**1. Introduction**

40 Ultra Wideband – Current Status and Future Trends

Inf. Theory. pp. 66–76.

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Ultra-wideband (UWB) has received significant attention for applications in target positioning and wireless communications recently. The extremely short pulses in turn generate a very wide bandwidth and offer several advantages, such as large throughput, covertness, robustness to jamming, lower power, and coexistence with current radio services. UWB not only can transmit a huge amount of data over a short distance at very low power, but also has the capability to pass through physical objects that tend to reflect signals with narrow bandwidth.

The extremely narrow pulse (usually in order of few nanoseconds to few hundred picoseconds) makes it possible to build radar with much better spatial resolution (usually 0.1 to 1 ft) and very short-range capability compared to other conventional radars. Also, the large bandwidth allows the UWB radar to get more information about the possible surrounding targets and detect, identify, and locate only the most desired target among others. The fine resolution makes the UWB radar beneficial for medical applications. The properties of short pulse indicate that the UWB signal can penetrate a great variety of biological materials such as organic tissues, fat, blood, and bone. Experiment results show that the signals with low center frequencies achieve better material penetration. Compared to a radar system with a pulse-length of one microsecond, a short Gaussian or Gaussian monopole pulse of 200ps in width has a wavelength in free space of only 60 mm, compared to 300m. Since the pulse length in conventional radar is significantly longer than the size of the target of interest, the majority of the duration of the returned signal is an exact replica of the radiated signal. Thus, the returned signal provides little information about the nature of the target. However, since the UWB pulse length is in the same order of magnitude with the potential targets, UWB radar reflected pulses are changed by the target structure and electrical characteristics. Those changes in pulse waveform provide valuable information

© 2012 Wang et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Wang et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 would not be otherwise distinguishable by the narrowband signals.

Radar Sensing Using Ultra Wideband – Design and Implementation 43

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

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

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

Two classes of UWB signals are utilized to transmit symbols in UWB system: carrier-free impulse signal, and carrier-based short sinusoidal signal. The impulse UWB signal is often represented using Gaussian (different orders of Gaussian derivatives), Rayleigh, or Hermitian pulse. The advantages of impulse signal are that the impulse-based transceiver architecture often very simple and consume the least amount of power due to its low pulse repetition rate and low duty cycle. However, the drawback for impulse-based signal is the

LNA and disable the sampling operation of the comparator for range finding.

**2. UWB radar architecture** 

architecture is discussed here.

the expected target, a delay time is chosen.

**3. UWB radar transmitter** 

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 1.

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

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 is consumed only during pulse transmitting period.
