Effect of nonlinear amplification on turbo coding gain

This paper presents results obtained from an end-to-end, proof-of-concept system that integrates a multi-constellation modem, turbo coding, and a nonlinear traveling wave tube amplifier (TWTA). Multilevel modulation schemes make it possible to provide high-speed data communications in a limited amount of spectrum. This improvement, however, comes at the price of increased linearity requirements for the end-to-end link. This constraint is especially important for the power amplifier, which is typically a nonlinear device. Forward error correction based on turbo codes provides a solution for the problems described above, and improves the BER by increasing the noise margin by over 5 dB. This paper presents measured BER curves for different modulations and turbo codes, taken at different power levels relative to saturation. The absolute coding gain is shown to be dependent on the nonlinearity. Limitations in the standard definition of the coding gain are identified and a new metric is proposed