60GHz mmWave moving into the mainstream
Article By : Dave Royer
Although 60GHz millimeter wave (mmWave) technology has historically seen its greatest use in military and scientific...
If you think of the different radio frequencies that our modern technologies use, you may struggle to think of any widespread users of the 60GHz frequency. Why haven’t we seen more widespread use of 60GHz until recently? The emergence of new applications such as those demonstrated by Ainstein at the annual Consumer Electronics Show (CES) in Las Vegas promise to soon bring 60GHz into the mainstream.
Brief history and overview of 60GHz
The 60GHz frequency is an Extremely High Frequency (EHF) which falls in the V Band, as designated by the IEEE (which includes frequencies ranging from 40GHz to 75GHz). The 60GHz frequency is also considered a millimeter-wave (mmWave) frequency. As a mmWave frequency, 60GHz offers benefits that haven’t been easily accessible to civilian use until recently.
Few developers and engineers are familiar with 60GHz applications, as it has historically seen its greatest use in military and scientific research applications. However, the 60GHz frequency is unlicensed and open for all to use and develop applications for (provided those applications meet the same Federal Communications Commission rules of non-interference that all other radio frequency applications must).
Benefits and challenges of working with the 60GHz frequency
Ongoing challenges
A few challenges remain from an engineering perspective that accompany the utilization of 60GHz frequency for radar systems.
One challenge exists at the edge of the field of view (FOV) wherein tracking a target can become difficult because the reflected signal strength is weaker than it is closer to the center of the FOV. This is a well-known challenge, however, and one that can be mitigated through the use of multiple radar system sensors.
Another challenge relates to the algorithm used by radar systems for deciding which objects are targets and which are clutter. Given a stationary target in a clutter rich environment (e.g., several people sitting for an extended period at a table), traditional Doppler domain processing may occasionally incorrectly classify a stationary target as clutter.
However, the potential benefits of radar systems designed for the 60GHz frequency may well outweigh these challenges.
Benefits
The 60GHz frequency offers potential benefits to those who develop applications for it. Chief among these isn’t inherent to the frequency itself, but a fact of the success – and crowding – of applications utilizing other radio frequencies: 60GHz is a relatively uncongested frequency, compared to the lower 2.4GHz and 5GHz frequencies typically used for Wi-Fi networks, which increasingly suffer performance issues owing to high congestion on these frequencies. 60GHz is largely free of these congestion concerns, and as a result promises better signal integrity.
A related benefit is that the unlicensed 60GHz frequency offers up to 9GHz of spectrum – many times greater bandwidth than what is available in the lower unlicensed frequencies. This also supports the case for greater signal integrity in the 60GHz frequency. In an editorial note to ACM’s SIGCOMM Computer Communication Review, L. Lily Yang of Intel Corporation said “…the abundance of the bandwidth in the unlicensed 60GHz band is unmatched in any of the lower frequency bands.”
Much of the excitement around 60GHz in the past decade has centered around wireless communications. However, for domains such as radar systems, the large bandwidth available in the 60GHz frequency can provide a much more precise and accurate radar reading, lending itself to use cases for radar that previously haven’t been widely deployed – people counting, building automation, perimeter security, and workplace safety, to name a few.
Another benefit of the 60GHz frequency derives from its short wavelength. The short wavelength of the 60GHz frequency is less capable of moving through walls and buildings (as 2.4GHz Wi-Fi signals do, for example). Historically this has been seen as a drawback, since the frequency has been viewed mostly through the lens of communications systems where we seek radio signals that travel through walls and building (cellular networks and Wi-Fi networks, for example). Thus, in many cases, allowing a signal to pass through walls is desirable. But, with the proliferation of radio signals flying through our airwaves today, there is a growing need on the part of many users for signals which remain contained to a defined space.
There are several factors that affect whether a signal can penetrate a solid object, such as a building's wall. Many of these relate to the wavelength of the signal. Whether or not a physical barrier such as a building's wall can be penetrated by a wireless signal depends in large part on the material the wall is made of. As long as the wall isn't made of a highly conductive metal, or an extremely dense material (brick, concrete, etc.), there typically aren't any issues with a Wi-Fi signal traveling from one room to another in a home.
The most common RF and microwave signals — AM/FM radio signals, television signals, cellular signals, and Wi-Fi signals, for example — feature a relatively long wavelength on the order of a few meters down to 12mm for Wi-Fi signals. For signals with this order of wavelength, a typical building wall is effectively "transparent", allowing them to pass through as easily as light passes through glass. In the case of higher frequency signals, though, walls begin to act less like light through a glass window and more like a closed door.
Signal penetration depth is described by the Beer-Lambert law, which says:
The Beer-Lambert law is related to the signal attenuation constant as such:
Where deltap is the penetration depth of signal into a surface. Alpha is found by calculating the attenuation constant, which is affected by the signal wavelength (lambda). Thus, as a signal’s wavelength decreases (i.e. higher frequency signal), its attenuation constant becomes larger, and its penetration depth decreases.
A 2014 study commission by the UK Office of Communications (the UK’s equivalent of the US FCC) looked at signal loss in buildings for different building materials and different frequencies. Figure 2 summarizes some of the study's findings, which are consistent with what theory predicts: energy loss (as measured in dB) of a signal increases rapidly and directly in proportion to a signal’s frequency.
Figure 1. Signal loss in buildings for different building materials and different frequencies. (Source: UK Office of Communications/2014 Aegis Systems technical study)
Radar systems utilizing the 60GHz frequency enable sensors to accurately sense range, velocity and angle of objects in a scene, all while providing the high resolution needed for industrial environments.
One frequent challenge for moving vehicles — construction machinery, mining equipment, and other specialty vehicles — as well as for unmanned aerial vehicles (UAVs), and industrial settings is how to track targets in these difficult environments. A target may be something as simple as a boulder blocking the path in a large open-pit mine, or it could be a human being which must be avoided by heavy industrial machinery in a manufacturing setting.
A device that operates at a lower frequency requires a larger antenna to properly send and receive signals than a similar device operating at a higher frequency. Because of this principle, radar systems utilizing the 60GHz frequency can be miniaturized and made smaller than similar devices utilizing the lower 2.4GHz frequency. Doing so opens up new potential uses of radar systems and radar sensors that would otherwise be infeasible due to physical constraints of large radar systems.
Solutions
By integrating the components needed for radar design, specialized system-on-chip (SoC) devices reduce the complexity of designing mmWave applications. Using these devices, developers can more effectively implement systems with multiple input multiple output (MIMO) technology.
MIMO is a concept most well-known in the domain of wireless communications but is also critical in the domain of radar systems design. By incorporating MIMO design into radar systems, a 3D image of an environment and targets can be developed. Utilizing multiple antennas in a radar system gives a more complete understanding of an environment. Antenna design in the 60GHz frequency also allows for a wide FOV by utilizing a smaller physical sensor size. To achieve a ±90° unambiguous FOV, the spacing between the four receive antennas was designed to be half wavelength, while the three transmit antennas are separated by a wavelength. This results in a 4×2 grid in both the elevation and azimuth planes.
Figure 2: Antenna design to achieve ±90° unambiguous FOV in 60GHz application.(Source: Ainstein)
The high bandwidth available in 60GHz applications means that readings from radar systems in this frequency can be made with a much higher resolution than is possible at lower frequencies. Traditional radar hasn’t needed to worry as much about high resolution, since it has historically been used mostly for detection of large objects, such as aircraft. But with the increasing prevalence of autonomous or semi-autonomous machines in our daily lives, the need for higher resolution detection for sensing smaller objects — light poles, buildings, or people, for example — has grown.
An example use case for 60GHz radar systems: people counting
People counting has grown to become an industry unto itself in the past decade, as retailers and hospitality properties seek to better understand and optimize their operations, and to deliver a better experience to their customers and employees.
People counting is a relatively mature application, which can be done using several different technologies. The most common existing technologies for people counting include video computer vision, infrared, thermal imaging, and Wi-Fi.
A major concern for many potential customers of video-based people counting solutions is privacy. This single concern frequently keeps them from deploying video-based people counting solutions, even when the organization could realize benefit from doing so. While video-based technologies offer accuracy and efficiency, they are entangled with privacy and safety concerns due to personally-identifiable information (PII)collected on specific individuals.
Radar systems utilizing the 60GHz frequency can provide the same information to a facility manager without the privacy concerns. Using radar alone, we can determine whether or not there are people in a room. This can be important for applications such as management of conferences rooms, smart buildings, hotels, and much more. For example, hotels can implement smart energy efficient smart lighting or temperature control based on room occupancy. And, we can do this without invading the privacy of the individuals in the room, because we aren't using a visualization tool: we're simply using a radar reflection to determine how many people are in the room, and how many people leave the room.
The standard algorithm for radar processing is the well-known Radar Equation:
Where:
Pr = Rx power
Pt = Tx power
Gt = Tx antenna gain
σ = scattering coefficient (an empirically measured or estimated value)
F = pattern propagation
Rt = distance from Tx to target
Rr = distance from target to Rx
Ar = effective aperture of Rx antenna as follows:
Where:
λ = Tx wavelength
Gr = Rx antenna gain
As is clear from the radar equation, a radar signal’s received power depends on the signal’s wavelength, which is inversely related to its frequency. Thus, higher frequency signals require greater Tx power and Tx antenna gain for the same received power. This is a significant design challenge, and one reason why use of the 60GHz frequency has been largely restricted to scientific and military applications until recently.
In real-world applications, additional controls are added to the general radar equation to improve filtering, to account for path loss, etc. Most radar systems providers also include their own proprietary algorithms for maximizing performance.
A new generation of smart sensors will dramatically improve utilization and functionality of commercial buildings as well as reduce operating costs with energy efficient solutions such as occupancy adjusted lighting and temperature controls. In the average commercial building, for example, HVAC consumes 39 percent of the energy budget. Smart sensors can reduce that spend by 18 percent, according to a recent report by the American Council for an Energy Efficient Economy (ACEEE).
As another example use case, a 2015 study found that proper usage of digital signage can reduce checkout waiting times in retail settings by up to 35 percent and increase overall sales volume by more than 30%. Achieving these results requires an intimate understanding of how people respond to the signage. By combining digital signage with footfall data collected from people counting technologies — how many people pass within view of digital signage, and how long they spend within view of that digital signage — retailers can be sure to capture more of this potential.
Wi-Fi based people counting solutions don’t provide a true count of people in a room, as they typically rely on received signal strength (RSS) measurements and/or actual Wi-Fi sessions. These technologies provide a concrete minimum number of people in a given area at a given time, and also extract an estimated total number of people using this baseline RSS data and statistical estimation methods. These methods tend to give a good estimate of foot traffic, but it is nonetheless an estimate. First, though most adults in the U.S. do own and carry a Wi-Fi enabled smartphone most of the time, the rate is still not 100%. Additionally, not all smartphone owners allow their devices to automatically probe for open networks (in many cases, precisely because of privacy and/or security concerns). Similarly, another person may have a Wi-Fi capable smart watch, smart phone, tablet, and laptop and connect them all to the network, again skewing the count. Wi-Fi based people counting also is limited in its ability to map the exact location and movement of people in a space. Moreover, these methods can leave individuals exposed to potential privacy breaches, despite claims to the counter, as demonstrated by Julien Freudiger in a 2015 ACM research paper, by Levent Demir in a 2013 INRIA research paper, and others.
Infrared imaging and thermal imaging can provide the necessary accuracy, but installation and operating costs for both can quickly become prohibitively expensive, as these systems require large amounts of power and run constantly.
Many office buildings have common conference rooms. Use of these rooms can become unwieldy to manage, even with a professional office manager. Sometimes people reserve a conference room on a calendar but end up not needing the space – but the calendar doesn’t get updated, leading to unused conference rooms. Other times, people may hold an impromptu meeting in a conference room they haven’t reserved. These are the types of situations that a radar-based people counting system helps to address.
An example of a real-world configuration for people counting for conference room management using 60GHz radar systems might look like the following. Three 60GHz radar systems modules can be placed in three separate conference rooms. Because these modules utilize radar systems technology, they can detect if a conference room is occupied or not (just like radar systems detect if an obstacle is nearby for a drone, or if another car is nearby in the case of an automobile).
Data are first processed on-board the radar systems modules, and are then transferred from the module to a microcontroller connected to a Wi-Fi network. This microcontroller can then be accessed with voice command via Amazon’s Alexa from anywhere in the world that has an Internet connection, and returns a response to the office manager as to whether or not a particular conference room is occupied.
–Dave Royer is the Director Of Engineering at Ainstein Inc.