Mário Pereira

Antenna design for underwater radio communications

In this paper we assess the influence of the conductivity of the medium on the radiation characteristics and input impedance of a loop antenna designed for underwater communications. The initial study is based on simulation of the antenna characteristics using FEKO electromagnetic (EM) software. Additionally, an equivalent circuit model of the antenna is also obtained and simulation of the input impedance on ADS software shows good agreement with the EM simulator. It is found that the radiation pattern, of the loop antenna changes significantly with the conductivity of the medium, from freshwater to seawater. The loop antennas were built with baluns and tested in freshwater, where the insertion loss between two identical antennas and the radiation pattern of one of them were also measured. The results show good agreement with the simulations.

Evaluation of Underwater IEEE 802.11 Networks at VHF and UHF Frequency Bands using Software Defined Radios

Radio Frequency (RF) communications suffer high attenuation underwater, limiting the range of standard IEEE 802.11 networks to a few centimeters underwater. The usage of custom RF solutions at lower frequencies to increase range entails high development costs, and proprietary hardware. This paper evaluates the performance of cost-effective, mass-market IEEE 802.11 networks underwater at different frequencies in the sub-GHz bands using Software Defined Radio platforms. Optimizations to the gr-ieee802.11 implementation, together with custom designed antennas, allowed the testing of frequencies in the 70-700 MHz range, in a large-scale freshwater tank, and in real-world, seawater conditions in Tagus river estuary. Results show a frequency-dependent communications range up to 5 m in freshwater and 1.8 m in seawater, with bitrates up to 550 kbit/s and delay under 45 ms. This proves the feasibility of IEEE 802.11 networks in the sub-GHz bands for applications such as video streaming from an autonomous underwater vehicle and underwater sensors.

Complexity checking of ARM programs, by deduction

In this paper we address two main questions: how to reason about the correctness of unstructured programs (particularly programs written in ARM Assembly) and how to use a proof-based system to check computational complexity of such programs. We approach the correctness issue by applying a flow sequentialization methodology and a formalized semantics of ARM instructions. An annotated ARM program is turned into a set of purely sequential programs, then each instruction is mapped into the corresponding formalized opcodes and finally Why3s VCGen is employed to generate proper Verification Conditions. Regarding complexity checking, we propose a methodology, based on the sequentialization process, by encoding instructions CPU-cost into their semantics and checking the programs calculated cost against user-supplied cost information (cost properties will be treated as normal functional annotations). Along with the formalization of correctness and complexity reasoning techniques, a prototype tool has been implemented and used to verify both the correctness and complexity of some practical examples.

Liquid Intersection Types

We present a new type system combining refinement types and th e expressiveness of intersection type discipline. The use of such features makes it possible t o derive more precise types than in the original refinement system. We have been able to prove severa l interesting properties for our system (including subject reduction) and developed an inference algorithm, which we proved to be sound.