**2.4. Monostatic radar MMICs**

All UWB radar sets reported so far use a bistatic antenna configuration. A monostatic UWB radar would significantly reduce the size of IR-UWB sensors because of the elimination of one antenna. However, implementation of rapid switching between the transmit and the receive path is difficult to realize in either this low-cost bipolar-only or CMOS technology. Here, a

16 Will-be-set-by-IN-TECH

Coherent detection receivers are based on the cross-correlation realized by feeding the received signal and the on-chip generated template impulse into a wideband analog four-quadrant multiplier and subsequent low-pass filtering. Fig. 18(a) shows the block diagram of the correlation receiver. The multiplier-based correlation is done in the RF domain, which leads to an energy efficient solution by omitting power-hungry wideband ADCs. In a radar setup, the transmit and receive clocks need to be phase adjusted, which in practice is done by a DDS board. The complete schematic of the UWB analog correlator circuit

(a) Diagram (b) Circuit Schematic

case of the energy detection above. The correlator consumes 35 mW.

**Figure 18.** Architecture of the correlation receiver system and the schematic of the correlator with a true

can be seen in Fig. 18(b). The core of the correlator is again a Gilbert cell which acts as a wide-band multiplier with proper template impulse amplitude applied to the switching quad. Capacitively shunted resistive emitter degeneration results in the necessary gain flatness over the whole UWB frequency band. The low-pass filters are formed by the load resistors (R1, R2) of the Gilbert-cell with the shunt capacitors (C1, C2) of the buffer, the same as shown for the

The complete receiver IC, including the differential LNA, the correlator and the template impulse generator, is shown in Fig. 19(a). It measures 0.43 X 0.61 mm2 and consumes a total DC power of 130 mW. To demonstrate the correlation performance of the receiver, the receiver IC was connected to a dipole-fed slot antenna, and placed at a distance of 20 cm from the transmitter. A small offset frequency of 100 Hz was introduced between the transmitter and receiver clocks, making the template impulses continuously sweep through the received signal. The measured cross-correlation can be seen in Fig. 19(b). More details of the correlation

All UWB radar sets reported so far use a bistatic antenna configuration. A monostatic UWB radar would significantly reduce the size of IR-UWB sensors because of the elimination of one antenna. However, implementation of rapid switching between the transmit and the receive path is difficult to realize in either this low-cost bipolar-only or CMOS technology. Here, a

*2.3.3. Correlation detection receiver*

multiplier, a low-pass filter and a buffer.

receiver can be found in [24].

**2.4. Monostatic radar MMICs**

**Figure 19.** Chip micrograph of the fully integrated correlation receiver and the measured normalized cross-correlation of the received impulse with the template impulse.

novel front-end concept based on a merged impulse generator/low noise amplifier, shown in Fig. 20(a) is proposed. In this design, the input of the differential low-noise amplifier is tied

**Figure 20.** Block diagram and chip photo of the proposed monostatic UWB radar front-end.

together with the output of a buffer amplifier following the impulse generator. An external monoflop and a bandgap reference circuit ensure that the LNA is disabled during the impulse emission. The LNA bias is recovered after the impulse has been transmitted, and it returns to full gain within 1.5 ns. The added parasitics of the buffer are included in the design of LNA.

Fig. 20(b) shows the fabricated IC. In the experimental test, the antenna terminal is connected to two short coaxial cables, each of which feeds into a 10 dB attenuator shorted at the far end. An approximately 1 ns delay is generated by the coaxial cable and attenuator, corresponding to a distance of 30 cm in air. The measured time domain trace at the output of the differential LNA can be seen in Fig. 21. The significant common-mode transients due to the bias switching are completely invisible owing to the balanced setup. The result clearly shows the received impulse echo. Due to a high isolation of the 'cold' low-noise amplifier, the crosstalk from the transmitted pulse is barely visible and will not influence the further processing.

**Figure 21.** Real time oscilloscope trace showing the functional test of the monostatic radar frontend, displaying transmitted pulse crosstalk and received pulse echo.

**Figure 22.** BER performance of PPM modulation for correlator and energy detection with AWGN

by the comb filter receiver presented in the following part.

to a spreading sequence. The UWB transmit signal *s*(*t*) can be written as

∞ ∑ *k*=−∞ *N*−1 ∑ *n*=0

where *p*(*t*) is a UWB impulse and *cn* is the spreading sequence. *Tc* is the period between two UWB impulses or 'chip period' and *Ts* = *NTc* is symbol period for communications or measurement period for radar/localization application. For communication, assuming a binary transmission, the impulse train of each data symbol can be modulated in different

*cn p*(*t* − *nTc* − *kTs*), (6)

457

UWB in Medicine – High Performance UWB Systems for Biomedical Diagnostics and Short Range Communications

*s*(*t*) =

The performance of the correlation receiver suffers from synchronization uncertainty and multipath propagation. This is due to the fact that the impulse used in UWB systems is very narrow and the impulse correlation receiver cannot capture all of the signal energy. On the other hand, the energy detector shows good performance also for non-perfect synchronization and multipath channel. We can conclude from the results that, with a performance trade-off, the energy detection is much more robust. Other challenges for implementing energy detection are multiuser capability and interference cancellation. These problems can be solved

The received signal power for medical applications are expected to be very small due to high attenuation in human tissue. We propose a receiver based on a comb filter to improve Signal-to-Noise ratio (SNR) before further processing. The comb filter is a feedback loop with an analogue delay and a constant loop gain of one for all frequencies. It is used to perform a coherent combination of the incoming UWB impulses. The feedback loop sums up the number of impulses used for the transmission of a data symbol/measurement and is reset after this. The coherent combination results in SNR improvement, interference suppression which come from different transmitters in a multiuser environment or narrowband interfering signals. Several UWB impulses are transmitted for one data symbol/measurement. One important feature of the concept is that the individual UWB impulses are weighted by +1 or -1 according

channel and multipath channel.

**3.2. Comb filter**
