R. Viola

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. >

A digital chip timing recovery loop for band-limited direct-sequence spread-spectrum signals

Migration towards a full-digital implementation of modems is currently one of the main trends in transmission systems design. The authors describe a noncoherent all-digital delay lock loop (DDLL) suited for chip timing synchronization in band-limited direct sequence spread spectrum (DS/SS) systems, and they thoroughly analyze its performance. The key features of this novel scheme are represented by its low-complexity processing section together with its good tracking capability. Analytical expressions for the DDLL S-curve and steady-state timing jitter are derived and confirmed by a time-domain computer simulation. Furthermore, the Mean Time to Lose Lock (MTLL) of the loop is evaluated and some numerical results are reported. The proposed chip timing synchronization scheme reveals also an improved tracking performance when compared to the traditional analog DLL for rectangular chip DS/SS signals. >

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>>

High efficiency voice activated CDMA mobile communication system based on master code synchronization

The authors propose an innovative code division multiple access (CDMA)-based mobile system for voice (and data) services. The system makes use of voice activation in order to increase the CDMA efficiency (useful transmitted information vs. occupied bandwidth). The synchronization problems are solved by distributing a master code in the forward link and using forced link activation in the return when no speech activity is detected. The blocking probability is minimized by means of multirate speech coding associated with a centralized network control. Bit error rate calculations show all the advantages of operating in a synchronous mode.<<ETX>>

Synchronous CDMA for satellite frequency-selective broadcasting channels: Performance and receiver structures

The performance of a synchronous code division multiple access (S-CDMA) system in the presence of frequency-selective propagation conditions is analyzed. The bit error rate performance of S-CDMA for a conventional receiver is derived, and numerical results are given for two types of channel. Two receiver schemes suited for multipath processing are discussed. Coherent RAKE receiver is first analyzed showing a combining gain at low carrier-to-multipath ratio (C/M). A better performing tapped delay line (TDL) equalizer based on minimum mean squared error (MMSE) is presented. Results show the promising performance of this technique, especially when channel parameters are slowly variant.<<ETX>>

Chip timing synchronization in an all-digital band-limited DS/SS modem

A noncoherent digital delay lock loop (D-DLL) suited for code tracking in direct-sequence/spread-spectrum (DS/SS) band-limited (BL) signals is presented and analyzed. The key feature of this tracking scheme is the requirement of only one example per chip to derive the loop error signal. The expression of the S-curve of the proposed scheme is derived theoretically and checked by a time-domain computer simulation. Moreover, the steady-state RMS chip timing jitter of the D-DLL is derived, and results are compared with computer simulations. The proposed synchronization scheme is shown to bear smaller chip timing jitter when compared to the analog scheme for rectangular shaped chips.<<ETX>>

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>>

Link performance optimization for digital satellite broadcasting systems

The authors introduce the concept of digital direct satellite broadcasting (D-DBS), which allows unprecedented flexibility by providing a large number of audiovisual services. The concept assumes an information rate of 40 Mb/s, which is compatible with practically all present-day transponders. After discussion of the general system concept, the results of transmission system optimization are presented. Channel and interference effects are taken into account. Numerical results show that the scheme with the best performance is trellis-coded 8-PSK (phase shift keying) modulation concatenated with a Reed-Solomon block code. For a net data rate of 40 Mb/s a bit error rate of 10/sup -10/ can be achieved with an equivalent bit energy to noise density of 9.5 dB, including channel, interference, and demodulator impairments. A link budget analysis shows how a medium-power direct-to-home TV satellite can provide multimedia services to users equipped with small (60-cm) dish antennas.<<ETX>>