Phase-Optimized Constant-Envelope Transmission (POCET) Modulation Method for GNSS Signals

Future GNSS systems introduce the requirement to multiplex several binary spreading codes on the same carrier. A constant-envelope signal is desirable to achieve efficient operation of the satellite transmitter. A new Phase-Optimized Constant-Envelope Transmission (POCET) modulation method is described that optimizes the multiplexing of several signal components, while maintaining specified power levels and phase relations, as perceived in the user’s correlation receiver. As is typical for previously proposed signal multiplexing approaches, the receiver for each component of the signal is assumed to correlate with its corresponding spreading code, without knowledge of the other codes. The approach is to compute an optimized, fixed table of transmitted phase values, one for each combination of bits of the spreading codes. The implementation of the satellite transmitter causes the carrier to be phase modulated by a complex rotation obtained by table lookup as a function of the code bits. Results computed by the new technique for cases of interest are presented and a comparison is made with the best known multiplexing schemes based on a combination of interlaced Majority Vote and Interplex techniques. Results show that the POCET technique has a reduction of 0.5 dB or greater in signal combining loss, making it an attractive alternative to existing schemes. The behavior of self-interference between the POCET modulation and its constituent components will also be described and compared to Interplex, majority vote, and hybrid approaches.