional fair scheduler under different degrees of correlation in the channel. In the present work we have considered a single-cell scenario.     

A Scheduling Algorithm with Dynamic Priority Assignment for WCDMA Systems

In third generation WCDMA systems, shared channels allow many users to jointly utilize a single Orthogonal Variable Spreading Factor (OVSF) code. In this paper, we propose a Scheduling Algorithm with Dynamic Priority Assignment (DPA) which is designed for the Downlink-Shared channel (DSCH) of 3G WCDMA systems and operates within a cross layer framework. The DPA scheduler has low computational complexity and is able to provide QoS differentiation among traffic flows based on their delay sensitivity. Through the cross layer framework, DPA takes into account the variations of the wireless channel, and exploits processing gain to improve transmission quality and enable service provisioning when possible. Additionally, by providing a guaranteed rate per traffic flow at each scheduling period, DPA can offer a deterministic delay bound to each connection when transmissions are reliable. Stochastic delay guarantees under transmission power limitations are also provided when the traffic flows are identical. Simulation results show that DPA outperforms Feasible Earliest Due Date (FEDD), a variation of EDD for wireless environments.     

Exploiting Multiuser Diversity With Imperfect One-Bit Channel State Feedback

It has been well recognized that significant throughput gains can be leveraged in multiuser wireless communication systems by exploiting multiuser diversity with a smart scheduler. This scheduler collects channel state information (CSI) from all users and allocates the resources to the user(s) experiencing favorable channel conditions. However, for a frequency-division-duplex system with a large number of users, how to efficiently collect the required CSI will be a challenging task, especially when the feedback links are of limited capacity. In this paper, we propose a scheduling algorithm to exploit multiuser diversity with possibly imperfect one-bit channel state feedback. The basic idea is to define a threshold lambda and let each user report one-bit information to the scheduler about the comparison between its measured channel fading level and lambda. Correspondingly, the scheduler uses these feedback bits to classify all users into two sets and assigns the channel to one user belonging to the set experiencing favorable channel conditions. Several implementation schemes are developed by attacking the optimization of lambda under different system configurations, covering both the case when the one-bit feedback is perfect and those when the one-bit feedback is imperfect. Computer simulations show that when the user number is large, say, more than ten users, the proposed scheduling supports significantly larger data rate over the round-robin scheduling, while in comparison with the optimum scheduling with complete CSI, the performance loss is limited if the one-bit feedback is of high reliability. In addition, our studies show that we can effectively enhance the robustness against feedback imperfectness by incorporating the feedback reliability into optimization of lambda     

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.     

Approximate Minimum BER Power Allocation for MIMO Spatial Multiplexing Systems

Precoding for multiple-input multiple-output (MIMO) spatial multiplexing generally requires high feedback overhead and/or high-complexity processing. Simultaneous reduction in transmitter complexity and feedback overhead is proposed by imposing a diagonal structural constraint to precoding, i.e., power allocation. Minimum bit-error rate (MBER) is employed as the optimization criterion, and an approximate MBER (AMBER) power-allocation algorithm is proposed for a variety of receivers, including zero-forcing (ZF), successive interference cancellation (SIC), and ordered SIC (OSIC). While previously proposed precoding schemes either require ZF equalization for MBER, or use a minimum mean-squared error (MMSE) criterion, we provide a unified MBER solution to power allocation for ZF, SIC, and OSIC receiver structures. Improved error-rate performance is shown both analytically and by simulation. Simulation results also indicate that SIC and OSIC with AMBER power allocation offer superior performance over previously proposed MBER precoding with ZF equalization, as well as over MMSE precoding/decoding. Performance under noisy channels and power feedback is analyzed. A modified AMBER algorithm that mitigates error propagation in interference cancellation is developed. Compared with existing precoding methods, the proposed schemes significantly reduce both transmit processing complexity and feedback overhead, and improve error-rate performance     

Improvements in W-CDMA: principles and experimental results

This article first reviews the channel structure and spreading code assignment for the physical layer and transport channel multiplexing along with a sophisticated rate-matching scheme that accommodates composite transport channels with various levels of quality of service (QoS) on one physical channel. Then, the key technologies of wideband ds-cdma (w-cdma) wireless access are presented and the results of experiments pertaining to these technologies are evaluated. Flexible system deployment is possible by employing inter-cell asynchronous operation with a three-step fast cell search method. The signal-to-interference power ratio (sir) measurement based fast transmit power control (tpc) guarantees the minimum transmit power according to the channel load and the changes in the link conditions due to fading. Furthermore, various diversity technologies are described such as pilot symbol-assisted (psa) coherent Rake combining, antenna diversity, site diversity (soft/softer handover), and transmit diversity in the forward link that are effective in decreasing the required transmit power, which results in increases system capacity. This article also presents link-capacity enhancing techniques such as using an interference canceller (IC) and adaptive antenna array diversity (AMD) receiver/transmitter. Experimental results are presented for an actual multipath fading channel that indicate the potential of the IC and aaad transceiver to decrease the mobile transmit power in the reverse link and interference from high rate users with high transmit power in the forward link.     

Downlink scheduling of multiuser MIMO systems with transmit beamforming

This article deals with downlink scheduling for multiuser multiple-input multiple-output （MIMO） systems, where the base station communicates with multiple users simultaneously through transmit beamforming. Most of the existing transmission schemes for multiuser MIMO systems focus on optimizing sum rate performance of the system. The individual quality of service （QoS） requirements （such as packet delay and minimum transmission rate for the data traffic） are rarely considered. In this article, a novel scheduling strategy is proposed, where we try to optimize the global system performance under individual QoS constraints. By performing scheduling into two steps, namely successive user selection and power allocation, the scheduler can achieve efficient resource utilization while maintaining the QoS requirements of all users. Extensive simulations and analysis are given to show the effectiveness of the proposed scheduler.     

Multiple Antenna Enhancements for a High Rate CDMA Packet Data System

A High Data Rate (HDR) system has been proposed for providing downlink wireless packet service by using a channel-aware scheduling algorithm to transmit to users in a time-division multiplexed manner. In this paper, we propose using multiple antennas at the transmitter and/or at the receiver to improve performance of an HDR system. We consider the design tradeoffs between scheduling and multi-antenna transmission/detection strategies and investigate the average Shannon capacity throughput as a function of the number of antennas assuming ideal channel estimates and rate feedback. The highest capacities are achieved using multiple antennas at both the transmitter and receiver. For such systems, the best performance is achieved using a multi-input multi-output capacity-achieving transmission scheme such as BLAST (Bell Labs Layered Space-Time) in which the transmitted signal is coded in space and time, and the receive antennas are used to resolve the spatial interference. In the second part of the paper, we discuss practical transmitter and receiver architectures using BLAST for approaching the theoretical gains promised by the capacity analysis. Because the terminal receivers will be portable devices with limited computational and battery power, we perform a computational complexity analysis of the receiver and make high-level assessments on its feasibility. We conclude that the overall computational requirements are within the reach of current hardware technology.     

Distributed approaches for exploiting multiuser diversity in wireless networks

In wireless fading channels, multiuser diversity can be exploited by scheduling users to transmit when their channel conditions are favorable. This leads to a sum throughput that increases with the number of users and, in certain cases, achieves capacity. However, such scheduling requires global knowledge of every user's channel gain, which may be difficult to obtain in some situations. This paper addresses contention-based protocols for exploiting multiuser diversity with only local channel knowledge. A variation of the ALOHA protocol is given in which users attempt to exploit multiuser diversity gains, but suffer contention losses due to the distributed channel knowledge. The growth rate of the sum throughput for this protocol is characterized in a backlogged system under both short-term and long-term average power constraints. A simple "fixed-rate" system is shown to be asymptotically optimal and to achieve the same growth rate as in a system with an optimal centralized scheduler. Moreover, asymptotically, the fraction of throughput lost due to contention is shown to be 1/e. Also, in a system with random arrivals and an infinite user population, a variation of this ALOHA protocol is shown to be stable for any total arrival rate, given that users can estimate the backlog.