Gil Wiechman Igal Sason An Improved Sphere-Packing Bound for Finite-Length Codes Over Symmetric Memoryless Channels. 1962-1990 2008 54 IEEE Transactions on Information Theory 5 http://dx.doi.org/10.1109/TIT.2008.920216 db/journals/tit/tit54.html#WiechmanS08