HF Radio Channel Research Articles

Letter: Frame alignment design for HF receivers

AbstractIn this letter the feasibility of the application of a frame alignment system based on a CFAR detection scheme over HF radio channels is investigated. The system performance is analyzed in terms of average holding time, average time for the detection of synchronization loss and average synchronization recovery time. The interdependence among the various parameters of the frame alignment scheme is investigated on a simulation basis.

Improved HF modem using convolutional coding

The paper describes the design of a digitally implemented modem for use over HF (2-30 MHz) radio channels. As a completely digital approach offers the prospect of a number of significant improvements, a new digital multifrequency-shift-keying (MFSK) modem has been developed that employs a novel digital processing procedure termed code-assisted bit synchronisation (CABS). Signal detection is achieved via a set of noncoherent correlators, whilst symbol synchronisation and error correction are performed using a soft-decision Viterbi decoder. Practical tests of modem show that it operates effectively under both additive white Gaussian noise and real HF channel conditions.

Speech processing techniques for HF radio security

The paper presents a review of the techniques used to date to provide secure duplex or half-duplex conversational speech communications over HF radio channels, and gives an assessment of likely future developments. The paper begins with a brief discussion of the mechanisms of speech production and hearing, with particular reference to the factors which are important for communications over the difficult HF medium. Various methods for speech scrambling based on time and frequency domain permutations are then described, and the effects of HF channel distortions on their speech quality performance is assessed. Digital vocoding techniques based on the long established channel vocoder and the more recent linear predictive coder are described, and their performance advantages over scrambling techniques are discussed. The importance of the type of modem employed in conjunction with vocoders is emphasised, particularly with respect to the techniques used to overcome the effects of fading and cochannel interference. In the future, continuing developments in semiconductor technology will have a significant impact on the amount of processing which can be performed within moderately priced equipment. The paper is concluded with some prognostications as to how the techniques used to provide narrowband secure speech will change to yield improved performance, both in terms of the speech quality and the reliability of obtaining communications over HF channels.

Wideband probing of the transauroral HF channel: Solar minimum

The Naval Research Laboratory HF Channel Prober is a high‐resolution, computer‐controlled, coded‐pulse sounder designed to study the time variable behavior of the wideband HF radio channel. Multispectral time histories of the channel pulse response and the channel scattering function are tools used in the study. Pulse response delay resolution down to 1 μs and unambiguous Doppler up to ±30 Hz are possible, depending on the choice of experiment format and operating parameters. Measurements were made quarterly between October 1985 and July 1986 on a 2300‐km transauroral path between a transmitter site at Frobisher Bay, Canadian Northwest Territories, and a receiver site near Rome, New York. Data are presented in the form of ionograms, pulse response time histories (power and coherent video), and channel scattering functions. Data for mid‐latitude and transauroral paths are compared, and their similarities and differences are discussed. The mid‐latitude channel is multimodal but is usually characterized by a single specular return per mode. The character of transauroral channel is quite variable, depending strongly on the degree of magnetic disturbance, the location of the midpath reflection point relative to the position of the auroral oval, and the mode. During quiet magnetic conditions, and with the path midpoint well below the auroral oval, channel behavior is shown to resemble a mid‐latitude channel at certain times and a specular multipath channel at other times (i.e., one where each mode comprises several distinct multipath components). Doppler spread in these cases tends to be small. At night, when the midpath point lies within the auroral oval, the return signal is diffuse, and the Doppler spread appears to be larger by an order of magnitude.

New gradient decision feedback equaliser algorithm

An efficient algorithm for a least-squares gradient lattice decision feedback equaliser is described. Simulation results for telephone channel and HF radio channel models are presented. Performance is compared with other decision feedback algorithms.

High‐resolution probing of the HF ionospheric skywave channel: F2 layer results

A computer‐controlled, coded‐pulse, step frequency, oblique sounder has been designed to probe the HF radio communication channel to a bandwidth of 1 MHz. The instrument monitors the complex pulse response of the channel over an extended period of time thus permitting simultaneous observation of the time and frequency behavior of the channel. A coherent time history presentation of the wide band data permits unequivocal identification of individual modes in situations involving several partially overlapping modes. Channel instantaneous bandwidth and mode Doppler shift can be estimated by inspection of the pulse response time history curves. Data presentation in the wide band time history format is also very informative in the description of signal fluctuation phenomena and in understanding their underlying causes. Measurements have been made over a short‐baseline (126 km), mid‐latitude path between an island off the southern California coast and the mainland. The response of the one‐hop F2 layer sky wave mode to a 1‐μs probing pulse is used to illustrate the unique capabilities of this instrument and the unusual effects that are observed with a high‐resolution probing pulse.

Comparison of DFE and MLSE Receiver Performance on HF Channels

Data communication at rates near or above 2 kbits/s on 3 kHz-baadwidth HF radio channels is subject to impairment from severe linear dispersion, rapid channel time variation, and severe fading. In this investigation, recorded 2.4 kbit/s QPSK signals received from HF channels were processed to extract a time-varying estimate of the channel impulse response. From the estimated channel impulse responses, performance-related parameters were computed for ideal matched filter reception, maximum-likelihood sequence-estimation (MLSE), and decision feedback equalization (DFE). The results indicated that the simpler DFE receiver suffered only a small theoretical performance degradation relative to the more complex MLSE receiver. Other HF channel impulse response statistics were also obtained to shed light on equalization and filter adaptation techniques.

Remote Medical Diagnosis System (RMDS) concept.

The goal of the Remote Medical Diagnosis System (RMDS) is to provide assistance to medical personnel on navy ships and at remote locations by providing an adequate consulting link with a physician at some other site. The RMDS utilizes navy communication links and commercial telephone lines for the exchange of patient physiological information and diagnostic/treatment advice. Narrow band-width (3 kHz) hf and uhf radio and satellite communication channels are utilized. The RMDS terminals perform the function of processing analogue physiological information (images, electrocardiograms, (ECG), auscultation, and voice) into a digital format suitable for transmission over the communication links. These terminals also perform the inverse operation of processing the received digital bit streams into analogue information for visual display. The digital data are transmitted at a rate of 2,400 bps.

Performance of selected block and convolutional codes on a fading HF channel

This paper describes the application of selected block and convolutional coding techniques to digital transmission over an HF radio channel. Performance results are presented for interleaved binary Bose-Chaudhuri-Hocquenghem (BCH) codes, interleaved two-stage concatenated codes, and diffuse convolutional codes. The performance results are based upon raw error data recorded in transmission over a 640-km HF path, with various data runs representing the typical effects of frequency-selective and non-selective fading, atmospheric impulse noise, and interference from other users of the HF band. The performance of binary BCH codes with bit interleaving is presented and the selection of a code to meet a specified performance criterion is described. The sensitivity of specific codes to changing channel conditions is presented, along with the cost, in terms of interleaving, of designing for one set of channel conditions rather than others. Concatenated codes are discussed as a means of effective error control on channels with clustered errors. Performance data are presented for selected two-stage codes used with inner-stage word interleaving and the effects of varying certain of the code parameters are shown. Emphasis is placed on code designs that use the inner-stage code only for error detection and the outer-stage code mainly for erasure filling with only a small amount of symbol error correction. The impact of error clustering on the performance of concatenated codes is discussed. Two diffuse convolutional codes are evaluated with the use of recorded error data in computer simulations of their threshold-decoding algorithms. Performance of various configurations of these convolutional codes is given and a comparison is made with that of a simple block coding scheme having the same decoder storage requirements. The various coding techniques studied are compared on the basis of achievable performance as well as certain factors affecting the feasibility of implementation.

Convolutional coding for channels with memory

This paper discusses the use of two types of convolutional codes, diffuse threshold-decoded codes and Gallager codes, on channels with memory (burst channels). The operation of these codes is explained and test results are given for a variety of equipments operated over phone line, HF radio, and troposcatter channels. Error propagation in the threshold-decoded codes is discussed and, in the Appendix, we prove that, for one important diffuse code, propagation is finite and small.