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We can say that UWB is the best and most advanced positioning technology available today, but where is the evidence? To answer this question, we need to look beyond the surface. This article will explore the inner workings of UWB technology and outline the differences between UWB and narrowband positioning methods.
Comparison of UWB and narrowband
There are several technologies suitable for indoor and outdoor positioning applications, but UWB is the most accurate, reliable and cost-effective; it is also generally more scalable. This can be clearly illustrated by comparing UWB technology with the most popular narrowband methods, which is what we will do.
It all comes down to bandwidth
From the outset, impulse radio UWB was designed to achieve high-precision range estimates while simultaneously communicating in both directions, so it can collect sensor data and control actuators.
Impulse radio is a form of UWB signaling that has properties that make it ideal for positioning and communication services in dense multipath environments.
In addition to its positioning capabilities, Qorvo UWB technology complies with the IEEE 802.15.4a standard and the recently released IEEE 802.15.4z standard. Therefore, in addition to centimeter-level ranging accuracy, developers also emphasized ensuring that the technology is stable and not affected by various interferences to achieve higher reliability. When formulating the standard, low power consumption and low cost factors were also considered, as well as the ability to support a large number of interconnected devices. Engineers had a vision when creating the standard: to enable every connected object to have "positioning awareness" capabilities.
The Federal Communications Commission (FCC) defines the UWB radio frequency range as 3.1 GHz to 10.6 GHz, with a minimum signal bandwidth of 500 MHz (see Figure 1). Unlike other radio technologies, UWB does not use amplitude or frequency modulation to encode the information transmitted by its signals. Instead, UWB uses a very narrow series of short pulses that encode the data using binary phase shift keying (BPSK) and/or pulse position modulation (BPM). The use of narrow pulses results in a transmission that exhibits wide bandwidth characteristics, which can extend range, reduce sensitivity to narrowband interference, and enable operation in the presence of multipath reflections.
Limitations of RSSI
In many applications today, location tracking is accomplished using the received signal strength indicator (RSSI). In RSSI applications, the strength of a radio signal varies as the inverse square of the distance from the transmitter in free space, as shown in Figure 2. As the signal moves away from the source, the signal strength decreases.
Figure 1: UWB spectrum
Figure 2: Signal source electric field
RSSI is used with Wi-Fi and Bluetooth 802.11 standards. Given the known transmit power of a transmitter device, the distance between devices can be predicted. However, these types of measurements also have drawbacks, which we will discuss next.
Location tracking using Bluetooth
Bluetooth location tracking, such as Bluetooth Low Energy (BLE) beacons, can be effective in some situations. Beacons are primarily used for proximity detection. They detect when a device (such as a phone) is within range and estimate the distance by distinguishing between strong and weak signal strength (RSSI).
The problem with this approach is that signal strength is not a good indicator of distance. If the signal strength is low, does it mean that the phone is far away from the beacon, or does it mean that there is a giant pillar between the beacon and the phone? As shown in Figure 3, each beacon has a varying degree of line of sight (LOS) to the receiving phone; each obstruction changes the overall accuracy of the distance measurement.
Figure 3: Beacon signal application (beacon and mobile device shown)
Device A can receive a very strong signal from the beacon on the conference room ceiling, but the wall significantly attenuates the signal from the beacon in the nearby corner outside the conference room, both of which are about the same distance from device A. Device B is not within LOS range of any beacon, so all signals are significantly attenuated, while device C is within LOS range of multiple beacons in the open office, so the signal strength is stronger because there is less attenuation.
A workaround to this problem is to use a method called "fingerprinting." Beacons installed in fixed locations a few meters apart measure the signal strength of other beacons at known locations. This signal strength information is stored in a fingerprinting database. The beacon can then determine the distance and location of the device by comparing its signal strength with the data in the fingerprinting database. The closest match is used to obtain the location measurement result.
There are many versions of fingerprinting, using a variety of complex algorithms. Keep in mind that these systems are just workarounds. They don't really solve the distance measurement problem with the accuracy of technologies like UWB.
Location tracking using Wi-Fi
Wi-Fi is the most commonly used radio signal for indoor positioning applications. It is still the most widely used indoor positioning technology and is often used in conjunction with BLE. The main advantage of Wi-Fi is that it is available in most public or private places.
However, using Wi-Fi signal strength to estimate distance faces the same challenges as Bluetooth. Some companies have developed alternative algorithms that attempt to use the time of flight (ToF) or time of arrival (ToA) of Wi-Fi signals to measure distance more accurately, but this cannot be directly implemented using standard Wi-Fi hardware.
ToF is a method of measuring the distance between two radio transceivers by multiplying the ToF of the signal by the speed of light. ToA is the point in time it takes for a radio signal to reach a remote receiver from a transmitter.
The accuracy of RSSI fingerprinting can be improved to some extent by adding more beacons to the network. Although the accuracy may increase a little, it does not improve the overall reliability of the measurement. In addition, if there are any changes in the floor plan, the fingerprinting database will need to be updated, which can be costly and time-consuming.
Why UWB is the best choice for indoor positioning and tracking
The inherent properties of UWB mean that it can achieve more accurate indoor positioning and distance measurement than other technologies.
As shown in Figure 4, UWB pulses (center and right) are only 2 nanoseconds (ns) wide, making them immune to interference and noise from reflected signals (multipath). The sharp edges of UWB radio frequency (RF) pulses enable accurate time of arrival and range determination in the presence of signal reflections and multipath effects common in everyday environments.
Figure 4: Comparison of a narrowband signal with a pulsed UWB direct path signal (blue) and reflected path signal (red).
When using UWB as a solution, reflected signals (grey) do not affect the direct signal (blue). The rise and fall times (edges) of IR-UWB signals (center and right) are shorter than those of standard narrowband signals (left), so the arrival time of the signal can be measured accurately. This also helps the UWB signal maintain its integrity and structure in the presence of noise and multipath effects.
Even under noisy conditions, the arrival time of a 2ns wide pulse radio UWB pulse is barely affected, as shown in Figure 4 (right). In contrast, as shown in Figure 5, the narrowband signal is significantly affected by noise.
Figure 5: How noise affects narrowband signals
We have experimented with ToF-based methods using narrowband radio technology. As shown in Figure 6, narrowband signals are very sensitive to multipath because reflected signals (dark grey) can destructively combine with the direct signal (light grey) to produce the final signal at the receiver (blue). This affects the time at which the signal crosses the threshold (used to measure ToA), reducing accuracy.
Figure 6: Narrowband signal with reflections
The accuracy advantage of UWB is very obvious. UWB is fully capable of measuring distance and location with an accuracy of 5 to 10 centimeters. In contrast, Bluetooth, Wi-Fi and other narrowband radio standards can only achieve meter-level accuracy. In addition, because UWB radio pulses are extremely short, under the multipath effect, the direct path signal will not overlap with the multipath signal, so the signal integrity and strength will not be damaged.
This shows that UWB has the following characteristics:
a. Ultra-precise, providing centimeter-level accuracy, 100 times more accurate than BLE and Wi-Fi;
b. Ultra-reliable, able to maintain signal integrity in the presence of multipath reflections;
c. Real-time, with latency 50 times lower than the Global Positioning System (GPS) and 3,000 times lower than standard beacons.
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