Use of pseudo-random codes in pulse compression

Abstract

Pseudo-random, or pseudo-noise (PN), codes are numeric sequences which appear random to the uninitiated observer yet are easily predictable to those who know the generation algorithm. Traditionally, PN codes have been generated by a hand wired sequence of binary shift registers, but are now created using programmable logic chips wired from user defined netlists. A properly implemented logic chip can generate PN codes of maximum length L=2n−1 where n is the number of shift register segments. The code’s period is limited only by the number of shift registers available. Bipolar PN codes have a normalized autocorrelation function that is unity at zero shift and 1/L at all other shifts. Thus PN codes can be used to amplitude modulate (AM) the ‘‘main bang’’ of an active RADAR or SONAR system. This modulation produces compression which vastly improves the range resolution and the sensitivity of the system. The properties of PN codes and their use in a simulated active SONAR system will be demonstrated using matlab programs.