Almost p-ary Sequences

Abstract

In this paper we study almost $p$-ary sequences and their autocorrelation coefficients. We first study the number $\ell$ of distinct out-of-phase autocorrelation coefficients for an almost $p$-ary sequence of period $n+s$ with $s$ consecutive zero-symbols. We prove an upper bound and a lower bound on $\ell$. It is shown that $\ell$ can not be less than $\min \{s,p,n\}$. In particular, it is shown that a nearly perfect sequence with at least two consecutive zero symbols does not exist. Next we define a new difference set, partial direct product difference set (PDPDS), and we prove the connection between an almost $p$-ary nearly perfect sequence of type $({\gamma }_1,{\gamma }_2)$ and period $n+2$ with two consecutive zero-symbols and a cyclic $(n+2,p,n,\frac{n-{\gamma }_2-2}{p}+{\gamma }_2,0,\frac{n-{\gamma }_1-1}{p}+{\gamma }_1,\frac{n-{\gamma }_2-2}{p},\frac{n-{\gamma }_1-1}{p})$ PDPDS for arbitrary integers ${\gamma }_1$ and ${\gamma }_2$. Then we prove a necessary condition on ${\gamma }_2$ for the existence of such sequences. In particular, we show that they don't exist for ${\gamma }_2\le -3$.