Seshadri Mohan

Sequential Coding Algorithms: A Survey and Cost Analysis

The cost of a number of sequential coding search algorithms is analyzed in a systematic manner. These algorithms search code trees, and find use in data compression, error correction, and maximum likelihood sequence estimation. The cost function is made up of the size of and number of accesses to storage. It is found that algorithms that utilize sorting are much more expensive to use than those that do not; metric-first searching regimes are less efficient than breadth-first or depth-first regimes. Cost functions are evaluated using experimental data obtained from data compression and error correction studies.

Two user location strategies for personal communications services

The vision of nomadic personal communications is the ubiquitous availability of services to facilitate the exchange of information between nomadic end users independent of time, location, on access arrangements. To realize this, it is necessary to locate users who move from place to place. The strategies commonly proposed are two-level hierarchical strategies, which maintain a system of home and visited databases/spl mdash/home location register (HLR) and visitor location register (VLR)/spl mdash/to keep track of user locations. Two standards exist for carrying out two-level hierarchical strategies using HLRs and VLRs. The standard commonly used in North America is the Electronics Industry Association/Telecommunications Industry Association (EIA/TIA) Interim Standard 41 (IS-41), and in Europe the Global System for Mobile Communications (GSM). The authors introduce these two strategies for locating users and provide a tutorial on their usage. Different forms of mobility in the context of PCS and a reference model for a PCS architecture are discussed. The user location strategies specified in the IS-41 and GSM standards are described, and then, using a simple example, a simplified analysis of the database loads generated by each strategy is presented. Also briefly discussed are possible modifications to these protocols that are likely to result in significant benefits by reducing query and update rate to databases and/or reducing the signaling traffic.<<ETX>>

Peak-To-Average Power Ratio Reduction in OFDM Systems: A Survey And Taxonomy

The objective of this survey is to provide the readers and practitioners in the industry with a broader understanding of the high peak-to-average power ratio (PAPR) problem in orthogonal frequency division multiplexing (OFDM) systems and generate a taxonomy of the available solutions to mitigate the problem. Beginning with a description of OFDM systems, the survey describes the most commonly encountered impediment of OFDM systems, the PAPR problem and consequent impact on power amplifiers leading to nonlinear distortion. The survey clearly defines the metrics based on which the performance of PAPR reduction schemes can be evaluated. A taxonomy of PAPR reduction schemes classifies them into signal distortion, multiple signaling and probabilistic, and coding techniques with further classification within each category. We also provide complexity analyses for a few PAPR reduction methods to demonstrate the differences in complexity requirements between different methods. Moreover, the paper provides insights into the transmitted power constraint by showing the possibility of satisfying the constraint without added complexity by the use of companding transforms with suitably chosen companding parameters. The rapid growth in multimedia-based applications has triggered an insatiable thirst for high data rates and hence increased demand on OFDM-based wireless systems that can support high data rates and high mobility. As the data rates and mobility supported by the OFDM system increase, the number of subcarriers also increases, which in turn leads to high PAPR. As future OFDM-based systems may push the number of subcarriers up to meet the higher data rates and mobility demands, there will be also a need to mitigate the high PAPR that arises, which will likely spur new research activities. The authors believe that this survey will serve as a valuable pedagogical resource for understanding the current research contributions in the area of PAPR reduction in OFDM systems, the different techniques that are available for designers and their trade-offs towards developing more efficient and practical solutions, especially for future research in PAPR reduction schemes for high data rate OFDM systems.

Mobility management for personal communications systems

Using a combination of empirical data and theoretical models, this paper develops a model of user behavior for a personal communications system environment. This model is used to analyze a mobility management strategy that combines automatic updates by the users-either when they make significant moves or when they go extended periods without network interaction; multiple hysteresis in the form of dynamic registration areas and delayed updates; and a focused paging strategy that minimizes the number of pages given a constraint on the time allowed to a page. Over a range of system and user parameters, the total paging and update traffic can be kept below 1 per 2000 user seconds. The impact on the users handset is less than ten brief updates per day. The total traffic is only a factor of three more than the minimum, immobile users case.

Control and management in next-generation networks: challenges and opportunities

This article discusses the challenges and opportunities associated with a fundamental transformation of current networks toward a multiservice ubiquitous infrastructure with a unified control and management architecture. After articulating the major driving forces for network evolution, we outline the fundamental reasons why neither the control infrastructure of the PSTN nor that of the present-day Internet is adequate to support the myriad of new services in next-generation networks. Although NGN will inherit heavily from both the Internet and the PSTN, its control and management architecture is likely to be radically different from both, and will be anchored on a clean separation between a QoS-enabled transport/network domain and an object-oriented service/application domain, with a distributed processing environment that glues things together and universally addresses issues of distribution, redundancy, and concurrency control for all applications. Finally, we allude to the transition issues and show how voice-over-packet services are emerging as the bootstrap application for marshaling in the NGN architecture.

A forwarding strategy to reduce network impacts of PCS

We propose a per-user forwarding strategy for locating users who move from place to place while using personal communications services (PCS). The forwarding strategy augments the basic location strategy proposed in existing standards such as GSM and IS-41, with the objective of reducing network signalling and database loads in exchange for increased CPU processing and memory costs. With the forwarding strategy, calls to a given user will first query the users home location register (HLR) to determine the first visitor location register (VLR) which the user was registered at, and then follow a chain of forwarding pointers to the users current VLR. This strategy is useful for those users who receive calls infrequently relative to the rate at which they change registration areas. We use a reference PCS architecture and the notion of a users call-to-mobility ratio (CMR) to quantify the costs and benefits of using forwarding and classes of users for whom it would be beneficial. We show that sender a variety of assumptions forwarding is likely to yield significant net benefits for certain classes of users, in exchange (possibly) for a small increase in mean call setup time. For instance, under certain cost assumptions, for users with CMR<1 forwarding can result in 20-60% savings over the basic strategy, with no increase in mean call setup time.

A Compact Polyimide-Based UWB Antenna for Flexible Electronics

In this letter, we present a compact ultrawideband (UWB) antenna printed on a 50.8-μm Kapton polyimide substrate. The antenna is fed by a linearly tapered coplanar waveguide (CPW) that provides smooth transitional impedance for improved matching. The proposed design is tuned to cover the 2.2-14.3-GHz frequency range that encompasses both the 2.45-GHz Industrial, Scientific, Medical (ISM) band and the standard 3.1-10.6-GHz UWB band. Furthermore, the antenna is compared to a conventional CPW-fed antenna to demonstrate the significance of the proposed design. A parametric study is first performed on the feed of the proposed design to achieve the desired impedance matching. Next, a prototype is fabricated; measurement results show good agreement with the simulated model. Moreover, the antenna demonstrates a very low susceptibility to performance degradation due to bending effects in terms of impedance matching and far-field radiation patterns, which makes it suitable for integration within modern flexible electronic devices.

Analyzing the trade off between implementation costs and performance: PCS Channel Assignment Strategies for Hand-off and Initial Access

YI-BING LIN, SESHADRI MOHAN, AND ANTHONY NOERPEL ersonal communications is expected to provide low-power/high-quality wireless access to the public switched telephone network (PSTN) [4,5]. The service area of a network providing personal communications services (PCS) is populated with a large number of ports, with each port providing coverage in its vicinity. Each port is assigned a group of channels (time slots, frequencies, spreading codes, or a combination of these) that can be either fixed or dynamic. This article assumes a fixed or quasi-static channel assignment [9]. The results may be extensible to dynamic channel assignment schemes [lo]. When a subscriber wishes to make or receive a phone call, the portable attempts to seize an available traffic channel for the call. For some PCS radio systems, the portable launches an access request on a common signaling channel and is then directed to a traffic channel (CT-2 Plus [14]). In other PCS radio systems the access attempt can be made directly on an available traffic channel (DECT [8], or Bellcore WACS [l]’). Owing to the limited number of servers or transceivers in a port, when a port is blocked there is no idle transceiver for the signaling channel. There is usually no provision (either no channel or no protocol, or both) for a portable to signal the need for a traffic channel to a blocked port and therefore access attempts cannot be queued by the network. If there is no available traffic channel or common signaling channel, the call is blocked. (An exception to this is the WACS system, which reserves a time slot channel called the system broadcast channel.) If there is an available traffic channel it is used for the call. The channel is released eitherwhen the call iscompleted or the portable (or the PCS subscriber) moves out of the coverage area. When a user moves from one coverage area to another while a call is in progress, a hand-off to the new port is required to maintain continuity and quality of the call. If the new port does not have an idle channel, the call may be dropped o r forced terminated. The forced termination probability is an important criterion in the performance evaluation of a PCS network. Forced termination of an ongoing call is considered less desirable than blocking of a new call attempt. Radio tech-

A Multiprocessor Architecture for the (M, L)-Algorithm Suitable for VLSI Implementation

The (M, L) -algorithm has been widely used in speech and image encoding. Recently, use of (M, L) -Iike algorithms has been suggested for decoding phase codes. With its ever-increasing use, there arises a need to explore architectures suitable for real-time applications. Toward this end, we present a multiprocessor architecture for the (M, L) algorithm that employs an SIMD (single instruction-multiple data) machine structure. The considerations involved in interconnection network design are discussed. The main functions of the network controller are switch state selection and synchronization. The number of switching elements required is significantly less than the elements required in the universal permutation network. These features make this architecture suitable for VLSI implementation. The tradeoff between number of processors and encoding time is also discussed.

Privacy and authentication protocols for PCS

This article describes the message flow due to authentication, voice privacy, and signaling message encryption expected to be incorporated in the EIA/TIAs cellular industry Interim Standard IS 41 Revision C. The algorithm for authentication and generation of voice privacy mask and signalling message encryption keys employed by the standard is based on private key cryptographic techniques that use a secret key (also known as shared secret data, or SSD) for authentication. Two schemes have been proposed in the standard. In the first one, the SSD is shared only between the handset and the authentication center. In the second, the SSD is also shared with the visited system. Compared to the first scheme, the second scheme requires a considerably reduced rate of accesses to network databases for authentication during call origination determination, thereby reducing call setup time. However, during registration, the second scheme requires additional database accesses compared to the first due to the need to get an up-to-date call history count from the previous visited system. We compare the two schemes with the use of a simple mobility model for users and study their impact on the traffic to network databases. Defining the user mobility rule as the number of registrations per hour per user, we show that as the user mobility rate increases from roughly 0.5 to 15, the effectiveness of the second scheme compared to the first varies from about a 66 percent improvement to about a 30 percent degradation, clearly implying that the mobility characteristics of the user population dictate the choice of the authentication scheme.