On the secrecy rate of limited feedback beamforming over multiple-input single-output wiretap channels

The secrecy rate of limited feedback beamforming is studied for a Multiple-Input Single-Output (MISO) wiretap channel with a multi-antenna eavesdropper. We first obtain the secrecy rate of limited feedback beamforming achieved at the legitimate receiver. We then derive a lower bound for the asymptotic secrecy rate in the large system limit. From this bound, we observe a threshold for the ratio of eavesdrop antennas to transmit antennas to obtain a positive secrecy rate. We further show that the secrecy rate loss due to limited feedback decays with the number of feedback bits per transmit antenna.