Time-dependent traveling wave tube model for intersymbol interference investigations

Summary form only given. To achieve increased data rates for near real-time transmission of large volumes of data, complex modulation and multiple access techniques are employed requiring minimum distortion of the signal as it is passed through the traveling wave tube amplifier (TWTA). Thus, intersymbol interference (ISI) becomes a major consideration as well as suspected causes such as reflections within the TWT. To experimentally investigate effects of physical TWTs on ISI would be prohibitively expensive, as it would require manufacturing numerous amplifiers, in addition to acquiring the required digital hardware. As an alternative, a fully 3D, time-dependent, TWT interaction model has been developed using the electromagnetic code MAFIA. The model includes user defined slow-wave circuit, input/output coupling, focusing and beam parameters allowing standard or novel TWTs to be investigated. Included are the comprehensive effects of frequency dependent nonlinear distortions (AM/AM and AM/PM); gain ripple; drive-induced oscillations; harmonic generation; intermodulation products; and backward waves. The user is able to define arbitrary excitation functions so that digital signals can be used as input and computational correlation of ISI with TWT parameters can be conducted. By choosing a MAFIA input signal having characteristics similar to those of the operational system, a direct description of the action of the nonlinear TWT is provided. The resulting data can be expected to possess superior predictive fidelity compared to system level models based on swept-tone, swept-amplitude data (e.g. as supplied by conventional frequency domain TWT codes). The latter data is partial because superposition does not apply in nonlinear devices, and interactions between frequency components in a realistic signal are not captured. Development of the 3D time-dependent MAFIA model will be presented and effects of the TWT on ISI will be demonstrated for various digital modulation sc- emes and data rates.