Decentralized dynamic power control for cellular CDMA systems

The control of transmit power has been recognized as an essential requirement in the design of cellular code-division multiple-access (CDMA) systems. Indeed, power control allows for mobile users to share radio resources equitably and efficiently in a multicell environment. Much of the work on power control for CDMA systems found in the literature assumes a quasi-static channel model, i.e., the channel gains of the users are assumed to be constant over a sufficiently long period of time for the control algorithm to converge. In this paper, the design of dynamic power control algorithms for CDMA systems is considered without the quasi-static channel restriction. The design problem is posed as a tradeoff between the desire for users to maximize their individual quality of service and the need to minimize interference to other users. The dynamic nature of the wireless channel for mobile users is incorporated in the problem definition. Based on a cost minimization framework, an optimal multiuser solution is derived. The multiuser solution is shown to decouple, and effectively converge, to a single-user solution in the large system asymptote, where the number of users and the spreading factor both go to infinity with their ratio kept constant. In a numerical study, the performance of a simple threshold policy is shown to be near that of the optimal single-user policy. This offers support to the threshold decision rules that are employed in current cellular CDMA systems.     

Capacity design and performance of call admission control incellular CDMA systems

Since code-division multiple-access (CDMA) capacity is interference limited, call admission control (CAC) must guarantee both a grade of service (GoS), i.e., the blocking rate, and a quality of service (QoS), i.e., the loss probability of communication quality. This paper describes the development of a new capacity design method based on these two concepts. Theoretical expressions for GoS and QoS as functions of traffic intensity and CAC thresholds are first derived from the traffic theory viewpoint, and then a design method using these expressions is presented. At that time, two strategies for CAC are assumed. One is based on the number of users, and the other is based on the interference level. Computer simulation results are presented that strongly support the proposed design method. Furthermore, numerical examples and a performance comparison of the two strategies considering various propagation parameters, nonuniform traffic distributions, and various transmission rates are shown     

Call admission control for CDMA mobile communications systems supporting multimedia services

The call admission control (CAC) belongs to the category of resource management. Since the radio spectrum is very scarce resource, CAC is one of the most important engineering issues for mobile communications. In this paper, we propose a CAC scheme for direct sequence code-division multiple-access cellular systems supporting mobile multimedia communications services. There are multiple call classes in multimedia services and the required signal-to-interference ratio (SIR) varies with call classes. Call admission decision in the proposed scheme is based on SIR measurement. We take account of the traffic asymmetry between uplink and downlink, which is the most important characteristic of multimedia traffic. In addition, the proposed scheme guarantees the priority of handoff call requests over new call requests. We evaluate the performance of the proposed scheme using Markov analysis. The performance measures which we focus on are the system throughput and the blocking probabilities of handoff calls and new calls. The outage probability of a call in progress is also calculated, which is the probability that the measured bit energy-to-noise density ratio of the call is smaller than the required value for maintaining adequate transmission quality. We present some numerical examples with practically meaningful parameter values and, as a result, show that the proposed CAC scheme can operate well in the mobile multimedia systems such as the International Mobile Telecommunications-2000 (IMT-2000) systems.     

等效频谱带宽分配的多媒体CDMA系统媒质接入控制

提出了一种基于等效带宽分配的媒质接入控制(MAC)方法,它针对CDMA系统是“误码率受限”型系统这一特点,将多媒体业务不同的误码率要求映射为等效带宽。系统根据流量动态地将等效带宽分配给不同业务。仿真结果表明,相比与已有的CDMA系统的MAC方法,本方法具有更好的吞吐量性能。     

Effects of diversity power control, and bandwidth an the capacityof microcellular CDMA systems

We evaluate the capacity and bandwidth efficiency of microcellular CDMA systems. Power control, multipath diversity system bandwidth, and path loss exponent are seen to have a major impact on the capacity. The CDMA system considered uses convolutional codes, orthogonal signalling, multipath/antenna diversity with noncoherent combining, and fast closed-loop power control on the uplink (portable-to-base) direction. On the downlink (base-to-portable), convolutional codes, BPSK modulation with pilot-signal-assisted coherent reception, and multipath diversity are employed. Both fast and slow power control are considered for the downlink. The capacity of the CDMA system is evaluated in a multicell environment taking into account shadow fading, path loss, fast fading, and closed-loop power control. Fast power control on the downlink increases the capacity significantly. Capacity is also significantly impacted by the path loss exponent. Narrowband CDMA (system bandwidth of 1.25 MHz) requires artificial multipath generation on the downlink to achieve adequate capacity. For smaller path loss exponents, which are more likely in microcellular environments, artificial multipath diversity of an order of as high as 4 may be needed. Wideband CDMA systems (10 MHz bandwidth) achieve greater efficiencies in terms of capacity per MHz     

Dynamic bandwidth recycle algorithm for OVSF–CDMA systems

This paper presents a dynamic bandwidth recycle algorithm in the downlink of a WCDMA system using orthogonal variable spreading factor codes (OVSF). It consists of a bandwidth recycle algorithm and a bandwidth reservation algorithm. The bandwidth recycle algorithm is used to recycle bandwidth from current serviced connections when the system does not have enough available bandwidth to support a bandwidth request. The bandwidth reservation algorithm is used to reduce transmission delay caused by suddenly increasing bandwidth requests. Four traffic classes, conversational, streaming, interactive, and background classes, defined by universal mobile telecommunication system (UMTS) are considered. Simulation results show that the bandwidth utilization and block rate are improved, the bandwidth guaranteed to conversational and streaming classes are protected, and the delay time of interactive and background classes are kept under an acceptable value even when the traffic load is heavy. Copyright © 2005 John Wiley & Sons, Ltd.     

The theoretical bandwidth advantage of CDMA over FDMA in a GaussianMAC

We develop an expression for the minimum extra bandwidth needed for a frequency-division multiple-access (FDMA) system to out perform its code-division (CDMA) counterpart uniformly (that is, for all rate n-tuples) in a Gaussian multiple-access channel (MAC). For equal-power sources, the behavior of this factor is as an iterated logarithm of the number of users; hence it increases slowly yet is unbounded. Asymmetric power cases are also studied and it is shown that the equal power scenario provides the least bandwidth expansion factor assuming constant constraint on total power     

Error-resilient video communications over CDMA networks with a bandwidth constraint.

We present an adaptive video transmission scheme for use in a code-division multiple-access network, which incorporates efficient bandwidth allocation among source coding, channel coding, and spreading under a fixed total bandwidth constraint. We derive the statistics of the received signal, as well as a theoretical bound on the packet drop rate at the receiver. Based on these results, a bandwidth allocation algorithm is proposed at the packet level, which incorporates the effects of both the changing channel conditions and the dynamics of the source content. Detailed simulations are done to evaluate the performance of the system, and the sensitivity of the system to estimation error is presented.     

IPv6-based dynamic coordinated call admission control mechanism over integrated wireless networks

Many wireless access systems have been developed recently to support users mobility and ubiquitous communication. Nevertheless, these systems always work independently and cannot simultaneously serve users properly. In this paper, we aim to integrate IPv6-based wireless access systems and propose a coordinated call admission control mechanism to utilize the total bandwidth of these systems to minimize the call blocking probabilities, especially the handoff call dropping probabilities. First, we propose an integrated hierarchical wireless architecture over IPv6-based networks to combine the wireless access systems including cellular systems (second-generation, General Packet Radio Service, or third-generation), IEEE 802.11 a/b/g WLAN, and Bluetooth. In the proposed architecture, mobile user can request a call with quality-of-service (QoS) requirements by any wireless network interfaces that can be accessed. When the proposed coordinated call admission control (CCAC) mechanism receives a request, it takes the QoS requirements of the incoming call and the available and reserved bandwidth of this wireless system into consideration to accept or reject this request. Besides, the mechanism can coordinate with other wireless systems dynamically to adjust the bandwidth reserved for handoff calls at each wireless system in this architecture so as to reduce the call blocking probabilities. Once the call is admitted, the mobile user is able to access heterogeneous wireless access networks via multiple interfaces simultaneously. Finally, we evaluate this system to show that the CCAC on the proposed architecture outperforms other mechanisms proposed before.     

A dynamic CDMA network for multicore systems

Code-division multiple-access (CDMA) is a data transmission method based on the spreading code technology, wherein multiple data streams share the same physical medium with no interference. A novel architecture for on-chip communication networks based on this approach is devised. The proposed design allows sharing coding resources among network?s users through the use of dynamic assignment of spreading codes. Data transmission latency is reduced by adopting a parallel structure for the coding/decoding circuitry. A 14-node CDMA network based on the proposed architecture is synthesised using 65 nm ST technology library. Performance analysis reveals that the proposed approach achieves significantly lower data packet latency compared to both conventional CDMA and packet switched network-on-chip implementations. Large area and power savings compared to existing approaches are also obtained.     

On call admission control in DS/CDMA cellular networks

Analytical models are proposed for various direct sequence code-division multiple-access (DS/CDMA) call admission control schemes. Many mathematical call admission models for DS/CDMA cellular networks have been proposed. However, they have shortcomings. First, by ignoring the stochastic traffic load variation or call blocking effect, they failed to sufficiently characterize the second moment of other-cell interference. This leads to inaccurate analysis of a real network. Second, the optimal control parameters were often obtained through an exhaustive search which was very time consuming. Finally, the estimation of system capacity in previous models was obtained by using a simple one-slope path-loss propagation model. However, it is well known that a two-slope path loss propagation model is needed in a line-of-sight (LOS) microcell propagation environment. We propose an analytical model for call admission to overcome these drawbacks. In addition, we combine a modified linear programming technique with the built analytical model to find better call admission control schemes for a DS/CDMA cellular network     

On multi‐cell admission control in CDMA networks

We consider a multi‐cell (MC) code division multiple access (CDMA) system that supports multiple service classes, including peak rate allocated and elastic ones. Peak rate allocated sessions—when admitted into the system—transmit at a constant bit rate, while elastic sessions can be slowed down at the expense of increasing their residency time. Admitted sessions cause an instantaneous bit rate‐dependent interference in neighbour cells. In this rather general setting, we propose a method to calculate the class‐wise blocking probabilities as the functions of the estimated so‐called inter‐cell coupling factors. In the paper this coupling factor is the ratio between the uplink path gains to different Node‐B:s (that can be easily obtained in a CDMA system from pilot measurement reports), but our model could include other coupling measures as well. We find that when these coupling factors are underestimated, the system may get into false states (FSs) or false rate states (FRSs) that lead to violating the noise rise threshold. As traffic becomes increasingly elastic, the probability of FSs decreases, but the probability of FRSs increases. Based on numerical results, we make the point that as the traffic becomes more elastic, avoiding the underestimation of these coupling factors as well as exercising MC admission control plays an increasingly important role in guaranteeing proper service quality. Copyright © 2007 John Wiley & Sons, Ltd.     

Power-based admission control for multiclass calls in QoS-sensitive CDMA networks

In this letter, we propose a power-based call admission control (CAC) scheme to accommodate multiclass traffic by directly extending the number-based CAC scheme in multicode CDMA networks, and develop some related mathematical properties. Against the conventional findings, we demonstrate that complete partitioning (CP) of the received signal power at a basestation for each traffic class can be an approach as useful as complete sharing (CS) in accommodating an appropriate number of users. The main advantage of CP scheme over CS scheme is its simplicity in resource management     

码分多址测控系统的功率调节

远近效应是码分多址测控系统中影响飞行器多址接收性能的重要因素。分析了远近效应在多站测控系统中对飞行器接收性能的影响,提出了在该系统中进行功率控制的必要性,给出了地面测控站功率调节的方法和策略,可作为多站应用的码分多址测控系统功率调节的参考思路。     

Effective interference and effective bandwidth of linear multiuserreceivers in asynchronous CDMA systems

The performance of linear multiuser receivers in terms of the signal-to-interference ratio (SIR) achieved by the users has been analyzed in a synchronous CDMA system under random spreading sequences. In this paper, we extend these results to a symbol-asynchronous but chip-synchronous system and characterize the SIR for linear receivers-the matched-filter receiver the minimum mean-square error (MMSE) receiver and the decorrelator. For each of the receivers, we characterize the limiting SIR achieved when the processing gain is large and also derive lower bounds on the SIR using the notion of effective interference. Applying the results to a power controlled system, we derive effective bandwidths of the users for these linear receivers and characterize the user capacity region: a set of users is supportable by a system if the sum of the effective bandwidths is less than the processing gain of the system. We show that while the effective bandwidth of the decorrelator and the MMSE receiver is higher in an asynchronous system than that in a synchronous system, it progressively decreases with the increase in the length of the observation window and is asymptotic to that of the synchronous system, when the observation window extends infinitely on both sides of the symbol of interest. Moreover, the performance gap between the MMSE receiver and the decorrelator is significantly wider in the asynchronous setting as compared to the synchronous case     

Multistage decoding for internally bandwidth efficient coded Poisson fiber-optic CDMA communication systems

We consider the structure and performance of a multistage decoding scheme for an internally bandwidth efficient convolutionally coded Poisson fiber-optic code division multiple access (CDMA) communication system. The decoder is implemented electronically in several stages in which in each stage, the interfering users' coded bit decisions obtained in the previous stage is applied for computing the likelihood of the coded symbols of the desired user. The first stage is a soft-input Viterbi decoder for the internally coded scheme, in which the soft-input coded symbol likelihood values are computed by considering the multiuser interference as a noise signal. The likelihood of coded symbol computed in each stage is then entered into the convolutional decoder for the next bit decisions. The convolutional codes that are used for demonstrating the performance of the multistage decoder are super orthogonal codes (SOCs). We derive the bit error rates (BERs) of the proposed decoder for internally coded Poisson fiber-optic CDMA systems using optical orthogonal codes (OOCs) along with both ON-OFF keying (OOK) and binary pulse position modulation (BPPM) schemes. Our numerical results indicate that the proposed decoding scheme substantially outperforms the single-stage soft-input Viterbi decoder. We also derive the upper bound on the probability of error of a decoder for the known interference case, which is the ultimate performance of a multiuser decoder, and compare the result with that of the soft-input Viterbi decoder.     

Joint admission/congestion control for wireless CDMA systemssupporting integrated services

A new data traffic control scheme is developed for maintaining the packet error rate (PER) of real-time voice traffic while allowing nonreal-time data traffic to utilize the residual channel capacity of the multi-access link in an integrated service wireless CDMA network. Due to the delay constraint of the voice service, voice users transmit their packets without incurring further delay once they are admitted to the system according to the admission control policy. Data traffic, however, is regulated at both the call level (i.e., admission control) and at the burst level (i.e., congestion control). The admission control rejects the data calls that will otherwise experience unduly long delay, whereas the congestion control ensures the PER of voice traffic being lower than a specified quality of service (QoS) requirement (e.g., 10 ^-2). System performance such as voice PER, voice-blocking probability, data throughput, delay, and blocking probability is evaluated by a Markovian model. Numerical results for a system with a Rician fading channel and DPSK modulation are presented to show the interplay between admission and congestion control, as well as how one can engineer the control parameters. The tradeoff of using multiple CDMA codes to reduce the transmission time of data messages is also investigated     

Power allocation and call admission control in multi‐class traffic DS‐CDMA systems with imperfect power control

We deal with power allocation (PA) and call admission control (CAC) under imperfect power control (IPC) in the reverse link of direct sequence‐code division multiple access systems for supporting multi‐class traffic. First, we briefly review the optimum PA scheme under perfect power control (PPC) and the CAC scheme subject to an outage constraint on the total composite received power. Then, we analyze the outage degradation due to the power control error when the optimum reference power levels under PPC are used. In order to mitigate the outage degradation, we would modify the reference power levels by incorporating a call dropping strategy and an outage‐lowering strategy into the optimum PA scheme under PPC. Also, we derive a constraint inequality to determine the reverse link capacity under IPC. Finally, through numerical analyses, we compute the modified reference power levels under IPC and evaluate the reverse link capacity under IPC. Copyright © 2006 John Wiley & Sons, Ltd.     

Effective bandwidth approach to connection admission control formultimedia traffic in ATM networks

The authors propose an effective bandwidth approach to connection admission control in ATM networks. The aggregate arrival traffic is accurately modelled by a two-state Markov modulated Poisson process (MMPP) via the matching of four important statistics. If the buffer is large, admission control can be achieved by computing the effective bandwidth of the two-state MMPP. Simulation tests show that approach is simple and results in higher utilisation compared with conventional methods     

Distributed power control in CDMA cellular systems

In wireless cellular communication, it is essential to find effective means for power control of signals received from randomly dispersed users within one cell. Effective power control will heavily impact the system capacity. Distributed power control (DPC) is a natural choice for such purposes, because, unlike centralized power control, DPC does not require extensive computational power. Distributed power control should be able to adjust the power levels of each transmitted signal using only local measurements, so that, in a reasonable time, all users will maintain the desired signal-to-interference ratio. In this paper, we review different approaches for power control, focusing on CDMA systems. We also introduce state-space methods and linear quadratic power control (LQPC) to solve the power-control problem. A simulation environment was developed to compare LQPC with earlier approaches. The results show that LQPC is more effective, and is capable of computing the desired transmission power of each mobile station in fewer iterations, as well as being able to accommodate more users in the system