Abstract
Orthogonal Frequency Division Multiplexing (OFDM) systems guarantee transmissions free from intersymbol interference (ISI) and interchannel interference (ICI) over frequency selective channels, provided that the channel delay spread is less than the duration of the cyclic-prefix (or the zero-padding guard interval). However, it is realistic to consider practical implementation scenarios in which the channel delay spread exceeds this limit so that both ISI and ICI do arise. We show that in such cases, the objective of symbol synchronization should be to maximize the ratio of the total useful received power over all subcarriers to the total power of ISI and ICI for the given channel realization, and that this aim can be achieved through a proper choice of the symbol timing. We also present a practical low-complexity synchronization scheme for this case and show that its performance tops the results obtained by the best existing correlation-based timing estimators. Furthermore, we determine upper bounds on the achievable signal-to-interference ratio based on the ideal assumptions of either perfect channel state information or at least knowledge of the channel statistical description, and show that they are very closely approached by the proposed method. © 2009 IEEE.

Abstract
We analyze the fundamental limits of secret-key agreement over MIMO quasi-static fading channels. In the low-power and high-power regimes, we establish closed-form expressions for secret-key capacity. In the low-power regime, we show that the optimal signaling strategy is independent of the eavesdropper's fading realization. The low-power secret-key capacity is achieved by transmitting along the direction corresponding to the maximal eigenvalue of the legitimate channel. By combining this signaling strategy with reconciliation and privacy amplification, one obtains a semi-blind key-distillation strategy in which the knowledge of the eavesdropper's fading is required for privacy amplification alone. © 2011 IEEE.

Abstract
Orthogonal frequency division multiplexing (OFDM) systems have enjoyed widespread adoption in high data rate wired and wireless networks, due to their ability to efficiently cope with slowly varying dispersive channels. This paper considers the information theoretic secrecy rates that are achievable by an OFDM transmitter/receiver pair in the presence of an eavesdropper that might either use an OFDM structure or choose a more complex receiver architecture. The analysis is performed through modeling of the OFDM system with an eavesdropper as a special case of a high dimensional multiple-input multiple-output (MIMO) wiretap channel, which allows the secrecy loss due to the OFDM structure constraints, and the information gain for an eavesdropper that uses a more complex receiver to be quantified. The results are expressed in terms of both ergodic rates and outage probabilities for multipath Rayleigh fading channels, and in terms of dependence on the signal to noise ratio (SNR) ratio between the main and eavesdropper channels. ©2010 IEEE.

Abstract
Orthogonal frequency division multiplexing (OFDM) has been established as the preferred modulation choice for highrate data transmission over dispersive channels, as it allows rejection of inter-symbol interference (ISI) and efficient utilization of the available spectrum. We address the issues of determining achievable secrecy rates and secrecy capacity for OFDM transmission in the presence of a generic eavesdropper. In doing so, we refrain from making the restrictive assumption made in previous works that the eavesdropper uses a standard fast Fourier transform (FFT)-based demodulator. First, the high SNR secrecy capacity under a total input power constraint is evaluated for both cyclic prefix and zero-padding suffix OFDM systems. It is shown that in both cases the result is sensibly lower than what we would get if the eavesdropper used an OFDM receiver. Then, optimal power allocation schemes are proposed for both types of OFDM systems and the secrecy rates are compared with results obtained by using existing power allocation methods described in the literature for parallel Gaussian wiretap channels and multiple-input multiple-output (MIMO) Gaussian wiretap channels. © 2010 IEEE.

Abstract
We describe the architecture of QuAKE (Quantum Advanced Key Exchanger), an experimental setup for quantum key distribution (QKD) over a free-space quantum channel based on the B92 protocol [1]. The system consists of a transmitter (Alice) and a receiver (Bob) that are connected through a free-space quantum channel over a distance of approximately 50 m, and are each driven by a field-programmable gate array (FPGA). The raw key shared by Alice and Bob is processed in four subsequent steps (i.e., sifting, channel estimation, key reconciliation and privacy amplification) which are implemented in Matlab. Finally, public discussion is implemented with the user datagram protocol (UDP) transport protocol running over the Internet protocol (IP) network protocol, while 802.11g underlies the physical layer transmission. © 2011 ACM.

Abstract
This paper considers the information theoretic secrecy rates that are achievable by an orthogonal frequency-division multiplexing (OFDM) transmitter/receiver pair in the presence of an eavesdropper that might either use an OFDM structure or choose a more complex receiver architecture. The analysis is made possible by modeling the system as a particular instance of a high dimensional multiple-input multiple-output wiretap channel. The secrecy capacity is formulated as a maximization problem under a trace constraint, and simple expressions are given for its high signal-to-noise (SNR) limit. The low rate limit of the secrecy outage probability is also evaluated under a fading channel model. As for the finite SNR case, the secrecy rates that can be achieved with particular inputs are considered. Numerical results are provided under a Rayleigh fading channel model and under dependence of the main and eavesdropper channels. The secrecy loss due to the OFDM structure constraints, and the information gain for an eavesdropper that uses amore complex receiver, are also considered.