The technical issues relevant to providing high-quality digital service over analog radio using 1.544-Mbit/s DS-1 data-invoice modems operating through analog supergroup channels with 256-QAM modulation are reviewed. By means of a typical example, the desirability of increasing the fading margin when operating over FM radio systems is demonstrated. This motivates the consideration of coded modulation as a means of extending the threshold of the modem System, so that operation of the modem can be achieved in supergroup channels located higher in the radio baseband. It is found that trellis coded modulation provides better improvement of threshold performance than lattice coded modulation when the actual coding gain realized near the R0channel bound is used rather than the high signalto-noise-ratio (SNR)asymptotic coding gains. Using Ungerboeck's eight-state trellis code [3], a coding gain of approximately 2 dB is found for operation at 28 dB SNR (1 dB from the R0bound). Furthermore, it is found that even at this low operating SNR (where error events at the coded modulation receiver ouput can be long), high-rate, interleaved Reed-Solomon codes, used for burst noise protection at high SNR, still provide over 2 dB of additional coding gain.

The application of the slotted ALOHA random access technique to a system using a spread spectrum waveform is described and analyzed. The technique takes advantage of the delay capture properties of spread spectrum waveforms and is shown to achieve excellent delay and throughput performance. System stability is maintained even in the face of severe fluctuations in user population or traffic loading such as may occur in a large population of mobile users.

Minimum shift keying (MSK) and offset keyed quadrature phase shift keying (OK-QPSK) modulation techniques are often proposed for use on nonlinear, severely band-limited communication channels because both techniques retain low sidelobe levels on such channels, while allowing efficient detection performance. A more detailed performance comparison of the two techniques on such channels is, therefore, of interest. In this paper a Markov process representation is developed which is applicable to either the MSK or OK-QPSK waveform. This representation is employed to illustrate the similarity between the modulation processes and to obtain the autocorrelations and power spectral densities of the two waveforms. This Markov process representation may be similarly employed with other modulation waveforms of the same class. The autocorrelations and power Spectral densities of MSK and offset QPSK provide initial insight to expected performance on band-limited channels. The results of a digital computer simulation are presented. The simulation compares the bit error rates (BER's) of MSK and offset QPSK on nonlinear, band-limited double-hop links such as encountered in satellite communications. The simmulation results are presented asE_{b}/N_{0}degradation with respect to ideal detection versus channel noise bandwidth. The error probability was used as a performance metric, and equal adjacent channel interference as a constraint. For the channels simulated, MSK is found to provide superior performance when the channel noise bandwidth exceeds about 1.1 times the binary data rate. For narrower bandwidths, offset QPSK provides superior performance.