A method for communication includes generating one or more data streams for transmission to respective target receivers (28), and one or more jamming streams. At least one parameter, selected from a group of parameters consisting of power ratios for allocation to the jamming streams and Modulation and Coding Schemes (MCSs) for assigning to the data streams, is calculated based on a criterion that is set to reduce a probability of the data streams being decoded by at least one eavesdropping receiver (32). The data streams and the jamming streams are transmitted using an antenna array (34) while applying the at least one parameter.