A wall strap tensioner for tensioning a wall strap of a metal wall. The proposed modified maximum-likelihood TBCC decoding technique preserves error correction performance of optimal maximum-likelihood based TBCC decoding, while the computational complexity is substantially decreased since a reduced number of decoding states has been evaluated. The air which has been heated by the heat exchanger is supplied to a hopper through a hot air supplying pipe so as to preheat and dry injection material in the hopper. The window blind panels are wound on the winding core in an overlapping manner.