Showing papers by "Barry G. Evans published in 1993"
TL;DR: In this paper, the results of a recent narrowband propagation experiment at high elevation angles have been further analyzed to determine fit coefficientS for the model proposed by Lutz et al. Results for the suburban and wooded environmental categories for elevation angles greater than 60 o are reported.
Abstract: The results of a recent narrowband propagation experiment at high elevation angles have been furthers analysed to determine fit coefficientS for the model proposed by Lutz et al. Results for the suburban and wooded environmental categories for elevation angles greater than 60 o are reported. An empirical model for L and S bands covering elevation angles from 20 to 80 o has beendeveloped. The model combines the empirical roadside shadowing model, and the empirical fading model derived from the results of a narrowband propagation campaign, and improves the existing CCIR model
27 citations
01 Apr 1993
TL;DR: This paper presents a systematic survey of the various techniques that are being adopted to mitigate the degradation experienced in speech quality as a result of corruption of the transmitted speech information by channel errors.
Abstract: The incorporation of low-bit-rate speech coders into emerging land and satellite mobile communication systems presents problems which previous communication systems have never encountered. One of the most important of these problems is the degradation experienced in speech quality as a result of corruption of the transmitted speech information by channel errors. This paper presents a systematic survey of the various techniques that are being adopted to mitigate this condition.
15 citations
TL;DR: In this paper, an adaptive linear fractional spaced equalization (LFSE) algorithm was proposed to cancel the interference in CDMA systems. But the proposed algorithm requires the explicit knowledge of the interfering spreading sequences.
Abstract: A method for adaptively cancelling the multiple-access interference in CDMA systems is addressed. The technique is based on adaptive linear fractional spaced equalisation (LFSE). The simulation results indicate that the LFSE offers significant gains over the conventional detector eliminating the near-far problem without the explicit knowledge of the interfering spreading sequences. Furthermore, LFSE performs despreading and multipath (RAKE) combining with no extra complexity.
8 citations
01 Aug 1993
TL;DR: In this paper, the results of a propagation campaign undertaken to characterize the fading nature of the LOS channel at high elevation angles are presented, where the propagation data were carried out at three RF frequencies corresponding to L (1.3 GHz), S (2.32/2.45 GHz), and Ku (10.4 GHz) bands.
Abstract: Signal propagation in the land mobile satellite (LMS) service is an important consideration due to its critical impact on the overall economic and commercial viability of the system. At frequencies allocated for LMS systems, shadowing of the line-of-sight (LOS) signal as well as multipath propagation phenomena can severely impair the link availability. In particular, as most of the studies have shown, the shadowing of LOS signal causes long and deep fades in a variety of mobile environments due to the inherent nature of the channel between the satellite and a mobile. Roadside obstacles, such as buildings, trees, utility poles etc., in the immediate vicinity of a mobile and the surrounding terrain are major sources of signal shadowing in LMS links. Therefore, a proper knowledge of link degradation is essential for cost-effective planning of a satellite based mobile communication system. The results of a propagation campaign undertaken to characterize the fading nature of LMS channel at high elevation angles is presented. It was envisaged that one of the most important physical variables contributing to the amount of LOS signal shadowing is the elevation angle of the satellite. At higher elevation angles to the satellite, less obstructions in the direct satellite-to-mobile path would therefore amount to statistically better link availability. Narrowband channel measurements were carried out at three RF frequencies corresponding to L (1.3 GHz), S (2.32/2.45 GHz), and Ku (10.4 GHz) bands. The campaign itself was divided into two phases to observe the effects of seasonal variation of foliage on the roadside trees. Phase measurements were carried out in September 1991 and in April 1992. Some important aspects from the statistical analysis of the propagation data are presented.
8 citations
Book Chapter•
02 Nov 1993TL;DR: In this paper, the authors present an overview of project R2074/Catalyst, an attempt to demonstrate the applicability of satellites in the provision of broadband island interconnection across Europe.
Abstract: The authors present an overview of project R2074/Catalyst, an attempt to demonstrate the applicability of satellites in the provision of broadband island interconnection across Europe. This project, which is part of the RACE II initiative, will set up a demonstrator network to implement the interconnection of various LAN architectures. The authors focus on a study of the expected user performance through the Catalyst network based upon mathematical and simulation models. They investigate the performance of a mix of client/server architectures using Novell NetWare and NEtwork File System protocols, which are supported by either the IEEE 802.3/Ethernet LANs, linked via LAN/ATM/satellite interface units. Areas of further investigation, and the implication of the author's analysis on the actual network architecture are discussed.
8 citations
28 Sep 1993
TL;DR: The multirate extension made to COSSAP to include frequency channelisation by a weighted overlap-add analyser structure is described.
Abstract: The multirate extension made to COSSAP to include frequency channelisation by a weighted overlap-add analyser structure is described. The model was used to simulate the channelisation of SCPC/FDMA signals in a multicarrier digital demodulator for a research project into on-board processing satellites. >
7 citations
06 Dec 1993
TL;DR: The authors propose the use of multi-rate speech and channel codecs in future PCS to control the excessive channel degradations that arise from co-channel interference.
Abstract: The mobile and personal communication systems (PCS) revolution is well underway. The major requirement of spectral efficiency in these systems often means the use of low bit rate speech and channel coders, rampant carrier frequency reuse with its associated problem of co-channel interference. The authors propose the use of multi-rate speech and channel codecs in future PCS to control the excessive channel degradations that arise from such interference. >
5 citations
27 Apr 1993
TL;DR: An excitation modeling based on a multitap adaptive or fixed codebook is proposed for low-delay encoding of speech signals, which can produce high-quality speech at bit rates but is still lacking the toll quality of LD-CELP at 16 kbit/s.
Abstract: An excitation modeling based on a multitap adaptive or fixed codebook is proposed for low-delay encoding of speech signals The algorithm can produce high-quality speech at bit rates of 8 kbit/s, but it is still lacking the toll quality of LD-CELP (low-delay code excited linear prediction) at 16 kbit/s A simplified vector quantization search procedure was incorporated to reduce the complexity with little subjective degradation The performance results indicated a need for several types of efficient excitation modeling, depending on the nature of the input signal >
4 citations
Proceedings Article•
01 Jan 1993
4 citations
22 Mar 1993
TL;DR: The DSP implementation and synchronisation algorithms for a low-bit-rate all-digital preambleless demodulator for mobile applications are described and the performance results are provided in terms of acquisition and bit-error-rate performances.
Abstract: The DSP implementation and synchronisation algorithms for a low-bit-rate all-digital preambleless demodulator for mobile applications are described. This demodulator has been developed and implemented for OQPSK modulation particularly but it can work equally well for BPSK and QPSK. The performance results of implemented demodulator are provided in terms of acquisition and bit-error-rate performances.<
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3 citations
01 Nov 1993
TL;DR: An overview of project R2074/CATALYST, an attempt to demonstrate the applicability of satellites in the provision of broadband island interconnection across Europe, and a study of the expected user performance through the CATALyST network based upon mathematical and simulation models.
Abstract: We present an overview of project
R2074/CATALYST, an attempt to demonstrate the
applicability of satellites in the provision of broadband
island interconnection across Europe. This project,
which is part of the RACE I1 initiative, will set up a
demonstrator network to implement the interconnection
of various LAN architectures. We focus on a study of
the expected user performance through the CATALYST
network based upon mathematical and simulation
models. We investigate the performance of a mix of
client/server architectures using Novel1 NetWare and
Network File System protocols, which are supported by
either the IEEE 802.3Ethernet LANs, linked via
LAN/ATM/satellite interface units. Areas of further
investigation, and the implication of our analysis on the
actual network architecture are discussed.
01 Jan 1993
TL;DR: Various design factors for mobile satellite systems, whose aim is to provide worldwide voice and data communications to users with hand-held terminals, are examined, and Orbit resonance is introduced and referred to throughout.
Abstract: Various design factors for mobile satellite systems, whose aim is to provide worldwide voice and data communications to users with hand-held terminals, are examined. Two network segments are identified - the ground segment (GS) and the space segment (SS) - and are seen to be highly dependent on each other. The overall architecture must therefore be adapted to both of these segments, rather than each being optimized according to its own criteria. Terrestrial networks are grouped and called the terrestrial segment (TS). In the SS, of fundamental importance is the constellation altitude. The effect of the altitude on decisions such as constellation design choice and on network aspects like call handover statistics are fundamental. Orbit resonance is introduced and referred to throughout. It is specifically examined for its useful properties relating to GS/SS connectivities.
TL;DR: This new method obviates the need for more powerful FEC schemes for transmission over noisy channels, and provides high resistance of the LSFs to channel errors, outperforming other schemes.
Abstract: The authors have exploited the ideas used in vector quantisation for error recovery of scalar quantised LSFs. The good performance of this method has provided high resistance of the LSFs to channel errors, outperforming other schemes by, possibly, a considerable margin. Better objective and subjective performances were obtained with this new method which obviates the need for more powerful FEC schemes for transmission over noisy channels. >
18 Apr 1993
TL;DR: In this paper, two new approaches based on a similar idea are proposed which significantly reduce the signalling overhead for low/medium Earth orbit satellite constellations, which results in an important increase in network efficiency for certain constellation types.
Abstract: Low/medium Earth orbit satellite constellations are expected to play a key role in any future global mobile communications system. Mobility management is examined for these constellations. When an advanced terrestrial type of approach is taken in examining user mobility, a location instability (or flip-flop) is found to occur. It is seen to result in a large signalling overhead. Two new approaches, both based on a similar idea, are proposed which significantly reduce the signalling overhead for such constellations. Rather than associating a mobile terminal with an exact location, the new approach is to associate it, through a single Earth station, which may or may not have an associated team of Earth stations, with what may loosely be described as a location area. Through this association the network can contact the terminal if required to do so. A slight average increase in call set-up time is the cost of the reduction in system signalling. The use of this new protocol for network mobility management results in an important increase in network efficiency for certain constellation types. >