Serially concatenated continuous phase modulation for satellite communications

In this paper, serially concatenated continuous phase modulation (SCCPM) is considered for the uplink of satellite communications. A three-step design procedure is proposed to optimize the association of the outer code and the CPM for a wide range of spectral efficiencies, ranging from 0.75 to 2.25 bit/s/Hz. Firstly, EXIT chart analysis is applied to derive general guidelines for choosing SCCPM parameters. A significant result is that a high-rate outer code is required to achieve good convergence threshold, given a spectral efficiency. At a second stage, union bounds to the error probability are considered to choose the outer code under the constraints arising from the EXIT charts analysis. From this analysis, extended BCH codes and extended Hamming codes are proposed as outer code for broadband and narrowband transmission, respectively. For the latter, double-binary convolutional codes and symbol interleaving is also proposed as a valid alternative. Finally, combining both EXIT charts and union bounds we optimize the association of code and CPM to achieve both low error rates and good convergence. The proposed concatenated structure offers very low error floors and good performance in the waterfall region for all considered spectral efficiencies. A significant improvement with respect to previous concatenated CPM schemes is shown.