摘要:

The basic characteristics of radio paths relevant to mobile radio communication at UHF are reviewed. Data gathered in a comprehensive survey of urban and suburban Melbourne were analysed to test for consistency with several simple models: Rayleigh envelope, a power spectrum model, log-normal distribution of mean, and the effects of distance and slope of terrain. Factors limiting the validity of some of the models are discussed. Other factors investigated were repeatability under varying weather conditions, the averaging required for reliable representation of local field strengths, and distributions associated with slow and fast fading patterns. A study of the applicability of space diversity showed that a saving of at least 75% of the transmitter power, for a given grade of service, could be made if diversity were used. Because of the number and complexity of the factors influencing the propagation to a vehicle, and because weather and time of day were not found to be significant influences, it was concluded that experimental assessment is a cost-effective method for determining accurately a service area. Examples of suitable data gathering phase are given.

DOI:10.1049/ip-f-1.1985.0067

出版商:IEE    

年代:1985

数据来源: IET

摘要:

In mobile radio services, an efficient utilisation of the frequency spectrum can be achieved by the small-cell concept. However, small cells lead to high hand-over rates of the radio link between the mobile and the different base stations in the vicinity of the cell boundary. The absolute field-strength levels and the differnces between them when received from various base stations are regarded as ‘hand-over criteria’. In the UHF range, however, the field-strength level is subject to large variations, even in flat terrain, owing to varying land usage. The Deutsche Bundespost has performed mobile radio measurements at 450 MHz in an area situated to the south of Frankfurt, about half-way between two base stations. The land usage in this area situated to towns, forests and open fields are mixed. The field-strength levels were recorded along all main roads in this region, having a total length of about 100 km. The measurements and statistical evaluation of more than 10 million samples are described in detail. The influence of the mixed land usage and the hand-over criteria is discussed with the aid of a 3 km drive.

DOI:10.1049/ip-f-1.1985.0068

出版商:IEE    

年代:1985

数据来源: IET

摘要:

A local wideband statistical model is presented for UHF mobile multipath propagation. The complex echo-envelope statistics have been derived from 10 MHz wideband pulse soundings at 436 MHz over a large area in suburban and urban localities. In the proposed model, the effects of Doppler-induced RF phase shifts have been considered. Echo paths are shown to fit a Poisson distribution for the excess time delays, while the echo envelopes conform to Rayleigh fading statistics. Correlated scattering, evident from echo path clusters in time-delay and echo-amplitude correlation coefficients in neighbouring path delay bins, is shown to be significant for excess path time delays less than 0.3 μs. Disagreements observed for local and nonlocal scattering in different localities are discussed. The implementation of a wideband multipath simulator based on the statistical model is described. Digital computer simulation of DPSK is used as an illustrative example to demonstrate its usefulness.

DOI:10.1049/ip-f-1.1985.0070

出版商:IEE    

年代:1985

数据来源: IET

摘要:

Predictions of bit error rate caused by a combination of impulsive and Gaussian noise are made for data communication systems using PSK, DPSK and FSK, with direct carrier modulation. The technique uses the amplitude probability distribution (APD) to characterise the noise at the input of the receiver demodulator. A direct numerical approach (with numerical integration, where appropriate) is used to produce predictions for the case where decisions are made by sampling the demodulated signal at times nominally at the centre of the received data symbols, there being a minimum amount of postdetector filtering. Error rates in mobile data communication systems have been measured at roadside locations and in the laboratory. In the latter case, an impulsive noise simulator capable of producing noise with specified statistical characteristics was used. Laboratory measurements have also been made with fading signals, and the prediction techniques have been extended to cover this case. Generally, agreement between theory and experiment is good, and it is concluded that the techniques ar applicable in any system where channel separations are well defined and, consequently, where IF filters are adequately standardised.

DOI:10.1049/ip-f-1.1985.0071

出版商:IEE    

年代:1985

数据来源: IET

摘要:

It is proposed to test the performance of mobile radio equipment by use of a fading simulator. This procedure has the advantages that the measurements can be made in laboratories, and that the random fading is reproducible in its statistical charcter. It is essential, however, to compare the results of fading simulation with field measurements. The fading simulator used is non-frequency-selective, and produces the two main random processes, Rayleigh and log-normal superimposed. The mobile measurements were made near Darmstadt, F.R.Germany. In most cases the results corresponded well with the simulation. The expected dependences of the signal/noise ratio and speed of the mobile on the bit error rates(BERs) were confirmed. Some measurements, however, revealed higher BERs than expected. Moreover, losses of data synchronism occurred. These irregularities are caused by heavy obstructions of the direct radio path and extremely low obstructions of the waves scattered from mountains, which were so far away that the excess delay can be of the order of magnitude of the clock period of the digital signal.

DOI:10.1049/ip-f-1.1985.0072

出版商:IEE    

年代:1985

数据来源: IET

摘要:

The paper surveys the digital modulation methods available for use in land mobile radio systems. The modulation methods are compared on the basis of their relative tolerances to additive white Gaussian noise, and brief consideration is also given to the equipment complexities involved. The emphasis of the paper is on basic techniques and the essential mechanisms by which they operate, with a view to clarifying the factors that contribute to a good tolerance to noise. Prominence is given to constant-envelope signals, such as FSK, PSK and the various developments of these. Consideration is also given to the introduction of correlation or dependence between neighbouring signal elements, by means of partial-response or correlative-coding techniques, or else by means of convolutional codes, these being effective methods of improving the tolerance to additive noise. Coherent, incoherent and noncoherent detectors are discussed, together with threshold-level and maximum-likelihood detection. However, the topics of carrier-phase and element-timing synchronisation are not considered here, nor are anydetails given of the detection processes. Finally, a selection is made of the techniques that are potentially the most cost-effective for land mobile radio systems.

DOI:10.1049/ip-f-1.1985.0073

出版商:IEE    

年代:1985

数据来源: IET

摘要:

The paper describes a simple modification to TTIB/FFSR signal processing which permits coherent demodulation of the received signal. This property is fundamental to the successful operation of some data modems used extensively in mobile radio. The operation of the modified processing is independent of the variable delay introduced by the transmitter and receiver RF/IF circuitry and by the propagation path.

DOI:10.1049/ip-f-1.1985.0074

出版商:IEE    

年代:1985

数据来源: IET

摘要:

A series of field measurements were conducted to examine the transmission characteristics of different digital land mobile systems in the VHF/UHF frequency bands. This paper briefly describes these measurements, and then focuses on the measured data that describe the bipolar coding/FM modulation with a bit rate of 4800 bit/s and an RF channel spacing of 25 kHz. The basic error characteristics of the channel are presented by (i) the average bit error rate, (ii) the gap length distribution and (iii) the error burst length distribution. The measured error pattern was used in conjunction with some known error control techniques, to examine their effectiveness in reducing the overall errors. In particular, the block coding techniques used in the AMPS system were evaluated and compared with other, simpler techniques having higher coding rates. The results are presented in the form of curves relating the packet error rate to the packet length.

DOI:10.1049/ip-f-1.1985.0075

出版商:IEE    

年代:1985

数据来源: IET

摘要:

The essential features of a novel adaptive slow frequency-hopping system are described. It is proposed that the system would operate in the heavily congested VHF private mobile radio bands by selecting currently inactive channels, hence increasing the overall spectral efficiency. A preliminary analysis of mutual interference is presented in full, the results of which indicate that a frequency-hopping system with a viable capacity could coexist with conventional private mobile radio networks.

DOI:10.1049/ip-f-1.1985.0076

出版商:IEE    

年代:1985

数据来源: IET

摘要:

Frequency assignment procedures in mobile radio systems generally aim at achieving a measure of intermodulation compatibility, and, in principle, provided sufficient numbers of channels are available and may be assigned with the minimum of restrictions, then this objective is achievable (at least as far as third-order products are concerned). However, in cellular systems, additional constraints are imposed by the need to multicouple cell centre base station transmitters on to a common an tenna, and this requirement sets a lower limit on the minimum frequency separation at cell centres, which, in turn, makes achievement of third-order compatible frequency plans extremely difficult. The paper examines the relationship between the minimum frequency separation for third-order compatibility and the number of available channels and cell cluster size, in order to identify optimum configurations.

DOI:10.1049/ip-f-1.1985.0077

出版商:IEE    

年代:1985

数据来源: IET