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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.

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.

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).

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.

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.

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.

A proper selection of links help obtain generalized loop networks with optimal or close to optimal diameter and connectivity. This was revealed in a study of the generalization of directed loop networks to loop-symmetric networks with N nodes, each node having in-degree and out-degree k such that 2 to the power k did not exceed N. Since their diameter is not more than twice the average distance, these networks are compact, making the symmetric networks reliable.

A modular architecture network for connecting several local area networks (LAN) at the hierarchical level is introduced along with the modified Spanning Tree routing scheme for facilitating improved reconfiguration times and a wider physical coverage. The bridged LANs are evaluated via Petri net concepts in terms of network availability, network performance and the traffic lost after network reconfiguration.

A new low-cost concept for a system-on-chip Bluetooth solution is proposed in this paper. The single chip includes all necessary baseband and RF parts to achieve full Bluetooth functionality and is implemented in a standard 0.25-[micro]m CMOS technology. The two-point modulation [Sigma] [Delta] fractional N phase-locked loop achieves a phase noise of -124 dBc/Hz at 3-MHz offset. The sensitivity of the embedded low-IF receiver is measured to be -82 dBm at a bit error rate of 0.1%. The power supply voltages for the digital and analog parts are internally regulated to 2.65 V. The maximum current consumption of the analog part is 60 mA. Index Terms--Communication systems, CMOS analog integrated circuits, CMOS digital integrated circuits, RF radio communication, RF receivers, RF transmitters, sigma-delta modulation, wireless LAN.

Characteristics of wideband indoor radio channel at 5.3 GHz were defined based on an extensive measurement campaign using a wideband channel sounder with 19 ns delay resolution. Pathloss exponents were 1.3-1.5 in LOS and 2.9-4.8 in non line of sight (NLOS). Large difference in NLOS exponents was due to different dominating propagation mechanisms in different types of building structures. The delay dispersion was characterized by cumulative distribution functions (cdf) of the rms delay spreads, the values for cdf = 0.9 varied from 20 to 180 ns in different setups in office building and large hall environments. The correlation functions of the radio channel in spatial and frequency domains were extracted. Small scale models for five typical indoor scenarios were developed using tapped delay lines. Index Terms--Indoor radio communication, microwave propagation, mobile antennas, mobile communication, wideband radio channel measurements, wideband radio channel modeling, wireless LAN.

Power and bandwidth efficient communication schemes are of major concerns in wireless communication systems. One kind of efficient pseudonoise code is proposed in this paper that provides combined power-bandwidth efficiency. A novel algorithm for verifying the power and bandwidth efficient properties of a pseudonoise code is proposed with systematic analysis on code parameters. Applications of the derived codes had been implemented on 802.11 DS/SS system, which performs 36% bandwidth reduction and 14% power consumption reduction. Several new pseudonoise codes with different code lengths are also proposed. Index Terms--Bandwidth reduction, CMOS implementations, low power design, pseudonoise code, spread-spectrum communication, wireless LAN.

Future mobile communication systems will require new medium access control protocols to attain an efficient multiplexing of different traffic sources while guaranteeing their requirements on quality of service. This paper investigates the performance of a modified packet reservation multiple access (PRMA) protocol, called PRMA with hindering states (PRMA-HS), for supporting voice and data transmissions in mobile cellular systems with high propagation delays. A scenario based on low earth orbit mobile satellite systems (LEO-MSSs) has been considered, but the analytical approach is general. We have obtained that PRMA-HS achieves a high capacity of voice sources in LEO-MSSs also in the presence of data traffic. A performance analysis has been carried out showing good agreement with simulation results. Finally, the PRMA-HS performance has been evaluated in the presence of different data traffic sources. Index Terms--Medium access control protocol, mobile satellite systems, packet reservation multiple access.

This paper provides network designers and operators with simple guidelines on traffic measurements and efficiency evaluation of various channel allocation schemes in digital mobile telecommunications networks. The paper evaluates the efficiency obtained by implementing the following channel allocation schemes: 1) fixed uniform channel allocation (FUCA); 2) fixed nonuniform channel allocation (FNCA); 3) dynamic channel allocation (DCA) where the number of frequency carriers is adaptive and dependent on the load; and 4) dynamic frequency/time channel allocation (DFTCA) (a new scheme which is the most efficient) where the number of channels is adaptive (based on the load), allowing two channels of the same frequency carrier to be used in two neighboring cells. The analysis is based on standard queuing models under the following assumptions: 1) Poisson call arrivals in each cell; 2) exponential call holding time; 3) exponential mobile travel time; and 4) exponential sojourn time of a mobile in a cell. Numerical results are presented to provide insight into accuracy of the models and efficiency gain by dynamic frequency time channel allocation under different traffic conditions (including conditions related to highway traffic).

This paper discusses connectivity between neuromorphic chips, which use the timing of fixed-height fixed-width pulses to encode information. Address-events ([log.sub.2] (N)-bit packets that uniquely identify one of N neurons) are used to transmit these pulses in real time on a random-access time-multiplexed communication channel. Activity is assumed to consist of neuronal ensembles--spikes clustered in space and in time. This paper quantifies tradeoffs faced in allocating bandwidth, granting access, and queuing, as well as throughput requirements, and concludes that an arbitered channel design is the best choice. The arbitered channel is implemented with a formal design methodology for asynchronous digital VLSI CMOS systems, after introducing the reader to this top-down synthesis technique. Following the evolution of three generations of designs, it is shown how the overhead of arbitrating, and encoding and decoding, can be reduced in area (from N to [radical] N) by organizing neurons into rows and columns, and reduced in time (from [log.sub.2] (N) to 2) by exploiting locality in the arbiter tree and in the row-column architecture, and clustered activity. Throughput is boosted by pipelining and by reading spikes in parallel. Simple techniques that reduce crosstalk in these mixed analog-digital systems are described. Index Terms--Asynchronous logic synthesis, interchip communication, spiking neurons, virtual wiring.

The capture effects for a wireless local area network system in the presence of multipath, shadowing and near-far effects have been examined. The performance of carrier-sense multiple-access/collision-avoidance (CSMA/CA) protocols with power capture, operating on channel impaired by Rayleigh fading, lognormal shadowing and near-far effects were analyzed. The throughput and packet delay in the three types of CSMA/CA protocols: the basic, SW ARQ and 4-WH CSMA/CA, were investigated.

Synchronization of concurrent activities by means of semaphores is widely adopted in industrial distributed systems. This paper presents some mechanisms which facilitate the efficient implementation of the popular semaphore concept in real-time networks based on the controller area network medium access technique. The advantages obtained by adopting the proposed solution are twofold. First, better performance can be obtained because most of the semaphore management mechanisms are moved from the application to the physical and data-link communication layers. Second, a fully distributed management scheme is implemented, and there is no need to place the semaphore control structures on a particular node in the network. Index Terms - Controller area network, fieldbuses, industrial LAN's, process synchronization, semaphores.