Background

Communications and location technologies are based primarily on the generation, transmission and reception of electromagnetic signals. In certain cases, the environment can distort, attenuate or even completely prevent the signals from propagating between transmitter and receiver (RF-, GPS-denied environments). Specifically, AC electromagnetic fields have a high attenuation in metals, solid materials (concrete, rock, soil, etc.) and water. The electromagnetic field propagating through a given material is attenuated exponentially (exponential attenuation law). The exponential attenuation with distance is three orders of magnitude smaller at 1 GHz than at 1 kHz signal in the same material. Another drawback is the long wavelength of the signal, and the large characteristic size of the antennae used for the most efficient RF link between transmitters and receivers. It has been shown that detecting the magnetic component of the electromagnetic field is beneficial at low frequencies. The advantage of using low-frequency magnetic signals in the presence of strong medium absorption comes at the price of reduced bandwidth (BW), leading to limited communication channel capacity and location accuracy. Low-frequency magnetic fields have a dipole nature, and the signal strength drops with the third power of the distance (cubic attenuation law), thus reducing the signal range.

Technology Overview

Researchers at the University of Colorado Boulder have proposed to develop a very-low-frequency (VLF, 3 kHz-30 kHz, extendable to 100 kHz) magnetic field-based quantum receiver for the part of the electromagnetic spectrum. The receiver system consists of (1) optically-pumped multiplexed RF magnetometer channels capable of signal detection in the VLF band, (2) noise cancellation scheme to suppress the noisy background and (3) a communication protocol. The multichannel system will reach noise floors below Earth’s background field, and more than 1 kHz bandwidth channels covering the VLF spectrum band. They will investigate a novel ambient background noise suppression technique using reference channels that relies on a prior knowledge of the frequency of phase-modulated signals. Potential impacts include short-range communications immune to jamming, and communications through absorbing media such as seawater, rock, sediment, collapsed infrastructure and other GPS-denied environments. Advantages of a VLF receiver include Increased communication range using low-frequency magnetic signals in the presence of strong medium absorption, Improved immunity to interference, as the source of interference will have to be within the communication range, and the fact that Magnetic signals are immune to multi-path.

Benefits

The receiver will offer:

Room-temperature operation
Small form factor and low power
Flexible hardware configuration offering scalar or vector measurement of magnetic component signals calibrated through fundamental constants
Multiplexed frequency channels, offering increased channel capacity compared to a single-channel system
500 fT/Hz1/2 channel noise floor, reachable only with limited-bandwidth/large-size resonant structures, or cryogenic systems
A possibility of a single-channel full-bandwidth sensor (at the expense of loss of sensitivity)
Novel scheme for ambient background noise suppression
Azimuthal orientation sensitivity

VLF (Very Low Frequency) Quantum Magnetic Field Receiver With Ambient Noise Suppression