What are UWB Radars?

UWB radars or Ultra Wideband Radars, are radars that use Ultra Wideband technology to transmit and receive short-time, low energy, wideband radio frequency signals reflected by target objects. The duration of a single pulse emitted by these radars is usually in the order of a few nanoseconds to a few hundred picoseconds. The pulse width of UWB signal in the time domain is inversely proportional to the bandwidth of the signal. This means that the narrower the pulse width, the wider the bandwidth of the signal.

The narrower the signal, the sharper the signal in time domain and therefore, UWB radars can provide cm-level location information between a transmitter and receiver within a short range (10-15 meters). The ranging and positioning information are significantly more accurate than Bluetooth beacons (however, not the newer Bluetooth direction finding feature) and Wi-Fi access points (APs) which has resulted in companies like Apple to incorporate UWB based radar technology into iPhones devices and AirTags. iPhone 11 was the first apple device to use this technology. UWB radars are ideal for location-based services, indoor ranging and positioning, radar target (stationary or moving) tracking, and in medical applications such as monitoring breathing and heart rate to determine whether a person is asleep or awake.

UWB radars operate over a wide frequency range from 3.1 to 10.6 GHz and the bandwidth of UWB signals is at least 20% of the center frequency or at least 500 MHz. For example, a UWB signal with a center frequency of 4 GHz utilizes a bandwidth of 1 GHz and a UWB radar device with a center frequency of 6 GHz has a bandwidth of 1.5 GHz. Because of the wide spectrum available for this technology, UWB radars can send pulsed data to the receiver very quickly. The narrower pulse width allows it to implement modulation techniques such as Pulse Position Modulation (PPM), On/Off Keying (OOK), and Pulse Amplitude Modulation (PAM).

Furthermore, the wide bandwidth allows radars to transmit signals with low power spectral density, thereby minimizing interference due to neighboring devices that operate in the same band as UWB radars. Regulators like the FCC have restricted the spectral density of UWB signals to -41 dBm/MHz allowing UWB radars to emit signals that are lower than spurious emissions from other devices in the proximity.

Image Credit: Eliko

In addition to minimizing interference from neighboring devices and other radars, UWB radar transmissions offer security in the sense that they are very difficult to detect. This is because the low power spectral density does not give a chance for the eavesdropper to determine whether the radar is transmitting or not as the transmitted pulse amplitude does not significantly differ from the existing ambient noise amplitude.

Working Principle of UWB Radars

The above figure shows a typical UWB radar transceiver block diagram that consists of a transmitter and receiver section. The transmitter section includes a pulse generator, power amplifier, modulator, mixer, and a transmitting antenna. The receiving section includes a receiving antenna, a low noise amplifier, a correlator (consisting of an integrator and a mixer), and a bandpass filter.

The pulse generator is driven by the oscillator and generates a pulse with a very narrow pulse width (typically on the order of 2 ns) in the time domain for transmission. The oscillator is responsible for driving the pulse generator with a desired waveform such as a Gaussian envelope (that has a narrow pulse width and looks like an impulse signal) and also determines the pulse repetition frequency (PRF) of the UWB radar. PRF is the rate at which the UWB pulses are transmitted in a unit of time. The signal is then amplified by a power amplifier to an extent while adhering to the low spectral density conditions imposed by the FCC. The signal is then fed to the antenna via a feed line and is then radiated into space towards the target object.

The signal, upon getting reflected from the target, reaches the receiver and is captured by the receiving antenna. Filters can be appropriately designed such that the reflections from other objects can be filtered to produce only the target reflection. The signal passes through the LNA to produce a signal with a low noise level compared to the received signal. The amount of reflection depends on the size of the reflecting surface and the distance of the target from the radar. The UWB radar captures the reflection emitted by every object within its field of view (FOV), and the distance between the radar and target directly corresponds to the time-position in the radar frame. UWB radars generally employ the Time of Flight (ToF) technique to estimate this distance since the time at which UWB pulses have arrived can be accurately determined as they are very narrow. Other techniques such as Time Difference of Arrival (TDoA) and Two Way Ranging (TWR) are also utilized depending on the application and environment.

Upon calculating the distance, other positioning techniques such as trilateration, triangulation, and others can be applied to determine the location of target. Fast DSP processors can be integrated to rapidly calculate the location at every instant of time. This allows the users to know the target location at any time.

The power consumed by the UWB radars is very low since the radar transmits the impulse only for a very short time period. And given its wide bandwidth, the radars can receive a relatively large amount of information about the target compared to conventional narrowband radars. The very narrow pulse width along with higher-order signal processing of impulse signals enable UWB radars to cleanly discriminate the target reflections from reflections produced by other surrounding objects.

These radars can operate both in Line of Sight (LoS) and Non Line of Sight (NLoS), though it produces more accurate results under LOS conditions. They can be useful in accurately locking on to targets in indoor environments where there are doors, walls, ceilings, and multiple rooms.

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