Showing papers on "Bandwidth expansion published in 1985"

Coherent Hybrid DS-SFH Spread-Spectrum Multiple-Access Communications

Abstract: The performance of synchronous and asynchronous hybrid direct-sequence/slow-frequency-hopped spread-spectrum multiple-access communications over additive white Gaussian noise channels is examined. Systems employing binary or quaternary phase-shift-keying modulation with coherent demodulation are investigated. Both deterministic and random signature sequences and frequency-hopping patterns are considered and several possible assignments for them are discussed. It is shown that the multiple-access capability of hybrid spread-spectrum is superior to that of pure frequency-hopped spread-spectrum, and inferior to that of pure direct-sequence spread-spectrum for systems with identical bandwidth expansion which employ the same data modulation and demodulation scheme and random hopping patterns and signature sequences.

Combined trellis coding with asymmetric MPSK modulation: An MSAT-X report

Abstract: Traditionally symmetric, multiple phase-shift-keyed (MPSK) signal constellations, ie, those with uniformly spaced signal points around the circle, have been used for both uncoded and coded systems Although symmetric MPSK signal constellations are optimum for systems with no coding, the same is not necessarily true for coded systems This appears to show that by designing the signal constellations to be asymmetric, one can, in many instances, obtain a significant performance improvement over the traditional symmetric MPSK constellations combined with trellis coding The joint design of n/(n + 1) trellis codes and asymmetric 2 sup n + 1 - point MPSK is considered, which has a unity bandwidth expansion relative to uncoded 2 sup n-point symmetric MPSK The asymptotic performance gains due to coding and asymmetry are evaluated in terms of the minimum free Euclidean distance free of the trellis A comparison of the maximum value of this performance measure with the minimum distance d sub min of the uncoded system is an indication of the maximum reduction in required E sub b/N sub O that can be achieved for arbitrarily small system bit-error rates It is to be emphasized that the introduction of asymmetry into the signal set does not effect the bandwidth of power requirements of the system; hence, the above-mentioned improvements in performance come at little or no cost MPSK signal sets in coded systems appear in the work of Divsalar

On binary differential detection for coherent lightwave communication

Abstract: Motivated by the communication problems caused by phase noise in those semiconductor lasers that may be used for fiber-optic data transmission, we consider heterodyned binary Differential Phase-Shift Keying (DPSK) in conjunction with high-rate (short time chip) redundancy as provided by repetition or by more complex coding techniques. In surprising contrast to repetitive coherent phase-shift keying where only a loss of a 2/π (2 db) in power is incurred in the limit of infinitely many infinitesimal time chips, we show that DPSK requires, in this limit, an infinite number of photons per bit. This is true regardless of the coding scheme used with the DPSK modulation. Next we find the bandwidth expansion that minimizes the number of received photons per bit required to hold the error rate at 10−9 for two situations: first for a simple repetition code, and then for a repeated (24, 12) Golay code with maximum likelihood detection. The performance of the latter is assumed to be representative of other optimally detected codes of the same rate, such as convolutional codes with Viterbi decoding. Explicit curves relating required photons per bit to the bandwidth expansion are given for B/R ratios of 0.01 to 10, where B is the laser linewidth and R is the data rate. An example of the results is that for B/R = 0.1 and a bandwidth expansion of 10, about 23 photons per bit are required for the repeated Golay code to perform as well as uncoded DPSK without phase noise (which requires 20 photons per bit for Pe = 10−9). If B/R = 0.01 the bandwidth expansion is reduced to 2, and 12 photons per bit are required, thus outperforming the phase-stable, but uncoded, situation.

Capacity of wideband TDMA digital mobile radio cellular schemes

Abstract: Wideband TDMA digital cellular mobile radio schemes can offer a capacity similar to that of analogue modulation schemes. To find the optimum coding to be used a balance must be made between bandwidth, power and number of channels. A comparison is made for an idealised system using QPSK transmission and biorthogonal coding. Coding with a bandwidth expansion of 2.7 is shown to provide a good capacity at low carrier/noise ratio and an acceptable error rate.

Simultaneous transmission of speech and data using an adaptive cancelling technique

Abstract: A new scheme to transmit data and analogue speech simultaneously over an analogue transmission line is presented. The transmitter simply combines speech and modulated voice-band data signals together while the receiver uses an adaptive cancelling technique to recover these signals from the received composite. The scheme works for phase-dispersive channels and without bandwidth expansion.

Direct sequence voice scrambling scheme—A new technique for secure communications

Abstract: Communication security is getting more and more important. Although the encryption of digitized speech is very attractive, direct scrambling of the analog speech waveforms continues to be an important approach due to a strong desire for the use of existing telephone networks with standard telephone bandwidth at acceptable speech quality and cost. In this paper, a new approach for analog speech scrambling, a Direct Sequence Voice Scrambling (DSVS) scheme, is proposed, in which the spread spectrum concept is borrowed but combined with a newly developed bandwidth reduction technique. The system design, performance analysis, practical considerations and simulation results are presented in detail in this paper. The analysis indicates that the DSVS scheme can provide the following advantages: small residual intelligibility, high degree of security, small bandwidth expansion, no time delay and only slight degradation of speech quality.