Attiya Mahmood

Impact of Array Mutual Coupling on Multiantenna Propagation-Based Key Establishment

While the impact of antenna mutual coupling on multiple-input multiple-output (MIMO) communication performance has been well documented, little work has appeared demonstrating how antenna coupling impacts the performance of multiantenna radios that use reciprocal electromagnetic propagation to establish secret encryption keys. This paper uses network theory to analyze the performance of such systems. The presentation includes an eavesdropper whose antennas may be very near and therefore coupled to the antennas of a legitimate node and further demonstrates the signal and noise model when the systems must remove the impact of nonreciprocal circuit elements through calibration. Representative results obtained with the model quantify the impact of coupling and the type of impedance matching used to connect the antennas to the radio circuitry on the system key establishment performance.

Impact of Propagation on the Vulnerability of Channel-Based Key Establishment

When generating encryption keys using the reciprocal propagation channel, the secure key rate bounding the number of bits that can be generated securely differs from the available key rate that ignores the presence of an eavesdropper. To explore the impact of propagation on this difference, this work formulates a model for computing the secure and available key rates for a multiuser scenario involving two legitimate nodes and a passive eavesdropper. Application of the model to a variety of different propagation conditions demonstrates that the secure and available key rates differ appreciably when the K-factor is large or when the separation between the eavesdropper and one of the legitimate nodes is small.

Data-dependent transmitter fingerprints for radio authentication

In radio frequency fingerprinting, observations of the impact of unique imperfections in the transmitter circuitry on the transmitted waveform are used to identify the device from which the data was transmitted. While this technique can be used to authenticate radios accessing a secure network, the security of the authentication can be enhanced if the transmitter fingerprint changes based on the transmitted data. This paper uses simulations and measurements to examine the behavior of a data-dependent filter response designed for such an application. The results show that the circuit holds promise for providing a data-dependent fingerprint useful for device authentication.

UAV attitude estimation using antenna arrays

This paper presents a new algorithm for estimating all three Euler angles that specify the relative attitude between two unmanned aerial vehicles based on multiple-input multiple-output (MIMO) radio transmissions. The algorithm uses direction-of-arrival estimates as well as estimates of the multi-polarized MIMO channel response to construct the coordinate frames describing the UAV attitudes. These coordinate frames then allow specification of the rotations required to align one UAV with the other. Simulations reveal that estimation errors are relatively small even for low signal-to-noise ratio.

Simple evaluation of the multi-user MIMO channel spatial covariance

This paper proposes a simple formulation for computing the full cross-covariance of multi-user multi-antenna multipath propagation channels where the multipath characteristics observed at two adjacent radios are correlated. The technique is applied to the case of key establishment based on reciprocal electromagnetic propagation to demonstrate the impact of a proximate eavesdropper on the key rate performance.

Radio circuitry calibration for reciprocal channel-based secret key establishment

Successful establishment of secret encryption keys based on bidirectional estimates of the multipath propagation channel depends critically on the reciprocity of the observed channel coefficients. Since typical radio circuitry is not reciprocal, calibration is required to remove the impact of non-reciprocal channel contributions. This paper reports on the impact of circuit non-reciprocity on key establishment performance and demonstrates calibration to recover the lost performance.

Secure key establishment in the presence of a passive eavesdropper: Impact of antenna coupling

While the establishment of encryption keys based on reciprocal electromagnetic propagation has received recent attention, work has so far ignored the impact of mutual coupling. This paper formulates the rate at which key bits can be generated incorporating the effects of antenna mutual coupling and a passive eavesdropper that is close to one of the radios.

The impact of array mutual coupling on propagation-based key establishment

Theoretical analyses and practical protocol implementations have demonstrated the concept of using reciprocal electromagnetic propagation as a basis for establishing secret encryption keys that enable secure communication between two radios. Recent studies have extended this concept, demonstrating that when multiple-antenna radios are used for such key establishment, the key rate, or the number of bits that can be generated for each channel estimate, can be increased (C. Chen and M. A. Jensen, IEEE Trans. Mobile Computing, 10, 205-215, 2011). However, these prior studies have largely ignored the impact of antenna mutual coupling on the achievable key establishment performance.

Network model for evaluating the performance of reciprocal channel-based key establishment with coupled antennas

Prior work on multi-antenna wireless communication has produced practical yet rigorous signal models that can account for the impact of antenna array mutual coupling and sophisticated circuit noise contributions. However, these models have not yet been extended to the case of systems that generate secret encryption keys based on the reciprocal electromagnetic propagation of multipath channels. This work performs this extension, building upon previously developed network models and augmenting them to accommodate considerations unique to the key establishment protocol. Simulation results demonstrate the impact on the achievable performance of mutual coupling and of the impedance matching network that interfaces the coupled array to the accompanying circuitry.