# Magnetic Communication Using High-Sensitivity Magnetic Field Detectors
## 2 Characteristics of Magnetic Field Detectors  
Presently, various magnetic sensors are commercially available. Most of the sensors measure the magnetic flux density of the magnetic field based on different sensing principles. For certain measurement tasks certain types of magnetic field sensors have been established. For example, giant magneto-resistive (GMR) sensors are used in reading heads of hard disks, whereas superconducting quantum interference devices (SQUIDs) are applied for measuring weak biomagnetic fields. In this section, the most common sensors are characterized regarding their suitability as a wideband receiver in a magnetic communication system. The crucial characteristics with respect to mobile communication purposes are summarized in Table 1.  
* SQUIDs are ultra-sensitive magnetic field sensors [11]. The sensors can measure magnetic flux densities on the order of fT (for example brain activity tracking by magnetoencephalography). To maintain superconductivity, SQUIDs need to operate within a few degrees of absolute zero. This is ensured by cooling the devices with liquid helium, which is certainly not possible in mobile applications. * The optical pumping resonance method exhibits very high sensitivity at low frequencies, but the sensor is large and is only able to detect the scalar (i.e., total) value of a magnetic field. The major problem regarding communication purposes is that the sensitivity of these magnetometers decreases rapidly for frequencies above 10 Hz [12]. This prevents communication at reasonable data rates. * Hall effect sensors are among the most popular magnetic field sensors. The sensors are very cheap and are mainly used as proximity indicators. They cannot be used for navigation purposes due to their low sensitivity [9]. Most properties of Hall effect sensors are suitable for magnetic communication systems. However, the high detection threshold results in very short communication ranges. Figure 1: Conventional coil-to-coil topology (**a**) and coil-to-AMR topology under investigation (**b**). * Fluxgate sensors can detect magnetic fields on the order of pT. Due to the small size and battery power supply, fluxgate magnetometers are qualified for mobile applications. The achievable data rate is limited by the upper frequency of a fluxgate sensor, which is about 10 kHz [12]. * Another well-known type of detector is magneto-resistive sensors. GMR and AMR sensors are implemented today in almost every smartphone, e.g. for compass functionality. Both sensor types require ultra-low power consumption and are cheap. The main advantage is the high bandwidth of up to 1 MHz and beyond. Due to the lower detection limit of AMR sensors, this method is favorable for communication systems. Beside the features mentioned in Table 1, depending on the application other characteristics should be addressed, such as linearity, demagnetization caused by high field strengths, and directivity of the magnetic field. In the following, the receiver coil of a conventional MI communication system (Figure 1a) and the AMR sensor as an example for the proposed topology (Figure 1b) are described in more detail.