Scheduling in multimedia CDMA wireless networks

Wireless systems in the future will have to provide multimedia services where different users have different physical-layer quality of service (QoS) requirements (e.g., bit energy per interference power density, E/sub b//N/sub 0/, or bit error rate and power constraints) and network-layer QoS requirements (e.g., delay bound, delay-jitter, throughput, and loss). We investigate the use of power control, processing gain and/or multiple codes, and scheduling in CDMA systems to accommodate these diverse service requirements. We first show that the instantaneous capacity region, given in terms of the set of user bit rates that can be supported simultaneously subject to peak power and E/sub b//N/sub 0/ constraints, is nonconvex. This suggests that by time-sharing the channel, one may be able to get better system throughput. We define the capacity region as the convex hull of the instantaneous capacity region and we show that it may be obtained by time sharing between operating points, where each user either uses full power or is silent (bang-bang control). We then consider the problem of scheduling so as to meet prespecified delay bounds or minimum service curve requirements for traffic streams, which are specified in terms of a traffic profile such as a sigma-rho constraint (enforced by a leaky bucket) and a guarantee that the system is stable.     

Power Allocation with Max–Min and Min–Max Fairness for Cognitive Radio Networks with Imperfect CSI

This paper consider the power allocation strategies in the cognitive radio (CR) system in the presence of channel estimation errors. As the user has different channel condition in CR systems, different amount of power resource is required to meets the QoS request. In order to guarantee the fairness of each CR user, ensure the interference from the primary user and other CR users meet the QoS requirement of the CR user and limit the interference that is caused by CR users on primary user within the range into the level that primary user can tolerate, we proposed some new power allocation schemes. The targets are to minimize the maximum power allocated to CR users, to maximize the minimum signal-to-interference-plus-noise ratio (SINR) among all CR users and to minimize the maximum outage probability over all CR users. The first power allocation scheme can be formulated using Geometric Programming (GP). Since GP problem is equivalent to the convex optimization problem, we can obtain the optimal solutions for the first scheme. The latter two power allocation schemes are not GP problems. We propose iterative algorithms to solve them. Simulation results show that proposed schemes can efficiently guarantee the fairness of CR users under the QoS constraint of the primary user and CR users.     

Distributed power control algorithm for cognitive radios with primary protection via spectrum sensing under user mobility

Substantial spectrum gains have been demonstrated with the introduction of cognitive radio however; such gains are usually short lived due to the increased level of interference to licensed users of the spectrum. The interference management problem is herein tackled from the transmitter power control perspective so that transmissions by cognitive radio network does not violate the interference threshold levels at the primary users as well as maintain the QoS requirements of cognitive radio users. We model the cognitive radio network for mobile and immobile users and propose algorithms exploiting primary radio environment knowledge (spectrum use), called power control with primary protection via spectrum sensing. The algorithm is briefly introduced for time invariant systems and demonstrated that it has the ability to satisfy tight QoS constraints for cognitive radios as well as meet the interference constraints for licensed users. We, however, further show that such assumption of terminal immobility in the power control algorithm would fail in cases where user mobility is considered, resulting in increased levels of interference to the primary as well as increased outages in cognitive radio network. We model the link gain evolution process as a distance dependent shadow fading process and scale-up the target signal to interference ratio to cope with user mobility. Since mobility driven power control algorithms for cognitive radios have not been investigated before, we therefore, propose a mobility driven power control framework for cognitive radios based on spectrum sensing, which ensures that the interference limit at the primary receiver is unperturbed at all times, while concurrently maintaining the QoS within the cognitive radio network as compared to static user cases. We also corroborate our algorithms with proof of convergence.     

Reservation-based distributed medium access in wireless collision channels

We consider an uplink wireless collision channel, shared by multiple mobile users. The medium access protocol incorporates channel reservation that relies on RTS (request-to-send) and CTS (clear-to-send) control packets. Consequently, collisions are reduced to the relatively short periods where mobiles request channel use. In our model, users individually schedule their channel requests, and the objective of each user is to minimize its own power investment subject to a minimum-throughput demand. Our analysis reveals that for feasible throughput demands, there exist exactly two Nash equilibrium points in stationary strategies, with one superior to the other uniformly over all users. We then show how this better equilibrium point can be obtained through distributed best-response mechanisms. Finally, we quantify and discuss the effect of the relative length of data and control periods on capacity, power and delay.     

Exploiting user profiles to support differentiated services in next-generation wireless networks

In the next-generation wireless network, user profiles such as the location, the velocity (both speed and direction), and the resource requirements of the mobile device can be accurately determined and maintained by the network on a per-user basis. We investigate the design of a wireless network architecture that exploits user profiles to maximize network efficiency and provide better quality-of-service (QoS) to different classes of users. In this article we provide implementation guidelines of such an architecture for the third-generation partnership project (3GPP) network. The key underlying primitive of the architecture is the use of both real-time and aggregate user profiles to perform advance resource reservation in the handoff target cells of the wireless cellular network. We identify various factors that can influence the efficiency of the resource reservation scheme, and through a simulation analysis of an example scenario we show the impact of these factors on the QoS that profiled users receive. The example scenario comprises two service classes: a high cost, profiled service with higher QoS; and a lower cost, non-profiled service with best-effort QoS. The results show that high QoS can be guaranteed to users who subscribe to the profiled service.     

QoS-controlling soft handoff based on simple step control and a fuzzy inference system with the gradient descent method

Quality of service (QoS) is an important criterion considered in mobile communication systems since it directly affects system users in terms of both call blocking and call quality. This paper aims to control the QoS in code-division multiple-access mobile communication systems by proposing two new practical methods applied to the conventional soft handoff (SHO) process based on a simple step-control method and a human-oriented information method of fuzzy inference system with classical mathematics of the gradient descent method. The output parameters selected to be controlled by adapting SHO thresholds are the trunk-resource efficiency (TRE), the forward link's average energy per bit to the interference plus noise power spectral density ratio (E/sub b//I/sub 0/), and the outage probability (P/sub out/). TRE is concerned with call blocking while E/sub b//I/sub 0/ is an indicator of the call quality. The number of the remaining channels of serving base station (CH/sub rm/), the moving average number of pilots in active set of any mobile station (no/sub BS/), and E/sub b//I/sub 0/ is or are selected to be one, two, or three of the control plant inputs, depending on the method applied. QoS-controlling SHO (TRE- and E/sub b//I/sub 0/-controlling SHO) can improve the system performance. However, which method that will be selected for use is according to the environment and the requirement of the system operator and/or users.     

Power control and channel allocation for real‐time applications in cellular networks

The next‐generation packet‐based wireless cellular network will provide real‐time services for delay‐sensitive applications. To make the next‐generation cellular network successful, it is critical that the network utilizes the resource efficiently while satisfying quality of service (QoS) requirements of real‐time users. In this paper, we consider the problem of power control and dynamic channel allocation for the downlink of a multi‐channel, multi‐user wireless cellular network. We assume that the transmitter (the base‐station) has the perfect knowledge of the channel gain. At each transmission slot, a scheduler allots the transmission power and channel access for all the users based on the instantaneous channel gains and QoS requirements of users. We propose three schemes for power control and dynamic channel allocation, which utilize multi‐user diversity and frequency diversity. Our results show that compared to the benchmark scheme, which does not utilize multi‐user diversity and power control, our proposed schemes substantially reduce the resource usage while explicitly guaranteeing the users' QoS requirements. Copyright © 2007 John Wiley & Sons, Ltd.     

阴影衰落信道下多波束卫星移动通信系统的动态信道分配策略

针对已有动态信道分配策略在阴影衰落环境中性能损失较大的问题,对卫星移动通信系统的阴影衰落信道模型进行分析,提出了一种动态信道分配策略。该策略结合用户的运动状态,通过在用户的预测运动轨迹上选取抽样点,将这些抽样点的平均干扰作为信道分配的指标,并采用链路质量约束避免了流量较大时的性能恶化。仿真结果表明提出的动态信道策略可在不增大阻塞率的前提下,在中低流量时将用户平均信噪比提高约0.5 dB。该策略可以应用于阴影衰落信道下的地面移动卫星通信系统,以提高用户的平均链路性能。     

Optimum receivers and low-dimensional spreaded modulation for multiuser space-time communications

The jointly optimum receiver is obtained for multi- user communications in a frequency nonselective Rayleigh-fading channel with N/sub T/ transmit antennas per user and N/sub R/ receive antennas. Based on a general analysis of quadratic receivers in zero-mean complex Gaussian vectors, asymptotically tight expressions (for high signal-to-noise ratio (SNR)) for the pairwise error probabilities are derived. Subsequently, it is shown that N/sub T/-dimensional single-user signaling suffices to provide full diversity order N=N/sub T/N/sub R/ for all the users. In other words, the presence of other users does not increase the minimum dimension required beyond what is needed for the single-user space-time channel. For the special case of low-rank "code-division multiple-access (CDMA)" signaling with N/sub T/=1 and provided the signatures of any two users are linearly independent, it is shown that the error probability of a K-user system asymptotically approaches single-user-like performance for every user. Remarkably, therefore, an increase in the number of users, and hence an increase in the aggregate spectral efficiency, does not require the users to transmit with more power to achieve single-user-like performance asymptotically. A signal design algorithm is proposed to illustrate this point and examples are given. These results are then generalized to the multiple transmit antenna case. A new (N/sub T/+1)-dimensional signaling strategy is proposed for the multiuser channel that leverages existing single-user space-time signal designs while ensuring full diversity order and single-user-like performance asymptotically for every user.     

Analysis of heterogeneous satellite networks with multi‐user detectors

In this work, a heterogeneous network, where both terrestrial‐ and multiple spot beam‐based geosynchronous satellite networks use the same sub‐channel, is investigated. Moreover, the satellite uses same sub‐channel to serve users across all spot beams. Apart from inter spot beam interference, satellite users also receive strong interfering signal from the terrestrial base station. The satellite receivers manage the inter system interference (ie, interference from terrestrial network) while the inter spot beam interference is mitigated through precoding at the gateway. To be specific, the following interference‐aware detectors are used to mitigate the inter system interference: successive interference canceller (SIC) and joint decoder (JD). The proposed decoders are shown to achieve better data rate and bit error rate (BER) than the interference ignorant decoder at moderate to strong interference levels. Although JD and SIC achieve same sum rate at moderate and strong interference levels, JD is shown to allow the terrestrial user achieve higher data rate than SIC.     

High-speed satellite mobile communications: technologies and challenges

Central features of future 4G mobile communication systems are high-speed data transmission (up to 1 Gb/s) and interactive multimedia services. For effective delivery of these services, the network must satisfy some stringent QoS metrics, defined typically in terms of maximum delay and/or minimum throughput. Mobile satellite systems will be fully integrated with the terrestrial cellular systems to provide ubiquitous global coverage to diverse users. The challenges for future broadband satellite systems, therefore, lie in the proper deployment of state-of-the-art satellite technologies to ensure seamless integration of the satellite networks into the cellular systems and its QoS frameworks, while achieving, as far as possible, efficient use of satellite link resources. The paper presents an overview of future high-speed satellite mobile communication systems, the technologies deployed or planned for deployment, and the challenges. Focusing in particular on nonlinear downlink channel behavior, shadowing and multipath fading, various physical channel models for characterizing the mobile satellite systems are presented. The most prominent technologies used in the physical layer, such as coding and modulation schemes, multiple-access techniques, diversity combining, etc., are then discussed in the context of satellite systems. High-speed and QoS-specific technologies, such as onboard processing and switching, mobility and resource management, IP routing and cross-layer designs, employed in the satellite systems are also discussed.     

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.     

Sum capacity of MIMO Gaussian broadcast channels with channel energy constraints

The motivation of this paper is to find the class of channels that provides the best sum capacity of a MIMO Gaussian broadcast channel with both transmit power and channel energy constraints when N/sub t//spl ges/KN/sub r/, where N/sub t/, N/sub r/, and K denote the number of transmit antennas, the number of receive antennas, and the number of users in the system, respectively. The best sum capacity is achieved when the user channels are mutually orthogonal to each other. For each individual user, equal energy is distributed to all non-zero spatial eigenmodes. Further, we optimize the number of non-zero eigenmodes for all users and the optimal power distribution among users. Although, we only study the case of N/sub t//spl ges/KN/sub r/ in this paper, we conjecture that similar results still hold for N/sub t/相似文献   

面向负载均衡的主动切换

为了解决超密集网络中用户在移动过程中由于小区间干扰和负载不均衡导致的用户服务质量下降的问题,本文研究了一种面向负载均衡的主动切换策略,设计了一种基于双门限的移动切换策略,包含小区内的切换与小区间的切换。通过优化小区内的切换门限,可以最小化系统总资源开销。通过优化小区间的切换门限,能够保证用户实时业务需求的同时均衡网络负载。通过预测用户未来大尺度信道信息,设计主动的双门限切换策略,可以有效降低切换延迟。仿真结果表明,与现有的切换策略相比,所提算法不仅提高了资源利用率,还降低了用户掉线概率与切换次数。      

Towards generalized ID‐based user authentication for mobile multi‐server environment

With the popularity of Internet and wireless networks, more and more network architectures are used in multi‐server environment, in which mobile users remotely access servers through open networks. In the past, many schemes have been proposed to solve the issue of user authentication for multi‐server environment and low‐power mobile devices. However, most of these schemes have suffered from many attacks because these schemes did not provide the formal security analysis. In this paper, we first give a security model for multi‐server environment. We then propose an ID‐based mutual authentication and key agreement scheme based on bilinear maps for mobile multi‐server environment. Our scheme can be used for both general users with a long validity period and anonymous users with a short validity period. Under the presented security model, we show that our scheme is secure against all known attacks. We demonstrate that the proposed scheme is well suitable for low‐power mobile devices. Copyright © 2011 John Wiley & Sons, Ltd.     

Vertical Handover Over Intermediate Switching Framework: Assuring Service Quality for Mobile Users

Ensuring quality of service (QoS) for the mobile users during vertical handover between IEEE 802.11 wireless local area networks (WLAN) and data network provided by Ultramodern Telecommunication Systems (UMTS) is one of the key requirements for seamless mobility and transfer of existing connections from one network to another. QoS fulfillment is a complex problem and requires participation of both the mobile users as well as the connection networks. The QoS assurance criteria for existing connections can be affected by fluctuations of data rates when a user moves from the high speed WLAN network to the low speed UMTS network, even in the presence of another WLAN network in its vicinity. This can happen if the alternate WLAN network is highly loaded. Therefore handover from a high speed network to a low speed network should be avoided, whenever possible. This paper proposes a QoS based handover procedure that prioritizes the existing connection over the new connections so that rate fluctuations due to handover can be avoided if there exist another WLAN network in the range of the mobile user. Whenever the possibility of handover is detected, a pre-handover bandwidth reservation technique is used to reserve bandwidth at the alternate WLAN networks to avoid QoS degradation. The proposed scheme is implemented in Qualnet network simulator and the performance is analyzed and compared with traditional handover techniques.     

一种适用于多用户OFDM系统下行链路的跨层设计调度算法

该文提出了一种适用于多用户正交频分复用(OFDM)系统下行链路的调度算法,基于跨层结构设计,并分别考虑了媒体接入控制(MAC)层的用户服务质量(QoS)要求、用户公平性、数据包队列状态信息以及物理层的信道状态信息(CSI)和功率约束等。多用户OFDM系统可以通过该调度算法充分利用MAC层和物理层的不同参数特性,在保证用户QoS的同时最大化频谱的利用效率。仿真结果表明,在小区内各个移动用户具有不同的接收SNR的情况下,该算法具有服务质量随信噪比波动平缓、丢包率低、实现复杂度低等特点。     

Scheduling in IEEE 802.16e mobile WiMAX networks: key issues and a survey

Interest in broadband wireless access (BWA) has been growing due to increased user mobility and the need for data access at all times. IEEE 802.16e based WiMAX networks promise the best available quality of experience for mobile data service users. Unlike wireless LANs, WiMAX networks incorporate several quality of service (QoS) mechanisms at the Media Access Control (MAC) level for guaranteed services for data, voice and video. The problem of assuring QoS is basically that of how to allocate available resources among users in order to meet the QoS criteria such as delay, delay jitter and throughput requirements. IEEE standard does not include a standard scheduling mechanism and leaves it for implementer differentiation. Scheduling is, therefore, of special interest to all WiMAX equipment makers and service providers. This paper discusses the key issues and design factors to be considered for scheduler designers. In addition, we present an extensive survey of recent scheduling research. We classify the proposed mechanisms based on the use of channel conditions. The goals of scheduling are to achieve the optimal usage of resources, to assure the QoS guarantees, to maximize goodput and to minimize power consumption while ensuring feasible algorithm complexity and system scalability.     

Mixed Power Control with Directed Assignment (MPCDA), a Method for Interference Reduction in Channelized Wireless Systems

Power control, often coupled with dynamic channel assignment, has been viewed as a promising answer to the challenge of reducing interference and increasing capacity. Indiscriminate use of power control, however, may exacerbate the near-far-end problem on the down link, and give rise to other complications when users are mobile. We propose a power control policy that can alleviate the near-far-end interference caused by the use of either the same channel or neighbor channels inside the same cell, while at the same time aiding in the reduction of interference from different cells through user matching. We present a heuristic algorithm for user matching, which is distributed and simple to implement. The method can be combined with an array of either fixed or dynamic channel assignment algorithms and applies to both circuit-based and packet-based traffic. It is ideal for fixed or slow circuit-based traffic and for packet-based traffic.A duality relationship is derived for the proposed power control policy between the signal-to-interference ratio of two interfering users experienced in the two communication directions. This relationship enables one to validate channel assignment decisions on both communication directions by analyzing only the decisions for one.     

Resource allocation for multiple classes of DS-CDMA traffic

We consider a packet data direct-sequence code-division multiple-access (DS-CDMA) system which supports integrated services. The services are partitioned into different traffic classes according to information rate (bandwidth) and quality of service (QoS) requirements. Given sufficient bandwidth, QoS requirements can be satisfied by an appropriate assignment of transmitted power and processing gain to users in each class. The effect of this assignment is analyzed for both a single class of data users and two classes of voice and data users. For a single class of data users, we examine the relationship between average delay and processing gain, assuming that ARQ with forward error correction is used to guarantee reliability. The only channel impairment considered is interference, which is modeled as Gaussian noise. A fixed user population is assumed and two models for generation of data packets are considered: (1) each user generates a new packet as soon as the preceding packet is successfully delivered and (2) each user generates packets according to a Poisson process. In each case, the packets enter a buffer which is emptied at the symbol rate. For the second traffic model, lowering the processing gain below a threshold can produce multiple operating points, one of which corresponds to infinite delay. The choice of processing gain which minimizes average delay in that case is the smallest processing gain at which multiple operating points are avoided. Two classes of users (voice/data and two data classes) are then considered. Numerical examples are presented which illustrate, the increase in the two-dimensional (2-D) capacity region achievable by optimizing the assignment of powers and processing gains to each class