In early standard wireless communications, beamforming was not typically used and the transmission of the RF signal was omnidirectional. Since the RF signal was dispersed evenly in all directions, interception was made easy for eavesdroppers because all they had to know was the bandwidth and carrier frequency. They simply 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 for security measures, such as data encryption for protection against intrusion. One area of research that continues to be actively investigated is providing communications 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 a beamformer 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. In traditional beamforming the channel is slowly time varying, the beamforming vector is also slowly time varying, and 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’s 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. With the array redundancy, it is possible to randomize the beamformer vector in way that does not inhibit having a focused transmission channel from Alice to Bob. Because of the random beamforming vector, Eve will not be able to estimate the channel. This creates built-in security at the PHY-layer, which will assist upper-layer security designs.