What is RF-PMD and How Can it be used as a sensor?

Abstract

Radio Frequency (RF) Polarization Mode Dispersion (PMD) refers to the polarization changes of RF signals after passing through space, reflecting off objects and recombining at the receiver. By using phase coherent signals, additional information can be obtained using Coherent Signal Dispersion (CSD) where the space domain can be leveraged.  The combination of RF PMD and CSD leads to a wide range of sensor possibilities in medical, defense and food processing applications.

Introduction: Use of Radio Frequencies

Electromagnetic waves which travel through space are ideal for remote non-contact sensing and for many reasons the radio frequency (RF) band is often the best choice. RF is ubiquitous, easy, inexpensive, and oftentimes license-free -- simplifying the hardware design, development, and deployment. Examples of radio frequency uses across the spectrum are shown in Figure 1.

Figure 1: Use of Radio Frequencies by frequency and wavelength.

RF can pass through many materials (wood, glass), can be blocked completely by others (metals), and can penetrate some objects (human body) at depths which are dependent upon the frequency. RF signals rarely, if ever, travel solely in a direct path from transmitter to receiver. Rather, RF signals reflect, refract, and scatter off objects in the area, creating new pathways for the signal.  These additional pathways that RF signals take before recombining at the receiver are collectively called multi-path.  Many RF data analysis methods are degraded by these common multi-path impairments, but PrīmSphera’s RF-PMD technique utilizes these impairments.  We leverage the pervasive nature of RF signals through coherent signal analysis to measure all changes within the propagation channel (Figure 2).

                                                           Figure 2: Multipath analysis

Figure 3: Poincaré Sphere

RF Polarization to Polarization Mode Dispersion (PMD)

The polarization of an RF signal is calculated from the amplitudes and phases at two orthogonal receive antennas. This relative and coherent comparison makes it highly sensitive to any changes that the RF signal may experience while propagating through the channel from transmitter (Tx) to receiver (Rx).

Polarization is numerically represented by four Stokes parameters, S0, S1, S2, S3.  S0  represents intensity when applied to light, while the other parameters define degree and orientation of polarization.  For RF signals we plot the three Stokes parameters, S1, S2, S3, on a Poincaré sphere to display them visually.  Every point on the sphere represents a unique polarization state with literally thousands and thousands of unique polarization states.  The poles represent circular polarization (right-handed or left-handed), while the equator represents linear polarization (from horizontal to vertical), and all other areas are elliptical polarization.  Figure 3 shows how the three Stokes parameters are plotted on Poincaré sphere.

However, polarization across a single frequency does not provide much information. Many ubiquitous RF signals have a measurable bandwidth (BW):

-          5G = up to 100MHz

-          WiFi BW = up to 80MHz

-          GPS BW = 15MHz

As these signals pass through multi-path channels, they will experience Polarization Mode Dispersion (PMD).  Their multiple sub bands will undergo differing polarization changes that are dependent upon their frequencies.  This 'spreading' across frequencies is called dispersion – similar to the optical rainbow that a glass prism creates from a single-beam of light.

An RF signal traveling a direct-path channel from Tx to Rx has no polarization change.  However, direct RF paths, with no multi-path, are extremely rare in the real world.

                  Figure 4: Frequency polarization changes

A single-band RF signal traveling a multi-path channel will experience a polarization change.  It leaves the Tx antenna with one polarization and arrives at the Rx antenna with another polarization.  The originating Tx polarization dot on the Poincaré sphere is received as another, single Rx dot on the sphere.  Interpreted individually, these dots are not all that helpful in understanding the propagation channel or any target within it.

A multi-band (bandwidth) RF signal traveling a multi-path channel will experience frequency-dependent polarization changes (i.e., dispersion across the multiple sub bands).  Once again, this frequency-dependent change in polarization across the signal's full bandwidth is called Polarization Mode Dispersion or PMD (Figure 4).  It can be leveraged to greatly improve remote sensing, among many other applications.  When plotted on the Poincaré sphere, the originating Tx polarization dot is received as a full, smoothly varying PMD curve, or PMD signature.

PrīmSphera’s patented technology leverages this PMD signature in two very important ways:

1. Capturing the full PMD signature allows for integration across frequency sub bands to significantly increase SNR sensing levels.  Of course, we can still integrate across time like traditional methods of improving SNR. However, the integration of the time-dimension limits the sensor's sampling rate.  Integration across PMD sub bands, the frequency-dimension, improves the sensor's SNR without reducing its sampling rate.

2. The PMD signature provides a fingerprint-like unique visualized display of the complete Tx-channel-Rx propagation path.  Any 'change' by a target properly illuminated with the RF signal will change the propagation multi-path and will thereby produce a measurable change in the PMD signature.

Polarization Mode Dispersion extended to Coherent Signal Dispersion
This PMD analysis has been extended to include other coherent Tx-Rx MIMO techniques.  Variations in antenna placement, for example, can be leveraged to analyze space-domain Coherent Signal Dispersion (CSD).  These additional CSD calculations are not merely redundant measurements of the PMD data.  Rather, they are all analyzed together in a coherent-matrix style to significantly improve sensing capabilities.  The figures below show multiple polarization transmitters and receivers in a Multi-In, Multi-Out (MIMO) system (Figure 5) as well as the associated Poincaré sphere output (Figure 6).

Figure 5: Polarization in a MIMO system

Figure 6: MIMO system output on a Poincaré sphere

Applications as a sensor:

Research at the University of Notre Dame and in collaboration with PrīmSphera has been conducted in the areas of standoff machinery health, debris intake classification, and human health condition monitoring. In machine health, RF PMD has demonstrated the ability to detect multiple vibration modalities on a single machine as well as multiple machines at once.  In aerospace applications, RF PMD has been able to classify objects that are typically sucked into a jet engine intake both in size, time of strike, and velocity. In the field of human monitoring, sensors leveraging RF PMD have shown the ability to monitor heart rate, respiratory rate, and changes in fluid level all without contacting the patient. Each of these areas will be discussed in more detail in future white papers

Conclusion:

Prīmsphera has exclusive commercialization rights to the fundamental patents on use of RF PMD and CSD held by the University of Notre Dame’s professor Dr. Thomas Pratt for a wide range of markets.  The company is actively developing hardware and software platforms so that RF PMD and CSD technology can be productized. This technology holds great promise for contact-less sensors that can be used in a wide range of applications.