Secrecy capacity of a multiple-antenna wiretap channel is studied in the lowsignal-to-noise ratio (SNR) regime. Expressions for the first and secondderivatives of the secrecy capacity with respect to SNR at SNR = 0 are derived.Transmission strategies required to achieve these derivatives are identified.In particular, it is shown that it is optimal in the low-SNR regime to transmitin the maximum-eigenvalue eigenspace of H_m* H_m - N_m/N_e H_e* H_e where H_mand H_e denote the channel matrices associated with the legitimate receiver andeavesdropper, respectively, and N_m and N_e are the noise variances at thereceiver and eavesdropper, respectively. Energy efficiency is analyzed byfinding the minimum bit energy required for secure and reliable communications,and the wideband slope. Increased bit energy requirements under secrecyconstraints are quantified. Finally, the impact of fading is investigated.
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