The performance of TSI-OQPSK (two-symbol-interval-offset QPSK) modems in nonlinearly amplified (saturated or hard-limited), adjacent channel interference (ACI) satellite earth station environment is studied. In closely spaced mobile satellite channels and higher frequency (≥ 14 GHz) earth stations, the effect of ACI on theP_{e} = f(E_{b}/N_{0})performance of faded systems may become very significant. Simulation results indicate that for the frequently used close channel spacing case (≤ 77 percent of the bit rate) and a practical fade depth (≥ 12 dB), our new TSI-OQPSK signal(n = 2)has 1 dB less degradation than IJF-OQPSK and 3 dB less than OQPSK, and that conventional QPSK could not operate in such an environment.

Based on Feher's nonlinear filter (NLF) invention, we developed a class of intersymbol-interference and jitter-free (IJF) baseband signals and JJF-OQPSK systems. In [14] and [18] we demonstrated the performance advantages of these systems over QPSK, OQPSK, and MSK systems. In this paper a new class of IJF two-symbol-interval (TSI) baseband pulse shapes is introduced. The spectral properties and the Peperformance of these TSI-OQPSK modulated systems is investigated. We show that in a hard-limited radio system the main lobe of then \geq 2, TSI-OQPSK is narrower and that the sidelobes, in the critical|(f-f_{c})T_{s}| < 2region, are lower than that of QPSK, OQPSK, MSK, andN = 1, IJF-OQPSK systems. A new filtering strategy, the sum of approximate eye (SAE), is also introduced. It is shown that SAE filters may reduce theE_{b}/N_{0}requirements of band-limited and hard-limited TSI-OQPSK modems.

New power and bandwidth efficient modulation techniques, named intersymbol interference and jitter-free (IJF)-QPSK and IJF-OQPSK, are presented. The properties of the IJF-QPSK and IJF-OQPSK signals in linear and nonlinear earth-station-satellite systems are studied. A finite-state Markov chain model is used to calculate power spectra of hard-limited IJF-QPSK and IJF-OQPSK. The model provides insights into the spectral spreading action of the ideal hard-limiter on the IJF-QPSK and IJF-OQPSK signals, Experimental, simulation, and theoretical results are in good agreement. The results indicate that the IJF-OQPSK modulated signal exhibits much less spectrum spreading than QPSK, OQPSK, and MSK. The error probability (Pe) performance of IJF-QPSK and IJFOQPSK in an additive white Gaussian noise and adjacent-channel interference environment is evaluated. The results show that the Peperformance of the IJF-OQPSK is superior to that of QPSK, OQPSK, and MSK in narrow-band nonlinear channels.