WiFi Sensing for Senior Independence

1. Introduction

Many modern wireless communication systems likeLTE, LTE-Advanced, 5G NR, IEEE 802.11n (WiFi 4),IEEE 802.11ac (WiFi 5) and IEEE 802.11ax (WiFi 6)are continuously sensing the state of the wireless channel through sounding signals (802.11, 2016; 802.11n,2009; 802.11ac, 2013; 802.11s, 2011). Sounding signals are mainly used to understand the environment and the wireless communication channel. This allows the communication system to deliver higher through-put rates, dynamically perform optimizations of the transmitted frame, improve the spectrum efficiency andin general, improve the robustness of the system. They also enable self-driven calibration systems and wire-less signal pre-compensation and post-compensation techniques, minimizing differences between the transmitted and received signals.

The sensing mechanisms used in modern wireless communication systems are always improving, generating fine-grained data in order to have a perfect understanding of the conditions of the wireless channelat any given point in time. The data captured during the sounding of the wireless systems is an excellent input for performing activity classification by applying digital signal processing (DSP) and machine learning(ML) techniques.

Aerial’s technologies rely on channel estimation measurements in order to perform device-free activity classification. The term “device-free” refers to the fact that the person, or group of people for whom activities are detected by Aerial, do not need to wear any device.People passively distort the sounding signals that travel between WiFi devices while Aerial translates those distortions into activities, e.g. walking, sitting, sleeping, falling. In particular, Aerial uses channel state information (CSI) and other metadata available in WiFi chipsets, starting from IEEE 802.11n (WiFi 4) onward.Since the previously mentioned sensing mechanisms are always improving, Aerial’s product offering is not only well supported in future WiFi platforms but it also increases the resolution of the data gathered by Aerial.As an example, the availability of higher bandwidth links and additional spatial streams in MIMO are ex-amples of continuous improvement in WiFi systems, which provides higher resolution data for the solutions developed at Aerial.

The goal of this white paper is to summarize the fundamentals behind the technology developed and deployed by Aerial during a Pre-commercial pilot with Telefónica, as well as sharing some of the results of this Study Case. Other publications within the IP portfolio produced by Aerial include: (Ghourchian, Allegue-Martinez, and Precup, 2017), (Chen et al.,2017), (Samadh et al., 2019), and (Ghourchian and Allegue-Martinez, 2017). Another contribution from Quantenna can be found in (Dash and Agrawal, 2019).

Figure 1
Figure 1 : Illustration of direct path and multipath betweenany two WiFi connected devices while acting as trans-mitter or receiver: (a) multiple direct path formedbetween each transmitter antenna and each receiverantenna and (b) example of multipath componentsfor only one transmitter antenna and one receiverantenna

2. The Sensing Infrastructure: Your Own WiFi network

The sensing infrastructure is the WiFi infrastructure that is already deployed in any home or business space.Before analyzing any particular WiFi device or network topology, Figure 1 illustrates the propagation mechanism of indoor WiFi signals. Suppose there is a WiFi transmitter and a receiver both with multiple antennas as in Figure 1. The communication system will exhibit a direct path between each transmitter antenna and each receiver antenna as illustrated in Figure 1(a). In addition to the direct path, there are also multipath components associated with each of the direct paths which generates a fine-grained grid of signals travelling along the space as shown in 1(b). A human being, a pet, or any object moving within the space illustrated in Figure 1 will distort the signals that are travelling be-tween transmitter and receiver. This fine-grained grid is described by the measurements that characterize the wireless channel. In the physical layer description of standards like IEEE 802.11n (WiFi 4), IEEE 802.11ac(WiFi 5), or IEEE 802.11ax (WiFi 6) these measurements are registered as CSI.

Figure 2
Figure 2: Example of a typical WiFi network in a residential environment and sensing area created by Aerial for only one of the wireless links

2.1 Brief description of CSI

The mechanism of collecting CSI measurements be-tween two WiFi devices, e.g. a Google mini connected to an access point (AP), can be summarized in a simple way, ignoring additive noise and/or interference, through the following equation in frequency domain:

where, Hk is the wireless channel matrix established be-tween the AP and the Google Home Mini as illustrated in Figure 2 with dimensionsNRXxNT X. The number of receiving and transmitter antennas are NRX and NT X, respectively. xk represents the sounding signal and yk is the received signal. The baseband equivalent channel Hk is a complex value and the wireless channel affects the phase and amplitude of the transmitted signals. The estimation of Hk is the under going measurement behind the CSI reports that are later ex-changed between WiFi devices. In other words, CSI is no more than a report that describes the relationship between a signal that a transmitter is sending, xk, and the signal received on the other end of the wireless link, i.e.yk. The exact relationship can be established because of the knowledge that the receiver device has about the training sequence emitted on the transmitter side. That relationship has different dimensions. One dimension is the number of spatial streams. The spatial streams can be seen as using parallel channels the signal could take to reach the receiver. A single stream is formed between each pair of transmitter and receiver antennas. This is an important concept from multiple-input multiple-output (MIMO) systems which is what the standards are based on. CSI reflects the behavior of those different spatial streams and, in general, the behaviour of the wireless channel.

Figure 3
Figure 3: An example of measurements of the amplitude of CSI (H(l)k)captured over 20 seconds (with sampling rate of 20 pks/s)for two different spatial streams representing No Motion or Empty, walking in Room1 and walking in Room2.

In order to visualize the concept explained above, Figure 3 shows an example of the amplitude of CSI for a wireless link established between two different WiFi devices. Figure 3 shows the amplitude of two channel coefficients versus subcarriers plotted over multiple frames (along rows) and three different tests (along columns). In the first test the CSI information is collected when no one is within the sensing area, i.e. an empty dwelling. The second test (second column) was recorded when a person was continuously walking in a room i.e. Room 1. In the third test (along the third column), the person was walking in a different room of the dwelling labelled as Room 2. For the three tests, two out of the sixteen available spatial streams are represented along the rows of figures in Figure 3

Figure 4
Figure 4: Example of a typical WiFi network in a residential environment and sensing area created by Aerial for two of the possible wireless links where Aerial agent is located on the home router

2.2 A Star Topology

In a typical WiFi deployment there are different types of network topologies that can be used. As illustrated in Figure 2, a sensing area is generated through at least two connected devices, for example, it could be the AP and the Google mini. Figure 4 shows the same space, although in this case two of the clients or station devices have been used to generate the sensing area created by the union of the areas that each of them cover. In this case, Aerial collects CSI measurements in the AP by using a small software agent with negligible footprint in the AP. The overall architecture of the solution is further discussed in Section 3.This example illustrates the ability of collecting CSI in a residential router by using available clients like a smart bulb and the Google mini. The WiFi network topology represented in Figures 2 and 4 is known asa star topology. Aerial can leverage this topology to generate multiple sensing areas.

The boundaries of the sensing area may not be clearly defined. In most cases, the specific shape of the sensing area is unknown since it will depend on the environ-ment (type of house, size, number and type of walls, etc.), the specific communication system generating the sensing area (hardware), the power levels employed by the devices involved and the distance between them.

2.3 Extended Star Topology

In 2020, an estimated 45 million Americans provided unpaid care to an adult. That’s around 1 in every 6 adults.

If there is any repeater in the WLAN, and the CSI in-formation is collected in that repeater, there could be a sensing area generated by the connection between the repeater and the router that the repeater is associated with. Another sensing area could be generated between the repeater and any other WiFi Client connected to that repeater, as shown in Appendix A Figure8. At the same time, the router that is also an AP, can be connected to other client stations generating other sensing areas. For multiple ISPs and WiFi vendors, this is the preferred option. In this way, a WiFi repeater can increase the WiFi coverage for larger houses. It is usually required in houses with more than1,500s.f. In this case, the AP or home router usually has a minimum of three to four antennas

2.4 Mesh Topology

Another deployment option often employed by ISPs and WiFi manufacturers is a mesh topology. In this topology, there are multiple APs involved where a mini-mum of two is required. Those APs are usually referred to as Mesh APs. The amendment to the IEEE 802.11standard to include mesh is 802.11s (802.11s, 2011).This topology splits the traffic load of the AP of a star topology deployment by deploying multiple APs. Each client can associate with the closest AP or, depending on the management software of the mesh, with an AP that is not the closest but has less traffic load and it is within a good coverage range. Usually, the wireless linke stablished between WiFi Clients and APs are stronger due to the presence of multiple APs. However, there is a logic (commonly translated into an additional software layer) behind this topology since there is traffic that needs to be circulated between the different APs in charge of transporting user data to the Internet.

Mesh is another topology that Aerial leverages to deploy human activity classification. As an example, the sensing areas generated by this topology can look like the one in Appendix A Figure 9. Notice that with only two devices where Aerial’s agent is installed on, four different sensing areas are generated by leveraging the existing devices in the network. There are three sensing areas generated by the APs composing the WiFi mesh network, and one sensing area generated by one of the STA devices that is connected to the closest AP mesh node.

Aerial support all the above-mentioned WiFi topologies which provides a wide flexibility for deployingWiFi sensing services independently of the particular topology deployed by the customers.

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