Carlo Elia

Bandlimited quasi-synchronous CDMA: a novel satellite access technique for mobile and personal communication systems

Recent trends in digital communications are opening commercial applications to code division multiple access (CDMA). A novel access technique based on bandlimited quasi-synchronous CDMA (BLQS-CDMA) is described, showing all the advantages of synchronizing conventional direct sequence CDMA to drastically reduce the effect of self-noise. Bandlimitation is achieved with no detection loss by means of Nyquist chip shaping, leading to a simple all-digital demodulator structure. Detection losses due to imperfect carrier frequency and chip timing synchronization are analytically derived and numerical results are checked by computer simulations. Impairments due to satellite transponder distortions are evaluated. The full digital modem structure is presented, together with possible applications to mobile and very small aperture terminal (VSAT) satellite communications. >

Performance evaluation of quasi-synchronous code division multiple access (QS-CDMA) for satellite mobile systems

The authors discuss and analyze the performance of a quasi-synchronous code-division multiple-access (QS-CDMA) mobile satellite communication system. Practical network synchronization schemes cannot guarantee perfect carrier frequency and code timing alignment. Therefore, bit error rate (BER) impairment due to frequency and chip timing errors is evaluated. Theoretical results on QS-CDMA performance are confirmed by a time-domain system computer simulation. In particular, QS-CDMA sensitivity to combined frequency/timing errors is addressed. The suitability of Gold codes for the proposed QS-CDMA mobile satellite network has been demonstrated.<<ETX>>

A new symbol timing and carrier frequency offset estimation algorithm for noncoherent orthogonal M-CPFSK

We present a new joint symbol timing and carrier frequency offset estimation algorithm for noncoherent orthogonal continuous-phase M-ary frequency-shift keying (M-CPFSK). Our algorithm performs nondata-aided feedforward processing of finite-length observations, and it is suited for an all-digital modem implementation based on a DSP or an ASIC processor. The algorithm exploits phase continuity of M-CPFSK in order to generate both a spectral component at the carrier frequency offset and a timing error signal. This is obtained without nonlinear transformations of the received signal involving noise/spl times/noise products. Thus, our algorithm can operate at a very low input signal-to-noise ratio. We discuss the operating range of our algorithm, and we show that no additional overhead (training sequence) in excess of the standard overhead of FDMA/TDMA packet transmission is required to resolve timing and frequency ambiguities. Moreover, we show that by differentially preceding the transmitted symbols, it is possible to eliminate automatically frequency ambiguities, at the price of a slight increase in the bit-error rate. An approximate mean-square error analysis of the estimators and simulation results prove that our algorithm provides good performance, even with a relatively short observation block length and large carrier frequency offset. Computer simulations show also that our algorithm is extremely robust to phase noise. These features make our algorithm a good candidate for satellite FDMA/TDMA applications in the 20-30 GHz band, with a large number of users and bursty transmission.

Analysis of satellite broadcasting systems for digital television

The concept of digital direct satellite broadcasting (D-DBS), which allows unprecedented flexibility by providing a large number of audio-visual services, is introduced. The concept assumes an information rate of about 40 Mb/s, which is compatible with practically all present-day transponders. After discussion of the general system concept, the optimization procedure is introduced, and results of the transmission system optimization are presented. Channel distortion and uplink/downlink interference effects are taken into account by means of a time domain system computer simulation approach. It is shown, by means of link budget analysis, how a medium-power direct-to-home TV satellite can provide multimedia services to users equipped with small (60-cm) dish antennas. >

Analysis of transmission schemes for satellite digital audio broadcasting

The European Space Agency (ESA) is studying different transmission techniques suited for satellite sound broadcasting services. The authors report preliminary results about the performance of two efficient transmission techniques, namely coded orthogonal frequency division multiplexing and coded quasi-orthogonal code division multiplexing. By means of a computer simulation approach, the performance of the two systems has been evaluated over the nonlinear satellite fading channel. Link budgets have been established for the case of highly elliptical orbit (HEO) and geostationary orbit (GEO) satellites. Results show that both approaches are suited for satellite radio broadcasting presenting similar impairments over the nonlinear satellite fading channel.<<ETX>>

A study for interactive TV via satellite

The digital compression revolution will offer the possibility of receiving hundreds of TV channels from the same orbital location. Only some of them will be dedicated to traditional television services. The largest part will be utilized for specialized services that will require user interaction. Interaction requires a “reverse channel” from the user site to the service provider premises. In this paper we study the feasibility of such a reverse channel via satellite. After discussing some system options, we focus on the simplest and most attractive one: where the users have the capability of sending information back to the service provider directly through a satellite link. For this solution, we provide a system outline and we present a coding and modulation scheme based on pragmatic-coded non-coherent orthogonal M-FSK concatenated with shortened Reed-Solomon codes which is able to meet the transmission requirements. Next, we address the problem of symbol-timing and carrier frequency offset open-loop estimation for short burst TDMA transmission with non-coherent M-FSK modulation. Finally, we discuss some particular network aspects of such a reverse channel.

Guest Editorial: Focus on Digital Signal Processing Techniques Applied to Space Communications

Digital Signal Processing is bringing about revolutionary changes in almost all areas of telecommunications. Satellite communications is no exception. As in other fields, DSP techniques not only allow improved implementation methods, but also open up possibilities which were not feasible using traditional techniques. The inherent sampling associated with digital processing in many cases leads naturally to the design of optimum estimation algorithms. Associated with this, the speed increases, power and mass reductions and reliability achievable with VLSI and the availability of high-level standard building blocks, such as DSP chips, ROMs, FFT processors etc. allow the exploitation of computational complexity, extending the usual power/bandwidth trade-off into a “third dimension”. The first applications of DSP to satellite communications were in the earth-stations, principally in modem implementation. Application to spacecraft payloads came later, partly because of the need to produce radiation-hard devices, qualified for the space environment and partly due to a conservative approach in a field where investments are large and reliability must be very high. The use of DSP techniques in space is now fully established however. Unlike many terrestrial applications, where the cost of VLSI implementation must be justified by large quantities of units required, the number of units required for space is almost always very small. The justifications for using VLSI in spacecraft are quite different from those on the ground. Cost per unit is typically not such an important issue, given the total system cost, as mass and power consumption. The small numbers of units required mean that appropriate design and fabrication techniques must be used. The Third International Workshop on Digitai Signai Processing Techniques Applied to Space Communications was held at ESTEC, the technical Centre of the European Space Agency (ESA), from 23‘(1 to 25‘h September 1992. It was organised by ESA and co-sponsored by EuteIsat and Inmarsat. 35 papers were presented on a wide variety of topics, covering theoretical developments, technology, design tools, new system concepts and descriptions of new DSP-based products. This focus section of the Transactions presents five papers, based on those presented at the workshop, selected to give an idea of the ground covered. Obviously, with such a broad field, we have had to leave out whole areas and we apologise to those speakers, some of them very eminent, whose contributions were vital to the success of the workshop, but which are not included here. Important DSP issues discussed at the workshop, but not treated in any detail in the papers selected for this issue include: the interaction between algorithms, architecture and implementation; digital antenna beamforming and the implementation of sophisticated onboard processing payloads. Given the severe power and/or bandwidth limitations associated with space communications, both sourceand channel-coding are of major importance. Increasingly, these make use of DSP techniques for their implementation. The paper by Musmann and Pirsch, which is a combination of two papers presented at the workshop addresses source

COFDM for broadcasting satellites in elliptical orbits

This paper analyses the performance of a coded orthogonal frequency division multiplexing (COFDM) based system for digital audio broadcasting using a highly inclined orbit satellite constellation. The sources of performance degradation considered are nonlinear amplification in the satellite transponder and multipath propagation for mobile reception. Despite its strong peak-to-mean amplitude variation, COFDM has been found to be very robust against nonlinear amplification. In the multipath channel, practical cases demonstrate low performance losses even in urban areas, when highly inclined elliptical orbits (HEO) are utilised.