On the Covering Radius of Some Modular Codes
  This paper gives lower and upper bounds on the covering radius of codes over $\Z_{2^s}$ with respect to homogenous distance. We also determine the covering radius of various Repetition codes, Simplex codes (Type $\alpha$ and Type $\beta$) and their dual and give bounds on the covering radii for MacDonald codes of both types over $\Z_4$. 
Manish. K. Gupta and C. Durairajan
cs.IT math.IT math.RA