摘要:

A description, pertinent to the HF communications engineer, of the high latitude ionosphere is given. The impact of this disturbed geophysical environment on communications systems is described, and some frequency management techniques to avoid potential system outages are outlined. Reference is made to long term ionospheric predictions and short term ionospheric forecasts.

DOI:10.1049/ip-i-2.1989.0001

出版商:IEE    

年代:1989

数据来源: IET

摘要:

The paper is a review of techniques for the transmission of medium-speed data over ionospheric channels. Typical applications are digital voice and high integrity data transmission using normal voice bandwidth circuits. The characteristics of the fading multipath propagation channel are described, as well as the relevant radio equipment limitations. An historical overview of modem designs is included from the early multitone designs to the present day. The two basic design approaches of multiple low-speed channels and a single high symbol rate channel with adaptive processing are described. The impact of new digital signal processing technology on modem implementation is noted. The performance of the different designs is compared, both on theoretical grounds and from the results of practical measurements. It is concluded that the single tone approach generally offers superior performance in terms of lower bit error rates, but the choice is highly dependent on the application.

DOI:10.1049/ip-i-2.1989.0002

出版商:IEE    

年代:1989

数据来源: IET

摘要:

A new modulation scheme for AM sound broadcasting is being studied which allows an additional data signal to be transmitted without significant envelope distortion or increase in bandwidth. The data signals carry information that can be used by specially-equipped receivers for various display and control functions. The proposed radio-data system concerns services for medium or high frequency broadcasting where AM predominates. A real-time implementation of the proposed system using a digital signal processor (TMS 320) is described, and experimental results for sinusoidal envelope modulation are presented. Measurements of the spectrum of the resulting signal show more than 40 dB rejection of out-of-band components, relative to the carrier.

DOI:10.1049/ip-i-2.1989.0003

出版商:IEE    

年代:1989

数据来源: IET

摘要:

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.

DOI:10.1049/ip-i-2.1989.0004

出版商:IEE    

年代:1989

数据来源: IET

摘要:

A method of time synchronisation has recently been developed for handling the combination of multipath propagation and fading experienced over HF radio links. The received demodulated signal is sampled here at a rate close to the Nyquist rate. The phase of the sampling instants does not critically affect the performance of the system, as long as it does not vary too rapidly with time; thus it is only necessary to control the sampling rate. Computer-simulation tests have been carried out with the given system on a 9600 bit/s serial data-transmission system. A 16-level QAM signal was transmitted over a model of an HF radio link, with two independent Rayleigh-fading sky waves, and satisfactory time synchronisation was achieved. Unfortunately it has been found that, under very severe fading conditions with three independent Rayleigh-fading sky waves, satisfactory operation is not always obtained with the time synchronisation algorithm. The paper describes various modifications that have been developed for improving the performance of the algorithm, one of these being a fundamental change in the algorithm itself. The paper then presents the results of computersimulation tests, comparing the performances of the original and modified algorithms, and uses these results to determine the most promising system.

DOI:10.1049/ip-i-2.1989.0005

出版商:IEE    

年代:1989

数据来源: IET

摘要:

Owing to the time-varying nature of the HF channel, error correcting codes of a fixed rate provide unnecessary correction power for much of the time, with consequently low data rates. The purpose of an adaptive coding scheme is to permit only the necessary degree of error correction to be applied to the transmitted information according to the channel conditions. Such a coding scheme, based on product codes, is introduced, and the improvement, compared to similar fixed rate codes, is demonstrated. A method of improving the code rate further is described, and the results of this and an interleaving procedure are also given.

DOI:10.1049/ip-i-2.1989.0006

出版商:IEE    

年代:1989

数据来源: IET

摘要:

The paper presents a differential detection scheme for noncoherent detection of digital FM signals received over bandwidth restricted channels. The new differential detection scheme observes the phase of the FM signal at centres of signalling intervals to derive the differential phase. Differential encoding of transmitted data is required for preventing error propagation. Numerical results show that the new differential detector yields a superior performance in narrowband channels where conventional differential detection becomes unusable. Error probabilities and optimum BT values are presented assuming that bandlimiting is caused solely by IF filtering.

DOI:10.1049/ip-i-2.1989.0007

出版商:IEE    

年代:1989

数据来源: IET

摘要:

Different interleaving procedures for high speed digital transmission are studied. The effect of multiplexing on the line signal is considered. Special attention is paid to digital sum variation and runlength properties, and also to spectral content for timing recovery. Multiplexing/demultiplexing synchronisation procedures have been examined. Emphasis has been placed on 5B6B, although some results can readily be extended to other block line codes of the familymBnB.

DOI:10.1049/ip-i-2.1989.0008

出版商:IEE    

年代:1989

数据来源: IET

摘要:

To meet increasing demand for mobile communications with a fixed spectrum and at reasonable cost, directional antennas have been used in cellular mobile radio systems. This paper reviews sectorised patterns that have been proposed so far, and then describes how to achieve high capacity systems by taking a new approach to the design of cellular patterns. Some novel patterns, with less than or equal to two hexagons per cluster are described. Performances are assessed in terms of the first decile carrier-to-cochannel interference (C/Ic) and traffic density for a given blocking probability. Cell site efficiency is also considered.

DOI:10.1049/ip-i-2.1989.0009

出版商:IEE    

年代:1989

数据来源: IET

摘要:

Multiple-access capability of a frequency-hopped spread-spectrum communication system employing the concatenated diversity/Reed-Solomon code is investigated. We present general asymptotic expressions for the achievable region and the normalised throughput. For the case of finite block length codes, a fairly accurate approximation to the achievable region, subjected to the constraint that the packet error on a given link is no greater thanP∩E, has been derived. Both side information and no side information cases are considered.

DOI:10.1049/ip-i-2.1989.0010

出版商:IEE    

年代:1989

数据来源: IET