# New bounds and constructions for constant weighted $X$-codes

## TL;DR

This paper investigates bounds and explicit constructions for constant weighted X-codes, which are essential for IC test response compression, improving known bounds and providing optimal or near-optimal code constructions.

## Contribution

It establishes new bounds on the maximum size of constant weighted X-codes and offers explicit constructions using combinatorial and algebraic methods, advancing the design of efficient test response compaction codes.

## Key findings

- Improved lower bounds for the maximum number of codewords in X-codes with specific parameters.
- Explicit constructions of constant weighted X-codes with optimal or near-optimal parameters.
- Enhanced bounds for a special class of X-codes introduced in prior work.

## Abstract

As a crucial technique for integrated circuits (IC) test response compaction, $X$-compact employs a special kind of codes called $X$-codes for reliable compressions of the test response in the presence of unknown logic values ($X$s). From a combinatorial view point, Fujiwara and Colbourn \cite{FC2010} introduced an equivalent definition of $X$-codes and studied $X$-codes of small weights that have good detectability and $X$-tolerance. An $(m,n,d,x)$ $X$-code is an $m\times n$ binary matrix with column vectors as its codewords. The parameters $d,x$ correspond to the test quality of the code. In this paper, bounds and constructions for constant weighted $X$-codes are investigated. First, we obtain a general result on the maximum number of codewords $n$ for an $(m,n,d,x)$ $X$-code of weight $w$, and we further improve this lower bound for the case with $x=2$ and $w=3$ through the probabilistic method. Then, using tools from additive combinatorics and finite fields, we present some explicit constructions for constant weighted $X$-codes with $d=3,7$ and $x=2$, which are optimal for the case when $d=3, w=4$ and nearly optimal for the case when $d=3,w=3$. We also consider a special class of $X$-codes introduced in \cite{FC2010} and improve the best known lower bound on the maximum number of codewords for this kind of $X$-codes.