The effects of imperfect carrier phase recovery on uncoded and trellis-encoded PSK (phase-shift keying) are compared. A recently proposed clock-aided carrier recovery scheme is analyzed and compared to the standard frequency multiplication circuit. Results obtained through a combination of analysis and computer simulation compare uncoded 4-PSK with trellis-encoded 8-PSK carrying two bits per channel symbol. The quality of the recovered carrier phase is evaluated using the ratio of tone power to background. The system error probability is evaluated and used to determine how accurate the time clock recovery must be in a clock-aided carrier recovery circuit in order to outperform classical recovery systems. It is shown that if the clock recovery system is accurate enough, clock-aided carrier recovery used with coded 8-PSK can compensate for the increased sensitivity of the latter to phase errors.>

The bit error rate (BER) of channels including a memoryless bandpass nonlinearity is evaluated by simulation. This would typically be an onboard travelling wave tube (TWT) amplifier in a satellite repeater, when the noise at the input of such a nonlinear element is non-negligible. The usual evaluation technique, error counting, requires a large amount of computer time if small error probabilities are to be estimated. It is shown that faster semianalytic procedures can be used, provided that a proper model for the nonlinear element is adopted. The output process is decomposed into a signal component plus an additional term representing an equivalent noise component, and an equivalent nonlinear transformation, relating the input useful signal to the output signal component, is derived. In addition, several modes for the probability density function (PDF) of the uplink noise component at the output of the transmission chain are discussed and compared. The procedure has been tested on a transparent satellite link using 4-CPSK modulation format. The results compare well with those of the error-counting technique if a composite rectangular PDF with exponential tails as adopted.>

The performance of a TFM scheme applied to a satellite downlink with an on-board regenerative repeater is evaluated by using a time domain simulation software package TOPSIM III. The TFM bit error rate in the presence of nonlinearities and adjacent channel interference is evaluated by means of semianalytic methods. The capabilities of convolutional codes for reducing the overall channel bit error rate are also investigated. Design guidelines are finally outlined for advanced satellite communication systems in which the joint use of nonconventional modulation schemes (like TFM) and of convolutional codes results in improved performance with respect to both bandwidth exploitation and system flexibility.

TOPSIM III is a flexible, easy to use, stand-alone Fortran package for the digital simulation of communication systems in the time domain. TOPSIM III provides a simple high-level language for the description of the system. Programs can be written following the same structure used in drawing the system block diagram. The paper discusses the time domain simulation approach used in the design of the package and describes its capabilities. The contents of the subprogram libraries are presented, describing some of the building blocks that can be used in the simulation as well as the performance evaluation facilities available in the package. Some simple examples illustrate the use of TOPSIM III as a tool for analysis and design of digital communication systems.

A mixed analytic and simulation method is described for the study of a DCPSK up-link used in a regenerative satellite transmission system. A great variety of impairments is considered and their effects on error probability are evaluated, thus emphasizing the performance evaluation in a "real situation." A case study is worked out in detail to stress the design-oriented approach.