Direct sequence spread spectrum code-division multiple-access (DS/CDMA) is receiving increasing attention for cellular communications systems. When the users are synchronized and special symbol-length sequences, such as Gold codes, are used, the mutual interference can be substantially reduced relative to a system with very long or purely random spreading sequences. It is shown that this approach degrades the performance of the code phase acquisition and tracking, however. This effect prevents the system from acquiring and tracking long before the data detection is affected by the multiuser interference.

The error probability of digital transmission through a frequency-selective channel with random multipath fading due to uncorrelated scatterers is calculated. The error probability is averaged with respect to the randomly varying delays, with a given delay power spectrum. Two coherent receivers, suboptimum without any knowledge of the channel and optimum with perfectly known channel, are compared. The results are applied to continuous-phase-modulation (CPM) signals.>

In digital angle modulation schemes the carrier is frequency modulated (continuous phase modulation) or phase modulated (digital phase modulation) by a waveform that is conventionally generated from the data sequence by a linear filtering operation. The concept is generalized by allowing a nonlinear dependence of the modulating signal on the data, making it possible to produce a signal with a narrower spectrum. The design procedure for the waveform is demonstrated for a signal with minimum out-of-band power. It is shown that the increase in bandwidth efficiency can be achieved without compromising the power efficiency of the signal.>

CLASS (communication link analysis and simulation system) is a software package developed for NASA to predict the communication and tracking performance of the tracking and data relay satellite system (TDRSS) services. This paper describes the capabilities of CLASS and the communication link model used, and discusses typical applications for CLASS.

An approach is presented for approximating the cumulative probability distribution of the acquisition time of the serial PN search algorithm. The results are applicable to variable as well as fixed dwell time systems. The theory is developed for the case where some a priori information on the PN code epoch is available (reacquisition problem or acquisition of very long codes). The special case of a search over the whole code is also treated. The accuracy of the approximation is demonstrated by comparing with published exact results for the fixed dwell time algorithm.

A set ofNidentical oscillators (clocks) are connected to form a mutually synchronous network. General matrix expressions are derived for the phase error spectrum at each oscillator when the network operates in the linear regime. For certain topologies of practical interest the phase error variance and the correlation between the errors at different oscillators is computed and compared in their behavior for largeN. The results presented assume no thermal noise and no delay between the oscillators. However, the theory can be easily extended to include these two effects.

This paper uses MAP estimation techniques to obtain carrier reconstruction circuits for unbalanced QPSK signals. In addition, the performance analysis of these circuits is carried out to the extent that a loop design can proceed. Optimal weighting coefficients in the receiving system are identified as functions of theI-Qpower and data rate splits. Finally, numerical results are presented in order to demonstrate the utility of the theory developed. This signal design will be used in the development of the Tracking and Data Relay Satellite System (TDRSS) and the Navigation Satellite Timing and Ranging Global Positioning System (NAVSTAR GPS).

This paper develops the theory for assessing the tracking performance and designing carrier synchronization loops which are to be matched to an unbalanced QPSK signal design. It is assumed that the power split and theIandQchannel data rates are arbitrary. Bit synchronization is assumed unavailable for both carrier reconstruction techniques which have been suggested via MAP estimation theory. Numerical results are presented along with their comparison with results given in Part I where bit timing was assumed known.

An analytical simulation package called LinCsim has been developed by LinCom Corporation for analytical verification of bit error rate and tracking service performance for bothS- andKu-band Shuttle user service through the Tracking and Data Relay Satellite System (TDRSS). The purpose of this effort was to provide a tool to assess overall system performances as well as the sensitivity to key subsystem parameters and to ensure compatibility of TDRSS and Shuttle transponder performance specifications. The evaluation is based on an analytical simulation, i.e., a combination of simulation techniques and analytical performance evaluation. This allows the use of a more realistic signal format than is possible with a purely analytical approach and maintains the advantage of short computation time. The Shuttle forward and return link models and the main features of the simulation are described. Sample results of the computer runs are provided for current power budgets and system design parameters.