Anthony Acampora

Multihop lightwave networks: a new approach to achieve terabit capabilities

A multihop approach for achieving concurrency in distributed lightwave networks with hundreds or thousands of Gb/s throughput, even while the users are limited to rates of 1 Gb/s or lower, is described. Multihop networks avoid two serious drawbacks of standard multichannel approaches: the requirement of wavelength-agile transmitters or receivers, and pretransmission coordination between users wishing to communicate. Although transmitting a packet from one user to another may require routing the packet through intermediate network interface units, the approachs connectivity is specifically designed to achieve efficient use of the channel bandwidth, allow modular growth of the network from small to large configurations, and provide a degree of network reliability.<<ETX>>

A Wireless Network for Wide-Band Indoor Communications

We propose and analyze a wide-band indoor communication system that uses radio as the transmission medium either on a stand-alone basis or to supplement a hard-wired network for those situations where complete portability is desired. One principal impairment to such a system is intersymbol interference caused by frequencyselective fading. A novel media-access scheme is proposed which permits the use of resource sharing, wherein a small pool of time slots is effectively shared among all users to provide added protection against channel impairments on an as-needed basis. Our results show that the use of resource sharing and diversity provide excellent protection against intersymbol interference caused by frequency-selective fading with negligible impact on throughput. Furthermore, resource sharing plus diversity can permit significantly higher data rates without large queueing delays. For example, a wireless network with a 10 Mbit/s data rate in a 10 MHz bandwidth using four antennas at the base station has a less than 10-4outage probability at a 10-4BER in buildings with less than 58 ns rms delay spread. A loading of 75 percent is permitted for a queueing delay of less than 20 packet transmission times all but 0.01 percent of the time.

Fundamental Conditions Governing TDM Switching Assignments in Terrestrial and Satellite Networks

We consider the traffic handling capability of a new switching architecture which generalizes upon the structure of a traditional time multiplex switching system. For the traditional approach, lowbandwidth end users are formed into groups, each of which shares a single high bandwidth time-division multiplexed (TDM) line into the central time multiplex switch. Each user synchronously generates packets of data in preassigned time slots at a rate consistent with its offered traffic, and the feeder for the TDM line serving a group of users merely time multiplexes the arriving packets prior to routing by the central switch; analogously, each output port of the switch feeds a demultiplexer which routes the packets to the appropriate user within its group. The generalized approach permits each user group to share some multitude of TDM lines interconnecting that group with the central switch, and the group multiplexers and demultiplexers are replaced by switches which route packets from users to TDM lines (and vice versa). For this structure, we derive a set of necessary and sufficient conditions on the user-touser offered traffic such that a valid, nonconflicting TDM assignment of packets-to-time slots exists. These conditions reveal that the constraints imposed by the three-tiered switching hierarchy do not limit the useable capacity of the switch. Consequently, with no loss of traffic bearing efficiency, it is possible to reduce the number of multiplexers used to serve the end-user population, achieve greater trunking efficiency since small user groups served by one TDM line are replaced by larger groups serving multiple lines, and modularly grow the system by adding TDM lines to each group commensurate with the traffic offered by that group. As a byproduct, it is shown that such a system designed to switch low-speed circuits of some particular data rate can, with no hardware change, switch circuits at lower rates (subrate switching). These conclusions have important ramifications for the design of terrestrial and satellite-based switching systems.

Analog Viterbi Decoding for High Speed Digital Satellite Channels

Analog means for implementing the Viterbi decoding algorithm at high data rates are presented. One approach employs sample-and-hold circuits and voltage adders to store and update the path metrics based upon maximum-likelihood decisions. Experimental results obtained from a breadboard realization of such a decoder are reported. An alternate approach employing tapped delay lines to store the analog channel waveform is also described. Analytical results pertaining to each implementation are presented.

The Use of Resource Sharing and Coding to Increase the Capacity of Digital Satellites

Resource sharing is a technique which can improve the circuit availability of digital satellites operating at frequencies above 10 GHz, without requiring large fade margins. In this paper, the resource sharing concept is generalized by fully exploiting the available clear-air carrier-to-noise ratio of the satellite link to achieve very high transmission capacity while maintaining low rain outage. During clear-air conditions, convolutional codes with a large channel signaling alphabet are employed to permit a high rate of information transfer. When the fade depth exceeds the built-in fade margin, the signaling alphabet is reduced and enough time slots are borrowed from a resource sharing reserved pool to maintain the data rate at the fade site. It is shown that this approach greatly diminishes the impact of rain attenuation on satellite communications. Effective utilized capacities exceeding 85 percent of that possible if it never rains are feasible, and the increase in capacity compared to a system not using resourcesharing protection is typically a factor of 3-10.

Three-dimensional ultrasonic vision for robotic applications

Considers a vision system that uses the echo of a transmitted ultrasonic pulse as the basis for identifying objects. The return of a single pulse from an object generates a three-dimensional acoustical characteristic signature across the aperture of a receiving antenna which can be directly used for object identification. Thus, there is no need to reproduce an accurate visual representation to recognize which of a class of objects is present. Since the signal bandwidth is narrow relative to that of an optical imaging system, faster execution should be possible. The need to remove clutter which can arise in a conventional imaging system, caused by the compression of three dimensions into two, is totally avoided. The authors calculate a fundamental limit (in the Shannon sense) on the number of objects that can be distinguished by an ultrasonic system and show that, for modest signal-to-noise ratios and an object space comparable to the beamwidth, an enormous number of different objects can still, in principle, be resolved. >

A Satellite System with Limited-Scan Spot Beams

A satellite system with several independent spot beams is considered, wherein each spot beam can be scanned over a limited portion of some large service region. The spectral band is totally reused in each spot beam, thereby providing high system capacity. Such a system combines the high antenna gain advantages of a fixed beam system while providing access to the satellite from any point within the overall service region. Moreover, the transmission resources of the satellite can be dynamically allocated to match time-varying changes of the possibly nonuniform terrestrial traffic patterns. The limited-scan antenna forms multiple spot beams, each of which scans only in one dimension; by alternating polarizations among scan strips, high isolation between beams is guaranteed. We show that nearly equal volumes of traffic can be served by each limited scan beam by proper orientation of the scan regions, and complete interconnectivity among all points in all scan regions is achieved easily. A practical antenna design to implement this concept is presented.

Maximum-Likelihood Decoding of Binary Convolutional Codes on Band-Limited Satellite Channels

Viterbi decoding of binary convolutional codes on bandlimited channels exhibiting intersymbol interference is considered, and a maximum likelihood sequence estimator algorithm is derived. This algorithm might be applied either to increase the allowable data rate for a fixed power transmitter or to reduce the required power for a fixed data rate. Upper and lower bounds on the bit error rate performance of several codes are found for selected values of the ratio of information rate to channel bandwidth, and results are compared against both conventional equalization techniques and the Shannon capacity limit. Results indicate that this algorithm can provide the power saving associated with low rate (highly redundant) codes without suffering the noise enhancement of linear equalization techniques.

Digital error rate performance of active phased array satellite systems

A model for a multiple spot beam digital satellite employing an active phased array antenna with frequency reuse is proposed and applied to study the power limited down-link bit error rate performance. Since the power amplifier preceding each antenna element is generally nonlinear, intermodulation distortion is produced among the various spot beams, degrading performance. Expressions for the bit error rate are derived, averaged over an ensemble of random arrays. Typical results indicate a degradation of 1-5 dB relative to a horn-reflector antenna of the same total effective aperture with a single channel per transponder, as the number of cochannel signals and the number of antenna elements are respectively varied between 10 and 30, and 500 and 10 000. The degradation is shown to result from nonlinearly induced signal suppression and cochannel interference.

Baseband Processing in a High-Speed Burst Modem for a Satellite-Switched TDMA System

We present block diagram designs for the baseband processing modules required to implement high-speed (600 Mbit/s) burst modems for use in TDMA satellite systems. Our main concern is to obtain designs which can be implemented with a minimum of high-speed (ECL) devices so as to reduce design and hardware costs. A major consideration in the modem operation is to accurately synchronize it to a reference frame-marker broadcast by a master earth station. Since frame mark recovery must be done at the full serial bit rate, the proposed design relies on the periodic nature of the frame-marker occurrences to enable utilization of short unique words and simple highSpeed electronics to provide extremely reliable marker recovery with virtually no false alarms (or missing markers), even on very noisy channels. Estimates of false alarm rates and the mean time to lock are given. Considerations of clock stability indicate that short-term (150 ms) instabilities must be on the order of a part in 107in order to maintain proper operation of the synchronization circuitry; such stability also minimizes guard time requirements between bursts from the several ground stations sharing a given transponder.