摘要:

The paper examines the possibility of private mobile radio users sharing the RF spectrum with the television broadcast channels on a mutual noninterference basis, with the aim of accommodating greater spectrum efficiency. A comparison is made between the use of conventional narrowband modulation methods, such as AM, FM and SSB, and the broadband direct-sequence spread-spectrum modulation schemes as contenders for a bandshared system. These comparisons are made using the criteria for establishing imperceptible visual interference on the TV screen for both modulation schemes. The results show that, for a base station sited in Central London with a service area typically that of the Greater London region, 16 MHz of bandwidth may be gained by the use of bandsharing, with either narrowband or spread-spectrum modulation schemes, for guaranteed interference-free TV reception. The paper also shows that, for the same base-station site covering the same area, a further 72 MHz of bandwidth could be opened up by the use of direct-sequence spread-spectrum transmissions, which would otherwise be unavailable for narrowband use. Such a system would also have the advantage that it could be made covert to eavesdroppers using conventional AM or FM receivers with only limited modification to existing transmitters and receivers, It is argued that spread-spectrum techniques can open up significant amounts of bandwidth, by bandsharing, not available to conventional modulation schemes in many urban environments where the normal TV-channel allocations are almost fully utilised, as they appear to be in the Greater London area.

DOI:10.1049/ip-f-1.1981.0043

出版商:IEE    

年代:1981

数据来源: IET

摘要:

The ratio of the average target-signal power to the average unwanted-signal power at the input of a radar detection receiver can be significantly increased by applying the method of clutter/interference suppression described in the paper. The polarisation state of the emissions, the object-backscatter field, and the interference field are all taken into account in a technique referred to as ‘virtual adaptation’ of the polarisation state of the transmit and receive antennas. This technique can be used to complement filtering techniques in the frequency and space domains. Attention is specifically drawn to the potential advantages of multinotch polarisation suppression filters of the ‘logic-product’ type, which might have applications in cases where there are uncertainties regarding the polarisation state of unwanted signals that are to be suppressed.

DOI:10.1049/ip-f-1.1981.0044

出版商:IEE    

年代:1981

数据来源: IET

摘要:

A new scheme for the adjustment of adaptive equaliser tap gains is developed. In the scheme the short-term averages of the stochastic gradients are used to reduce the self-noise arising from the application of noisy gradient estimates. The new algorithm permits the application of larger step sizes, and therefore results in faster convergence. Some theoretical results are presented and compared with results obtained by computer simulation.

DOI:10.1049/ip-f-1.1981.0045

出版商:IEE    

年代:1981

数据来源: IET

摘要:

Ground-wave propagation over a curved earth with a troposphere whose refractive index varies with height is considered. Series expansions are derived whose first term is the Sommerfeld flat-earth formula. These expansions generalise previous results by the inclusion of the effects of the inhomogeneous refractive-index variation. The methods described are induded in the ground-wave-propagation program GRWAVE.

DOI:10.1049/ip-f-1.1981.0046

出版商:IEE    

年代:1981

数据来源: IET

摘要:

Ground-wave propagation over a curved earth with a troposphere whose refractive index varies with height is considered with application to the CCIR reference exponential atmosphere. The residue-series and geometrical-optics methods are described. A ground-wave-propagation program GRWAVE has been written, and new propagation curves for frequencies from 10kHz to 30MHz are given. An interpretation in terms of an effective earth's radius is supplied.

DOI:10.1049/ip-f-1.1981.0047

出版商:IEE    

年代:1981

数据来源: IET

摘要:

A new adaptive equaliser for the minimisation of mean-square error has been developed for use over a wide range of transmission channels. The equaliser comprises a transversal filter (with variable tap gains) and a processor. Using the processor, the tap-gain-adjustment information is derived from blocks of samples taken from the channel output. The processor operates independently of the channel speed and can provide variable step size for adjusting the tap gains of the transversal filter and flexibility in the implementation of the equaliser in terms of the channel speed and the equaliser realisation. Analytical and computer-simulation results are presented showing the equaliser performance.

DOI:10.1049/ip-f-1.1981.0048

出版商:IEE    

年代:1981

数据来源: IET

摘要:

Sequential and unsequential algorithms for the radar detection of border penetration are investigated. Operating characteristics and energy requirements are determined for both algorithms. The sequential approach is shown to provide a substantial reduction in energy requirements in comparison with the unsequential approach. This improvement depends on the position of the target and the required quality of the algorithm.

DOI:10.1049/ip-f-1.1981.0049

出版商:IEE    

年代:1981

数据来源: IET

摘要:

The concept and some experimental results of the phased-array system ELRA are presented, with emphasis on signal processing for target search. Multiple beams together with a variable number of pulses in each beam position are used for sequential detection and also for search in clutter by applying a recursive filter against stationary target echoes. New considerations concerning the practical application of sequential detection for multiple range elements, even in cluttered areas, are discussed.

DOI:10.1049/ip-f-1.1981.0050

出版商:IEE    

年代:1981

数据来源: IET

摘要:

Frequency-modulated quasisynchronous (QS) mobile radio systems are of benefit to PMR users already committed to FM in the solution of radio coverage problems. The paper analyses the performance of a conventional FM receiver in the overlap area of two QS transmitters and gives graphs quantifying this performance as a function of system parameters. The paper suggests design criteria for a practical system and shows, on the basis of speech-intelligibility tests, that satisfactory performance of a QS FM system is possible throughout the coverage area, except at those pockets where the two received carriers have approximately equal strength. Here, a receiver modification is proposed which can result in marked improvement in the quality of reception.

DOI:10.1049/ip-f-1.1981.0051

出版商:IEE    

年代:1981

数据来源: IET

摘要:

A suite of computer programs has been developed to calculate the power spectral density of a wide range of transmission codes. Suitable user interfaces are provided for the automatic construction of the state diagram, the encode function and the codework matrices. The output includes the attractive analytical formula in closed-cosine form, the autocorrelation function and plots, in colour, for Dirac delta and raised-cosine pulses. The paper presents results on the analysis of zero-mean-disparity block codes, general binary/tertiary codes with multiaphabets and an adaptive code based on the Howells-Woodman Miller codes.

DOI:10.1049/ip-f-1.1981.0052

出版商:IEE    

年代:1981

数据来源: IET