Showing papers by "Barry G. Evans published in 1992"
TL;DR: The overall analysis of the propagation data indicates that the fading character of the LMS channel is significantly dependent on the surrounding environment, especially at higher path elevation angles.
Abstract: Results from a multiband propagation experiment for a high elevation angle land-mobile satellite (LMS) channel are presented. A small helicopter was used to fly the transmitter module, transmitting frequencies in the L, S, and Ku bands. A vehicle equipped with the corresponding receivers and data acquisition system was followed by the helicopter on the selected routes. The measurement campaign was undertaken in two phases in September 1991 and in April 1992. The results show considerable decrease in signal attenuation when the path elevation angle is high due to reduced shadowing. In general, the attenuation increases with the increasing radio frequency. Foliage density has also been found to influence the fade levels, especially at higher path elevation angles. The overall analysis of the propagation data indicates that the fading character of the LMS channel is significantly dependent on the surrounding environment. >
39 citations
TL;DR: This paper proposes the use of transmission to two satellites separated in azimuth and elevation in order to improve the reliability of throughput and investigates the improvements by building upon the model proposed by Lutz for land mobile satellite communications via one satellite.
Abstract: There are certain data services that could use transmit-only land-mobile satellite communication terminals. Such terminals would not be able to receive acknowledged messages and transmit them onward. Thus it is of interest to study alternative means of improving the system reliability. In this paper we propose the use of transmission to two satellites separated in azimuth and elevation in order to improve the reliability of throughput.
We have investigated the improvements by building upon the model proposed by Lutz for land mobile satellite communications via one satellite, and extended this using measurements of the shadowing coefficient in different areas (suburban, countryside and city) to two separated satellites. The measured data is used to produce an empirical model of the equivalent shadowing coefficient variations with the separation angle variation between two satellites.
The results show good agreement with the shadowing coefficients produced by the Lutz model for a single satellite. The degree of improvement produced by using two separated satellites in the different environments, in terms of bit rate and probability of the received power is given.
22 citations
01 Nov 1992
TL;DR: A design approach is introduced for efficient structures based on the minimisation of the total number of delay units, and a reduction by a factor of L on the number of data stores can be achieved.
Abstract: Recent developments on multirate system design show that a rational (L/M) sampling rate conversion system can be reconfigured into a three-dimensional polyphase structure. However, the delay requirements associated with each subfilter path in such a structure have not sufficiently been considered. The paper presents a detailed analysis of the delay requirements, and shows that a reduction by a factor of L on the number of data stores can be achieved. A design approach is introduced for efficient structures based on the minimisation of the total number of delay units. Two transformation rules are derived which directly lead to a homogeneous system configuration. This approach is general and simple so that any rational sampling rate conversion system can be specified by a set of parameters calculated by a small computer program. A hardware implementation is also presented, which demonstrates the usefulness of the present approach. >
20 citations
22 Mar 1992
TL;DR: In this paper, the authors proposed a non-geostatiobery orbit satellite appearing at high elevation angles (>500) are likely to be a more viable and suitable alternative for land mobile satellite systems in many parts of Europe.
Abstract: Propagation characteristics of land mobile satellite (LMS) links play an important role in the design and viability of such a system. In the mobile environment, considerable degradation of the received signal may occur due to the shadowing and multipath effects. This results from the nature of the surrounding terrain in which the mobile terminal operates. Consequently, to provide communication facility to mobile users with reasonable link reliability would require large margins in most cases. The severity of propagation conditions can be related to the elevation angle of the transmitting source, as almost all satellite systems operate on line-of-sight communication link. Therefore, at low elevation angles adverse propagation conditions become even worse. As a result, geostationery satellites do not provide an optimum solution, particularly for land mobile satellite systems in many parts of Europe. Non-geostatiobery orbit satellites appearing at high elevation angles (>500) are likely to present a more viable and suitable alternative [I]. It is anticipated that advantages gained in terms of reduction in propagation degradations will outweigh the complexities added to the system due to such a choice. Presently, the scarcity of propagation data above 500 makes it difficult to quantify the advantages of any system using such a system architecture [a.
11 citations
14 Apr 1992
TL;DR: A MB-LPC vocoder operating at 2.4 and 1.2 kb/s exploits the advantages of both time and frequency domain speech coding to product natural sounding, good quality speech.
Abstract: At 4.8 kb/s and below, conventional code excited linear prediction (CELP) does not provide the appropriate degree of periodicity. It has been suggested that good quality low bit rate speech can be obtained from frequency domain techniques. A MB-LPC vocoder operating at 2.4 and 1.2 kb/s exploits the advantages of both time and frequency domain speech coding to product natural sounding, good quality speech. This includes a new frequency domain post-filter which attenuates the noise in the formant nulls and enhances the synthesized speech. >
8 citations
25 Jun 1992
TL;DR: A novel accessing scheme which can efficiently support mixed (voice+data) service requirements on the future systems is proposed and proved to operate efficiently in mixed traffic environments.
Abstract: The importance of the data services for radio communication is growing. The paper proposes a novel accessing scheme which can efficiently support mixed (voice+data) service requirements on the future systems. For this work some comprehensive computer models were developed. Through these models, the proposed scheme was proved to operate efficiently in mixed traffic environments. Further improvements were achieved by utilizing the pauses/listening periods in speech for transmitting delay-insensitive data messages. A description is included of the algorithm, and the results of the simulations are presented. >
6 citations
23 Mar 1992
TL;DR: The authors present an 8 kb/s low delay speech coder which is based on the code excited linear prediction (CELP) algorithm, and the latest test results indicate a performance equal to that of the GSM at worst case.
Abstract: The authors present an 8 kb/s low delay speech coder which is based on the code excited linear prediction (CELP) algorithm. They concentrate on analyzing the problem areas in backward predictive systems and solutions to alleviate these problems. The authors report on a forward linear predictive coding adaptive scheme to eliminate such distortions. Another approach is to use a windowing technique in the frequency domain on the error signal in analysis-by-synthesis procedures. The adaptation of the long term predictor (LTP) coefficients is also considered in a backward and forward mode for differential pitch coding of the LTP lags. The role of the excitation gain update rate is also examined in providing higher quality and better adaptation of the gains for an efficient vector quantization procedure. An 8-kb/s coder with a buffering delay of 4 ms has been informally subjectively tested, and the latest test results indicate a performance equal to that of the GSM at worst case. >
5 citations
25 Jun 1992
TL;DR: A MB-LPC vocoder operating at 2.4 kb/s and below is presented which exploits the advantages of both time- and frequency-domain speech coding to produce natural sounding, good quality speech.
Abstract: Accurate representation of periodic speech segments is essential for synthesizing high quality digital speech at very low bit rates For bit rates at 48 kb/s and below, conventional code excited linear prediction (CELP) does not provide the appropriate degree of periodicity It has been suggested that good quality low bit rate speech can be obtained from frequency domain techniques such as STC, NMR and MB-LPC vocoders A MB-LPC vocoder operating at 24 kb/s and below is presented which exploits the advantages of both time- and frequency-domain speech coding to produce natural sounding, good quality speech This also includes a new frequency domain post-filter which attenuates the noise in the formant nulls and enhances the synthesized speech >
4 citations
01 Jan 1992
TL;DR: In this article, the problem of carrier acquisition of OQPSK signals in mobile satellite communication is addressed where large carrier errors are encountered due to Doppler shift, and a carrier acquisition and tracking technique suitable for all-digital implementation is presented.
Abstract: The problem of carrier acquisition of OQPSK signals in mobile satellite communication is addressed where large carrier errors are encountered due to Doppler shift. A Carrier acquisition and tracking technique suitable for all-digital implementation is presented. The method is independent of clock timing which often hinders carrier acquisition in offset modulation schemes in the presence of large initial frequency uncertainties.
3 citations
TL;DR: In this article, a multiband propagation measurement campaign for the high elevation angle land mobile satellite channel is reported, where simultaneous narrowband sounding of the channel has been carried out in suburban, wooded and open areas of the UK using a helicopter-mounted platform to simulate the satellite signal at various elevation angles.
Abstract: Results of a recent multiband propagation measurement campaign for the high elevation angle land mobile satellite channel are reported. Simultaneous narrowband sounding of the channel has been carried out in suburban, wooded and open areas of the UK using a helicopter-mounted platform to simulate the satellite signal at various elevation angles. Propagation related link degradations in the land mobile-satellite channel have been observed to be less severe when the path elevation angle is increased or radio frequency decreases.
01 Oct 1992
TL;DR: This paper reviews some basic equations of motion and coverage of satellites in earth orbit as well as examining satellite constellation design techniques, and proposes a PCN oriented satellite constellation with some unique properties.
Abstract: This paper reviews some basic equations of motion and coverage of satellites in earth orbit as well as examining satellite constellation design techniques. Specific orbit properties are highlighted and their applicability to constellation design and earth coverage are then investigated. A PCN oriented satellite constellation is then proposed with some unique properties. The proposed constellation scheme allows for a reduction in overall system cost in two ways. Firstly, simplification in satellite design result in satellite development costs being reduced and secondly, because each satellite will be designed for a particular orbit, constellation lifetime can be significantly extended.