▁channel ▁fl uct u ations ▁due ▁to ▁f ading ) ▁and ▁explo it ▁the ▁difference ▁between ▁the ▁channel ▁to ▁a ▁legit imate ▁receiver ▁and ▁the ▁channel ▁to ▁an ▁e aves dro pper ▁to ▁benefit ▁the ▁legit imate ▁receiver . ▁More ▁recent ▁theoretical ▁results ▁are ▁concerned ▁with ▁determ ining ▁the ▁secre cy ▁capacity ▁and ▁optimal ▁power ▁allocation ▁in ▁broadcast ▁f ading ▁channels . ▁There ▁are ▁cave ats , ▁as ▁many ▁capac ities ▁are ▁not ▁comput able ▁unless ▁the ▁assumption ▁is ▁made ▁that ▁Alice ▁knows ▁the ▁channel ▁to ▁E ve . ▁If ▁that ▁were ▁known , ▁Alice ▁could ▁simply ▁place ▁a ▁null ▁in ▁E ve ' s ▁direction . ▁Sec re cy ▁capacity ▁for ▁M I MO ▁and ▁multiple ▁coll ud ing ▁e aves dro ppers ▁is ▁more ▁recent ▁and ▁on going ▁work , ▁and ▁such ▁results ▁still ▁make ▁the ▁non - use ful ▁assumption ▁about ▁e aves dro pper ▁channel ▁state ▁information ▁knowledge . ▁ ▁Still ▁other ▁work ▁is ▁less ▁theoretical ▁by ▁attempting ▁to ▁compare ▁implement able ▁schemes . ▁One ▁physical ▁layer ▁encryption ▁scheme ▁is ▁to ▁broadcast ▁artificial ▁noise ▁in ▁all ▁directions ▁except ▁that ▁of ▁Bob ' s ▁channel , ▁which ▁basically ▁j ams ▁E ve . ▁One ▁paper ▁by ▁Neg i ▁and ▁Go el ▁details ▁its ▁implementation , ▁and ▁Kh isti ▁and ▁W orn ell ▁computed ▁the ▁secre cy ▁capacity ▁when ▁only ▁statistics ▁about ▁E ve ' s ▁channel ▁are ▁known . ▁ ▁Par allel ▁to ▁that ▁work ▁in ▁the ▁information ▁theory ▁community ▁is ▁work ▁in ▁the ▁anten na ▁community , ▁which ▁has