Conventional wireless communications could be threatened by an eavesdropper with a sufficiently sensitive receiver and unlimited computational resources, or may reach the channel capacity in the near future. Recent research into a new digital modulation technique termed Direct Antenna Modulation (DAM) shows that DAM is a potential solution to the aforementioned problems. Direction-dependency, which describes the manner of signal transmission, is the most important attribute of a DAM system. Direction-dependent transmission can provide extra protection from a physical-layer source against security attack. Various transmission schemes are discussed in this work, and it is shown that accurate demodulation can be prevented from eavesdropping in the following two scenarios: first, when the angular separation between eavesdropper and intended recipient is very small; second, when one or two eavesdropping directions are pre-known. In addition, DAM system can be configured to have extra channel resources by introducing space as an additional domain for multiplexing. With the technique of space multiplexing, the transmitter can send independent data streams towards multiple receivers located at various transmission directions simultaneously. An algorithmic method is also presented to provide space multiplexing with a relatively low system cost
