Spread Spectrum Codes for Continuous-Phase Modulated Systems

We study the theoretical performance of a combined approach to demodulation and decoding of binary continuous-phase modulated signals under repetition-like codes. This technique is motivated by a need to transmit packetized or framed data bursts in high noise regimes where many powerful, short-length codes are ineffective. In channels with strong noise, we mathematically study the asymptotic bit error rates of this combined approach and quantify the performance improvement over performing demodulation and decoding separately as the code rate increases. In this context, we also discuss a simple variant of repetition coding involving pseudorandom code words, based on direct-sequence spread spectrum methods, that preserves the spectral density of the encoded signal in order to maintain resistance to narrowband interference. We describe numerical simulations that demonstrate the advantages of this approach as an inner code which can be used underneath modern coding schemes in high noise environments.