Gary R. Lomp

Effect of limiting the downlink power in CDMA systems with or without forward power control

This paper investigates the effects of limiting the downlink transmit power in cellular CDMA communications systems. The composite signal transmitted in downlink is the sum of all the signals intended for different users. The amplitude of the composite baseband signal depends on the data bits, the spreading waveforms of the users, and the forward power control algorithm, and it determines the transmit power. Two of the important constraints when designing the RF circuits are the average transmit power and the peak transmit power. These two parameters are often specified in terms of the peak-to-average power ratio. One way to reduce the peak-to-average power ratio is to clip the composite baseband signal before modulating it. The analysis in this paper investigates how the signal-to-noise ratio at the remote receiver changes as a result of this clipping. The results show that the peak-to-average power ratio can be made significantly smaller without notable signal-to-noise ratio degradation.

The FAVR meteor burst communication experiment

The authors describe the meteor burst communications experiment performed by SCS Telecom, Martin Marietta Energy Systems, and the 11th Air Force. The experiment consisted of transmitting data and voice over a meteor burst channel between Anchorage and Kotzebue using SCS Telecoms FAVR modem. The FAVR algorithm is described. It is a variable rate modem capable of operating at bit rates up to 92 kbit/s using a 40-kHz bandwidth. Forward error correction, trellis decoding, and nonlinear equalization are used. Simulation and experimental results are shown and compared.<<ETX>>

High data rate meteor-burst communications

Results of an experimental study of the meteor-burst channel which illustrates its time-varying bursty nature are presented. In addition, the authors describe an innovative approach to data transmission over meteor-burst systems, that is, the use of the feedback-adaptive variable-rate (FAVR) system, which allows the maximum amount of information to be transmitted over each meteor-burst channel. For underdense meteor channels the FAVR performance is compared with that of optimum system, i.e. a system capable of changing its bit rate instantaneously to channel conditions so as to maintain a constant SNR in each bit, and with that of a constant optimum-rate/burst system. It is shown that FAVR can result in a throughput increase of 17 times over that of the optimal fixed-rate system.<<ETX>>

Communicating through a constant modulus interferer using multi-user detection

In this paper, we consider techniques for communicating in the presence of interference. In particular, we consider the simple scenario of a BPSK communication system in the presence of a constant modulus (CM) interferer. Using computer simulations and analytical approximations, we compare the performances of four detectors for recovering the communications signal in an additive white Gaussian noise non-fading environment. We first consider a simple sign detector for equiprobable BPSK symbols, ignoring the presence of the interferer. We then study a novel geometric detector, which takes advantage of the CM property of the interferer. We next examine a successive interference cancellation (SIC)-based detector, which first estimates the interferer, then subtracts it from the received observation, and finally decodes the residual using a sign detector. Lastly, we consider the optimal multi-user (MU) detector, which decodes both the communications signal and interferer concurrently. Comparing the bit error rates (BERs) of the four detectors as a function of signal-to-interference ratio (SIR) or its inverse, the jammer-to-signal ratio (JSR), we find that for low to medium JSRs the geometric detector provides the best performance versus complexity trade-off. For large JSRs, we show that the SIC detector is superior, yielding the same performance as the optimal MU detector at reduced complexity. In fact, for large JSRs, the SIC detector in the presence of the interferer performs as well as a conventional BPSK (sign) detector when no interferer is present.

The projection code with trellis coded FM for meteor burst channels

The performance of a high rate projection code used with a trellis coded FM system operating in a meteor burst channel is described. Meteor burst communication has wide ranging applications, usually limited to low bit rates. The system described has been developed for the USAF Alaskan Air Command and can transmit at bit rates up to 90 kbit/s in a 40 kHz bandwidth. The projection code is a geometrical code which yields very large coding gain with relatively low complexity. Results are presented to illustrate the benefits derived by the use of the projection code.<<ETX>>

Meteor burst communications LPI applications

A technique that enhances the performance of meteor-burst communications is described. The technique, the feedback adaptive variable rate (FAVR) system, maintains a feedback channel that allows the transmitted bit rate to mimic the time behavior of the received power so that a constant bit energy is maintained. This results in a constant probability of bit error in each transmitted bit. Experimentally determined meteor-burst channel characteristics and FAVR system simulation results are presented.<<ETX>>

High Data Rate Meteor-Burst Communications

The meteor burst communications channel offers a largely untapped means of communications that can alleviate congestion in many existing communications systems. It also provides some interesting advantages over other beyond-line-of-sight systems, including lower susceptibility to atmospheric perturbations, a limited footprint, and relatively low cost. Past system applications have been quite limited, however, due to the random nature of the meteor burst phenomenon which has resulted in relatively low throughput and lengthy wait-times.This paper presents the results of an experimental study of the meteor burst channel which illustrates its time-varying bursty nature. In addition, the authors describe an innovative approach to data transmission over meteor burst systems, ie, the use of the Feedback Adaptive Variable Rate (FAVR) system which allows the maximum amount of information to be transmitted over each meteor burst channel.For underdense meteor channels the FAVR performance is compared to an optimum s...

Nonbinary forward error correction for military communications

The authors present an alternative nonbinary coding scheme that is an extension of a previously developed binary code, called the yjr projection code. The projection code is characterized by simple implementation and high speed, making it attractive for low-cost error-control systems. Projection codes are capable of efficiently correcting both random and burst errors and are well suited to ARQ operation and incremental parity systems. The basic projection code construction is reviewed, and key features of the nonbinary version are discussed. Performance data and hardware complexity results are presented. Analysis and simulations were used to obtain preferred code structures and determine error rates. The results show that projection codes correct a considerable percentage of errors, exceeding the maximum number of errors that is always corrected.<<ETX>>

Adaptive HF for voice and data

Summary form only given. The authors describe an adaptive HF system for voice and video. The system uses information obtained from a link quality analysis and/or ionospheric soundings to determine the appropriate link and frequency over which to communicate. In addition, the system can be frequency hopped, thereby reducing the probability of being jammed. The improved performance obtained using frequency-agile modulation techniques was measured, and it is shown that the FH (frequency hopping) modem described is capable of achieving low error rates at low signal-to-noise ratio and with narrow bandwidth.<<ETX>>