Selection on the combination of M-sequence codes in alternate transmission for extension of measurable distance

Abstract

In M-sequence pulse compression for acoustic distance measurement, a continuous signal modulated by the cyclic binary code in an M-sequence is transmitted. Then, the received signal is correlated with one cycle of the transmitted code. The time of flight for the estimation of the propagation distance is determined from the peak appeared in the cross-correlation function when both codes match. The temporal resolution of the measurement and the maximum measurable distance are determined by the cycle length of the transmitted signal and in a trade-off relationship. Alternate transmission of different M-sequence codes and cross-correlations with each code can extend the measurable distance without degradation of the temporal resolution. However, truncation noise in each code and truncated interference noise between codes are generated around correlation peaks. The patterns of these noises are varied by the combination of codes. Therefore, contamination by these noise can be suppressed by selecting suitable codes.