This document summarises the results of several research campaigns over the past seven years. The main connecting theme is the physical layer of widely deployed sensors in Pervasive Computing domains. In particular, we have focused on the RF-channel or on ambient audio. The initial problem from which we started this work was that of distributed adaptive transmit beamforming. We have been looking for a simple method to align the phases of jointly transmitting nodes (e.g. sensor or IoT nodes). The algorithmic solution to this problem was to implement a distributed random optimisation method on the participating nodes in which the transmitters and the receiver follow an iterative question-and-answer scheme. We have been able to derive sharp asymptotic bounds on the expected optimisation time of an evolutionary random optimiser and presented an asymptotically optimal approach. One thing that we have learned from the work on these physical layer algorithms was that the signals we work on are fragile and perceptive to physical environmental changes. These could be obstacles such as furniture, opened or closed windows or doors as well as movement of individuals. This observation motivated us to view the wireless interface as a sensor for environmental changes in Pervasive Computing environments. Another use of physical layer RF-signals is for security applications. We are currently working to further push these mentioned directions and novel fields of physical prototyping. In particular, the calculation of mathematical operations on the wireless channel at the time of transmission appears to contain good potential for gains in efficiency for communication and computation in Pervasive Computing domains.
Another sensor class utilised for context-based device authentication is the RF-channel. present a technique to authenticate co-located devices based on RF-measurements since channel measurements from devices in near proximity are sufficiently similar to authenticate devices against each other @cite_116 . utilise physical layer features to derive secret keys for a pair of devices @cite_107 . In the absence of interference and non-linear components, transmitter and receiver experience identical channel response @cite_191 . This information is utilised to generate a secret key among a node pair. Since channel characteristics are spatially sharply concentrated and not predictable at a remote location @cite_126 , an eavesdropper is not capable of guessing information about the secret. This scheme was validated in an indoor environment in @cite_205 . Although we consider the keys generated by this scheme as strong, it does not preserve spatial properties. A device at arbitrary distance could pretend to be a nearby communication partner.
@cite_107 @cite_205 @cite_191 @cite_116 @cite_126
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We propose an enhanced physical-layer authentication scheme for multi-carrier wireless systems, where transmission bursts consist of multiple frames. More specifically, it is based on the spatial variability characteristic of wireless channels, and able to work with moderate terminal mobility. For the authentication of the first frame in each data burst, the legal transmitter uses the saved channel response from the previous burst as the key for authentication of the first frame in the next burst. The key is obtained either via feedback from the receiver, or using the symmetric channel property of a TDD system. Then the authentication of the following frames in the burst is performed either by a Neyman-Pearson hypothesis test, or a least-squares adaptive channel estimator. Simulations in a typical indoor building show that the scheme based on the Neyman-Pearson test is more robust against terminal mobility, and is able to detect spoofing attacks efficiently with small system overhead when the terminal moves with a typical pedestrian speed. The wireless medium contains domain-specific information that can be used to complement and enhance traditional security mechanisms. In this paper we propose ways to exploit the spatial variability of the radio channel response in a rich scattering environment, as is typical of indoor environments. Specifically, we describe a physical-layer authentication algorithm that utilizes channel probing and hypothesis testing to determine whether current and prior communication attempts are made by the same transmit terminal. In this way, legitimate users can be reliably authenticated and false users can be reliably detected. We analyze the ability of a receiver to discriminate between transmitters (users) according to their channel frequency responses. This work is based on a generalized channel response with both spatial and temporal variability, and considers correlations among the time, frequency and spatial domains. Simulation results, using the ray-tracing tool WiSE to generate the time-averaged response, verify the efficacy of the approach under realistic channel conditions, as well as its capability to work under unknown channel variations. Physical-layer authentication can address physical layer vulnerabilities and security threats in wireless sensor networks, and has been considered as an effective complementary enhancement to existing upper-layer authentication mechanisms. In this paper, to advance the existing research and improve the authentication performance, we propose a novel physical layer assisted authentication scheme for mobile wireless sensor networks. In our proposed scheme, we explore the reciprocity and spatial uncorrelation of the wireless channel to verify the identities of involved transmitting users and decide whether all data frames are from the same sender. In our proposed scheme, a new method is developed for the legitimate users to compare their received signal strength (RSS) records, which avoids the information from being disclosed to the adversary. Our proposed scheme can detect the spoofing attack even in a high dynamic environment. We evaluate our scheme through experiments under indoor and outdoor environments. Experiment results show that our proposed scheme is more efficient and achieves a higher detection rate as well as keeping a lower false alarm rate. New approaches that generate secret keys from radio channel characteristics have been recently proposed. The security of these schemes usually relies on the reciprocity principle which states that the channel fluctuations can be a source of common randomness for two communicating peers and these fluctuations can't be measured by any eavesdropper. A validation of these properties for indoor environments is presented in this work. The shared keys are created by measuring the reciprocal channel features and converting this information to binary vectors using a quantization algorithm. This paper addresses the problem of quantization. It identifies an important weakness of existing key generation algorithms and it shows that the secret bits extraction has a significant impact on the robustness and security of these algorithms. A new adaptive quantization algorithm for secret key generation is presented. This method has the advantages to create sufficient long secret keys with a high key agreement ratio between authorized users without revealing information to the attacker. The new scheme is experimentally validated using Ultra Wide Band technology.