The feasibility of a personal communications network using portable terminals that can provide 4.8 kb/s voice communications to a hub station via Ka-band geosynchronous satellite has been investigated. Tradeoffs are examined so that the combined system of hub and gateway earth stations, the satellite, and the personal terminals can provide a competitive service in terms of cost, availability, and quality. A baseline system that uses a spacecraft with approximately 140 spot beams to cover the contiguous US (CONUS) and 5 W power amplifiers in each beam is described. Satellite access in both the forward and return directions uses frequency-division multiple-access/code-division multiple-access (FDMA/CDMA) with a chip rate of 2.5 Mchip/s.>

The advanced communications technology satellite (ACTS) is an experimental communications satellite being developed by NASA's Lewis Research Center. Low burst rate (LBR) traffic stations will access the ACTS multibeam communications package (MCP) through two hopping beams that can be directed at certain cities and areas in the continental United States. An onboard baseband processor (BBP) demodulates uplink traffic received via two up-link (30 GHz) hopping beams, switches the traffic between uplink and downlink beams at baseband, and then remodulates the traffic for retransmission at 20 GHz via two downlink hopping beams. This study concentrates on the demand assigned operation of the ACTS LBR system where the onboard switch is remote from both the traffic stations and the centralized master control station (MCS). Network control uses inbound and outbound orderwire channels and a BBP control channel thereby allowing the MCS to coordinate assignment of individual 64-kb/s channels in the spacecraft. Models are developed to simulate the dynamics of the demand assignment process in order to verify call blocking behavior, to predict control channel loads, and to evaluate alternative algorithms for burst time plan rearrangement that becomes necessary to minimize blocking under conditions of high-traffic intensity.>

Time-domain simulation is used to evaluate NTSC and multiplexed analog component (MAC) television transmission over the Communications satellite channel using analog frequency modulation. The simulation program is used to measure transmission impairments caused by thermal noise, distortion resulting from spectral truncation by the band-limited channel, AM-to-PM conversion in the satellite TWTA, and the onset of impulse noise in the FM demodulator as the receiver approaches threshold. Although subjective testing is necessary before final comparisons can be made, the simulation results give insight into the sources of transmission impairments and the tradeoffs among certain transmission parameters.

Computer modeling of satellite communications channels is a valuable adjunct to analytical modeling and hardware simulation for predicting and verifying communications link performance. In the computer simulation approach, sampled signals are created in the computer and operated on by algorithms that simulate the effects of filtering, channel nonlinearities, interference, and noise. After passage through the simulated channel, the distorted signals are demodulated to determine the performance degradation introduced by the channel. This paper describes the approach used in simulating communications channels on the computer. Examples are given that apply to the communications satellite channel; however, the techniques described are applicable to general channel configurations.

Carrier phase and symbol timing synchronization performances are determined for QPSK transmission over the nonlinear communications satellite channel with several typical modem filtering configurations. A computer simulation is used that models conventional or offset QPSK transmission, the modem filters, the nonlinear satellite TWTA, and the receive modem synchronizers. These simulations are used to obtain errors in carrier phase and symbol timing estimates caused by both thermal noise and the modulation-pattern-induced jitter. The statistics of these errors are obtained for several channel filtering combinations and TWTA backoff levels. These experiments are extended to examine the overall effect of synchronization errors on bit-error probability for one particular implementation of the synchronizers.

An analysis has been performed to examine the tradeoffs between cost and performance for digital data transmission via the communications satellite channel. The model relates both cost and performance to the major parameters of the spacecraft and the earth station. A combination of M-ary PSK transmission, and forward error correction (FEC) is selected to maximize communications efficiency (q in bps/Hz) for the prevailing carrier-to- noise ratio on the link. A data link control protocol using automatic repeat request (ARQ) as an error recovery technique is assumed. The overall performance measure computed herein is the annual cost per “effective” bit per second throughput over the link. Trends in cost/bps are shown for several satellite models as a function of information bit rate (Rb), and earth station figure of merit (G/T).

The discrete Fourier transform (DFT) is applied as a coarse estimator of the frequency of a sine wave in Gaussian noise. Probability of anomaly and the variance of the estimation error are determined by computer simulation for several DFT block sizes as a function of signal energy-to-noise density ratio\mathcal{E}/N_0. Several data windows are considered, but uniform weighting gives the best performance.

The phase-slipping behavior of second-order squaring and quadrupling phase-locked loop configurations is investigated by digital computer simulation. Statistics are obtained for the mean time between batches of phase slips and the duration of the batches, as a function of the effective loop signal-to-noise ratio (SNR).

Suboptimal stopping rules are developed for the sequential detection of one ofMorthogonal signals. One procedure is derived from the information measures associated with sequential probability ratio tests (SPRTs). The resultant stopping rules are relatively simple functions ofMand the error probabilities desired for the sequential tests.