Ultrawideband impulse radio systems offer two kinds of processing gains that can be adapted based on the interference level in the system so that quality of service requirements are fulfilled. An adaptive assignment scheme for two types of multiple-access parameters in cluster-based wireless sensor networks is investigated. A mathematical framework is developed for asynchronous communications using a Gaussian approximation method to model the multiple-access interference in two cases: one with fixed frame duration, where the goal is to increase the average throughput, and the other with fixed symbol duration, where the goal is to increase the network lifetime. Extension of the analysis to multipath channels is carried out, and the validity of the Gaussian approximation is investigated using the Kullback-Leibler distance.

In time hopping impulse radio, Nfpulses of duration Tcare transmitted for each information symbol. This gives rise to two types of processing gains: (i) pulse combining gain, which is a factor Nf, and (ii) pulse spreading gain, which is Nc= Tf/Tc, where Tfis the mean interval between two subsequent pulses. This paper investigates the tradeoff between these two types of processing gains in the presence of timing jitter. First, an additive white Gaussian noise (AWGN) channel is considered, and approximate closed-form expressions for bit error probability (BEP) are derived for impulse radio systems with and without pulse-based polarity randomization. Both symbol-synchronous and chip-synchronous scenarios are considered. The effects of multiple-access interference (MAI) and timing jitter on the selection of optimal system parameters are explained through theoretical analysis. Finally, a multipath scenario is considered, and the tradeoff between processing gains of a synchronous impulse radio system with pulse-based polarity randomization is analyzed. The effects of the timing jitter, MAI, and interframe interference (IFI) are investigated. Simulation studies support the theoretical results.

Due to the fine delay resolution of typical ultra wideband (UWB) systems, Rake receivers need to combine samples from a large number of multipath components (MPCs) in order to collect sufficient signal energy for reliable decisions. In addition, these samples need to be combined optimally in order to minimize bit error probability. The optimal linear minimum mean square error (MMSE) combining scheme might require inversion of a large matrix depending on channel and system parameters. Therefore, suboptimal algorithms with lower computational complexity but close-to-optimal performance are desirable. In this paper, a low-complexity combining scheme is proposed for that purpose, which divides samples into a number of groups and performs MMSE combining in two steps. Performance of this two-step combining scheme is investigated theoretically and by simulations.

In an ultra-wideband (UWB) impulse radio (IR) system, a number of pulses, each transmitted in an interval called a "frame," is employed to represent one information symbol. Conventionally, a single type of UWB pulse is used in all frames of all users. In this paper, IR systems with multiple types of UWB pulses are considered, where different types of pulses can be used in different frames by different users. Both stored-reference (SR) and transmitted-reference (TR) systems are considered. First, the spectral properties of a multipulse IR system with polarity randomization is investigated. It is shown that the average power spectral density is the average of the spectral contents of different pulse shapes. Then, approximate closed-form expressions for the bit-error probability of a multipulse SR-IR system are derived for Rake receivers in asynchronous multiuser environments. The effects of both interframe interference (IFI) and multiple-access interference (MAI) are analyzed. The theoretical and simulation results indicate that SR-IR systems that are more robust against IFI and MAI than a "conventional" SR-IR system can be designed with multiple types of ultra-wideband pulses. Finally, extensions to multipulse TR-IR systems are briefly described.

We present an analysis of the time-of-arrival (TOA), time-difference-of-arrival (TDOA), angle-of-arrival (AOA) and signal strength (SS) based positioning methods in a non-line-of-sight (NLOS) environment. Single path (line-of-sight (LOS) or NLOS) propagation is assumed. The best geolocation accuracy is evaluated in terms of the Cramer-Rao lower bound (CRLB) or the generalized CRLB (G-CRLB), depending on whether prior statistics of NLOS induced errors are unavailable or available. We then show that the maximum likelihood estimator (MLE) using only LOS estimates and the maximum a posteriori probability (MAP) estimator using both LOS and NLOS data can asymptotically achieve the CRLB and the G-CRLB, respectively. Hybrid schemes that adopt more than one type of position-pertaining data and the relationship among the four methods in terms of their positioning accuracy are also investigated.

Network routing in wireless ad hoc networks is liable to attacks that may have a grave impact on network operations. Such attacks can be targeted at the route discovery process or the data packet forwarding process. Although the protection of route discovery is a critical prerequisite to ensure the robustness of the routing process, secured route discovery by no means eliminates attacks on routing. We, accordingly, propose a secure data forwarding protocol that detects faulty links in the packet forwarding process, which enables the corresponding sources to progressively route packets over non-faulty paths.

Propagation of Internet worms is a serious problem in our highly information oriented society. In this paper, we propose a stochastic model for Internet worm propagation to evaluate its dependability measures quantitatively. More precisely, the deterministic kill-signal model is reformulated based on a continuous-time Markov chain. We define some dependability measures and derive the recursive computation algorithms to assess them. In numerical experiments, we investigate the behavior of actual Internet worms with real infection data and characterize their propagation.

Convex relaxations of the optimal finger selection algorithm are proposed for a minimum mean square error (MMSE) RAKE receiver in an impulse radio ultra-wideband system. First, the optimal finger selection problem is formulated as an integer programming problem with a non-convex objective function. Then, the objective function is approximated by a convex function and the integer programming problem is solved by means of constraint relaxation techniques. The proposed algorithms are suboptimal due to the approximate objective function and the constraint relaxation steps. However, they can be used in conjunction with the conventional finger selection algorithm, which is suboptimal on its own since it ignores the correlation between multipath components, to obtain performances reasonably close to that of the optimal scheme that cannot be implemented in practice due to its complexity. The proposed algorithms leverage convexity of the optimization problem formulations, which is the watershed between 'easy' and 'difficult' optimization problems.

In this paper, we present an unbalanced key establishment scheme for a heterogeneous wireless network environment. We use powerful access points as multiple security managers to configure low-end reduced functional devices (RFD) on site. Our scheme is proposed to set up the master key between a roaming RFD node and a security manager during the handoff process, which exploits the difference in device capabilities, and puts the cryptographic burden where resources are less constrained. The scheme authenticates the device identities in a flexible manner, based on asymmetric certificates. A TESLA certificate is deployed to enable efficient certificate verification on RFD nodes. Our protocol offloads the expensive public key operations from the low-end RFD nodes to powerful security managers, and has less computation complexity than all the other authenticated key agreement protocols that adopt elliptic curve cryptography.

While deploying the next generation of optical networks with a mesh topology, telecommunications carriers are being confronted with a choice between wavelength switches that can switch traffic at SONET STS-48 (2.5 Gbps) granularity and subwavelength grooming capable switches that can switch at STS-1 (51 Mbps) granularity. The former consumes high fragmented/unused capacity to support low capacity end-to-end circuits using high capacity STS-48 channels (given current subwavelength traffic levels) while the latter may require relatively complicated hardware design that decreases switch scalability. Two-tier network architectures combine the benefits of STS-1 and STS-48 switches by using an upper tier of STS-48 switches for routing and restoration and a lower tier of STS-1 switches for grooming efficiency. A partial two-tier architecture, where STS-1 switches are restricted to a subset of the network nodes, has been shown in to closely match the grooming benefits of a full lower STS-1 tier. We furnish a detailed upper hound analysis of how the fragmented/unused capacity in STS-48 channels (fragmentation loss) varies with the grooming capability of a network for arbitrary traffic scenarios. We show that the upper bounds derived in this paper are in agreement with results obtained using efficient routing and grooming algorithms discussed. Because the bounds obtained do not make any assumptions about traffic and are easy to compute, they are suited for incorporation into a network engineering tool for deciding strategic placement of STS-1 switches in partial two-tier networks. Our work is not biased towards any particular network architecture but aims to analyze the grooming efficiency of two-tier networks.

In time hopping impulse radio, N/sub f/ pulses of duration T/sub c/ are transmitted for each symbol. This gives rise to two types of processing gain: (i) pulse combining gain, which is a factor N/sub f/, and (ii) pulse spreading gain, which is N/sub c/ = T/sub f//T/sub c/, where T/sub f/ is the mean interval between two subsequent pulses. This paper investigates the trade-off between these two types of processing gain with and without random polarity codes in the presence of timing jitter. Approximate expressions for bit error probability are derived for both coded and uncoded systems over additive white Gaussian noise channels and are used as the criterion to choose optimal N/sub f/ and N/sub c/ values. The effects of timing jitter and multiple access interference on the selection of optimal system parameters are explained through theoretical analysis. Simulation studies support the theoretical results.

In this paper, we consider the problem of routing in an adversarial environment, where a sophisticated adversary has penetrated arbitrary parts of the routing infrastructure and attempts to disrupt routing. We present protocols that are able to route packets as long as at least one nonfaulty path exists between the source and the destination. These protocols have low communication overhead, low processing requirements, low incremental cost, and fast fault detection. We also present extensions to the protocols that penalize adversarial routers by blocking their traffic.

Ultrawideband (UWB) is an attractive physical layer technology for wireless sensor networks due to its unique characteristics. Flexibility in adjusting the processing gain of UWB systems makes it possible to tune the data rate and transmission range to fulfill the requirements of specific applications. Conventional systems assign identical multiple access parameters to all users regardless of the signal-to-interference plus noise ratio of the received signal. An adaptive assignment scheme for multiple access parameters in cluster based wireless sensor networks is investigated. First, an orthogonal time hopping sequence construction is proposed for synchronous communications (downlink), where the number of pulses per symbol are adjusted to meet the bit error rate requirement of an application. Then, adaptation of multiple access parameters in asynchronous scenarios (uplink) is evaluated using a Gaussian approximation method to model the multiple access interference in two cases: one with fixed frame duration, where the goal is to increase the average throughput, and the other with fixed symbol duration, where the goal is to increase the network lifetime. Finally, a mathematical framework is developed for approximating the interference when the number of pulses per symbol and the frame duration vary.

Flooding is an important tool in the routing operations of wireless ad hoc networks with applications to both topology/route discovery and data packet forwarding. We present two "guaranteed delivery" flooding protocols. The protocols "guarantee" successful delivery of packets from a source router to any nonfaulty router in a network that is connected by at least one nonfaulty path to the source router, even if the behavior of faulty routers and links is arbitrary and malicious. The first protocol is based on digital signatures. It improves earlier work by preventing the adversary from overwhelming a victim router with spurious digital signatures. The second protocol is based on the TESLA broadcast authentication protocol.

The performance of a binary phase shift keyed random time-hopping impulse radio system with pulse-based polarity randomization is analyzed. The effects of multiple access interference are investigated for both chip-synchronous and asynchronous systems. It is shown that the performance of a chip-synchronous system is the same as that for the symbol-synchronous case studied in E. Fisher and H. V. Poor (Oct. 2-4, 2002). The asynchronous system is modelled as a chip-synchronous system with uniformly distributed timing jitter on the transmitted pulses of interfering users. This extends the analytical technique developed for the chip-synchronous case to the asynchronous case. An approximate closed-form expression for the probability of error, expressed in terms of the autocorrelation function of the transmitted pulse, is derived for the asynchronous case. The analysis shows that the chip-synchronous assumption can result in over-estimating the error probability, and hence that the system design based on this approximation will he on the safe side. The degree of over-estimation mainly depends on the autocorrelation function of the UWB pulse and signal-to-interference-plus-noise-ratio (SIR) of the system. Simulations studies support this approximate analysis.

Clos networks, which are ubiquitous in large capacity switches, have been widely studied for unicast connection requests. But most of the results for unicast do not hold when the switch has to support multicast connections. Our interest is specifically in bicast connections which are required for setting up backup protected circuits in optical backbone networks. In this work we have proposed two approximation methods to estimate the blocking probability of a switch that is not strictly non-blocking in the presence of bicast connections. We have considered two ways of realizing a bicast connection - by splitting the circuit at the outer stage or at the middle stage. We have also carried out simulations to be compared with our analytical results and found that they closely match. Furthermore, we have compared two routing strategies - one randomly selects an available middle stage block and the other selects one of the highly loaded ones, and found that the latter significantly reduces blocking.

We investigate the problem of joint frequency offset and channel estimation for OFDM systems. The complexity of the joint maximum likelihood (ML) estimation procedure motivates us to propose an adaptive MLE algorithm which iterates between estimating the frequency offset and the channel parameters. Pilot tones are used to obtain the initial estimates and then a decision-directed technique provides an effective estimation technique. The joint modified (averaged) Cramer-Rao lower bounds (MCRB) of the channel coefficients and frequency offset estimates are derived and discussed. It is shown that, for the case of a large number of subcarriers in the OFDM system, there is approximately a 6 dB loss in the frequency offset estimate lower bound due to the lack of knowledge of the channel impulse response (CIR). The degradation of the CIR lower bound is less severe and depends on the channel delay spread. We show both analytically and by simulation, that the channel estimate accuracy is less sensitive to unknown frequency offset than the frequency offset estimation is affected by the unknown CIR. Comprehensive simulations have been carried out to validate the effectiveness of the adaptive joint estimation algorithm.

The time-of-arrival (TOA), time-difference-of-arrival (TDOA) and signal strength (SS) methods have been widely accepted as three principal techniques for positioning a mobile station (MS) in a wireless communication system. To the best of our knowledge, previous studies tend to treat these methods separately, and less analytical results on their relationship have been reported. We first examine the link between the TOA and TDOA methods. We provide an analytical explanation for the claim that, given a set of BS locations and an MS position, the TOA method should achieve a higher positioning precision than its TDOA counterpart. However, the two positioning methods may attain the same level of accuracy under certain conditions. We then investigate the tradeoff between the accuracy limits of the TOA and SS based methods, which leads to our proposal of a hybrid distance estimation scheme that combines both TOA and SS data.

The combination of multiple-antenna and orthogonal frequency division multiplexing (OFDM) provides reliable communications over frequency selective fading channels. We investigate this approach and focus on the application of space-time block codes (STBC) and space-frequency block codes (SFBC) in OFDM systems. We compare the performance of maximum likelihood (ML), zero forcing (ZF) and conventional detection algorithms. We show that ZF provides a good trade-off between computational complexity and performance. The problem of channel estimation in STBC-OFDM and SFBC-OFDM systems is also studied, including the derivation of the Cramer-Rao lower bound (CRLB). Since knowledge of the channel is required to coherently decode STBC-OFDM and SFBC-OFDM, we propose an iterative channel estimation algorithm based on the EM algorithm that requires very few pilot symbols. The CRLB can be achieved by the channel estimation algorithm.

Estimating a channel that is subject to frequency selective Rayleigh fading is a challenging problem in an orthogonal frequency division multiplexing (OFDM) system. We propose an enhanced channel estimation algorithm that combines the EM-based algorithms proposed previously and a least squares polynomial fitting (LSPF) approach. The combined algorithm can efficiently estimate the channel response of an OFDM system operating in an environment with multipath fading and additive white Gaussian noise (AWGN). The algorithm can improve the channel estimate obtained from the EM-based algorithms by polynomial fitting. Simulation results show that the bit error rate (BER) as well as the mean square error (MSE) of the channel can be improved by the algorithm. In particular, with these additional computations and demodulation delay, the MSE can be made smaller than the Cramer-Rao lower bound (CRLB).

Time-sensitive mobile commerce is vulnerable to message authentication delays. Significant power consumption incurred by cryptography is another limiting factor of most mobile devices. In this paper, we present a scalable mobile cryptosystem, which installs a group key and an elliptic curve private/public key pair in each device to enable both symmetric key and public key cryptography. Scalable key establishment protocols and secure routing protocols with scalable authentication are proposed to make tradeoffs between security and energy, according to different mobile applications.

Performance of both orthogonal frequency division multiplexing (OFDM) and single carrier (SC) systems suffers from a carrier frequency offset (CFO) due to Doppler effect and the carrier instability between the transmitter and the receiver. We investigate the bit error rate (BER) performance degradation of OFDM and SC systems due to the frequency offset in an additive white Gaussian noise (AWGN) channel as well as multipath Rayleigh fading channels. We consider three effects to the BER degradation, i.e., phase shift, useful power decrease and intercarrier interference (ICI). We also derive the approximate expressions of BER under binary phase shift keying (BPSK) and quaternary phase shift keying (QPSK) for both OFDM and SC systems in the presence of CFO. In general, SC is more robust to CFO in the AVVGN channel than OFDM in terms of BER, while both of them suffer similarly from CFO in the multipath Rayleigh fading channels assuming the the same CFO.

Existing algorithms for estimating the model parameters of an explicit-duration hidden Markov model (HMM) usually require computations as large as O((MD/sup 2/ + M/sup 2/)T) or O(M/sup 2/ DT), where M is the number of states; D is the maximum possible interval between state transitions; and T is the period of observations used to estimate the model parameters. Because of such computational requirements, these algorithms are not practical when we wish to construct an HMM model with large state space and large explicit state duration and process a large amount of measurement data to obtain high accuracy. We propose a new forward-backward algorithm whose computational complexity is only O((MD + M/sup 2/)T), a reduction by almost a factor of D when D > M and whose memory requirement is O(MT). As an application example, we discuss an HMM characterization of access traffic observed at a large-scale Web site: we formulate the Web access pattern in terms of an HMM with explicit duration and estimate the model parameters using our algorithm.

Geolocation in an non-line-of-sight (NLOS) environment is an important issue in wireless communications. Several empirical approaches have been proposed in recent years. However, two fundamental questions still remain unanswered: what is the achievable geolocation accuracy in the NLOS environment and how to attain it. In this paper, we derive a closed-form expression of the Cramér-Rao Lower bound (CRLB) for NLOS geolocation, which we believe is new and answers the first question. Its physical interpretation turns out to be very helpful to better understanding of NLOS geolocation mechanism. We discuss some numerical examples based on simulation experiments.

We have investigated the indoor geolocation based on signal strength modeling. Linear, compensated linear, and multiple regression methods have been applied to set up signal strength models by using simulated data. We have also analyzed this modeling method to better understand the relation between the location error and the signal strength error. Some important results have been obtained to help us determine proper placement of access points (APs) and evaluate the range of location error. A simulation experiment has been conducted based on typical parameters of IEEE802.11b MAC.

We apply joint maximum likelihood (ML) estimation to orthogonal frequency division multiplexing (OFDM) systems and develop a simple receiver structure that gives joint ML estimates of a multipath channel and the transmitted data sequence. Simulation results have confirmed good performance of our algorithm. For a two-path or three-path slow fading channel, our algorithm converges to the case with known channel parameters.

This paper investigates the packet error rate (PER) and the throughput performance employing code spreading (CS), in which a very low-rate channel coding is used instead of spreading by the Walsh-Hadamard channelization code, associated with Type-l Hybrid ARQ with packet combing (i.e., Chase combining) in the W-CDMA reverse link. Simulation results elucidate that the required average received signal energy per bit-to-background noise power spectrum density ratio (E/sub b//N/sub 0/) at the average PER of 1% employing CS-CDMA coupled with turbo coding using the coding rate of R = 1/15 and with convolutional coding using R = 1/16 is decreased by approximately 0.5 to 1.0 dB compared to that employing the DS-CDMA scheme with R = 1/2 in a 2-path Rayleigh fading channel. We also clarify that the achievable throughput employing CS-CDMA using turbo coding coupled with Chase combining and with two-branch antenna diversity reception is increased by approximately 10 to 20% compared to that with DS-CDMA. Consequently, we conclude that the code spreading associated with turbo coding is a very promising technique in the reverse-link of broadband packet wireless access beyond IMT-2000 as well as of W-CDMA packet access because a higher channel coding gain is expected thanks to the frequency (path) diversity effect.

We propose an EM-based algorithm to efficiently detect transmitted data in an OFDM system as well as estimating the channel impulse response (CIR). The maximum likelihood estimate of CIR is obtained by using channel statistics (their means and covariances) via the expectation-maximization (EM) algorithm. This algorithm can improve signal detection and the channel estimation accuracy by making use of pilot symbols to obtain an initial estimate for the iteration. Simulation results show that the bit error rate (BER) can be significantly reduced by this algorithm, and validate its good convergence and robust properties.

We address the problem of downlink detection in a mobile radio time division/code division multiple access multirate communication system employing linear modulation. We focus on the detection of a group of intracell codes (ranging from a single one to all the active codes) rejecting both interference coming from the complementary set of undesired intracell codes and co-channel intercell interference. We investigate efficient implementations of linear nonadaptive multiuser detection realized by either joint or separate intersymbol interference and multiple access interference (MAI) mitigation using the zero-forcing or minimum mean square error criteria. The proposed detection schemes employ a tunable-complexity structured description of the MAI for the purpose of detection and interference mitigation. Specifically, the receivers always envision an intracell interference mitigation and data detection capability, while intercell interference is treated differently depending on operating environments. If a statistical description of the intercell interference is available, the receiver realizes group detection in the presence of possibly nonwhite Gaussian noise. Soft hand-over procedures are also proposed wherein direct suppression of intercell interference is possible as well as group detection of the data of the neighboring cells. A unified and finite complexity implementation of the proposed detection schemes based on a sliding window formulation is provided. The numerical results validate the proposed receiver structures showing that a structured description of the observation always leads to a detector with superior performance.

Iterative decoding is used to achieve backward compatible performance improvement in several existing systems. Concatenated coding and iterative decoding are first set up using composite mappings, so that various applications in digital communication and recording can be described in a concise and uniform manner. An ambiguity zone detection (AZD) based iterative decoder, operating on generalized erasures, is described as an alternative for concatenated systems where turbo decoding cannot be performed. The described iterative decoding techniques are then applied to selected wireless communication and digital recording systems. Simulation results and utilization of decoding gains are discussed.

In this paper, we address the problem of the downlink detection in a mobile radio system with hybrid time-division and code-division multiple access, wherein the deployment of orthogonal variable spreading factor codes allows for multirate communication. Accounting for a multipath propagation channel, we focus on the detection of data carried by a group of codes (ranging from a single code, to all active intracell codes) destined to the same mobile station and on the simultaneous rejection of the interference due to the complementary set of active intracell codes. We tackle both the problems of intersymbol interference (ISI) suppression, and multiple access interference (MAI) mitigation, and show when it is convenient to treat these issues together. Specifically, the feasibility of linear detectors based on the zero forcing and the minimum mean square error (MMSE) criterion is investigated, with the aim to mitigate both ISI and MAI. A unified low-complexity formulation based on a sliding window algorithm is proposed for this class of linear detectors. The numerical results validate the proposed receiver structure, indicating that the attainable performance is very close to the single user lower bound up to moderate system loads.

We propose two maximum likelihood based semi-blind block channel estimation and signal detection algorithms for multipath channels with additive Gaussian noise. The algorithms are based on the Baum-Welch (1972) algorithm and the segmental k-means algorithm for hidden Markov models (HMMs). By making use of a training signal, the algorithms are applied block-wise to sequential disjoint subintervals of the whole observation interval. We study the effects of block length in terms of the bit error rate (BER), the mean square error (MSE) of the estimated channel impulse response, and its Cramer-Rao lower bound. Our simulation results show that the BER performance does not suffer even for a short block length when a good initial estimate is available.

The criterion that existing prefetch schemes apply in prefetching documents from the origin servers into a proxy Web server is usually the probability of each document being accessed in the near future or the popularity of the document. This criterion is not optimum in minimizing the average access latency or maximizing the average hit probability because the factors that affect the latency and the hit probability also include the response time and the updating cycle of the documents. We derive expressions for the average latency, hit probability, cache capacity and required bandwidth for a general prefetch scheme. A new prefetch scheme that combines access probability, response time and updating cycle to determine the lowest average latency or the highest hit probability is proposed. Finally, some numerical results are presented. The required parameters of the prefetch scheme can be simply derived from the log data of the cache. Thus, our scheme can be implemented in practice.

We propose a new method to design an interleaver with practical size for turbo codes. In order to construct an optimal interleaver, we first set up the following design rules: (i) to suppress the interleaver correlation; (ii) to break up self-terminating weight-2 input sequences; and (iii) to avoid edge effects. We then synthesize a new type of interleaver structure, starting with the conventional block interleaver and applying simple transformation steps. The resulting interleaver can create turbo codes with relatively small block size that yield better performance than previously known interleavers. Advantages of the new interleaver are demonstrated by conducting comprehensive comparisons based on both theoretical analyses and computer simulations.

We attempt to improve the performance of an existing wireless packet transmission system that uses coding and transmitter diversity. We view it as a concatenated system, system constructed by "concatenation" of encoding blocks and interleavers at the transmitter. The transmitter structure is improved using a better interleaving scheme, the receiver architecture by applying an iterative decoder with AZD (ambiguity zone detection) structure. This receiver uses generalized erasures, which are resolved in an iterative process. Using this framework, overall encoding and decoding gain of almost 9 dB is achieved with small increase in complexity and decoding delay.

We study error correcting which are constructed by using permutation and in conjunction with concatenation. These include concatenated codes, product codes and turbo codes as special cases. We then extend our approach to a continuous channel, by adopting AZD (ambiguity zone detection) at the receiver, which introduces generalized erasures. Iterative decoding can significantly improve the performance of a coded system that contains a channel with memory (e.g., spectral shaping, ISI, multipath delay), modulation with memory (e.g., trellis coded modulation, continuous phase modulation) and/or diversity. Moreover, it is implementable with low decoding complexity and requires a small decoding delay. Using this framework, we propose two architectures for iterative receivers in an existing wireless packet transmission system. These receivers are fully backward compatible with the existing transmitter, have low decoding complexity and delay, and offer a decoding improvement of several dBs.

We present analytical results characterizing the effects of multipath interference on an FM subcarrier data channel. A closed form expression for the FM discriminator output (or the instantaneous received frequency) is obtained when there are M reflected path signals in addition to the primary path signal. We derive the power spectral density when the multipath interference is weak, and determine the conditions under which multipath interference causes nonlinearities in the instantaneous frequency of the received signal. In order to verify our analytical results, we have built an SPW simulator of HSDS (the high speed data service) system, and we have observed close agreement between our analysis and simulations. The analytical techniques presented could potentially be applied to other FM subcarrier systems, as well as to digital wireless systems that adopt FSK or its variants. Hence, our results could be useful in understanding noncoherent reception of the GSM digital cellular system, for example.

We consider a statistical multiplexer model, in which each of the K sources is a Markov modulated rate process (MMRP). This formulation allows a more general source model than the well studied "on-off" source model in characterizing variable bit rate (VBR) sources such as compressed video. In our model we allow an arbitrary distribution for the duration of each of the M states (or levels) that the source can take on. We formulate Markov modulated sources as a closed queueing network with M infinite-server nodes. By extending our earlier results we introduce an M-dimensional diffusion process to approximate the aggregate traffic of such Markov modulated sources. Under a set of reasonable assumptions we then show that this diffusion process can be expressed as an M-dimensional Ornstein-Uhlenbeck (O-U) process. The queueing behavior of the buffer content is analyzed by applying a diffusion process approximation to the aggregate arrival process. We show some numerical examples which illustrate typical sample paths, and autocorrelation functions of the aggregate traffic and its diffusion process representation. Simulation results validate our proposed approximation model, showing good fits for distributions and autocorrelation functions of the aggregate rate process and the asymptotic queueing behavior. We also discuss how the analytical formulas derived from the diffusion approximation can be applied to compute the equivalent bandwidth for real-time call admission control, and how the model can be modified to characterize traffic sources with long-range dependence.

This paper describes the performance analysis of a class of two-connected multihop shufflenets, known as generalized shuffle networks. The topology of such networks is described mathematically by the equation N=kn, where N is the total number of nodes in the network, k the number of stages in the network and n the number of nodes in each stage. Compared to classical shufflenets, the definition of generalized shuffle networks allows a larger number of feasible network structures that are realizable for a given network size N. In attempting to find an optimum network structure, network characteristics are discussed and system performance is evaluated. Important relationships and interdependencies among the various network parameters are developed to facilitate cross-structural comparison.

Multihop networks with wavelength-division multiplexing (WDM) are one possible way to conduct high data-rate communication. We provide an in-depth study of the generalization of the well-known shuffle network for ultrafast multihop lightwave communication. In the classical definition of a shuffle network, i.e., N=kp/sup k/ where N is the number of nodes and k is the number of stages with nodes of degree p, the realizable values of N are very sparse and many of the intermediate values of N are not realizable. We use a new definition of the shuffle network, N=nk, where n is the number of nodes per stage, which was originally proposed by Krishna and B. Hajek (1990) as the shuffle-ring network. Based on this definition, we divide the shuffle networks into two classes: extra-stage and reduced-stage. We derive an exact model and an approximate model of the expected number of hops for various network topologies. The results can be used to determine an optimal network topology when given a value of N.

This paper considers a mixed-media packet-switched computer communication network which consists of a low-delay terrestrial store-and-forward subnet combined with a low-cost high-bandwidth satellite subnet. We show how to route traffic via ground and/or satellite links by means of static, deterministic procedures and assign capacities to channels subject to a given linear cost such that the network average delay is minimized. Two operational schemes for this network model are investigated: one is a scheme in which the satellite channel is used as a slotted ALOHA channel; the other is a new multiaccess scheme we propose in which whenever a channel collision occurs, retransmission of the involved packets will route through ground links to their destinations. The performance of both schemes is evaluated and compared in terms of cost and average packet delay tradeoffs for some examples. The results offer guidelines for the design and optimal utilization of mixed-media networks.

Modeling and performance prediction are becoming increasingly important issues in the design and operation of computer communications systems. Complexities in their configuration and sophistications in resource sharing found in today's computer communications demand our intensive effort to enhance the modeling capability. The present paper is intended to review the state of affairs of analytic methods, queueing analysis techniques in particular, which are essential to modeling of computer communication systems. First we review basic properties of exponential queueing systems, and then give an overview of recent progress made in the areas of queueing network models and discrete-time queueing systems. A unified treatment of buffer storage overflow problems will be discussed as an application example, in which we call attention to the analogy between buffer behavior and waiting time in theGI/G/1queue. Another application deals with the analysis of various multiplexing techniques and network configuration. An extensive reference list of the subject fields is also provided.

We develop here an approximate method for the design and performance prediction of a multiaccess communication system which employs the ALOHA packet-switching technique. Our model is based on the use of a diffusion process approximation of an ALOHAlike system (with or without time-slotting). A simple closed-form solution for the variableQ(t), a variant of the number of backlog messages at timet, is given in terms of a few system and user parameters. Final results are expressed in terms of ordinary performance measures such as throughput and average delay. Several numerical examples are given to demonstrate the usefulness of the approximation technique developed.

Research into queuing networks and their applications to computer systems is in a state of prosperity. The object of this paper is to discuss the computational aspect of separable queuing networks. Separable networks constitute that class of models for which a solution can be computed efficiently for fairly large problems. Open networks do not pose any computational problem. It is the case of closed networks where the subject of numerical algorithms becomes an issue.

The practical value of queueing theory in engineering applications such as in computer modeling has been limited, since the interest in mathematical tractability has almost always led to an oversimplified model. The diffusion process approximation is an attempt to break away from the vogue in queueing theory. The present paper introduces a vector-valued normal process and its diffusion equation in order to obtain an approximate solution to the joint distribution of queue lengths in a general network of queues. In this model, queueing processes of various service stations which interact with each other are approximated by a vector-valued Wiener process with some appropriate boundary conditions. Some numerical examples are presented and compared with Monte Carlo simulation results. A companion paper, Part II, discusses transient solutions via the diffusion approximation.

Quite often explicit information about the behavior of a queue over a fairly short period is wanted. This requires solving the nonequilibrium solution of the queue-length distribution, which is usually quite difficult mathematically. The first half of Part II shows how the diffusion process approximation can be used to answer this question. A transient solution is obtained for a cyclic queueing model using the technique of eigenfunction expansion. The second half of Part II applies the earlier results of Part I to modeling and performance problems of a typical multiprogrammed computer system. Such performance measures as utilization, throughput, response time and its distribution, etc., are discussed in some detail.

Modems for digital communication often adopt the so-called correlative level coding or the partial-response signaling, which attains a desired spectral shaping by introducing controlled intersymbol interference terms. In this paper, a correlative level encoder is treated as a linear finite-state machine and an application of the maximum-likelihood decoding (MLD) algorithm, which was originally proposed by Viterbi in decoding convolutional codes, is discussed. Asymptotic expressions for the probability of decoding error are obtained for a class of correlative level coding systems, and the results are confirmed by computer simulations. It is shown that a substantial performance gain is attainable by this probabilistic decoding method.

The relationship between distance measures and asymptotic relative efficiency is discussed. It is shown that the ratio of the Bhattacharyya distance orJdivergences of two test statistics is equivalent to asymptotic relative efficiency. Two-input systems are discussed as examples, and the performances of the polarity coincidence correlator (PCC) and the correlator are discussed in terms of the distance measures of reduced data.