Shortened Regenerating Codes

For general exact repair regenerating codes, the optimal trade-offs between the storage size and repair bandwidth remain undetermined. Various outer bounds and partial results have been proposed. Using a simple chain rule argument, we identify nonnegative differences between the functional repair and the exact repair outer bounds, and one of the differences is then bounded from below by the repair data of a shortened subcode. Our main result is a new outer bound for an exact repair regenerating code in terms of its shortened subcodes. In general, the new outer bound is implicit and depends on the choice of shortened subcodes. For the linear case, we obtain explicit bounds.