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This paper studies a multi-antenna wiretap channel with a passive eavesdropper and an external helper, where only quantized channel information regarding the legitimate receiver is available at the transmitter and helper due to finite-rate feedback. Given a fixed total bandwidth for the two feedback channels, the receiver must determine how to allocate its feedback bits to the transmitter and helper. Assuming zero-forcing transmission at the helper and random vector quantization of the channels, an analytic expression for the achievable ergodic secrecy rate due to the resulting quantization errors is derived. While direct optimization of the secrecy rate is difficult, an approximate upper bound for the mean loss in secrecy rate is derived and a feedback bit allocation method that minimizes the average upper bound on the secrecy rate loss is studied. A closed-form solution is shown to be possible if the integer constraint on the bit allocation is relaxed. Numerical simulations indicate the significant advantage that can be achieved by adaptively allocating the available feedback bits. [ABSTRACT FROM PUBLISHER]

In this letter, we study on-off opportunistic beamforming for the multiuser downlink channel with a passive eavesdropper. Two opportunistic scheduling schemes exploiting multiuser diversity are investigated which require limited feedback of the effective signal-to-noise ratio (SNR) from the legitimate users. For the two scheduling schemes, we derive new closed-form expressions for the ergodic secrecy rate with on-off beamforming over Rayleigh fading channels. Numerical results are provided to verify our analytical results and illustrate the impact of multiuser diversity on the secrecy performance. [ABSTRACT FROM PUBLISHER]

This letter investigates masked beamforming schemes for multiuser multiple-input multiple-output (MIMO) downlink systems in the presence of an eavesdropper. With noisy and outdated channel state information (CSI) at the base station (BS), we aim to maximize the transmit power of an artificial noise, which is broadcast to jam any potential eavesdropper, while meeting individual minimum mean square error (MMSE) constraints of the desired user links. To this end, we adopt a Bayesian approach and derive an average MSE uplink-downlink duality with imperfect CSI. Using the duality, a robust beamforming algorithm is proposed. Simulation results show the effectiveness of the proposed scheme. [ABSTRACT FROM PUBLISHER]