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The optimization of the channel-aware ALOHA protocol for the finite population random access network model is discussed. Results suggest that the optimal transmission policy is nonrandomized and piecewise constant for every user.

A single-stage equivalent circuit is proposed to model microwave bandpass filters (BPFs) fabricated in low-temperature co-fired ceramic (LTCC) technology up to several times its passband frequency. The equivalent circuit adopts a modified T topology with the expandable multilayer resonators to achieve an extremely large bandwidth. It can be efficiently established from the measured S-parameters using direct extraction or rational approximation. As a result, the modeled S-parameters for a 2.45-GHz wideband local area network (WLAN) LTCC BPF show good agreement with the measured results over a wide frequency range up to 8.5 GHz. Such a broadband model can be used to accurately predict the suppression of harmonics and interferences in system simulation of the WLAN front-end modules. Index Terms--Equivalent-circuit model, low-temperature co-fired ceramic (LTCC) bandpass filter (BPF), wideband local area network (WLAN) BPF.

Advancing technology in wireless communication offers users anytime, anywhere access to information and network resources without restricting them to the fixed network infrastructure. Mobile computing represents a shift in the [...]

Three-dimensional (3-D) ray-tracing and analysis is used to characterize the path angles in a rectangular building in which all interior walls are parallel or perpendicular to the exterior walls. Path angles are recorded for 500 random transmitter and receiver locations, maintaining constant range throughout a particular building model. The main contribution of this paper is the observation that when all of the path angles from all 500 trials are measured relative to one building wall and collectively analyzed in a signal strength distribution, clustering is clearly apparent. While this angle clustering in the ensemble of channels is not the same as clustering in a single channel trial, it may partially account for the clusters observed in single trials. As a practical matter, these results may impact the use and placement of directional antennas for wireless LANs in buildings of this type. Index Terms--Indoor radio communication, local area networks, multipath channels, propagation.

In multispot-diffuse multiple-input-multiple-output (MSD-MIMO) system configuration, a communications channel can be considered virtually ideal at data rates of hundreds of megabits per second. Thus, the main concern is power efficiency. We propose a transceiver optical design that creates a reconfigurable transmitter output and independent communications channels. The transmitter employs multiple light sources that can be turned on and off, independently. In this way, an optical signal is provided only where it is needed. The receiver utilizes an imaging optical system and a segmented photodetector, thus performing direction diversity reception. We show that when maximum ratio combining is employed on the received electric signals and power-efficient modulation schemes such as L-level pulse-position modulation are used, significant power savings can be achieved. We also investigate the vulnerability of MSD-MIMO links to shadowing and blockage and their effects on communications cell size and required transmit power in a cellular configuration. Index Terms--Infrared local area networks (LANs), optical wireless communications, transceiver design.

Secure Sockets Layer: How it Works and Why We Need it. Ryan Laus, Central Michigan University, Park Library, 300 E. Preston, Mount Pleasant, MI 48859 Secure Sockets Layer, or SSL, [...]

Temporal skewing, a hitherto unexamined effect of chromatic dispersion on multiwavelength optical code-division multiple-access (MW-O-CDMA) systems, is explored through simulation of realistic networks. In addition, two novel methods for combating the deleterious effects of temporal skewing are proposed: optimum threshold detection and code-pattern preskewing. It is shown that for a practical system configuration with optimum threshold detection, MW-O-CDMA networks can maintain design bit error rate performance within an order of magnitude for link lengths of up to approximately 500 m. Our results show that the effect merits careful attention in the design and realization of any MW-O-CDMA system based on two-dimensional codes. Code pattern preskewing presents the possibility of completely eradicating the impact of temporal skewing. Index Terms--Chromatic dispersion, local-area networks, multiwavelength optical networks, optical code-division multiple access, temporal skewing.

In the wireless indoor environment, the channel may vary slowly as users and the interferers may move at slow speeds. A frequency-hopped code-division multiple-access system can adapt to the different interference levels in hopping patterns and assign the slots different bit rates to increase the system capacity. We show that the maximum throughput bit-rate/channel-assignment problem is NP-hard. Several practical ad hoc bit-allocation algorithms are designed based on the insights derived from exhaustive searches. The algorithms that achieve the most system capacity perform interference avoidance. Users concentrate their throughput in a small fraction of the slots with low interference by transmitting large signal constellations, while avoiding the channels with large interference. Simulations show that the flexibility of users to adjust their bit rates to the interference environment can significantly increase the system capacity. Index Terms--Code-division multiple access, power control, wireless LAN.

Transmission Control Protocol (TCP) is the most commonly used transport protocol on the Internet. All indications assure that mobile computers and their wireless communication links will be an integral part of the future internetworks. In this paper, we present how regular TCP is well tuned to react to packet loss in wired networks. We then define mobility and the problems associated with it. We discuss why regular TCP is not suitable for mobile hosts and their wireless links by providing simulation results that demonstrate the effect of the high bit error rates of the wireless link on TCP performance. We discuss and illustrate the problems caused by the mobility of hosts using a graph tracing packets between fixed and mobile hosts. We then present a survey of the research done to improve the performance of TCP over mobile wireless networks. We classify the proposed solutions into three categories: link layer, end-to-end and split. We discuss the intuition behind each solution and present example protocols of each category. We discuss the protocols functionality, their strengths and weaknesses. We also provide a comparison of the different approaches in the same category and on the category level. We conclude this survey with a recommendation of the features that need to be satisfied in a standard mobile TCP protocol. Categories and Subject Descriptors: A. 1 [General Literature]: Introductory and Survey; C.4 [Computer Systems Organization]: Performance of Systems--Reliability, availability, and serviceability; C.2.2 [Computer--Communication Networks]: Network Protocols; K. 6.3 [Management of Computer and Information Systems]: Software Management--Software development, software maintenance, software selection; K. 6.4 [Management of Computer and Information Systems]: System Management--Quality assurance General Terms: Design, Performance, Standardization Additional Key Words and Phrases: standard TCP, mobile TCP, wireless TCP, TCP performance, wired networks, mobile wireless networks, mobility, base station, mobile host, comparison of TCP implementations, link layer, end-to-end, split TCP, snoop, Reno, New-Reno, SACK, I-TCP, WTCP, MTCP, M-TCP, WAP

A class of generalized multicarrier direct sequence code-division multiple-access (MC DS-CDMA) schemes is defined and its performance is considered over multipath Nakagami-m fading channels. The spacing between two adjacent subcarriers of the generalized MC DS-CDMA is a variable, allowing us to gain insight into the effects of the spacing on the bit error rate (BER) performance of MC DS-CDMA systems. This generalized MC DS-CDMA scheme includes the subclasses of multitone DS-CDMA and orthogonal MC DS-CDMA as special cases. We present a unified analytical framework for determining the exact average BER of the generalized MC DS-CDMA system over generalized multipath Nakagami-m fading channels. The optimum spacing of the MC DS-CDMA system required for achieving the minimum BER is investigated and the BER performance of the system having optimum spacing is evaluated. The resultant BER is compared with that of both multitone DS-CDMA and orthogonal MC DS-CDMA.

In ad hoc networks, the hidden- and the exposed-terminal problems can severely reduce the network capacity on the MAC layer. To address these problems, the ready-to-send and clear-to-send (RTS/CTS) dialogue has been proposed in the literature. However, MAC schemes using only the RTS/CTS dialogue cannot completely solve the hidden and the exposed terminal problems, as pure "packet sensing" MAC schemes are not safe even in fully connected networks. We propose a new MAC protocol, termed the dual busy tone multiple access (DBTMA) scheme. The operation of the DBTMA protocol is based on the RTS packet and two narrow-bandwidth, out-of-band busy tones. With the use of the RTS packet and the receive busy tone, which is set up by the receiver, our scheme completely solves the hidden- and the exposed-terminal problems. The busy tone, which is set up by the transmitter, provides protection for the RTS packets, increasing the probability of successful RTS reception and, consequently, increasing the throughput. This paper outlines the operation rules of the DBTMA scheme and analyzes its performance. Simulation results are also provided to support the analytical results. It is concluded that the DBTMA protocol is superior to other schemes that rely on the RTS/CTS dialogue on a single channel or to those that rely on a single busy tone. As a point of reference, the DBTMA scheme out-performs FAMA-NCS by 20-40% in our simulations using the network topologies borrowed from the FAMA-NCS paper. In an ad hoc network with a large coverage area, DBTMA achieves performance gain of 140% over FAMA-NCS and performance gain of 20% over RI-BTMA. Index Terms--Ad hoc networks, busy tone, exposed-terminal, FAMA, hidden-terminal, MAC, MACA, MACAW, medium access control, RTS/CTS.

Multi-rate CDMA is a potentially attractive multiple access method for future broad-band multimedia wireless networks that must support integrated voice/data traffic. The primary impairment for such multi-rate systems is the multipath nature of radio channels that results in intra-user inter-chip interference (ICI) and multi-user interference (MUI) between different users' symbols. Explicit knowledge of the channel is typically needed for high performance detectors (such as coherent demodulation). In this work, we propose a subspace method for channel estimation in multi-rate CDMA systems. A unified signal model that applies to three multi-rate CDMA schemes proposed in the literature is developed. The computational complexity for multi-rate scenarios is large and variable--accordingly, a modified approach is devised that offers performance/complexity trade-offs. Performance analysis is conducted based on a close-form expression for the mean square error of the estimator, supported by simulation results that investigate the effectiveness of our method. Index Terms--Blind channel estimation, code division multiple access, multi-code, multi-rate, variable chip rate, variable processing gain.

Most comparisons between single carrier and multicarrier modulations assume frequency-domain linear equalization of the channel. In this paper we propose a new frequency-domain decision feedback equalizer (FD-DFE) for single carrier modulation, which makes use of a data block transmission format similar to that of the orthogonal frequency-division multiplexing with cyclic prefix (OFDM). The scheme is a nonadaptive DFE where the feedforward part is implemented in the frequency domain, while feedback signal is generated by time-domain filtering. Through simulations in a HIPERLAN-2 scenario, we show that FD-DFE yields a capacity very close to that of OFDM. This result is also confirmed by analytical derivations for a particular case. Furthermore, when no channel loading is considered, FD-DFE performs closely to OFDM for the same averaged frame error rate in a coded transmission. Design methods of the FD-DFE are investigated and a reduced complexity technique is developed, with the result that FD-DFE and OFDM have a similar computational complexity in signal processing. Index Terms--Decision feedback equalizers, signal processing, wireless LAN.

In this letter, we consider the density of the zeros of random polynomials with nonzero mean correlated Gaussian coefficients, We show that for most communication problems the original result by Hammersley can be expressed in a simpler form. In order to illustrate the usefulness of the presented theory, two applications are considered. First, the zeros of frequency-selective Ricean fading channels are investigated. Second, the zeros of the transfer function corresponding to channel impulse response estimates obtained by a least-squares approach are calculated. For both applications implications for equalizer design are discussed. Index Terms--Channel estimation, equalization, random polynomials, wireless channels.

The attenuation, depolarization, and fluctuation of a microwave signal going through a tree canopy are investigated by developing a Monte Carlo based coherent scattering model. In particular, the model is used to analyze the performance of global positioning system (GPS) receivers under tree canopies. Also the frequency and time-domain channel characteristics of a forest are investigated when a transmitter is outside and a receiver is inside a forest. A fractal algorithm (Lindenmayer system) is used to generate the structure of coniferous or deciduous trees whose basic building blocks are arbitrarily oriented finite cylinders, thin dielectric needles, and thin dielectric disks. Attenuation and phase change of the mean field through foliage is accounted for using Foldy's approximation. Scattering of the mean field from individual tree components and their images in the underlying ground plane are computed analytically and added coherently. Since tree trunks and some branches are large compared to the wavelength and may be in the close proximity of the receiver, a closed-form and uniform expression for the scattered near-field from dielectric cylinders is also developed. Monte Carlo simulation of field calculation is applied to a cluster of trees in order to estimate the statistics of the channel parameters, such as the probability density function (pdf) of the polarization state of the transmitted field, path loss, and the incoherent scattered power (the second moment of the scattered field), as a function of the observation point above the ground. Index Terms--Channel simulation, propagation, vegetation.

A novel design of smart antenna system with adaptive beamforming capability is introduced for broad-band wireless communication. To achieve high data throughput of the multi-antenna system, a parallel analog--digital (A/D) signal processing scheme is proposed. The essential idea is to realize the real-time beamforming using a heterodyne RF and IF circuitry. The bottleneck of digital signal processor (DSP) I/O and processing speed is thus relieved, while the advanced signal processing capability of the DSP chip is utilized. Based on this idea, a 5.8 GHz smart antenna receiver is implemented. Various experiments are carried out to examine the direction of arrival (DOA) estimation, beam synthesis, and bit error rate (BER) performances of the system. A 20-Mb/s data throughput using binary phase shift keying (BPSK) modulation is demonstrated for this eight-element adaptive array. Index Terms--Adaptive antennas, broad-band communication, direction of arrival (DOA) estimation, phased arrays, wireless local area network (WLAN).

We examine the properties and applications of a general class of multi-wavelength optical code-division multiple access codes in high-speed optical local area networks. We demonstrate that multiple pulse per row codes with optimum threshold detection admit maximization of the number of simultaneous users and spectral efficiency. The code design problem is also greatly simplified when the optimum threshold detection is employed. Based on our results, we propose a simple and low-cost receiver to enable true real-time network optimization. Index Terms--Correlation receiver, optical code-division multiple access, optical communication networks, optimum threshold detection, real-time scalable networks.

An efficient all-optical code-division-multiplexing (AOCDM) technique is proposed to support multirate data communications and local-area-network (LAN) interconnections with multiple protocols. To achieve this goal, we use a strict optical orthogonal code (OOC) in multirate AOCDM systems to guarantee that both cross- and autocorrelation constraints are minimum (i.e., 1) for incoherent optical processing. In contrast, the use of a conventional OOC may result in correlation constraints of 2, which in turn can degrade system performance. Moreover, implementation issues on AOCDM systems are discussed. These include the design of low-cost optical transmitters and the trade-off among coherence time, spectral width, and pulse width for AOCDM systems. It is shown that multirate AOCDM systems have a high operation flexibility to support data communications and LAN interconnections of both equal and multiple bit rates. For multirate data transmissions, the proposed system can have a better bandwidth efficiency and a lower bit error rate than a system that uses a conventional OOC. OCIS codes: 060.2330, 060.2360, 230.1150, 140.5960, 230.3120, 030.1670.

A transmission structure of many parallel conductors is capable of propagating many distinct quasi-TEM modes [3]-[6]. A field theory of such modes was developed for a theoretically infinite number of very thin conductors placed on a dielectric substrate [1]. The current paper extends this work to include a large number of thin parallel conductors embedded within finite dielectric slabs. The theory is illustrated by the following three structures: a large number of parallel wires embedded in dielectric slabs with substrates of infinite width, both with and without a ground plane; and a large number of thin wires embedded in a finite dielectric slab without a ground plane. This extension to a finite dielectric structure is obtained using a superposition method where both vertical and horizontal surfaces are considered infinite in turn. A major application of the derived formulation is the investigation of velocity distributions and crosstalk within these structures. Comparison of these structures with ones where the conductors are on the substrate [1] (as opposed to embedded) indicates how the placement and structure of the conducting channel affects the modal velocities and crosstalk. Index Terms--Crosstalk, dielectrics, Green's function, modal velocity, model spectrum, quasi-TEM modes, ribbon cable, transmission structures.

We present analytical and simulation models that obtain the minimum number of required communication channels for subscriber units and their allocations so as to increase the level of the grade of service (GOS) per user of fixed wireless access communication systems (FWA or WACS). On the basis of the proposed analytical and simulation models, a new methodology of using user radio ports (RPs) is introduced for evaluating the optimal number of RP systems, their allocation policy, and their spatial distribution for different configurations. This methodology enables us to increase the efficiency of FWA services on the basis of GOS maximization. Index Terms--Balanced and unbalanced link matrix, controlled resources search, fixed wireless access, grade of service, predefined order search, random search.

Characteristics and synchronization of two semiconductor lasers with optoelectronic feedback is examined. Communication strategies, with and without channel noise, are discussed.

The authors examine the problem of mitigating interference and fading in multiple access communication systems. Topics include adaptive interference suppression, adaptive arrays, and coding schemes.

A novel beam combination method for wide-band digital indoor communication systems such as wireless LAN's and wireless ATM's is proposed. Crossed fan beams are used at the base station (BS) and personal stations (PS's). The beam combination method provides better transmission quality than the traditional pencil-beam combination and eliminates complex beam control in the BS. Maximum dispersion distributions in a transmission frequency bandwidth are calculated by a three-dimensional (3-D) indoor propagation delay simulation algorithm. A prototype fan-beam antenna for the PS is fabricated and measured at 20 GHz. Index Terms - Fan-beam antennas, high-speed wireless communication, indoor propagation, wireless LAN.

We propose an optical code division multiple-access (OCDMA) system that uses in-fiber chirped moire gratings (CMG's) for encoding and decoding of broadband pulses. In reflection the wavelength-selective and dispersive nature of CMG's can be used to implement wavelength-encoding/temporal-spreading OCDMA. We give examples of codes designed around the constraints imposed by the encoding devices and present numerical simulations that demonstrate the proposed concept.

The design of a scalable optical local area network for multiprocessing systems is described. Each workstation has a parallel-fiber-ribbon optical link to a centralized complementary metal-oxide silicon (CMOS) switch core, implemented on a single compact printed circuit board (PCB). When the Motorola Optobus fiber technology is used, each workstation has a data bandwidth of 6.4 Gbits/s to the core. A centralized switch core interconnecting 32 workstations supports a 204-Gbit/s aggregate data bandwidth. The switch core is based on a conventional broadcast-and-select architecture, implemented with parallel CMOS integrated circuits (IC's). The switch core scales well; by incorporation of the CMOS optoelectronic IC's with optical input-output, the electrical core can be reduced to a single-chip optoelectronic IC with terabit capacities. A prototype of an optoelectronic switch core has been fabricated and is described. The appeal of the architecture includes its reliance on commercially available parallel-fiber technology, its reliance on the well-developed markets of local area networks and networks of workstations, and its smooth scalability from the electrical to optical domains as technology matures. OCIS codes: 060.0060, 060.2310, 060.2330, 060.4250, 060.4510.

This paper describes the pilot implementation of a local network by using a hybrid medium access control (MAC)layer protocol, called group implicit token-carrier sense multiple access with collision detection (GIT-CSMA/CD) developed for hard real-time industrial local area networks (LAN's). This protocol outperforms standard CSMA/CD and token-passing protocols, as well as other hybrid protocols, in terms of mean and maximum packet delays versus throughput. Also, GIT-CSMA/CD offers bounded packet delay and a great degree of adaptation to the varying requirements imposed by the user applications. A seven-node pilot network was set up and its experimental performance evaluated. The implementation of the network node is based on the existing microcontroller technology (INTEL 80C152 Communication Controller) and field programmable gate array (FPGA) logic (XILINX XC3190). The measurements carried out in real time on a 1-Mb/s LAN yielded reproducible results. The comparison between the experimental and the simulation results showed excellent agreement. Index Terms - Control networks, CSMA/CD, hard real-time MAC-layer protocols, hybrid protocols, network node implementation, performance evaluation, real-time LAN's, token passing.

A polynomial solution is suggested to address the problem of minimizing job completion time by allocating task interaction graphs to heterogeneous computers. All processor pairs have the same interprocessor communication cost, to make them appropriate for fully interconnected multiprocessor systems and local area network systems. The availability of adequate communication capacity and homogeneous processors leads to an optimal polynomial solution which can be generalized to produce two faster heuristics.

We applied for the first time the ATM technology and the LAN Emulation protocol in an existing data acquisition system. The distributed data acquisition system, based on standard host computers connected via TCP/IP over ethernet to processors in VME crates was upgraded using ethernet to ATM bridges and ATM switches. The maximum data rates in the new situation are a factor of five higher than in the previous architecture because the VME processors have individual bottleneck-free network connections to the host computer. This system scales with the available ATM network bandwidth and host computer CPU power. Index Terms - ATM, CLIP, LANE, data acquisition.

Low-power and high-speed algorithms and architectures for complex adaptive filters are proposed. The architectures are obtained through the implementation of algebraic and algorithm transformations. A fine-grained pipelined architecture for the weakened algorithm is then developed using the relaxed lookahead transformation. The architecture is used as a receive equalizer in a communication system for a data rate of 51.84 Mb/s over 100m of UTP-3 wiring in an asynchronous transfer mode-local area network environment.

We report an experimental nondirected optical link for short-range, indoor data transmission at 50 Mb/s. The system uses on-off keying (OOK) and achieves low bit-error rates (BER's) in the presence of intersymbol interference, background light noise, and shadowing, with a range of 2.9 m in a skylit room. The transmitter produces an eye-safe Lambertian pattern at 806 nm with an average power of 474 mW. The receiver utilizes a hemispherical concentrator with a hemispherical bandpass optical filter, a 1-[cm.sup.2] silicon p-i-n photodiode, and a high-impedance hybrid preamplifier to achieve a high signal-to-noise ratio (SNR). A high-pass filter is used to mitigate fluorescent light noise, with quantized feedback removing the resulting baseline wander. A decision-feedback equalizer provides resistance to intersymbol interference due to multipath. The system and its components are characterized, and compared to theory. We observe that decision-feedback equalization yields a reduction of multipath power penalties that is in good agreement with theory. Index Terms - Infrared, wireless communications, local area networks.

The low-cost self aligned multichip module (MCM)-D based optical data link (ODL) demonstrates coupling efficiencies greater than those for actively aligned optical subassemblies. The LED integral-lens design affects the coupling efficiency and the tolerance of the coupling components of the link. The flip-chip bonding, solder-bump and mechanical self-alignment schemes, and the plastic components of the optical link provide a complete self-alignment to the MCM-ODL. The piece part tolerances that help in maintaining a coupling efficiency of 70% are detailed.

A synchronous bandwidth allocation scheme to facilitate real-time message throughput in token ring networks wired according to the fiber distributed data interface (FDDI) standard is introduced. The scheme addresses the inherent problem of message-delivery delays being larger than medium-access delays to enable FDDI networks by computing and assigning a synchronous bandwith to a specific application. In this way, FDDI networks are enabled to support the synchronous traffic of distributed multimedia applications.

A multistage switch connection approach to improve the performance of large local computer networks is devised. The single large LAN is partitioned into smaller networks and these networks are interconnected by a Delta multistage butterfly switch. The performance of the single large network and the proposed partitioned network have been modeled and simulated using SIMSCRIPT II.5. The scheme has been applied to the token bus topology as a case study. It has been shown that the throughput of the proposed partitioned network is increased and there is a significant decrease in the average packet delay compared to the single large network. Key Words: LAN, partitioning, simulation, token bus, switch connection, Delta switch, performance evaluation, clustering techniques.

In a ring LAN, performance characteristics such as message transfer time, throughput, and bandwidth utilization depend on its station permutation. As ring LANs adopt fiber optics or other high speed transmission media, extend their geographical coverage, and hence contain more stations, such a dependency becomes more significant. This paper formulates the ring performance optimization problem associated with such a dependency, proves the NP-completeness of the problem, and establishes an integer programming model for solving the problem.