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In early standard wireless communications the transmission of the RF signal was dispersed evenly in all directions. This made interception of the RF signal easy for those eavesdroppers because all they had to know was the bandwidth and carrier frequency. They just needed an energy detector to determine if the received signal was just noise or if it was signal plus noise. Therefore, the probability of intercept was relatively high and had to rely on the upper layers security measures, such as data encryption for protection against intrusion. An area of research that grown recently is providing security at the physical layer.
With the advent of smart antennas, wireless communications became more intelligent (or aware of its surroundings), and energy efficient. Smart antennas could use transmit beamforming to focus the RF signals in the direction of the user they wanted to communicate with. Also beamforming could be used on the receive side for rejecting any directional interferes. This situation made interception much more difficult for the eavesdropper since is he is no longer in direct line-of-sight of the communication. But a good eavesdroper will still be able to intercept this signal with intelligent signal processing. This is because in traditional beamforming, the channel is slowly time varying, then beamforming vector is also slowly time varying, thus channel can be determined by the eavesdropper.
Let's introduce an Alice, Bob and Eve scenario. Alice wants to communicate securely with Bob, without Eve being able to successfully intercept the signal. As discussed previously even with beamforming, Eve will still be able to determine Alice communication channel with Bob. The proposed solution to this problem is to randomize the array transmission in such a manor that the Alice to Bob channel is deterministic for Bob, but is perceived to be random by Eve. Due to the array redundancy is possible to randomize the beamforming vector in way that does not inhibit having focused transmission channel from Alice to Bob. Because of the random beamforming vector, Eve will not be able to estimate channel. This creates built-in security at the PHY-layer, which will assists upper-layer security designs.