This paper summarizes the important aspects of digital modem (modulator-demodulator) design for linear and nonlinear satellite channels using time-division multiple access (TDMA). The modem performance degradations caused by transmission anomalies, such as amplitude and delay distortions, effects of single and cascaded nonlinearities, and band-limiting effects in nonlinear channels, have been examined using the test data gathered from various field test programs using INTELSAT IV satellites and laboratory simulations using both INTELSAT IV and INTELSAT V parameters. A systematic methodology has been prescribed to reduce the effect of transmission anomalies on the performance of the modems. This paper consists of three main sections; essential features of TDMA modem design, transmission anomalies and their effects on modem performance, and optimization of modem performance in the presence of transmission deviations. The latter includes a brief description of an advanced maximum-likelihood sequence detection (MLSD) technique for application to the nonlinear channel.

The performance of 90- and 108-Mbit/s 8-phase CPSKTDMA channels through a 36-MHz-bandwidth transponder has been evaluated. At the optimum operating point, the error rate of the 90Mbit/s channel is improved by about one order of magnitude relative to that of the 108-Mbit/s channel, and the corresponding carrier power saving is about 3.5 dB. Measurements through a linearized TWT indicate that such a TWTA may be driven harder than a conventional TWTA with a substantial saving in the transponder RF power output. The potential of 8-phase CPSK-TDMA signal transmission through band-limited satellite channels using linearized TWTA's onboard the satellite and modems of the latest technology appears attractive.

An experimental investigation has been performed to assess the feasibility of 8-phase phase-shift-keyed (PSK) signal transmission through a 6-GHz band microwave radio link with a 30-MHz nominal RF bandwidth. Only a portion of this bandwidth is currently being utilized to transmit one analog TV program or 1800channel frequency-division-multiplexed (FDM)/FM telephony. The link has been modified to have a 40-MHz bandwidth for this experiment. The digital modem is a 36-MHz bandwidth, 8-phase coherent PSK (CPSK), 36-Msymbol/s (108 Mbit/s) system. The work which has been performed includes the following: 1) study of the link bit error rate as a function of link carrier-carrier-to-noise ratio(C/N)with the transmitter TWT power output as a parameter; 2) study of the effects of an adjacent 1800-channel carrier (center-to-center separation of 36 MHz) on the error rate of the PSK carrier; and 3) perturbation of the 1800-channel FM carrier top channel noise power ratio (NPR) by an adjacent-channel 8-phase carrier (center-to-center separation of 36 MHz). In addition, experiments have been conducted through a four hop test simulator to examine the cumulative effect of transmission deviations such as group delay and TWT AM-PM transfer effect on the error rate performance for two different TWT power outputs, namely 2 and 4 W.