Direct Sequence (DS) Code Division Multiple Access (CDMA) is a promising technology for wireless environments with multiple simultaneous transmissions because of several features: asynchronous multiple access, robustness to frequency selective fading, and multipath combining. The capacity

How to control hand-off drops is a very important Qualityof -Service (QoS) issue in cellular networks. In order to keep the hand-off dropping probability below a pre-specified target value (thus providing a probabilistic QoS guarantee), we design and evaluate predictive and adaptive schemes for the bandwidth reservation for the existing connections' handoffs and the admission control of new connections. We first develop a method to estimate user mobility based on an aggregate history of hand-offs observed in each cell. This method is then used to predict (probabilistically) mobiles' directions and hand-off times in a cell. For each cell, the bandwidth to be reserved for hand-offs is calculated by estimating the total sum of fractional bandwidths of the expected hand-offs within a mobility-estimation time window. We also develop an algorithm that controls this window for efficient use of bandwidth and effective response to (1) time-varying traffic/mobility and (2) inaccuracy of mobility...

In this paper, we address the problem of the performance analysis of image watermarking systems that do not require the availability of the original image during ownership verification. We focus on a statistical approach to obtain models that can serve as a basis for the application of the decision theory to the design of efficient detector structures. Special attention is paid to the possible nonexistence of a statistical description of the original image. Different modeling approaches are proposed for the cases when such a statistical characterization is known and when it is not. Watermarks may encode a message, and the performance of the watermarking system is evaluated using as a measure the probability of false alarm, the probability of detection when the presence of the watermark is tested, and the probability of error when the information that it carries is extracted. Finally, the modeling techniques studied are applied to the analysis of two watermarking schemes, one of them defined in the spatial domain, and the other in the direct cosine transform (DCT) domain. The theoretical results are contrasted with empirical data obtained through experimentation covering several cases of interest. We show how choosing an appropriate statistical model for the original image can lead to considerable improvements in performance

Applications for the location of subscribers of wireless services continue to expand. Consequently, location techniques for wireless technologies are being investigated. With code-division multiple access (CDMA) being deployed by a variety of cellular and PCS providers, developing an approach for location in CDMA networks is imperative. This article discusses the applications of location technology, the methods available for its implementation in CDMA networks, and the problems that are encountered when using CDMA networks for positioning. ireless location has received considerable attention over the past few years. A recent Report and Order issued by the U.S. Federal Communications Commission (FCC) in July 1996 requires that all wireless service providers, including cellular, broadband PCS, and wide-area SMR licensees, provide location information to Emergency 911 (E-911) public safety services [1]. These

We consider the problem of scheduling CDMA data users on the forward link. The goal is to meet their QoS requirements defined in terms of probabilistic packet delay bounds. The constraint is the limit on the total forward link transmit power. Each user's channel condition is characterized by the forward link power required to achieve a unit data rate. This paper extends the work reported in [1], in which several simplifying assumptions were made, including the assumption that channel conditions are constant in time. In this work, we study a more realistic scenario, in which transmission rates can only be chosen from a discrete finite set, rate scheduling can only be done at discrete scheduling intervals, and, most importantly, the users' channel conditions may vary in time.

Transmit power control is a central technique for resource allocation and interference management in spread-spectrum wireless networks. With the increasing popularity of spread-spectrum as a multiple access technique, there has been significant research in the area in recent years. While power control has been considered traditionally as a means to counteract the harmful effect of channel fading, the more general emerging view is that it is a flexible mechanism to provide Quality-of-Service to individual users. In this paper, we will review the main threads of ideas and results in the recent development of this area, with a bias towards issues that have been the focus of our own research. For different receivers of varying complexity, we study both questions about optimal power control as well as the problem of characterizing the resulting network capacity. Although spread-spectrum communications has been traditionally viewed as a physical-layer subject, we argue that by suitable abstr...

This paper presents call admission control and bandwidth reservation schemes in wireless cellular networks that have been developed based on assumptions more realistic than existing proposals. In order to guarantee the handoff dropping probability, we propose to statistically predict user mobility based on the mobility history of users. Our mobility prediction scheme is motivated by computational learning theory, which has shown that prediction is synonymous with data compression. We derive our mobility prediction scheme from data compression techniques that are both theoretically optimal and good in practice. In order to utilize resource more efficiently, we predict not only the cell to which the mobile will handoff but also when the handoff will occur. Based on the mobility prediction, bandwidth is reserved to guarantee some target handoff dropping probability. We also adaptively control the admission threshold to achieve a better balance between guaranteeing handoff dropping probability and maximizing resource utilization. Simulation results show that the proposed schemes meet our design goals and outperform the static-reservation and cell-reservation schemes. Paper submitted to Computer Networks. This paper is based on a paper presented at IEEE Infocom 2001, Anchorage, Alaska, April 2001. Technical subject area: call admission control, bandwidth reservation, mobility prediction. Please address all correspondence to Professor Victor Leung at the above address. This work was supported by a grant from Motorola Canada Ltd., and by the Canadian Natural Sciences and Engineering Research Council under grant CRDPJ 223095. Mobility-Based Predictive Call Admission Control and Bandwidth Reservation in Wireless Cellular Networks Yu 1 I.

This work focuses on the problem of efficient exploitation of the available bandwidth in order to provide high bit rates on the wireless link, as will be required in future wireless systems interfacing to broadband fixed networks. In particular, the uplink of a CDMA system with two user classes is considered. One of the classes consists of delay intolerant users requiring support for a constant information bit rate, while the other consists of delay tolerant users needing support for an information bit rate larger than a given value. It is assumed that when not transmitting information, both classes maintain synchronization contact with the base station at a given rate. The objective is to maximize the throughput of the delay tolerant users, while ensuring that the interference to other cells is as low as possible by minimizing the sum of all the transmit powers used by the mobiles. Two transmission modes for the delay tolerant users are considered. In the first mode, all the users are...

Multiple-access interference (MAI) in a code-division multiple-access (CDMA) system plays an important role in performance analysis and characterization of fundamental system limits. In this paper, we study the behavior of the output MAI of the minimum mean-square error (MMSE) receiver employed in the uplink of a direct-sequence (DS)-CDMA system. We focus on imperfect power-controlled systems with random spreading, and establish that in a synchronous system 1) the output MAI of the MMSE receiver is asymptotically Gaussian, and 2) for almost every realization of the signatures and received powers, the conditional distribution of the output MAI converges weakly to the same Gaussian distribution as in the unconditional case. We also extend our study to asynchronous systems and establish the Gaussian nature of the output interference. These results indicate that in a large system the output interference is approximately Gaussian, and the performance of the MMSE receiver is robust to the randomness of the signatures and received powers. The Gaussianity justifies the use of single-user Gaussian codes for CDMA systems with linear MMSE receivers, and implies that from the viewpoints of detection and channel capacity, signal-to-interference ratio (SIR) is the key parameter that governs the performance of the MMSE receiver in a CDMA system.

Spread spectrum techniques and collision avoidance multiple access protocols are combined to form a new set of medium access protocols for mobile wireless ad hoc networks. The Request-to-Send and Clear-to-Send message dialogue solves the "hidden terminal" and the "exposed terminal" problems and speeds up the retransmission. The spreading code assignment avoids disruption of any ongoing transmission by an intruder. Simulation results confirm that a higher channel throughput is achieved by the new protocols even in a dense network. I. INTRODUCTION Designing an efficient and effective medium access control (MAC) protocol with collision avoidance capability in a mobile ad hoc network is a very challenging task because the network is self-organizing without the aid of wired backbone or centralized network control. The existing MAC protocols can be classified into ALOHA [1], carrier sense medium access (CSMA) [2], multiple access with collision avoidance (MACA) [3], MACAW [4], floor acquis...