**2. UWB radar architecture**

42 Ultra Wideband – Current Status and Future Trends

**Figure 1.** Pulse reflection and transmission diagram

is consumed only during pulse transmitting period.

1.

such as shape and material properties about the targets. Discrimination of target using higher order signal processing of impulse signals can distinguish between materials that

To work as UWB radar, the UWB transmitter sends a narrow pulse toward a target and an UWB receiver detects the reflected signal. This is a very simple algorithm of radar sensing which has been widely used. For biomedical radar, the target is, for example, a human heart. When the UWB pulse in propagation encounters an boundary of two types of medium with different dielectric properties, a portion of the incident electromagnetic energy is reflected back to the original medium with a reflection angle *<sup>r</sup>* θ (zero reflection angle if the incident wave path is parallel to the normal line), while the other portion continues propagating through the next medium. The analogy of the transmission of UWB pulse is shown in Figure

> θ *r* θ*i*

Unlike ultrasound device, which is being widely used at the present time that requires direct skin contact, the UWB makes imaging internal organ movements without invasive surgical or direct skin contact possible. Another advantage in using UWB technology is that the UWB transceiver is simple and occupies a very small chip area as it does not require complicated frequency recovery system as in the narrow bandwidth transceiver. In addition, power consumption of the impulse based UWB systems is extremely low because the power

θ*t*

would not be otherwise distinguishable by the narrowband signals.

When designing a UWB radar transceiver system, two design aspects need to be considered: architecture and implementation. Different architecture set the fundamental performance capabilities of the design, and good implementation choices improves radar performances. Impulse radar detection range depends on radiated energy, transmitter and receiver design, target size, and signal processing. Among various UWB transceiver architectures, the impulse-based energy detection UWB transceiver architecture is discussed here.

An example of the impulse-based energy detection transceiver architecture is shown in Figure 2. In this architecture, the transmitter sends a pulse train toward the target. The interface between two medias produces a partial reflection. Then the receiver detects and samples this particular type of reflected pulse train, and the decision circuit makes the final decision. Pulses are diffracted and scattered by different tissue layers and organs in human body. Channel distortion and power loss easily destructs the reflected pulses and make them undistinguishable. The rang-gate is designed to look for the destined reflected pulse rather than wait and receive every reflected pulse from every location and try to identify the expected return pulse, which in many cases are very week and tangled with other return pulses. The receiver samples only the pulses arriving at the receiver during a very narrow time window after pulse transmission, as shown in Figure.3. By estimating the distance of the expected target, a delay time is chosen.

This proposed transceiver architecture enormously reduces the circuitry complexity and power consumption. The transmitter consists of a modulator, a pulse generator, and a variable gain amplifier (VGA) driver. An on-off keying (OOK) modulation scheme is used to modulate the pulse. The VGA and driver are used to amplify output and match output impedance. The receiver consists of a low noise amplifier (LNA), a correlator, an integrator, a clocked voltage comparator, and a delay controller. The input clock train and control signal are modulated to a sequence of clock pulse, which then enters the pulse generator to produce a pulse train. This pulse train is passed onto a driver amplifier and then to an UWB antenna. The reflected pulse is caught by the antenna in the non-coherent receiver and amplified by a LNA. The signal then is squared by a multiplier at the asynchronous receiver. The squared output is then fed into an integrator and clocked comparator to boost up the voltage and reconstruct the signals. The range controller uses logic gates to switch on/off the LNA and disable the sampling operation of the comparator for range finding.
